GENOME WIDE ASSOCIATION OF HEAT TOLERANCE LOCI OF WHEAT IN HOTSPOTS OF SUDAN AND SYRIA

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ABSTRACT

One hundred and eighty-nine (189) wheat genotypes were evaluated in multi-environments (Tel-hadya (Syria), Dongola (Sudan) and Wadmedani (Sudan)) for heat tolerance from 2011 to 2012. Genomic mapping of the quantitative trait loci underlying heat tolerance in the crop was also performed. The field experiment was laid out in an alpha lattice design. The data obtained were subjected to restricted maximum likelihood (REML) for generation of best linear unbiased estimates (BLUEs). The heat tolerance study in the two seasons (early and late) in Tel-hadya, Syria showed that days to heading, days to maturity and grain filling duration, plant height and grain yield were significantly (p <0 .05="" 0.297="" 0.449="" 0.68="" 0.793="" 0.804="" 0.908="" 2.593="" 2.893t="" 20="" 25cm.="" 58="" 60="" 68="" 88="" a="" across="" additive="" admedani="" all="" ammi="" analyses="" analysis="" and="" approximately="" associated="" association="" at="" ate="" biomass="" broad="" but="" canopy="" coefficient="" compared="" consistently="" crop="" days="" decay="" decreased="" detected="" direct="" disequilibrium="" dongola="" due="" duration="" early="" effect="" effects="" environment-="" environments="" estimates="" few="" fifth="" filling="" first="" for="" fourth="" from="" gen101="" gen117.="" gen118="" gen135="" gen155="" gen="" genome="" genotypes="" germplasm="" grain="" ha="" had="" harvest="" he="" heading="" heat="" height="" heritability="" in="" index="" influence="" interaction="" kernel="" late="" ld="" like="" linkage="" loci="" loss="" low="" main="" many="" mapping="" markers="" maturity="" moderate="" most="" multiplicative="" negative="" observed="" obtained="" of="" on="" one="" ongola="" other="" path="" performance="" plant="" positive="" pronounced="" quantitative="" range="" ranged="" ranked="" ranking="" reduced="" respectively.="" revealed="" season.="" season="" second="" sense="" seven="" showed="" sites.="" sites="" span="" specific.="" stability="" stress="" structure="" studied="" sub-populations.="" sudan.="" t="" tel-hadya="" temperature="" terms="" than="" that="" the="" their="" third="" thousand="" three="" to="" tolerance="" top="" trait="" traits="" two="" values="" varied="" wadmedani="" was="" weight="" were="" wheat="" when="" while="" wide="" with="" yield.="" yield="" yielding="">

TABLE OF CONTENTS

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

APPENDICES

ABSTRACT

INTRODUCTION

LITERATURE REVIEW

Botany and Adaptation of Wheat

Genome of Wheat

Importance of Wheat

Effects of Heat stress on Wheat Crop

Bases for Screening Wheat for Heat Tolerance

Molecular Markers

Genetic Mapping Approaches

MATERIALS AND METHODS

RESULTS

DISCUSSION

CONCLUSION

REFERENCES

APPENDICES

INTRODUCTION

Bread wheat (Triticum aestivum L.) is unarguably one of the world’s most important and widely consumed cereal crop (Asif et al., 2005; Bushuk, 1998). The flour is used for making bread, biscuits, confectionary products, noodles, wheat gluten among others. The world population is expected to reach about 9 billion by the end of the 21^st century, and it has been predicted that the demand for cereals, especially wheat, will increase by approximately 50% by 2030 (Borlaug and Dowswell, 2003).Wheat production attracts increasing attention globally owing to its importance as a staple food crop, such that the availability of wheat and wheat products are seen as a food security issue in many countries. This has led to growing of wheat in many parts of the world even where it was not formerly grown. Paliwal et al. (2012) indicated that wheat is one of the most broadly adapted cereals. Although wheat is a thermo sensitive long day crop that requires relatively low temperature for its optimal yield, it is being grown in the tropics and subtropics despite the relatively high temperature that is associated with the areas (Rehman et al., 2009). In spite of the growing attention on the crop globally, Ali (2011) reported that its production in many regions of the world is below average because of adverse environmental conditions. High temperature which imposes heat stress on wheat is a major limitation to its productivity in arid, semi- arid, tropical and subtropical regions of the world (Ashraf and Harris, 2005). It affects the different growing stages of the crop especially during anthesis and grain filling (Rehman et al., 2009) leading to poor grain yield and quality. This is exacerbated by the increasing temperature associated with global warming, thus breeding for high temperature tolerance in wheat is a major challenge globally.

A detailed understanding of the genetics and morpho-physiology of heat tolerance and use of effective breeding strategies to address the situation would be ideal. Sikder and Paul (2010) reported that identification of wheat varieties suitable for heat stressed condition would be an important step toward achieving high yield potentials in wheat. Major gains have been achieved in the improvement of economic traits of wheat through conventional breeding, and more recently; through marker assisted selection (MAS) that has transformed plant breeding. Advances in molecular technologies have resulted in the mapping and identification of quantitative trait loci (QTLs) controlling traits of importance in wheat, thereby permitting improvement beyond the upper limit of conventional breeding approaches. The two most commonly used approaches in mapping and identification of QTLs are bi-parental and association mapping (AM). Association mapping, which is more recent, has been utilized in overcoming some limitations associated with bi-parental mapping approach in exhaustive genomic dissection of putative QTLs of interest in plants. These limitations that are associated with bi-parental mapping approach are time consuming in generation of mapping population from a cross between two parents, low recombination events in the mapping population which leads to poor mapping resolution, and detection of only few QTL, among others. AM has the potential to identify a single polymorphism within a gene that is responsible for phenotypic differences (Braulio et al., 2012).

Although significant variation for heat tolerance exists among wheat germplasm (Reynolds et al., 1994; Joshi et al., 2007a, b), no direct selection criteria for heat tolerance are available (Paliwal et al., 2012). This is probably because of lack of detailed understanding of the morphological, physiological and genetic bases for heat tolerance in wheat. Phenotypic selection for heat tolerance has been performed using grain filling duration (Yang et al., 2002); one thousand grain weight, canopy temperature depression (Reynolds et al., 1994a; Ayeneh et al., 2002) and grain yield. Despite these attempts, Ortiz et al. (2008) and Ashraf (2010) reported that breeding for heat tolerance using trait- based selection is still in its infancy stage and warrants more attention.

Wheat developmental phases such as ear emergence, anthesis and maturity are controlled by three groups of vernalization (Vrn), photoperiod (Ppd), and the earliness per se genes (Kosner and Pankova, 1998) and their expression plays a significant role in wheat adaptation to different locations (Gororo et al., 2001). These three sets of genes together influence flowering time, and the suitability of genotypes for flowering under particular environmental conditions (Snape et al., 2001; Dubcovsky et al., 2006). Differences in flowering time could be of vital physiological implication in heat tolerance in wheat crop. The paucity of knowledge of the underlying physiological basis of heat tolerance as well as the genomic regions associated with heat tolerance in wheat prompted this research. This study was carried out to characterize the physiological bases of heat tolerance and identify QTLs/genomic regions underlying these traits in a.....

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