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 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 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.

中文翻译：

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由 CCoAOMT 酶介导的木质素合成是水稻对过量铜的耐受性所必需的

摘要 木质素积累是对重金属毒性的普遍反应；然而，潜在的机制尚不清楚。我们分离了两个水稻木质素缺陷突变体（ldm1 和 ldm2）；两者都显示出降低的机械强度和木质素积累。此外，两个突变体中木质素的丁香基和愈创木酰基单位的水平都降低了。两个突变体都比野生型植物对过量铜更敏感，这可能是由于突变体叶鞘和根中的铜含量高于野生型植物。此外，在 ldm1 和 ldm2 突变体中，Cu 在细胞壁中的保留被改变。基因型分析证实，ldm1 在 OsCCoAMT1 的最后一个外显子和 ldm2 在 OsCCoAMT20 的第二个外显子中具有 T-DNA 插入。一致地，两个突变体在所有检查的组织中都显示出显着降低的 CCoAOMT 酶活性，其催化甲基从 S-腺苷酸-L甲硫氨酸 (SAM) 转移到咖啡酰-CoA 或 5-羟基阿魏酰-CoA 的苯碳 3 位。此外，OsCCoAOMT1 和 CCoAOMT20 的表达以及 CCoAOMT 活性在过量 Cu 下叶鞘和根中上调，表明由 OsCCoAOMT1 和 OsCCoAOMT20 介导的木质素合成在适应过量 Cu 胁迫中起重要作用。基于木质素在植物细胞壁中螯合重金属的作用，我们推测增强木质素合成可能是减轻铜离子毒性的一种适应性策略。OsCCoAOMT1 和 CCoAOMT20 的表达以及 CCoAOMT 活性在叶鞘和根中在过量 Cu 下上调，表明由 OsCCoAOMT1 和 OsCCoAOMT20 介导的木质素合成在适应过量 Cu 胁迫中起重要作用。基于木质素在植物细胞壁中螯合重金属的作用，我们推测增强木质素合成可能是减轻铜离子毒性的一种适应性策略。OsCCoAOMT1和CCoAOMT20的表达和CCoAOMT活性在过量Cu条件下在叶鞘和根中上调，表明由OsCCoAOMT1和OsCCoAOMT20介导的木质素合成在适应过量Cu胁迫中起重要作用。基于木质素在植物细胞壁中螯合重金属的作用，我们推测增强木质素合成可能是减轻铜离子毒性的一种适应性策略。