Carotenoids are plastid isoprenoid pigments that play critical roles in light harvesting, photoprotection, and phytohormone biosynthesis. They are also vitamin-A precursors and antioxidant molecules important for human nutrition. Apples (e.g. Malus x domestica Borkh), one of the most widely consumed fruits with high nutrient levels, have a very low carotenoid concentration in flesh, compared with other fruits and vegetables. This could be explained by a deficiency in carotenoid synthesis/accumulation and/or accelerated degradation. We analysed the contribution of M. domestica cv. 'Fuji' phytoene synthase (PSY) in the biosynthesis of carotenoids and determined that among four MdPSY genes present in the organism, MdPSY2 and MdPSY5 are highly expressed in leaves and during fruit ripening in line with an increment in carotenoid content in fruits. Furthermore, two representative polymorphic MdPSY2 variants were found, one with a Tyr358Phe substitution (MdPSY2_F) and the other that additionally has a six-amino-acid deletion in the signal peptide (MdPSY2_CG). MdPSY2, MdPSY5, MdPSY2_F and MdPSY2_CG are all localised in plastids. Interestingly, the polymorphic MdPSY2_F and MdPSY2_CG variants show lower enzymatic activity than the wild-type form in a heterologous complementation assay, which could be attributed to the Tyr358Phe substitution close to the active-site pocket, as was suggested by 3-D modelling analysis. The presence of polymorphic MdPSY2 variants with lower enzymatic activity could be partially responsible for the low carotenoid content in Fuji apple fruits.

中文翻译：

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MdPSY2 变体和 MdPSY5 八氢番茄红素合酶的功能表征和计算机模拟

类胡萝卜素是质体类异戊二烯色素，在光收集、光保护和植物激素生物合成中起关键作用。它们也是对人类营养很重要的维生素 A 前体和抗氧化剂分子。与其他水果和蔬菜相比，苹果（例如 Malus xdomestica Borkh）是食用最广泛的水果之一，具有高营养水平，果肉中的类胡萝卜素浓度非常低。这可以通过类胡萝卜素合成/积累和/或加速降解的不足来解释。我们分析了 M.domestica cv 的贡献。'富士'八氢番茄红素合成酶 (PSY) 在类胡萝卜素生物合成中的作用，并确定在生物体中存在的四个 MdPSY 基因中，MdPSY2 和 MdPSY5 在叶子和果实成熟期间高度表达，与果实中类胡萝卜素含量的增加一致。此外，发现了两种代表性的多态性 MdPSY2 变体，一种具有 Tyr358Phe 取代（MdPSY2_F），另一种在信号肽中另外具有 6 个氨基酸缺失（MdPSY2_CG）。MdPSY2、MdPSY5、MdPSY2_F 和 MdPSY2_CG 都位于质体中。有趣的是，多态性 MdPSY2_F 和 MdPSY2_CG 变体在异源互补测定中显示出比野生型形式更低的酶活性，这可能归因于靠近活性位点口袋的 Tyr358Phe 取代，正如 3-D 建模分析所表明的那样。