Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, D-14476 Potsdam-Golm, Germany.
Plant Physiol. 2022 Jan 20;188(1):637-652. doi: 10.1093/plphys/kiab428.
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.
虾青素（3,3'-二羟基-β,β-胡萝卜素-4,4'-二酮）是一种高价值的类胡萝卜素，是自然界中最有效的抗氧化剂之一。除了在水产养殖中的大规模应用外，该色素还可用作食品补充剂，以及化妆品和用于治疗与活性氧有关的疾病的药物中的活性成分。通过核质和质体工程，虾青素合成的生化途径已被引入到不会自然合成这种色素的种子植物中。在质体转化植物中，虾青素的积累率最高，但大量生产虾青素会导致严重的生长迟缓。那么，是什么限制了虾青素的积累水平，又是什么导致了突变表型呢？目前还不得而知。在这里，我们通过在烟草（Nicotiana tabacum）质体中合成一种合成核糖开关来诱导虾青素的合成，从而解决了这些问题。结果表明，即使在未诱导的状态下，虾青素的积累水平也与组成型表达途径的质体转化植物相似。重要的是，诱导型植物表现出与野生型相似的生长特性，而核糖开关的诱导则进一步增加了虾青素的积累。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，这表明虾青素的积累受到质体隔离能力的限制。
