The Maize Phytoene Synthase Gene Family: Overlapping Roles for Carotenogenesis in Endosperm, Photomorphogenesis, and Thermal Stress Tolerance      

Abstract

Carotenoids are essential for photosynthesis and photoprotection; they also serve as precursors to signaling molecules that influence plant development and biotic/abiotic stress responses. With potential to improve plant yield and nutritional quality, carotenoids are targets for metabolic breeding/engineering, particularly in the Poaceae (grass family), which includes the major food crops. Depending on genetic background, maize (Zea mays) endosperm carotenoid content varies, and therefore breeding-enhanced carotenoid levels have been of ongoing interest. The first committed step in the plastid-localized biosynthetic pathway is mediated by the nuclear-encoded phytoene synthase (PSY). The gene family in maize and other grasses contains three paralogs with specialized roles that are not well understood. Maize endosperm carotenoid accumulation requires PSY1 expression. A maize antibody was used to localize PSY1 to amyloplast envelope membranes and to determine PSY1 accumulation in relation to carotenoid accumulation in developing endosperm. To test when and if PSY transcript levels correlated with carotenoid content, advantage was taken of a maize germplasm diversity collection that exhibits genetic and chemical diversity. Total carotenoid content showed statistically significant correlation with endosperm transcript levels at 20 d after pollination for PSY1 but not PSY2 or PSY3. Timing of PSY1 transcript abundance, previously unknown, provides critical information for choosing breeding alleles or properly controlling introduced transgenes. PSY1 was unexpectedly found to have an additional role in photosynthetic tissue, where it was required for carotenogenesis in the dark and for heat stress tolerance. Leaf carotenogenesis was shown to require phytochrome-dependent and phytochrome-independent photoregulation of PSY2 plus nonphotoregulated PSY1 expression.

Timing and Biosynthetic Potential for Carotenoid Accumulation in Genetically Diverse Germplasm of Maize

“…For the carotenoid biosynthetic pathway, we identified two CrtISO (for carotene isomerase) genes in maize on chromosomes 2 and 4, adding to previously isolated pathway genes for PDS (for phytoene desaturase), ZDS (for zetacarotene desaturase), LCYE (for lycopene e-cyclase), and LCYB (for lycopene b-cyclase), all of which are single copy in maize and other grasses (Buckner et al, 1990;Li et al, 1996Li et al, , 2008aLi et al, , 2008bMatthews et al, 2003;Singh et al, 2003;Harjes et al, 2008;Supplemental Table S1). In contrast to two maize CrtISO genes, only one was found in rice, sorghum, and Arabidopsis (Arabidopsis thaliana; Supplemental Table S1; Supplemental Fig.…”

“…The validated germplasm subset was then used to assess the influence of other genes in the pathway collection described above using paralog-specific primers (Supplemental Table S2). Transcript levels were quantified in developing endosperm 10 to 25 d after pollination (DAP; Supplemental (Li et al, 2008b). Transcript levels at each developmental stage were then statistically compared with carotenoid content of the mature kernel.…”

“…To validate the utility of the core subset to yield predictive results, we used the collection to test the PSY gene family, which contains one member, PSY1, known to control flux to endosperm carotenoids. Only PSY1, of three paralogs, showed the expected correlation of transcripts and carotenoid content in endosperm (Li et al, 2008b). The validated germplasm subset was then used to assess the influence of other genes in the pathway collection described above using paralog-specific primers (Supplemental Table S2).…”

“…1). Extensive studies have implicated phytoene synthase (PSY), the first committed enzyme, as rate controlling for endosperm carotenoids (Randolph and Hand, 1940;Palaisa et al, 2003;Gallagher et al, 2004;Wong et al, 2004;Pozniak et al, 2007;Li et al, 2008aLi et al, , 2008b. However, upstream precursor pathways may also positively influence carotenoid accumulation (Matthews and Wurtzel, 2000;Mahmoud and Croteau, 2001), while downstream degradative pathways may deplete the carotenoid pool (Galpaz et al, 2008).…”

Alleviation of Cadmium Toxicity in Pisum sativum L. Seedlings by Calcium Chloride

“…As an antioxidant, they scavenge 1 O 2 to inhibit oxidative damage and quench triplet sensitizer (3Chl * ) and excited chlorophyll (Chl * ) molecule to prevent the formation of 1 O 2 to protect the photosynthetic apparatus. Carotenoids also serve as precursors to signaling molecules that influence plant development and biotic/abiotic stress responses (Li et al, 2008).…”