Key Laboratory of Horticultural Plant Biology of MOE (Ministry of Education), Huazhong Agricultural University, Wuhan, Hubei 430070, China.
Department of Biological Sciences, Lehman College, The City University of New York, 250 Bedford Park Boulevard West, Bronx, New York 10468, United States.
J Agric Food Chem. 2020 Oct 28;68(43):12048-12057. doi: 10.1021/acs.jafc.0c04740. Epub 2020 Oct 19.
Carotenoids are a large class of structures that are important in human health and include both provitamin A and nonprovitamin A compounds. Vitamin A deficiency is a global health problem that can be alleviated by enriching provitamin A carotenoids in a range of food crops. Suitable plants for biofortification are those with high levels of the provitamin A biosynthetic precursor, lycopene, which is enzymatically converted by lycopene β-cyclase (LCYB) to β-carotene, a provitamin A carotenoid. Crops, such as citrus, naturally accumulate high levels of provitamin A and other health-promoting carotenoids. Such plants may have useful genes to expand the synthetic biology toolbox for producing a range of phenotypes, including both high provitamin A crops and crops with unique compositions of health-promoting carotenoids. To examine enzyme variants having different activity levels, we introduced two citrus LCYB alleles into tomato, a plant with fruit rich in lycopene. Overexpression in tomato of the stronger allele of the citrus chromoplast-specific lycopene β-cyclase () produced "golden" transgenic tomato fruits with 9.3-fold increased levels of β-carotene at up to 1.5 mg/g dry weight. The use of the weaker allele, , also led to enhanced levels of β-carotene but in the context of a more heterogeneous composition of carotenoids. From a synthetic biology standpoint, these allelic differences have value for producing cultivars with unique carotenoid profiles. Overexpression of the citrus genes was accompanied by increased expression of other genes encoding carotenoid biosynthetic enzymes and increased size and number of chromoplasts needed to sequester the elevated levels of carotenoids in the transgenic tomato fruits. The overexpression of the citrus genes also led to a pleiotropic effect on profiles of phytohormones and primary metabolites. Our findings show that enzyme variants are essential synthetic biology parts needed to create a wider range of metabolic engineering products. In this case, strong and weak variants of LCYB proved useful in creating dietary sources to alleviate vitamin A deficiency or, alternatively, to create crops with a heterogeneous composition including provitamin A and healthful, nonprovitamin A carotenoids.
类胡萝卜素是一大类结构，对人类健康很重要，包括维生素 A 前体和非维生素 A 化合物。维生素 A 缺乏是一个全球性的健康问题，可以通过在各种粮食作物中富集维生素 A 前体类胡萝卜素来缓解。适合生物强化的植物是那些具有高水平的维生素 A 生物合成前体番茄红素的植物，番茄红素可以通过番茄红素β-环化酶（LCYB）酶促转化为β-胡萝卜素，即维生素 A 前体类胡萝卜素。柑橘等作物天然积累高水平的维生素 A 和其他促进健康的类胡萝卜素。这些植物可能具有有用的基因，可以扩展合成生物学工具包，用于生产一系列表型，包括高维生素 A 作物和具有独特组成的促进健康的类胡萝卜素的作物。为了研究具有不同活性水平的酶变体，我们将两个柑橘 LCYB 等位基因引入番茄中，番茄是一种果实富含番茄红素的植物。在番茄中过表达柑橘质体特异性番茄红素β-环化酶（）的强等位基因，产生了“金色”转基因番茄果实，β-胡萝卜素水平提高了 9.3 倍，高达 1.5mg/g 干重。使用较弱的等位基因，也导致了β-胡萝卜素水平的提高，但在类胡萝卜素组成更为混杂的情况下。从合成生物学的角度来看，这些等位基因差异对于生产具有独特类胡萝卜素谱的品种具有价值。柑橘基因的过表达伴随着编码类胡萝卜素生物合成酶的其他基因的表达增加，以及质体大小和数量的增加，这些质体需要将转基因番茄果实中升高的类胡萝卜素隔离。柑橘基因的过表达也导致了植物激素和初级代谢物谱的多效性效应。我们的研究结果表明，酶变体是创造更广泛代谢工程产品所必需的合成生物学部件。在这种情况下，LCYB 的强和弱变体在创造饮食来源以缓解维生素 A 缺乏症方面非常有用，或者，创造具有包括维生素 A 和有益的非维生素 A 类胡萝卜素在内的混杂组成的作物。
