Expression levels of vacuolar ion homeostasis-related genes, Na+ enrichment, and their physiological responses to salt stress in sugarcane genotypes

Sugarcane is a sugar-producing crop widely grown in tropical regions in over 120 countries of the world. Salt-affected soil is one of the most significant abiotic constraints that inhibit growth and crop productivity, and, consequently, reduce sucrose concentration in the stalk. The present study investigated vacuolar ion homeostasis, Na+ accumulation, and physiological and morphological adaptations under salt stress in two different sugarcane genotypes (salt-tolerant K88-92 and salt-sensitive K92-80) under greenhouse conditions. Na+ was rapidly absorbed by the root tissues of both sugarcane genotypes within 3–7 days of 150 mM NaCl treatment, as confirmed by the results of CoroNa Green fluorescence staining. In addition, the rate of Na+ translocation from roots to shoots was evidently reduced, leading to lower amount of Na+ in the leaf tissues. At the cellular level, expression of ShNHX1 (vacuolar Na+/H+ antiporter), ShV-PPase (vacuolar H+-pyrophosphatase), and ShV-ATPase (vacuolar H+-ATPase) was upregulated in salt-stressed plants for the compartmentation of Na+ into the vacuoles of root cells. Interestingly, sucrose, glucose, and fructose in root tissues of salt-stressed sugarcane cv. K88-92 were increased by 10.61, 5.58, and 1.81 folds, respectively, over the control. Total soluble sugars in the roots and free proline in the leaves of sugarcane cv. K88-92 (salt-tolerant) were enriched by 3.08 and 1.99 folds, respectively, when plants were exposed to 150 mM NaCl, leading to maintain better photosynthetic abilities, net photosynthetic rate (Pn), stomatal conductance (gs), transpiration rate (E), and water use efficiency (WUE) in sugarcane cv. K88-92 than those in cv. K92-80. The study concludes that Na+ compartmentation in the root tissue acts as a major defense mechanism in sugarcane, especially in salt-tolerant genotype.