| Aluminum (Al) is one of the most important factors limiting plant growth in acid soils. Although it has been considered that Al-induced secretion of organic acids from roots is a main mechanism for some Al-tolerance plants, so far the knowledge about the mechanisms responsible for Al-induced secretion of organic acids is still very limited. In present studies, two cultivars of stylo (Stylosanthes Spp), Reyan No.2 and Reyan No. 7, were objected to compare the difference of Al-tolerance between cultivars and exudation of organic acid from the roots to elucidate the mechanism responsible for Al-tolerant diversity in stylos. Furthermore, on the base of study on the characteristics of Al-induced secretion of citrate in stylo, the factors regulated citrate secretion under Al stress, in terms of anion channels, protein synthesis, metabolizion and transportion of organic acids and cell signal transduction, were investigated in Reyan No.2. The results were shown as follows:There was great genetic variation in Al tolerance between cultivars of stylo. More inhibition of root elongation and stronger staining on the surfaces of roots with eriochrome cyanine R were found in Reyan No.2 contrasting to that of Reyan No.7 after exposure of roots to Al3+ (10, 20, 30μmol·L-1) solution, which indicated that Reyan No.2 was an Al-tolerance cultivars. Aluminum-induced secretion of citrate from roots was more significantly in Reyan No.2. In other way, Al content increased significantly when citrate exudation was inhibited by the inhibitors of anion channels, protein synthesis and respiration. These results demonstrated that Al-reduced secretion of citrate from roots was an important mechanism for the difference in Al-tolerance between cultivars of stylo.The amount of citrate secreted from the segment of 0.5-1.0cm from root apex was greater than other segment(0-0.5, 1.0-1.5, 1.5-2.0cm). It was illuminated that citrate was secreted mainly from 0.5-1.0cm root segment in stylo under Al stress.Phosphor (P) absence could also induce secretion of citrate from roots in stylo, and the amount came to the most from the 8th and 10th day of P deficiency treatment. The result showed that secretion of citrate from roots in stylo did not the special response to Al stress.Another, there was obvious a lag between Al-induced secretion of citrate and the initial to Al treatment in stylo. The secretion of citrate was limited after the treatment with Al for 0.5, 2, 4 hours, while the secretion enhanced significantly from the 8th hour of Al treatment. It indicated that citrate in stylo was secreted from roots under Al stress in pattern II.The anion channels inhibitors including PG, NIF, DIDS and A-9-C in solution (10μmol·L-1) could all inhibit Al-induced secretion of citrate. Secondly, the inhibition of citrate secretion was found after addition of protein synthesis inhibitor (CHM, 25 or 50μmol·L-1) in Al solution as well as pre-treatment of CHM before Al treatment and the treatment with CHM after Al treatment. Furthermore, respiration inhibitor (NaN3, 10μmol·L-1) or the inhibitor (PITC, 10μmol·L-1) of citrate carrier on mitochondorial membrane all inhibited the exudation. On the other hand, the treatment with heterotrimeric G protein activator (CTX) enhanced [Ca2+]cyt signal in root tip surface cells and the exudation of citrate during the course of Al-induced secretion of citrate, while the inhibitor (PTX) of heterotrimeric G protein depressed effectively Ca2+ signal transduction and the secretion of citrate. All the results demonstrated that: (1) Anion channels involved in Al-induced secretion of citrate in stylo, de novo synthesis and activation of an anion channel proteins might be required for the secretion; (2) Both the respiration of root tips and citrate transportation crossing mitochondorial membrane impacted the secretion of citrate under Al stress. (3) Heterotrimeric G protein would intervene in the signal transduction of Al stress and participated in the regulation of Al-induced secretion of citrate in stylo. |
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