A neighboring aromatic-aromatic amino acid combination governs activity divergence between tomato phytoene synthases

First author: Hongbo Cao; Affiliations: Cornell University (康奈尔大学): Ithaca, USA

Corresponding author: Li Li

Carotenoids exert multifaceted roles to plants and are critically important to humans. Phytoene synthase (PSY) is a major rate-limiting enzyme in the carotenoid biosynthetic pathway. PSY in plants is normally found as a small enzyme family with up to three members. However, knowledge of PSY isoforms in relation to their respective enzyme activities and amino acid residues important for PSY activity is limited. In this study, we focused on two tomato (Solanum lycopersicum) PSY isoforms, PSY1 and PSY2, and investigated their abilities to catalyze carotenogenesis via heterologous expression in transgenic Arabidopsis and bacterial systems. We found that the fruit-specific PSY1 was less effective in promoting carotenoid biosynthesis than the green tissue-specific PSY2. Examination of the PSY proteins by site-directed mutagenesis analysis and 3D structure modeling revealed two key amino acid residues responsible for this activity difference, and identified a neighboring aromatic-aromatic combination in one of the PSY core structures as being crucial for high PSY activity. Remarkably, this neighboring aromatic-aromatic combination is evolutionarily conserved among land plant PSYs except PSY1 of tomato and potato (Solanum tuberosum). Strong transcription of tomato PSY1 likely evolved as compensation for its weak enzyme activity to allow for the massive carotenoid biosynthesis in ripe fruit. This study provides insights into the functional divergence of PSY isoforms and highlights the potential to rationally design PSY for the effective development of carotenoid-enriched crops.

类胡萝卜素对于植物具有多个方面的作用，而且对于人类也非常重要。八氢番茄红素合酶PSY是类胡萝卜素生物合成通路上的一个主要限速酶。植物中的PSY是一个比较小的基因家族，最多不过3个基因成员。然而，对于不同的PSY基因与其各自的酶活性以及氨基酸残基对于PSY活性的重要性还不清楚。本文中，作者主要研究了两个番茄的PSY基因PSY1和PSY2，通过在拟南芥和细菌系统中异源表达这两个PSY基因研究其催化类胡萝卜素合成的能力。作者发现果实特异性表达的PSY1催化类胡萝卜素合成的能力要逊色于绿色组织特异性表达的PSY2。通过对PSY蛋白定点诱变分析和3D结构建模，作者发现这种差异主要由两个关键氨基酸引起的，并且在其中一个PSY核心结构中鉴定到了一个相邻芳香蛋白组合对于PSY的高活性至关重要。并且该相邻芳香蛋白组合在除了番茄和土豆的PSY1基因外所有其它陆地植物的PSY中都是保守的。番茄PSY1基因的高表达似乎是为了弥补其酶活较低，从而保证成熟果实中大量的类胡萝卜素合成。本文的研究为PSY基因的功能分化提供了新的视野，并且也为有效开发富含类胡萝卜素作物时PSY基因的合理设计提供了理论基础。

通讯：Li Li (https://plbrgen.cals.cornell.edu/people/li-li/)

个人简介：英国雷丁大学，博士。

研究方向：作物营养质量提升的遗传方法开发。

doi: https://doi.org/10.1104/pp.19.00384

Journal: Plant Physiology

First Published: June 20, 2019