ROP2-RIC1 Signaling Pathway Is Involved In Arabidopsis Salt Stress Tolerance

Microtubules are highly dynamic subcellular structure.They respond to plant growth and development signals(such as plant hormones)and environmental cues(such as illumination,gravity,biotic and abiotic stresses),function in plant growth,morphogenesis,and plant responses to environment signals.Accumulating evidence shows that microtubules not only function in the downstream of signal transduction pathways,but also in the upstream of signal transduction pathways.Previous reports showed that microtubules have an important role in the plant tolerance under saltstress.The reorganization of microtubules under salt stress is required for Arabidopsis surviving under high salinity.Hence,it is of interest to study the mechanisms of how plants regulate their microtubule dynamic in response to salt stress.ROP signaling participates in multiple distinct signaling pathways in plant growth and development.In addition,evidence is also reported to show that ROP signaling is involved in plant innate immunity.RIC1 belongs to a plant-specific family of ROP effectors,and is identified as a microtubule-associated protein that mediates a ROP6 signaling pathway to regulate cortical microtubule organization,but the function of RIC1 on microtubules is suppressed by ROP2.This study reveals a ROP2-RIC1 pathway that participates in the response to salt stress by regulating the microtubule reorganization in Arabidopsis.Observations showed that ric1-1(Col-0)mutant exhibited a higher survival rate than wild type under salt stress.Confocal observations revealed that knockout of RIC1 promoted the recovery of depolymerized microtubules induced by salt treatment,so that microtubule reorganization was enhanced under salt stress.Besides the down regulation of RICl expression level,the activity of RIC1 on microtubules was also affected by active ROP2.Overexpression of constitutively active rop2(CA-rop2)increased seedling survival rate under salt stress.In addition,the rop2-1 ric1-1(Col-0)double mutant rescued the salt-sensitive phenotype of rop2-1,indicating that ROP2 functions in salt tolerance through RIC1.During salt stress,the increased activity of ROP2 was observed,which was corresponding to the time course of microtubule recovery,indicating that the regulation of ROP2 activity is closely related to microtubule dynamic changes under salt stress.The observation of GFP-RIC1 in pavement cells showed that the fluorescence intensity of GFP-RIC1 on microtubules decreased but increased on the plasma membrane after salinity treatment.However,the translocation of GFP-RIC1 was not observed in rop2-1 mutant,indicates that the translocation of RIC1 induced by salinity is mediated by ROP2.Nevertheless,phenotype analysis demonstrated that ROP6,which also actives RIC1,does not participate in Arabidopsis salt stress tolerance.In addition,experiments also show that such ROP2-RIC1 pathway still functions in 100 mM NaCl treatment,suggesting that this pathway is truly a responsive pathway for Arabidopsis survival under salt stress.This study demonstrated that salt stress signal actives ROP2,and activated ROP2 inhibits RIC1's function on microtubules,and the recovery of microtubules after the depolymerization induced by salt stress is therefore enhanced,which is beneficial to the seedling survival under salt stress.This study elucidates the importance of ROP2 signaling in abiotic tolerance,and reveals a novel mechanism for plant salt tolerance,provides new evidence for better understanding of plant response to abiotic stress.