The role of sucrose metabolizing proteins in hyperosmotic stress tolerance in sweet sorghum (Sorghum bicolor L. Moench)

Sugar accumulation is a common metabolic response under drought stress conditions. However, studies on the underlying molecular mechanisms of sugar accumulation under stress remain restricted. This study explores the role of sucrose metabolizing proteins in conferring tolerance to drought-induced hyperosmotic stress, and ultimately osmotic adjustment in sorghum. The effect of stress on sugar content, enzyme activity and gene expression was investigated. Sweet sorghum varieties (ICSV213 and ICSB338) differing in levels of drought tolerance were subjected to a 10-day water deficit period. Brix plant sap analysis indicated elevated total soluble sugar levels under water stress conditions in both varieties with ICSV213 demonstrating a higher brix content than ICSB338. HPLC analysis gave a decrease in sucrose levels and an increase in glucose and fructose concentrations in both varieties with ICSV213 demonstrating higher hexose levels. Enzyme activity levels of invertase, sucrose phosphate synthase and sucrose synthase were found to increase under stress in both varieties with ICSV213 invertase displaying the highest activity when compared to other sucrose metabolizing enzymes. Transcriptional expression of invertase and sucrose phosphate synthase (SPS) genes was significantly up regulated in ICSV213 under stressed conditions, whereas sucrose synthase (Susy) levels remained low in both varieties. Data obtained for sorghum variety ICSV213 points towards a relationship between hyperosmotic stress tolerance and the accumulation of solutes and sucrose metabolism proteins. Consequently variety ICSV213 may therefore be an excellent target for future development of hyperosmotic stress tolerant sweet sorghum.