Integrated transcriptomics and metabolomics reveals that regulating biosynthesis and metabolism of HCN and GABA plays a key role in drought resistance of wild soybean

Wild soybeans [Glycine soja (Siebold & Zucc.)] grown in arid areas develop adaptive traits and they are important wild plant resources. In order to reveal the drought-tolerance mechanism in wild soybean, we studied the changes of growth parameters, photosynthetic physiology, and ion physiology of two wild soybean ecotypes under drought stress, and integratedly analyzed the transcriptome and metabolome changes in their leaves. Drought-tolerant wild soybean had greater root/shoot ratio, water use efficiency, and K+ and Ca2+ contents than common wild soybean. In drought-tolerant wild soybean, the gene encoding (R)-mandelonitrile lyase in the pathway of hydrogen cyanide (HCN) biosynthesis and metabolism and the genes encoding serine O-acetyltransferase and aminocyclopropanecarboxylate oxidase in the pathway of 1-aminocyclopropane-1-carboxylate oxidation to produce HCN were down-regulated. In HCN biosynthesis and metabolic pathways of drought-tolerant wild soybean, the content of cysteine as a raw material was significantly reduced, while the content of 3-cyanoalanine as a product was significantly increased. In addition, drought-tolerant wild soybeans up-regulated genes for glutamate synthase and glutamate decarboxylase during synthesis of 4-aminobutyric acid (GABA), accumulating aspartic acid, asparagine, glutamic acid, and GABA; and down-regulated the gene for 4-aminobutyrate-pyruvate transaminase in GABA metabolism. Integrated analysis showed that drought-resistant wild soybean weakened the production of HCN and enhanced the utilization of HCN to avoid HCN toxicity under drought stress. At the same time, it also accumulated GABA to maintain carbon/nitrogen balance and amino acid metabolism. This study provides insights into the genetic mechanism of adaptation to drought by wild soybean, which will be helpful for screening of excellent wild plant resources.