February 13, 2015

Zhou et al. "Arabidopsis OR proteins are the major posttranscriptional regulators of phytoene synthase in controlling carotenoid biosynthesis "

Xiangjun Zhou, Ralf Welsch, Yong Yang, Daniel Álvarez, Matthias Riediger, Hui Yuan, Tara Fish, Jiping Liu, Theodore W. Thannhauser, and Li Li

Phytoene synthase (PSY) has long been considered the rate limiting step in the formation of carotenoids, C40 isoprenoid compounds which act as auxiliary pigments in photosynthesis, quenching highly excited chlorophyll and preventing photooxidative damage to chloroplasts. In the human diet, carotenoids are considered both essential nutrients (beta-carotene serves as provitamin A) as well as generally beneficial antioxidant neutraceuticals due to their electron donating (and free radical scavenging) properties. Increasing the accumulation of carotenoids in crops has been a biotechnological goal for some time, and it is clear that PSY modulation will be an important part of this process. But the control of flux though the long and fairly complex carotenoid biosynthetic pathways is tightly regulated, resisting obvious attempts at increasing carotenoid accumulation through simple overexpression. Here Zhou et al. provide an important advance in understanding the posttranscriptional regulation of PSY in plants, which is mediated by the OR (Orange) protein and a small group of partially redundant, related proteins (OR-like) that regulate PSY activity through protein-protein interactions using a combination of yeast 2-hybrid assays, bimolecular fluorescence complementation, and mutant analysis.

Early Edition: Xiangjun Zhou, doi: 10.1073/pnas.1420831112
The original article can be found here

From the article:

Carotenoids are indispensable natural pigments to plants and humans. Phytoene synthase (PSY), the rate-limiting enzyme in the carotenoid biosynthetic pathway, and ORANGE (OR), a regulator of chromoplast differentiation and enhancer of carotenoid biosynthesis, represent two key proteins that control carotenoid biosynthesis and accumulation in plants. However, little is known about the mechanisms underlying their posttranscriptional regulation. Here we report that PSY and OR family proteins [Arabidopsis thaliana OR (AtOR) and AtOR-like] physically interacted with each other in plastids. We found that alteration of OR expression in Arabidopsis exerted minimal effect on PSY transcript abundance. However, overexpression of AtOR significantly increased the amount of enzymatically active PSY, whereas an ator ator-like double mutant exhibited a dramatically reduced PSY level. The results indicate that the OR proteins serve as the major posttranscriptional regulators of PSY. The ator or ator-like single mutant had little effect on PSY protein levels, which involves a compensatory mechanism and suggests partial functional redundancy. In addition, modification of PSY expression resulted in altered AtOR protein levels, corroborating a mutual regulation of PSY and OR. Carotenoid content showed a correlated change with OR-mediated PSY level, demonstrating the function of OR in controlling carotenoid biosynthesis by regulating PSY. Our findings reveal a novel mechanism by which carotenoid biosynthesis is controlled via posttranscriptional regulation of PSY in plants. 

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