ORHis, a Natural Variant of OR, Specifically Interacts with Plastid Division Factor ARC3 to Regulate Chromoplast Number and Carotenoid Accumulation

Abstract

Chromoplasts are colored plastids that synthesize and store massive amounts of carotenoids. Chromoplast number and size define the sink strength for carotenoid accumulation in plants. However, nothing is known about the mechanisms controlling chromoplast number. Previously, a natural allele of Orange (OR), OR His , was found to promote carotenoid accumulation by activating chromoplast differentiation and increasing carotenoid biosynthesis, but cells in orange tissues in melon fruit and cauliflower OR mutant have only one or two enlarged chromoplasts. In this study, we investigated an OR His variant of Arabidopsis OR, genetically mimicking the melon OR His allele, and found that it also constrains chromoplast number in Arabidopsis calli. Both in vitro and in vivo experiments demonstrate that OR His specifically interacts with the Membrane Occupation and Recognition Nexus domain of ACCUMULATION AND REPLICATION OF CHLOROPLASTS 3 (ARC3), a crucial regulator of chloroplast division. We further showed that OR His interferes with the interaction between ARC3 and PARALOG OF ARC6 (PARC6), another key regulator of chloroplast division, suggesting a role of OR His in competing with PARC6 for binding to ARC3 to restrict chromoplast number. Overexpression or knockout of ARC3 in Arabidopsis OR His plants significantly alters total carotenoid levels. Moreover, overexpression of the plastid division factor PLASTID DIVISION 1 greatly enhances carotenoid accumulation. These division factors likely alter carotenoid levels via their influence on chromoplast number and/or size. Taken together, our findings provide novel mechanistic insights into the machinery controlling chromoplast number and highlight a potential new strategy for enhancing carotenoid accumulation and nutritional value in food crops.

TGD5 is required for normal morphogenesis of non‐mesophyll plastids, but not mesophyll chloroplasts, in Arabidopsis

“…Although the molecular machinery of mesophyll chloroplast division is well understood, little is known about the division of non‐mesophyll plastids (Chen et al ., 2018). Strikingly, however, a recent study showed that a variant of OR (ORANGE) protein modulates chromoplast division in cauliflower curds and melon fruits, but not chloroplast division in leaves, by specifically binding with the chloroplast division regulator ARC3, thereby inhibiting the interaction between ARC3 and the division regulator PARC6 (Sun et al ., 2020). We are now beginning to better understand the underlying mechanisms of plastid morphogenesis, including plastid division and stromule formation, that are specific to non‐mesophyll (or non‐green/non‐photosynthetic) plastids.…”

A Multifunctional Modulator Coordinates Nuclear Transcription and Plastid Metabolism and Proliferation

“…In addition to promoting chromoplast differentiation, OR mut arrests chromoplast division in cauliflower curd and melon mesocarp, but not chloroplast division in leaves; thus, only one or two chromoplasts per cell are present in mutants. This contrasts with the corresponding wild-type cells, which contain numerous plastids (Sun et al, 2020). Introduction of the relevant OR mutation into Arabidopsis mimics this phenomenon in the calli, resulting in the formation of only one or two chromoplasts per cell in contrast to wild-type cells, which contain numerous leucoplasts.…”

“…Because most plastid proteins are encoded in nuclear DNA, which is translated in the cytosol before the proteins are translocated into plastids (Archibald, 2015), plastid activities are regulated largely by both nuclear transcriptional changes and post-translational modulation of plastid proteins. Although the mechanism coordinating plastid differentiation and division is largely unknown, recent studies have revealed that ORANGE (OR) protein is targeted to both the nucleus and plastids and acts as a multifunctional modulator of nuclear transcription and plastid metabolism and division during plastid differentiation (Sun et al, 2019(Sun et al, , 2020.…”

Multi-strategy engineering greatly enhances provitamin A carotenoid accumulation and stability in Arabidopsis seeds

“…OR as a conserved protein in the plant kingdom is the major post-translational regulator of PSY for carotenogenesis (Park et al 2016;Welsch et al 2018;Zhou et al 2015). Its natural variants, such as OR His , have additional functions to promote chromoplast biogenesis and development (Chayut et al 2017;Sun et al 2020b). Ectopic expression of OR His greatly promotes carotenoid accumulation in various plants (Kim et al 2021;Yazdani et al 2019;Yuan et al 2015a).…”