LAUSR

Chromoplasts are the colored plastids that synthesize and store massive amounts of carotenoids. Chromoplast number and size define the sink strength for carotenoid accumulation in plants. Yet nothing is known about the mechanisms controlling chromoplast number. Previously, we showed that a natural allele of Orange (OR), ORHis, promotes carotenoid accumulation by activating chromoplast differentiation and increasing carotenoid biosynthesis. However, orange tissues in melon fruit and cauliflower OR mutant have only one or two enlarged chromoplasts. Here, we investigated an ORHis variant of Arabidopsis OR, genetically mimicking the melon ORHis allele, and found that it also constrains chromoplast number in Arabidopsis calli. Both in vitro and in vivo evidences demonstrate that ORHis specifically interacts with the Membrane Occupation and Recognition Nexus (MORN) domain of ACCUMULATION AND REPLICATION OF CHLOROPLASTS3 (ARC3), a crucial regulator of chloroplast division. ORHis was further shown to interfere with the interaction between ARC3 and PARALOG OF ARC6 (PARC6), another key regulator of chloroplast division, suggesting a role of ORHis in competing with PARC6 for binding to ARC3 to restrict chromoplast number. Over-expression or knockout of ARC3 in Arabidopsis ORHis plants significantly alters total carotenoid levels. Moreover, upregulation of the plastid division factor PLASTIDDIVISION1 greatly enhances carotenoid accumulation. These division factors likely alter carotenoid levels via their influence on chromoplast number and/or size. Together, our findings provide novel mechanistic insights into the machinery controlling chromoplast number and reveal a potential new strategy for enhancing carotenoid accumulation and nutritional value in food crops.