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Rice is the most tolerant staple crop to aluminium (Al) toxicity, which is a limiting stress for grain production worldwide. This Al tolerance is the result of combined mechanisms that are triggered in part by the transcription factor ASR5. ASRs are dual target proteins that participate as chaperones in the cytoplasm and as transcription factors in the nucleus. Moreover, these proteins respond to biotic and abiotic stresses, including salt, drought and Al. Rice plants with silenced ASR genes are highly sensitive to Al. ASR5, a well-characterized protein, binds to specific cis elements in Al responsive genes and regulates their expression. Because the Al sensitive phenotype found in silenced rice plants could be due to the mutual silencing of ASR1 and ASR5, we investigated the effect of the specific silencing of ASR5. Plants with artificial microRNA silencing of ASR5 present a non-transformed phenotype in response to Al because of the induction of ASR1. ASR1 has the same subcellular localization as ASR5, binds to ASR5 cis-regulatory elements, regulates ASR5 regulated genes in a non-preferential manner and might replace ASR5 under certain conditions. Our results indicate that ASR1 and ASR5 act in concert and complementarily to regulate gene expression in response to Al. [ABSTRACT FROM AUTHOR]

ABSTRACT Among cereal crops, rice is considered the most tolerant to aluminium (Al). However, variability among rice genotypes leads to remarkable differences in the degree of Al tolerance for distinct cultivars. A number of studies have demonstrated that rice plants achieve Al tolerance through an unknown mechanism that is independent of root tip Al exclusion. We have analysed expression changes of the rice ASR gene family as a function of Al treatment. The gene ASR5 was differentially regulated in the Al-tolerant rice ssp. Japonica cv. Nipponbare. However, ASR5 expression did not respond to Al exposure in Indica cv. Taim rice roots, which are highly Al sensitive. Transgenic plants carrying RNAi constructs that targeted the ASR genes were obtained, and increased Al susceptibility was observed in T1 plants. Embryogenic calli of transgenic rice carrying an ASR5-green fluorescent protein fusion revealed that ASR5 was localized in both the nucleus and cytoplasm. Using a proteomic approach to compare non-transformed and ASR-RNAi plants, a total of 41 proteins with contrasting expression patterns were identified. We suggest that the ASR5 protein acts as a transcription factor to regulate the expression of different genes that collectively protect rice cells from Al-induced stress responses. [ABSTRACT FROM AUTHOR]