Salicylic acid in plant salinity stress signalling and tolerance

Soil salinity is one of the major environmental stresses affecting crop production worldwide, costing over $27Bln per year in lost opportunities to agricultural sector and making improved salinity tolerance of crops a critical step for sustainable food production. Salicylic acid (SA) is a signalling molecule known to participate in defence responses against variety of environmental stresses including salinity. However, the specific knowledge on how SA signalling propagates and promotes salt tolerance in plants remains largely unknown. This review focuses on the role of SA in regulation of ion transport processes during salt stress. In doing this, we briefly summarise a current knowledge on SA biosynthesis and metabolism, and then discuss molecular and physiological mechanisms mediating SA intracellular and long distance transport. We then discuss mechanisms of SA sensing and interaction with other plant hormones and signalling molecules such as ROS, and how this signalling affects activity of sodium and potassium transporters during salt stress. We argue that NPR1-mediated SA signalling is pivotal for (1) controlling Na⁺ entry into roots and the subsequent long-distance transport into shoots, (2) enhancing H⁺-ATPase activity in roots, (3) preventing stress-induced K⁺ leakage from roots via depolarisation-activated potassium outward-rectifying channel (KOR) and ROS-activated non-selective cation channels, and (4) increasing K⁺ concentration in shoots during salt stress. Future work should focus on how SA can regulate Na⁺ exclusion and sequestration mechanisms in plants.