School of Advanced Agriculture and Bioengineering, Yangtze Normal University, Fuling, 408100, China.
State Key Laboratory for Conservation and Utilization of Subtropical Agro-Bioresources/ Guangdong Litchi Engineering Research Center, College of Horticulture, South China Agricultural University, Guangzhou, 510642, China.
BMC Plant Biol. 2019 Feb 7;19(1):62. doi: 10.1186/s12870-019-1658-5.
Maturation of litchi (Litchi chinensis) fruit is characterized by dramatic changes in pigments in the pericarp and flavor compounds in the aril. Among them, the biosynthesis of anthocyanins is most noticeable. Previous studies showed that LcMYB1 and LcbHLH transcription factors participated in regulating the anthocyanin biosynthesis in litchi. However, the roles of other MYB factors remain unclear.
In this study, we cloned and characterized the function of LcMYB5, a novel R2R3-MYB identified from litchi transcriptome. Although LcMYB5 was constitutively expressed in litchi tissues and its expressions was not correlated with tissue coloration, overexpression of LcMYB5 resulted in enhanced biosynthesis of anthocyanins in tobacco and petunia concurrent with the up-regulation of their endogenous bHLHs and key structural genes in anthocyanin precursor biosynthesis. These results indicate that LcMYB5 is an R2R3 transcriptional factor regulates anthocyanin biosynthesis either by directly activating the expression of key structural genes such as DFR or by indirectly up regulating the expressions of endogenous bHLH regulators. More interestingly, the pH values in petals and leaves from transgenic lines were significant lower than those in both untransformed tobacco and petunia, indicating LcMYB5 is also associated with pH regulation. The expressions of LcMYB5 and its bHLH partner LcbHLH1 were consistent with the expression of putative tissue acidification gene LcPH1, and the changes in malic acid provided further evidence for the close relationship between LcMYB5 and tissue acidification.
Taking together, our study indicated that LcMYB5 is involved in not only anthocyanin biosynthesis but also tissue acidification.
荔枝果实的成熟过程以果皮中色素和种皮中风味化合物的显著变化为特征。其中，花青素的生物合成最为显著。先前的研究表明，LcMYB1 和 LcbHLH 转录因子参与调节荔枝中的花青素生物合成。然而，其他 MYB 因子的作用尚不清楚。
在这项研究中，我们从荔枝转录组中克隆并鉴定了一个新的 R2R3-MYB，即 LcMYB5，并对其功能进行了研究。尽管 LcMYB5 在荔枝组织中持续表达，且其表达与组织着色无关，但过表达 LcMYB5 导致烟草和矮牵牛中花青素的生物合成增强，同时上调其内源 bHLHs 和花青素前体生物合成的关键结构基因。这些结果表明，LcMYB5 是一种 R2R3 转录因子，通过直接激活 DFR 等关键结构基因的表达或间接上调内源 bHLH 调节因子的表达来调节花青素的生物合成。更有趣的是，转基因系花瓣和叶片的 pH 值显著低于未转化的烟草和矮牵牛，表明 LcMYB5 也与 pH 值调节有关。LcMYB5 和其 bHLH 伴侣 LcbHLH1 的表达与假定的组织酸化基因 LcPH1 的表达一致，而苹果酸的变化为 LcMYB5 与组织酸化之间的密切关系提供了进一步的证据。
综上所述，我们的研究表明，LcMYB5 不仅参与花青素的生物合成，还参与组织酸化。
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