The Plant Hormones: Their Nature, Occurrence, and Functions

Abstract

Plant hormones are a group of naturally occurring, organic substances which influence physiological processes at low concentrations. The processes influenced consist mainly of growth, differentiation and development, though other processes, such as stomatal
movement, may also be affected. Plant hormones1 have also been referred to as ‘phytohormones’ though this term is infrequently used.

Full text

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A. INTRODUCTION
A1. The Plant Hormones: Their Nature,
Occurrence, and Functions
Peter J. Davies
Department of Plant Biology, Cornell University, Ithaca, New York 14853,
USA. E-mail: pjd2@cornell.edu
INTRODUCTION
The Meaning of a Plant Hormone
Plant hormones are a group of naturally occurring, organic substances which
influence physiological processes at low concentrations. The processes
influenced consist mainly of growth, differentiation and development, though
other processes, such as stomatal movement, may also be affected. Plant
hormones1 have also been referred to as ‘phytohormones’ though this term is
infrequently used.
In their book Phytohormones Went and Thimann (10) in 1937 define a
hormone as a substance which is transferred from one part of an organism to
another. Its original use in plant physiology was derived from the
mammalian concept of a hormone. This involves a localized site of synthesis,
transport in the bloodstream to a target tissue, and the control of a
physiological response in the target tissue via the concentration of the
hormone. Auxin, the first-identified plant hormone, produces a growth
response at a distance from its site of synthesis, and thus fits the definition of
a transported chemical messenger. However this was before the full range of
what we now consider plant hormones was known. It is now clear that plant
hormones do not fulfill the requirements of a hormone in the mammalian
sense. The synthesis of plant hormones may be localized (as occurs for
animal hormones), but it may also occur in a wide range of tissues, or cells
within tissues. While they may be transported and have their action at a
distance this is not always the case. At one extreme we find the transport of
1 The following abbreviations are used throughout this book with no further definition: ABA,
abscisic acid; BR, brassinosteroid; CK, cytokinin; GA gibberellin; IAA, indole-3-acetic acid

Nature, occurrence and functions
2
cytokinins from roots to leaves where they prevent senescence and maintain
metabolic activity, while at the other extreme the production of the gas
ethylene may bring about changes within the same tissue, or within the same
cell, where it is synthesized. Thus, transport is not an essential property of a
plant hormone.
The term ‘hormone’ was first used in medicine about 100 years ago for a
stimulatory factor, though it has come to mean a transported chemical
message. The word in fact comes from the Greek, where its meaning is ‘to
stimulate’ or ‘to set in motion’. Thus the origin of word itself does not
require the notion of transport per se, and the above definition of a plant
hormone is much closer to the meaning of the Greek origin of the word than
is the current meaning of hormone used in the context of animal physiology.
Plant hormones2 are a unique set of compounds, with unique metabolism
and properties, that form the subject of this book. Their only universal
characteristics are that they are natural compounds in plants with an ability to
affect physiological processes at concentrations far below those where either
nutrients or vitamins would affect these processes.
THE DISCOVERY, IDENTIFICATION AND QUANTITATION OF
PLANT HORMONES.
The Development of the Plant Hormone Concept and Early Work.
The plant hormone concept probably derives from observations of
morphogenic and developmental correlations by Sachs between 1880 and
1893. He suggested that "Morphological differences between plant organs
are due to differences in their material composition" and postulated the
existence of root-forming, flower forming and other substances that move in
different directions through the plant (10).
At about the same time Darwin (3) was making his original observations
on the phototropism of grass coleoptiles that led him to postulate the
existence of a signal that was transported from the tip of the coleoptile to the
bending regions lower down. After further characterizations by several
workers of the way in which the signal was moved, Went in the Netherlands
was finally able to isolate the chemical by diffusion from coleoptile tips into
agar blocks, which, when replaced on the tips of decapitated coleoptiles,
resulted in the stimulation of the growth of the decapitated coleoptiles, and
their bending when placed asymmetrically on these tips. This thus
demonstrated the existence of a growth promoting chemical that was
2 The term "plant growth substance" is also used for plant hormones but this is a rather vague
term and does not describe fully what these natural regulators do - growth is only one of the
many processes influenced. The international society for the study of plant hormones is
named the "International Plant Growth Substance Association" (IPGSA). While the term
plant growth regulator is a little more precise this term has been mainly used by the
agrichemical industry to denote synthetic plant growth regulators as distinct from endogenous
growth regulators.

P. J. Davies
3
synthesized in the coleoptile tips, moved basipetally, and when distributed
asymmetrically resulted in a bending of the coleoptile away from the side
with the higher concentration. This substance was originally named
Wuchsstoff by Went, and later this was changed to auxin. After some false
identifications the material was finally identified as the simple compound
indoleacetic acid, universally known as IAA (11).
Discovery of Other Hormones
Other lines of investigation led to the discovery of the other hormones:
research in plant pathogenesis led to gibberellins (GA); efforts to culture
tissues led to cytokinins (CK); the control of abscission and dormancy led to
abscisic acid (ABA); and the effects of illuminating gas and smoke led to
ethylene. These accounts are told in virtually every elementary plant
physiology textbook, and further elaborated in either personal accounts (9,
11) or advanced treatises devoted to individual hormones (see book list at the
end of the chapter) so that they need not be repeated here. More recently
other compounds, namely brassinosteroids (Chapters B7 and D7), jasmonates
(Chapter F1) (including tuberonic acid, Chapter E5), salicylic acid (Chapter
F2), and the peptides (Chapter F3) have been added to the list of plant
hormones, and these are fully covered in this book for the first time.
Polyamines, which are essential compounds for all life forms and important
in DNA structure, have also been categorized as plant hormones as they can
modulate growth and development, though typically their levels are higher
than the other plant hormones. However, as little further understanding of
their exact function in plants at the cellular and molecular levels has been
added in the last few years, no individual chapter has been devoted to
polyamines in this edition (a chapter on polyamines can be found in the
previous edition (4): 2E Chapter C1).
It is interesting to note that, of all the original established group of plant
hormones, only the chemical identification of abscisic acid was made from
higher plant tissue. The original identification of the others came from
extracts that produced hormone-like effects in plants: auxin from urine and
the fungal cultures of Rhizopus, gibberellins from culture filtrates of the
fungus Gibberella, cytokinins from autoclaved herring sperm DNA, and
ethylene from illuminating gas. Today we have at our disposal methods of
purification (such as high performance liquid chromatography: HPLC,
following solid phase extraction: SPE cartridges) and characterization (gas
chromatography-mass spectrometry: GC-MS, and high performance liquid
chromatography-mass spectrometry: HPLC-MS) that can operate at levels
undreamed of by early investigators (Chapter G1). Thus while early
purifications from plant material utilized tens or even hundreds of kilograms
of tissues, modern analyses can be performed on a few milligrams of tissue,
making the characterization of hormone levels in individual leaves, buds, or
even from tissues within the organs much more feasible. Thus it is not
surprising to see the more-recently discovered hormones being originally

Nature, occurrence and functions
4
identified within plant tissues. Nonetheless only brassinosteroids were
identified following investigations of plant growth effects, with the discovery
of jasmonates, salicylic acid and peptide hormones deriving from work on
insect and disease resistance.
Immunoassay (see 2nd edition, Chapter F2) is also used for hormone
quantitation, though is considered much less precise because of interfering
effects of other compounds and cross reactivity. Immunoassay columns can,
however, permit the very precise isolation of plant hormones prior to more
rigorous physico-chemical characterization. While the exact level and
location of the hormones within the individual tissues and cells is still largely
elusive (Chapter G1), huge strides have been made in analyzing and
localizing the expression of genes for hormone biosynthesis using sensitive
techniques such as PCR (polymerase chain reaction), or the expression, in
transgenic plants, of marker genes driven by promoters of one or more steps
in the biosynthetic process. The location of hormone action in tissues and
cells has also been investigated by examining the location of marker gene
expression driven by promoters of genes known to be induced by the
presence of hormone (e.g. Chapter A2).
THE NATURE, OCCURRENCE, AND EFFECTS OF THE PLANT
HORMONES
Before we become involved in the various subsequent chapters covering
aspects of hormone biochemistry and action it is necessary to review what
hormones do. In subsequent chapters some or most of these effects will be
described in more detail, whereas others will not be referred to again. It is
impossible to give detailed coverage of every hormonal effect, and the reader
is referred to the book list at the end of this chapter. The choice of topics for
subsequent chapters has been determined largely by whether there is active
research in progress in that area. Over the last few years there has been
active progress in elucidating the biosynthesis, signal transduction and action
of almost every hormone. Thus whereas previously the progress in
understanding the action of one hormone was much better than that of
another we now find increased understanding of hormone action across the
board. A good case in point is cytokinin, where we now know much more
about perception, signal transduction (Chapter D3) and action (Chapter C3)
than just a few years ago. In fact progress on understanding one hormone as
opposed to another has been leapfrogging: whereas the action of auxin at the
physiological level was one of the first to be understood (Chapter C1) we still
do not understand the connection between auxin signal transduction (Chapter
D1) and its final action in inducing cell elongation, and while the
identification of the auxin receptor was previously regarded as established,
this is now regarded as far less certain. By contrast, after two decades of
relatively little advance in the understanding of brassinosteroids, or even
much interest in these compounds, following their discovery by extraction

P. J. Davies
5
from Brassica pollen and the demonstration of growth activity in a bean
petiole bioassay, the entire biosynthetic pathway has been elucidated
(Chapter B6), receptors identified (Chapter D7), mutants characterized and
crosstalk with other hormones investigated (Chapter B7).
The effects produced by each hormone were initially elucidated largely
from exogenous applications. However in more and more cases we have
evidence that the endogenous hormone also fulfills the originally designated
roles, and new functions are being discovered. Such more recent evidence
derives from correlations between hormone levels and growth of defined
genotypes or mutants, particularly of the model plant Arabidopsis, or from
transgenic plants. In other cases it has not yet been conclusively proved that
the endogenous hormone functions in the same manner.
The nature, occurrence, transport and effects of each hormone (or
hormone group) are given below. (Where there is no specific chapter on the
topic in this edition but a reference in the second edition of this book (4) this
is indicated with the notation ‘2E’.) It should, however, be emphasized that
hormones do not act alone but in conjunction, or in opposition, to each other
such that the final condition of growth or development represents the net
effect of a hormonal balance (Chapter A2) (5).
Auxin
Nature
Indole-3-acetic acid (IAA) is the main auxin in most plants.
Compounds which serve as IAA precursors may also have auxin activity
(e.g., indoleacetaldehyde). Some plants contain other compounds that
display weak auxin activity (e.g., phenylacetic acid). IAA may also be
present as various conjugates such as indoleacetyl aspartate (Chapter B1)).
4-chloro-IAA has also been reported in several species though it is not clear
to what extent the endogenous auxin activity in plants can be accounted for
by 4-Cl-IAA. Several synthetic auxins are also used in commercial
applications (2E: G13).
Sites of biosynthesis
IAA is synthesized from tryptophan or indole (Chapter B1) primarily in leaf
primordia and young leaves, and in developing seeds.

Nature, occurrence and functions
6
Transport
IAA transport is cell to cell (Chapters E1 and E2), mainly in the vascular
cambium and the procambial strands, but probably also in epidermal cells
(Chapter E2). Transport to the root probably also involves the phloem.
Effects
! Cell enlargement - auxin stimulates cell enlargement and stem growth
(Chapter D1).
! Cell division - auxin stimulates cell division in the cambium and, in
combination with cytokinin, in tissue culture (Chapter E2 and 2E: G14).
! Vascular tissue differentiation - auxin stimulates differentiation of
phloem and xylem (Chapter E2).
! Root initiation - auxin stimulates root initiation on stem cuttings, and
also the development of branch roots and the differentiation of roots in
tissue culture (2E: G14).
! Tropistic responses - auxin mediates the tropistic (bending) response of
shoots and roots to gravity and light (2E: G5 and G3).
! Apical dominance - the auxin supply from the apical bud represses the
growth of lateral buds (2E: G6).
! Leaf senescence - auxin delays leaf senescence.
! Leaf and fruit abscission - auxin may inhibit or promote (via ethylene)
leaf and fruit abscission depending on the timing and position of the
source (2E: G2, G6 and G13).
! Fruit setting and growth - auxin induces these processes in some fruit
(2E: G13)
! Assimilate partitioning - assimilate movement is enhanced towards an
auxin source possibly by an effect on phloem transport (2E: G9).
! Fruit ripening - auxin delays ripening (2E: G2 & 2E:G12).
! Flowering - auxin promotes flowering in Bromeliads (2E: G8).
! Growth of flower parts - stimulated by auxin (2E: G2).
! Promotes femaleness in dioecious flowers (via ethylene) (2E: G2 &
2E: G8).
In several systems (e.g., root growth) auxin, particularly at high
concentrations, is inhibitory. Almost invariably this has been shown to be
mediated by auxin-produced ethylene (2, 7) (2E: G2). If the ethylene
synthesis is prevented by various ethylene synthesis inhibitors, the ethylene
removed by hypobaric conditions, or the action of ethylene opposed by silver
salts (Ag+), then auxin is no longer inhibitory.
Gibberellins (GAs)
Nature
The gibberellins (GAs) are a family of compounds based on the ent-
gibberellane structure; over 125 members exist and their structures can be
found on the web (Chapter B2). While the most widely available compound
is GA3 or gibberellic acid, which is a fungal product, the most important GA

P. J. Davies
7
in plants is GA1, which is the GA primarily responsible for stem elongation
(Chapters A2, B2, and B7). Many of the other GAs are precursors of the
growth-active GA1.
Sites of biosynthesis.
GAs are synthesized from glyceraldehyde-3-phosphate, via isopentenyl
diphosphate, in young tissues of the shoot and developing seed. Their
biosynthesis starts in the chloroplast and subsequently involves membrane
and cytoplasmic steps (Chapter B2).
Transport
Some GAs are probably transported in the phloem and xylem. However the
transport of the main bioactive polar GA1 seems restricted (Chapters A2 and
E5).
Effects
! Stem growth - GA1 causes hyperelongation of stems by stimulating both
cell division and cell elongation (Chapters A2, B7 and D2). This
produces tall, as opposed to dwarf, plants.
! Bolting in long day plants - GAs cause stem elongation in response to
long days (Chapter B2, 2E: G8).
! Induction of seed germination - GAs can cause seed germination in
some seeds that normally require cold (stratification) or light to induce
germination (Chapter B2).
! Enzyme production during germination - GA stimulates the production
of numerous enzymes, notably α-amylase, in germinating cereal grains
(Chapter C3).
! Fruit setting and growth - This can be induced by exogenous
applications in some fruit (e.g., grapes) (2E: G13). The endogenous role
is uncertain.
! Induction of maleness in dioecious flowers (2E: G8).
Cytokinins (CKs)
Nature
CKs are adenine derivatives characterized by an ability to induce cell
division in tissue culture (in the presence of auxin). The most common

Nature, occurrence and functions
8
cytokinin base in plants is zeatin. Cytokinins also occur as ribosides and
ribotides (Chapter B3).
Sites of biosynthesis
CK biosynthesis is through the biochemical modification of adenine (Chapter
B3). It occurs in root tips and developing seeds.
Transport
CK transport is via the xylem from roots to shoots.
Effects
! Cell division - exogenous applications of CKs induce cell division in
tissue culture in the presence of auxin (Chapter C3; 2E: G14). This also
occurs endogenously in crown gall tumors on plants (2E: E1). The
presence of CKs in tissues with actively dividing cells (e.g., fruits, shoot
tips) indicates that CKs may naturally perform this function in the plant.
! Morphogenesis - in tissue culture (2E: G14) and crown gall (2E: E1)
CKs promote shoot initiation. In moss, CKs induce bud formation (2E:
G1 & G6).
! Growth of lateral buds - CK applications, or the increase in CK levels in
transgenic plants with genes for enhanced CK synthesis, can cause the
release of lateral buds from apical dominance (2E: E2 & G6).
! Leaf expansion (6), resulting solely from cell enlargement. This is
probably the mechanism by which the total leaf area is adjusted to
compensate for the extent of root growth, as the amount of CKs reaching
the shoot will reflect the extent of the root system. However this has not
been observed in transgenic plants with genes for increased CK
biosynthesis, possibly because of a common the lack of control in these
systems.
! CKs delay leaf senescence (Chapter E6).
! CKs may enhance stomatal opening in some species (Chapter E3).
! Chloroplast development - the application of CK leads to an
accumulation of chlorophyll and promotes the conversion of etioplasts
into chloroplasts (8).

P. J. Davies
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Ethylene
Nature
The gas ethylene (C2H4) is synthesized from methionine (Chapter B4) in
many tissues in response to stress, and is the fruit ripening hormone. It does
not seem to be essential for normal mature vegetative growth, as ethylene-
deficient transgenic plants grow normally. However they cannot, as
seedlings, penetrate the soil because they lack the stem thickening and apical
hook responses to ethylene, and they are susceptible to diseases because they
lack the ethylene-induced disease resistance responses. It is the only
hydrocarbon with a pronounced effect on plants.
Sites of synthesis
Ethylene is synthesized by most tissues in response to stress. In particular, it
is synthesized in tissues undergoing senescence or ripening (Chapters B4 and
E5).
Transport
Being a gas, ethylene moves by diffusion from its site of synthesis. A crucial
intermediate in its production, 1-aminocyclopropane-1-carboxylic acid
(ACC) can, however, be transported and may account for ethylene effects at
a distance from the causal stimulus (2E: G2).
Effects
The effects of ethylene are fully described in 2E: G2. They include:
! The so called triple response, when, prior to soil emergence, dark grown
seedlings display a decrease in tem elongation, a thickening of the stem
and a transition to lateral growth as might occur during the encounter of
a stone in the soil.
! Maintenance of the apical hook in seedlings.
! Stimulation of numerous defense responses in response to injury or
disease.
! Release from dormancy.
! Shoot and root growth and differentiation.
! Adventitious root formation.
! Leaf and fruit abscission.
! Flower induction in some plants (2E: G8).
! Induction of femaleness in dioecious flowers (2E: 8).
! Flower opening.
! Flower and leaf senescence.
! Fruit ripening (Chapters B4 and E5).
Abscisic acid (ABA)
Nature
Abscisic acid is a single compound with the following formula:

Nature, occurrence and functions
10
Its name is rather unfortunate. The first name given was "abscisin II"
because it was thought to control the abscission of cotton bolls. At almost
the same time another group named it "dormin" for a purported role in bud
dormancy. By a compromise the name abscisic acid was coined (1). It now
appears to have little role in either abscission (which is regulated by
ethylene; 2E: G2) or bud dormancy, but we are stuck with this name. As a
result of the original association with abscission and dormancy, ABA has
become thought of as an inhibitor. While exogenous applications can inhibit
growth in the plant, ABA appears to act as much as a promoter, such as in
the promotion of storage protein synthesis in seeds (Chapter E4), as an
inhibitor, and a more open attitude towards its overall role in plant
development is warranted. One of the main functions is the regulation of
stomatal closure (Chapters D6 and E3)
Sites of synthesis
ABA is synthesized from glyceraldehyde-3-phosphate via isopentenyl
diphosphate and carotenoids (Chapter B5) in roots and mature leaves,
particularly in response to water stress (Chapters B5 and E3). Seeds are also
rich in ABA which may be imported from the leaves or synthesized in situ
(Chapter E4).
Transport
ABA is exported from roots in the xylem and from leaves in the phloem.
There is some evidence that ABA may circulate to the roots in the phloem
and then return to the shoots in the xylem (Chapters A2 and E4).
Effects
! Stomatal closure - water shortage brings about an increase in ABA
which leads to stomatal closure (Chapters D6 and E3).
! ABA inhibits shoot growth (but has less effect on, or may promote, root
growth). This may represent a response to water stress (Chapter E3; 2E:
2).
! ABA induces storage protein synthesis in seeds (Chapter E4).
! ABA counteracts the effect of gibberellin on α-amylase synthesis in
germinating cereal grains (Chapter C2).
! ABA affects the induction and maintenance of some aspects of
dormancy in seeds (Chapters B5 and E4). It does not, however, appear

P. J. Davies
11
to be the controlling factor in ‘true dormancy’ or ‘rest,’ which is
dormancy that needs to be broken by low temperature or light.
! Increase in ABA in response to wounding induces gene transcription,
notably for proteinase inhibitors, so it may be involved in defense
against insect attack (2E: E5).
Polyamines
Polyamines are a group of aliphatic amines. The main compounds are
putrescine, spermidine and spermine. They are derived from the
decarboxylation of the amino acids arginine or ornithine. The conversion of
the diamine putrescine to the triamine spermidine and the quaternaryamine
spermine involves the decarboxylation of S-adenosylmethionine, which also
is on the pathway for the biosynthesis of ethylene. As a result there are some
complex interactions between the levels and effects of ethylene and the
polyamines.
The classification of polyamines as hormones is justified on the
following grounds:
! They are widespread in all cells and can exert regulatory control over
growth and development at micromolar concentrations.
! In plants where the content of polyamines is genetically altered,
development is affected. (E.g., in tissue cultures of carrot or Vigna,
when the polyamine level is low only callus growth occurs; when
polyamines are high, embryoid formation occurs. In tobacco plants that
are overproducers of spermidine, anthers are produced in place of
ovaries.)
Such developmental control is more characteristic of hormonal compounds
than nutrients such as amino acids or vitamins.
Polyamines have a wide range of effects on plants and appear to be
essential for plant growth, particularly cell division and normal morphologies.
At present it is not possible to make an easy, distinct list of their effects as for
the other hormones. Their biosynthesis and a variety of cellular and
organismal effects is discussed in 2E Chapter C1. It appears that polyamines
are present in all cells rather than having a specific site of synthesis.
Brassinosteroids
Brassinosteroids (Chapters B6 and D7) are a range of over 60 steroidal
compounds, typified by the compound brassinolide that was first isolated
from Brassica pollen. At first they were regarded as somewhat of an oddity
but they are probably universal in plants. They produce effects on growth
and development at very low concentrations and play a role in the
endogenous regulation of these processes.
SPERMIDINE
H2N(CH
2)3NH (CH2)4NH2

Nature, occurrence and functions
12
Effects
! Cell Division, possibly by increasing transcription of the gene encoding
cyclinD3 which regulates a step in the cell cycle (Chapter D7).
! Cell elongation, where BRs promote the transcription of genes encoding
xyloglucanases and expansins and promote wall loosening (Chapter D7).
This leads to stem elongation.
! Vascular differentiation (Chapter D7).
! BRs are needed for fertility: BR mutants have reduced fertility and
delayed senescence probably as a consequence of the delayed fertility
(Chapter D7).
! Inhibition of root growth and development
! Promotion of ethylene biosynthesis and epinasty.
Jasmonates
Jasmonates (Chapter F1) are represented by jasmonic acid (JA) and its
methyl ester.
They are named after the jasmine plant in which the methyl ester is an
important scent component. As such they have been known for some time in
the perfume industry. There is also a related hydroxylated compound that
has been named tuberonic acid which, with its methyl ester and glycosides,
induces potato tuberization (Chapter E5). Jasmonic acid is synthesized from
linolenic acid (Chapter F1), while jasmonic acid is most likely the precursor
of tuberonic acid.
Effects
! Jasminates play an important role in plant defense, where they induce
BRASSINOLIDE
OH
OH
O
HO
HO
O
H
COOH
O
JASMONIC ACID

P. J. Davies
13
the synthesis of proteinase inhibitors which deter insect feeding, and, in
this regard, act as intermediates in the response pathway induced by the
peptide systemin.
! Jamonates inhibit many plant processes such as growth and seed
germination.
! They promote senescence, abscission, tuber formation, fruit ripening,
pigment formation and tendril coiling.
! JA is essential for male reproductive development of Arabidopsis. The
role in other species remains to be determined.
Salicylic Acid (SA)
Salicylates have been known for a long time to be present in willow bark, but
have only recently been recognized as potential regulatory compounds.
Salicylic acid is biosynthesized from the amino acid phenylalanine.
Effects
! Salicylic acid (Chapter F2) plays a main role in the resistance to
pathogens by inducing the production of ‘pathogenesis-related proteins’.
It is involved in the systemic acquired resistance response (SAR) in
which a pathogenic attack on older leaves causes the development of
resistance in younger leaves, though whether SA is the transmitted
signal is debatable.
! SA is the calorigenic substance that causes thermogenesis in Arum
flowers.
! It has also been reported to enhance flower longevity, inhibit ethylene
biosynthesis and seed germination, block the wound response, and
reverse the effects of ABA.
Signal Peptides
The discovery that small peptides could have regulatory properties in plants
started with the discovery of systemin, an 18 amino acid peptide that travels
in the phloem from leaves under herbivore insect attack to increase the
content of jasmonic acid and proteinase inhibitors in distant leaves, so
protecting them from attack (Chapters F1 and F3). Since then, over a dozen
peptide hormones that regulate various processes involved in defense, cell
OH
COOH
SALICYLIC ACID

Nature, occurrence and functions
14
division, growth and development and reproduction have been isolated from
plants, or identified by genetic approaches (Chapter F3). Among these
effects caused by specific peptides are:
! The activation of defense responses.
! The promotion of cell proliferation of suspension cultured plant cells.
! The determination of cell fate during development of the shoot apical
meristem
! The modulation of root growth and leaf patterning in the presence of
auxin and cytokinin
! Peptide signals for self-incompatability.
! Nodule formation in response to bacterial signals involved in nodulation
in legumes.
Are the More-Recently-Discovered Compounds Plant Hormones?
Two decades ago there was a heated discussion as to whether a compound
had to be transported to be a plant hormone, and could ethylene therefore be
a plant hormone. To this Carl Price responded: “Whether or not we regard
ethylene as a plant hormone is unimportant; bananas do…” 3. Hormones are
a human classification and organisms care naught for human classifications.
Natural chemical compounds affect growth and development in various ways,
or they do not do so. Clearly brassinosteroids fit the definition of a plant
hormone, and likely polyamines, jasmonates salicylic acid and signal
peptides also can be so classified. Whether other compounds should be
regarded as plant hormones in the future will depend on whether, in the long
run, these compounds are shown to be endogenous regulators of growth and
development in plants in general.
A Selection of Books on Plant Hormones Detailing their Discovery and
Effects
Abeles FB, Morgan PW, Saltveit ME (1992) Ethylene in Plant Biology. Academic Press, San
Diego
Addicott FT (ed) (1983) Abscisic acid. Praeger, New York
Arteca RN (1995) Plant Growth Substances, Principles and Applications. Chapman and Hall,
New York
Audus LJ (1959) Plant Growth Substances (2E) L. Hill, London; Interscience Publishers New
York. (Editors note: the 2nd edition of Audus contains a lot of information on auxins that
was cut out of the later, broader, 3rd edition and it is therefore still a valuable reference.)
Audus LJ (1972) Plant Growth Substances (3E). Barnes & Noble, New York
Crozier A (ed) (1983) The Biochemistry and Physiology of Gibberellins. Praeger, New York
Davies PJ (ed) (1995) Plant Hormones: Physiology, Biochemistry and Molecular Biology.
Kluwer Academic, Dordrecht, Boston
Davies WJ, Jones HG (1991) Abscisic Acid: Physiology and Biochemistry. Bios Scientific
Publishers, Oxford, UK
Hayat S, Ahmad A (eds) (2003) Brassinosteroids: Bioactivity and Crop Productivity. Kluwer
Academic, Dordrecht , Boston
3 Carl A. Price, in Molecular Approaches to Plant Physiology

P. J. Davies
15
Jacobs WP (1979) Plant Hormones and Plant Development. Cambridge University Press
Khripach VA, Zhabinskii VN, de Groot AE (1999) Brassinosteroids: A New Class of Plant
Hormones. Academic Press, San Diego
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