MsWRKY44 regulates Mg-K homeostasis of shoots and promotes alfalfa sensitivities to acid and Al stresses

Mg-K homeostasis is essential for plant response to abiotic stress, but its regulation remains largely unknown. MsWRKY44 cloned from alfalfa was highly expressed in leaves and petioles. Overexpression of it inhibited alfalfa growth, and promoted leaf senescence and alfalfa sensitivities to acid and Al stresses. The leaf tips, margins and interveins of old leaves occurred yellow spots in MsWRKY44-OE plants under pH4.5 and pH4.5+Al conditions. Meanwhile, Mg-K homeostasis was substantially changed with reduction of K accumulation and increases of Mg as well as Al accumulation in shoots of MsWRKY44-OE plants. Further, MsWRKY44 was found to directly bind to the promoters of MsMGT7 and MsCIPK23, and positively activated their expression. Transiently overexpressed MsMGT7 and MsCIPK23 in tobacco leaves increased the Mg and Al accumulations but decreased K accumulation. These results revealed a novel regulatory module MsWRKY44-MsMGT7/MsCIPK23, which affects the transport and accumulation of Mg and K in shoots, and promotes alfalfa sensitivities to acid and Al stresses. Environmental Implications At soil pH<5, the form of Aluminum (Al) changes from non-toxic Al(OH)3 and Al(OH)3•H3PO4 to soluble Al3+ ions, that are absorbed by plant roots and quite toxic to plant growth and development. This study identified an acid and Al responsive MsWRKY44 gene from alfalfa which activated the expressions of MsMGT7 and MsCIPK23 to form a precise regulatory module to maintain the Mg-K ion homeostasis under normal condition, but disrupt the Mg-K homeostasis under excess H+ and Al3+ condition. Based on this, the phytotoxicity of acid and acid-Al stresses in plants was partly revealed at the physiological, biochemical and molecular.