Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto, 606-8502, Japan.
Graduate School of Environmental and Life Science, Okayama University, Okayama, 700-8530, Japan.
Plant J. 2019 Nov;100(4):693-705. doi: 10.1111/tpj.14462. Epub 2019 Sep 5.
Capsaicinoids are unique compounds that give chili pepper fruits their pungent taste. Capsaicinoid levels vary widely among pungent cultivars, which range from low pungency to extremely pungent. However, the molecular mechanisms underlying this quantitative variation have not been elucidated. Our previous study identified various loss-of-function alleles of the pAMT gene which led to low pungency. The mutations in these alleles are commonly defined by Tcc transposon insertion and its footprint. In this study, we identified two leaky pamt alleles (pamt and pamt ) with different levels of putative aminotransferase (pAMT) activity. Notably, both alleles had a Tcc transposon insertion in intron 3, but the locations of the insertions within the intron were different. Genetic analysis revealed that pamt , pamt and a loss-of-function pamt allele reduced capsaicinoid levels to about 50%, 10% and less than 1%, respectively. pamt and pamt encoded functional pAMT proteins, but they exhibited lower transcript levels than the functional type. RNA sequencing analysis showed that intronic transposons disrupted splicing in intron 3, which resulted in simultaneous expression of functional pAMT mRNA and non-functional splice variants containing partial sequences of Tcc. The non-functional splice variants were more dominant in pamt than in pamt . This suggested that the difference in position of the intronic transposons could alter splicing efficiency, leading to different pAMT activities and reducing capsaicinoid content to different levels. Our results provide a striking example of allelic variations caused by intronic transposons; these variations contribute to quantitative differences in secondary metabolite contents.
辣椒素类化合物是赋予辣椒果实辛辣味道的独特化合物。辣椒素类化合物在不同辣味品种中的含量差异很大，从低辣度到极辣度不等。然而，这种数量变化的分子机制尚未阐明。我们之前的研究鉴定了各种导致低辣度的 pAMT 基因的功能丧失等位基因。这些等位基因的突变通常由 Tcc 转座子插入及其足迹定义。在这项研究中，我们鉴定了两个具有不同潜在氨基转移酶（pAMT）活性的漏 pamt 等位基因（pamt 和 pamt ）。值得注意的是，这两个等位基因在 3 号内含子中都有 Tcc 转座子插入，但插入的位置不同。遗传分析表明，pamt 、pamt 和一个功能丧失的 pamt 等位基因分别将辣椒素类化合物的水平降低到约 50%、10%和不到 1%。pamt 和 pamt 编码功能性 pAMT 蛋白，但它们的转录水平低于功能性类型。RNA 测序分析表明，内含子转座子破坏了 3 号内含子的剪接，导致功能性 pAMT mRNA 和含有 Tcc 部分序列的非功能性剪接变体同时表达。非功能性剪接变体在 pamt 中比在 pamt 中更为优势。这表明内含子转座子位置的差异可能会改变剪接效率，从而导致不同的 pAMT 活性，并将辣椒素类化合物的含量降低到不同水平。我们的结果提供了一个引人注目的例子，即内含子转座子引起的等位基因变异；这些变异导致次生代谢物含量的定量差异。
