The Role Of Sa In Modulation Of Aluminum-Induced Citrate Efflux And Oxidative Stress In The Roots Of Cassia Tora L.

Cassia tora L. has been reported to be an Al-tolerant species due to its specifical secretion of a large amount of citrate from roots in response to Al. The present investigation focused on the Al-induced citrate efflux and its relation to root elongation inhibition, and the possible alleviation of Al toxicity in Cassia tora L by exogenous SA. It was observed that the treatment of seedlings with 50 mol/L Al for 12h led to a remarkable inhibition of root growth, which was accompanied by sufficient efflux of citrate out of roots and increase in the citrate content in the root tips. Activities of citrate synthase (CS) increased while those of aconitase (Aco) decreased during the Al stress. This might be the reason for the Al-induced increase in citrate efflux.To better understand the regulatory process leading to the citrate efflux, the possible involvement of salicylic acid(SA) in regulating Al-induced citrate release in C. tora was identified. The response of citrate efflux to exogenous SA was concentration-dependent. Application of SA at 5 mol/L in solution containing 20 mol/LAl increased citrate efflux to the level 1.76-fold higher than in 20 mol/L Al treatment alone. However, inhibition of citrate release was observed when SA concentrations were increased to more than 20 mol/L. Increased citrate efflux due to the SA treatment was associated with decreased inhibition of root growth and Al content in root tips, suggesting that exogenous SA could confer Al tolerance by increasing citrate efflux. We also examined CS and Aco activities, however, activities of both enzymes remained unaffected. These results indicated that SA-promotion of Al-induced citrate efflux might be not correlated with increases in citrate production. The endogenous SA concentrations were significantly enhanced in the presence of Al at levels of 10-50 mol/L.We also found that the MDA, O2-, and H2O2 contents in the root tips of C. tora increased in response to Al, which suggested the Al-induced oxidative stress. Application of 5 mol/LSA in solution containing 20 mol/L Al decreased MDA, O2-, and H2O2 contents as well as permeability of plasma membrane, while activities ofSOD, CAT and APX were found to be low, as compared with the controls of 20 mol/L Al alone. These results indicated that SA alleviation of Al toxicity was not involved in the antioxidant enzyme system.Taken together, a possible mechanism for SA improving Al-tolerance of C.tora is proposed: treatments with SA promoted the Al-induced increase in citrate efflux, and this resulted in the decreased accumulation of Al in root tips, thereby causing a limited oxidative stress and root growth inhibition.