Reducing PSY Activity Fine Tunes Threshold Levels of Acis-Carotene-derived Signal That Regulates the PIF3/HY5 Module and Plastid Biogenesis

ABSTRACTPHYTOENE SYNTHASE (PSY) is a rate-limiting enzyme catalysing the first committed step of carotenoid biosynthesis, and changes in PSY gene expression and/or protein activity alter carotenoid composition and plastid differentiation in plants. Here we identified four genetic variants ofPSY(psy−4,psy−90,psy−130andpsy−145) using a forward genetics approach that rescued leaf virescence phenotypes displayed by theArabidopsisCAROTENOID ISOMERASE (CRTISO) mutantccr2(carotenoid and chloroplast regulation 2) when grown under a shorter photoperiod. The four non-lethal mutations affected alternative splicing, enzyme-substrate interactions, and PSY:ORANGE multi-enzyme complex binding, constituting the dynamic posttranscriptional fine-tuning of PSY levels and activity without changing localization to the stroma and protothylakoid membranes.psygenetic variants did not alter overall total xanthophyll or cis-carotene accumulation inccr2 yetreduced specific acyclic linearcis-carotenes linked to the biosynthesis of a yet-to-be-identified apocarotenoid signal.ccr2 psyvariants modulated the ratio of PHYTOCHROME-INTERACTING FACTOR 3/ELONGATED HYPOCOTYL 5 (PIF3/HY5), displayed a normal PLB formation in etioplasts, and chlorophyll accumulation during seedling photomorphogenesis. Thus, suppressing PSY activity and impairing PSY:ORANGE protein interactions reveals how threshold specificcis-carotene levels can be fine-tuned through holoenzyme-metabolon interactions to control plastid development.HighlightsManipulation of the PHYTOENE SYNTHASE catalytic activity in concert with its regulatory protein, ORANGE, reduces threshold levels of acyclic linearcis-carotenes that signal control over plastid biogenesis in dark and light grown Arabidopsis seedlings