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Barley (Hordeum vulgare vulgare L.) is an ancient and important cereal grain crop. It ranks fifth among all crops in dry matter production in the world today (129 M mt, 2002-2005 mean), Baik and Ullrich (2008). Barley was presumably first used as human food but evolved primarily into a feed, malting and brewing grain due in part to the rise in prominence of wheat and rice. In recent times, about two-thirds of the barley crop has been used for feed, one-third for malting and about 2% for food directly. However in extreme climates barley remains a principal food source today, e.g., Himalayan nations, Ethiopia, and Morocco (Baik and Ullrich 2008), although in the tropical and arid region barley production is limited by drought especially the Middle East (Smith, 1998).
Drought is by far the most important environmental stress in agriculture and studies have reported reduced biomass production in barley due to drought which caused a reduction in the amount of radiation intercepted; especially during anthesis which is mostly associated with more rapid leaf senescence (Jamieson et al 1995). Although efforts have been made to improve barley productivity under water-limiting conditions, natural selection has favored mechanisms for adaptation and survival of wild occurring barley (Hordeum spontaneum K) under adverse conditions making it a rich source of new germplasm which breeding activity should direct selection towards increasing the economic yield of cultivated barley. Wild barley is the immediate evolutionary progenitor of cultivated barley. Crosses between cultivated and wild barley species occur naturally, are easily produced, and are fully fertile; transferring genetic traits from wild to cultivated barley is a simple procedure (Gunasekera et al 1994). The greater genetic diversity in wild barley and its occurrence in a wide range of habitats (including areas of extreme water stress) suggest that the genetic resources in wild barley may be exploited for the improvement of cultivated barley (Gunasekera et al 1994).
Again, fundamental research has provided significant gains in the understanding of the physiological and molecular responses of plants generally to water deficits, but there is still a large gap between yields in optimal and stress conditions. Minimizing the 'yield gap' and increasing yield stability under different stress conditions are of strategic importance in guaranteeing food for the future. Cattivelli et al (2008). This essay however will try to put forward a crop improvement plan for developing drought resistance barley crop for arid and semi arid region based on the following objectives. (1)Trait analysis which includes: to identify specific physiological traits which are associated with the maintenance of photosynthesis at low water potential in wild barley genotypes and to identify genetic diversity in wild barley with regards to cellular level of drought resistance, (2) To select genetic resource in wild barley which include: to ascertain whether some wild barley genotypes demonstrate greater drought resistant traits than that present in cultivated varieties. (3) Dissection of identified genetic variability using genomic tools e.g. Linkage mapping which include the usual method of mapping the gene(s) responsible for the expression of trait of interest. (4) Characterization and categorization of identified genetically complex abiotic stress responses traits which include: a rapid discovery of genes by large-scale partial sequencing of selected cDNA clones or expressed sequence tags (EST). (5) Back crossing or introgression of drought resistance trait from the wild type to the cultivated barley population.
Phenotypic analysis: Physiological and Morphological trait for Barley
Although it certainly is a debatable issue as what the exact definition for drought resistance should be however (Gunasekera et al 1994) defines 'drought resistance' in a specific sense as the ability to maintain growth and photosynthesis at low leaf water potential. Other studies has characterized the cellular-level water stress responses of osmotic adjustment and co-incident protoplast volume maintenance at low water potential as having a significant impact on the maintenance of photosynthesis and crop growth under water deficit conditions. Gunasekera et al (1994) reported the differential capacity for osmotic adjustment in a number of cultivated barley genotypes under polyethylene glycol-induced water stress. He has reported that these genotypes display a negative correlation between stress-induced growth reduction and osmotic adjustment. Therefore, it will be interesting to examine the potential range for physiological acclimation to water stress in a number of wild barley genotypes, and to compare this drought resistance parameter (i.e. physiological acclimation to low leaf water potential) to that displayed by cultivated barley genotypes. This proposal will adopt the method described by Gunasekera et al (1994) for analyzing the physiological and morphological trait for Barley.
Selection of genetic resource
Fig.1. Gunasekera et al (1994).
One of the objectives of this breeding proposal is to evaluate the potential of wild barley germplasm as a genetic source of 'drought resistance' for use in breeding programs for cultivated barley. To facilitate this evaluation, the physiological response of two cultivated varieties of barley, to stress was compared with the response of the high and low acclimation wild barley accessions. This comparison is represented in Fig. 1. Above according to Gunasekera et al (1994), individual genotype responses of the high acclimation accessions were grouped together, as were the negative acclimation accessions, for this analysis. Gunasekera et al (1994). As shown in Fig. 1, the high acclimation genotypes, grouped together, showed significantly different responses than the group of negative acclimation genotypes, with regard to change in extent of volume reduction at low water potential (Fig. 1A), stress-induced osmotic adjustment (Fig. 1B), and extent of photosynthetic inhibition at low water potential (Fig. 1C). The high and negative acclimation responses for each of the related physiological parameters shown in (Fig.1), allows for only two replications (i.e. responses of Noga and Ruth) in a statistical evaluation of cultivated barley response to stress, the three physiological parameters shown in the figure above are calculated as change in response of stressed versus control plants Gunasekera et al (1994). When analyzed in this fashion, the cultivated barley lines as a group did not demonstrate any statistically significant differences from the wild barley genotypes grouped as high acclimators. Osmotic adjustment was evaluated in a different fashion; allowing statistical evaluation of genotype differences, by comparing the't' s at 100% relative water capacity (RWC)of stressed plants. The analysis does indicate that the high acclimating wild barley genotypes may have a greater capacity to maintain metabolic function at low water potential. As shown in Fig. 1, Noga plants appeared to show a greater capacity to acclimate to stress than plants of the Ruth cultivar. However, all of the high acclimating wild barley genotypes displayed a lower water potential at 100% RWC under stress than Noga, Gunasekera et al (1994).
Dissection of identified genetic variability using genomic tools
The establishment of genetic linkage maps provides the basis for mapping the gene(s) responsible for the expression of traits of interest in a barley population. For example in wheat, such maps have also corroborated cytological evidence of major chromosome rearrangements and have allowed the comparative mapping among related species (Zhao et al 2008). Molecular markers have been considered an important biotechnology tools for enhancing the magnitude of plant breeding. From the conceptualization and delineation of perspectives in their use in breeding programmes, the methodologies have ample applications: characterization of genetic diversity, introgression of exogenous genetic material for diversity increment, advancement in novel varieties release, diagnostics or selection tools. In several cereal species, genetic linkage maps have allowed the identification of regions controlling some traits related to the response to drought Zhao et al (2008). The method of linkage mapping construction proposed in this essay is according to Chen et al (2008) and (Baum et al 2003).
Characterization and identification of genetically complex abiotic stress responses traits
Physiological, morphological and developmental changes which confer drought tolerance in plants must have a molecular genetic basis. Identification and genetic mapping of QTL for specific drought tolerance traits combined with the mapping of candidate genes has been shown to be a useful approach towards dissecting the genetic basis of drought tolerance (Diab et al 2004). Large expressed sequence tag (ESTs) databases have been obtained at different developmental stages from tissues and organs of plants exposed to a variety of environmental conditions thus mapping of differentially expressed sequences can be useful for the identification of candidate genes controlling important traits (Diab et al 2004). If validated with accurate phenotyping and properly integrated in marker-assisted breeding programs, this approach can facilitate gene pyramiding and accelerate the development of drought tolerant genotypes (Diab et al 2004). Therefore the method of finding if the marker segregation within the population is associated with the segregation of the target traits proposed in this breeding plan is described by Diab et al (2004).
Back crossing or introgression of drought resistance trait from wild type to the cultivated barley population.
Several studies have endeavored to improve yield under dryland conditions using wild barley (Hordeum spontaneum) as a potential source of alleles for drought tolerance (Ashraf 2009). In one of the works (Baum et al 2003) developed and evaluated a population by backcrossing cultivated barley with wild type to detect wild barley alleles involved in producing high yield under drought conditions, they found out that six QTL from the wild barley were responsible for enhanced yield under water limiting conditions (Ashraf, 2009).the breeding plan therefore will adopt Baum et al (2003) introgression method to establish drought resistant lines of cultivated barley.
Drought stress is one of the major limitations to wheat productivity. To develop crop plants with enhanced tolerance of drought stress, a basic understanding of physiological, biochemical and gene regulatory networks is essential. Various functional genomics tools such as RFLP, SNP, EST, Micro array, Bio informatics etc have helped to advance our understanding of stress signal perception and transduction, and of the associated molecular regulatory network.
Trait analysis which includes: to identify specific physiological and morphological traits which are associated with the maintenance of photosynthesis at low water potential in wild barley genotypes and identify genetic diversity in wild barley with regards to cellular level of drought resistance.
Selection of genetic resource in wild barley which include: to ascertain whether some wild barley genotypes demonstrate greater drought resistant traits than that present in cultivated varieties. ↓
Dissection of identified genetic variability using genomic tools e.g. Linkage mapping which include the usual method of mapping the gene(s) responsible for the expression of trait of interest.↓
Characterization and categorization of identified genetically complex abiotic stress responses traits: (genes discovery by large-scale partial sequencing of selected cDNA clones or expressed sequence tags EST).↓
Introgression of drought resistance trait from the wild type to the cultivated barley population ↓