Dysfunction of the 4-Coumarate:Coenzyme A Ligase 4CL4 impacts aluminum resistance and lignin accumulation in rice

The root cell wall is the first and primary target of aluminium (Al) toxicity. Monocots such as rice can accumulate appreciated levels of hydroxycinammic acids (HCAs) to modify and cross-link hemicellulose and/or lignin of the cell wall. Nevertheless, it is unclear whether this HCAs-mediated modification of the cell wall is important for Al accumulation and resistance. We previously isolated and characterized a rice mutant ral1 (resistance to aluminum 1) that shows enhanced Al resistance. In this study, we cloned RAL1 and found that it encodes the 4-Coumarate:Coenzyme A ligase 4CL4, an enzyme putatively involved in the lignin biosynthesis. Mutation of RAL1/4CL4 reduces lignin content and increases the accumulation of its substrates 4-coumaric acid (PA) and ferulic acid (FA). We demonstrate that altered lignin accumulation is not required for the enhanced Al resistance in ral1/4cl4 mutants. We found that the increased accumulation of PA and FA can reduce the Al binding to the hemicellulose and consequently enhance Al resistance in ral1/4cl4 mutants. Al stress is able to trigger the PA and FA accumulation, which is likely caused by the repression of the expression of RAL1/4CL4 and its homologous genes. Our results thus reveal that Al-induced PA and FA accumulation is actively and positively involved in Al resistance in rice through the modification of the cell wall and thereby the reduced Al binding to the cell wall.

Keywords: Oryza sativa; 4CL4; aluminium resistance; cell wall; hydroxycinammic acids; lignin.