Citrus Biotechnology: Current Innovations and Future Prospects

Abstract

Citrus is a valuable fruit crop worldwide. It not only provides essential minerals
and vitamins but is also of great commercial importance. Conventional research
has contributed a lot to the improvement of this fruit plant. Numerous improved
varieties have been developed through conventional breeding, mutational breeding,
polyploidization and tissue culture yet pathogens continue to emerge at a consistent
pace over a wide range of citrus species. Citriculture is vulnerable to various biotic
and abiotic stresses which are quite difficult to be controlled through conventional
research. Biotechnological intervention including transgenesis, genome editing, and
OMICS offers several innovative options to resolve existing issues in this fruit crop.
Genetic transformation has been established in many citrus species and transgenic
plants have been developed having the ability to tolerate bacterial, viral, and fungal
pathogens. Genome editing has also been worked out to develop disease-resistant
plants. Likewise, advancement in OMICS has helped to improve citrus fruit through
the knowledge of genomics, transcriptomics, proteomics, metabolomics, interactomics,
and phenomics. This chapter highlights not only the milestones achieved
through conventional research but also briefs about the achievements attained
through advanced molecular biology research.

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
Chapter
Citrus Biotechnology: Current
Innovations and Future Prospects
GhulamMustafa, MuhammadUsman, Faiz AhmadJoyia
and Muhammad SarwarKhan
Abstract
Citrus is a valuable fruit crop worldwide. It not only provides essential minerals
and vitamins but is also of great commercial importance. Conventional research
has contributed a lot to the improvement of this fruit plant. Numerous improved
varieties have been developed through conventional breeding, mutational breeding,
polyploidization and tissue culture yet pathogens continue to emerge at a consistent
pace over a wide range of citrus species. Citriculture is vulnerable to various biotic
and abiotic stresses which are quite difficult to be controlled through conventional
research. Biotechnological intervention including transgenesis, genome editing, and
OMICS offers several innovative options to resolve existing issues in this fruit crop.
Genetic transformation has been established in many citrus species and transgenic
plants have been developed having the ability to tolerate bacterial, viral, and fungal
pathogens. Genome editing has also been worked out to develop disease-resistant
plants. Likewise, advancement in OMICS has helped to improve citrus fruit through
the knowledge of genomics, transcriptomics, proteomics, metabolomics, interac-
tomics, and phenomics. This chapter highlights not only the milestones achieved
through conventional research but also briefs about the achievements attained
through advanced molecular biology research.
Keywords: citriculture, conventional research, transgenesis, genome editing,
multi-OMICS
. Introduction
Citrus is one of the most diverse members of the family Rutaceae and is the
leading tree fruit crop in the world. Citrus comprises different species of edible
fruits like mandarins (Citrus reticulata Blanco), sweet oranges (C. sinensis Osbeck),
grapefruit (C. paradisi Macf.), acid limes (C. aurantifolia Swingle), and sweet limes
(C. limettioides), lemons (C. limon Burmf.) and their hybrids including tangerines,
tangelos, tangors, etc. [1].
Citrus is being widely cultivated in the sub-tropical, tropical, and temperate
regions of the world. Global citrus production is 157 million tons per annum
from an area of 15 million hectares. About 50% of the area and production of cit-
rus is being contributed by the northern hemisphere of the world. China (28%)
and the Mediterranean regions (25%) are the major contributors to global citrus
production followed by Brazil (13%). China is leading in grapefruit and manda-
rin production. Among Mediterranean countries, Spain is leading in global citrus

Citrus

production (6 million tons) including mandarins, oranges, limes, lemons and
exports. Brazil is leading in global fresh sweet orange and its juice production.
Mexico and India are major lime producers [2]. Pakistan’s share in global citrus
production is quite low (1.6%) which includes mandarin and sweet oranges as
major species whereas limes, grapefruit, and lemons have less production and are
dealt as minor species. The global citrus industry is facing many biotic (Citrus
greening, Citrus tristeza virus, sudden death, citrus canker, and Phytophthora)
and abiotic stress (salinity, drought, and temperature
fluctuations) which have a direct impact on fruit crop production and yield [3].
. Origin and diversification
Citrus and other genera including Poncirus, Clymenia, Fortunella, Eremocitrus,
and Microcitrus belong to tribe Citreae and sub-tribe Citrineae and are considered
as true citrus [4]. Classification in citrus has been controversial since ancient times
due to vast morphological diversity, interspecific and intergeneric sexual compat-
ibility. However, molecular biology tools have revealed four species including
mandarins, citron (C. medica), pummelos (C. grandis Linn.), and wild cultivar of
papeda (C. micrantha Wester) as the true parental species that have contributed to
the development of other species during the process of evolution [5, 6]. Based on
phylogenetic and genomic studies it is revealed that mandarin originated in China,
Vietnam, and Japan whereas citron was originated in northeast India and China.
Pummelo originated in Indonesia and Malay whereas C. micrantha was originated
in the Philippines [6]. Other citrus species including sweet oranges, grapefruit,
lime, lemon, sour oranges, and hybrids (tangelos and tangors) have developed
from these ancestral species through random hybridization and natural mutation
events [7].
Among citrus genetic resource centers, major collections are found in the USA,
China, Spain, France, Japan, and Brazil where a large number of wild species, their
relatives, old and new varieties, and breeding lines are conserved [8]. In Pakistan,
citrus genetic resources are conserved mainly in the field as orchards or germplasm
units in Sargodha, Faisalabad, and Sahiwal in different academic and research
institutes.
. Conventional approaches for crop improvement
Citrus breeders have been using different approaches for their improvement
including conventional breeding, mutation breeding, polyploidization and in vitro
culture tools particularly somatic hybridization which has played an essential role
in developing new somatic hybrids. These techniques have contributed towards
the selection and development of new potential cultivars and are still being used
as important fundamental tools for the development of genetically diverse germ-
plasm which could be further screened and characterized using modern breeding
technologies.
. Classical and mutation breeding
Though conventional breeding has limitations in citrus due to its complex
reproductive behavior, nucellar embryony, long juvenility, sterility, sexual incom-
patibility, and endogametic depression [9, 10]. However, still, many hybrids have
been developed by conventional breeding and recovered using in vitro tools.

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Citrus Biotechnology: Current Innovations and Future Prospects
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Mutation breeding has played a pivotal role in fruit crop improvement including
citrus and has developed several mutants with improved phenotypic and genotypic
traits [11]. Spontaneous or induced mutants do not have intellectual property rights
(IPR) related issues that have to be faced in the case of conventional breeding and
transgenics [12]. Both spontaneous and induced mutations have enhanced genetic
diversity in existing varieties and have provided the raw material for making selec-
tions for the novel horticultural traits [13]. About 3365 mutant varieties belonging
to 170 plant species have been released including citrus and 20 other fruit species
[14]. Among continents, Asia is leading with 2052 mutants released followed by
Europe (960 mutants). Among countries, China (817), Japan (479), India (341) and
the USA (139) are leading in mutant development whereas Pakistan has released 59
mutants in different crops [15]. In citrus, a total of 15 mutants have been released
since 1970 including mandarins and clementine (6), sweet oranges (6), grapefruit
(2), and lemon (1) [10]. Pakistan has registered a single mutant variety in citrus,
a Kinnow mandarin induced mutant having less number of seeds and named it as
“NIAB Kinnow” in 2017.
The rate of spontaneous mutations has been much higher in citrus compared
with other fruit crops, however, due to random genetic alterations it has been
difficult to identify and utilize such mutants [16, 17]. Induced mutations using
different irradiation sources including gamma rays (physical mutagens) and various
chemical compounds have enhanced the frequency of genetic variability. Physical
mutagens or ionizing radiations have been more commonly used for inducing
genetic diversity, chromosomal aberrations, and point mutations. About 70% of
the mutant varieties have been developed using physical mutagens [18]. In fruit
crops, physical mutagens have altered key horticultural traits like seedlessness,
precocious bearing, and dwarfism [19–21]. Other traits include fruit ripening time,
fruit skin and flesh color, fruit aroma, self-compatibility, pathogen resistance, and
fertility restoration in sterile hybrids. Among physical mutagens, gamma rays have
been most used for the development of mutants due to their shorter wavelength
and greater penetration [22], however, the ion beam is getting more popular and is
being widely used due to its greater efficiency and precision compared with gamma
rays [23]. Among chemical mutagens, ethyl methanesulfonate (EMS), diethyl
sulfate (DES), ethylenimine (EI), sodium azide (SA) has been most frequently
used for reliable and gene-specific mutations. A comprehensive review of the role
of mutation breeding in mandarins and lime crop improvement has been discussed
[24, 25]. Irradiation and chemical mutagen treatment of seeds and budwood have
been commonly used by breeders for inducing variation followed by selection and
clonal propagation. Mutation breeding applications have been reported in different
fruit crops including papaya, peach, pear, grapes, sweet and sour cherries, banana,
plum, almond [26], apple [27], and rough lemon [28]. Natural bud mutants include
Washington navel orange, most of the early grapefruit varieties including Marsh,
Foster, Shamber, Salustiana sweet orange, and Shamouti orange have originated
as bud sports. Now there are several commercial seedless varieties including Daisy
SL, Kinnow SL, Fairchild SL, and Tango that have been developed from their seedy
parents through mutation breeding and are being commercially cultivated [29].
Other commercial mutants in citrus include sweet orange varieties Jin Cheng [30],
Kozan [21], and NIAB Kinnow mandarin [31]. In grapefruit, Rio Red and Star Ruby
are two induced mutants that have obtained commercial significance due to their
better fruit color and seedlessness, respectively [32]. These are leading grapefruit
varieties in Texas, USA. Star Ruby is the leading variety in Turkey, South Africa,
Australia, and Spain. Rio Red is the main cultivar in China, India, and Argentine
[33]. In Pakistan, Shamber is the main grapefruit variety that needs to be replaced
with other potential candidate varieties like Star Ruby, Rio Red, and Flame [10].

Citrus

Conclusively mutation breeding has shown its enormous potential in citrus crop
improvement particularly in economically important horticultural traits. However,
it is a slow and long-term process and takes more time to detection of desirable
phenotypic variability. Utilization of modern breeding tools including molecular
markers, advanced methods for phenotypic screening like Targeting Induced Local
Lesions IN Genomes (TILLING) [34], using targeted mutagenesis and genome
editing technologies [35] like Clustered Regularly Interspaced Short Palindromic
Repeats (CRISPR/Cas9) could enhance the efficiency and cost-effectiveness of
variety development having novel traits in citrus.
. Ploidy manipulations
Polyploid organisms have a greater number of chromosomes compared with
their diploid progenitors. Breeders have utilized polyploidization for the investiga-
tion of inheritance patterns in genes of interest. Polyploids have shown tremendous
success in nature due to higher heterozygosity, less inbreeding depression, and
more tolerance to biotic and abiotic stress conditions compared with their diploid
progenitors [36–38]. The duplicated genes may evolve new functionalization
during evolution [39]. Polyploids have been reported in many fruit crops including
grapes, apples, strawberry, and citrus [40, 41], however, the frequency of sponta-
neous polyploid events is quite low and breeders prefer to induce hyperploidy using
different chemicals.
Among chemical mutagens, colchicine is mostly used for the induction of poly-
ploids due to its more reliability, higher efficacy, and cost-effectiveness. Colchicine is
an alkaloid derived from Colchicum autumnale (meadow saffron). It is used for induc-
ing chromosome doubling or developing tetraploids by restricting the chromosomal
segregation at metaphase in meiosis [42, 43]. Other methods of polyploid induction
include interploid hybridization [44], unreduced gamete formation [45, 46], and
endosperm culture [47, 48].
Members of the subfamily Aurantioidae including Citrus, Fortunella, and
Poncirus are mainly diploid having chromosome number 2n = 18 [49]. The occur-
rence of spontaneous polyploids in citrus is known since the 1940s [50]. Important
spontaneous polyploids include triploid Tahiti lime [51], Triphasia desert lime
[52], Clausena excavata [53], tetraploid mandarins [54], sweet oranges [41, 42] and
grapefruit [43]. In spontaneous polyploids, triploids and tetraploids are believed
to be formed by doubling of chromosomes in nucellar cells and fertilization of the
unreduced gametophytes [55, 56]. Polyploids have been induced using colchicine
in several citrus species and tetraploids produced have been used for interploid
crossing to develop triploid progenies that are usually seedless due to irregular
distribution of the chromosomes during cell division particularly gamete forma-
tion and formation of unreduced gametes. In interploid crossing, the formation of
tetraploids in addition to triploids indicates the predominant formation of the unre-
duced (2n) gametes which may be formed by the first division restitution (FDR)
or second division restitution (SDR) during meiosis. Production of 2n gametes was
predominantly via SDR in lemon [44, 45] and monoembryonic Orah mandarin
[57]. Higher tetraploid: triploid ratio in the progeny of the interploid hybridization
indicates greater production of the 2n megagemtophytes in that cultivar which is
promising to produce a greater number of polyploids.
. In vitro culture: somatic hybridization
Plant tissue culture tools offer advantages related to efficient regeneration,
propagation, and crop improvement in citrus and other horticultural crops.

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Citrus Biotechnology: Current Innovations and Future Prospects
DOI: http://dx.doi.org/10.5772/intechopen.100258
Endosperm cultures have been used for the development of triploids in citrus [48].
In interploid and wide hybridizations, the progeny may be sterile or have under-
developed or shriveled seeds with viable embryos. The embryo rescue technique
has shortened the breeding cycle and many plants have been recovered from these
embryos through in vitro culture in different citrus species [45]. Similarly, micro-
grafting is another tool in which a miniature bud is grafted under aseptic conditions
on in vitro raised rootstocks and micrografted plants have been reported in many
citrus varieties [58]. Micrografting is also useful for the production of virus-free
citrus plants.
Another highly promising and most widely used approach is somatic hybridiza-
tion which is utilized to overcome sexual incompatibility and to enhance genetic
variability by combining nuclear and organelle (chloroplast and mitochondria)
genomes followed by their characterization for hybrid confirmation and variability
assessment [59]. The organelle genomes are known to encode genes related to
photosynthesis and male sterility and new hybrids could be developed having novel
genetic recombinations. Somatic hybrids may be developed through electrofusion
of plant embryogenic protoplasts predominantly with mesophyll protoplasts. The
plant progeny having nuclear origin could be characterized and separated using
flow cytometry and molecular markers [60].
Protoplast fusion of distantly related citrus species bypasses the biological barri-
ers and develops allopolyploids that could not be obtained through classical breed-
ing. Somatic hybridization is an important tool and has been widely used in citrus
scion and rootstock breeding. The first intergeneric allotetraploid somatic hybrid
of Trovita sweet orange and Poncirus trifoliata was reported by Ohgawara et al. [61]
followed by several interspecific and intergeneric hybrids in citrus from the USA
[62], Japan [63], and other citrus-producing countries. Triploids were also reported
from interspecific and intergeneric somatic hybridization of Citrus species, kum-
quats (Fortunella japonica), and Poncirus trifoliata by protoplast fusion [64]. Fusion
of protoplasts from the haploid lines and diploid cultivars may also yield triploids
and hundreds of triploids and tetraploids were developed and planted for field
evaluations [65]. Polyethylene glycol may also be used to induce regeneration in
the fused protoplast cultures as reported in Willow leaf mandarin (embryogenic
parent) and Duncan grapefruit and sweet orange (mesophyll parents). The regener-
ated plants were identified as alloplasmic cybrids [66].
Polyploids developed through somatic hybridization have also shown enhanced
tolerance to abiotic and biotic stress conditions. Allotetraploids of cv. FlhorAG1
(FL-4x) developed by somatic hybridization of diploid Poncirus and Citrus showed
greater tolerance to cold and higher light conditions compared with parents (dip-
loid) and their tetraploids [36]. Kumquats (Fortunella species) chloroplast have
demonstrated higher resistance to canker in diploid kumquats and their tetraploid
somatic hybrids developed with other citrus species including grapefruit [37].
. Innovative approaches/technologies
. Transgenesis
Since the advent of recombinant DNA technology, transgenesis has proved its
significance, and 190.4 million hectares of transgenic crops were grown in more
than 29 countries in 2019. They have significantly contributed to food security,
climate change mitigation, sustainability thus uplifted the lives of 17 million biotech
farmers worldwide. The first transgenic plant was developed in the 1980s and was
available as commercial food in the 1990s. More than 400 transformation events

Citrus

have been approved so far wherein 356 events have been approved for crop plants,
23 for ornamentals, 22 for fruit plants, and 2 for trees. Hence a wide range of plant
species (maize, cotton, canola, papaya, rice, tomato, sweet pepper, squash, popular,
petunia, sugarcane, alfalfa, and citrus) have been engineered for various valu-
able traits i.e. insect resistance, herbicide tolerance [67], abiotic stress tolerance,
improved nutritional value, and disease resistance. In addition to nuclear transfor-
mation plastid genome has also been targeted and has proved to be of more value as
multiple genes can be introduced at a specific target site, the transgene is contained
owing to maternal inheritance, and hyperexpression of the transgene, etc. [68, 69].
Citrus is an economically important fruit crop worldwide. It not only provides
essential minerals and vitamins but is also of great commercial importance.
Conventional research has contributed a lot to the improvement of this fruit yet
serious problems are evolving which are difficult to tackle with these conventional
approaches [70]. Juvenility, sexual incompatibility, high heterozygosity, apomixes,
large plant size, and nucellar polyembryony, and certain other biological limita-
tions hinder the improvement of these plant species through conventional breed-
ing. Genetic manipulation through advanced innovative techniques is a potential
approach to improve crop plants as well as fruit species. Though citrus species
are recalcitrant to transformation and subsequent rooting, yet consistent efforts
by the researchers have resolved these bottlenecks and proficient protocols have
been established. Likewise, various transformation methods i.e. Agrobacterium-
mediated transformation [71], biolistic transformation [72], and chemically
assisted uptake of recombinant DNA by protoplasts [73] have been attempted to
introduce genes of agronomic value as well as to strengthen it against bacterial,
viral, and fungal pathogens (Figure ).
Genetic manipulation of vegetatively propagated crops like citrus is very tricky
as the expression of transgenes over a long period during numerous cycles of graft
propagation should be stable.
The first attempt to produce transgenic citrus was made in the 1980s wherein
protoplast transformation was attempted but it was not successful. The first authen-
tic report was published by Kaneyoshi et al. [74] who reported transforming NPT II
and GUS genes into trifoliate orange through Agrobacterium. Epicotyls of the afore-
mentioned citrus species were used to transform with the selectable marker gene as
well as reporter gene and more than 25% transformation efficiency was achieved.
Likewise, Yao et al. [72] reported the first successful transformation through gene
gun. They transformed tangelo (C. reticulata × C. paradisi) embryogenic cells.
Since genetic transformation has successfully been performed in different
species and hybrids including Carrizo citrange, Washington naval orange, Poncirus
trifoliata, Sour orange, Mexican lime, sweet orange, Citrus reticulata [75], and a
valuable rootstock, swingle citrumelo. Similarly, protocols have been optimized
for the genetic transformation of different citrus species by using different explant
tissues including seeds, embryogenic cells, epicotyls, embryogenic cells, callus,
nodal stem segments, and protoplasts. The most responsive explant tissue has been
epicotyl from the in vitro germinated seedlings and is preferably used for genetic
transformation research. Duncan grapefruit was successfully transformed through
Agrobacterium for the first time using epicotyl and confirmation of the transgene
(NPTII and GUS) integration was carried in the resultant 25 transgenic plants by
histochemical staining, PCR, and Southern blot hybridization. Transgenic grape-
fruit, sweet orange, and citrange plants were developed using epicotyls as target
explant whereas selection was carried out on kanamycin [76]. Epicotyl has also been
used for Agrobacterium-mediated transformation of citrange and sweet orange
[77]. In addition, callus, as well as suspension cultures derived from different parts
of flower and seed, have also been attempted to transform. The transformation

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Citrus Biotechnology: Current Innovations and Future Prospects
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efficiency attained, in this case, was lower than 0.5%. Genetic transformation has
also been optimized in pomelo (Citrus grandis) and sour orange wherein internodal
stem segments were used as explants and a promising transformation efficiency was
achieved (91%) [78].
The biolistic transformation has also been performed successfully in tangelo
(C. paradisi Macf. x C. reticulata Blanco) using nucellar embryogenic cells raised
from the suspension culture and more than 15 stable whereas 600 transient trans-
genic lines were attained per bombardment. The transformed calli cells showed
proficient growth on kanamycin selection medium and positive GUS activity but
were not able to regenerate into plants. Calli treatment with 0.3M osmoticum
sorbitol and 0.3M mannitol appeared to have positive effects for enhanced
transformation efficiency for stable and transient transformation. Thin epicotyl
segments of germinated seedlings were also targeted through the biolistic gun and
more than 93% transformation efficiency was attained for transient transgene
integration in C. citrange. The incubation of explant on culture medium before
bombardment appeared to have profound effects on transformation efficiency
which was further improved [79].
Since transgenic technology is the most reliable intervention having the mas-
sive potential to improve the citrus crop. The introduction of alien genes is only
possible through this technology so any of the desired traits can be engineered.
Figure 1.
Schematic sketch showing the importance of conventional and advanced innovative approaches for the
improvement of different citrus species.

Citrus

Recent research indicates that citrus growing farmers are facing severe problems
due to biotic and abiotic stresses i.e. salinity, cold, drought, and diseases. Hence,
the development of improved citrus varieties is direly needed to get a quality crop.
Various citrus species have been engineered with alien genes to combat abiotic
stresses including salinity and drought. Expression of HLA2 gene, isolated from
yeast imparted salinity tolerance and resultant transgenic plants were able to toler-
ate a higher level of the salts as compared with non-transformants [80]. PsCBL and
PsCIPK derived from Pisum sativum were transformed into Citrus sinensis and Citrus
reticulata by targeting calli derived from mature seed. Bacterial strain LBA4404 was
used to induce infection in the target calli cells. The putative transformants showed
better performance as compared with control plants for salinity and drought toler-
ance when tested under in vitro conditions [81].
Citrus paradisi was transformed to improve carotenoid content by manipulating
the genes involved in carotenoid biosynthesis i.e. phytoene synthase, lycopene-
β-cyclase, and phytoene desaturase. The multigene transgenic citrus plants were
aimed to supplement human nutrition with vitamin A along with antioxidants.
Similarly, fruit juice quality has been attempted to improve in Valencia orange, a
valuable variety that is majorly grown for its juice. Degradation of TSPME (ther-
mostable pectin methylesterase) deteriorates the quality of the juice. This TSPME
is encoded by the CsPME gene. The protoplasts were isolated from embryogenic
suspension cultures and transgene was introduced through the PEG mediated
transformation method [82] aiming at down-regulation of the CsPME resulting in
the citrus with improved juice quality.
Citriculture is prone to be infected by a wide range of diseases that are controlled
by chemicals, a drastic non-environmentally friendly strategy. Different types of
viral, bacterial, and fungal pathogens infect these plants resulting in drastic losses
to crop production and quality of the produce. A range of transgenic citrus lines
have been developed varying from fully resistant to susceptible to the diseases. Coat
protein (p25) from the CTV (Citrus Tristeza Virus) was expressed in Mexican lime
and 33% of the transgenic plants were found to be resistant, neither symptoms
appeared nor the viral load was detected. Accumulation of siRNA (small interfering
RNA) in the transgenic lines resulted in resistant phenotype and plants were able
to withstand viral infection [83]. Expression of viral coat protein (part of p gene
and the 3UTR), in the sense and antisense orientation also delayed viral infection in
grapefruit [84].
Phytophthora is a noxious fungal pathogen that has been reported to infect a
wide range of citrus species. Among these Phytophthora parasitica and Phytophthora
citrophthora cause more severe damage in the citrus orchards and nurseries all over
the world [85]. Expression of bo gene (bacterio-opsin) in Rangpur lime rootstock
showed an elevated level of tolerance against Phytophthora parasitica infection. It
was observed that expression of the aforementioned gene led to induce expression
of defense-related proteins; chitinase, salicylic acid, and glucanase. Hence plants
with an elevated level of transgene expression showed greater resistance against
the oomycetic fungi including Phytophthora parasitica. Transgenic citrus plants
expressing the tomato PR gene showed an enhanced survival rate in the presence
of pathogen (P. citrophthora). Transgenic grapefruit plants were able to better
withstand citrus scab infection when transformed with the attE gene encoding for
antimicrobial peptide [86].
Transgenic technology has also played a pivotal role to tackle another noxious
disease in citrus i.e. Huanglongbing (HLB) which is supposed to be caused by
phloem-restricted Gram-negative bacteria; Candidatus Liberibacter americanus,
Candidatus Liberibacter asiaticus, and Candidatus Liberibacter africanus [87].
Various genetic strategies have been tested to develop HLB-resistant citrus lines

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Citrus Biotechnology: Current Innovations and Future Prospects
DOI: http://dx.doi.org/10.5772/intechopen.100258
with decreased susceptibility to the pathogen. These include the expression of
anti-microbial peptides from a bacterial, fungal, plant, or animal origin and
engineering host-pathogen interaction pathways. The expression of antimicrobial
proteins under phloem-specific promoters has been an effective strategy to control
this phloem-resident pathogen. Overexpression of synthetic cecropin B gene under
phloem specific promoter resulted in reduced bacterial population after one year
of inoculation and no disease symptoms appeared even after two years of inocula-
tion [88]. Overexpression of modified methionine under double 35S promoter also
appeared to have an inhibitory effect on bacterial growth and lowered down the
CLas (Candidatus Liberibacter asiaticus) titer in the roots of transgenic Carrizo
citrange (rootstock). Further, newly emerging leaves from the rough lemon, grafted
on this transgenic rootstock, also had a non-detectable bacterial titer. Expression
of AtNPR1 and chimeric proteins (ThioninLBP and Thionin1-D4E1) demonstrated
elimination of CLas providing tolerance against HLB infection [89].
Another economically important disease, the citrus canker has also been
addressed through transgenesis resulting in enhanced tolerance against
Xanthomonas citri. Engineering sweet orange genome with Xa gene showed a
significant reduction in disease severity upon inoculation in three lines Hamlin,
Pera, and Natal. Expression of the Xa gene under its promoter appeared to
be more effective in disease resistance when expressed in highly susceptible
Anliucheng sweet orange [90]. Transgenic Carrizo citrange and sweet orange
plants were developed by introducing RpfF from X. fastidiosa which encodes for a
quorum-sensing factor and can disrupt bacterial communication by reducing acti-
vation of virulence factors, thus enhancing the ability to tolerate pathogen infec-
tion. Similarly, the expression of AMP sarcotoxin from flesh fly also enhanced
tolerance against X. citri [91].
. Genome editing
Genome editing through CRISPR-Cas9 has emerged as a breakthrough technol-
ogy for the precise modification and manipulation of targeted genomic DNA. It has
extensively been exploited by several research groups [92] and certain successes
have been achieved. Three major sequence-specific engineered nucleases that have
so far been used for genome editing are CRISPR-Cas (clustered regularly inter-
spaced short palindromic repeats), TALENs (transcription activators such as effec-
tor nucleases), and ZFNs (zinc finger nucleases). Among these, the CRISPR-Cas9
editing system has been established in many plant species through gene activation,
repression, mutation, and epigenome editing in wheat, rice, maize, tomato, potato,
carrot, apple, grape, and citrus. Even a few of the genome-edited crops have been
approved for commercial cultivation. Through this technology, field crops as well
fruit crops can not only be manipulated for improved agronomic traits but can also
be manipulated for improved nutritional value [93].
For citrus, genome editing research is at infancy, yet few successes have been
achieved by editing its genome for enhanced resistance against diseases. The CRISPR-
Cas9 system was firstly used to target the CsPDS gene in Duncan grapefruit and sweet
orange. The target gene was successfully modified through a transient expression
method, Xcc-facilitated agroinfiltration [94]. The modified CsPDS sequence was not
detectable in the leaves of sweet orange indicating that CRISPR/Cas9 has induced the
desired mutation successfully.
Most of the research studies were carried out to target the LOB (LATERAL
ORGAN BOUNDARIES 1) gene which has been characterized as a citrus susceptibil-
ity gene for Xanthomonas citri. The said gene has been explored to be upregulated by
TAL (transcription activator-like) effector PthA4 which binds to the EBE (effector

Citrus

binding elements) in the promoter region of LOB1 thus activates expression of this
canker-susceptibility gene [95]. Mutation in the single allele of effector binding ele-
ments of the LOB gene resulted in minor alleviation of canker symptoms in Duncan
grapefruit. Anyhow, a mutation in effector binding elements of both of the alleles
of LOB1 promoters alleviated canker symptoms to great extent thus showing a high
degree of resistance in Wanjincheng orange [96]. Another research group explored
that editing the coding region of LOB1 in Duncan grapefruit, through the CRISPR-
Cas9 system also provides resistance against X. citri infection.
A marker gene for pathogen triggered immunity (CsWRKY22) was knocked out
in Wanjincheng orange and the resultant mutant plant showed a decreased level of
susceptibility to the canker disease [97]. In addition to the CRISPR-Cas9 system,
another improved genome editing system (CRISPR/Cas12a (Cpf1) has also been
used to edit the CsPDS gene in the Duncan grapefruit gene. It appeared to be a more
efficient editing system with lower off-target effects thus will prove a great mile-
stone in citrus genome editing [98]. These studies indicate that CRISPR-mediated
genome editing can be a promising pathway to generate disease-resistant citrus
cultivars [99].
. Multi-Omics technology: An integrated approach and useful strategy
for the improvement of the citrus crop
MultiOMICS including genomics, transcriptomics, proteomics, metabolo-
mics, interactomics, and phenomics approaches have massive potential for citrus
improvement just like other crops and fruits. In all disciplines of OMICS, various
techniques can be utilized for genome analyses, transcripts, proteins, metabolites,
and interactions between different molecules to indicate the molecules which may
result in crop improvement.
. Genomics
The field of genomics is a highly applicable part of Omics technologies. It
is based on sequencing technologies and the analysis of subsequent genome
sequences. Many advanced techniques in genomics for example sequence determi-
nation DNA, marker-assisted selection, and transition from marker-assisted selec-
tion to genomic selection assist in quick varietal development. Genome sequencing
technologies have brought about a revolution in the field of biology. It has also
transformed the citrus breeding that helps to understand a relationship between
the genetic makeup and response towards various abiotic and biotic stresses like
Alternaria brown spot [100].
A specific pathotype of Alternaria alternata (Fr.) Keisel is a disease with heavy
losses [101]. It causes necrosis and resultant lesions on the surface of fruits and
young leaves. It leads towards defoliation and fruit drop [102]. Thus, exploitation of
innate genetic resistance appears to be the most applicable and effective strategy of
disease control. Currently, the control is primarily based on the application of 4–10
sprays of toxic and environmentally injurious fungicide per year [103]. Such limita-
tions are compelling farmers to find alternative cultivars with resistant ones [104].
Usually, the female parent transmits the 2n gamete in 2x × 2x citrus crosses
[105–108]. Cuenca et al. [109] recognized ABS resistance locus containing genomic
region by using BSA-genome scan combined with HTA based. Trait segregation in
crosses between two heterozygous ABS-susceptible or between heterozygous ABS-
susceptible parents and resistance was used to confirm the recessive inheritance of the
ABS resistance in triploid populations. ABSr locus was first located at 10cM from a


Citrus Biotechnology: Current Innovations and Future Prospects
DOI: http://dx.doi.org/10.5772/intechopen.100258
centromere based on segregation of 368 SDR 2n gametes. A genomic region containing
several markers with a high probability (> 99%) of association with phenotype varia-
tion was identified on chromosome III by performing BSA over 93 triploid hybrids
from a Fortune × Willow leaf population. This identified region contains 25 significant
SNP markers within an interval of 13.1cM. The size of the genomic region among these
two markers is 15Mb. Linkage genetic mapping was performed on identified genomic
regions by developing new SNP and SSR markers. A 268-diploid mapping population
was performed by Cuenca et al. [110] from a heterozygous-susceptible × resistant
hybridization. Fine mapping was performed to confirm the location of ABSr locus in
a region of 1.1cM between the markers SNP05/SNP06/SNP07/AT21 (at 0.7cM) and
SNP08 (at 0.4cM). Another region containing eight genes with NBS-LRR repeats was
identified by the SNP08 marker and considered ABS resistance genes.
In citrus plant, molecular markers are linked to some agronomic traits, e.g.
SSR markers are linked to Citrus tristeza virus resistance from Poncirus trifoliate,
PCR assay for the anthocyanin content of pulp [111], AFLP markers are associated
with polyembryony [112] and RAPD markers are associated to dwarfism and fruit
acidity [113]. Some other characteristics such as salinity tolerance and nematode
resistance are linked to QTLs [114]. The selection of resistant genotype at the early
growth stage was improved by the newly developed SNP08 marker. This marker
was mapped at 0.4cM from the ABD resistance gene and it has role in avoiding the
selection of susceptible varieties. On the other side of the gene, some new markers
were also identified at 0.7cM from the ABSr locus. Combining these new markers
with SNP08, the probability of selection of resistant genotype was increased by
0.0028%. This marker appeared to be very helpful in the selection of resistant and
susceptible genotypes and for analyzing the resistant germplasm to configure the
ABS genes. So, it is a very valuable tool for the selection of susceptible heterozygous
cultivars which may be used as breeding parents allowing manipulation of genetic
diversity in citrus and prevents susceptible homozygous genotypes.
About 40 mandarin genotypes (susceptible and resistant) were tested by the
SNP08 marker and were used as breeding parents. An ultimate association was
observed between response to Alternaria infections and SNP08 marker. Recently
SNP08 is used in breeding programs of citrus performed at CIRAD and IVIA for the
selection of ABS-resistant citrus genotypes. About 2187 resistant hybrids were selected
from 4517 total hybrids rising from 10 different parental combinations by using the
SNP08 marker since its development. This analysis was very helpful to prevent the
growth of more than 2000 susceptible lines which were removed at the early growth
stage after selection so, a lot of time, cost, personnel, and resources were saved.
. Proteomics and metabolomics approaches
Proteomics is the comprehensive analysis of all the proteins found in a cell. It
includes the identification of proteins, their location in the cell, their interactions
with other proteins and other biological components in the cell, and most impor-
tantly post-translational modifications that a protein undergoes in the cell [115].
Metabolites are referred to as the last product of any biological activity in a cell and
are found in very small quantities [116]. Metabolites are small molecules including
intermediates of various metabolic reactions, signaling molecules, hormones, and
other regulatory products found in a cell. Hence, metabolomics is defined as the
study of metabolites of a cell [117, 118]. It is estimated that around five thousand
metabolites are found in any cell depending upon the physical and chemical com-
plexity of that cell [119].
Huanglongbing (HLB) is considered one of the most devastating citrus diseases
that affect not only the production but also the quality of citrus fruit and its juice.

Citrus

Using a combination of proteomics and metabolomics approaches it was found that
in symptomatic fruit, the expression of proteins found in the cytoplasm for glycoly-
sis, in mitochondria involved in the tricarboxylic acid (TCA) cycle, and in chloro-
plasts for the synthesis of amino-acids was downregulated. Similar downregulation
was observed for genes involved in terpenoid metabolism for example valencene,
limonene, 3-carene, linalool, myrcene, and aterpineol in fruit found on infected
trees. Similar phenomena were observed for sucrose and glucose. Hence, the off-fla-
vor found in symptomatic fruits was linked to the downregulation of the above genes
and a decrease in the levels of the abovementioned secondary metabolites [120].
In another study, comparative iTRAQ proteomic profiling was carried out using
the fruits of sweet orange which was grafted on sensitive and tolerant rootstocks
infected by CaLas. The results showed that symptomatic fruit on sensitive rootstock
exhibited a greater number of differentially expressed (DE) proteins as compared
to the healthy fruit on a similar rootstock. It was also found that the expression level
of various defense-related proteins was reduced in symptomatic fruit on sensitive
rootstock, particularly the proteins related to the jasmonate biosynthesis, is signal-
ing, protein hydrolysis, and vesicle trafficking. Hence, it was concluded that the
down-regulation of these proteins is likely to be linked with the sensitivity of citrus
to the CaLas pathogen [121].
. Interactomics and metabolomics and phenomics
Interactomics bears a broad scope as it may cover a complete set of interactions
in a cell [122]. It covers every type of interaction among interacting molecules
including proteins and other molecules. It is a well-known fact that the Protein–
protein interactions are major of all cellular processes [123].
To designate the complete phenotype of a plant, the term phenome is used.
Similarly, a phenotype encloses a group of traits that are liable to be distinguished
either by utilizing modern science analytical techniques or by a naked eye evalu-
ation. These traits can also be attributed to being an interaction between external
factors (environment) and Genotype. David Houle also termed phenomics as the
collection of data from varying backgrounds and dimensions in a single entity
[124]. Phenomics involves both “extreme phenotyping,” referring to a compre-
hensive selection of a wide range of valid and correct phenotypes, and “phenome
analysis” indicating towards an analysis of specimen and correlation between
syndication of genotype and phenotype.
Plant phenomics utilizes screening of large populations to analyze genetic
mutations found in the population for a specific trait (drought, salinity, or high-
temperature stress tolerance). Various types of imaging techniques are employed
in the phenotyping of plants for various growth and developmental processes.
The techniques include visible-light imaging [125], Thermographic imaging [126],
Hyperspectral imaging, Chlorophyll fluorescence, X-ray, MRI, PET [127].
Using the phenomics approach and tools, we can study the traits regarding plant
growth, leaf growth, root growth, and architecture of soil/root interaction, etc.
This extensive use of phenomics and its integration into OMICS is the need of the
hour to combat food security issues and overcome adverse effects of climate change
on crop production.
. Conclusions
Conventional research has played a pivotal role in the improvement of citrus.
Enhanced heterozygosity has helped in the development of genetically diverse


Citrus Biotechnology: Current Innovations and Future Prospects
DOI: http://dx.doi.org/10.5772/intechopen.100258
Author details
GhulamMustafa1, MuhammadUsman2, Faiz AhmadJoyia1
and Muhammad SarwarKhan1*
1 Centre of Agricultural Biochemistry and Biotechnology (CABB), University of
Agriculture, Faisalabad, Pakistan
2 Institute of Horticultural Sciences, University of Agriculture, Faisalabad, Pakistan
*Address all correspondence to: sarwarkhan_40@hotmail.com
germplasm in most of the citrus species and numerous varieties have been released
for commercial cultivation. However, with the advent of modern biotechnologi-
cal tools, the period involved in crop improvement through indirect mutagenesis
and polyploidization could be further reduced and enhancing cost-effectiveness.
Transgenic technology and OMICS have great potential to improve this fruit crop.
MultiOMICS, integrative-OMICS, or panOMICS technologies may result in better
crops having better agronomic traits, enhanced yield potential, and less prone to
insect pests. It will ultimately lead towards food security and poverty alleviation.
Various OMICS technologies have been used for crop improvement, yet their
integrated use will further strengthen the application of this robust technology.
Still, there are many challenges associated with tolerant varieties which need to be
fine-tuned. Moreover, three thousand reports of enhanced drought and salinity
tolerance in wheat, sorghum, canola and rice are present but none of them is in use
by farmers. A fundamental reason for this is that salinity and drought are complex
multigenic traits. So, to induce tolerance in plants every gene needs to be fine-tuned
precisely. However, their evaluation in the field is a long way, and distribution at the
commercial level is also a hurdle in their production.
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms
of the Creative Commons Attribution License (http://creativecommons.org/licenses/
by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.

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