Chloroplast-to-chromoplast transition envisions provitamin A biofortification in green vegetables

The carotenoids available in food are vital dietary micronutrients for human health. Plants synthesize and accumulate different carotenoids in plastids in a tissue-specific manner. The level of β-carotene (provitamin A) and other nutritionally important carotenoids is substantially low in the green tissues such as leaves compared to the fruits and roots. In photosynthetic tissues, chloroplasts can accumulate a moderate level of carotenoids, mainly to facilitate photosynthesis and environmental stress tolerance. However, chromoplasts from the storage tissues such as tomato fruit and carrot root can synthesize and accumulate carotenoids to a substantially higher level. A synthetic biology approach that utilizes a transient expression of bacterial phytoene synthase (crtB) gene in the photosynthetic leaves can induce the transition of chloroplasts into chromoplasts. The plastid-localized heterologous expression of crtB in leaves can induce the overaccumulation of phytoene, triggering the chloroplast-to-chromoplast transition; therefore, enhancing the biosynthesis and accumulation of carotenoids, including provitamin A. The transition of chloroplasts into chromoplasts, however, altered the photosynthetic thylakoids, consequently reducing the photosynthetic efficiency and plant growth. An efficient metabolic engineering strategy is desirable to enhance the production of targeted carotenoids in leaves without perturbing the photosynthetic efficiency and plant growth. Collectively, a synthetic biology strategy that triggers the transformation of chloroplasts into chromoplasts in photosynthetic tissues unfolds new avenues for carotenoid biofortification in the leafy food and vegetable crops, which can increase the dietary intake of carotenoids, therefore, combating the crisis of vitamin A deficiency.