l-Theanine, a non-proteinaceous amino acid abundantly present in tea (Camellia sinensis), contributes to the umami flavor of tea and has beneficial effects on human health. While key l-theanine biosynthetic genes have been well documented, their transcriptional regulation remains poorly understood. In this study, we determined the l-theanine contents in tea leaves of two cultivars at three developmental stages and investigated the expression patterns of the l-theanine biosynthetic genes CsGS1 and CsGS2. Additionally, we identified an R2R3-MYB transcription factor, CsMYB73, belonging to subgroup 22 of the R2R3-MYB family. CsMYB73 expression negatively correlated with l-theanine accumulation during leaf maturation. We found that CsMYB73, as a nuclear protein, binds to the promoter regions of CsGS1 and CsGS2 via MYB recognition sequences and represses the transcription of CsGS1 and CsGS2 in tobacco leaves. Collectively, our results demonstrate that CsMYB73 is a transcriptional repressor involved in l-theanine biosynthesis in tea plants. Our findings might contribute to future tea plant breeding strategies.

中文翻译：

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R2R3-MYB 转录因子 CsMYB73 消极调节茶 (Camellia sinensis L.) 中的 l-茶氨酸生物合成

L-茶氨酸是一种大量存在于茶（Camellia sinensis）中的非蛋白质氨基酸，有助于增加茶的鲜味，对人体健康有益。虽然关键的 l-茶氨酸生物合成基因已被充分记录，但它们的转录调控仍然知之甚少。在这项研究中，我们测定了两个品种在三个发育阶段的茶叶中的 l-茶氨酸含量，并研究了 l-茶氨酸生物合成基因 CsGS1 和 CsGS2 的表达模式。此外，我们确定了 R2R3-MYB 转录因子 CsMYB73，属于 R2R3-MYB 家族的第 22 亚组。CsMYB73 表达与叶成熟过程中 l-茶氨酸的积累呈负相关。我们发现 CsMYB73 作为一种核蛋白，通过 MYB 识别序列与 CsGS1 和 CsGS2 的启动子区域结合，并抑制烟叶中 CsGS1 和 CsGS2 的转录。总的来说，我们的结果表明 CsMYB73 是一种参与茶树 l-茶氨酸生物合成的转录抑制因子。我们的发现可能有助于未来的茶树育种策略。