Characterization of Na⁺ signaling pathways in roots of Arabidopsis thaliana

Soil salinization threatens agricultural production as high salt levels reduce yields. Because of their sessile lifestyle, plants must be able to continuously monitor environmental cues to initiate directed root growth towards favorable conditions and away from unfavorable ones, such as saline soils. In this investigation, cytosolic Ca²⁺, surface pH, and extracellular reactive oxygen species reporters were utilized to characterize the initial signaling events in roots of Arabidopsis thaliana associated with the addition of 3 mM Na⁺ cations. Confocal laser scanning microscopy allowed for real-time monitoring of these secondary messengers. The results indicate that the cellular environment is rapidly modified following the addition of a low, non-stressing concentration (3 mM) of Na⁺, suggesting that the root has mechanisms in place to monitor the salinity status of the surrounding medium. These results show that Na⁺ constitutes a novel signaling molecule to which Arabidopsis can respond. To identify proteins that are involved in sodium sensing, surface pH was monitored in mutants with defects in salt overly sensitive (SOS) proteins. The SOS pathway was originally identified as a salinity stress responder that was critical to plant survival under high salt conditions; however, here it is implicated in the sensing of 3 mM NaCI. The results of this study may direct future research towards the identification of proteins that link Na⁺ perception to these secondary signals, and ultimately to the generation of salt-tolerant crops