Identification of quantitative trait loci (QTL) for drought tolerance and leaf senescence in juvenile barley

Abstract

Drought stress as a trait with increasing importance in the background of climate change is an important factor limiting barley yield. Induced by drought, leaf senescence may occur prematurely, leading to a stop of photosynthesis and to an early translocation of stored assimilates into grains. For barley breeding, the identification of quantitative trait loci (QTL) involved in drought stress and leaf senescence may be an advantage as reliable phenotyping for drought stress is difficult to achieve. Therefore, the aim of the present thesis was to identify markers associated to drought stress response and drought stress induced leaf senescence in juvenile barley through genome wide association studies (GWAS), which will facilitate efficient marker based selection procedures. In a first step, a screening method was developed for analysing drought stress response and early leaf senescence in juvenile barley. Next, in semi controlled greenhouse pot experiments 156 winter barley genotypes were analysed in early developmental stages under control and drought stress treatment. Drought application started at the primary leaf stage and continued for a four weeks stress period. These experiments were used for phenotyping six physiological parameters (biomass yield, leaf colour, electron transport rate at photosystem II, osmolality, content of free proline and total content of soluble sugars), as well as for gene expression analysis of genes involved in drought stress and leaf senescence. Significant genotypic and treatment effects were detected for all phenotypic traits and gene expression data. Based on these data and on 3,212 SNP markers of the Illumina 9k iSelect Chip, GWAS were conducted to detect QTL and expression QTL (eQTL). In total, 47 significant QTL were identified for the traits analysed under drought stress conditions and 15 significant eQTL were found for the relative expression of the 14 genes involved in these traits. Under drought stress conditions, two major QTL regions overlapping for different traits such as biomass yield and leaf colour were detected on chromosome 2H at 50 cM and on chromosome 5H at 45 cM. In these QTL, genes coding for proteins involved in drought stress or leaf senescence were identified. Four of these genes showed a differential expression and thus, eQTL were detected. One eQTL for TRIUR3 coincides with the phenotypic QTL on chromosome 5H. After validation respective markers BOPA1_9766-787 and SCRI_RS_102075 may be used in future barley breeding programmes for improving tolerance to drought stress and leaf senescence.