The lignin synthesis mediated by CCoAOMT enzymes is required for tolerance against excess Cu in Oryza sativa

Abstract Lignin accumulation is a general response to heavy metal toxicity; however, the underlying mechanism is unclear. We isolated two rice l ignin- d efective m utants (ldm1 and ldm2); both showed reduced mechanical strength and lignin accumulation. Also, the levels of the syringyl and guaiacyl units of lignin were decreased in both mutants. Both mutants were more hypersensitive to excess Cu than wild-type plants, possibly resulting from more Cu in leaf sheaths and roots of the mutants than that that of wild-type plants. Moreover, Cu retention in the cell wall was altered in the ldm1 and ldm2 mutants. Genotypic analysis confirmed that ldm1 has a T-DNA insert in the last exon of OsCCoAMT1 and ldm2 in the second exon of OsCCoAMT20. Consistently, both mutants displayed significantly decreased activity of CCoAOMT enzymatic activity in all tissues examined, which catalyzed the transfer of methyl groups from S-adeosyl-Lmethionin (SAM) to benzene carbon 3 position of caffeoyl-CoA or 5-hydroxyferuloyl-CoA. Furthermore, the expression of OsCCoAOMT1 and CCoAOMT20 and CCoAOMT activity were upregulated in leaf sheaths and roots under excess Cu, suggesting that lignin synthesis mediated by OsCCoAOMT1 and OsCCoAOMT20 plays an important role in the adaptation to excess Cu stress. Based on the role of lignin in the sequestration of heavy metals in the plant cell wall, we speculate that enhancement of lignin synthesis may be an adaptive strategy to alleviate the toxicity of Cu ions.