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Light stress is the most uncharacterized and less studied among the various types of abiotic stresses experienced by the plant systems. Plants, being sessile organisms, cannot escape from such stresses, and one of the mechanisms of adaptation under such hostile circumstances is mediated through the altered regulation of phytohormones. In this book chapter, we have presented an exhaustive literature-based study on the different kinds of light stress encompassing light quality and type, the basic mechanism of perception of UV-B (the most harmful) rays by the plant system, the general metabolites which get upregulated under stress, and then a detailed excerpt on the role of phytohormones like auxin, gibberellic acid, cytokinins, ethylene, and abscisic acid under such conditions. Based on this account, our chapter also aims at integrating the perception of light stress-signaling pathway with the phytohormone-signaling networks, thus providing the idea of a universal cross talk occurring in plant cells, exposed to a variety of light stresses.
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Plant Hormones under Challenging
Environmental Factors
Golam Jalal Ahammed Jing-Quan Yu
Plant Hormones under
Challenging Environmental
ISBN 978-94-017-7756-8 ISBN 978-94-017-7758-2 (eBook)
DOI 10.1007/978-94-017-7758-2
Library of Congress Control Number: 2016941869
© Springer Science+Business Media Dordrecht 2016
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Golam Jalal Ahammed
Department of Horticulture
Zhejiang University
Hangzhou , People’s Republic of China
Jing-Quan Yu
Department of Horticulture
Zhejiang University
Hangzhou , People’s Republic of China
This book is dedicated to the Institute of
Vegetable Science of Zhejiang University for
advancing vegetable research and industry in
Pref ace
In nature, plants are often exposed to different environmental adversities. Although
success in crop cultivation involves good care and management, some environmen-
tal factors at extreme states pose serious challenges that ultimately limit crop pro-
duction. Abiotic stress is one of the primary causes of crop losses worldwide. The
stress caused by abiotic factors alters normal plant metabolism leading to various
negative effects on plant growth, development, and productivity. To survive, sessile
plants have evolved a wide array of molecular programs to perceive environmental
stimuli rapidly and adapt accordingly. In recent years, much progress has been
achieved in unraveling the complex stress response mechanisms, particularly the
involvement of different phytohormones in stress perception and signal transduc-
tion. Phytohormones are the most fascinating features of plant system that precisely
regulate growth, development, and responses to stresses. In addition to normal regu-
latory functions, classical phytohormones such as auxins , cytokinins , gibberellins ,
abscisic acid , and ethylene could induce stress tolerance to various abiotic factors.
New plant hormones such as jasmonates, salicylates, brassinosteroids, strigolac-
tones, etc., have also been implicated in plant growth and stress adaptation. Although
the in-depth mechanisms of phytohormone-mediated stress tolerance still remain
largely unknown, plant growth regulators or hormone analogues are being used to
manage different environmental adversities. Nonetheless, there is still a remarkable
gap between theory and practice in terms of large-scale fi eld application.
In this book, we tried to provide a unique compilation of the roles of phytohor-
mones in the response of plants to abiotic stresses considering heat, cold, drought,
salinity, fl ooding, soil acidity, heavy metals, light, and ozone as an individual envi-
ronmental hazard in each chapter. The physiological and molecular mechanisms
controlling phytohormone-mediated tolerance to a single abiotic stress and interac-
tions among them are discussed for relevant cases. In the last chapter, genetic engi-
neering aspects of phytohormone metabolism along with major challenges and
future research directions are suggested. Much attention has been paid to adhere
with the focus in each chapter that enabled the authors to avoid repetition of similar
issues. It is worth mentioning that all authors of this book have recently contributed
original research articles in the fi eld of phytohormone research. The chapters are
written at the levels intended to be useful to students (senior undergraduate and
postgraduate) and researchers in plant physiology, biochemistry, and biotechnology.
Although minor editorial changes were adopted, author’s justifi cation was kept
intact in each chapter. However, some errors may still exist in the book, and thus we
would greatly appreciate reader’s feedback for potential improvement in future edi-
tion. We wish to thank all the authors who joined this book project by contributing
their valuable works. We extend our sincere thanks to Springer Science+Business
Media, especially Mr. Zachary Romano (editor, biochemistry and molecular biol-
ogy, Springer New York), Mr. Abbey (Xiaojin) Huang (assistant editor, medicine
and biological sciences, Springer Beijing Offi ce), and all the other staff members of
Springer involved in this book project for their generous cooperation.
Hangzhou, Zhejiang, People’s Republic of China Golam Jalal Ahammed
Jing-Quan Yu
1 Role of Hormones in Plant Adaptation to Heat Stress ........................ 1
Golam Jalal Ahammed , Xin Li , Jie Zhou , Yan-Hong Zhou ,
and Jing-Quan Yu
2 Involvement of Plant Hormones in Cold Stress Tolerance .................. 23
Joanna Lado , Matías Manzi , María Martha Sainz ,
Mariana Sotelo , and Lorenzo Zacarías
3 Hormonal Interactions Underlying Plant Development
under Drought ......................................................................................... 51
Maria Elizabeth Abreu , Paulo Tamaso Mioto ,
and Helenice Mercier
4 Participation of Phytohormones in Adaptation to Salt Stress ............ 75
Agnieszka Waśkiewicz , Olimpia Gładysz , and Piotr Goliński
5 Roles of Phytohormones in Morphological and Anatomical
Responses of Plants to Flooding Stress ................................................. 117
Zhongyuan Hu , Xiaohua Qi , Mingfang Zhang , Xuehao Chen ,
and Mikio Nakazono
6 Phytohormonal Responses to Soil Acidity in Plants ............................ 133
Marjorie Reyes-Díaz , Elizabeth Maria Ulloa-Inostroza ,
Jorge González- Villagra , Alexander Gueorguiev Ivanov ,
and Leonid Vladimir Kurepin
7 Use of Phytohormones for Strengthening Metal(loid)
Phytoextraction: Limitations and a Case Study .................................. 157
Meri Barbafi eri
8 Plant Responses to Light Stress: Oxidative Damages,
Photoprotection, and Role of Phytohormones ...................................... 181
Aditya Banerjee and Aryadeep Roychoudhury
9 Involvement of Phytohormones in Plant Responses to Ozone ............ 215
Elisa Pellegrini , Alice Trivellini , Lorenzo Cotrozzi ,
Paolo Vernieri , and Cristina Nali
10 Engineering Phytohormones for Abiotic Stress
Tolerance in Crop Plants ........................................................................ 247
Vinay Kumar , Saroj Kumar Sah , Tushar Khare , Varsha Shriram ,
and Shabir Hussain Wani
Index ................................................................................................................. 267
Maria Elizabeth Abreu Department of Botany, Institute of Biosciences , University
of São Paulo , São Paulo , SP , Brazil
Golam Jalal Ahammed Department of Horticulture , Zhejiang University ,
Hangzhou , People’s Republic of China
Aditya Banerjee Post Graduate Department of Biotechnology , St. Xavier’s
College (Autonomous) , Kolkata , West Bengal , India
Meri Barbafi eri National Research Council – Institute of Ecosystem Studies ,
Pisa , Italy
Xuehao Chen Department of Horticulture, School of Horticulture and Plant
Protection , Yangzhou University , Yangzhou , People’s Republic of China
Lorenzo Cotrozzi Department of Agriculture, Food and Environment , University
of Pisa , Pisa , Italy
Olimpia Gładysz Department of Inorganic Chemistry , Wrocław Medical
University , Wrocław , Poland
Piotr Goliński Department of Chemistry , Poznań University of Life Sciences ,
Poznań , Poland
Jorge González-Villagra Programa de Doctorado en Ciencias de Recursos
Naturales , Universidad de La Frontera , Temuco , Chile
Zhongyuan Hu Laboratory of Germplasm Innovation and Molecular Breeding,
Institute of Vegetable Science , Zhejiang University , Hangzhou , People’s Republic
of China
Alexander Gueorguiev Ivanov Department of Biology and the Biotron Centre for
Experimental Climate Change Research , Western University , London , ON , Canada
Tushar Khare Department of Biotechnology, Modern College , S. P. Pune
University , Pune , India
Vinay Kumar Department of Biotechnology, Modern College , S. P. Pune
University , Pune , India
Leonid Vladimir Kurepin Department of Biology and the Biotron Centre for
Experimental Climate Change Research , Western University , London , ON , Canada
Joanna Lado Instituto Nacional de Investigación Agropecuaria (INIA) , Salto ,
Xin Li Tea Research Institute , Chinese Academy of Agricultural Sciences ,
Hangzhou , People’s Republic of China
Matías Manzi Ecofi siología y Biotecnología, Departamento de Ciencias Agrarias
y del Medio Natural , Universidad Jaume I de , Castellón de la Plana , Spain
Helenice Mercier Department of Botany, Institute of Biosciences , University of
São Paulo , São Paulo , SP , Brazil
Paulo Tamaso Mioto Department of Botany, Institute of Biosciences , University
of São Paulo , São Paulo , SP , Brazil
Mikio Nakazono Graduate School of Bioagricultural Sciences , Nagoya University ,
Nagoya , Japan
Cristina Nali Department of Agriculture, Food and Environment , University of
Pisa , Pisa , Italy
Elisa Pellegrini Department of Agriculture, Food and Environment , University of
Pisa , Pisa , Italy
Xiaohua Qi Department of Horticulture, School of Horticulture and Plant
Protection , Yangzhou University , Yangzhou , Jiangsu , People’s Republic of China
Marjorie Reyes-Díaz Departamento de Ciencias Químicas y Recursos Naturales,
Facultad de Ingeniería y Ciencias , Universidad de La Frontera , Temuco , Chile
Aryadeep Roychoudhury Post Graduate Department of Biotechnology , St.
Xavier’s College (Autonomous) , Kolkata , West Bengal , India
Saroj Kumar Sah Department of Biochemistry and Molecular Biology,
Entomology and Plant Pathology , Mississippi State University , Starkville , MS ,
María Martha Sainz Laboratorio de Bioquímica, Departamento de Biología
Vegetal, Facultad de Agronomía , Universidad de la República , Montevideo ,
Varsha Shriram Department of Botany, Prof. Ramkrishna More College , S. P.
Pune University , Pune , India
Mariana Sotelo Laboratorio de Bioquímica, Departamento de Biología Vegetal,
Facultad de Agronomía , Universidad de la República , Montevideo , Uruguay
Alice Trivellini Institute of Life Science , Scuola Superiore Sant’Anna , Pisa , Italy
Elizabeth Maria Ulloa-Inostroza Programa de Doctorado en Ciencias de
Recursos Naturales , Universidad de La Frontera , Temuco , Chile
Paolo Vernieri Department of Agriculture, Food and Environment , University of
Pisa , Pisa , Italy
Shabir Hussain Wani Division of Genetics and Plant Breeding , SKUAST-K ,
Srinagar , Jammu and Kashmir , India
Agnieszka Waśkiewicz Department of Chemistry , Poznań University of Life
Sciences , Poznań , Poland
Jing-Quan Yu Department of Horticulture , Zhejiang University , Hangzhou ,
People’s Republic of China
Lorenzo Zacarías Instituto de Agroquímica y Tecnología de Alimentos , Consejo
Superior de Investigación Científi ca (CSIC) , Valencia , Spain
Mingfang Zhang Laboratory of Germplasm Innovation and Molecular Breeding,
Institute of Vegetable Science , Zhejiang University , Hangzhou , People’s Republic
of China
Jie Zhou Department of Horticulture , Zhejiang University , Hangzhou , People’s
Republic of China
Yan-Hong Zhou Department of Horticulture , Zhejiang University , Hangzhou ,
People’s Republic of China
Jing-Quan Yu is a Professor at the Department of
Horticulture, Zhejiang University, Hangzhou, China,
and the Director of the Key Laboratory of Horticultural
Plants Growth, Development and Quality Improvement,
Agricultural Ministry of China. He has completed his
M.Sc. in Agrochemistry in 1991 from Shimane
University, Japan, and Ph.D. in Bio-resource Chemistry
in 1994 from Tottori University, Japan. Afterward, he
worked as a postdoc scientist at Shimane University. In
1995, he returned to China and joined Zhejiang
University as Associate Professor. His fi elds of
specialization include environmental stress, plant
growth, development and its regulation, allelopathy,
and monocropping obstacle. Prof. Yu is one of the leading scientists in the area of
brassinosteroid research. He is a pioneer in popularizing fi eld-level application of
brassinosteroids for environmental stress management in China. Prof. Yu has been
awarded a number of honors such as Excellent Youth Instructor (2007), Yangtze
River Scholar (2007), National Natural Science Award (2006), Science and
Technology Advancement Award (2006), Science and Technology Advancement
Award of Zhejiang Province (fi rst class, 2005), National Outstanding Youth Scholar
(2002), and so on. He has been serving as an editor for the International Journal of
Botany and Acta Horticulturae Sinica . He has published more than 150 research
articles in peer-reviewed journals.
About the Editors
Golam Jalal Ahammed is a Research Associate at the
Department of Horticulture, Zhejiang University,
Hangzhou, China. He obtained his B.Sc. (Hons.) in
Agriculture in 2004 and M.S. in Horticulture in 2006
from Bangladesh Agricultural University,
Mymensingh, Bangladesh. Dr. Ahammed received his
Ph.D. in Olericulture with major focus on plant stress
physiology and hormonal regulation in 2012 from
Zhejiang University, China. His major research inter-
ests include plant stress physiology, phytohormones,
climate change effect on plants, and environmental
pollution. He has authored over 30 research papers in
peer-reviewed journals. Dr. Ahammed is a regular
reviewer for a number of prestigious journals. Moreover, he has edited special issues
for Current Protein & Peptide Science . Dr. Ahammed has been awarded
an International Young Scientist Fellowship in 2015 by the National Natural Science
Foundation of China. He is also a recipient of China Postdoctoral Science Foundation
Award (56
th and 58
th ). Currently, Dr. Ahammed is actively engaged in unraveling
hormonal involvement in iron homeostasis under elevated carbon dioxide and inter-
action with cadmium in tomato.
About the Editors
181© Springer Science+Business Media Dordrecht 2016
G.J. Ahammed, J.-Q. Yu (eds.), Plant Hormones under Challenging
Environmental Factors, DOI 10.1007/978-94-017-7758-2_8
Chapter 8
Plant Responses to Light Stress: Oxidative
Damages, Photoprotection, and Role
of Phytohormones
Aditya Banerjee and Aryadeep Roychoudhury
Abstract Light stress is the most uncharacterized and less studied among the vari-
ous types of abiotic stresses experienced by the plant systems. Plants, being sessile
organisms, cannot escape from such stresses, and one of the mechanisms of adapta-
tion under such hostile circumstances is mediated through the altered regulation of
phytohormones. In this book chapter, we have presented an exhaustive literature-
based study on the different kinds of light stress encompassing light quality and
type, the basic mechanism of perception of UV-B (the most harmful) rays by the
plant system, the general metabolites which get upregulated under stress, and then
a detailed excerpt on the role of phytohormones like auxin, gibberellic acid, cytoki-
nins, ethylene, and abscisic acid under such conditions. Based on this account, our
chapter also aims at integrating the perception of light stress-signaling pathway
with the phytohormone-signaling networks, thus providing the idea of a universal
cross talk occurring in plant cells, exposed to a variety of light stresses.
Keywords Light quality Light stress UV-B rays Plant hormone Stress
8.1 Introduction
The environmental or abiotic factors are the major regulators of plant growth and
distribution across the globe. These abiotic factors include soil water content, salin-
ity, heavy metal content, and even the quantity of the incoming light striking the
plants. A range of tolerance against these abiotic factors is exhibited by each plant
A. Banerjee A. Roychoudhury (*)
Post Graduate Department of Biotechnology , St. Xavier’s College (Autonomous) ,
30 Mother Teresa Sarani , Kolkata 700016 , West Bengal , India
species. The moment the limit of tolerance is exceeded, the plant experiences stress
and upregulates the stress-responsive pathways. This ultimately leads to the survival
of the stress-tolerant species and susceptibility or death of the sensitive species in
the area under stress. Hence, these factors cumulatively dictate the geography of
plant distribution. Light stress is the chief atmospheric stress acting upon plants
which mainly inhibit growth via disrupting the photosynthetic pathway (Greenberg
et al. 1989 ; McKenzie et al. 2003 ; Seppelt 2005 ). Phytohormones are the diverse
chemical molecules which play prime roles in activating stress-responsive pathways
required to guard against the potential damages caused by light stress (Effendi et al.
2013 ; Hayes et al. 2014 ). In this chapter, we would highlight the various light stress
responses that are activated by the series of phytohormones, which on a broader
level do have the possibility to act in tandem through a cross talk in their pathways.
The type and quality of light stress affecting plant growth, the major signaling cas-
cade involved in their perception, and the major effectors to combat light stress have
also been dealt with in this chapter. The major form of light stress is of course the
ultraviolet B (UV-B) light of shorter wavelength (280–320 nm). Such short wave-
lengths are normally fi ltered by the ozone layer in the stratosphere. However, due to
the alarming increase in pollutants like chlorofl uorocarbons (CFCs), the ozone layer
is getting rapidly depleted resulting in harmful UV rays reaching the earth surface
and posing a threat to plant growth (Arróniz-Crespo et al. 2004 ). Though several
plants (species and cultivars), depending on their location, remain unaffected by
small increases in UV-B rays, the overall constitution of the ecosystem obviously
gets altered. This has remained a phenomenon of immense concern than loss of
agricultural crops that are more easily manipulated.
8.2 Types of Light Affecting Plant Growth
8.2.1 UV Rays and Their Effects on Plant Growth
Solar UV radiation reaching the earth’s surface is composed of UV-A (320–400 nm)
and part of UV-B (280–320 nm), while most of the UV-B and all UV-C (<280 nm)
radiation are absorbed by the ozone layer (Fig. 8.1 ). Over the last decades, depletion
in the ozone layer has increased the level of solar UV-B radiation reaching the earth,
and now approximately 0.5 % of the total solar radiation accounts from
UV-B. Exposure to UV light has been stated as a major deterrent of evolution to
land (Arróniz-Crespo et al. 2004 ). The major effects of such exposure can be chlo-
rophyll degradation and photoinduced DNA damages. Lignin, which is absent in
bryophytes, plays a role in protecting the cells against UV light. The shifting of
radiation to shorter wavelengths increases its damaging potential, and the factor by
which the biologically effective radiation is increased by this effect is termed the
radiation amplifi cation factor (RAF) (Bornman
1991 ). Plant species with habitats
around the higher latitudes are more prone to UV- B stress. This is because in
A. Banerjee and A. Roychoudhury
comparison to the tropics, the temperate and higher latitude regions experience
larger relative increase of effective UV-B on slight thinning of the ozone layer.
Measurement of viability of species in varying latitudes has substantiated such
inference (McKenzie et al.
2003 ).
Response to UV-B stress leads to the accumulation of UV-absorbing biomole-
cules like fl avonoids, along with an increase in leaf thickness, lignin content, and
surface refl ectivity of the leaves. Such adaptations often are accompanied by stunted
growth and reduced photosynthetic activity. Bleaching of leaves together with
degraded cell membranes have also been documented as a result of UV-B exposure.
Measurement of net and partial photosynthesis, fl ash-induced absorption changes,
and fi ber optic microprobes have been used to quantify the internal photosyntheti-
cally active radiation (PAR) of leaves, phytoluminography, and even to monitor
chemiluminescence (Seppelt 2005 ). Different partial reactions of photosynthetic
electron transport chain has led to the identifi cation of UV-sensitive targets like the
reaction center of photosystem II (PSII), the light-harvesting complex (LHC), the
acceptor site of PSII, and the donor site of PSII. Experiments have proved that on
exposure to UV-B radiation, the functional integrity between the water-splitting
complex and P680 on the oxidizing side of PSII is lost (Gill et al.
2015 ). Greenberg
et al. ( 1989 ) showed that the chloroplast protein D1 (Q
B ) also gets rapidly degraded
by UV-B rays. Thus, apart from being a prime target of atrazine herbicides, D1 is
rays X-rays UV rays Infrared
stress; ROS
Fig. 8.1 Different types of light rays having varying wavelengths constitute the solar radiations.
Out of these, the UV- B rays have the maximum potential to cause damages to both fl oral and faunal
communities. The major photoinduced damages occurring in plants exposed to UV-B stress have
been listed in the fi gure
8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
also affected upon UV-B exposure, probably by the semiquinone anion radical.
Since the Mn cluster of water oxidation is the most fragile component of the electron
transport chain, protein matrix absorption of UV-B rays or other redox components
have the ability to dissociate the Mn cluster and altogether inactivate
Plants have often been observed to respond differently in greenhouses with sub-
optimal PAR in contrast to those growing under normal sunlight. The fi rst example
of such interaction between PAR levels and UV-B radiation was forwarded in
Phaseolus vulgaris (Bornman 1991 ). Here, the bean plants were grown under three
levels of visible light (230, 500, and 700 μmol m
−2 s −1 referred to as low light (LL),
medium light (ML), and high light (HL)), with or without enhanced UV-B radiation .
Resistance to UV-B rays was observed in plants grown under HL conditions, while
the LL conditions elicited increased response to UV stress. The ML plants exhibited
intermediate response patterns. Signifi cant increase in leaf thickness and pigment
changes were observed in plants under all three conditions with least leaf thickness
in the LL-grown plants. This was due to plant growth under suboptimal greenhouse
conditions which did not completely mimic the natural environmental conditions.
Hence, UV-B-exposed LL-grown plants did not show pronounced induction of
UV-B absorbing compounds, and the leaf refl ectivity was the highest in the blue and
red regions of the spectrum, as derived by using bifurcated optical fi bers. This indi-
cates a phenomenon which decreases the penetration of PAR. In another experi-
ment, the two species of Brassica , one from a northern latitude origin ( B. campestris )
and the other from southern latitude ( B. carinata ), were grown under high visible
light (1800 μmol m
−2 s −1 ) supplemented with UV-B radiation. B. campestris showed
maximum sensitivity to UV-B radiation with 45 % increase in leaf thickness and
decrease in chlorophyll content. Though the internal PAR of the leaves changed
with respect to the controls leading to attenuation of the transmitted light, the scat-
tered light within the leaf in the palisade and spongy mesophyll tissue considerably
increased (Berli et al. 2011 ). This observation shows that exposure to UV-B rays not
only alters the anatomical features of the leaves but also their microenvironment
thereby infl uencing photosynthesis.
Aromatic amino acids like phenylalanine, tyrosine, and tryptophan as well as
histidine, cysteine, and cystine absorb in the UV-B region and hence can be direct
targets of UV-B rays. Photooxidation of tyrosine to dityrosine and conversion of
tryptophan to N-formyl kinurenine have been reported. The latter photoproduct so
formed has the ability to interact with DNA after absorbing UV-A rays. Though
cysteine is a poor absorber in the UV-B region, it can undergo photolysis at high
quantum effi ciency, leading to the splitting of the covalent disulfi de bonds into reac-
tive sulfhydryl groups. Hence, the tertiary structure of the entire protein is disturbed
leading to aggregate formation. Characteristic examples include ribulose 1,
5-bisphosphate (Rubisco), ATPase, violaxanthin de-epoxidase, and protein subunits
of PSI and PSII. Plant cell membranes are rich in phospholipids and glycolipids,
consisting of unsaturated fatty acids (FAs) which are destroyed by UV-B radiation
in the presence of oxygen (Hollosy
2002 ).
A. Banerjee and A. Roychoudhury
UV-B rays have the ability to destroy the genomic DNA by initiating the production
of cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6–4) pyrimidinone dimers
(6–4 PPs). Formation of such adducts lead to the inability of the polymerases to access
such bulky regions, resulting in disrupted replication, transcription, and fi nally cell
death. To reverse such abrupt cellular failure, plants like Arabidopsis contain photoly-
ases which have substrate specifi city for either CPDs or 6–4 PPs and utilize UV-A and
blue light to monomerize the pyrimidine dimers (Frohnmeyer and Staiger 2003 ; Gill
et al. 2015 ). Cyanobacteria like Anabaena, Nostoc , and Scytonema contain a myco-
sporin-like amino acid called shironine. It has a cyclohexanone or cyclohexenimine
chromophore conjugated with an amino acid or its imino alcohol accumulates in
response to UV-B stress evidently during the daily light period (Sinha et al. 2001 ).
8.2.2 Gamma Rays and Their Effects on Plant Growth
Irradiation of plant cells with gamma rays resulted in dissolution of the pectin
through increased activity of polygalacturonase and pectin methyl esterase.
However, the cells in apple treated with calcium were less prone to such cell wall
alterations, particularly due to the formation of stable cementing material calcium
pectate. A very high radiation dose of 5 kGy fi nally resulted in a completely disin-
tegrated middle lamella (Kovacs and Keresztes 2002 ). Gamma irradiation at expo-
sures above 0.2 kGy in banana resulted in dilations between thylakoid membranes
and a loss of granal stacking. Exposure to 0.2 kGy radiations retarded the fruit
softening and yellowing, while higher gamma ray exposure values accelerated such
changes. Marked decrease in sensitivity to exogenously applied ethylene was
reported in banana under irradiation exposures between 0.6 and 1.0 kGy (Kovacs
and Keresztes 2002 ). In another experiment, chlorophyll and carotene in acetone-
ethanol solutions were irradiated with 60 krad of gamma rays and the optical
absorption was measured. The results were expressed by an opacity term, which
was derived from the ratio of the incident to the emergent light intensities . The per-
sistent decrease of the opacity behavior of the chlorophyll and carotene solutions
was recorded (Ramiarez-Nina et al. 1998 ). This indicates that following irradiation,
fewer organic dye molecules remain to absorb light at a specifi c wavelength.
Chlorophyll in silicon oxide matrix was also irradiated with 0, 30, and 60 krad, and
surprisingly smaller changes were observed in irradiated gel samples than in solu-
tions (Ramiarez-Nina et al. 1998 ). Such results were observed probably due to the
presence of fewer free radicals in the gel structure in comparison to the large amount
in the liquid sample solutions. Cia et al. ( 2007 ) reported the use of gamma irradia-
tion to evade anthracnose which is the main post-harvest disease in papaya. It was
perhaps a novel interpretation which showed the use of these harmful rays toward
benefi cial plant growth instead of generating stress in the plant. It was concluded
that doses of 0.75 and 1 kGy of gamma irradiation reduced the lesion size caused by
Colletotrichum gloeosporioides (causal organism of anthracnose) and anthracnose
incidence in papaya fruit (Cia et al. 2007; Patrícia et al. 2007).
8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
8.2.3 Low Light Intensity and Its Effects on Plant Growth
Low light intensity is about 40–50 % of natural light. Hence, it is about 40–60 % less
than the light intensity during the dry season. The growth pattern and productivity
of two indica rice genotypes of kharif and three of rabi were assessed under low
light intensity. It was observed that growth of plants under shade affected the dry
matter accumulation at all stages. Such effects were critical from primordial initia-
tion onward. Within 26–9 °C range of mean maximum temperatures, the expression
of the panicles appeared to be a function of the light intensity. As a result of expo-
sure to low light intensity and shading from the primordial initiation stage, the grain
and panicle yields in the rice plants were suffi ciently reduced. Low light intensity
was depicted to be a major constraint for higher yields during monsoons, as low
yields of 3.2–4.4 tons ha
−1 were recorded in rice cultivars which yielded 8–10 tons
−1 under exposure to high light (Retkute et al. 2015 ). Thus, it seems that low light
intensity causes stress by insuffi cient supply of energy-rich photons which fuel the
photosystems and regulate photosynthesis. Insuffi cient photosynthesis results in
insuffi cient food production for the autotrophic plants leading to unusual retarded
growth patterns and lowered productivity. This has led to the hypothetical possibil-
ity of the probable existence of a cross talk between the signaling cascade perceiv-
ing low light intensity and the quantitative trait loci encoding plant productivity.
Boo et al. ( 2000 ) showed that the rice plants had similar oxidative stress profi les
under drought, when incubated with 5-aminolevulinic acid (ALA) in dark and sepa-
rately exposed to low light intensity. This has been accredited to the formation of
singlet oxygen in the plants both under drought stress and after incubating with
ALA.Treatment of the rice plants with ALA resulted in high accumulation of proto-
chlorophyllide which under low light intensity photodynamically generated singlet
oxygen (Boo et al. 2000 ).
Vantuyl et al. ( 1985 ) reported the incidence of fl ower bud abortion in Asiatic
hybrid lilies exposed to low light intensity. It was found that these lilies required
additional light to prevent fl ower bud abscission and fl ower bud blasting during the
winter season when natural light intensity is quite low. Among the cultivars,
Connecticut King and Enchantment appeared to be the most sensitive to low light
conditions. Low light intensity was also reported to upregulate ethylene biosynthe-
sis especially during the critical stage. This is because ethylene is the essential hor-
mone which initiates abscission and hence abortion of the fl ower buds. According
to Durieux et al. (
1983 ), a short period with very low light intensity at the critical
stage for bud abscission should be suffi cient for a disproportionate increase in per-
centage of fl ower bud abortion. Artetxe et al. (
2002 ) however reported an unusual
result where low light intensity levied the oxidative stress in duckweeds ( Lemna
minor ) under cadmium (Cd) and zinc (Zn) stress and rendered them tolerant to these
heavy metals. The low light (LL)-treated plants exhibited reduced symptoms of Cd-
and Zn-induced cytotoxicity with no effect on the relative chlorophyll contents.
Such surprising results were unexpected as we have discussed in the previous
instances that the LL plants are less protected toward oxidative stresses. Along with
A. Banerjee and A. Roychoudhury
increasing the phytochelatin content, LL plants showed enhanced Glu accumulation
particularly to be channelized for glutathione biosynthesis. Large increases in total
ascorbate, tocopherol, and xanthophylls -cycle pigments were also observed which
altogether launched an antioxidative response (Artetxe et al. 2002 ).
8.2.4 High Light Intensity and Its Effects on Plant Growth
Severe photoinactivation and photodamage of photosynthetic apparatus and degra-
dation of photosynthetic proteins have been documented in plants exposed to light
intensity which is abnormally higher than what is utilized in photochemistry.
Degradation of D1 protein of PSII, large subunit (LSU) of the Rubisco, and
decreased levels of PSI polypeptides like PsaA, PsaB, and PsaC are the major det-
rimental effects of high light intensity (Jiao et al. 2004 ). Higher plants which are
often exposed to such detrimental doses of high light intensity during summer have
evolved photoacclimation strategies. A series of dynamic alterations in the chloro-
plast occurs involving a reduction in the number and size of light-harvesting com-
plexes and accumulation of D1 proteins. Light stress response thus includes the
modifi cation of the stoichiometry, content, and activity of PS complexes like chlo-
rophyll levels, activity of the electron transport chain, plastocyanin content, ATP
synthase activities, Rubisco LSU synthesis, and transcriptional regulation of PSII/
PSI ratio (Jiao et al. 2004 ; Pfannschmidt et al. 1999 ). Energy transition between
PSII and PSI is also controlled by regulating the synthesis and activity of the light-
harvesting complex II (LHCII) (Allen 2003 ). Jiao et al. ( 2004 ) reported high light
stress-induced synthesis of the β-isoform of the chloroplast ATP synthase coupling
factor (CF1) as a positive response during chloroplast photoacclimation in Brassica
rapa . The gametophytes and sporophytes of Laminaria saccharina exposed to high
light stress (500 μmol m
−2 s −1 for two hours) exhibited photoinhibition of photosyn-
thesis with fast kinetics (Hanelt et al. 1997 ). Maximum photodamage was recorded
in the younger sporophytes, some of which did not recover fully even after 12 h of
exposure to lowered light intensity. However, the kinetics of recovery in the old
sporophytes and gametophytes showed a fast and a slow phase. Hanelt et al. ( 1997 )
inferred that the fast phase was indicative of a decline in the photoprotective pro-
cesses following stress, and the slow phase corresponded to steady recovery from
stress. It was also investigated and seen that the resistance of the older sporophytes
and gametophytes was not only due to an increase in the light absorbing pigments
like xanthophylls but also due to a rapid change in the thallus structure. Laminaria
sporophytes became multilayered when their size exceeded one centimeter along
with thickening of the blades. In this way, by a mechanism of self-shading, the
plants have the ability to protect their chloroplasts by rearranging them into a low-
absorbing position in the cytosol. Similar cases have also been reported in Porphyra
purpurea where the effect of photoinhibition decreased due to self-shading of over-
lapping thalli (Hanelt et al.
1997 ). The effect of high proton fl ux density (PFD) was
studied in the desert resurrection plant Selaginella lepidophylla to analyze the
8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
chlorophyll fl uorescence under such extreme conditions (Eickmeier et al. 1993 ).
Signifi cant reductions in the intrinsic fl uorescence yield and photochemical effi -
ciency of PSII were observed in hydrated physiologically competent stems under
extremely high intensity of light (2000 μmol m
−2 s −1 ). However, the recovery of the
resurrection plants under low PFD was rapid. Borya nitida are poikilohydrous
angiosperms known for their tolerance to extremes of temperatures and also to
intense visible radiation. Exposure to two hours of high light intensity (650 μmol
−2 s −1 ) resulted in no photoinhibition in Borya , while 70 % photoinhibition occurred
under exposure to low light intensity (120 μmol m
−2 s −1 ) (Retkute et al. 2015 ). Thus,
it was inferred that Borya ourish best under harsh extreme habitat probably by
maintaining a high basal level of antioxidative front to tackle oxidative stress.
Morphological adaptations like compact rounded leaves and their orientations to
afford maximum refl ectance also aid Borya in tackling high light stress. Funk et al.
( 2010 ) reported that Brassica nigra (invasive) and Encelia californica (native) were
tolerant to high light stress than Ricinus , Salvia , Artemisia californica (native), or
Nicotiana glauca (invasive).
Such inferences could be drawn from the observations that Brassica and Encelia
both were resistant to photoinhibition in response to high light stress.
8.3 Quality of Light Affecting Plant Growth
8.3.1 Blue Light Affecting Plant Growth
The quality of light is one of the most crucial variables affecting photosynthetic
parameters and concentrations of the phytochemicals in plants (Whitelam and
Halliday 2007 ). High pressure sodium (HPS) lamps are often used to produce sup-
plementary blue light for the plants. In case of the greenhouses, the light-emitting
diodes (LEDs) are procured for service (Paradiso et al. 2011 ). Blue light (having a
bandwidth 420–460 nm) in a cross talk with phytochrome signaling regulates sto-
matal opening, mainly via potassium and chloride uptake and malate biosynthesis.
Blue light activates the proton pumping ATPase which aids in ion uptake. The
C-terminal phosphorylation by a Ser/Thr kinase activates the ATPase. An action
spectrum was exhibited by the blue light-specifi c stomatal opening with a maxi-
mum peak at 450 nm and two minor peaks at 420 and 470 nm (Talbott et al. 2003 ).
The role of phytochrome in blue light-mediated stomatal opening was hypothesized
from the fact that the absorption spectrum of phytochrome extends into the blue
region of the spectrum. In the orchid genus Paphiopedilum , phytochrome plays a
direct role in stomatal opening (Talbott et al. 2002 ). Photosynthesis-dependent
stomatal opening is absent in this genus as the leaves have minute amounts of
chlorophyll. The chloroplastic carotenoid and zeaxanthin have been identifi ed as
the putative photoreceptors of blue light in the guard cells. Hence, the
Arabidopsis mutants npq1 , which failed to accumulate zeaxanthin due to defective
violaxanthin de-epoxidase, lacked blue light-specifi c responses (Talbott et al.
2003 ).
A. Banerjee and A. Roychoudhury
Kinoshita et al. ( 2001 ) reported that the double mutants of phot1 and phot2 exhib-
ited more impaired blue light response than the single mutants alone. It was also
suggested that the stomata from the double mutant would show blue light response
at higher fl uence rates of blue light (Kinoshita et al. 2001 ).
In another instance, it was seen that predawn and high light intensity treatment
supplemented with blue light decreased the quantum yield of PSII and enhanced the
accumulation of phenolics , avonoids, and pigments in Lactuca sativa (Ouzounis
et al. 2015 ). Here two cultivars of lettuce, viz., Batavia (green) and Lollo Rossa
(red), were subjected to variable light intensities with or without supplemented blue
light. Though the total fresh and dry weights were not affected in the plants under
blue light, these were more compact in growth and development. Difference in vari-
etal response to blue light was also observed as the red cultivar under blue light
showed reduced quantum yield of PSII with an increase in non-photochemical
quenching. However, the green lettuce variety exhibited no such difference except
an increase in the stomatal conductance. These results obviously indicate the fact
that high light levels not only trigger photoprotective heat dissipation in the plant
system but also the specifi c spectral composition of the light itself at low intensities
(Ouzounis et al. 2015 ).
8.3.2 Red and Far-Red Light Affecting Plant Growth
Red light has a bandwidth of 620–640 nm and often acts as a pivotal regulator in
plant physiological functions. Alyabyev et al. ( 2002 ) showed that at high tempera-
ture, the rate of oxygen consumption of summer wheat seedlings exposed to blue
light decreased by 40–45 % when compared with the control plants under similar
conditions. The heat production rate of wheat seedlings exposed to blue light was
also higher than those grown under red light. This was the fi rst report of a compara-
tive analysis between the red and blue light-mediated responses which aided in
understanding the contrasting signaling mediated by them. Depending on the qual-
ity of light exposure, differential rates of respiration were observed in plants grow-
ing in optimum temperature. The excised roots of plants exposed to blue and white
light exhibited slightly lowered rate of respiration in comparison to those under red
light. However, high temperature treatment reduced the respiration rate in all plants
with the lowest inhibition in the blue light-treated plants. Such results possibly indi-
cate greater stability achieved by the plants under blue light at higher temperatures
(Alyabyev et al. 2002 ). This observation also corresponds to the fact that the rate of
oxygen uptake of roots is dependent on the spectral composition of the light it receives.
Such correlation might be because the plant tissues are highly light conducting and
photosensitive (Kim et al. 2007 ). Electron transport and activation of enzymes
related to respiratory metabolism and membrane stabilization are often regulated by
blue light which can be accredited to such correlation with respiratory rate.
Far-red (FR) light generally has an emission peak at 740 nm and half bandwidth
of 25 nm. It has a correlation with the plant photoreceptors. The chief plant
8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
photoreceptors are phytochromes and cryptochromes which regulate the circadian
clock in plants. In Arabidopsis , ve phytochromes (Phy A-E) have been identifi ed
and characterized, of which PhyA is predominant in etiolated seedlings and PhyB
in the light-grown plants (Qin et al. 2010 ). Identifi cation of SUB1, a calcium-bind-
ing protein, has strengthened the possibility of a cross talk between the phyto-
chrome- and cryptochrome-mediated signaling networks (Guo et al. 2001 ). Qin
et al. ( 2010 ) depicted the involvement of calcineurin B-like protein (CBL)-
interacting protein kinase 14 (CIPK14) in the PhyA-mediated FR light inhibition of
greening in Arabidopsis seedlings. CIPK14 was also found to affect the expression
of Protochlorophyllide Oxidoreductase ( POR ) genes like PORA , PORB , and PORC
in Arabidopsis . Greening was not initiated in the cipk14 seedlings under FR light
even after 15 h of exposure to white light. However, the phyA seedlings showed
greening within 0.5 h of white light exposure. Expression of CIPK14 appeared to be
regulated by both the circadian clock and PhyA, thus indicating the role of CIPK14 in
the FR inhibition of seedling greening mediated by PhyA by negatively regulating
the PhyA-dependent repression of the POR genes (Qin et al. 2010 ).
Far-red elongated hypocotyls 3 (FHY3), also known as chloroplast division 45
(CPD45), acts as a crucial factor in FR light-signaling pathway in Arabidopsis . The
FHY3/CPD45 mediates perception of FR light and regulates chloroplast division.
ARC5 is a nuclear gene encoding a dynamin-related protein involved in chloroplast
division (Gao et al. 2003 ). The chloroplast division mutant arc5-3 had no defect in
FR light sensing, while constitutive overexpression of ARC5 aided in recovering
from chloroplast division defects but not from those in FR light signaling in the
cdp45 mutants (Chang et al. 2015 ). Constitutive overexpression of FHY1 repaired
the fallacies in FR signaling, but not in the chloroplast division mechanism in cdp45
mutants. Chang et al. ( 2015 ) hence reported that FHY3/CDP45 regulates FR light
signaling and chloroplast division together through parallel activation of FHY1 and
ARC5 independently. Apart from these two pathways, CDP45 also acts as an impor-
tant regulator of circadian rhythm and other important biological processes essential
for proper plant growth and development. This fact was supported by the observa-
tion that CDP45 regulates the expression of Early Flower4 ( ELF4 ) and interacts in
the CCA1/LHY-TOC1 circadian clock feedback circuit via association with the
ELF4 promoter (Li et al. 2011 ). Thus, the link between photomorphogenesis, circa-
dian rhythm, and FR signaling is probably through FHY3/CPD45 which acts as a
crucial node in the entire gene regulatory pathway and even acts as a transcription
factor (TF) to transcribe the downstream effector genes. Kegge et al. ( 2015 ) reported
that the emission of total volatile organic compounds (VOCs) was reduced in
Hordeum vulgare cv. Alva under low red and FR light conditions when compared
with their control counterparts. The basic notion of this study was to investigate the
effect of light quality on the emission of VOCs which act as crucial signaling mol-
ecules in plant-plant interactions. Thus, the altered VOC emission by Alva cultivars
surprisingly affected the carbon (biological) allocation in the receiver plants of
another Hordeum cultivar named Kara (Kegge et al. 2015 ).
Blue, red, and far-red light signaling visually does not seem to cause much stress
like the UV-B rays because they do not harm the morphological queues of plants.
A. Banerjee and A. Roychoudhury
However, from the point of regulating plant growth patterns, they do trigger essen-
tial photomorphogenesis which may have common patterns under true light stress.
In this section, we have tried to highlight only such phytoregulations mediated by
the quality of light, reaching the plant absorptive surfaces.
8.4 Signaling Pathways Involved in UV-B Stress
Since light stress is more focused mainly on the UV-B mediated stress, in this sec-
tion we would briefl y discuss the receptive pathway related with UV-B-mediated
response in plants (Fig. 8.2 ). The major UV-B photoreceptor is UVR8 which is a
seven-bladed β-propeller protein fi rst identifi ed in the Arabidopsis mutants which
showed hypersensitivity to UV-B light (Li et al. 2013 ). Apart from the sequence
homology of this receptor with Regulator of Chromatin Condensation 1 (RCC1)
found in humans, UVR8 is the prime regulator of multiple genes involved in UV
damage repairs. Such results have been concluded after exhaustive transcriptome
studies of UVR8 (Brown et al. 2005 ). An interesting phenomenon was identifi ed
that the UVR8 proteins are constitutively expressed irrespective of UV-B light
stress. The UV-B rays stimulate the distribution of UVR8 in the cell mainly concen-
trating them in the nucleus (Kaiserli and Jenkins 2007 ). The UV-B stress was still
required for the constitutively nuclear-localized proteins. Though the biological
UV-B stress
Light Responsive
Genes DELLAs
Low red:
stress ABA
Fig. 8.2 The light-signaling pathway involved in UV-B perception and the potential cross talk
among the major phytohormones, ABA, auxin, and GA with this light receptive pathway
8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
signifi cance of UVR8 still remains largely unknown, it has been depicted that this
protein can even act as a potential TF by associating with the promoter region of
Elongated Hypocotyl 5 ( HY5 ) (Cloix and Jenkins 2008 ). HY5 is a nuclear bZIP TF
involved in the regulation of photomorphogenesis under a wide spectrum of wave-
lengths including blue, red, and FR lights. HY5 is involved in the upregulation and
downregulation of light-responsive genes through association with the promoters of
the annotated genes (Zhang et al. 2011 ). Oravecz et al. ( 2006 ) showed that HY5 is
a positive effector in UV-B-mediated signaling because the hy5 mutants displayed
sensitivity to UV-B. New alleles like cop1 and uvr8 were identifi ed by Favory et al.
( 2009 ) by using HY5p: LUC reporter construct. Involvement of other TFs like HY5
is also possible as a subset of UVR8, and Constitutive Photomorphogenic 1 (COP1)-
dependent genes were not upregulated by HY5/HY5 Homolog (HYH) (Feher et al.
2011 ).
Favory et al. ( 2009 ) reported the importance of the physical interaction between
UVR8 and COP1, an E3 ubiquitin ligase. The C-terminus of UVR8 consists of a 27
amino acid region, which, when digested, destroyed the UVR8 activity. Single
amino acid changes in UVR8 and COP1 proteins resulted in an abrogated direct
interaction between them, due to which the entire UV-B-mediated response path-
way became aberrant (Favory et al. 2009 ; Li et al. 2013 ). COP1 is a conserved
RING fi nger E3 ubiquitin ligase involved in plant development, mammalian cell
survival, growth, and metabolism (Lau and Deng 2012 ). However, in this discussion
we would deal mainly with the aspects of COP1 as a central repressor in light sig-
naling. It functions as an E3 ligase, targeting different photomorphogenesis-
promoting proteins like HY5 and HYH for degradation under visible light (Li et al.
2013 ). However, under UV-B stress, COP1 accumulates in the nucleus and forms
protein complexes with four WD40-repeat proteins. Suppressor of PhyA-105
(SPA1), SPA2, SPA3, and SPA4 is not obligatory for proper UV-B-mediated stress
response (Zhu et al. 2008 ). Structural stabilities and similar levels of UVR8 in the
wild-type and cop1-4 mutants depict that COP1 does not stabilize UVR8, as it does
in case of other photoreceptors like phytochromes and cryptochromes (Favory et al.
2009 ; Jang et al. 2010 ).
Rizzini et al. ( 2011 ) reported that a UVR8 dimer is the major UV-B receptor and
proposed that a number of conserved tryptophan residues like W285 have key roles
as light sensors in UVR8. In vivo experiments showed that the mutation of either
tryptophan (W233 or W285) residues to alanine totally abolished the photomorpho-
genic responses mediated by UVR8 in Arabidopsis mutants. This is because absorp-
tion of UV-B photons by these two tryptophans dissociates the salt bridges, resulting
in destabilization of the dimeric form and signal initiation (Christie et al. 2012 ).
Redimerization of UVR8 has been seen to be mediated by Repressor of UV-B
Photomorphogenesis (RUP1) and RUP2. This dimerization negatively regulates the
stress-responsive cascade as UVR8 and COP1 cannot mutually interact with each
other (Heijde and Ulm 2013 ). RUP1 and RUP2 are members of the WD40-repeat
protein superfamily. Members of this superfamily consist of at least one copy of a
conserved motif (WD40 motif: 40 amino acids long and typically ending with tryp-
tophan and aspartate residues). RUP1 and RUP2 have been reported to contain
A. Banerjee and A. Roychoudhury
seven WD40 repeats with no other apparent domains (Gruber et al. 2010 ). RUP1
and RUP2 have been reported to be upregulated under UV-B stress in a UVR8-,
COP1-, and HY5-dependent manner (Gruber et al. 2010 ). Thus, if RUP1 and RUP2
are considered as negative regulators in the UV-B-mediated stress-responsive path-
way, then the direct interaction of these proteins with UVR8 to promote its dimer-
ization can be seen as a direct interaction of the negative regulators at the
photoreceptor level (Li et al. 2013 ). It has been postulated that since COP1 is essen-
tial for the UV-B responsive expression of RUP1 and RUP2, there should be suffi -
ciently high basal level of RUPs, if the RUP-mediated dimerization of UVR8 is
COP1 independent, as hypothesized by some groups (Gruber et al. 2010 ; Heijde
and Ulm 2013 ).
FR Elongated Hypocotyl 3 (FHY3) and its homologue FR Impaired Response 1
(FAR1) are transposase-derived TFs, which, apart from participating in PhyA-
mediated signaling, are also involved in multiple development processes like circa-
dian rhythm, chloroplast development, chlorophyll biosynthesis, and shoot
branching (Stirnberg et al. 2012 ; Tang et al. 2012 ; Li et al. 2013 ). These processes
are regulated by light in association of FHY3/FAR1, with PhyA in vivo. Other TFs
which have been identifi ed to work in tandem with FHY3/FAR1 are HY5,
Phytochrome-Interacting Factor1 (PIF1), Circadian Clock Associated 1 (CCA1),
and Late Elongated Hypocotyl (LHY) (Tang et al. 2012 ). The tandem functioning
of these TFs with FHY3/FAR1 has presented a possibility of a cross talk existing in
the light-signaling pathway, where FHY3/FAR1 may be the central node. Huang
et al. ( 2012 ) reported that FHY3 and HY5 physically interact with their cis- elements
in the COP1 promoter. This means that COP1 is inducible by UV-B light stress.
FHY3 acts as a positive regulator in light stress-mediated signaling, as the fhy3
mutants exhibited distorted UV-B-induced hypocotyl growth and high UV-B sensi-
tivity. On the contrary, FAR1 is not obligatory for the UV-B photomorphogenetic
pathway, because it was monitored that the far1 mutants were devoid of any appar-
ent impairments, while the fhy3/far1 double mutants also showed no further impair-
ment of the UV-B-induced hypocotyl growth (Huang et al. 2012 ). Li et al. ( 2013 )
have suggested that HY5 promotes COP1 expression through a positive feedback
loop. This is based on the fact that though FHY3 and HY5 both positively regulate
COP1 gene expression, the accumulation of HY5 under UV-B exposure requires
COP1. Another emerging signaling intermediate of the UV-B photomorphogenetic
pathway is the Salt Tolerance (STO also referred to as BBX24) which is a B-box Zn
nger protein. STO is a negative regulator of the UV-B-responsive signaling cas-
cade, interacting with COP1 and repressing the transcriptional activities of HY5
(Jiang et al. 2012 ). The UV-B stress-responsive pathway is still a major focus of
research for several scientifi c groups as some schools of scientists fi nd the emerging
players in this signaling cascade as potent targets of developing transgenic plants
with considerable tolerance toward UV-B stress.
8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
8.5 Major Metabolites Which Confer Resistance
Against Light Stress
8.5.1 Flavonoids
Phenolic compounds constitute one of the most important groups among the bioac-
tive compounds found in plants since they have diverse functioning in signaling and
defense pathways. Flavonoids and hydroxycinnamic acids are the major phenolic
compounds found in fruits and berries determining the color, aroma, astringency,
and antioxidant properties (He and Giusti 2010 ). The avonols, anthocyanins, and
proanthocyanidins (PA) constitute the major fl avonoids found in fl owers and fruits.
These fl avonols, apart from serving as visual signals for pollinators in fl ower and
fruit dispersal, act as photoprotectants by scavenging the free radicals formed under
UV-B exposure (Bogs et al. 2007 ). Suppressed fl avonoid biosynthesis has been
reported in fruits which have been shaded from sunlight. In Litchi chinensis , fruit
bagging treatments resulted in lowered accumulation of anthocyanins as well as the
anthocyanin biosynthetic genes like chalcone synthase ( LcCHS ), chalcone isomer-
ase ( LcCHI ), avones 3-hydroxylase ( LcF3H ), dihydrofl avonol 4-reductase
( LcDFR ), anthocyanidin synthase ( LcANS ), and UDP-glucose: fl avonoid
3-O-glucosyltransferase ( LcUFGT ). These genes were again reported to be upregu-
lated on debagging and exposure of fruits to sunlight (Wei et al. 2011 ). The purpose
is to inhibit chlorosis and bleaching on exposure to light, which can only be done if
these genes are particularly upregulated. Light has also been reported to regulate
avonoid biosynthesis in strawberry ( Fragaria X ananassa ), peach/nectarine
( Prunus persica ), pear ( Pyrus pyrifolia ), and apple ( Malus X domestica ) (Zoratti
et al. 2014 ; Sun et al. 2014 ). Agati et al. ( 2013 ) reported the signifi cance of fl avo-
noids in their ability to scavenge ROS and control the development of individual
organs and the whole plant. The chloroplast-located fl avonoids have been portrayed
as hydrogen peroxide and singlet oxygen scavengers, so that they can prohibit pro-
grammed cell death under light stress. The vacuolar fl avonoids, together with the
antioxidant enzymes, like peroxidases and ascorbic acid, form a second antioxidant
system. The fl avonols have also been depicted as the key developmental regulators,
as they have the ability to mediate auxin transport and catabolism. Thus, Agati et al.
( 2013 ) concluded that UV- B photoprotection is just one of the essential functions of
avonoids which have several other myriad roles in plant growth regulation.
However, the most important functions of fl avonoids arise when the plants face
UV-B stress. During such stress, there is a burst in reactive oxygen species (ROS)
production, resulting in various damages which we have discussed before. Koyama
et al. ( 2012 ) reported a decrease in the fl avonol content in the skin of grape berries
(under UV shield). However, the cinnamic acid and PA levels were much less
affected. In Sauvignon blanc grape berries, substantial increase in fl avonols like
quercetin and kaempferol glycosides were detected by Liu et al. ( 2014 ) when the
fruits were exposed to UV-B light stress. Of the four Vitis vinifera avonol synthase
( VvFLS ) genes, two were found to be transcriptionally active and only one (VvFLS4)
responded to UV-B mediated stress (Liu et al. 2014 ).
A. Banerjee and A. Roychoudhury
Peng et al. ( 2013 ) reported the expression of MdMYBA and the related anthocyanin
pathway genes in apple skin in response to UV-B stress. Martinez-Luscher et al.
( 2014 ) showed that the anthocyanin and fl avonol contents were high in red grapevine
variety (cv. Tempranillo) exposed to UV-B stress. Qualitative differences were also
found in the fl avonol profi les in the UV-B treated fruits when compared to that of the
untreated fruits (Martinez-Luscher et al. 2014). Accumulation of fl avan- 3-ol and high
transcript levels of related genes was observed in the developmental stages (3–11
weeks after fl owering) of Cabernet Sauvignon grapevine variety, exposed to UV-A
irradiation (Zhang et al. 2013 ). However UV-B and UV-C irradiations gave rise to
same results only in berries of 7–11 weeks of fl owering, based on which Zhang et al.
( 2013 ) inferred that fl avan-3-ol accumulation increased on exposure to UV light
stress, only in berries which were still in their developmental phases, but not in those
which were mature. Enhanced transcript levels of fl avonoid biosynthesis genes like
DFR and UFGT were observed in the skin of nectarines (cv. Stark Red Gold) on expo-
sure to white light, supplemented with UV light for about 72 h (Ravaglia et al. 2013 ).
Castagna et al. ( 2014 ) also reported fl avonol accumulation in the fl esh of tomato fruits
at green mature stage, harvesting them following UV-B exposure. Namli et al. ( 2014 )
observed an increase in the total fl avonoids, phenolics , and hypericin content in the
in vitro cultures of Hypericum retusum var. Aucher exposed to UV radiations. When
the cultured plantlets were exposed to UV-B rays for 15, 30, 45, and 60 min, the high-
est total phenolics, fl avonoids, and hypericin accumulation (43.17 ± 0.8; 35.09 ± 0.8;
2.7 ± 0.05 mg g −1 , respectively) was achieved at 45 min of UV-B exposure, whereas
the contents of these metabolites in the naturally grown plants were 23.33 ± 0.9,
18.62 ± 0.3, and 1.6 ± 0.01 mg g −1 respectively (Namli et al. 2014 ).
Scattino et al. ( 2014 ) observed the increased accumulation of fl avonols and antho-
cyanidin glycosides in two cultivars of peach where the transcript levels of the struc-
tural phenylpropanoid and fl avonoid pathway genes were consistent with the levels of
the detected metabolites (Scattino et al. 2014 ). Crupi et al. ( 2013 ) reported that the
accumulation of stilbene cis- and trans-piceid along with quercetin-3-O- galactoside
and quercetin-3-O-glucoside was enhanced in grape berry skin under UV-C stress.
The accumulation was higher by about three folds than the control berries which were
not exposed to UV-C light stress. The radical-scavenging properties were found to be
higher in the papaya fruits exposed to UV-C irradiation. Such antioxidative activity
was found to be mediated by an enhanced level of fl avonoids (Rivera-Pastrana et al.
2014 ). Thus, in the above excerpt, we have provided a brief discussion on the varied
cases where fl avonoid accumulation has been depicted as a result of mainly UV stress.
From all these results, it is clearly implicated that fl avonoids do have important roles
as antioxidants considering generation of plant tolerance against light stress (Fig. 8.3 ).
8.5.2 Xanthophylls
The xanthophyll cycle consists of light-dependent conversions of three oxygenated
carotenoids in a cyclic reaction which involves de-epoxidation of the diepoxide
violaxanthin, via the monoepoxide antheraxanthin to the epoxide free form of
8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
zeaxanthin. This is followed by an epoxidation sequence in the reverse direction
(Demmig-Adams and Adams 1992 ). It was found that this xanthophyll cycle acts as
a thermal dissipator in plants when preventing overheating of particular tissues
which are more exposed to light (Fig. 8.3 ). The leaves of Euonymus kiautschovicus
Loesener experienced a wide degree of light stress mainly in response to different
levels of incident photon fl ux densities at similar photosynthetic capacities among
the leaves. The intrinsic PSII effi ciency, non-photochemical fl uorescence quenching
and the levels of zeaxanthin along with antheraxanthin in leaves have been consid-
ered as functions of actual light stress (Demmig-Adams and Adams 1996 ). Thus,
under a given degree of light stress, the same conversion state of the xanthophyll
cycle, accompanied by constant level of energy dissipation, was found, irrespective
of the usage of species or conditions, causing light stress. Xanthophyll-independent
energy dissipation was not reported, since all increases in thermal dissipation were
associated with simultaneous increases in the levels of zeaxanthin and antheraxan-
thin in these leaves (Demmig-Adams and Adams 1996 ).
8.5.3 Glucosinolates
Schreiner et al. ( 2014 ) recently reported the production of defense-related compounds
like glucosinolates in members of Brassicaceae, on exposure to UV light stress
(Fig. 8.3 ). The glucosinolates are sulfonated thioglycosides which share a common
glycone moiety with a variable aglycone side chain. Based on this, the glucosinolates
can be differentiated into aliphatic, indolyl, and aromatic glucosinolates (Schreiner
et al. 2014 ). UV-B-mediated increased glucosinolate content has been reported in
Light stress
Flavonoids Xanthophylls Glucosinolates
Cell Death
enzymes and
Fig. 8.3 The major metabolites which accumulate in plant tissues exposed to light stress and their
major roles in generating plant tolerance against photodamages and light stress
A. Banerjee and A. Roychoudhury
Brassica oleracea var. italica (broccoli), Arabidopsis thaliana , and Tropaeolum
majus . UV-B doses led to high accumulation of mainly aliphatic methylsulfi nylal-
kyl glucosinolates and indole 4-methoxy-indol-3-ylmethyl glucosinolate in broccoli
and Arabidopsis and an aromatic glucosinolate in Nasturtium . It was also reported
that multiple exposures of high (up to 0.9 KJ m
−2 d −1 ) doses of UV-B did not elicit a
stronger response in glucosinolate accumulation than that which occurred as a result
of lower UV-B doses. UV-C irradiation in broccoli fl orets resulted in high accumu-
lation of 4-methoxy-indol-3-ylmethyl glucosinolate, 4-hydroxy-indol-3-ylmethyl
glucosinolate, and 4-methylsulfi nylbutyl glucosinolate. Unripe Nasturtium green
seeds exhibited sixfolds enhanced level of benzyl glucosinolate accumulation,
whereas the mature leaves exhibited only threefolds increase in the same metabolite
content (Schreiner et al. 2014 ).
8.6 Phytohormone-Mediating Plant Stress Tolerance
Against Light Stress
Plants have been depicted to possess the intrinsic ability to tackle stress by altering
its physiological growth parameters. Such alterations are often brought about by a
dynamic change in the concentration of phytohormones. It has been reported that
plants can regulate their physiological responses by changing the concentration of
indole-3-acetic acid (IAA) , cytokinin (CTK), abscisic acid (ABA) , ethylene, and
other minor phytohormones. In this section, we would mainly highlight the conse-
quences of light stress, especially mediated by UV rays on the phytohormone levels
and the resultant effects on the plant system. In Fig. 8.2 , we have tried to refl ect a
possible model of cross talk among the plant signaling pathways involved in UV-B
perception and three phytohormones, viz., auxin, gibberellic acid (GA), and ABA.
8.6.1 Auxin
Liu and Zhong ( 2009 ) emphasized the fact that UV radiation reduces the concentra-
tion and activity of IAA through photodegradation. This has a direct effect on the
cell physiology as the cells can no longer metabolize and utilize IAA. Witztum and
Keren ( 1978 ) observed the signifi cant decrease in IAA levels in the fronds of
Spirodela oligorhiza exposed to UV light stress. The daughter colonies of the
stressed fronds exhibited a high percentage of abscission. Such physiological decay
could be reversed if the stressed fronds were imbibed in appropriate IAA solutions.
However, this resulted in a strong release of ethylene, which fi rst provided the pos-
sibility of a cross talk between auxin- and ethylene-signaling pathways during UV
stress. Yang et al. ( 1993 ) inferred that exogenous application of IAA of appropriate
concentrations could result in growth of plants exposed to UV-B radiation. Liu and
Zhong (
2009 ) also reported that the IAA content in the leaves of UV-B stressed
8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
Trichosanthes kirilowii was lower than that of the control plants. Another auxin
with a much stable structure than IAA is the α-naphthalene acetic acid (NAA) which
acts as an artifi cial plant growth regulator and is not degraded by IAA oxidase. NAA
causes acidifi cation of the cell wall medium and formation of proteins, thus promot-
ing total plant growth (Wang 2000 ). In T. kirilowii exposed to UV-B rays, addition
of NAA promoted an increase in height and leaf area. This data represents the fact
that NAA replaces the decrease in the endogenous IAA levels under light stress. It
has also been found that UV-B radiation alters the ability of the cells to utilize NAA,
and so the alleviation effect of NAA on the damages caused by UV-B stress is lim-
ited. It was also seen that UV-B radiation decreased the gibberellic acid 1/3 (GA1/3)
content in the leaves of T. kirilowii. Addition of NAA reduced the effect of UV-B on
the endogenous content of GA , because it was observed that the GA content was
higher in the T3 plants where NAA has been exogenously applied than in the T2
plants (Liu and Zhong 2009 ). UV-B also reduced the leaf area, and hence the pro-
duction of less leaf area under such UV-B stress has been portrayed as a defensive
mechanism against this abiotic stress . Reduction of cell size accompanied with con-
ditional change in leaf structure, reduction in cell number through decreased cell
division, and expansion have been accredited to such reduced leaf area under UV-B
stress (Hofmann et al. 2001 ). Irina et al. ( 2004 ) observed that in Pisum sativum , the
leaf area and biomass decreased after UV-B exposure and accounted the inactiva-
tion of CTKs to be responsible for such a phenomenon. However, Liu and Zhong
( 2009 ) found that zeatin riboside (ZR) levels increased in the leaves of T. kirilowii
plants under UV-B stress after addition of NAA.
In previous sections, we have clearly discussed the signaling pattern involved in
UV-B-mediated response in plants. Our main intention is to merge this signaling
with those of the phytohormones which occur in tandem to tackle stress. Hayes
et al. ( 2014 ) showed that the UV-B photoreceptor UVR8 provides an unambiguous
sunlight signal that inhibits shade avoidance responses in Arabidopsis by antagoniz-
ing the phytohormones like auxin and GA. UV- B has also been reported to trigger
the degradation of the TFs like phytochrome-interacting factor 4 (PIF4) and PIF5
and stabilization of the growth repressors like DELLA. Such UV-B-mediated
responses block the auxin biosynthetic pathway, via a dual mechanism (Hayes et al.
2014 ). Leivar and Quail ( 2011 ) inferred that the PIF proteins act as potent signaling
hubs , regulating the auxin and GA activity to control plant development and growth.
However, the cross talk and mechanism of interaction of these pathways are not
completely known, and hence further research is required to investigate the intricate
happenings between the UV-B-mediated signaling cascade and the phytohormone
biosynthetic pathways.
Hectors et al. ( 2012 ) reported that auxin is a component of the regulatory system
that controls both UV-mediated accumulation of fl avonoids and UV-induced photo-
morphogenesis. The leaf area of Arabidopsis Col-0 plants raised under low doses of
UV radiation (0.56 KJ m
−2 day −1 ) decreased by about 23 % on average when com-
pared with the control plants, and the level of free auxin also declined in the stressed
young leaf tissues. The auxin infl ux mutant axr4-1 and the auxin biosynthesis
mutant nit1-3 exhibited stronger morphogenetic responses than the control plants
A. Banerjee and A. Roychoudhury
with higher levels of decrement in the leaf area. It was also inferred that auxin medi-
ated UV acclimation in the plants through the regulation of fl avonoid concentration
and fl avonoid-glycosylation pattern and controlled the UV stress-induced photo-
morphogenesis (Hectors et al. 2012 ).
Lin et al. ( 2002 ) reported the relation between auxin and polyamine content in
the leaves of Nancheum (NC) and Shan You63 (Sy63). The three major enzymes
like arginine decarboxylase (ADC), ornithine decarboxylase (ODC), and
S-adenosylmethionine decarboxylase (SAMDC) were found to be increased in both
the plants. However, in the leaves of IR65600-85, only the ADC and ODC activities
increased by 115.93 % and 14.45 %, respectively, but the SAMDC activity decreased
surprisingly by 33.01 % on exposure to UV-B radiation for one to two weeks. In late
treatment time course (21–28 days), the activities of ADC and ODC substantially
increased in the leaves of Sy63, NC, and IR65600-85 under UV-B stress. However,
the SAMDC content was reduced by 40.06 %, 19.20 %, and 38.21 % in Sy63, NC,
and IR65600-85, respectively, when all were exposed to UV-B stress . The activity
of polyamine oxidase was also low in the plants which ultimately resulted in high
accumulation of polyamines especially putrescine. Apart from these results, Lin
et al. ( 2002 ) also reported that after exposure of the plants to UV-B stress for 7–28
days, the IAA and GA1/3 contents decreased by 58.92 % and 45.48 %, respectively,
in the leaves of Sy63; by 43.31 % and 56.20 %, respectively, in the leaves of NC; and
by 38.60 % and 47.33 %, respectively, in the leaves of IR65600-85. Another contra-
dictory result was observed for CTKs, when it was found that the ZR levels reduced
in the leaves of the three plants with UV-B treatment for 7–14 days, but peaked in
the late courses of exposure (21–28 days). However, the production of the “univer-
sal stress hormone” ABA was quite high. So, a low ratio of IAA/ABA, GA1/3/ABA,
and ZR/ABA led to the suppressed growth of the plants so that more resources
could be channelized to tackle the damages caused by UV-B stress (Lin et al. 2002 ).
Zhang et al. ( 2014 ) showed that PhyB plays a major role as photoreceptor in
sensing the ratio of red to FR light in order to mediate the shade avoidance response
(SAR). Change in homeostasis of phytohormones like auxin and strigolactone has
been partially held responsible for SAR, though the link between PhyB and hor-
monal homeostasis has not been unveiled. Constans-like 7 (COL7) plays a role in
increasing the branching number under high red: FR, but not under low red: FR,
from which it was inferred that COL7 can be involved in the PhyB-mediated
SAR. COL7-induced branching proliferation was suppressed by mutating PhyB and
it was also reported that COL7 inhibits auxin biosynthesis by elevating the mRNA
expression of Superroot2 ( SUR2 ) which encodes a suppressor of auxin biosynthetic
pathway. This suppression of auxin synthesis occurred only under high red: FR and
not under low red: FR. Thus, it can be said that following photoexcitation, PhyB
stabilizes COL7, which regulates the light perceived changes in auxin biosynthesis
(Zhang et al. 2014 ). In another report, Effendi et al. ( 2013 ) found that the auxin
receptor, auxin binding protein (ABP) is involved in the direct regulation of plasma
membrane activities, including the number of PIN proteins and auxin effl ux trans-
port. A steady-state level of ABP1 and auxin-inducible growth capacity are main-
tained by the phytochromes under red light. The hypocotyl lengths were larger in
8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
abp1-5 and abp1/ABP1 mutants exposed to FR light, but not in the null mutant of
Transport-Inhibitor-Response1 (tir1-1) auxin receptor. Auxin transport has been
depicted to be an important condition for FR-induced elongation. So, naphthylphtha-
lamic acid (NPA) which is an auxin transport inhibitor reduced elongation more
strongly in the low red: FR light-enriched white light than in the high red: FR light-
enriched white light condition (Effendi et al. 2013 ). It was also found that decreased
phytochrome action occurred in conjunction with auxin transport in the abp1
mutants, because after adding NPA, hypocotyl gravitropism was inhibited on expo-
sure to both red and FR light. The abp1-5 mutant lines exhibited a reduction in the
transcription of FR light-induced genes, which included several genes regulated by
auxin and shade. The same set of genes under the same conditions was found to be
even lower in the abp1/ABP1 mutants. In the tir1-1 and PhyA-211 mutants, the
shade- induced gene expression was reported to be greatly attenuated. These data
obviously support the fact that ABP1 directly or indirectly plays a role in auxin-
mediated light signaling (Effendi et al. 2013 ). Zhao et al. ( 2013 ) reported that high-
intensity blue light-induced hypocotyl phototropism is mediated by phototropins
mainly through the regulation of the cytosolic calcium concentration in Arabidopsis .
It was seen that the addition of an inhibitor of auxin effl ux carrier resulted in the
inhibition of phototropism and cytosolic calcium bursts, which obviously points to
the role of polar auxin transport in high blue light-induced responses. Phytochrome
kinase substrate (PKS1), the phototropin-related signaling element, has been depicted
to interact physically with the phototropins, auxin effl ux carrier PIN Formed 1
in vitro, and with calcium-binding protein, Calmodulin4 in vivo (Zhao et al. 2013 ).
Intact auxin signaling regulates proper brassinosteroid (BR)-mediated responses
in plants, as has been ascertained from several genetic and physiological studies
(Zhou et al. 2013 ). It was demonstrated that high BR concentration induced the dif-
ferential growth of etiolated hypocotyls, resulting in variable morphological altera-
tions. However, the dominant mutant of IAA19, msg2 , and Auxin Responsive
Factor7 (ARF7) mutant, arf7 , were insensitive to the BR effect and could suppress
enhanced BR-mediated downstream signaling. Reduced BR response in msg2 was
verifi ed from systemic microarray analysis, from which Zhou et al. ( 2013 ) nally
inferred that BR employs auxin signaling components IAA19 and ARF7 to trigger
the downstream responses. Since the relation of light quality with hypocotyl growth
has already been correlated in the previous sections, it can be predicted that BR,
along with auxin, entail pivotal signifi cance in the light-mediated responses. Tao
et al. ( 2008 ) reported the existence of an aminotransferase named TAA1 which cata-
lyzes the accumulation of indole-3-pyruvic acid (IPA) from L -tryptophan. This reac-
tion leads to the rapid accumulation of auxin through a previously uncharacterized
pathway. Such rapid accumulation of auxin is required to initiate the multiple altera-
tions at the morphological level associated with shade avoidance (Tao et al. 2008 ).
Among the early auxin-responsive genes, the Aux/IAA genes are the best character-
ized. These genes encode short-lived transcriptional repressors which regulate
downstream responses. Singla et al. ( 2006 ) found 15 expressed sequence tags
(ESTs) in wheat by screening available databases, and these ESTs exhibit high
homology with the Aux/ IAA homologues in the other related species. TalAA1 is one
A. Banerjee and A. Roychoudhury
such Aux/IAA gene, which encodes a protein containing all the four conserved
domains, characteristic of the Aux/IAA proteins. Expression of TalAA1 is regulated
by light intensities and is tissue specifi c and auxin inducible. TalAA1 is upregulated
in the presence of calcium ions and is also induced by BR, thus showing that the
interplay and cross talk between hormones is essential for plant growth and devel-
opment under light stress (Singla et al. 2006 ). Based on the fact that cryptochromes
participate in several photomorphological responses in seed plants, Imaizumi et al.
( 2002 ) analyzed the functions of cryptochrome in the moss Physcomitrella patens .
The reason of choosing this bryophyte has been accredited to the fact that this is a
well-characterized species, where gene replacement occurs at a high frequency by
homologous recombination. In Physcomitrella , two cryptochrome genes and single
and double disruptants of these genes were generated. It was surprisingly found that
the induction of side branching on protonema and gametophore induction and
development are regulated via cytochrome-mediated signaling. The disruptants
exhibited higher sensitivity to exogenous auxin than the wild type, accompanied
with an altered expression pattern of the auxin-inducible genes, especially when the
disruptant lines were exposed to blue light. These data from the reports of Imaizumi
et al. ( 2002 ) indicate the potential of cytochrome-mediated light signaling in
repressing the auxin-mediated downstream cascades in order to restructure the plant
developmental pattern. The detailed excerpt of the various roles of auxin under light
stress or variable light quality does illustrate the importance of these phytohormones
in maintaining the plant metabolic homeostasis under aforesaid conditions. We have
also tried to highlight the still uncharacterized versions of cross talks and interplay
among the phytohormones like auxin, CTK, and BR which are even more crucial
for the proper generation of plant responses against light stress and light quality.
8.6.2 Cytokinins (CTKs)
CTKs play crucial roles in regulating plant growth, metabolism, and development
under both normal and stress conditions. It has been seen that during desiccation
stress arising out of salinity, drought, variations of temperatures, and even under
heavy metal toxicity, the levels of CTK alter rapidly (Atanasova et al. 2004 ).
According to Buchanan-Wollaston et al. ( 2003 ), the plants experience senescence-
like symptoms under stress conditions due to the burst in the production of ROS.
The CTKs have often been portrayed as antioxidants due to their anti- senescence
property. Vaseva et al. ( 2006 ) studied the changes in CTK oxidase/dehydrogenase,
involved in CTK biosynthesis in plants exposed to UV-B stress. The authors chose
two pea cultivars based on phenotypic differences; the cv. “Scinado” was taller and
fast growing, whereas the cv. “Manuela” showed slow growth, accompanied by
shorter stems and broader leaves. The initial level of CTK content in the leaves of
both the varieties suffi ciently differed from each other. The control Manuela leaves
exhibited lower CTK concentration compared to Scinado. However, the CTK
oxidase/dehydrogenase (CKX) behaved differently in the leaves of plants after
8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
exposure to UV-B radiation. In Manuela, UV-B radiation completely inhibited CKX
activity which led to lowered CTK content, except only the phosphorylated forms.
On the contrary, the leaves of Scinado exhibited a completely opposite trend, lead-
ing to increased accumulation of CTK due to high CKX activity (Vaseva et al.
2006 ). The CTK ribosides, cis-zeatin (cisZ), and isopentyl adenosine riboside
monophosphate (iPRP) levels were unaffected by irradiation in the Manuela roots,
though a drastic decrease in the isopentyl adenine (iP) titer (more than 2.5-fold
compared to the control) was recorded. The Scinado roots exhibited lowered CTK
content with the exception of cisZ, accompanied with very low CKX activity (about
tenfolds inhibition compared to the control) after exposure to UV-B stress. Thus
Vaseva et al. ( 2006 ) reported the better adaptability of the Scinado cultivars under
UV-B stress due its high CTK content, mainly because of the fact that CTKs have
antioxidant and anti-senescence properties. It was found that the Manuela cultivars
are more prone to UV-B stress. Thus, apart from inferring that a high CTK level is
a prerequisite for better UV-B stress adaptability, it was also predicted that the dif-
ferent CKX responses in Scinado and Manuela could be due to the presence of dif-
ferent alleles controlling the studied processes in their genomes (Vaseva et al. 2006 ).
8.6.3 Ethylene
The response of the plants to tackle light stress or to adapt to the quality of light to
which it is exposed depends on a complex network of interactions among multiple
phytohormones. Weller et al. ( 2015 ) identifi ed a mutant with greatly increased leaf
expansion and delayed petal senescence by screening for pea mutants showing
altered photomorphogenesis under red light. The mutant was quite insensitive to
ethylene due to the occurrence of a nonsense mutation in the single pea orthologue
of the ethylene-signaling gene Ethylene Insensitive2 ( EIN2 ). Phytochrome-defi cient
plants having ein2 mutation had the ability to reverse the effects of ethylene over-
production. Increase in leaf expansion under monochromatic red: FR or blue light
was recorded in the ein2 mutant seedlings. That ein2 enhances both phytochrome-
and cytochrome-dependent responses in a LONG1-dependent manner was con-
rmed from the interaction of ein2 with PhyA and PhyB and also from the long1
mutants (Weller et al. 2015 ). However, it was inferred that ethylene was not the
limiting factor for the development of seedlings in darkness or under high- irradiance
white light. It was also concluded that ethylene signaling was responsible for the
constrained leaf expansion during de-etiolation in pea and that the downregulation
of ethylene biosynthesis might be linked with the photomorphogenetic development
of the plant mediated by phytochromes and cryptochromes (Weller et al. 2015 ).
UV- B mediates stomatal closure via production of hydrogen peroxide and also
affects ethylene biosynthesis. He et al. ( 2011 ) linked UV-B stress response and eth-
ylene activity, based on the fact that both induce stomatal closure through the pro-
duction of hydrogen peroxide. So, it is possible that the UV-B-mediated stomatal
closure actually occurs via ethylene-mediated hydrogen peroxide production. This
A. Banerjee and A. Roychoudhury
probability was investigated in Vicia faba by epidermal strip bioassay, laser scan-
ning confocal microscopy, and assays of ethylene production (He et al. 2011 ). The
report became interesting when the experiments showed that ethylene might be epi-
static to UV-B radiation in stomatal movement, because stomatal closure, induced
by UV-B stress, was inhibited when the ethylene biosynthesis and signaling cascade
was interfered. It was also observed that on exposure to UV-B stress, the ethylene
accumulation preceded the hydrogen peroxide production which also supported the
hypothesis of auxin-mediated stomatal regulation on exposure to UV-B radiation.
He et al. ( 2011 ) also suggested that such stomatal closure occurs via a peroxidase-
dependent hydrogen peroxide production, mediated by auxin because the inhibitors
for peroxidase, but not for NADPH oxidase, strongly inhibited hydrogen peroxide
production upon UV-B radiation.
The following report by Becatti et al. ( 2009 ) showed the effect of UV-B shielding
on ethylene production for the ripening in tomato fruits, along with the ethylene-
mediated carotenoid accumulation in the ripened fruits, exposed to UV-B rays. The
authors, therefore, selected rin and nor tomato mutants, which were unable to pro-
duce ethylene required for fruit ripening in the cv. Alisa Craig, which were culti-
vated under control and UV-B depleted conditions till fruit ripening. The requirement
of functional rin and nor genes was declared essential after observing that following
UV-B depletion, the ethylene production decreased in Alisa Craig. The carotenoid
content in the ripened fruits was found to be controlled by UV-B-mediated plant
responses, either in an ethylene-dependent or ethylene-independent manner (Becatti
et al. 2009 ). In another fi nding, it was seen that PhyB-1-mediated ethylene produc-
tion increased only with high photosynthetic photon fl ux density (PPFD) and high
red: FR light ratio in Sorghum (Finlayson et al. 2007 ). Enhanced ethylene produc-
tion promoted shade avoidance by reducing the leaf blade, leaf sheath elongation,
though the total shoot elongation was hindered. Finlayson et al. ( 2007 ) also reported
that PhyA could participate in the transduction of shade signals in light-grown
Sorghum plants by triggering ethylene-mediated responses.
8.6.4 Abscisic Acid (ABA)
ABA is the universal stress hormone which has been reported to be upregulated
under almost all forms of abiotic stress . It can be clearly deciphered that UV-B
stress causes several morphological and molecular damages to the plant system, and
hence like other abiotic stresses, ABA level is expected to enhance under such con-
ditions. The downstream signaling cascade mediated by ABA has been depicted to
be involved in triggering downstream stress-responsive genes which encode protein
products having potential roles as stress relievers. One such product is the
OsWRKY89 which is ABA inducible. This protein encoding gene OsWRKY89 was
identifi ed in rice which has been depicted to regulate responses during UV-B stress.
Increased wax deposition on the surfaces of leaves was reported in UV-B-stressed
transgenic plants overexpressing OsWRKY89 . The increased wax deposition
8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
drastically reduced the percentage of UV-B transmittance through the leaves.
Further researches are required to create a database on the involvement of multiple
WRKY proteins regulating the responses induced by radiation stress (Banerjee and
Roychoudhury 2015 ).
Liu et al. ( 2009 ) also suggested the accumulation of ABA in plants exposed to
UV-B stress , and the authors also documented that ABA might be responsible for
the corresponding decrease in the levels of IAA , GA, and CTKs . The NAA, IAA,
GA3, and 6-benzyladenine (6-BA) have functions contradicting those of ABA and
hence can decrease the total endogenous ABA levels. In Trichosanthes kirilowii , as
explained earlier, UV-B stress leads to reduced accumulation of IAA and GA1/3,
whereas the levels of ZR and ABA signifi cantly increased. Thus, it was concluded
by Liu et al. (2009) that growth regulation in T. kirilowii exposed to UV-B stress is
not only mediated by altered IAA concentrations or activities but also by the inte-
grated changes in concentrations and activities of GA and ZR which affected the
endogenous levels of ABA. This observation was in line with those of Tossi et al.
( 2012 ) who inferred that an increase in endogenous ABA concentration is a univer-
sal response to UV-B stress. It was also proposed that the induction of common
signaling components like ABA, nitric oxide, and calcium bursts in plant and ani-
mal cells exposed to UV-B radiation points toward the evolution of a general mech-
anism in divergent multicellular organisms to tackle the damages caused by high
doses of UV-B stress (Tossi et al. 2012 ). In another experiment, it was observed that
ABA concentration increased by 100 % in the UV-B-irradiated leaves of Zea mays ,
whereas the maize viviparous14 ( vp14 ) mutant, due to a defective ABA biosynthesis
pathway, was hypersensitive to UV-B stress (Tossi et al. 2009 ). However, the UV-B-
mediated damages were attenuated both in the wild-type and vp14 mutants after
exogenous ABA treatments. It was also suggested that UV-B perception induced an
increase in the ABA concentration which stimulated NADPH oxidase activity,
hydrogen peroxide generation, and also a nitric oxide synthase-like-dependent
mechanism leading to increased nitric oxide production, required to maintain cel-
lular homeostasis under stress (Tossi et al. 2009 ). Gil et al. ( 2012 ) investigated the
terpene profi les as determined by gas chromatography with electron impact mass
spectrometry (GC-EIMS) analysis of in vitro cultured plantlets of Vitis vinifera
exposed to fi eld-like UV-B dosage. In young leaves exposed to low UV-B dosage,
notable increases were found in the levels of membrane-associated triterpenes like
sitosterol, stigmasterol, and lupeol. On the other hand, antioxidants like diterpenes α
and γ tocopherols, phytol, the sesquiterpene E-nerolidol and the monoterpenes
carene, α-pinene and terpinolene cumulatively accumulated in the leaves under high
dosages of UV-B stress. Along with the increase in the cellular concentration of
these antioxidant terpenes, rise in the levels of the sesquiterpene phytohormone,
ABA, was also recorded (Gil et al. 2012 ).
Berli et al. ( 2010 ) exposed one-year-old fi eld-grown plants of Vitis vinifera to
PAR along with weekly supplemental sprays of ABA. They observed that on exog-
enous ABA treatment, the levels of UV-B-absorbing fl avonols, quercetin, and
kaempferol signifi cantly increased. Though the levels of two hydroxycinnamic
acids named caffeic and ferulic acids remained unaffected on exposure to UV-B
A. Banerjee and A. Roychoudhury
stress , their levels increased on exogenous ABA treatments. Such UV-B-independent
increased accumulation was also true for the cell membrane β-sitosterol, solely by
exogenous ABA treatments. However, simultaneous treatments of both UV-B radia-
tion and exogenous ABA were required to upregulate the activities of antioxidant
enzymes like catalase, ascorbate peroxidase, and also carotenoids (Berli et al. 2010 ).
Berli et al. ( 2011 ) reported that in the UV-B-exposed grapevine leaf tissues, the
ABA levels increased suffi ciently to trigger the biosynthesis of phenols that fi lter
the harmful radiations and act as potential antioxidants. It was found that when the
grapevine plantlets were exposed to both high and low dosages of UV-B irradiation
and weekly exogenous supplements of 1 mM ABA (+ ABA) or water (− ABA), the
reduction of UV-B delayed the development and maturation of berries, whereas
the + UV-B and + ABA treatments hastened sugar and phenol accumulation.
However, individual treatments of + UV-B or + ABA reduced berry growth and the
sugar content per berry, without affecting the concentration of sugar at harvest
(Berli et al. 2011 ). Assays performed also exhibited that the ABA levels in the berry
skins were high in the + UV-B and + ABA combined treatment which led to hastened
berry ripening, thus indicating toward the possibility of some role of ABA in regu-
lating fruit ripening under stress conditions. Under such treatments, the berry skin
phenols increased with a change in the anthocyanin and non-anthocyanin profi les,
thus enhancing the proportion of phenols which display high antioxidant properties
(Berli et al. 2011 ).
ABA produced by the vascular parenchyma cells has been depicted to regulate the
bundle sheath cell (BSC)-specifi c expression of Ascorbate Peroxidase2 ( APX2 ) in the
leaves of Arabidopsis exposed to high light stress. ABA mediates APX2 expression by
triggering the combined activation of Sucrose Non-fermenting-1- related protein
kinase, SnRK2.6 (Open Stomata1 protein kinase), protein phosphatase2C ABA
Insensitive2 (ABI2), and Gα (GPA1)-regulated signaling cascades (Gorecka et al.
2014 ; Galvez-Valdivieso et al. 2009 ). The degree of susceptibility of the BSCs to
photoinhibition under high light stress is regulated by the ABA-activated signaling
network through infl uential non-photochemical quenching. However, except the
guard cell responses to initiate stomatal closure, no major ABA-mediated response in
the transcriptome was detected in the whole leaves exposed to high light stress
(Gorecka et al. 2014 ). Piskurewicz et al. ( 2009 ) reported that under the canopy, seed
germination was slowed by FR light along with the inactivation of the photoreceptors.
Such conditions elicited a decrease in the GA content and an increase in the ABA
level. Seed germination is generally promoted by GA , via the proteasome-mediated
degradation of the DELLA repressors, whereas ABA inhibits seed germination via
stimulation of the ABI repressors. The link between phytochrome- mediated light
responses and the GA/ABA concentration ratio has not yet been clearly deciphered.
However, it has been depicted that exposure to FR light stabilizes the DELLA factors
like GAI, RGA, and RGL2, which in turn stimulated ABA synthesis, thus inhibiting
seed germination through the production of ABI proteins. Such transcription of GAI
and RGA was mediated by the basic helix-loop- helix TF named PIL5. Under low GA
concentration, high GAI and RGA levels inhibited seed germination. Interestingly
under white light, GAI and RGA were expressed under the RGL2 promoter and could
8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
substitute RGL2 to trigger ABA synthesis. Testa rupture was inhibited by the DELLA
proteins, while ABI3 blocked endosperm rupture thus prohibiting seed germination
under low GA levels, induced by FR light (Piskurewicz et al. 2009 ). FR and red light
perceived by phyA and phyB in tomato were found to function antagonistically in
mediating cold tolerance. This antagonism was regulated by the ABA- and JA- related
genes and the C-repeat binding factor (CBF) stress-signaling pathway (Wang et al.
2015 ). Chen et al. ( 2008 ) observed that HY5 mediated ABA responses during seed
germination, early seedling growth, and root development in Arabidopsis. ABI5 tran-
scription is activated through the association of HY5 with the promoter of the ABI5
gene. Translated ABI5 acts as a crucial TF to upregulate the late embryogenesis abun-
dant ( lea ) genes in seeds. It was found, via chromatin immunoprecipitation assays,
that ABA further enhances the binding of HY5 with the ABI5 promoter and the over-
expression of ABI5 solely restored ABA sensitivity in the hy5 mutants. Chen et al.
( 2008 ) identifi ed a possible integration of light and ABA-signaling pathways which
helped young seedlings to develop tolerance toward abiotic stresses . After the percep-
tion of light, the phytochromes promoted the expression of Light-Harvesting
Chlorophyll a/b protein encoded by the PSII LHCB genes. Staneloni et al. ( 2008 )
fused the LHCB promoter to a reporter gene in etiolated Arabidopsis seedlings and
exposed them to continuous FR light. This was done only to activate phytochromes
and not photosynthesis. The seedlings were also treated with exogenous ABA. The
authors found a motif containing the core CCAC sequence in the LHCB promoter
which is required for ABI4 binding, thus facilitating the ABA-mediated downregula-
tion of the associated gene. However, the ACGT sequence containing G-box was not
found in the promoter sequence. Thus, Staneloni et al. ( 2008 ) proposed a model in
which hydrogen peroxide, produced in the chloroplasts under high light conditions, is
associated with the ABA-mediated signaling to altogether regulate LHCB expression.
Thus, it could be clearly seen that ABA have tremendous roles to play in the plant
system in order to mediate stress-responsive and stress-tolerance signals, which aid
the plant to better tackle light-related abiotic stresses .
8.7 Conclusions
Several breakthrough outcomes have been recorded over the past years regarding
plant tolerance toward abiotic stress. Among these, the most uncharacterized and
less studied is the light-associated stress and their effects and responses in plants.
Though sunlight is obligatory for plant growth and development via photosynthesis,
it contains harmful rays which mainly fall within the UV-B range of the spectrum.
The plant nuclear DNA is an inherently unstable biomacromolecule which can be
spontaneously damaged metabolically or through the generation of several stress-
responsive harmful factors. Light stress which includes high and low light intensity
along with the ROS- generating UV-B rays have such disastrous effects, leading to
the ultimate degeneration of genomic integrity. Phytohormones are the major chem-
ical regulators which determine alterations in the patterns of plant morphogenesis at
A. Banerjee and A. Roychoudhury
molecular and physiological levels. Recent researches have emphasized the fact that
such phytohormones, especially auxin and ABA, have vital roles to play in light
stress-mediated responses. Our chapter has also covered the integrated signaling of
UV-B perception of the plant and the downstream cross talks present among the
auxin-, CTK -, GA- , ethylene-, and ABA-signaling cascades. However, a proper
blueprint containing clean demarcations and accurate knowledge of the responsible
factors participating in such cross-talk signaling has not yet been created. Thus, the
future perspectives in this fi eld are bright enough, since light stress cannot be regu-
lated as such, along with increasing CFCs and ozone layer depletion. In spite of
such constraints, crop development and large-scale production of food crops are
required for the ever growing population, a large fraction of which suffer from “hid-
den hunger.” The breeders cannot avoid and vanquish land stretches based only on
the constraint that they experience high doses of light stress. This brings forth the
need of transgenic technology which can create stress-tolerant varieties, viable
under such stress conditions, thereby increasing the overall yield and crop produc-
tion. Hence the inquisitive science community is on the verge to better understand
the phytohormone responses in plants exposed to a variety of light stresses, so that
the biosynthetic or downstream genes regulated by these hormones can be targeted
for up- or downregulation, in order to create stress-tolerant transgenic cultivars for
human consumption.
Acknowledgements The nancial support from Science and Engineering Research Board
(SERB), Department of Science and Technology, Government of India through the research grant
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of India, through the project [38(1387)/14/EMR-II] to Dr. Aryadeep Roychoudhury is gratefully
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8 Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role…
... Light is an important environmental factor and fluctuation in light quality and intensity are responsible for alterations in plant growth (Banerjee and Roychoudhury 2016). The downregulation of light incited genes was observed in Arabidopsis strigolactone -deficient mutants but it was enhanced after the GR24 supplementation. ...
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Strigolactones are a group of carotenoid-derived phytohormones which are synthesized in plastids and cytosol. Strigolactones also act as molecular cue that assist plants to liaise with surroundings. These active biomolecules are also called as non-traditional phytohormones or plant growth regulators. Strigolactones are generated in roots mainly and released into rhizosphere, although their synthesis has been reported in other plant parts in trace amount. More than one thousand types of strigolactones are reported in plants but only 30 strigolactones have been identified yet. Strigolactones play a significant role in regulation of biochemical processes such as seed germination, plant growth and development (root system architecture, shoot branching and tillering), and delaying leaf senescence during plant survival under environmental constraints. Strigolactones usually involve oxidative responses in plant cells and promote osmolytes production in stressed cells for maintenance of homeostasis under environmental stress, including salt, light, temperature, drought, nutrient deficiency, and heavy metals. The present review highlights biosynthesis and functions of strigolactones, their crosstalk with other phytohormones and functions in abiotic stress resistance mechanism. Application of strigolactones for plant growth under harsh conditions provides channel for development of novel technologies which are environmentally benign and sustainable. Further in-depth investigations are required about the exogenous treatment or transgenic approaches for the accumulation of strigolactones inside the plants which can be a potential strategy for protection of plants against multiple environmental stresses.
... We have evaluated the malic enzyme sensitivity under varying concentrations (saturated and limited) of malic acid. Interestingly, as noted earlier, light illumination may increase the cellular concentration of pyruvic acid in the bundle sheath, with an optimum value in maize [33]. This is accompanied by enzyme activities releasing CO 2 levels over ten-fold compared to the ambient concentration. ...
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We revealed the functional characterization of C4-NADP-malic enzyme (NADP-ME), extracted and partially purified from maize (Zea mays L. cv. Kaveri 50). The leaf discs were previously activated under 1000–1200 µE m−2 s−1, incubated in bicarbonate (2.0 mM) solution, and subjected to salt stress (100 mM NaCl). Initially, salt stress was evident from the accumulations of proline, chlorophyll content, carbohydrate profile, and Hill activity influencing the C4 enzyme. Primarily, in illuminated tissues, the activity of the enzyme recorded a reduced trend through salinity irrespective of light and darkness compared to the control. On illumination, the kinetic parameters such as Vmax of the enzyme increased by 1.36-fold compared to in the dark under salinity whereas Km was decreased by 20% under the same condition. The extent of light induction was proportionate to limiting (0.01 mM) and saturated (4.0 mM) malate concentrations for enzyme activity. Moreover, the catalytic properties of the enzyme were also tested on concomitant responses to activator (citrate and succinate) and inhibitor (oxalate and pyruvate) residues. The sensitivity to light and dark effects was also tested for reducing agents such as dithiothreitol, suggesting the effect of the changes in redox on the regulatory properties of the enzyme. The ratio of enzyme activity under light and darkness in the presence or absence of a reducing agent was concomitantly increased with varying malate concentrations. At the molecular level, protein polymorphism of the enzyme represented minor variations in band intensities, however, not in numbers through salinity subjected to light and darkness. Therefore, salinity-induced changes in the decarboxylation reaction, evident by NADP-ME activity, may be based on the redox property of regulatory sites and sensitivity to light and darkness.
... Light-mediated regulation of molecular/biochemical processes and physiological responses in an organism is called photoregulation . Several members of the light signaling pathway including photoreceptors, transcription factors and components of degradation complex play roles in regulating genes involved in flavonoids and carotenoids biosynthesis (Banerjee and Roychoudhury, 2016;Guidi et al., 2016;Yadav et al., 2020). The underlying mechanistic details about the light signaling components regulating the synthesis and accumulation of secondary metabolites have expanded over the last few years. ...
... These pigments prevent the oxidation of the D1 protein of PSII complex by rapid quenching of the excited state of chlorophyll. Banerjee and Roychoudhury (2016) reported that SP induces the synthesis of photo-protective agents such as anthocyanin which improves ROS scavenging and plant protection. Drought stress causes the autopepsia and deformed the chloroplast structure, which can be recovered in melatonin (MT) SP in rapeseed. ...
Wheat is one of the most cultivated crops, and its production is greatly affected by abiotic stresses such as drought. Transcription factors that regulate multiple downstream genes have a good potential to improve stress tolerance due to their ability to modulate various response pathways. Many members of the C1-2i C2H2 zinc finger protein (ZFP) sub-family are up-regulated in response to abiotic stress and specific members were shown to improve oxidative and abiotic stress tolerance in different plant species. Our previous work demonstrated that the most up-regulated TaZFP, TaZFP1B, improves drought tolerance and oxidative stress protection. Based on their strong response to different abiotic stresses, five additional candidate TaZFPs were selected for functional characterization by reverse genetics in three wheat lines, using the four-component Barley Stripe Mosaic Virus system. Of the five candidates studied, TaZFP13D was the only gene that increased drought stress tolerance (resistance to wilting and improved relative water content) in three different wheat lines and significantly improved antioxidant enzymes activities resulting in lower accumulation of superoxide and hydrogen peroxide. Moreover, TaZFP13D down-regulation decreased drought stress tolerance suggesting that this gene does not have a redundant function within the TaZFPs family.
Pulses are climate-smart grain legumes important to nutritional security and sustainable agriculture. Abiotic stresses take a heavy toll in pulse production, and genetic engineering offers a solution to add adaptive traits in the germplasm. Abiotic stresses being mostly polygenic are difficult to manipulate and require a thorough understanding of the underlying mechanism. Impact of abiotic stresses in eight different pulses, genetic mechanism involved, and transgenics approach adopted for enhancing the stress tolerance in those pulses are discussed. Traits engineered in chickpea (drought and salt tolerance), pigeon pea (salt tolerance), mung bean (salt and cold tolerance), urdbean (salt and drought tolerance and aluminum toxicity), cowpea (salt tolerance), field pea (salt, frost, and heat tolerance), common bean (drought tolerance), and lentil (cold and freezing tolerance) and resulting phenotypes are also discussed. Currently, only two transgenic pulses for biotic stress (insect resistance for cowpea and golden mosaic virus in common bean) are commercialized. Climate change poses various challenges, and genetic engineering and emerging genome editing techniques for abiotic stress-adaptive traits shall play a crucial role in abiotic stress management.
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The global population is growing at a rapid pace that is directly proportional to increasing food demand. Besides productivity is likely to stay steady and perhaps even decline due to climate change and various other abiotic factors including UV. This stress is significantly enhanced by environmental contamination such as halogenated hydrocarbons. Since plants are regarded as sessile organisms, they are unable to escape exposure to potentially stressful environmental conditions. Mild stress conditions can result in active acclimation responses, whereas according to research more severe stress conditions can result in metabolic abnormalities. Changes in the amount and ratio of phytohormones produced by plants are among the initial responses of plants to environmental stressors. Sunlight is a key characteristic of photosynthesis that allows crops to fine-tune their morphological traits, growth, and metabolic processes. A part of solar radiation known as ultraviolet light has a substantial influence on the development and metabolism of plants. The buildup of antioxidants and secondary metabolites in plants is a frequent acclimatization response to a variety of environmental stresses in the environment. When it comes to metabolites, UV light has a significant impact on the levels of a diverse spectrum of substances such as phenolic, terpenoid, and alkaloid compounds. In this chapter, we illustrated the levels of several of these phytohormones’ responses because of ultraviolet radiation.KeywordsPhytohormoneCerealsBiostimulantsMetabolitesUltraviolet rays
Rice (Oryza sativa L.) is one of the most important staple food crops that is cultivated in South East Asia. This crop is afected by a wide range of biotic and abiotic factors, each of which is contributing to a change in its physiology, biochemistry, and genetic makeup. Ozone is produced in the troposphere as a result of reactions between abiotic factors, such as oxides of nitrogen and carbon, and UV radiation. These reactions lead to the production of a wide variety of volatile organic compounds. Rice scientists have expressed a great deal of concern regarding the impact of ozone on rice, which has necessitated the development of strategies to combat the problem. The recent advances in rice genomics have led to the discovery of molecular biology approaches such as marker-assisted selection involving quantitative trait loci linked to genes that confer tolerance to ozone stress. This trait is thought to be controlled by a large number of loci with medium efects rather than by a single locus with a large efect. The current review is an efort to provide information on the physiological, biochemical, and molecular responses of rice towards elevated ozone tolerance and also to refect the available strategies to minimize the effect.
One of the challenges being faced by the fruit crop production globally is that regional climate regimes are becoming more unpredictable every year. The exact impact of these abiotic stresses on fruit crops is difficult to predict and it depends on numerous other climacteric factors that are all always counted in predictive models. Plant hormones can improves tolerance against abiotic stresses by inhibiting or reducing the active oxygen species (AOS) accumulation, electrolyte leakage, induce the expression of stress-specific genes, etc. Among the major hormones produced by the plants, ABA, SA, JA and ET are known to play major roles in mediating plant defence response against various abiotic stresses. Typically ABA has major role in contrast to abiotic stresses because environmental condition such as drought, heat stress, cold, chilling, salinity and heavy metals are known to trigger increase in ABA levels. Crosstalk networks among phytohormones mediated by diverse key regulators have been extensively investigated and documented in various research investigations. Many canvasser's recommended the use of phytohormones to counteract abiotic stress because of increasing stress tolerance through conventional breeding methods requires time and a diverse cultivation environment for validation of performance and heritability. In this review, we summarize the roles of the major phyto-hormones present in fruit crops in regulating abiotic stresses responses with special focus on the significance of crosstalk between various hormones at biochemical and molecular level in generating a sophisticated and efficient stress response.
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WRKY proteins are emerging players in plant signaling and have been thoroughly reported to play important roles in plants under biotic stress like pathogen attack. However, recent advances in this field do reveal the enormous significance of these proteins in eliciting responses induced by abiotic stresses. WRKY proteins act as major transcription factors, either as positive or negative regulators. Specific WRKY factors which help in the expression of a cluster of stress-responsive genes are being targeted and genetically modified to induce improved abiotic stress tolerance in plants. The knowledge regarding the signaling cascade leading to the activation of the WRKY proteins, their interaction with other proteins of the signaling pathway, and the downstream genes activated by them are altogether vital for justified targeting of the WRKY genes. WRKY proteins have also been considered to generate tolerance against multiple abiotic stresses with possible roles in mediating a cross talk between abiotic and biotic stress responses. In this review, we have reckoned the diverse signaling pattern and biological functions of WRKY proteins throughout the plant kingdom along with the growing prospects in this field of research.
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CPD45 (chloroplast division45),which is also known as FHY3 (far-red elongated hypocotyl3), is a key factor in the far-red light signaling pathway in Arabidopsis. We previously showed that FHY3/CPD45 also regulates chloroplast division. Because light is also a regulator of chloroplast development and division, we sought to clarify the relationship between far-red light signaling and chloroplast division pathways. We found that the chloroplast division mutant arc5-3 had no defect in far-red light sensing, and that constitutive overexpression of ARC5 rescued the chloroplast division defect, but not the defect in far-red light signaling, of cpd45. fhy1, which is defective in far-red light signaling, exhibited normal chloroplast division. Constitutive overexpression of FHY1 rescued the far-red light signaling defect, but not the chloroplast division defect, of cpd45. Moreover, ARC5 and FHY1 expression were not affected in fhy1 and arc5-3, respectively. Based on these results, we propose that FHY3/CPD45 regulates far-red light signaling and chloroplast division in parallel by activating the expression of FHY1 and ARC5 independently. This work demonstrates how relationships between different pathways in a gene regulatory network can be explored.
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Background and aims: Volatile organic compounds (VOCs) play various roles in plant-plant interactions, and constitutively produced VOCs might act as a cue to sense neighbouring plants. Previous studies have shown that VOCs emitted from the barley (Hordeum vulgare) cultivar 'Alva' cause changes in biomass allocation in plants of the cultivar 'Kara'. Other studies have shown that shading and the low red:far-red (R:FR) conditions that prevail at high plant densities can reduce the quantity and alter the composition of the VOCs emitted by Arabidopsis thaliana, but whether this affects plant-plant signalling remains unknown. This study therefore examines the effects of far-red light enrichment on VOC emissions and plant-plant signalling between 'Alva' and 'Kara'. Methods: The proximity of neighbouring plants was mimicked by supplemental far-red light treatment of VOC emitter plants of barley grown in growth chambers. Volatiles emitted by 'Alva' under control and far-red light-enriched conditions were analysed using gas chromatography-mass spectrometry (GC-MS). 'Kara' plants were exposed to the VOC blend emitted by the 'Alva' plants that were subjected to either of the light treatments. Dry matter partitioning, leaf area, stem and total root length were determined for 'Kara' plants exposed to 'Alva' VOCs, and also for 'Alva' plants exposed to either control or far-red-enriched light treatments. Key results: Total VOC emissions by 'Alva' were reduced under low R:FR conditions compared with control light conditions, although individual volatile compounds were found to be either suppressed, induced or not affected by R:FR. The altered composition of the VOC blend emitted by 'Alva' plants exposed to low R:FR was found to affect carbon allocation in receiver plants of 'Kara'. Conclusions: The results indicate that changes in R:FR light conditions influence the emissions of VOCs in barley, and that these altered emissions affect VOC-mediated plant-plant interactions.
Living organisms are subject to fluctuating environmental conditions. Whereas most animals are able to move away from unfavourable conditions, plants are sessile and so must cope with whatever comes their way. Of all the environmental cues that challenge the developing plant, light can probably be considered to be the most important. In addition to its key role in plant metabolism, and hence almost all life on Earth, where it drives the process of photosynthesis, light energy also acts to regulate plant growth and development. Light quantity, quality, direction and diurnal and seasonal duration regulate processes from germination, through seedling establishment to the architecture of the mature plant and the transition to reproductive development. These developmental responses of plants to light constitute photomorphogenesis. This volume is designed to provide the reader with state-of-the-art accounts of our current knowledge of the major classes of higher plant regulatory photoreceptors and the signal transduction networks that comprise plant developmental photobiology. Consideration is also given to the ways in which knowledge of plant photoreceptors and their signalling networks can be exploited, for instance to improve the quality and productivity of commercially-grown plants. The book is directed at researchers and professionals working in plant molecular biology, plant physiology and plant biochemistry.
Light signaling and phytohormones both influence plant growth, development and stress responses; however, crosstalk between these two signaling pathways in response to cold remains underexplored. Here, we report that far-red (FR) and red (R) light perceived by phytochrome A (phyA) and phyB positively and negatively regulated cold tolerance, respectively, in tomato (Solanum lycopersicum), which were associated with the regulation of levels of the phytohormones such as abscisic acid (ABA) and jasmonic acid (JA), and transcript levels of ABA- and JA-related genes and the C-REPEAT BINDING FACTOR (CBF) stress signaling pathway genes. A reduction in the R/FR ratio did not alter cold tolerance, ABA and JA accumulation, and transcript levels of ABA- and JA-related genes and the CBF pathway genes in phyA mutant plants; however, those were significantly increased in wild type and phyB plants with the reduction in the R/FR ratio. Even though low R/FR treatments did not confer cold tolerance in ABA (not)- and JA (spr2)-deficient mutants, it up-regulated ABA accumulation and signaling in the spr2 mutant, with no effect on JA levels and signaling in the not mutant. Foliar application of ABA and JA further confirmed that JA functioned downstream of ABA to activate CBF pathway in light quality-mediated cold tolerance. It is concluded that phyA and phyB function antagonistically to regulate cold tolerance that essentially involves FR light-induced activation of phyA to induce ABA signaling and subsequently JA signaling, leading to an activation of the CBF pathway and a cold response in tomato plants.
The blue light receptors termed cryptochromes mediate photomorphological responses in seed plants. However, the mechanisms by which cryptochrome signals regulate plant development remain obscure. In this study, cryptochrome functions were analyzed using the moss Physcomitrella patens. This moss has recently become known as the only plant species in which gene replacement occurs at a high frequency by homologous recombination. Two cryptochrome genes were identified in Physcomitrella, and single and double disruptants of these genes were generated. Using these disruptants, it was revealed that cryptochrome signals regulate many steps in moss development, including induction of side branching on protonema and gametophore induction and development. In addition, the disruption of cryptochromes altered auxin responses, including the expression of auxin-inducible genes. Cryptochrome disruptants were more sensitive to external auxin than wild type in a blue light–specific manner, suggesting that cryptochrome light signals repress auxin signals to control plant development.
Plant responses to light involve a complex network of interactions among multiple plant hormones. In a screen for mutants showing altered photomorphogenesis under red light, we identified a mutant with dramatically enhanced leaf expansion and delayed petal senescence. We show that this mutant exhibits reduced sensitivity to ethylene and carries a nonsense mutation in the single pea ortholog of the ethylene signalling gene EIN2. Consistent with this observation, the ein2 mutation rescues the previously described effects of ethylene overproduction in mature phytochrome deficient plants. In seedlings, ein2 confers a marked increase in leaf expansion under monochromatic red, far-red or blue light, and interaction with phyA, phyB and long1 mutants confirms that ein2 enhances both phytochrome- and cryptochrome-dependent responses in a LONG1-dependent manner. In contrast, minimal effects of ein2 on seedling development in darkness or high-irradiance white light show that ethylene is not limiting for development under these conditions. These results indicate that ethylene signalling constrains leaf expansion during de-etiolation in pea and provide further evidence that down-regulation of ethylene production may be an important mechanism mechanism in the broader control of photomorphogenic development by phytochrome and cryptochrome. Copyright © 2015, Plant Physiology.