Diurnal regulation of SOS Pathway and Sodium Excretion Underlying Salinity Tolerance of Vigna marina

Vigna marina (Barm.) Merr. is adapted to tropical marine beaches and has an outstanding tolerance to salt stress. Given there are growing demands for cultivating crops in saline soil or with saline water, it is important to understand how halophytic species are adapted to the saline environments. Here we sequenced the whole genome of V. marina with longreads, and performed a forward genetic study to identify QTLs involved in salt tolerance. As the QTL region harbored SOS1 , encoding plasma membrane Na ⁺ /H ⁺ antiporter, we traced the sodium dynamics in V. marina using the positron emitting tracer imaging system (PETIS) and revealed that it actively excretes sodium from the root. In addition, the sodium excretion was faster in the light period and slower in the dark period, indicating it is under diurnal regulation. The following comparative transcriptome analyses revealed that the SOS pathway plays a key role in the diurnal regulation of sodium excretion. Furthermore, we demonstrated that, under a condition of mild salt stress, the plants with the diurnally regulated SOS pathway outperformed those with the constitutively activated one.