Abstract:

This research aimed to explore the molecular mechanisms， metabolic pathways and physiological differences of responses to drought and salt stress in sweet sorghum， in order to strengthen the scientific information available to enhance sweet sorghum forage crop husbandry and breeding. Seedlings of sweet sorghum cultivar Liaotian No.1 were used to provide the plant material for study. Moderate drought stress was induced using 10% PEG-6000 solution and salt stress was simulated using 0.9% NaCl solution. Photosynthetic gas exchange parameters， endogenous hormone content， content of organic osmotic regulatory substances and antioxidant enzyme activities were determined on days 2 and 7 of stress exposure. Concurrently， transcriptome sequencing and bioinformatics analyses were carried out on seedling leaves， and qRT-PCR was used to verify the sequencing results. For sweet sorghum seedlings under salt stress， photosynthetic parameters， endogenous auxin concentration and degree of cytokinin inhibition were higher than under drought stress； Soluble sugar content was significantly higher under drought stress than under salt stress； Antioxidant enzyme activities and abscisic acid concentrations were higher under salt stress than under drought stress， indicating different physiological mechanisms of drought and salt stress response in sweet sorghum. Under drought stress on day 2， transcriptome sequencing identified 922 up-regulated and 975 down-regulated genes while under salt stress there were 2047 up-regulated and 1714 down-regulated genes. At day 7 under drought stress there were 157 up-regulated and 54 down-regulated genes while under salt stress there were 795 up-regulated and 722 down-regulated genes. Forty drought stress response genes and 493 salt stress response genes were also identified. Gene ontology （GO）-based enrichment analysis revealed that sweet sorghum seedlings under drought and salt stress were significantly enriched in pathways involving plant response to stress. Kyoto encyclopedia of genes and genomes （KEGG） enrichment analysis showed that genes up-regulated in response to drought stress tended to be involved in endoplasmic reticulum processing and shear body metabolism pathways， while salt stress response genes were generally associated with plant hormone signal transduction metabolic pathways. Analysis of the genes related to photosynthesis， plant hormone signal transduction， antioxidant enzymes and starch and sucrose metabolic pathways showed that the expression patterns of these genes were consistent with the observed changes in physiological indicators. Therefore， sweet sorghum adaptation to salt stress lags behind response drought stress at the transcriptional level. Meanwhile， the tolerance of sweet sorghum seedlings to moderate salt stress was lower than that under moderate drought stress. Soluble sugars played an important role in the resistance to drought stress of sweet sorghum seedlings. The joint regulation of plant hormone signal transduction and antioxidant enzyme activity changes was the key to the resistance of sweet sorghum seedlings to salt stress.

Key words: sweet sorghum seedlings, drought stress, salt stress, physiology, transcriptomic analysis