| Al toxicity is a major limited factor for crop production and quality in acid soil, and it has become the focus of ongoing research in the area of plant stress physiology and genetics. At present, however, Al toxicity mechanisms in plant are still not clearly understood. Barley, which was used as experimental material in this study, is one of the most Al-sensitive crops. Al toxicity-induced ultraweak luminescence (UL), sister-chromatid exchanges (SCEs), programmed cell death (PCD) in root tips of barley, Al-resistant genetic engineering and the role of root border cells in Al toxicity were studied respectively, in order to explore some new pathways for Al toxicity, Al-resistant mechanism and genetic improvement.The determination of Al toxicity and Al-resistant ability in plant root-tip using UL is still not reported. Our experiments show that, according to root relative elongation rates (RERs), 30 barley cultivars can be divided into three types such as Al-resistant cultivars, intermediate cultivars, and Al-sensitive cultivars. However, according to root tip relative UL rates (RULRs), these cultivars can be divided into five types such as highly Al-resistant cultivars, Al-resistant cultivars, intermediate cultivars, Al-sensitive cultivars, and highly Al-sensitive cultivars. These results suggest that RULRs as an Al toxicity indicator is more sensitive than RERs. There was a statistically significant and weak positive correlation between RERs and RULRs (r=0.63295), indicating that it is possible to estimate barley Al-resistant ability using UL.Al-induced sister-chromatid exchanges (SCEs) were first time determined. Our experiments show that Al-induced SCE rates are higher in cv. 2000-2 than in cv. Humai 16 at 20 umol/L Al level, indicating that there is a negative correlation between Al-induced SCEs and Al-resistant ability in barley. Under different Al concentrations, there are same change curves of SCE rates hi cv. 2000-2 and cv. Humai 16, but their critical concentrations, which can induce the highest SCE rates, were significantly different from each other. Critical concentrations of cv. 2000-2 and cv. Humai 16 are 10 and 320 umol/L Al respectively. These results suggest that SCEs are very sensitive to Al toxicity hi the root tips of barley and can be also served as a sensitive indicator in Al phytotoxicity. The Al-resistant ability of barley may be assessed by their critical concentrations. Further studies show Al toxicity indirectly induces SCEs through inducing ROS.Al accumulation on cell wall of root tips is the primary prerequisite of toxic effect of Al on root tips in plants. According their RERs, there is a statistically significant difference (PO.01) between cv. 2000-2 and cv. Humai 16. Morin fluorescent staining shows that fluorescence in the root tips of cv. 2000-2 is more intense than that in the root tips of cv. Humai 16, indicating that more Al is accumulated on the root tips of cv. 2000-2 than cv. Humai 16. 4-h Al treatment significantly enhanced PME activity hi root tips of cv. 2000-2 and after 24 h Al treatment, PME activity significantly decreased. However, PME activity did not show significant changes in cv. Humai 16. These results suggest that Al accumulation on cell wall is very closely related to PME activity on the root tips, and PME activity has an important role in Al toxic sensitivity.We first reported Al-induced programmed cell death in the root-tip cells of barley. During Al treatment or recovery culture without Al, the irreversible inhibition of cell elongation or growth was resulted from cell death. 0.1-5.0 mmol/L Al can induce DNA ladders in the root-tips (treated for 8 h) and suspension cells (treated for 24 h) of barley, but no apoptotic bodies were observed. Therefore, cell death caused by Al is called as Al-induced PCD. The results of UL show Al induced PCD possibly via a ROS-activated signal transduction pathway. Irreversible inhibition induced by Al show that cells were doomed to death whether Al stress or not, once the cell death program was activated in Al-t...
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