Synthetic conversion of leaf chloroplasts into carotenoid-rich plastids reveals mechanistic basis of natural chromoplast development

Abstract

Plastids, the defining organelles of plant cells, undergo physiological and morphological changes to fulfill distinct biological functions. In particular, the differentiation of chloroplasts into chromoplasts results in an enhanced storage capacity for carotenoids with industrial and nutritional value such as beta-carotene (provitamin A). Here, we show that synthetically inducing a burst in the production of phytoene, the first committed intermediate of the carotenoid pathway, elicits an artificial chloroplast-to-chromoplast differentiation in leaves. Phytoene overproduction initially interferes with photosynthesis, acting as a metabolic threshold switch mechanism that weakens chloroplast identity. In a second stage, phytoene conversion into downstream carotenoids is required for the differentiation of chromoplasts, a process that involves a concurrent reprogramming of nuclear gene expression and plastid morphology for improved carotenoid storage. We hence demonstrate that loss of photosynthetic competence and enhanced production of carotenoids are not just consequences but requirements for chloroplasts to differentiate into chromoplasts.

Lycopene β-cyclase expression influences plant physiology, development, and metabolism in tobacco plants

“…In this way, any changes in carotenoid composition always lead to abnormal plastid development (Park et al, 2002;Welsch et al, 2000). Recently, the expression of a bacteria CrtB (PSY) gene (in Arabidopsis, lettuce, tobacco, and zucchini), and as a consequence of that, a burst in phytoene accumulation, was shown to stimulate differentiation of chloroplast into chromoplast suggesting that indeed carotenoid content influences chloroplast development (Llorente et al, 2020). Interestingly, we observed increased chloroplast number in our transplastomic tobacco lines (Table S4), which is in line with the increased chromoplast number observed in tomato fruits expressing the citrus LCYB gene (Zhu et al, 2020).…”

Comparative analysis of chloroplast genomes of cultivars and wild species of sweetpotato (Ipomoea batatas [L.] Lam)

“…Chloroplasts are key organelles of plants. In addition to their well-known function in photosynthesis, chloroplasts are also involved in important biological processes such as plant immunity and crop quality [ 9 , 10 ]. The genetic transformation of chloroplasts has become a hotspot in genetic engineering [ 11 ].…”

Carotenoid and tocopherol fortification of zucchini fruits using a viral RNA vector

“…We previously observed that inoculation of zucchini plants with a ZYMV-based vector that expressed P. ananatis crtB (ZYMV-crtB) caused infected leaves to turn bright yellow 5 . We have, more recently, understood that this phenotype is a consequence of the heterologous crtB enzyme inducing phytoene accumulation beyond a threshold that triggers transformation of leaf chloroplasts into chromoplasts, which is accompanied by the accumulation of high levels of downstream carotenoids 6 . To test whether we could trigger the chloroplast-to-chromoplast transformation in fruits, with concomitant carotenoid overaccumulation, we grew seedlings of a zucchini cultivar (MU-CU-16) 13,14 , which produces marketable dark green uniformly cylindrical fruits, and mechanically inoculated the plants (n=3) with ZYMV-crtB at one-week intervals (Fig.…”

“…Virus-based expression of transcription factors Delila and, particularly, Rosea1 from Antirrhinum majus has been shown to lead to massive accumulation of anthocyanins in plant tissues 3,4 . Similarly, virus-based expression of Pantoea ananatis phytoene synthase (crtB), the enzyme catalyzing the first step of carotenoid biosynthesis in this soil bacterium, has been demonstrated to lead to a substantial accumulation of phytoene and other carotenoids in plant tissues 5,6 . Virus-based co-expression of crtB and a Crocus sativus carotenoid cleavage dioxygenase (CCD2L) induced large accumulation in N. benthamiana leaves of the apocarotenoid crocins and picrocrocin, which naturally accumulate in saffron stigma and are main constituents of the valued spice 7 .…”