The high-value carotenoid astaxanthin (3,3′-dihydroxy-β,β-carotene-4,4′-dione) is one of the most potent antioxidants in nature. In addition to its large-scale use in fish farming, the pigment has applications as a food supplement and an active ingredient in cosmetics and in pharmaceuticals for the treatment of diseases linked to reactive oxygen species. The biochemical pathway for astaxanthin synthesis has been introduced into seed plants, which do not naturally synthesize this pigment, by nuclear and plastid engineering. The highest accumulation rates have been achieved in transplastomic plants, but massive production of astaxanthin has resulted in severe growth retardation. What limits astaxanthin accumulation levels and what causes the mutant phenotype is unknown. Here, we addressed these questions by making astaxanthin synthesis in tobacco (Nicotiana tabacum) plastids inducible by a synthetic riboswitch. We show that, already in the uninduced state, astaxanthin accumulates to similarly high levels as in transplastomic plants expressing the pathway constitutively. Importantly, the inducible plants displayed wild-type–like growth properties and riboswitch induction resulted in a further increase in astaxanthin accumulation. Our data suggest that the mutant phenotype associated with constitutive astaxanthin synthesis is due to massive metabolite turnover, and indicate that astaxanthin accumulation is limited by the sequestration capacity of the plastid.

中文翻译：

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核糖开关介导的质体中虾青素生物合成操纵子的诱导表达


高价值的类胡萝卜素虾青素（3,3'-二羟基-β,β-胡萝卜素-4,4'-二酮）是自然界中最有效的抗氧化剂之一。除了在养鱼业中大量使用外，该色素还可以用作食品补充剂以及化妆品和治疗与活性氧相关疾病的药物中的活性成分。虾青素合成的生化途径已通过核工程和质体工程引入到种子植物中，而种子植物并不天然合成这种色素。转质体植物中的积累率最高，但虾青素的大量生产导致了严重的生长迟缓。限制虾青素积累水平的因素以及导致突变表型的原因尚不清楚。在这里，我们通过合成核糖开关诱导烟草（Nicotiana tabacum）质体中虾青素的合成来解决这些问题。我们表明，在非诱导状态下，虾青素积累到与组成型表达该途径的转质体植物中相似的高水平。重要的是，诱导型植物表现出类似野生型的生长特性，核糖开关诱导导致虾青素积累进一步增加。我们的数据表明，与组成型虾青素合成相关的突变表型是由于大量代谢物周转所致，并表明虾青素的积累受到质体封存能力的限制。