Coordinated regulation of photosynthesis and sugar metabolism in guar increases tolerance to drought

Abstract

Drought is an inevitable environmental constraint to plant productivity. Cyamopsis tetragonoloba commonly known as guar is an economically important crop used as vegetable, and for its seed gum. We examined the complicated network of sugar interactions and regulation of photosynthesis in four varieties of guar subjected to water stress. A presumptive mechanistic approach considering growth and development, physiological attributes, and gene expression facet was evaluated. Decreased photosynthesis results in reduced equivalents (NADPH2+) for CO2 assimilation through the Calvin cycle, which furthermore generates oxidative stress and accumulates antioxidant enzymes. The scavenging of ROS was more in RGC-986 and HG-563 than RGC-471 and Varsha variety. Chlorophyll fluorescence studies indicate a significant reduction in values of photochemical quantum yield, electron transport rates of photosystem I and II, photochemical quantum yield due to regulated energy dissipation, and fraction of energy that is passively dissipated in the form of heat and fluorescence. For a better understanding of the behavioural pattern in response to drought, we studied the relative gene expression of enzymes regulating the sugar metabolism. The study suggested that at the onset of drought the genes of PS II machinery and Calvin cycle were co-ordinately regulated efficiently in RGC-986 and HG-563 than in RGC-471 and Varsha. However, the gene expression levels of sugar transporter enzymes in leaves of guar varieties increased under water stress. Gene expression analysis of sugar metabolic pathways reveals the increased production of glucose, fructose and fructan in HG-563 and RGC-986 in comparison to Varsha and RGC-471. These results indicate that induced level of sugar metabolic genes under drought leads to the activation of its tolerance mechanism. Furthermore, this study significantly highlights that how carbon flux is diverted to sugar metabolism via a sequence of molecular events in relation to its physiological status, under water deficit conditions in guar leaves.