SA is a very promising compound for regulation of abiotic stresses because application of the appropriate concentration of SA enhances tolerance to abiotic stresses as described above. However, in certain cases, contradictory results are obtained in investigating the effect of exogenous SA application and that of endogenous SA levels. The effects of SA are dependent not only on the concentration of applied SA but also on the method of application. Still, several questions remain unanswered. It is unclear if exogenous SA application directly or indirectly increases endogenous SA levels and whether the effect of SA is connected with ROS production. Furthermore, no SA receptor has been identified. SA signaling and abiotic stress signaling could be connected and may contribute to the complex signal trans-duction network. The clarification of these questions could lead to a better understanding of the precise role of SA in abiotic stress responses.
Acknowledgements This work was supported, in part, by Special Coordination Funds for Promoting Science and Technology, and by a Grant-in-Aid for Young Scientists (B, No. 20780046; B, No. 21770032) from the Ministry of Education, Culture, Sports, Science and Technology.
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