wheat hybridization.pdf

WHEAT Wheat Classification: Common wheat, sp. Kingdom: Plantae - Plants Subkingdom: Tracheobionta - Vascular plants Superdivision: Spermatophyta - Seed plants Division: Magnoliophyta - Flowering plants Class: Liliopsida - Monocotyledons Subclass: Commelinidae Order: Cyperales Family: Poaceae - Grass family Genus: Triticum - wheat Species: Triticum aestivum - common wheat Other Species: T. aestivum, T. aethiopicum, T. araraticum, T. boeoticum, T. carthlicum, T. compactum, T. dicoccoides, T. dicoccon, T. durum, T. ispahanicum, T. karamyschevii, T. macha, T. militinae, T. monococcum, T. polonicum, T. spelta, T. sphaerococcum, T. timopheevii, T. turanicum, T. turgidum, T. Urartu, T. vavilovii, and T. zhukovskyi. Introduction: It is the most important human food grain and ranks second in total production as a cereal crop behind maize and the third being rice. Wheat is the staple food for over ten billion people in as many as 43 countries of the world. Wheat provides nourishment to 35% of world population. Wheat cultivation has traditionally been dominated by Bangladesh. Hexaploid wheats, widely grown to day thought to have evolved before 7000 BC in an area from just south of the Caspian Sea in Northern Iran Eastward into Northern Afghanistan. De Candolle believed – Valley of Euphrates and Tigris. But, Vavilov Origin of Durum wheat probably Abyssinia Soft wheat groups. In the region of Western Pakistan, SW Afghanistan, and Some parts of mountainous Babshara. Seed enterprises consider ‘wheat seed’ to be of secondary importance, since it is a self-pollinating crop and the grain can be used as seed, farmers tend to replant their own seed. In last ten years significant efforts have been made for commercial exploitation of hybrid wheat through the use of gametocide and CMS lines. Today France and Italy are at the verge of commercial release of such wheat hybrids for grain purposes. The exact place and date of the origin of wheat plant that we recognize today is unknown. Globally, it is the most important human food grain. The flour obtained from wheat is unique. Wheat grain is a staple food used to make flour for leavened, flat and steamed breads; cookies, cakes, pasta, noodles and couscous. Fermentation to make beer, Alcohol, vodka. Bio-fuel- extraction of ethanol from straw. The husk of the grain, separated when milling white flour, is bran. Wheat is planted to a limited extent as a forage crop for livestock and the straw can be used as fodder. Floral morphology: Floral structure: The inflorence of wheat is called spike of spikelet (15-20 no.) Spikelets are borne on zig zag rachis in two alternate rows with a terminal spikelet. The spikelets are sessile and each spikelet contains 3-7 florets. Only lateral florets are fertile & the central ones may be sterile. At the base of each spikelet, there are two oppositely placed empty glumes. Each floret comprises of a lemma ending in awn, a palea, 2 lodicules, androecium & gynoecium. The androecium consists of 3 stamens with thin filament & large bilobed anthers. Gynoecium consists of monocarpellary superior ovary with 2 feathery stigma. Floral biology: Floral biology Main Culm flowers first & the tillers bloom later in order of their formation. Flowering starts at approximately 2/3 from the base and proceeds in both the directions. Blooming remains throughout the day & it takes 3-5 days for completion. Flower opening is usually during warmer part of the day i.e. , between 9 am to 2 pm and peak period between 10 am to 1 pm Anther dehiscence takes place simultaneously & hence the crop is highly self- pollinated ( < 1% cross pollination). Monocot species like wheat have caryopsis (cereal grains) as propagation units. Caryopses are single-seeded fruits in which the testa (seed coat) is fused with the thin pericarp (fruit coat). Cereal grains have highly developed embryos and in cereal grains the triploid endosperm consists of the starchy endosperm (dead storage tissue) and the aleurone layer (living cells). Organs of the cereal embryo are: coleoptile (shoot sheath), scutellum, the radicula & the coleorhizae (root sheath). Seed Seed development Stages in Wheat Kernels at various stages during grain filling: a) kernel at watery ripe b) kernel at late milk c) kernel at soft dough d) kernel at hard dough showing loss of green color e) kernel ripe for harvest Physiological maturity. When the kernels have attained maximum dry weight it is physically matured. Note the green color is gone from the peduncle and head parts. Breeding Objectives:       High grain yield, Early maturity, Photo and thermo insensitive varieties Resistance to disease like rust, smut and leaf spots. Response to high doses of fertilizers Dwarf and lodging resistant varieties. Hybridization Techniques of Wheat: Emasculation: The spike enclosed in leaf sheath or partially emerged is selected for emasculation. The awns and tips of spiklets are cut off to avoid obstacle in the process of emasculation and pollination, similarly the central sterile flower also removed with forceps. The requisite numbers of spikelet are kept on the spike and with the half of forceps the glumes are separated and three young immature greenish yellow anthers are removed from each flower and the flower bagged. Pollination: On the next morning between 9.00 am to 11.00 am the pollen grain collected desired protected plant in petridish and dusted on stigma of emasculated flower with the help of hair brush. The spike is covered with bag after pollination and labelled again. Breeding Methods: 1) Introduction: The green revolution is successful in the world due to introduction of Norin-10 variety (dwarfing gene) developed in Japan. The variety Norin-10 was never important variety in Japan. The seed sample received in 1946 to Washington State University and Crosses were made in 1948 worht Brevor – 14 and the genotype become main source of two Norin-10 dwarfing gene. Then Dr. N.E. Borlaug (Father of green revolution) who engineered development of semi dwarf wheat. Or his work he was awarded a Nobel peace Prize in 1970. In India, the dwarf wheat varieties were importance from Mexico, Sonoro-64, and Larma Rojo- 64 A in 1965-66. Latter on made green revolution successful India. 2) Pure Line Selection: In this method individual progenies are evaluated and promising progenies are finally selected old Indian tall varieties E.g. N-P-4, N-P-6, N-P-12, PB-12, PB-11 were developed by pure line selection. 3) Pedigree Method: The most common method used in self-pollinated crops is pedigree method of selection. The crosses are made between complimentary lines and records are maintained of selections made over number of generations. The procedure provide selection opportunities generation after generations. It allow breeder to identify bet combination with considerable uniformity. The hybrid bulk selection method is relatively inexpensive, in which generations are advanced without selection till F5 to F6 and much material can be handled, nut often difficulty is isolation of superior recombination. To overcome, this difficulty single decent method of selection is used in which population remain constant over segregation generations. 4) Back Cross Method: This method is used when variety otherwise is good, high yielding but deficient in simply inherited trait. The obvious effect of this method the production potential of improved variety is fixed at the level of recurrent variety. Recently identified donors always are used in back cross breeding programme. Stem Rust: Resistance gene- Sr2 From variety Hope. Leaf Rust: Resistance gene – Lr 13 from variety Sonalika. 5) Multiline Breeding: It is extension of back cross breeding and could be called Multilateral backcrossing. It consist of spontaneous back cross programme to produce isogenic lines for resistance to disease, in back ground of some recurrent parent. Each isogenic line will be similar to recurrent parent but they will differ for resistance to various physiological farms of diseases. A mixture of these isogenic lines is called multiline variety. 6) Mutation Breeding: This method is used in depleted gene pool situation. Chemical mutagenes EMS provide broad spectrum genetic changes with lesser sterility effects, as compared to X ray or particular mutation. Varieties developed are 1) NP836, Sarbati Sonora, Pusa larma, etc. are examples of induced mutation and NP-11 is the examples of spontaneous mutation. Hybrid Wheat Breeding Problems: Hybrid wheat breeding is not commercially successful through cytoplasmic genetic male sterile lines are available due to following problems. • • • Inadequate heterosis over wide range of environment. Inadequate genetically controlled fertility restoration. High cost of hybrid seed production. Heterosis in wheat The main goal of hybrid breeding is to systematically exploit heterosis. Heterosis of a hybrid is expected to increase with the genetic divergence between its parents (Melchinger, 1999). Consequently, grouping of lines into genetically divergent heterotic pools is of paramount importance to make maximum use of heterosis (Reif et al., 2005). Genetically divergent groups are not expected to exist in wheat elite germplasm adapted to a particular target environment, because of the intensive exchange of elite lines. Use of lines from different target environments has been suggested as a method to promote genetic diversity among pools (Koekemoer et al., 2011). However this approach is complicated by the different requirements for vernalization, photoperiod, quality, and frost tolerance. Consequently, sophisticated solutions are required to develop genetically distinct groups with high heterotic combining ability for grain yield combined with high end-use quality (Longin et al., 2012). An improved understanding of the underlying genetic mechanisms of heterosis represents a key step towards a systematic development of complementary groups of lines exhibiting high heterosis. Heterosis and hybrid performance of complex agronomic traits such as grain yield is very probably influenced by many loci. Genomic selection has been suggested to predict the phenotype for traits that are controlled by multiple genes with small effects. In this approach, a large number of markers distributed across the genome are used simultaneously to train a prediction model (Meuwissen et al., 2001). Genomic selection has been used successfully to predict hybrid performance in wheat (Zhao et al., 2013). Using diversity in floral traits to breed ‘male’ and ‘female’ ideotypes Redesigning the wheat flower will be important for efficient production of hybrid seed. While it is a complex procedure, this is an achievable target, as our understanding of the control of floral architecture has greatly improved over the past few years (Barazesh and McSteen, 2008; Thompson and Hake, 2009). Wheat flowers are composed of spikelets which are made up of bract-like organs, glumes, and florets. The lemma and palea envelop the male and female reproductive organs. At anthesis, rapid swelling of a small organ located at the base of the floret, called the lodicule, opens the floret and exposes the anthers and pistil for pollination, a state called chasmogamy. Wheat flowers are largely cleistogamous, and pollen is shed before or just after flowers start opening. Stiff glumes, lemmas, and paleas are often found in common wheat varieties, and are associated with traits that prevent flower opening and kernel shattering (Vogel, 1941; Zhang et al., 2009). Figure: Structure of wheat flowers and spikes. (A) Wheat spikelet. (B) floret. (C) Palea and reproductive tissues.(D) Lodicule and female reproductive tissues. (E) Secale cereale (rye) floret. (F) Spikelets of various Triticeae species, from left to right: rye, T. monococcum ssp. boeticum, three T. aestivumvarieties (Chinese spring, Magenta, and Kite) and T. aestivum landrace. (G) Spikes of various Triticeae species, from left to right: rye, T. monococcum ssp. boeticum, T. turgidum ssp. Durum, and five T. aestivum (bread wheat) varieties (Chinese Spring, Cadoux, Ghurka, amd Sentinel, Kite). Bars, 5mm (A–C, E, F); 2mm (D); 5cm (G). A, awn; An, anther; G, glume; L, lemma; Lo, lodicule; O, ovary; P, palea; S, stigma. Ideally, both male and female parental plants for hybrid seed production would possess open flowering spikelets and the following desirable traits to achieve cross-pollination. Large lodicules, a soft lemma, and palea in well-spaced spikelets along long spikes (Murai et al., 2002) would also enable each floret to open widely. The male ideotype plant would be tall with long extruded anthers producing large quantities of long-life pollen able to disperse metres away. In comparison, the female ideotype would be a shorter plant with multiple chasmogamous florets to maximize pollen reception. Stigmatic hairs would be long, fully extruded and receptive for extended periods. Most importantly, the female ideotype should be male sterile and/or self-incompatible. Therefore preventing self-pollination and ensuring cross-fertilization for commercial hybrid seed production. This also allows row interplanting or mixed planting of male and female parental lines. Finally, the flowering time of male and female plants should be synchronized. Figure: Differences between fertile and sterile wheat flowers and spikes. (A) Fertile (left) and sterile (right) spikes of wheat. (B) Fertile spikelet. (C) Glumes and lemmas removed from the fertile spikelet in (B). (D) Sterile spikelet. Arrows indicate stigmas extruded from the floret. (E) Glumes and lemmas removed from the sterile spikelet in (D). The unfertilized ovary expands horizontally opening the floret. Bar, 5mm. Variation and heritability estimates for most of these traits in cultivated wheat are often moderate to high (Virmani and Edwards, 1983), suggesting that much progress can be made in improving the cross-pollinating ability of inbred parents derived from breeding populations. For several of these traits, the genetic control appears to be simple, implying that single or few genes are responsible for the phenotype. The diversity among grass inflorescences is a result of variation in the identity and determinacy of floral meristems produced throughout inflorescence development. A range of synthetic wheats, where variation in floral morphology has been reported, is available (e.g. Chhabra and Sethi, 1991; Murai et al., 2002; Yang, 2010). Mutants and associated genomic and genetic resources widely seen as a model for wheat, have provided powerful new tools for the isolation and characterization of genes controlling developmental processes of flower formation (Druka et al., 2011). Some close relatives of wheat, such as rye (Secale cereale L.), are obligate outcrossers and possess a floral architecture that enhances cross-pollination. Barley mutants showing altered floral development, which could be exploited for the molecular identification of wheat floral genes Manipulation of these genes in wheat could help either enhance cross-pollination or increase hybrid seed set per spike (Takahashi, 1972; Larsson, 1985). Class Name Developmental effect com1.a Compositum Branched spikelet dub1 Double seeds 1 Fasciation of the floret (wide lemma trait) resulting in the Class Name Developmental effect formation of double (dub1) and triple-kernel mutants Flo Extra floret A single adventitious floral bud (spikelet) occasionally arise below the central bud and form an extra floret Int Intermedium spike Extra spikelet Laxatum Rachis internodes; conversion of the lodicules into anthers in lax-a but the extra anthers are deficient Dense spike Dense or compact spike; rachis internode length Multi-ovary The lodicules of the mov1 mutant become somewhat leafy or sepallike, stamens are partially or completely converted into pistils Multiflorus1/supernumerary florets Number of floral buds increase, 2 or more florets are produced within a spikelet, the alternating florets face each other and the multi-floreted structure is contained within a pair of glumes Lax Dsp mov1/mov2 mul1/vrs4 Changes in floral meristems have also been important in crop domestication. For example, the domestication gene Q is a major regulator of floral architecture and has resulted in the introduction of free-threshing characteristics and a compact spike in cultivated wheat (Simons et al., 2006). Such floral characters have inadvertently reduced cultivated wheat’s ability to cross-pollinate. Genes controlling floral architecture in wheat Floral development of monocotyledonous and dicotyledonous species can be explained by an ABCDE model whereby organ identities are determined by a specific class or a combination of classes of genes (Coen and Meyerowitz, 1991). Some of these regulatory genes are well conserved across species. It is anticipated that this model could be partially translated to wheat floral development (Ciaffi et al., 2011) and be exploited for the purpose of redesigning the floral architecture of male and female plants. There are a number of cereal genes, such as various MADS box genes, involved in floral determinacy and differentiation of the glume, lemma, and lodicule (Sreenivasulu and Schnurbusch, 2012). For example, OsMFO1 regulates palea and lodicule identity in rice (Ohmori et al., 2009), and TaQ is involved in the determination of glume shape, lodicule size, and other floral traits in wheat (Simons et al., 2006). These genes are potential targets for manipulating wheat’s floral architecture. Transcription factors that alter flower morphology in wheat as potential molecular targets for allele mining. Class APETALA2 APETALA3 AGAMOUS -LIKE6-like MADS box Wheat and barley genes HvCly1,TaQ TaAP3, HvAP3 TaMADS16, HvA GL6 Orthologue gene Orthologue s and developme ntal role Referenc es indeterminant spikelet1 (ids1, maize) Lodicule size; spike morphology ; number of florets per spikelet; sex determinati on in the tassel and branching in inflorescenc es. Nair et al. (2010); Chuck et al.(2007) SUPERWOMAN1 OsSPW1 Identity of stamens and lodicules Nagasawa et al. (2003) MOSAIC FLORAL ORGANS1(OsMFO1/M ADS6) Identity of lodicules and ovules; lodicule size Ohmori et al. (2009) Class Wheat and barley genes AGAMOUS TaWAG SHORT INTERNOD ES KN1-like homeobox Orthologue gene Orthologue s and developme ntal role Referenc es Stamen developme nt Meguro e t al. (2003) Awn elongation; pistil morphology HvLks2 TaWKNOX1, HvB KN3 Meristem identity; number of flower in the spikelet Yuo et al. (2012) Takumi et al. (2000); Osnato et al. (2010) The wheat domestication gene Q encodes an AP2-like transcription factor, which is also a possible miR172 target. The Q gene influences the number of florets per spikelet and might regulate lodicule development as a possible orthologue of barley Cly1. The recessive q allele present in diploid wheat, hexaploid wheat mutants, and natural variants results in an elongated rachis and reduced number of florets per spikelet (Zhang et al., 2011), in contrast to cultivated wheat with the dominant Qallele. However, it is currently not clear whether Q controls determinacy of the spikelet meristem or heterochronic development of the floral meristem (Shitsukawa et al., 2009). Although some interesting targets could be modified for improving hybrid seed production in wheat, this survey also raises several issues. Firstly, most studies to date have determined that strong alleles of major genes regulate floral organ identity, a trait that should not be modified in male and female ideotypes unless it could be reverted in the F1 plants. Thus, it is required to identify alleles associated with quantitative differences in organ size to manipulate floral architecture. Secondly, how is it possible to modify floral organs without losing valuable domestication traits such as freethreshing (Simons et al. 2006)? Pleiotropic effects of floral genes on phenology and other plant structures are well described. For example, a single amino acid substitution in the Q gene changes properties of the transcription factor, which in turn affects the expression of many downstream genes, explaining its pleiotropic nature (Simons et al., 2006). It is required to identify new genes and alleles that regulate floral development without altering the ultimate fate of meristematic cells. Fertility control systems An effective hybrid seed production system requires a reliable and cheap system for forcing outcrossing. This depends on blocking self-pollination by inducing male sterility or selfincompatibility. Several options are explored in wheat. Chemical hybridizing agents (CHAs) The term CHA describes this class of chemicals in hybrid seed production that cause male sterility, depending on mode of action and dosage, can sometimes lead to female sterility (McRae, 1985). An advantage inherent to CHA use is that male sterility can be induced in the female inbred parent by simply spraying a chemical, therefore significantly reducing production costs. The use of CHAs allows the production of a high number of parental combinations for estimating germplasm combining ability. Figure: Hybrid breeding systems that utilize pollination control. (A) Chemical hybridizing agents (CHAs). (B) CMS-based hybrid breeding system. (C) XYZ-like hybrid wheat breeding system 4E-ms (D) Dual-component Barnase/Barstar transgenic system in which tapetal cell-specific expression (Ta) of Barnase (bar) induces male sterility. (E) Split barnase in allelic repulsion transgenic system of inducing male sterility. (F) Pro-herbicide (1) and herbicide (2) transgenic pollination control systems. (G) SPT uses a transgenic maintainer line for the propagation of a homozygous male-sterile mutant mother line (ms45). CHA is only useful for commercial hybrid seed production if it selectively induces male and not female sterility, is genotype independent, and has systemic activity and persistence to allow for different stages of maturity among the treated plants. Because rain, wind, and heat can reduce the efficacy of CHA application in the field, it is important that the period of application is broad enough to overcome these negative environmental conditions. The CHA must be non-phytotoxic and nonmutagenic, environmentally safe, economic to synthesize, practical to apply, and flexible in the dosage to permit a secure margin for application. Finally, CHAs must not affect F1 seed quality and seedling or plant vigour. Because of such stringent prerequisites, few CHAs have being taken up by commercial seed companies (Virmani and Edwards, 1983; Cisar and Cooper, 2002). The earliest report of CHA use in wheat was maleic hydrazide (Hoagland et al., 1953) followed by anti-lodging and height-reducing agents like ethephon (Ethrel) (Rowell and Miller, 1971), gibberellins (Porter and Wiese, 1961), and RH531 and RH532 (Jan et al., 1974, 1976). All these chemicals showed strong phytotoxic effects and inadequate male sterility across a range of environments and their commercial use was considered too risky. This led to the development of next-generation CHAs such as fenridazon-potassium (RH-0007, HYBREX®) (Mizelle et al., 1989), the sogital compound SC2053 (Orsan) (Wong et al., 1995), azetidine-3- carboxylic acid (WL 84811) from Shell, clofencet (Genesis®) from Monsanto, and sintofen (Croisor®100) from Saaten Union Recherche. RH-007 was used for commercial production in the USA and Europe for a limited time. Because it only worked in select genotypes and in a narrow application window and was therefore deemed commercially high risk (Cisar and Cooper, 2002). WL84811 was used in Europe, the USA, South Africa, China, Australia and New Zealand until it was discontinued because toxic residues were detected in F1 seed produced on treated plants (Pickett, 1993). Genesis® was used in wheat for commercial hybrid seed production in the USA and Europe until 2007. Croisor®100, a plant growth regulator (EFSA, 2010), is the only CHA currently being used in Europe for commercial production of hybrid wheat. Although the modern CHAs are effective across a broad range of genotypes and have reduced phytotoxicity, their commercial deployment is still hindered by a narrow window for application, which is subject to the prevailing environmental conditions. Cytoplasmic male sterility (CMS) CMS in plants is based on rearrangements of mitochondrial DNA, which lead to chimaeric genes and can result in the inability to produce fertile pollen (Hanson and Bentolila, 2004). For example, recent sequencing of the first CMS-derived mitochondrial genome (K-type) from wheat revealed novel fusions between open reading frames (CMS-ORFs) of low sequence homology with genuine proteincoding genes (Hanson and Bentolila, 2004; Liu et al., 2011). Exactly how these CMS-ORFs induce male sterility is still unclear, although they have been identified as disrupting tapetal cell or microspore function by invoking oxidative stress responses (Karpova et al., 2002; Pring et al., 2006; Fujii and Toriyama, 2008, 2009). CMS can arise both spontaneously and following mutagenesis, or be the result of interspecific, intraspecific, and intergeneric crosses (Kaul, 1988). In cultivated wheats, CMS lines can be created by initially crossing common wheat as the pollen donor to wild wheat (e.g. Triticum timopheevii Zhuk.) or related species such as Aegilops, Hordeum, and Secale, and then backcrossing to common wheat. According to Adugna et al. (2004), cytoplasm from as many as 35 species can be transferred to common wheat, and among these complete to partial sterility has only been observed for 20 species. The effect of male sterility-inducing cytoplasm in wheat can be counteracted by nuclear-encoded fertility-restorer (Rf) genes. Rf genes are classified as either sporophytic or gametophytic in action depending on the affected tissues. Sporophytic Rfs are more practical for hybrid breeding because heterozygotes (Rfrf) produce 100% viable pollen grains whereas only 50% of pollen grains are viable in gametophytic heterozygotes. This can, in the F1, have undesired yield penalties. Molecular cloning of Rfs from a range of species has revealed that they often encode proteins containing a common degenerate motif called a pentatricopeptide repeat (PPR). These are sequence-specific RNA-binding proteins, which in some cases can directly bind CMS-ORFs, typically suppressing their transcription and translation. The recent identification of disrupted untranslated region structures flanking wheat K-type CMS mitochondrial gene sequences (Choi et al., 2012) is suggestive of the corresponding wheat Rf gene sequences encoding PPR proteins. Growing bioinformatics capabilities are now supporting the prediction of RNA recognition specificities derived from knowledge of PPR tracts (Barkan et al., 2012). Coupled with the complete K-type CMS mitochondrial genome sequence, this information may now help elucidate exactly which PPR gene sequence(s) can act as Rfs. This is an important step towards being able to identify, develop, and deploy molecular markers for tracking key genes responsible for CMS expression and full fertility restoration within hybrid breeding programmes. In wheat, two or three major restorer loci are required for complete fertility restoration (Bahl and Maan, 1973). According to Ma and Sorrells (1995), the universal expression of as many Rf genes as possible seems to be beneficial for obtaining stable and high fertility restoration. In order to maintain a male sterile line, it must be crossed to a sister line (called the maintainer line), which has the identical nuclear genotype but a fertile cytoplasm derived from an elite adapted line. The maintainer line carries recessive restorer allele (rf); therefore, when this male-fertile line is crossed to a sterile CMS plant, it creates sterile progeny. For commercial hybrid seed production, a male-sterile line must be crossed to a line carrying dominant restorer alleles with excellent pollinator qualities. This is necessary for producing fertile F1 seed. Here, it is unimportant if the pollen donor exhibits alien or native cytoplasm. The only prerequisite is that the genotype is homozygous for Rf gene(s). In general, CMS is a relatively inflexible system that is only feasible for hybrid seed production when CMS mutants and effective fertility restorers are available in a given crop and if the CMS mutation is not associated with yield penalties or other undesirable phenotypic effects. Moreover, CMS systems are frequently sensitive to environmental factors, particularly temperature and photoperiod. To date, only T. timopheevii Zhuk.-derived male-sterile cytoplasms have been used for commercial production of wheat hybrids (Longin et al., 2012). However, this cytoplasm has undesirable side effects that are environment dependent (Baier et al., 1978). These include incomplete fertility restoration and shrivelled F1 seed, which taken together can compromise hybrid yield. Commercially, this compromised heterotic advantage must still be sufficient to compensate for increased production and marketing costs that is inherent to such a complicated breeding strategy. Despite the many challenges, engineering CMS could significantly reduce the labour-intensive exercise of incorporating sterility-inducing cytoplasm(s) into breeding materials, a significant costprohibitive step to hybrid seed production. Self-incompatibility Self-incompatibility (SI) is a biological mechanism that prevents self-pollination in open-pollinated species. Although wheat is fully self-fertile, SI is widespread in the grasses, and cereal rye (S. cereale L.), a close relative of wheat, is an obligate outbreeder. In all grass systems studied, gametophytic SI is controlled by two multiallelic loci, S and Z (reviewed by Langridge and Baumann, 2008). The interaction of two genes means that SI in the grasses has several features that differentiate it from the more common, single-locus systems. Of particular importance are differences in reciprocal crosses and the varying levels of compatibility (namely, the percentage of compatible pollen) between two plants. Compatibility can range from 0 to 50, 75, or 100%, depending on the genotypes. For example, if a cross is made between plants with the genotype S1.1 Z1.2 as female and S1.2 Z1.3 as the pollen donor, 75% compatible pollen grains will be scored (as four pollen genotypes are produced, three compatible S1Z3, S2Z1, and S2Z3, and S1Z1, which is incompatible), whereas the reciprocal cross will show 50% of the pollen as compatible (two pollen genotypes: S1Z1, which is incompatible and S1Z2, which is compatible). Therefore, it may be possible to generate a SI wheat if the requisite genes can be identified in a close relative and introgressed into the wheat genome. Genic male sterility systems The utilization of mutations in nuclear-encoded genes, known as nuclear (NMS) or genic (GMS) male sterility can greatly broaden the choice of parental lines when compared with CMS systems. They also avoid negative alloplasmic and cytoplasmic effects on yield, as well as problems associated with complete fertility restoration. Mutations in nuclear-encoded genes that cause male sterility can occur spontaneously or be induced. Historically, spontaneous mutants have been observed and retained by wheat breeders; examples include Pugsley’s, Langzhou, BNY-S, and Taigu, which affect Ms1 (4BS) Wtms1 (2B), and Ms2 (4DS) fertility loci (Pugsley and Oram, 1959). Mutations can also be induced through exposure to physical (e.g. γ-rays and X-rays) or chemical (e.g. ethylmethane sulphonate) mutagens. Examples of ionizing radiation-induced male-sterile wheats are Probus (ms1b) and Cornerstone (ms1c), whereas ethylmethane sulphonate-induced male-sterile wheats include FS2 (ms1d), FS20 (ms5, 3AL), and KS87UP9 (ms3, 5AS) (Fossati and Ingold, 1970). Depending on the mutated locus, they are either dominant (Ms2, Ms3) or recessive (ms1, ms5), and can be classified as being conditional or non-conditional, depending on whether environmental factors revert fertility. Comparable to CMS, conditional GMS can be temperature and/or photoperiod dependent, with mutations classified as being either thermo-, photoperiod- or photo-thermo-sensitive GMS (PTGMS). An example of the utility of PTGMS is the two-line hybrid rice system, which takes advantage of a mutation in O. japonica cv. Nongken 58S. This system has been used successfully for grain production in China since 1995 (Mei et al., 1999). However, the deployment of a PTGMS two-line hybrid wheat system (BS20, C49S) in China has been limited by two factors. Firstly, effective fertilityrestoring germplasm seems to be restrictive, and secondly, certain climatic regions are not conducive to sterility expression. Limitations inherent to conditional GMS can be overcome by the use of non-conditional GMS mutants. However difficulties arise in the maintenance, multiplication, and selection of pure malesterile populations, a necessity in large-scale production of hybrid seed. One way to overcome the problem of large-scale production of male steriles is by breeding lines in which the male-sterile mutant locus is tightly linked to a visual marker. An example of this is the dominant GMS mutant Ms2 locus, which is linked by 0.19 cM to the Rht10 dwarfing locus (Bing-Hua and Jing-Yang, 1986; Yang et al., 2009). The dwarfing locus facilitates the identification of tall male fertiles from dwarf male steriles. However, large-scale hybrid production is limited by the need to manually remove tall male fertiles from the female stand. Problems associated with propagating pure stands of male steriles can also be circumvented through the use of cytogenetic chromosomal manipulation coupled with recessive non-conditional GMS mutants such as XYZ-like three-line systems. Islam and Driscoll (1984) observed the expression of a normally latent fertility gene when using the ms1c mutant allele. A similar phenomenon may affect the ms1gmutant used in the 4Ems system. However, only three of ten independent Y lines, when selfed, generated Z lines that were completely male sterile (Zhou et al., 2006; Zhou and Wang, 2007). Genetic modification (GM) systems for hybrid breeding Despite the development of different CHA, CMS, and GMS systems in wheat over the last 60 years, each has serious drawbacks in either F1 fertility restoration or in providing complete male sterility in the female inbred parent under a range of environmental conditions. The first description of the application of recombinant DNA technologies for engineering a wheat fertility control system was in 1997. De Block et al. (1997) created a dominant GMS system that relied on tapetal cell ablation induced through the targeted expression of a cytotoxic bacterial ribonuclease (Fig. 4D). This RNase is encoded by the barnase gene, an integral component of Bayer’s Seedlink® system for commercial hybrid canola production. Seedlink® couples glufosinate resistance (LibertyLink®) with the sterilityinducing properties of the barnase gene allowing in-field selection of male-sterile female parents. F1 fertility restoration is achieved through the highly specific inactivation of RNase activity by the Barstar protein (Fig. 4D), introduced via the male inbred parent. This type of dual-component dominant system is limited by the requirement for transgenes in each crossing partner, which results in extra breeding time. Recent fine-tuning of barnase-mediated sterility has led to the development of a recessive split system for wheat, where the barnase gene is encoded in two non-overlapping fragments at complementary loci. Termed allelic repulsion, RNAase is expressed when complementary barnase gene fragments are co-expressed in the same tissues. Targeted expression in tapetal cells induces cell death and subsequent male sterility. Introducing complementary barnase‘isoloci’ derivatives into the same individual creates male-sterile females, which can easily be propagated by crossing to a homozygous single ‘isolocus’ maintainer line. Linking the ‘isoloci’ not present in the maintainer line to herbicide resistance allows easy selection of malesterile progeny. Although this split-gene system harnesses many advantages found in classic GMS systems, herbicide selection is disadvantageous because it requires overplanting and eliminating half the sown plants in order to attain a pure stand of male-sterile female inbreds. Extra seed handling adds to hybrid seed production costs. Chemically induced GM systems In recent decades, the search for an ideal CHA has contributed to the development of many inducible molecular systems where chemical application can control fertility through the action of a transgene. Conditional chemical fertility control systems have been developed around both synthetic and naturally occurring phytotoxic agents. Examples include herbicides as well as naturally occurring plant hormones like abscisic acid, jasmonic acid, ethylene, and cytokinins. For example, the herbicide-catalysing properties of a cytochrome P450 induces sterility in tobacco through the action of a tapetal cell-specific promoter. Tapetal cell death results in male sterility (O’Keefe et al., 1994). A limitation to its commercial deployment has been the finding that CYP105A1 expression can at times generate unwanted pleiotropic affects due to the disruption of brassinosteroid signalling and homeostasis. Despite these issues, progress has been made in the targeted modification of P450s for altered specificity and activity. Through chemical spraying and fertility control are subject to short biological windows for application and environmental factors such as wind and rain, which at times can compromise efficacy. Conditional male fertility is therefore preferred over conditional male sterility, thus ensuring male-sterile female inbred parents are inherently 100% sterile. Compromised male fertility restoration is indeed acceptable when it is only required for propagating the female inbred parent. Although this would be an ideal chemical-based system, one has yet to be commercially developed. Transgenic construct driven non-GM systems The use of transgenes to control fertility is clearly advantageous in reducing hybrid seed production costs. However, all of the technologies described above generate hybrid seed that is transgenic. Burdensome regulatory requirements for commercial release and restrictions to world trade of GM crops have spurred the development of new breeding approaches that use transgenic systems but generate non-transgenic seed. These approaches are being developed within both the public and private sectors and are finding application to both fertility control and heterosis breeding (Lusser et al., 2012). The techniques used encompass genome editing nucleases, oligonucleotide-directed mutagenesis, and RNA-dependent DNA methylation. Genome editing nuclease-based technologies harness a cell’s endogenous mechanism to repair induced DNA double-stranded breaks by homologous recombination and non-homologous end joining. These site-specific lesions can be induced in a host cell either transiently or stably through the design, synthesis, and expression of artificial zinc finger nucleases, transcription activator-like effector nucleases, or meganucleases (Curtin et al., 2012). Synthetic nucleases are finding application in targeted gene inactivation, addition of genes of interest, gene replacement, and trait stacking. Cultural Practices for Seed production of Wheat Planning for wheat seed production: Planning for wheat seed production Land to be used for seed production of wheat should be: Free of volunteer plants. The field should be well drained, free of weeds. The soil neither too acidic not too alkaline. Long interval of Crop rotation is desirable previous cropping. The crop should be planted on a field with a known history to avoid contamination from volunteer plants, noxious weeds and soil-borne diseases that are potentially seed transmitted. A wheat seed crop should never immediately follow wheat, unless the wheat crop in the previous season was of the same variety and of the same or higher generation. Two year rotation for flag smut and seed gall nematode is suggested where applicable. Land requirement: Isolation requirement: Isolation requirement Normally a self-pollinated crop (Clistogamous ) 1-4 %. Cross pollination sometime occurs. It is sufficient to isolate seed fields with a strip of 3 meters all around which is planted with a non-cereal crop, or left uncroped. In cases where variety is susceptible to diseases caused by Ustilago spp. ( eg . loose smut) an isolation distance of 180 meters between seed field and other fields of wheat is recommended. It is also require only 150 m isolation from other wheat fields where in loose smut infection is in excess of 0.1% in the case of foundation seed production and 0.5 % in the case of certified seed production. Future perspectives Hybrid wheats are seen to have higher agronomic potential than line varieties due to improved grain and straw productivity and yield stability under harsh environmental conditions (Longin et al., 2012). Rapid developments in wheat genomics, understanding of gene function, and the targeted modification of plant phenotypes using GM technologies is likely to increase the efficiency of hybridization and therefore aid in the development of more cost-effective hybrid seed production systems. Deployment of novel transgenic constructs to drive non-GM hybrid breeding systems may be a step towards alleviating public concern over GM crops. 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