Abstract
Melatonin (MT) is a multifunctional molecule in animals and plants and is involved in defense against salinity stress in various plant species. In this study, MT pretreatment was simultaneously applied to the roots and leaves of sweet potato seedlings [Ipomoea batatas (L.) Lam.], which is an important food and industry crop worldwide, followed by treatment of 150 mM NaCl. The roles of MT in mediating K+/Na+ homeostasis and lipid metabolism in salinized sweet potato were investigated. Exogenous MT enhanced the resistance to NaCl and improved K+/Na+ homeostasis in sweet potato seedlings as indicated by the low reduced K+ content in tissues and low accumulation of Na+ content in the shoot. Electrophysiological experiments revealed that exogenous MT significantly suppressed NaCl-induced K+ efflux in sweet potato roots and mesophyll tissues. Further experiments showed that MT enhanced the plasma membrane (PM) H+–ATPase activity and intracellular adenosine triphosphate (ATP) level in the roots and leaves of salinized sweet potato. Lipidomic profiling revealed that exogenous MT completely prevented salt-induced triacylglycerol (TAG) accumulation in the leaves. In addition, MT upregulated the expression of genes related to TAG breakdown, fatty acid (FA) β-oxidation, and energy turnover. Chemical inhibition of the β-oxidation pathway led to drastic accumulation of lipid droplets in the vegetative tissues of NaCl-stressed sweet potato and simultaneously disrupted the MT-stimulated energy state, PM H+–ATPase activity, and K+/Na+ homeostasis. Results revealed that exogenous MT stimulated TAG breakdown, FA β-oxidation, and energy turnover under salinity conditions, thereby contributing to the maintenance of PM H+–ATPase activity and K+/Na+ homeostasis in sweet potato.
Highlights
Soil salinity, a major environmental stress, disturbs normal growth, cellular ion homeostasis, and metabolic pathways in plants and decreases the crop yield by approximately 20% in irrigated lands (Shabala et al, 2016)
To further elucidate the mechanism through which MT functions in plants exposed to salinity stress, we investigated the effects of exogenous MT on K+/Na+ homeostasis and lipid metabolism in salinized sweet potato [Ipomoea batatas (L.) Lam.], which is an important food and industry crop worldwide, through a combination of multiple analytical techniques
MT enhanced the chlorophyll content, relative water content, and the antioxidant enzyme activities in Xu 32 leaves after 7 days of salinity stress (Supplementary Figure S1). These results indicated that salt tolerance in Xu 32 seedlings was improved by exogenous MT
Summary
A major environmental stress, disturbs normal growth, cellular ion homeostasis, and metabolic pathways in plants and decreases the crop yield by approximately 20% in irrigated lands (Shabala et al, 2016). Maintaining an appropriate cytosolic potassium–sodium ratio under high salinity condition is an important determinant of plant salt tolerance (Anschütz et al, 2014). Salttolerant species (such as halophytes) or crop varieties exhibit high K+ retention capacity in the root and leaf tissues (Anschütz et al, 2014; Kumari et al, 2015). Scholars have confirmed the strong positive correlation between cellular K+ retention and salt tolerance of the whole plant in a broad range of species, including barley (Chen et al, 2005), wheat (Cuin et al, 2008), poplar (Sun et al, 2009b), sweet potato (Yu et al, 2016), Brassica species (Chakraborty et al, 2016), and halophytes (Bose et al, 2015). High PM H+–ATPase activity under salinity condition contributes to the maintenance of low depolarized membrane potential, thereby decreasing the opening of depolarizationactivated K+ outward-rectifying channels and K+ efflux triggered by high salinity (Shabala and Pottosin, 2014)
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