Genotypic Difference Among Plant Species In Response To Aluminum Stress And Mechanisms Of Aluminum Resistance In Buckwheat (Fygopyrum Esculentum Moench)

Aluminum toxicity is one of the major factors limiting crop production in acid soils worldwide. In China, 21% of the total cultivated land is acidic, extending over 15 provinces in southern China. Selection and breeding of crops offers an alternative to lime application in acid soils, esp. where lime application is not economically feasible. At present, genotypic differences among plant species in response to Al stress were screened. The underlying mechanisms contributing to genotypic difference among buckwheat cultivars and the effect of cycloheximide. a broad protein synthesis inhibitor, on Al-induced oxalate efflux from buckwheat roots were investigated. In addition, preliminary experiment was conducted in order to investigate the effect of Al stress on root and leaf proteins. The results were summarized as followings.The genotypic differences among plant species or cultivars within the same species were substantial. The ability to resist Al stress is in the increasing order of wheat, triticale, buckwheat, and rye. The greatest genotypic variation of triticale in response to Al stress implys that hexaploid triticale possessed more genetic variation in their resistance to Al stress than other species. The marked genotypic differences make it possible to develop hybrids suitable for acid soils. In addition, Al exclusion from root tips may play a pivotal role in Al resistance in triticale. Using the plant materials with contrast Al tolerance as found in this study will help to clarify mechanisms of Al phytotoxicity and resistance in plants.Here, we demonstrated that there were considerable differences in Al resistances among the cultivars of buckwheat and found an extreme sensitive cultivar (Shanxi). Furthermore, Al resistance in this species correlated closely with growth inhabits of origin, indicating that adapted mechanisms leading to high Al resistance had been evolved in this species. Oxalic acid secretion from roots is considered to be an important mechanism for aluminum (Al) resistance in buckwheat. However, no correlation was found between Al-induced oxalic acid secretion and Al-induced inhibition of root elongation. The two cultivars showing the greatest difference in resistance were Shanxi (Al-sensitive) and Jiangxi (Al-resistant) and these were compared in more detailed studies. The dependence of oxalic acid secretion on external Al concentration and the time-course for secretion were similar in both cultivars. Furthermore the variation in Al-induced oxalic acid efflux along the root was similar showing a 10-fold greater from the apical 0-5 mm region than from the 5-10 mm region. These results suggest that both Shanxi and Jiangxi possess an equal capacity for Al-dependent oxalic acid secretion and that the variation in Al resistance among different cultivars cannot be explained by this mechanism. However after longer treatments in Al (10 days) the concentration of P and Al in the roots of the Al-resistant cultivar Jiangxi were significantly higher than those in Shanxi. These results suggest that while Al-dependent oxalic acid secretion might contribute to the overall high resistance to Al stress of buckwheat this response cannot explain the variation in tolerance between cultivars. We present evidence suggesting the greater Al resistance of Jiangxi is related to the immobilization and detoxification of Al by P in the root tissues.Organic acid secretion has been shown to play a central role in detoxifying aluminum both internal and external by forming chelating compounds. It was demonstrated that oxalate secretion plays a key role in the tolerant mechanism to aluminum stress in buckwheat, and was presumed be secreted through anion channels located on the plasma membrane. However, the question remains equivocal whether these channels are preexist in the plasma membrane or induced do novo synthesis by Al stress. Our results indicated that cycloheximide does not affect the secretion ofoxalate in buckwheat in response to Al stress, indicating that the activation of channels rather than de novo syn...