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Synthetic Conversion of Leaf Chloroplasts Into Carotenoid-Rich Plastids Reveals Mechanistic Basis of Natural Chromoplast Development

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Synthetic Conversion of Leaf Chloroplasts Into Carotenoid-Rich Plastids Reveals Mechanistic Basis of Natural Chromoplast Development Synthetic conversion of leaf chloroplasts into carotenoid-rich plastids reveals mechanistic basis of natural chromoplast development Briardo Llorentea,b,c,1, Salvador Torres-Montillaa, Luca Morellia, Igor Florez-Sarasaa, José Tomás Matusa,d, Miguel Ezquerroa, Lucio D’Andreaa,e, Fakhreddine Houhouf, Eszter Majerf, Belén Picóg, Jaime Cebollag, Adrian Troncosoh, Alisdair R. Ferniee, José-Antonio Daròsf, and Manuel Rodriguez-Concepciona,f,1 aCentre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, Campus UAB Bellaterra, 08193 Barcelona, Spain; bARC Center of Excellence in Synthetic Biology, Department of Molecular Sciences, Macquarie University, Sydney NSW 2109, Australia; cCSIRO Synthetic Biology Future Science Platform, Sydney NSW 2109, Australia; dInstitute for Integrative Systems Biology (I2SysBio), Universitat de Valencia-CSIC, 46908 Paterna, Valencia, Spain; eMax-Planck-Institut für Molekulare Pflanzenphysiologie, 14476 Potsdam-Golm, Germany; fInstituto de Biología Molecular y Celular de Plantas, CSIC-Universitat Politècnica de València, 46022 Valencia, Spain; gInstituto de Conservación y Mejora de la Agrodiversidad, Universitat Politècnica de València, 46022 Valencia, Spain; and hSorbonne Universités, Université de Technologie de Compiègne, Génie Enzymatique et Cellulaire, UMR-CNRS 7025, CS 60319, 60203 Compiègne Cedex, France Edited by Krishna K. Niyogi, University of California, Berkeley, CA, and approved July 29, 2020 (received for review March 9, 2020) Plastids, the defining organelles of plant cells, undergo physiological chromoplasts but into a completely different type of plastids and morphological changes to fulfill distinct biological functions. In named gerontoplasts (1, 2). particular, the differentiation of chloroplasts into chromoplasts The most prominent changes during chloroplast-to-chromo- results in an enhanced storage capacity for carotenoids with indus- plast differentiation are the reorganization of the internal plastid trial and nutritional value such as beta-carotene (provitamin A). structures, together with a concurrent loss of photosynthetic Here, we show that synthetically inducing a burst in the production competence and overaccumulation of carotenoid pigments (1–3, of phytoene, the first committed intermediate of the carotenoid 5, 7, 8). The remodeling of the internal plastid structures generates pathway, elicits an artificial chloroplast-to-chromoplast differentia- an increased metabolic sink capacity but it also promotes carot- tion in leaves. Phytoene overproduction initially interferes with enoid biosynthesis. The control of chromoplast differentiation PLANT BIOLOGY photosynthesis, acting as a metabolic threshold switch mechanism appears as a very promising strategy for improving the nutritional that weakens chloroplast identity. In a second stage, phytoene con- and health benefits of crops (5–9). The overall process is known to version into downstream carotenoids is required for the differentiation involve changes in gene expression (e.g., via retrograde signaling of chromoplasts, a process that involves a concurrent reprogramming of nuclear gene expression and plastid morphology for improved ca- from plastids to the nucleus), hormonal regulation, protein quality rotenoid storage. We hence demonstrate that loss of photosynthetic control, and plastid protein import (1, 3, 5). However, very few competence and enhanced production of carotenoids are not just con- inducers of chromoplast development have been identified to sequences but requirements for chloroplasts to differentiate into date. Orange (OR) chaperones are among the best characterized, chromoplasts. Significance carotenoid | chromoplast | differentiation | phytoene | synthetic Carotenoids are natural pigments whose properties as provi- lastids comprise a group of morphologically and functionally tamin A and health-promoting phytonutrients make them ideal Pdiverse plant organelles capable of differentiating from one targets for biofortification. Here, we show that plastids spe- plastid type to another in response to developmental and envi- cialized in carotenoid overaccumulation named chromoplasts ronmental stimuli (1, 2). Such plastidial conversions are essential can be synthetically produced in plant tissues that do not to sustain many fundamental biological processes and largely naturally develop them. We further demonstrate that differ- contribute to cell specialization in the different plant tissues. entiation of chromoplasts from leaf chloroplasts not just causes Among the different plastid types, chromoplasts are of great but requires both a reduction in photosynthetic activity and a importance in nature and agriculture because of their capacity to stimulation of carotenoid biosynthesis in a process hardwired accumulate high levels of carotenoids, plant pigments of iso- to a major reprogramming of global gene expression and cell metabolism. The synthetic system that we report here should prenoid nature that provide color in the yellow to red range allow to boost the nutritional quality of green vegetables and (3–5). Carotenoids such as beta-carotene (provitamin A) are forage crops once their photosynthetic activity is dispensable health-promoting nutrients that animals cannot synthesize but (e.g., just before harvesting). take up in their diets. They are also added-value compounds widely used in cosmetics, pharma, food, and feed industries as Author contributions: B.L., S.T.-M., L.M., I.F.-S., E.M., J.-A.D., and M.R.-C. designed re- natural pigments and phytonutrients (4, 6). search; B.L., S.T.-M., L.M., I.F.-S., J.T.M., M.E., L.D., F.H., E.M., B.P., J.C., and A.T. performed Chromoplasts differentiate from preexisting plastids such as research; B.L., S.T.-M., L.M., I.F.-S., J.T.M., M.E., B.P., J.C., A.T., A.R.F., J.-A.D., and M.R.-C. contributed new reagents/analytic tools; B.L., S.T.-M., L.M., I.F.-S., J.T.M., M.E., L.D., F.H., proplastids (i.e., undifferentiated plastids), leucoplasts (i.e., un- E.M., B.P., J.C., A.T., A.R.F., J.-A.D., and M.R.-C. analyzed data; and B.L. and M.R.-C. wrote colored plastids in nonphotosynthetic tissues), and chloroplasts the paper. (i.e., photosynthetic plastids). Chloroplasts transform into chro- The authors declare no competing interest. moplasts during the development of many flowers and fruits, but This article is a PNAS Direct Submission. only a few plant species differentiate chromoplasts in leaves (1, Published under the PNAS license. 5). The yellow to red colors that some leaves acquire as they 1To whom correspondence may be addressed. Email: [email protected] or senesce (e.g., in the autumn or when they are exposed to con- [email protected]. tinuous darkness) are due to chloroplast carotenoids becoming This article contains supporting information online at https://www.pnas.org/lookup/suppl/ visible when the chlorophylls degrade. This senescence process, doi:10.1073/pnas.2004405117/-/DCSupplemental. however, does not involve the transformation of chloroplasts into www.pnas.org/cgi/doi/10.1073/pnas.2004405117 PNAS Latest Articles | 1of8 Downloaded by guest on September 24, 2021 but they only work in some tissues, and the specific mechanism by which they promote chromoplast differentiation remains unclear (5). The experimental manipulation of chromoplast differentiation for fundamental studies and biotechnological applications there- fore requests a much better understanding of the mechanisms regulating this process. Results The Bacterial crtB Enzyme Induces the Transformation of Leaf Chloroplasts into Plastids of Chromoplast Features. The first committed step of the carotenoid pathway is the conversion of geranylgeranyl di- phosphate (GGPP) to phytoene, catalyzed by phytoene syn- thase (referred to as PSY in plants and crtB in bacteria). We previously found that the virus-mediated expression of a bac- terial crtB gene in tobacco (Nicotiana tabacum and Nicotiana benthamiana), tomato (Solanum lycopersicum), Arabidopsis thaliana, and several other plants caused leaf yellowing due to increased accumulation of colored endogenous carotenoids downstream of phytoene (10). When the production of crtB was optimized using appropriate viral vectors for specific target hosts, an intense and widespread yellow phenotype was achieved in ed- ible leaves such as those of lettuce (Lactuca sativa)(Fig.1A)and green vegetables such as zucchini (Cucurbita pepo) (Fig. 1B). These results illustrate the potential of this approach to boost the nutritional value of green (i.e., chloroplast-containing) plant tis- sues, which are particularly recalcitrant to carotenoid enrichment. To further investigate the mechanism underlying the characteristic yellow phenotype of crtB-expressing tissues, we used transmission electron microscopy (TEM) to analyze plastid ultrastructure in leaves from N. tabacum and Arabidopsis plants treated with crtB- harboring viral vectors. Yellow sectors in infected leaves contained plastids with a distinctive ultrastructure that were absent in empty vector controls (Fig. 2 A and B). Very similar plastids were ob- Fig. 1. Virus-mediated production of crtB causes leaf yellowing due to ca- served when the crtB gene was transiently expressed from Agro- rotenoid overaccumulation. (A) Lettuce at 12 d postinoculation (dpi) with a bacterium tumefaciens-delivered vectors in agroinfiltrated N. crtB-expressing Lettuce mosaic virus (LMV)-derived vector or an empty benthamiana leaves (Fig. 2C). These plastids were devoid of the control. (B) Zucchini from plants at 14 dpi
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