Physiological, Biochemical Responses And Signal Transduction In Roots From Upland Rice Under Salt Stress

In the present study,we used rice(Yujing-6)as control to study the physiological, biochemical responses and signal transduction of upland rice(Zhenghan-2)in salt stress.The main results showed that,under salt stress,the extents of the relative membrane permeability and lipid peroxide in Zhenghan-2 roots were lower than that in Yujing-6 roots.These results suggested that Zhenghan-2 had higher salt tolerance than Yujing-6 did.The further study showed that antioxidant enzyme activities in Zhenghan-2 roots were dramatically enhanced compared to Yujing-6 roots;and the contents of ROS(O₂^·-and H₂O₂)in Zhenghan-2 roots were lower than those in Yujing-6 roots under salt stress.These higher activities of antioxidant enzymes in Zhenghan-2 roots could facilitate to scavenge excessive ROS,and thus decreased the oxidative damage of Zhenghan-2 in salt stress.In control conditions,Na⁺ content was similar between Zhenghan-2 and Yujing-6.Na⁺ levels increased in the roots of both rices,but it increased higher in Yujing-6 than that in Zhenghan-2.These data suggested that Zhenghan-2 might have a better mechanism for controlling the low Na⁺ to K⁺ ratio than Yujing-6 did.Under salt stress,Ca²⁺percentage increased in Zhenghan-2 roots;in contrast,it decreased in Yujing-6 roots.Therefore,the increased Ca percentage in Zhenghan-2 might be important for salt resistance.Under salt stress, alternative pathway rate increased in both rice roots,and it was higher in Zhenghan-2 roots compared with that in Yujing-6 roots.The higher alternative pathway of Zhenghan-2 could facilitate to decrease ROS levels induced by salt stress.Pentose phosphate pathway(PPP)increased in both rice roots,and it was higher in Zhenghan-2 than that in Yujing-6.A similar profile was found in glucose-phosphate dehydrogenase(G6PDH)activity,which was the key enzyme of PPP.Thus,the enhanced PPP could produce more NADPH for the tolerance of Zhanghan-2 to salt tolerance.H₂O₂,as a signal,was involved in salt tolerance of upland rice roots. Exogenous H₂O₂(1 mM)markedly enhanced the activities of antioxidant enzymes (SOD,POD,CAT,APX)in Zhenghan-2 roots;in Yujing-6 roots,SOD,CAT and APX activities increased but POD activity did not.Under salt stress,antioxidant enzyme activities increased in both rice roots.Under NaCl+DPI(NADPH oxidase enzyme inhibitor)treatment,the NaCl-induced H₂O₂ accumulation was abolished,and the antioxidant enzyme activities also decreased to the control levels in both rice roots. These results suggested that H₂O₂ was involved in regulating antioxidant enzyme activities in both rice roots under salt stress,and Zhenghan-2 had a stronger H₂O₂ signal transduction pathway.In summary,Zhenghan-2 had higher salt tolerance than Yujing-6,which might due to its higher antioxidant enzyme activities and alternative pathway rate to decrease ROS and maintain cellular redox status;a better mechanism for controlling cellular ion homeostasis;as well as a stronger H₂O₂ signal transduction pathway in salt stress.The long-term adaptation of Zhenghan-2 to drought stress might result in its higher salt tolerance,it was known as the 'cross-tolerance' phenomenon in nature.