| Numerous studies showed that salt stress usually destroyed the ionic homeostasis, water balance and redox equilibrium in plant tissues. Whether plants can survive in saline conditions is directly related to the ability of maintaining intracellular K+/Na+homeostasis and redox equilibrium, which are regulated by a complex signaling network. Recently, a large number of studies have shown that SA are not only involved in the regulation of biotic resistance, but also plays an important role in inducing abiotic stress resistance. However, the role of SA in plant responses to salt stress is still controversial. It has been demonstrated that the signaling molecules, such as Ca2+, H2O2, and NO, were involved in the regulation of K+/Na+homeostasis in plants. However, the role of SA in mediating K+/Na+homeostasis is still unknown. Furthermore, to our knowledge, almost all the researches on the effects of SA on the response to salt stress focused on herbaceous plants, while the reference on the role of SA on woody plants is still limited.In this study, using hydroponic seedlings of L. ruthenicum, we investigated the changes of K+, Na+and H+fluxes in L. ruthenicum roots by using the Non-invasive Micro-test Technique, to clarify the role of SA in mediating K+/Na+homeostasis in plants under salt stress. In addition, we determined the changes of ROS levels (O2-,H2O2), antioxidant enzyme activities (SOD, CAT, GPX etc.) and antioxidants contents (ASA, GSH) in the leaves and roots of L. ruthenicum seedlings under saline conditions to investigate the role of SA in mediating ROS homeostasis under salt stress.The main results are as follows:1. NMT data showed that the Na+efflux and H+influx in L. ruthenicum roots increased significantly after a relatively long-term salt stress (in saline environment of100mM NaCl for3days). While, the salt-induced Na+efflux in root tissues was inhibited by amiloride (a Na+/H+antiporter inhibitor) and the salt-induced K+efflux was inhibited by TEA (a K+channels blocker). These results indicated that the PM Na+/H+antiport system in L. ruthenicum roots was activated by salt stress, which promoted Na+efflux in roots. Meanwhile, we found that the onset of salt shock caused a transient shift from H+influx to net efflux, indicating a strong H+pump activity in L. ruthenicum roots, which will help to reduce K+loss and maintain intracellular K+/Na+homeostasis. On the other hand, salt stress activated the antioxidant defense system in L. ruthenicum roots and leaves (particularly at the initial stage of salt stress). H2O2contents gradually increased, while the O2-contents significantly reduced at the initial stage of salt stress. Both the leaves and the roots of L. ruthenicum had no obvious salt injury symptoms after exposure to200mM NaCl for7days. Therefore, the salt-induced H2O2may play a signaling role in regulation of K+/Na+homeostasis and ROS homeostasis under salt stress in L. ruthenicum.2. Salt stress significantly induced SA accumulations, especially its conjugated form. However, the salt-induced SA accumulation could be completely abolished by uniconazole, an inhibitor of benzoic acid2-hydroxylase. These results indicated that SA might be synthesized via phenylalanine pathway in L. ruthenicum seedlings under salt stress. After SA pretreatment for24h, both free SA and conjugated SA accumulated in L. ruthenicum tissues. We proposed that L. ruthenicum roots can effectively absorb exogenous SA and transport it to the leaves. On ther other hand, although free SA was added to hydroponic cultures, conjugated SA rather than free SA increased to a greater extent in the tissues of L. ruthenicum, implying that part of free SA was converted to conjugated form.3. After a long-term salt exposure (100mM NaCl for15days), as compared with the control plants, the K+contents decreased significantly in roots, stems and leaves of SA pre-treated seedlings, but Na+contents increased, which led to a decreased K+/Na+ratio. The NMT data showed that SA pre-treatment significantly inhibited the salt-induced Na+efflux and H+influx, which was mediated by PM Na+/H+antiporter. On the contrary, when the salt-induced SA accumulation was inhibited by uniconazole, Na+/H+exchange was remarkably promoted after salt exposure for3days. These results suggested that SA may constrain the PM Na+/H+antiporter when L. ruthenicum seedlings were exposed to salt stress. Meanwhile, SA completely inhibited the net K+efflux under either saline or non-saline conditions for3days, which was mediated by K+channels. Contrarily, when salt-induced SA accumulation was inhibited by uniconazole, a lesser K+efflux were observed either after a transient salt shock or a relatively long-term salt stress. Taken these results together, we speculate that SA may affect K+flux of roots by inhibiting the K+channels under either saline or non-saline conditions.4. SA pre-treatment induced an increase in O2-contents in both roots and leaves when L. ruthenicum were exposed to200mM NaCl, although SOD activity increased significantly as well. However, the H2O2contents in L. ruthenicum roots and leaves significantly decreased at the finally stage of salt stress. SA pre-treatment induced a decrease in CAT and GPX activities in saline conditions, as well as caused a disorder in ASA-GSH cycle when the plant exposed to high salinity. However, the opposite results were obtained when the salt stress induced SA were inhibited by BA2H inhibitor. Therefore, we suggest that increasing SA accumulation negatively affect the response of L. ruthenicum tissues to200mM NaCl by an imbalance in antioxidant metabolism. |
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