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Rice (Oryza sativa) is the most aluminum (Al)-resistant crop species among the small-grain cereals, but the mechanisms responsible for this trait are still unclear. Using two rice cultivars differing in Al resistance, rice sp. japonica 'Nipponbare' (an Al-resistant cultivar) and rice sp. indica 'Zhefu802' (an Al-sensitive cultivar), it was found that Al content in the root apex (0-10 mm) was significantly lower in Al-resistant 'Nipponbare' than in sensitive 'Zhefu802', with more of the Al localized to cell walls in 'Zhefu802', indicating that an Al exclusion mechanism is operating in 'Nipponbare'. However, neither organic acid efflux nor changes in rhizosphere pH appear to be responsible for the Al exclusion. Interestingly, cell wall polysaccharides (pectin, hemicellulose 1, and hemicellulose 2) in the root apex were found to be significantly higher in 'Zhefu802' than in 'Nipponbare' in the absence of Al, and Al exposure increased root apex hemicellulose content more significantly in 'Zhefu802'. Root tip cell wall pectin methylesterase (PME) activity was constitutively higher in 'Zhefu802' than in 'Nipponbare', although Al treatment resulted in increased PME activity in both cultivars. Immunolocalization of pectins showed a higher proportion of demethylated pectins in 'Zhefu802', indicating a higher proportion of free pectic acid residues in the cell walls of 'Zhefu802' root tips. Al adsorption and desorption kinetics of root tip cell walls also indicated that more Al was adsorbed and bound Al was retained more tightly in 'Zhefu802', which was consistent with Al content, PME activity, and pectin demethylesterification results. These responses were specific to Al compared with other metals (CdCl(2), LaCl(3), and CuCl(2)), and the ability of the cell wall to adsorb these metals was also not related to levels of cell wall pectins. All of these results suggest that cell wall polysaccharides may play an important role in excluding Al specifically from the rice root apex.

Cell wall is the first physical barrier to aluminum (Al) toxicity. Modification of cell wall properties to change its binding capacity to Al is one of the major strategies for plant Al resistance; nevertheless, how it is regulated in rice remains largely unknown. In this study, we show that exogenous application of putrescines (Put) could significantly restore the Al resistance of art1, a rice mutant lacking the central regulator Al RESISTANCE TRANSCRIPTION FACTOR 1 (ART1), and reduce its Al accumulation particularly in the cell wall of root tips. Based on RNA‐sequencing, yeast‐one‐hybrid and electrophoresis mobility shift assays, we identified an R2R3 MYB transcription factor OsMYB30 as the novel target in both ART1‐dependent and Put‐promoted Al resistance. Furthermore, transient dual‐luciferase assay showed that ART1 directly inhibited the expression of OsMYB30, and in turn repressed Os4CL5‐dependent 4‐coumaric acid accumulation, hence reducing the Al‐binding capacity of cell wall and enhancing Al resistance. Additionally, Put repressed OsMYB30 expression by eliminating Al‐induced H2O2 accumulation, while exogenous H2O2 promoted OsMYB30 expression. We concluded that ART1 confers Put‐promoted Al resistance via repression of OsMYB30‐regulated modification of cell wall properties in rice. [ABSTRACT FROM AUTHOR]