Germplasm Characterization And Molecular Dissection Of Drought Tolerance In Sesame (Sesamum Indicum L.)

Sesame?Sesamum indicum L.?is one of the oldest oilseed crops widely grown in Africa and Asia for its high-quality nutritional seeds.It is emerging as an alternative cash crop for smallholders in developing countries,especially in West Africa.Most of the growing areas of sesame are located in drought-prone areas with limiting rainfall,hence,its cultivation is often challenged by drought which constitutes the major abiotic stress impairing sesame productivity.Unfortunately,sesame is an understudied crop and the level of drought tolerance of the most of the cultivars is unknown.More importantly,drought tolerance in sesame has not been yet investigated through molecular approaches,leading to relatively a meagre progress in efforts to develop highly tolerant varieties.The objectives of this study were to provide necessary genetic and genomic information for instigating breeding programmes towards enhancing drought tolerance in sesame cultivars.Firstly,in order to get insight into the genetic relationships of the West African materials,96sesame accessions were collected from 22 countries of Asia and Africa including seven West African countries.They were genotyped using 33 polymorphic simple sequence repeat markers.Large genetic variability was found within the germplasm and five subpopulations were identified according to the geographical origins.Results suggested that selecting sesame materials from the different subpopulations is essential to efficiently exploit the diversity in breeding programmes.Secondly,with the aim to identify some drought tolerant genotypes and important traits to be targeted in screening for drought tolerance in a large sesame germplasm collection,ten genotypes genetically diversified were selected from West Africa and screened using 21 biochemical,physiological,agromorphological and seed nutritional traits under severe drought stress.Clustering analysis and PCA ranking value revealed that WC17,WC18,and WC14 were drought tolerant;WC12,WC13,WC06,and WC03 were moderately drought tolerant while,WC02,WC10,and WC08 were sensitive to drought.Strong accumulation?²00%ratio stress/control?of biochemical markers including SOD,APX,CAT and proline could be regarded as important indicators for selecting drought resistant genotypes.Thirdly,two contrasting genotypes?drought tolerant?DT?and drought sensitive?DS??were subjected to progressive drought stress to uncover the biochemical,physio-anatomical and transcriptome changes in the root leading to drought tolerance.Results showed that DT relied on a well-functioning taproot while DS displayed a disintegrated root due to collapsed cortical cells.This was attributed to a high accumulation of osmoprotectants and a strong activity of antioxidant enzymes especially POD in DT based on both biochemical dosage and RNA-seq of DT.Furthermore,61 candidate genes conferring high drought tolerance were discovered and may constitute useful resources for drought tolerance improvement.The AP2/ERF gene family was highly active in the sesame transcriptome in response to drought stress and then was subsequently thoroughly examined.A total of 132 AP2/ERF genes were identified in the whole sesame genome and classified into five subfamilies.Based on qRT-PCR analysis,20 genes were significantly up-/down-regulated under drought stress,of which the highly induced gene AP2si16 could be considered as a potential candidate gene for further functional validation and utilization in sesame improvement programmes for drought tolerance.Finally,to shed light on the genetic basis of drought tolerance in sesame,genome-wide association studies?GWAS?were performed for five traits related to drought tolerance including time-course wilting level,relative capsule number,relative stem length,survival rate and relative seed yield using 1M SNPs.A total of 400 worldwide sesame accessions were characterized for two consecutive years.The results revealed 969 significant SNPs(P<7.8×10^-6)of which 24%were constitutively identified in both years.These significant SNPs were resolved to eight QTLs distributed on four linkage groups.Two major genomic regions associated with various traits related to drought tolerance were discovered in sesame.The peak SNPs contributed individually from 2.21%to 10.34%of the phenotypic variance.Accumulation of favorable alleles strongly enhances the survival rate of the plants.Seventeen candidate genes with diverse functions were discovered around GWAS peaks by integrating functional annotation,RNA-seq,qRT-PCR and genetic transformation analyses.