The Effect Of Post-anthesis Drought Stress On Malt Barley Quality And Its Mechanisms

Post-anthesis drought stress is a major factor causing instability and deterioration of malt quality in China as well as in the world.It is imperative and important to identify or create the elite germplasm and reveal the mechanisms of genetic regulation of malt quality in response to drought stress for developing the new malt barley cultivars with drought tolerance and quality stability.In this study 143 barley genotypes,including cultivated and Tibetan wild barley were used.At first the genotypes differing distinctly in drought stress tolerance were identified under a field trial.Then the genotypes with significant difference were used for transcriptome and metabolome analysis to compare their different response of malt quality to post-anthesis drought stress.Moreover,the genetically transformated materials were obtained by overexpression,RNAi and gene editing technologies to determine the functions of the key genes regulating malt qualities.The current studies revealed the mechanisms of genetic regulation of the main malt qualities under drought stress,and provided theoretical basis and genetic resources for high-quality malt barley breeding.The main results are as follows:1.The influence of drought stress on malt quality of the wild and cultivated barleysAs a major abiotic stress,drought causes instability and deterioration of malt barley quality.There is a distinct difference among barley cultivars in the response of the main malt quality traits to drought stress.This study identified some Tibetan wild barley accessions with relatively less change in malt quality traits under drought.Then the impact of post-anthensis drought stress on grain weight and some malt quality traits were examined,including total protein,protein fractions andβ-glucan contents,limit dextrinase andβ-amylase activities in 4 barley genotypes（2 Tibetan wild and 2cultivated barleys）.Drought treatment reduced grain weight,β-glucan content,and increased total protein content,β-amylase activity.These changes differed among barley genotypes and treatments,and were closely associated with the change of kernel weight.It was indicated that Tibetan wild barley remained less change than the cultivated barley in malt quality under drought stress.Relatively stable kernel weight under water stress should be highlighted in malt barley breeding in order to reduce the effect of water stress on malt quality.2.Transcriptome and metabolome analysis revealed regulatory networks and key genes controlling barley malt quality in responses to drought stressIn this study,transcriptome and metabolome analysis were performed on the grains of Tibetan wild accession,XZ166（drought tolerant）and cultivated barley,ZU9（drought sensitive）under post-anthesis drought stress.The results showed that drought stress enhanced H₂O₂ and heat shock protein accumulation in the two barley genotypes,with XZ166 showing greater capacity than ZU9 in scavenging H₂O₂ and reducing misfolded protein accumulation.Moreover,XZ166 was more efficient in redistributing assimilates stored in the vegetative tissues into the developing grains.After re-watering to relieve drought stress,XZ166 can further modify auxin transport and ethylene signaling,enhancing redistribution of assimilates into developing grains.Transcriptome comparisons and weighted correlation network analysis（WGCNA）identified some key genes regulating the responses of malt quality traits under drought stress,such as LRR-RLK.Under drought stress,the metabolism of carbohydrate and nitrogen in barley grains changed,and some key metabolic pathways including citric acid cycle were affected,some of which were related to defense responses.3.Identification and functional analysis of LRR-RLK genes involved in drought stress tolerance in barleyReceptor-like protein kinases with leucine-rich repeats（LRR-RLKs）form a large cluster of receptor-like kinases（RLKs）and play the important roles in plant growth,immunity,and stress resistance.Although a lot of plant LRR-RLK genes were already identified in many species,but their roles in barley remain to be clarified.The present study obtained 251 barley LRR-RLK genes from its genome,which could be further clustered into 20 subgroups.Drought treatment on XZ166 and ZU9 was conducted to compare the genotypic difference in the expression of LRR-RLK genes in response to drought stress.Most differentially expressed LRR-RLK genes under drought stress were up-regulated in the drought-tolerant genotype XZ166,while down-regulated in the drought sensitive genotype ZU9.Moreover,it was found there are many Me JA-and ABA-responsive cis-acting elements in the promoter region of these differently-expressed LRR-RLK genes.4.Functional analysis of LRR-RLK gene involved in drought stress tolerance in barleyThe biological functions of LRR-RLK genes in response to drought stress are still unclear.Previous studies found that HVPXY1,a member of barley LRR-RLK gene family,plays a key role in response to drought stress of barley grains.Further cloning and transgenic analysis of the HVPXY1 gene were carried out to understand its role in response to drought stress.The results showed that expression of HVPXY1 gene was enhanced by drought,Me JA,ABA and H₂O₂,indicating that the gene might be involved in ABA,Me JA and H₂O₂ signal transduction and a series of drought responses.Enhancement of the HVPXY1 gene expression could improve the tolerance of barley grains to drought stress,which functioned positively for stabilizing malt quality.