A foliar pigment-based bioassay for interrogating chloroplast signalling revealed that carotenoid isomerisation regulates chlorophyll abundance

Transcriptome Analysis Revealed ZmPTOX1 Is Required for Seedling Development and Stress Tolerance in Maize

Plant seedling morphogenesis is considerably related to photosynthesis, pigment synthesis, and circadian periodicity during seedling development. We identified and cloned a maize zebra or crossbanding leaves mutant wk3735, which produces pale white kernels and was identified and plays a role in the equilibrium of the Redox state the in/out of ETC by active oxygen scavenging. Interestingly, it produces the zebra leaves during the production of the first seven leaves, which is apparently different from the mutation of homologs AtPTOX in Arabidopsis. It is intriguing to investigate how and why yellow crossbands (zebra leaf phenotype) emerge on leaves. As expected, chlorophyll concentration and photosynthetic efficiency both significantly declined in the yellow sector of wk3735 leaves. Meanwhile, we observed the circadian expression pattern of ZmPTOX1, which was further validated by protein interaction assays of the circadian clock protein TIM1 and ZmPTOX1. The transcriptome data of yellow (muW) and green (muG) sectors of knock-out lines and normal leaves of overexpression lines (OE) at the 5th-leaf seedling stage were analyzed. Zebra leaf etiolated sections exhibit a marked defect in the expression of genes involved in the circadian rhythm and rhythmic stress (light and cold stress) responses than green sections. According to the analysis of co-DEGs of muW vs. OE and muG vs. OE, terms linked to cell repair function were upregulated while those linked to environmental adaptability and stress response were downregulated due to the mutation of ZmPTOX1. Further gene expression level analyses of reactive oxygen species (ROS) scavenging enzymes and detection of ROS deposition indicated that ZmPTOX1 played an essential role in plant stress resistance and ROS homeostasis. The pleiotropic roles of ZmPTOX1 in plant ROS homeostasis maintenance, stress response, and circadian rhythm character may collectively explain the phenotype of zebra leaves during wk3735 seedling development.

Reducing PHYTOENE SYNTHASE activity fine-tunes the abundance of a cis-carotene-derived signal that regulates the PIF3/HY5 module and plastid biogenesis

PHYTOENE 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. Four genetic variants of PSY (psy-4, psy-90, psy-130, and psy-145) were identified using a forward genetics approach that rescued leaf virescence phenotypes and plastid abnormalities displayed by the Arabidopsis CAROTENOID ISOMERASE (CRTISO) mutant ccr2 (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 post-transcriptional fine-tuning of PSY levels and activity without changing localization to the stroma and protothylakoid membranes. psy genetic variants did not alter total xanthophyll or β-carotene accumulation in ccr2, yet they reduced specific acyclic linear cis-carotenes linked to the biosynthesis of a currently unidentified apocarotenoid signal regulating plastid biogenesis, chlorophyll biosynthesis, and photomorphogenic regulation. ccr2 psy variants modulated the PHYTOCHROME-INTERACTING FACTOR 3/ELONGATED HYPOCOTYL 5 (PIF3/HY5) ratio, and displayed a normal prolamellar body formation in etioplasts and chlorophyll accumulation during seedling photomorphogenesis. Thus, suppressing PSY activity and impairing PSY:ORANGE protein interactions revealed how cis-carotene abundance can be fine-tuned through holoenzyme-metabolon interactions to control plastid development.

Subfunctionalization of D27 Isomerase Genes in Saffron

Chromoplasts and chloroplasts contain carotenoid pigments as all-trans- and cis-isomers, which function as accessory light-harvesting pigments, antioxidant and photoprotective agents, and precursors of signaling molecules and plant hormones. The carotenoid pathway involves the participation of different carotenoid isomerases. Among them, D27 is a β-carotene isomerase showing high specificity for the C9-C10 double bond catalyzing the interconversion of all-trans- into 9-cis-β-carotene, the precursor of strigolactones. We have identified one D27 (CsD27-1) and two D27-like (CsD27-2 and CsD27-3) genes in saffron, with CsD27-1 and CsD27-3, clearly differing in their expression patterns; specifically, CsD27-1 was mainly expressed in the undeveloped stigma and roots, where it is induced by Rhizobium colonization. On the contrary, CsD27-2 and CsD27-3 were mainly expressed in leaves, with a preferential expression of CsD27-3 in this tissue. In vivo assays show that CsD27-1 catalyzes the isomerization of all-trans- to 9-cis-β-carotene, and could be involved in the isomerization of zeaxanthin, while CsD27-3 catalyzes the isomerization of all-trans- to cis-ζ-carotene and all-trans- to cis-neurosporene. Our data show that CsD27-1 and CsD27-3 enzymes are both involved in carotenoid isomerization, with CsD27-1 being specific to chromoplast/amyloplast-containing tissue, and CsD27-3 more specific to chloroplast-containing tissues. Additionally, we show that CsD27-1 is co-expressed with CCD7 and CCD8 mycorrhized roots, whereas CsD27-3 is expressed at higher levels than CRTISO and Z-ISO and showed circadian regulation in leaves. Overall, our data extend the knowledge about carotenoid isomerization and their implications in several physiological and ecological processes.