Inhibition of the maize salt overly sensitive pathway by ZmSK3 and ZmSK4

Soil salinity is a worldwide problem that adversely affects plant growth and crop productivity. The salt overly sensitive (SOS) pathway is evolutionarily conserved and essential for plant salt tolerance. In this study, we reveal how the maize shaggy/glycogen synthase kinase 3-like kinases ZmSK3 and ZmSK4, orthologs of brassinosteroid insensitive 2 in Arabidopsis thaliana, regulate the maize SOS pathway. ZmSK3 and ZmSK4 interact with and phosphorylate ZmSOS2, a core member of the maize SOS pathway. The mutants defective in ZmSK3 or ZmSK4 are hyposensitive to salt stress, with higher salt-induced activity of ZmSOS2 than that in the wild type. Furthermore, the Ca^(2+) sensors ZmSOS3 and ZmSOS3-like calcium binding protein 8 (ZmSCaBP8) activate ZmSOS2 to maintain Na^(+)/K^(+) homeostasis under salt stress and may participate in the regulation of ZmSOS2 by ZmSK3 and ZmSK4. These findings discover the regulation of the maize SOS pathway and provide important gene targets for breeding salt-tolerant maize.

The Salt Overly Sensitive （SOS） Pathway： Established and Emerging Roles

Soil salinity is a growing problem around the world with special relevance in farmlands. The ability to sense and respond to environmental stimuli is among the most fundamental processes that enable plants to survive. At the cellular level, the Salt Overly Sensitive （SOS） signaling pathway that comprises SOS3, SOS2, and SOS1 has been proposed to mediate cellular signaling under salt stress, to maintain ion homeostasis. Less well known is how cellularly heterog- enous organs couple the salt signals to homeostasis maintenance of different types of cells and to appropriate growth of the entire organ and plant. Recent evidence strongly indicates that different regulatory mechanisms are adopted by roots and shoots in response to salt stress. Several reports have stated that, in roots, the SOS proteins may have novel roles in addition to their functions in sodium homeostasis. SOS3 plays a critical role in plastic development of lateral roots through modulation of auxin gradients and maxima in roots under mild salt conditions. The SOS proteins also play a role in the dynamics of cytoskeleton under stress. These results imply a high complexity of the regulatory networks involved in plant response to salinity. This review focuses on the emerging complexity of the SOS signaling and SOS protein functions, and highlights recent understanding on how the SOS proteins contribute to different responses to salt stress besides ion homeostasis.

ESCRT-I Component VPS23A Sustains Salt Tolerance by Strengthening the SOS Module in Arabidopsis

The Salt-Overly-Sensitive(SOS)signaling module,comprising the sodium-transport protein SOS1 and the regulatory proteins SOS2 and SOS3,is well known as the central salt excretion system,which helps protect plants against salt stress.Here we report that VPS23A,a component of the ESCRT(endosomal sorting complex required for transport),plays an essential role in the function of the SOS module in conferring plant salt tolerance.VPS23A enhances the interaction of SOS2 and SOS3.In the presence of salt stress,VPS23A positively regulates the redistribution of SOS2 to the plasma membrane,which then activates the antiporter activity of SOS1 to reduce Na+accumulation in plant cells.Genetic evidence demonstrated that plant salt tolerance achieved by the overexpression of SOS2 and SOS3 dependeds on VPS23A.Taken together,our results revealed that VPS23A is a crucial regulator of the SOS module and affects the localization of SOS2 to the cell membrane.Moreover,the strong salt tolerance of Arabidopsis seedlings conferred by the engineered membrane-bound SOS2 revealed the significance of SOS2 sorting to the cell membrane in achieving its function,providing a potential strategy for crop salt tolerance engineering.