Role of RAB5 GEF in maintaining potassium levels under sodium chloride stress.

Salt stress is one of the major factors impacting crop productivity worldwide. Through a variety of effector and signaling pathways, plants achieve survival under salinity stress by maintaining high cytosolic potassium/sodium ion (K/Na) ratios, preventing Na cytotoxicity, and retaining osmotic balance. Ras-related protein 5 (Rab5) members are involved in the trafficking of endosomes to the vacuole or plasma membrane (PM). The vacuolar protein sorting- associated protein 9 () encodes the single guanine nucleotide exchange factor (GEF) that activates all three known Rab5 proteins in . Previous work from our group has reported the critical function of for the operation of salt-induced endocytic pathway, as well as the expansion of endomembrane compartments under saline stress conditions. Here we show an additional role of in plant response to salt stress via maintenance of K status of the cell rather than Na homeostasis. Our results show that roots from mutant, subjected to 100 mM NaCl, display alterations in transcript levels of genes involved in the K homeostasis pathway. Concurrent with the observed sensitivity of mutant under NaCl stress, exposure to low K environments resulted in growth retardation, and reduced rate of endocytosis. Furthermore, mutant displays reduced expression of auxin reporter, Direct Repeat-5 (DR5), and alterations in polarity and abundance of auxin efflux carrier PIN- FORMED2 (PIN2). Imposition of NaCl stress was found to be restrictive to the elongation capacity of cells in the root elongation zone of mutant. Together our results indicate that alterations in K homeostasis and associated cellular changes causing increased cell wall pH, contribute to diminished root growth and compromised survival of mutant under salt stress conditions.