ORHis, a natural variant of OR, specifically interacts with plastid division factor ARC3 to regulate chromoplast number and carotenoid accumulation

Authors: SUN, TIANHU - Cornell University; YUAN, HUI - Cornell University; CHEN, CHENG - Michigan State University; KADIRJAN-KALBACH, DEENA - Michigan State University; MAZOUREK, MICHAEL - Cornell University; OSTERYOUNG, KATHERINE - Michigan State University; Li, Li

Submitted to: Molecular Plant
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2020
Publication Date: 3/26/2020
Citation: Sun, T., Yuan, H., Chen, C., Kadirjan-Kalbach, D., Mazourek, M., Osteryoung, K., Li, L. 2020. ORHis, a natural variant of OR, specifically interacts with plastid division factor ARC3 to regulate chromoplast number and carotenoid accumulation. Molecular Plant. https://doi.org/10.1016/j.molp.2020.03.007.
DOI: https://doi.org/10.1016/j.molp.2020.03.007

Interpretive Summary: Carotenoids are critical for human nutrition and health. Chromoplasts are the key organelle for carotenoid biosynthesis and accumulation. Chromoplast numbers define the sink strength for carotenoid accumulation in plants. Yet nothing is known about how chromoplast number is controlled. A better understanding of the control mechanism will have signficiant impact on carotneoid enrichment and nutrition improvement in crops. Here, we discover that ORHis, a natural variant of OR that promotes carotenoid accumulation, constrains chromoplast number via directly interacting with the central chloroplast division factor ARC3 to interfere with the chromoplast number. We also demonstrate that manipulation of plastid division factors significantly affects carotenoid levels in plants. This study provides not only the mechanistic insights into the machinery controlling chromoplast or non-green plastid number, but also a novel strategy and new genetic tools to enrich carotenoids with enhanced nutritional value in food crops.

Technical Abstract: 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 PLASTID DIVISION1 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.