Mohamed Najeb Barakat

Molecular mapping of QTLs for wheat flag leaf senescence under water-stress

A segregating population from the cross between drought sensitive (Variant-2) and drought tolerant (Cham-6) genotypes was made to identify molecular markers linked to wheat (Triticum aestivum L.) flag leaf senescence under water-stress. From 38 random amplified polymorphic DNA (RAPD) primers, 25 inter-simple sequence repeat (ISSR) primers and 46 simple sequence repeat (SRR) primers, tested for polymorphism among parental genotypes and F2 population. Quantitative trait locus (QTL) for flag leaf senescence was associated with 1 RAPD marker (Pr9), 4 ISSR markers (Pr8, AD5, AD2 and AD3), and 1 SSR marker (Xgwm382) and explained 44, 50, 35, 31, 22 and 73 % phenotypic variation, respectively. The genetic distance between flag leaf senescence gene and Pr9 was 10.0 cM (LOD score 22.9). The markers Pr8, AD5, AD2 and AD3 had genetic distances of 10.5, 14.6, 15.6 and 18.1 cM, respectively (LOD scores 22.6, 17.8, 17.5 and 14.6). The genetic distance between Xgwm382 was 3.9 cM (LOD score 33.8). Therefore, the RAPD, ISSR and SSR markers linked to the QTL for the drought-induced flag leaf senescence can be further used in breeding for drought tolerance in wheat.

Identification of new TRAP markers linked to chlorophyll content, leaf senescence, and cell membrane stability in water-stressed wheat

In order to identify target region amplification polymorphism (TRAP) markers linked to three physiological traits in wheat (Triticum aestivum L.), the segregating F4 population from the cross between drought-sensitive (Yecora Rojo) and drought-tolerant (Pavon 76) genotypes was made. The parents and 150 F4 families were evaluated phenotypically for drought tolerance using two irrigation treatments [2.5 and 7.5 m3(H2O) m−2(soil)]. Using 40 different TRAP primer combinations tested for polymorphism in parental and F4 family genotypes, the results revealed that quantitative trait locus (QTL) for chlorophyll content was associated with TRAP 5, TRAP 14, and TRAP 20 and explained 18, 16, and 23 % phenotypic variation, respectively. The genetic distance between chlorophyll content QTL and TRAP 5, TRAP 14, and TRAP 20 were 12.3, 19.8, and 13.6 cM, respectively. QTL for flag leaf senescence was associated with TRAP 2, TRAP 3, TRAP 15, and TRAP 16 and explained 33, 27, 28, and 23 % phenotypic variations, respectively. The genetic distance between flag leaf senescence QTL and TRAP 2, TRAP 3, TRAP 15, and TRAP 16 were 9.4, 14.7, 18.1, and 17.3 cM, respectively. QTL for cell membrane stability was associated with TRAP 8, TRAP 9, and TRAP 37 and explained 27, 30, and 24 % phenotypic variation, respectively. The markers TRAP 8, TRAP 9, and TRAP 37 had genetic distances of 17.0, 10.0, and 9.0 cM, respectively. Therefore, these TRAP markers can be used in breeding for drought tolerance in wheat.

Mapping of QTLs associated with abscisic acid and water stress in wheat

A segregating F4 population from the cross between drought sensitive (Yecora Rojo) and drought tolerant (Pavon 76) genotypes was made to identify molecular markers linked to a wheat (Triticum aestivum L.) abscisic acid (ABA) content at two water regimes. The parents and 150 F4 lines were evaluated phenotypically for drought tolerance using two irrigation treatments [0.25 and 0.75 m3(H2O) m−2(soil)]. Forty different target region amplification polymorphism (TRAP) primer combinations, 98 different sequence-related amplified polymorphism (SRAP) primer combinations, and 400 simple sequence repeat (SSR) primers were tested for polymorphism among the parental genotypes and the F4 lines. Seven loci in the F4 lines treated with the drought stress were identified. Single quantitative trait loci (QTLs) were located on chromosomes 1B, 2A, 3A, 5D, and 7B and each of them explained from 15 to 31 % of phenotypic variance with a LOD value of 7.2 to 15.7. Five QTLs were located on chromosome 4A and six QTLs on chromosome 5A. In control (well-watered) F4 lines, two QTLs were mapped on chromosome 3B and one QTL on each chromosome 5B and 5D. Statistically the most significant groups of QTLs for the ABA content were identified in the regions of chromosomes 3B, 4A, and 5A mostly near to Barc164, Wmc96, and Trap9 markers. Therefore, these markers linked to QTLs for the drought-induced ABA content can be further used in breeding for drought tolerance in wheat.

Anther culture response in wheat (Triticum aestivum L.) genotypes with HMW alleles

The objective of this study was to develop a simple anther culture protocol for a range of Saudi wheat genotypes. Seven wheat genotypes were evaluated in anther culture on five different medium protocols for their ability to initiate callus and green plants. The estimates of significance for the effects of genotypes, and medium protocols used, and their interactions on callus induction, callus weight and shoot formation derived from anther explants indicated that the in vitro traits were significantly influenced by the genotypes, medium protocols, and their interactions. The percentage of explants that developed calli ranged from 0.41% (Lang) to 15.39% (Irena) averaged across the five medium protocols with an average 4.45%. The genotype Irena produced the highest average means of shoot formation (69.65%) across medium protocols. The genotype Yecora Rojo (13.73%) was significantly inferior to all other tested genotypes for shoot formation.

The genetic basis of spectral reflectance indices in drought-stressed wheat

Drought imposes a major constraint over the productivity of wheat, particularly in arid and semi-arid production zones. Here, the genetic basis of spectral reflectance indices was investigated in drought-stressed wheat by comparing, under two contrasting moisture regimes, the performance of an F6 recombinant inbred line (RIL) population bred from a cross between the drought tolerant cultivar Pavon76 and the sensitive cultivar Yecora Rojo. The parents and RILs were genotyped with respect to both a set of microsatellite (SSR) loci and a number of known drought-responsive genes. In all, 28 quantitative trait loci (QTL) controlling dry weight per plant, water content of the above-ground biomass, leaf water potential, canopy temperature, and spectral reflectance indices traits were identified. The loci were distributed over 11 chromosomes, belonging to each of the three wheat sub-genomes. There were important location-flanking markers Barc109 and Barac4 on chromosome 5B relating to dry weight per plant accumulation under the limited irrigation regime. The same region-harbored QTL associated with leaf water potential, canopy temperature, and ratio index under the limited irrigation regime. Linkage between the known drought-responsive genes and aspects of the drought response was established. Some of QTL were of substantial enough effect for their linked markers to be likely usable for the marker-assisted breeding of drought tolerance in wheat.

Identification of new SSR markers linked to leaf chlorophyll content, flag leaf senescence and cell membrane stability traits in wheat under water stressed condition

Segregating F4 families from the cross between drought sensitive (Yecora Rojo) and drought tolerant (Pavon 76) genotypes were made to identify SSR markers linked to leaf chlorophyll content, flag leaf senescence and cell membrane stability traits in wheat (Triticum aestivum L.) under water-stressed condition and to map quantitative trait locus (QTL) for the three physiological traits. The parents and 150 F4 families were evaluated phenotypically for drought tolerance using two irrigation treatments (2500 and 7500 m3/ha). Using 400 SSR primers tested for polymorphism in testing parental and F4 families genotypes, the results revealed that QTL for leaf chlorophyll content, flag leaf senescence and cell membrane stability traits were associated with 12, 5 and 12 SSR markers, respectively and explained phenotypic variation ranged from 6 to 42%. The SSR markers for physiological traits had genetic distances ranged from 12.5 to 25.5 cM. These SSR markers can be further used in breeding programs for drought tolerance in wheat.

Application of Molecular Breeding for Development of the Drought-Tolerant Genotypes in Wheat

Drought is one of the most important environmental challenges growers have to face around the world. Drought is the cause for large grain losses every year, especially in developing countries, and the current trend in global climate change will likely lead to further losses. The worldwide water shortage and uneven distribution of rainfall makes the improvement of drought tolerance especially important (Lou and Zhang, 2001). Breeding for drought tolerance is a major objective in arid and semiarid regions of the world due to inadequate precipitation, shortage of irrigation water and high water demand for crop evapotranspiration in such climates. Little progress has been made in characterizing the genetic determinants of drought tolerance, because it is a complex phenomenon (Tripathy et al, 2000). Breeding for water stress tolerance by traditional methods is a time consuming and considered inefficient procedure. Improving the drought tolerance of a crop is difficult for a breeder because yield usually has a relatively low heritability even under ideal condition and an unpredictably variable water supply reduces heritability (Blum, 1988). Drought tolerance is now considered by both breeders and molecular biologists to be a valid breeding target. In the past, breeding efforts to improve drought tolerance have been hindered by its quantitative genetic basis and our poor understanding of the physiological basis of yield in water-limited conditions (Passioura, 2002). Recently, Tuberosa and Saliva (2007) reported that genomics based approaches provide access to agronomically desirable alleles present at quantitative trait loci (QTLs) that affect such responses, thus enabling us to improve the drought tolerance and yield of crops under water limited conditions more effectively. Compared to conventional approaches, genomics offers unprecedented opportunities for dissecting quantitative traits into their single genetic determinants (QTLs), thus paving the way to marker-assisted selection (MAS) (Ribaut et al,2002; Morgante and Salamini, 2003) and eventually, cloning of genes at target QTLs (Salivi and Tuberosa, 2005).Recently, Tuberosa