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BREEDING FOR DIFFERENT FLOWER FORMS IN ORNAMENTAL
CROPS: A REVIEW
Shreedhar Beese*, S.R. Dhiman, Puja Sharma, Sapna Kaushal, Neha Grace Angel Kisku,
Divesh Thakur, Poonam Sharma, Aman Guleria, Oyem Kombo and Medalis Pala
Department of Floriculture and Landscape Architecture, Dr. Y.S Parmar, University of Horticulture and Forestry Nauni,
Solan - 173 230, Himachal Pradesh, India.
*Corresponding author E-mail : shridhar.rb38@gmail.com, ORCID: https://orcid.org/0009-0000-5125-4957
(Date of Receiving-16-06-2024; Date of Acceptance-31-08-2024)
Flowering plants are extremely important in our daily lives due to their aesthetic value. The distinctiveness
of various flower types is greatly valued, with double blossoms having more ornamental value than their
single counterparts. Researchers have presented a novel ABCDE model that builds on the classic ABC
model and discovered critical transcriptional variables to identify floral organs. In this new model, A+E
designate sepals, A+B+E denote petals, B+C+E represent stamens, C+E indicate carpels, and D+E symbolize
ovules. To breed cultivars with novel flower forms, a range of technologies are used, including hybridization,
mutation, polyploidy, and genetic engineering. The genetic control of single, semi-double and double flower
forms can be attributed to either a single gene or numerous genes. It is possible to successfully develop
double flowers by carefully choosing the right hybridization techniques. The selection of mutants with
modified apparent characteristics, such as altered flower color, shape, size, leaf form and growth habit, also
becomes feasible through induced mutagenesis. By doubling the number of chromosomes, polyploidy
breeding doubles plant size, leaf size, branch development and flower components. Genetic engineering has
made it possible to manipulate a variety of features though biotechnological developments like RNAi,
CRES-T, CRISPR/Cas9 and miRNA. These traits include flower color, fragrance, resistance to abiotic stress,
disease and pest resistance, alteration of plant and flower form and architecture, flowering time and post-
harvest longevity. The shapes of plants like torenia, chrysanthemum, morning glory, petunia, orchids,
gentian, cyclamen and rose plants, among others have been successfully altered using these techniques. In
spite of the abundance of these techniques, only a small number of cultivars have been created for commercial
purpose.
Key words : Ornamental crops, ABCDE flower model, Flower form, Mutation, Polyploidy, Genetic engineering.
Plant Archives Vol. 24, No. 2, 2024 pp. 2201-2215 e-ISSN:2581-6063 (online), ISSN:0972-5210
Plant Archives
Journal homepage: http://www.plantarchives.org
DOI Url : https://doi.org/10.51470/PLANTARCHIVES.2024.v24.no.2.317
ABSTRACT
Introduction
Our daily lives depend heavily on ornamental plants,
both for their aesthetic value and for their capacity to
improve the environment in which we used to grow. Their
value lies in their striking morphology, captivating flower
colors, and unique shapes. Among these characteristics,
floral form is particularly important for attractive plants.
The development of novel flower forms in the phonotype
is regularly given priority in breeding programs, whether
it takes the form of semi-double or double flowers, which
have greater ornamental value than single flowers, or
changes in specific floral parts like petals, sepals, or
serrations. Such alterations carry substantial commercial
value due to their distinctiveness. Engineered traits are
valuable to either the consumer or the producer.
Advancements in our understanding of floral
development have been achieved through the study of
model plants such as Arabidopsis and Antirrhinum majus,
which have elucidated key transcriptional factors
responsible for identifying floral organs (Bowman et al.,
2202 Shreedhar Beese et al.
1989; Coen and Meyerowitz, 1991). Phenotypes with
unusual flower forms or double flowers are typically
prized for their higher ornamental value than their single
counterparts. A key feature of beautiful blooming species
is the presence of double flowers, which are distinguished
by the presence of double whorls of petals as a result of
excessive development or the conversion of other floral
organs into petals. The broad diversity of cultivated
double-flowered species that exist today are the result of
human selection for aesthetically pleasing features, which
has been a major factor in the development of double
flowers (Abbo et al., 2014; Ross-Ibarra et al., 2007).
It’s interesting to note that many double-flowered
ornamental crop types have single-flowering wild
forebears as their ancestors (Liu et al., 2013). This
historical journey serves to highlight the double blossoms’
ongoing relevance and attractiveness in decorative
horticulture.
The distinctive identity of organs within the four floral
verticils results from the intricate interplay of at least
three distinct types of gene products, each with its unique
functions. The roles of functions A and C are crucial for
defining the identities of the perianth verticils and the
reproductive verticils, respectively, in the ABC model.
These roles are mutually exclusive, so when one is absent,
the other takes over and determines the identification of
every floral verticil. The differentiation between stamens
and carpels inside the tertiary verticil is similarly guided
by the B function, as it plays an important role in separation
between petals and sepals within the secondary verticil.
The transition from the vegetative phase to the
reproductive phase marks a significant change in the life
cycle of the plant, maybe one of the most important ones
because successful reproduction depends on its proper
execution. The inflorescence meristem, which will
eventually give rise to a cluster of flowers or, in some
cases, a single flower, begins to grow during this phase.
This morphogenetic change is affected by both internal
and environmental variables. Specific prerequisites must
be met, for example, the plant must have a certain number
of leaves and reach a certain level of total biomass.
Environmental conditions, such as a particular photoperiod,
also play a vital role in triggering this transformation. Plant
hormones are essential in coordinating this process, with
gibberellins playing an essential part.
The ABCDE Model: Understanding Flower
Development Stages
The ABCDE Model is a framework for the
specification of floral organs in flower development.
Fig. 1 : Phases of flower development.
The apical meristem changes its program from
generating leaves to flowers to flower tissue at a certain
stage of plant development. At this stage the apical
meristem (AM) develops into an inflorescence meristem
(IM). Numerous primordia’s are formed by the IM, and
these later give rise to sepal, petal, carpel and stamen.
The Molecular and Developmental Basis of Flower
Development as Illustrated by the ABC Model. The ABC
model of flower development provides a theoretical
framework for knowing how blooming plants control the
patterns of gene expression in their meristems, which
ultimately result in the creation of a reproductive organ
called a flower. Three significant physiological shifts are
involved in this complicated process. First, the beginning
of flowering is marked by the plant’s transformation from
sexual immaturity to sexual maturity. Second, the apical
meristem’s function changes from a vegetative meristem
to that of an inflorescence. Finally, the flower’s individual
organs start to grow and differentiate. The ABC model
delves into the molecular and developmental genetics
underpinning the latter phase, providing insights into the
biological mechanisms responsible for the formation of
flower organs.
Fig. 2 : Mutation in Floral organ identify genes.
Breeding for different Flower Forms in Ornamental Crops 2203
According to the ABCDE model of flower formation
(Coen and Meyerowitz, 1991; Rijpkema et al., 2010),
floral organ identity is controlled by five different classes
of homeotic genes, which are designated as A, B, C, D
and E. The interaction of A and E-class proteins results
in the development of sepals as the fundamental floral
organs in the first whorl, according to the quartet models
for floral organ specification (Coen and Meyerowitz, 1991;
Smaczniak et al., 2012). The A, B and E-class proteins
work together to determine petals in the second whorl.
While the third whorl sees the participation of B, C, and
E-class proteins in the determination of stamens and the
fourth whorl relies on C and E-class proteins for carpel
specification.
in genes which comprise the MADS-box gene family.
The DNA-binding MADS domain is encoded by the
MADS-Box genes. The MADS-Box has a length
between 168 and 180 base pairs. A class of MAD-Box
genes includes homeotic genes. The MADS-Box gene
family got its name as an acronym referring to the four
founding members:
MCM1 from the budding yeast, Saccharomyces
cerevisiae
AGAMOUS from the thale cress Arabidopsis
thaliana
DEFICIENS from the snapdragon Antirrhinum
majus
SRF from the human Homo sapiens
The development of male and female gametophytes,
floral organ identification, flowering time determination,
embryo and seed development, root, flower, and fruit
development are all the key components of plant
development that are regulated by MADS-Box genes.
Origins and Applications the ABC model of flower
development was initially conceptualized by George
Haughn and Chris Somerville in 1988. It became an
innovative framework to clarify the complex genetic
pathways bringing about the establishment of floral organ
identity in two different plant groups: the Rosids, illustrated
by Arabidopsis thaliana, and the Asterids, illustrated by
Antirrhinum majus. Sepals, petals, stamens and carpels
are all arranged in four separate whorls in each of these
species. These floral organs’ identities are determined
by the distinctive expression patterns of particular
homeotic genes within each whorl. According to this
model, sepals are identified by the expression of the A
gene alone, whereas the identity of petals is determined
by the co-expression of the A and B genes. While carpels
only need the activation of the C genes, the B and C
genes each play crucial roles in defining the identity of
stamens. Notably, the regulatory network is further
complicated by the reciprocal antagonistic behavior of
the A and C genes (Haughn George W., Somerville, Chris
R., 1988).
When a particular gene, like the A gene, is not
expressed, it is clear how important homeotic genes are
in determining the identity of an organ. According to
Bowman et al. (1991), a flower in Arabidopsis lacking
the expression of the A gene consists of three verticils:
one with carpels, another with stamens, and a third with
carpels. Researchers use a variety of strategies to
investigate how genes operate, such as reverse genetics
procedures that involve the creation of transgenic plants
Fig. 3 : ABCDE Model of flower Development (Dornelas and
Dornelas, 2005).
With the exception of the class A gene APETALA2
(AP2), the cloning of ABCDE homeotic genes in
Arabidopsis has shown that these genes contain MADS-
box transcription factors (Jofuku et al., 1994). The class
A MADS-box gene in Arabidopsis is AP1, class B genes
include AP3, PISTILLATA (PI), class C gene
is AGAM OUS (AG), class D gene
compriseSEEDSTICK ( STK), SHATTE RPROOF1
(SHP1) and SHP2 and class E genes in the plant are
SEPALLATA1 (SEP1), SEP2, SEP3, and SEP4.
According to research, class E genes play partially
overlapping roles in determining the identities of sepals,
petals, stamens and carpels, with D-class proteins and
E-class proteins dictating the ovule identity (Mandel et
al., 1992; Jack et al., 1992; Goto and Meyerowitz, 1994;
Yanofsky et al., 1990; Favaro et al., 2003; Pinyopich et
al., 2003; Pelaz et al., 2000; Ditta et al., 2004). The
amazing diversity of floral morphologies seen in
angiosperms has been produced through the
diversification of MADS-box genes over evolutionary time
(Litt and Kramer, 2010). And the MADS-Box Genes
are the MADS box is a conserved sequence motif found
2204 Shreedhar Beese et al.
with mechanisms for gene silencing through RNA
interference. Alternatively, to identify and clone the desired
gene, forward genetics methods like genetic mapping are
employed to investigate the phenotypes of flowers
displaying structural defects. These aberrant flowers
might have alleles of the gene under inquiry that are
overexpressed or inactive (Somerville and Somerville,
1999).
Two additional functions, D and E have also been
proposed in addition to the previously stated A, B and C
functions. Function D is responsible for specifying the
identity of the ovule, a distinct reproductive function that
occurs separately from the determination of carpels,
which takes place prior to ovule development. On the
other hand, Function E is linked to a physiological need
that all floral verticils must meet. Although initially
described as essential for the development of the three
innermost verticils, its broader definition suggests its
necessity in all four verticils. As a result, the loss of
Function E causes the floral organs of the three outermost
verticils to change into sepals, while the loss of Function
D results in ovule structures that resemble leaves It should
be noted that the gene products in charge of functions D
and E are also MADS-box genes (Hong, 2005).
Genetic analysis of floral development using
homeotic mutants
Homeotic mutations are the changes that take place
in homeotic genes. These genes encode transcription
factors that help to create the overall body plan by
regulating how body components are identified and
organizing development. The remarkable conversion of
one body part into another displayed by homeotic mutants,
offers important new information about how genes
regulate development. Homeotic mutations have proved
crucial in unraveling the floral development model in the
model plant Arabidopsis thaliana.
Arabidopsis produces homeotic mutant flowers when
the mutation occur in ABC genes. Notably, type A and C
genes have reciprocal antagonistic behavior, whereby the
expression of the C gene’s activity results from the loss
of function in the A gene (Bowman et al., 1991). The
calyx and corolla are the primary targets of A gene
mutations, which result in the formation of carpels in place
of sepals and stamens in place of petals. One example of
this transformation is seen in the A. thaliana APETALA2
(AP2) mutant. The corolla and stamens, on the other
hand, are affected by mutations in the B gene, resulting
in the formation of sepals rather than petals and carpels
rather than stamens. In A. thaliana, mutants like
APETALA3 and PISTILLATA exhibit this behavior.
Stamens and carpels, which are reproductive organs,
are directly impacted by mutations in the C gene, leading
to the formation of petals in place of stamens and sepals
in place of carpels. This change is best illustrated by the
A. thaliana AGAMOUS (AG) mutant (Bowman et al.,
1989 and Bowman et al., 1991). Therefore, the loss or
alteration of C gene activity is crucial for producing an
excess of petals and, as a result, the development of double
blooms. The phyllody phenotype in Rosa chinensis cv.
Viridiflora, according to Yan et al. (2016), is connected
to the up-regulation and ectopic expression of RcSOC1
and A-class genes, together with the down-regulation of
B, C and E class genes involved for floral organ identity.
The complex regulatory networks that control floral
growth and the emergence of distinctive flower shapes
are clarified by this research.
Techniques for detecting differential expression
Cloning investigations have been conducted on the
DNA within genes associated with the altered homeotic
functions observed in the aforementioned mutants. These
research used serial analysis to examine the patterns of
gene expression at different phases of floral development,
and the results show that the ABC model predicts many
of these patterns.
Surprisingly, these genes exhibit traits of transcription
factors, which is compatible with their function in
controlling gene expression. As was expected, a collection
of factors found in yeasts and mammalian cells and these
transcription factors have structural similarities. The term
MADS, which stands for the variety of elements included
in this category is used to refer to this group as a whole.
Although it is still conceivable that additional components
are involved in the complex control of gene expression,
MADS factors have been found in every plant species
tested, which is significant (Taiz and Zeiger, 2002).
Genes exhibiting type-A function
Function A is predominantly represented in two
important genes in Arabidopsis thaliana: APETALA1
(AP1) and APETALA2 (AP2). While, AP2 is a member
of the exclusive AP2 family of genes that are found only
in plants, AP1 is categorized as a MADS-box type gene
(Bowman, 1989). Surprisingly, it has been shown that
AP2 and the co-repressor TOPLESS (TPL) work
together to suppress the C-class gene AGAMOUS (AG)
in developing floral buds. The shoot apical meristem
(SAM), which shelters the latent stem cell population
throughout the adult life of Arabidopsis, is notable because
it does not express AP2. This raises the possibility that
TPL works in concert with another A-class gene in the
SAM to suppress AG. As a type A gene, AP1 is essential
Breeding for different Flower Forms in Ornamental Crops 2205
for identifying the identity of sepals and petals as well as
for the growth of the floral meristem. On the other hand,
AP2 participates in the growth of ovules and even leaves,
acting not only in the first two whorls of floral organs but
also exerting its influence on the remaining two whorls.
Genes exhibiting type-B function
Two MADS-box genes, APETALA3 (AP3) and
PISTILLATA (PI), give rise to type-B function of A.
thaliana. The homeotic transformation of petals into
sepals and of stamens into carpels is brought on by a
mutation in one of these genes. This also occurs in its
orthologs in A. majus, which are DEFICIENS (DEF)
and GLOBOSA (GLO) respectively (Schwarz-Sommer,
1990). Eudicotyledonous angiosperms have four separate
whorls of floral parts that include sepals, petals, stamens,
and carpels. According to the ABC model, the identity of
these organs is governed by the action of homeotic genes
A, A+B, B+C and C, respectively.
However, in contrast to the typical arrangement of
sepals and petals in eudicots, many plants within the
Liliaceae family exhibit a unique floral morphology with
two nearly identical external petal-like whorls known as
tepals. A modified ABC model was brought up in 1993
by van Tunen et al. to clarify the floral structure of the
Liliaceae family. According to this revised hypothesis,
class B genes are expressed in whorls 1 as well as whorls
2 and 3, not just those two. Because of the expression of
class A and class B genes, whorls 1 and 2 have petaloid
features.
The cloning and characterization of homologs of the
Antirrhinum genes GLOBOSA and DEFICIENS in a
Liliaceae species, the tulip Tulipa gesneriana, allowed
for the experimental validation of this theoretical
paradigm. These genes were found to be expressed in
whorls 1, 2 and 3. Homologs GLOBOSA and
DEFICIENS were also identified and characterized in
Agapanthus praecox ssp. orientalis (Agapanthaceae),
a phylogenetically distant species from the model
organisms. Both of these genes, ApGLO and ApDEF,
areencoded proteins of 210 to 214 amino acids each.
These sequences were connected to the monocotyledon
B gene family through phylogenetic analysis. Studies using
in situ hybridization further demonstrated that whorls 1,
2, and 3 express both ApGLO and ApDEF. Collectively,
these data offer strong support for the modified ABC
model’s alignment with the floral development mechanism
in Agapanthus species, which has been reported in
Liliaceae species. This information sheds insight on the
remarkable diversity of floral structures among different
plant families.
Genes exhibiting type-C function
The C function in Arabidopsis thaliana is controlled
by the single MADS-box type gene AGAMOUS (AG).
In additional to contributing to form the floral meristem,
AG is essential for determining the identities of both
stamen and carpels (Bowman, 1989). As a result,
androecium and gynoecium are absent in the phenotype of
AG mutants, which is instead defined by the development
of petals and sepals in place of these structures.
Additionally, the flower ’s center is still poorly
differentiated, which causes the whorls of petals and
sepals to repeatedly grow.
Genes exhibiting type-D and E functions
The identification of D function genes in 1995 signaled
a significant advancement in the study of flowers. Despite
some similarities with C function genes, these genes,
which are members of the MADS-box protein family,
have a different function than the ones previously
mentioned. These two genes are referred to as FLORAL
BINDING PROTEIN7 (FBP7) and FLORAL
BINDING PROTEIN1L (FBP1l). They were discovered
to be extremely important for ovule formation in Petunia.
Subsequently, equivalent genes were discovered in
Arabidopsis, where they are likewise responsible for
controlling the growth of the carpel, ovule, and structures
related to seed dispersal.
In 1994, a novel function in the floral development
model was discovered as a result of intriguing phenotypes
discovered in Petunia RNA interference investigations.
The three innermost whorls of floral organs were once
thought to grow primarily as a result of the E function.
However, later studies enhanced our knowledge of its
function in floral development by showing that its
expression is necessary in each floral whorl.
Strategies for improving flower shape include
Hybridization : It defined as the process of crossing
two or more plants with distinct genetic backgrounds,
plays a pivotal role in the creation of new crop varieties.
Combining desired features into a single variety is an
efficient way to increase genetic diversity and to take
theadva ntage of hybrid vigor. The best strategy for
achieving distinctive flower shapes is to cross two diverse
types. Depending on the genetic makeups involved, the
results of such crossings can produce a diverse spectrum
of morphologies, including single, double and semi-double.
A single gene or a number of genes may control the genetic
makeup of the many flower forms found in ornamental
crops. Breeders can create new cultivars with the
appropriate floral kinds by carefully choosing compatible
2206 Shreedhar Beese et al.
parent genotypes.
For instance, as shown by Debener in 1999, a
dominant allele governs the inheritance of the double
flower shape in roses. Therefore, it is important to cross
cultivars having double-type flowers with suitable single-
flower cultivars in order to produce new double-type
cultivars. According to Chen et al. (2012), a recessive
gene governs the double flowering trait in Catharanthus
roseus. Furthermore, they suggested that single flowers,
whether in a homozygous or heterozygous state, are
controlled by a dominant allele. These genetic discoveries
direct the breeding efforts to produce unique flower
shapes in ornamental crops. Crossing two diverse forms
is the best approach to create a unique shape. The
outcome may take on a variety of shapes, including single,
double, and semi-double, depending on genetic make-up.
For example: Tuberose. In order to create new cultivars
of the double flower form in roses, cultivars with double
flower forms must be crossed with suitable single flower
form cultivars since the inheritance of the double flower
form is regulated by a dominant allele. Hybridization
between single and double cultivars was conducted by
Debener et al., 1999 and Shen et al. in 1987. Numerous
single and few double plants were produced in the progeny
of reciprocal crosses between single and double cultivars.
Hybridization techniques are essential in plant
breeding to create new and improved cultivars. Two
common methods of hybridization are intervarietal
hybridization (intraspecific) and interspecific hybridization
(intrageneric):
Intervarietal Hybridization (Intraspecific): In this
method, crosses are made between plants from two
different varieties of the same species. It is an effective
method for enhancing both cross-pollinated and self-
pollinated crops. This approach is frequently used to
create cultivars for a variety of flowering plants, such as
Chrysanthemum, Gladiolus, Rose, Bougainvillea, Hibiscus,
and Camellia. “April Blush,” “April Dawn,” “April Rose,”
and “April Snow” are a few cultivars that were created
through intervarietal hybridization.
Interspecific Hybridization (Intrageneric): In this
approach, plants from two different species within the
same genus are crossed. In order to create new cultivars
of plants like verbena, petunia, orchid, bougainvillea, lilium
and amaryllis, interspecific hybridization is used.
Interspecific hybridization is commonly seen in species
crosses as those between the lilium, orchid, Hemerocallis,
Victoria amazonica and Bougainvillea spectabilis and
Bougainvillea glabra.
These methods of hybridization enables plant breeders
to introduce genetic diversity and create new cultivars
with desirable traits, ultimately contributing to the
enhancement of ornamental crops and their aesthetic
appeal. Chrysanthemum: Huang et al. (2011) conducted
interspecific hybridization experiments involving
Chrysanthemum morifolium varieties ‘Hongxinju’ and
‘Xinbaiju’ as females and Chrysanthemum indicum and
Chrysanthemum nankingense as males. Bud pollination
was found to enhance heterozygosity in these interspecific
hybrids, resulting in aneuploid chromosomes and significant
character segregation. Cheng et al. (2010) employed
ovary rescue techniques to create six interspecific hybrids
by crossing Dendranthema morifolium ‘rm20-12’ (2n-
54) with its wild diploid relative, Dendranthema
nankingense (2n=18)
Comparing these hybrids to their D. morifolium
parent, the cold tolerance of these offspring was much
higher. Chrysanthemum cultivars can be improved by
interspecific hybridization. Gerbera: The history of gerbera
breeding dates back to the late 19th century in Cambridge,
England, when R.I. Lynch crossed two South African
species, G. jamesonii and G. viridifolia, resulting in the
creation of G. cantabrigiensis, known today as G.
hybrida. The descendants of these two species are the
source of the majority of commercially grown gerbera
cultivars. Gerbera cultivars come in a wide range of sizes,
colors, and shapes, including white, yellow, orange, red,
and pink. Dianthus caryophyllus, or the carnation, has
the potential to be crossed with a number of different
wild species, according to research. Between D.
silvestris, D. knappi, D. sequierii, D. carthusianorum,
and D. caryophyllus, successful crossings were seen
that produced seeds and hybrid plants.
Mutation breeding : involves sudden heritable
changes that occur in an organism, deviating from
Mendelian principles of segregation and recombination.
Mutants are those individuals that display these heritable
alterations. De Vries first proposed the idea of mutation
in the year 1900. Gene, chromosomal, or cytoplasmic
alterationscaninducedmutationsorcausethemtooccur
spontaneously.
Mutagens are agents, either physical or chemical,
that artificially induce mutations. Physical mutagens like
as alpha rays, beta rays, X-rays, gamma rays, neutrons,
and UV rays can all result in these mutations. Chemical
mutagens encompass a range of substances such as 5-
bromouracil, 5-chlorouracil, mustard gas, sulphur mustard,
nitrogen mustard, ethyl methane sulphonate (EMS),
methyl methane sulphonate (MMS), ethylene oxide,
ethylene imine, azasorine, mitomycin C and streptonigrin,
Breeding for different Flower Forms in Ornamental Crops 2207
among others. These substances are used to purposefully
cause mutations for various breeding and research goals.
Induced mutation
Mutation breeding in plants : Mutation induction
techniques find a particularly favorable application in
ornamental plants due to the ease of monitoring
economically important traits such as flower
characteristics and growth habits after mutagenic
treatment. Numerous kinds of ornamental plants are
heterozygous and reproduce vegetatively, which makes
it easier to find, pick out, and preserve mutants in the M1
generation. For instance, the moss rose originated as a
mutant of Rosa centifolia and approximately 5,819 rose
cultivars have been developed through bud mutations.
The 50% of rhododendron and chrysanthemum cultivars
are the result of spontaneous or artificial mutations. The
cultivar ‘Faraday’ of tulip contained the first known floral
mutation. Due to their variety of economically valuable
features, which can be easily tested and defined after
treatment, ornamental plants are very advantageous
systems for mutagenic treatments. The identification,
selection, and retention of mutants in the M1 generation
are also made possible by the heterozygous nature of
many ornamental plants and their frequent vegetative
propagation (Schum and Preil, 1998). Induced mutations
observed in ornamental plants encompass a wide range
of features, including flower characteristics (such as
color, size, morphology, and fragrance), leaf traits, growth
habits, and physiological attributes such as alterations in
photoperiodic responses, early flowering, increased
flowering, improved post-harvest longevity, and enhanced
tolerance to biotic and abiotic stress factors (Schum and
Preil, 1998).
The morphology of flowers and inflorescences can
also be significantly affected by the mutation induction.
In some cases, it has resulted in an increase in flower
size, although more commonly, mutagenic treatments have
led to undesired reductions in flower size. Additionally, it
has been observed that mutation induction can alter petal
form, which can occasionally lead to ornamental
advancements. Additionally, mutagenic treatments have
been shown to change petal counts in both the upward
and downward direction. There have been reports of
larger whorls of ligulate florets and changes from ligulate
to tubular florets in the Compositae family as a result of
mutagenic treatments.
Polyploidy breeding in ornamental crops : Crop
species with genetic chromosomal number ‘n’ are
referred to as haploid, while those with somatic
chromosome number (2n) are referred to as diploid.
Euploid crop species are those that have a somatic
chromosomal number that is a direct multiple of the basic
number. Aneuploid crop species, on the other hand, are
those whose somatic number is not an exact multiple of
the basic number. According to the multiplicity of the
basic number, euploids can be further divided into
monoploid (x), diploid (2x), triploid (3x), tetraploid (4x),
hexaploid (6x), octaploid (8x), and so on. Polyploid refers
to species that are more complex than diploids.
Autopolyploids are polyploids with the same genome
number and allopolyploids are those with differing genome
numbers.
Haploids are typically weaker and infertile, whereas
other polyploids show traits including larger plant parts,
Fig. 4 : Common Mutagens used in plant mutation induction.
2208 Shreedhar Beese et al.
Table 1 : Transformation of Flower Morphology in Flower crops through hybridization.
Crop name Ploidy Level Species/ Cultivars
Amaranthus Tetraploid Amar Tetra
Amaryllis Triploid Kiran
Tetraploid Samrat, Tetra Apricot, Tetra Starzynski
Hexaploid A. belladona
Heptaploid A. blumenvia
Antirrhinum Tetraploid Tetra Giant, Tetra Guilt, Velvet Beauty, Red Shades
Anthurium Diploid A. Andreanum, A. hookerii, A. magnificum
Triploid A. Scandens
Tetraploid A. Digitatum, A. wallisii
Bougainvillea Triploid Cypheri, Temple Fire, Lateritia, Perfection, Poultoni Special
Tetraploid Crimson King, Princess, Mahara, Magnifica, Shubhra, Mrs. McClean,
President Roosevolt, Lady Mary Baring, Thimma, Zakariana
Aneuploid Begum Sikander, Wajid Ali Shah, Chitra
Carnation Tetraploid D. chinensis
Dahlia Tetraploid D. imperialis
Octaploid D. variabilis, D. coccinea, D. rosea
Day Lily Diploid Barbara Mitchell Ruffled, Master Piece, Ruffled Perfection
Tetraploids Tetra Apricot, Tetra Peach, Crestwood Series, Wedding Band
Gladiolus Triploid Manmohan, Monohar, Manhar, Mukta, Manisha, Mohini
Pentaploid G. psittacinus
Aneuploid Archana, Arun
Jasmine Triploid J. primulinum, J. sambac, J. grandiflorum
Tetraploid J. flexile, J. angustifolium
Lily Triploid Lilium tigrinum
Marigold Diploid T. erecta, T. tenuifolia
Tetraploid T. patula, T. minuta, T. biflora, T. remotiflora
Triploid Seven Star, Showboat, Nugget
Narcissus Triploid N. pseudonarcissus hispanicus
Hexaploid N. bulbocodium
Diploid N. pseudonarcissus, N. poeticus
Orchid (Dendrobium) Amphidiploids Jacquelin Thomas Y 166
Orchid (Phalaenopsis) Tetraploid Riverbend
Orchid (Oncidium) Tetraploid Popcorn
Orchid (Spathoglottis) Tetraploid Lion
Orchid (Vanda) Tetraploid Atherton, Juliet, Hula Girl, Wood Lawn
Petunia Double Haploid Mitchel
Autotetraploid State Fair, Old Mexico
Table 1 continued...
Breeding for different Flower Forms in Ornamental Crops 2209
larger cells and slower growth rates as compared to
diploids. In diploid species, monosomics hardly persist,
whereas nullisomics rarely flourish in polyploid species.
A form of allopolyploid called amphidiploid has two copies
of each genome within. Tetraploid plants are distinguished
by their vigor, strong vegetative growth, thicker leaves,
and larger blooms. Triploids exhibit a combination of both
hybrid vigor and polyploidy-induced vigor.
Increasing a species’ chromosome count by
polyploidy breeding is a highly successful way to add
genetic variations, especially when there are few natural
variants. Chromosome doubling produces genetic
diversity, which can be used to improve breeding
strategies. This strategy has been extensively used in a
variety of crops to improve plant characteristics and
produce novel forms with better plant architecture, hence
supplying useful data for additional breeding efforts and
cultivar improvement (Mata, 2009).
Induced polyploidy often results in an increase in the
size and dimensions of numerous plant parts, such as
leaves, branches, floral components, fruits, and seeds
(Chopra, 2008). Particularly tetraploids exhibit increased
vigor and greater stature. Colchicine and oryzalin are the
main drugs used to induce polyploidy, and the dosage and
length of the treatment are very important. Numerous
studies have investigated this aspect, with Gantait et al.
(2011) found that tetraploid Gerbera jamesonii Bolus
cv. Sciella had strong plant growth, longer stem length
and larger blossom diameter. Similar to this, Hanzelka
and Kobza (2001) also noted a bigger bloom of diameter
(4.20 cm) after colchicine-induced polyploidy.
The production of polyploids involves several methods
and factors, resulting in tetraploids and higher levels of
polyploidy:
Regeneration Methods : Polyploids, including
tetraploids, can be induced through regeneration
techniques. This process often includes heat and cold
treatments applied to germinating seeds.
Chemical Induction : Polyploidy can also be induced
chemically using substances such as colchicine, nitrous
oxide, oryzaline, trifluralin and phosphoric amide.
Somatic Mutation : Some instances of polyploidy
are the result of somatic mutations, where disruptions in
mitosis lead to chromosome doubling. For example, this
phenomenon has been observed in Primula kewensis.
Unreduced Gametes : Polyploidy can occur when
unreduced gametes (eggs and sperm) unite. These
gametes have not undergone normal meiosis and still
maintain a 2n chromosome constitution.
Environmental Factors : Polyploidy tends to be
more frequent at high altitudes, high latitudes, and in wet
soils and meadows.
In vitro Polyploidization : Recently, in vitro
polyploidization methods have been developed and
employed to expedite heterosis breeding in ornamental
plants. This method speeds up the process of inducing
polyploidy and decreases the number of aberrant plants.
Polyploidy in ornamental plants has made significant
advances. The evolution of numerous decorative species
and cultivars, such as tulips, dahlias, anthuriums,
bougainvilleas, lilies, cactus, primulas, narcissus and roses,
has been significantly influenced by polyploidy. Various
ornamental crops, including marigolds, petunias,
snapdragons, portulacas, chrysanthemums, calendulas
and lilies have been reported to include induced tetraploids.
These initiatives have improved and increased the variety
of decorative plant types.
Primula Diploid P. frondose
Tetraploid P. farinosa
Hexaploid P. scotica
Octaploid P. Scandinavica
Rose Diploid R. gigantea, R. multiflora, R. wichuriana, R. chinensis, R. moschata
Triploid R. bourboniana, Prema, Surekha, Surya
Tetraploid R. gallica, R. damascena, R. foetida
Trisomic Mohini
Stock Aneuploid Snow Flake
Tulip Triploid T. lanata, T. stellata
Tetraploid T. clusiana, T. stellata
Pentaploid T. clusiana
Table 1 continued...
2210 Shreedhar Beese et al.
Table 2 : Induced mutation for change in flower morphology in flower crops.
S. no. Crop Mutagen Character References
1Begonia rex Gamma rays Shape mutants Buiatti et al. (1990)
2 Bougainvillea Gamma rays Variegated flower Srivastava et al. (2002)
3 Carnation Heavy ion beams Change from serrate to rounded petals Okamura et al. (2003)
4Catharanthus roseus Gamma rays Development of four petals instead of five El-Mokadem et al. (2014)
5 Chrysanthemum Gamma rays Tubular ray florets Tubular and Flat shaped Banerji and Datta (1992),
floretsSpoon-shaped, tubular and irregular ray Misra et al. (2003),
floretsReduced flower head size Kumari et al. (2013)
6 Cyclamen Ion-beams Typical petal mutant Sugiyama et al. (2008)
7 Dahlia X-rays Development of white tip Broertjes and Ballego
(1967)
8 Dendrobium Orchid Gamma rays Narrow, elongated or broad or curled petals, Ariffin and Basiran (2000)
Veinous sepals. Short and broad lip.Small flower
etc.
9 Gerbera Gamma rays Change in flower morphology Jain et al. (1998)
10 Hibiscus Gamma rays Single flower type Banerji and Datta (1986)
11 Hydrangea Ion-beams Seeds Kudo et al. (1998)
12 Limonium Ion beam Shoot cultures Ogawa et al. (2014)
13 Petunia MMS, MNNG Dissected and Dentate corolla Mahna and Garg (1989
14 Pelargonium Ion-beams Buds Yu et al. (2016)
15 Prunus Ion- beams Scions Hayashi et al. (2019)
16 Rose Ion-beams X-rays Change in no of petal, shape, and size Increase Murugesan et al. (1993),
and decrease in petal number Yamaguchi et al. (2003),
Van and Broertjes (1989)
17 Spiraea Ion beam Seeds Iizuka et al. (2001)
18 Torenia hybrida Gamma rays Erose petal margins, extra petals, extrastamens Suwanseree et al. (2011),
or missing petals Serrate petal margins Nishijima and Shima (2006)
19 Tuberose Gamma rays Large flower size Patil et al. (1975)
20 Tulip X-rays Parrots, fringed and double Van and Broertjes (1989)
Achievements of polyploidy in ornamentals
In a variety of ornamental crops, polyploidy has been
a key factor in determining how species and cultivars
have evolved. There are numerous well-known plants in
this category, including tulips, dahlias, anthuriums,
bougainvilleas, lilies, cactus, primulas, narcissus, roses,
and more. Notably, induced tetraploids have been
successfully produced in a number of ornamental species,
including lilies, calendulas, chrysanthemums, petunias,
snapdragons and marigolds. Those achievements have
greatly increased the variety and quality of ornamental
plant varieties.
Genetic Modification of Ornamental plants
The ability to create novel varieties through the use
of gene transfer techniques makes genetic modification
an intriguing opportunity for breeders of ornamental plants.
In some cases, creating new ornamental varieties through
traditional methods like hybridization or mutagenesis can
be exceptionally challenging, time-consuming, or even
unfeasible, especially when dealing with completely sterile
varieties such as orchids. Genetic modification offers a
promising alternative avenue for enhancing these
varieties.
Breeders now have the ability to introduce features
that are challenging or impossible to breed for using
traditional techniques. These characteristics cover a wide
variety of potential outcomes, such as alterations in floral
color, smell, resistance to abiotic stressors, disease
resistance, pest resistance, adaptations to plant and flower
structure, modifications to flowering period, and
lengthening post-harvest longevity, among others. Notable
examples of ornamental plants that have benefited from
genetic modification include Chrysanthemum, Torenia,
Cyclamen, Petunia and more.
Breeding for different Flower Forms in Ornamental Crops 2211
The act of introducing a specific DNA sequence,
usually a gene, into an organism without fertilization or
conventional crossbreeding is known as genetic
transformation. Transgenic plants are referred to be plants
that have undergone genetic modification. This technique
increases the potential for genetic improvement by
allowing the regulated integration of nucleic acids into
the recipient genome.
Along with improvements in tissue culture techniques
and genetic engineering, numerous genetic transformation
approaches have been developed. The use of
Agrobacterium tumefaciens, particle acceleration
(biolistics), polyethylene glycol treatment, electroporation,
silicon carbide fiber-mediated transformation
(siliconization), silica carbonate microparticles, microlaser
techniques, micro- and macro-injection techniques, and
direct DNA application are some of the methods covered
by these techniques. These methods each have particular
benefits and uses in the area of genetic modification for
ornamental plants.
The successful development of the first transgenic
petunia during the 1990s was a crucial turning point for
the field of genetic engineering of ornamental plants.
Rose, chrysanthemum and carnation are three of the most
widely grown cut flower crops that have undergone
genetic transformation. For Rosa hybrida cv. ‘Royalty,’
the transformation procedure comprised co-cultivating
the plant material with Agrobacterium. After that, friable
embryogenic callus formed, and the plants were then
converted by embryogenesis.
Agrobacterium has also been used to genetically alter
Chrysanthemum grandiflora and indicum. The infection
of either leaves or peduncles led to the regeneration of
transformed plants through organogenesis or the formation
of transformed callus capable of generating transformed
plants. Transformed Dianthus caryophyllus (carnation)
cultivars were produced by co-cultivating leaves, petals,
or stems with Agrobacterium, followed by either direct
or indirect organogenesis. Other flowering plants that
have undergone Agrobacterium-mediated transformation
include Narcissus, Gladiolus, Lilium longiflorum, L.
leichlnii var. maximowiczii and Tulipa. Other flower
crops, such as Gerbera, Dendrobium, Antirrhinum,
Anthurium, Eustoma and Pelargonium, have also been
investigated for genetic modification. For genetic
alteration in ornamental plants, a variety of methods have
been used, including:
a. RNAi or Gene Silencing
b. Chimeric Repressor Gene-Silencing Technology
(CRES-T)
c. MicroRNA
These methods provide many ways to control gene
expression and produce desirable features in ornamental
plants, creating new opportunities for the development
of ornamental variety.
RNAi or Gene silencing is a powerful method for
silencing gene expression, offering a straightforward
approach to regulate gene function. Both transcriptional
gene silencing (TGS) and posttranscriptional gene
silencing (PTGS), commonly known as RNA interference
(RNAi) are separate methods by which this unique gene
regulation mechanism might lower transcript levels. When
mRNA is broken down into short RNAs, which in turn
activate ribonucleases to target homologous mRNA of a
particular gene, gene silencing occurs. The resulting
phenotypes may resemble an allelic series of mutants or
genetic null mutants.
These methods are frequently used for loss-of-
function investigations because they enable the precise
silencing of specific genes, inhibiting the manifestation of
specified features. For instance, in a work by Noor et al.
(2014), two C-class MADS-box genes, pMADS3 and
FBP6 were targeted by virus-induced gene silencing
(VIGS) to induce double flower development in four
cultivars of Petunia hybrida. In flowers exposed to
pMADS3/FBP6-VIGS, the results revealed a complete
conversion of stamens into petaloid tissues combined with
a considerable increase of upper limb-like tissues, giving
the flowers a decorative aspect. Additionally, according
to Heijmans et al. (2012a), flowers in fbp6/fbp6
pMADS3-RNAi plants showed a full conversion of these
methods are frequently used for loss-of-function research
because they enable the precise silencing of specific
genes, prohibiting the expression of specific traits For
instance, in a work by Noor et al. (2014), two C-class
MADS-box genes, pMADS3 and FBP6, were targeted
by virus-induced gene silencing (VIGS) to induce double
flower development in four cultivars of Petunia hybrida.
In flowers exposed to pMADS3/FBP6-VIGS, the results
revealed a complete conversion of stamens into petaloid
tissues combined with a considerable increase of upper
limb-like tissues, giving the flowers a decorative aspect.
In addition, flowers in fbp6/fbp6 pMADS3-RNAi plants
showed a complete conversion of carpels into secondary
flowers, giving them a voluminous appearance, according
to Heijmans et al. (2012a). Numerous other plants, such
as Japanese gentian (Nakatsuka et al., 2015),
Thalictrum thalictroides (Galimba et al., 2012),
Phalaenopsis orchids (Hsieh et al., 2013) and
Aquilegia (Gould and Kramer, 2007) have been the
2212 Shreedhar Beese et al.
subject of similar investigations for double bloom
production. These results show how flexible and
successful RNAi is for modifying gene expression and
phenotypic features in ornamental plants.
Micro RNA is a class of small non-coding RNA
molecules, typically about 22 nucleotides in length, that
are present in eukaryotes. These molecules are essential
for the post-transcriptional control of gene expression as
well as RNA silencing. Nearly all biological and metabolic
activities involve miRNAs. There have been many
miRNAs found that are closely linked to plant
architecture. For instance, miR156 has been linked to
the control of plant architecture, according to research
by Jiao et al. (2010). According to Carle et al. (2007)’s
investigation of another miRNA, miR319, snapdragon
plants’ leaves and petals’ morphology is influenced.
CRISPR/Cas9 Technology : The regularly spaced,
clustered short palindromic repeats, the (CRISPR)/
CRISPR-associated protein (Cas) system has become a
potent genome-editing tool for precisely altering DNA
sequences in particular places. It offers great ways to
genetically improving floricultural crops. The CRISPR/
Cas9 system plays an important role in agricultural crops,
such as enhancing flowering characteristics including
colour modification, extending the shelf life of flowers,
promoting flower initiation and development and using
genome editing tools to alter the colour of decorative
foliage. Cas9/CRISPR gene editing could be helpful in
creating new cultivars with improved essential oils and
smell, among many other beneficial characteristics (Sirohi
et al., 2022). A CRISPR/Cas tool that can separate from
the Cas9/sgRNA construct to prevent comparable
changes by CRISPR/Cas generates stable gene
mutations. CRISPR/Cas gene is a rapid and precise
genetically engineered crop technology that produces
crops resistant to abiotic and biotic stresses, viruses,
fungus and bacteria in a fraction of the time compared to
crops generated using conventional methods, which take
ten to fifteen years to achieve resistance. Thus, CRISPR/
Cas is a helpful tool for producing agricultural products
in a sustainable manner. This technology has been used
to successfully modify plant characteristics. This has been
used to successfully modify plant characteristics.
Gene knockouts have been the primary application
of CRISPR/Cas9 technology in plants. Additionally
Petunia hybrid,Chrysanthemum morifolium,
Dendrobium officinale, Torenia, Ipomoea nil, Lilium
longiflorum,Lilium pumilumand Phalaenopsis
equestrishave all benefited from its successful use in
generating gene knockouts in ornamental plants to induce
genetic alterations. According to these research,
ornamental plants can effectively undergo mutagenesis
produced by CRISPR/Cas9. Using the traditional
Mendelian segregation, the modification generated is
accur ateand may be inherited by future generations
(Sirohi et al., 2022).
CRISPR/Cas systems are easy to use, dependable
and capable of multiplex targeting, they provide many
benefits for the establishment of resistance in agricultural
crops. Because of their high precision and efficiency,
these systems hold considerable promise for overcoming
the limits of conventional breeding for the development
of resistance. CRISPR/Cas9 still has a lot of restrictions
despite its many benefits and wide range of applications.
The direct targeting of S genes may result in some fitness
Fig. 5 : Mechanism of Gene silencing in RNA interference
technology.
Fig. 6 : Chimeric Repressor Gene-Silencing Technology
(CRES-T) induces phenotype.
Breeding for different Flower Forms in Ornamental Crops 2213
cost due to their connection with other desired genes,
particularly the genes governing plant growth and
development. There are only a few notable barriers that
could prevent the CRISPR/Cas9 system from being
effective in the development of disease resistance.
Furthermore, any interruption of the S gene may interfere
with the product’s pathway and eventually, the pathways
of many additional products. Without taking into account
species boundaries, editing can produce desired S gene
mutants in the majority of interesting plants for breeding.
It is anticipated that more S genes will be identified,
increasing the number of potential targets for genome
editing.”Offtargetmutations”areamajorconstraintas
wellandarenowa cornerstoneofattemptstoenhance
the CRISPR system, especially in the transgene-free
agricultural production process. Off-target genome editing
is the term for DNA alterations at random and nonspecific
locations that can happen via gRNA misguides or in a
gRNA-independent way (Ahmad et al., 2020). Beyond
the technological difficulties of bringing CRISPR/Cas9-
developed crops from lab to the field, other barriers include
ambiguous legal frameworks, disagreements over
intellectual property rights and acceptability by customers.
A number of useful Cas9-based applied approaches have
emerged that allow scientists to quickly improve plants.
Finally, it should be noted that the CRISPR/Cas9 system
is an effective tool for genetically engineering crops.
Conclusion
The creation of novel flower forms in ornamental
plants is a primary breeding objective, as it significantly
enhances their commercial value due to their
distinctiveness. It has been established that floral organ
identity is determined by five classes of homeotic genes,
denoted as A, B, C, D and E. To generate a new form,
modifications of these genes are necessary. Various
strategies, including hybridization, mutation, polyploidy
induction, and genetic engineering, can be employed to
achieve unique flower forms, including double flowers.
In contemporary breeding efforts, innovative techniques
such as RNA interference (RNAi), Chimeric Repressor
gene-Silencing Technology (CRES-T), microRNA
(miRNA) modulation, CRISPR/Cas9 technology and
other gene-silencing approaches are available. These
methods enable the targeted silencing of specific genes,
thereby facilitating the development of plants with altered
flower forms. Despite the array of techniques at our
disposal, the commercial development of ornamental
varieties through genetic transformation remains relatively
limited.
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