Mechanisms of aluminum induced growth inhibition based on the inhibitory effects of aluminum on sugar transport in plants

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A major factor to inhibit crop production in acidic soils is aluminum (Al) ion. Thus the elucidation of mechanisms of Al toxicity and tolerance will be expected to provide basic information necessary for breeding Al-tolerant crops. In this study, these mechanisms were investigated in both cultured cell system and whole plant systems, focusing on sugar metabolism which is necessary for cell elongation and root growth. The findings are as follows:1. Aluminum ion inhibits sugar uptake and stimulates sugar consumption in tobacco cells, which could lead to an increase in water potential and then the inhibition of cell elongation.2. In a whole plant system, the translocation of photoassimilates is stimulated towards the root under Al stress.3. The Al-induced cell death process is different from the sugar-starvation-induced cell death, and involves the production of reactive oxygen species, vacuolar collapse and changes of cytosolic calcium concentration in tobacco cells.4. Another physiological meaning of the Al-triggered malate secretion in wheat is to maintain the growth capability of root under Al stress. The repetitive sequences in the upstream of the ALMT1 gene, encoding the Al-activated malate transporter, seem to control the expression level of the gene. The membrane topology of ALMT1 protein was elucidated.5. Isolation and characterization of Al-sensitive mutants of rice at germination stage were performed.These results indicate novel Al symptoms related to energy metabolism. Molecular and genetical aspects of these symptoms will be studied, and the roles of Al will be elucidated in the sugar metabolism related to the organic acid secretion to support Al tolerance and the sugar metabolism related to germination.