Studies On Mechanisms Of Aluminum Toxicity Tolerance In Tibetan Wild Barley

Aluminum (Al) toxicity is a main constraint of crop growth and yield formation in acid soils. Overcoming of the problem caused by Al toxicity is quite important for fully utilizing land resource and increasing crop production in the world. Therefore, Understanding of physiological and molecular mechanisms of Al tolerance in crops is imperative. Tibetan annual wild barley(Hordeum vulgare subsp. spontaneum), recognized as one of the ancestors of cultivated barley, is a previous plant germplasm, rich in genetic diversity and elite stress-tolerant alleles. In the present study, the physiological and genetic characteristics of Tibetan wild barley were investigated to make sight into the Al3+-tolerant mechanisms. The main findings were summarized as follows:1. The effect of acid soil stress on ionomic change in barley90barley genotypes, including wild and cultivated barleys, were planted in the acid soil with pH below4.5and limed soil, to determine growth and ionomic difference among genotypes under the two soil conditions. There was no significant difference in root aluminum (A1) concentration between the plants on the acid and limed soils. However, the plants grew in the acid soil had higher shoot Al concentration than those grew in the limed soil. Moreover, the genotypes tolerant to acid soil showed lower shoot Al concentration than sensitive ones. Loading analysis indicated that shoot Al concentration is the main factor contributed to shoot ionomic change under Al stress. Root phosphorus (P) concentration was higher for the plants subjected to acid soil than those to limed soil, but opposite was true for shoot P concentration. It may be suggested that Al-phosphate complex was formed in root surface for plants in response to Al stress, preventing P transportation from roots to shoots. Al stress disordered the relationships between S, K, Zn and other elements in shoots, resulting in disturbance of shoot ionic homostesis. 2. Genome-wide association analysis of aluminum tolerance in cultivated and Tibetan wild barleyA genome-wide association analysis (GWAS) was performed by using four root parameters reflecting Al tolerance and469DArT markers distributed evenly on all7barley chromosomes after making evaluation of A1tolerance of110Tibetan wild accessions and56cultivated cultivars. Population structure and cluster analysis revealed that a wider genetic diversity was present in Tibetan wild barley. Linkage disequilibrium (LD) decayed more rapidly in Tibetan wild barley (9.30cM) than in cultivated barley (11.52cM), indicating that GWAS may provide higher resolution in Tibetan group. Two novel Tibetan group-specific loci, bpb-9458and bpb-8524were identified. They were associated with relative longest root growth (RLRG), and located on chromosomes2H and7H, and could explain12.9%and9.7%of the phenotypic variation, respectively. Moreover, a common locus bpb-6949, localized at0.8cM away from a candidate gene HvMATE, was detected in both wild and cultivated barleys, and it was significantly associated with total root growth (TRG). The present results showed that Tibetan wild barley could provide elite germplasm and novel genes for barley Al-tolerant improvement.3. The effect of A1stress on root morphology, antioxidative enzymes and organic acid secretion in barleyThe responses of root morphology, antioxidative enzymes and organic acid secretion of wild barley accessions X29, X9and cultivated cultivar Dayton to Al stress were examined. More inhibition of root elongation, larger root diameter and smaller root surface area were observed for X9in comparison with other two genotypes. Under Al stress, the activities of antioxidative enzymes were greatly affected, and malondialdehyde (MDA) content was greatly increased in X9. There was no significant difference in Al3+-induced citrate secretion or HvMATE expression between X29and X9, but Dayton showed much higher HvMATE expression level. The amount of oxalate and phosphate secreted in X29was higher than that in X9under Al stress. It may be suggested that Al3+-induced oxalate and phosphate efflux could be a possible Al3+-resistant mechanism, which need to be proved by more evidences.4. Metabolites and metabolic pathway in response to Al stressUsing gas chromatography-mass spectrometry (GC-MS), we investigated the metabolites levels of X29, X9and Dayton under Al stress. A total of79key metabolites were identified and the adaptive metabolic pathway was constructed. The concentrations of ROS cleaners, such as proline, Ferulic acid,4-hydroxy-benzoic acid and4-hydroxy-3-methyl benzoic acid, increased greatly in Dayton roots, in response to increased membrane lipid peroxidation. Higher concentrtions of carbohydrate metabolic intermediates, such as glucose-6-P and fructose-6-P in X29could be a physilogical basis for Al tolerance. Maintainance of high level in root phosphate and oxalate concentrations, which are then secreted into rhizophere and cultrue solution, alleviating Al stress, could be a Al3+-tolerant mechanism in X29. But the hypothesis should be confirmed by further study.