Browsing by Subject "Photomorphogenesis"
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Item Conserved modulation of the constitutive photomorphogenic 1 E3 ubiquitin ligase activity by the bHLH transcription factors, phytochrome interacting factors(2016-05) Xu, Xiaosa; Huq, Enamul; Roux, Stanley J; Vokes, Steven A; Lloyd, Alan M; Linder, Craig RAs sessile organism, plants are informed of the time of the day and their place of growth by a collection of photoreceptors that detect changing intensity, quality, and direction of light in the environment. Among these photoreceptors, phytochromes (A, B, C, D, E) are the major ones to drive a developmental switch for initial emergence of seedlings from subterranean darkness into sunlight, called plant photomorphogenesis. Previous studies have identified many regulators in the phytochorme-mediated photomorphogenesis pathway. Among them, CONSTITUTIVELY PHOTOMORPHOGENIC 1/ SUPPRESSOR OF PHYTOCHROME A (COP1/SPA) complex and PHYTOCHROME INTERACTING FACTORs (PIF1, 3, 4, 5, 7, 8) are key negative regulators that can suppress photomorphogenesis individually. However, the functional relationships between the COP1-SPA and the PIFs are still unknown. Here in my dissertation project, I showed that PIFs have nontranscriptional roles by acting as cofactors of the COP1 E3 Ubiquitin ligase to enhance the trans-ubiquitination and subsequent degradation of the substrates of COP1, including LONG HYPOCOTYL 5 (HY5), LONG HYPOCOTYL IN FAR-RED 1 (HFR1) and a newly identified substrate HECATE 2 (HEC2), to suppress photomorphogenesis. HFR1 also promotes the degradation of PIF1 in the dark via direct heterodimerization to trigger rapid seed germination upon light exposure. The reciprocal co-degradation between PIF1 and HFR1 is dependent on the ubi/26S-proteasome pathway in vivo. In addition, the cop1 and pif1, 3, 4, 5 mutant combinations showed overproliferation of stigmatic tissues phenotype similar to HEC overexpression plants. Biochemical and genetic evidence showed that HECs are highly abundant in the cop1 pifs mutant flowers. Moreover, HECs negatively regulate the PIFs’ binding activity to the G-box regions of promoters of flower pattern genes, SEP1 and SEP3. Taken together, these data revealed the conserved modulation of the COP1 Ubiquitin E3 ligase activity by PIFs, uncovered a suicidal co-degradation mechanism between the HFR1 and PIF1 to fine tune seed germination and seedling development, and demonstrated a novel function of COP1 and PIFs in regulating flower pattern development.Item Light regulation and functional characterization of Phytochrome Interacting Factor 1 (PIF1) in Arabidopsis(2012-05) Zhu, Ling; Huq, Enamul; Jansen, Robert; Lloyd, Alan; O'Halloran, Theresa; Roux, StanleyPlants sense light intensity, quality and direction through a group of photoreceptors to modulate their growth and development. One family of photoreceptor is called phytochromes (phys) that perceives red and far red light. Phys transduce light signals via a sub-family of the basic Helix-Loop-Helix (bHLH) transcription factors called Phytochrome Interacting Factors (PIFs). PIFs function as negative regulators in the phy-mediated light signaling pathways. In darkness, PIFs regulate downstream gene expressions to inhibit photomorphogenesis. Upon light exposure, PIFs are phosphorylated and poly-ubiquitylated prior to their rapid degradation through the 26S proteasome pathway. One of the PIFs, PIF1, has the highest affinity for both phyA and phyB and also displayed the fastest degradation kinetics under both red and far red light. Here we showed that PIF1 directly and indirectly regulates key genes involved in chlorophyll biosynthesis to optimize the greening process in Arabidopsis. PIF1 binds to a G-box (CACGTG) DNA sequence element present in its direct target genes (e.g., protochlorophyllide oxidoreductase C, PORC) in darkness and regulates their expression. Structure-function studies revealed two separate regions called APB and APA necessary for binding to phyB and phyA, respectively, located at the amino-terminus and a novel phosphorylation site at the carboxy-terminus of PIF1. Both amino- and carboxy-terminal regions are necessary for the light-induced degradation of PIF1. However, the DNA binding is not necessary for the light-induced degradation of PIF1. Using a targeted systems biology approach, we identified new factors, HECATE proteins that promote photomorphogenesis by negatively regulating the function of PIF1. Moreover, we employed an unbiased genetic screening using luciferase imaging system to identify new mutants defective in the light-induced degradation of PIF1. The cloning and characterization of these mutants will help identify the factors, such as the kinase and E3 ligase, responsible for the light-induced degradation of PIF1. Taken together, these data revealed detail mechanisms of how PIF1 negatively regulates photomorphogenesis and how light induces rapid degradation of PIF1 to promote photomorphogenesis.Item The regulation of phytochrome interacting factor1 and its role in light signaling(2009-05) Castillón, Alicia; Huq, EnamulPlants modulate their growth and development according to the prevailing light conditions. To detect light signals plants have an array of photoreceptors including the phytochromes which monitor the red and far-red light regions of the light spectrum. Phytochromes regulate gene expression in response to light in part by physically interacting with nuclear-localized bHLH transcription factors called PHYTOCHROME INTERACTING FACTORS (PIFs). PIFs are known to function as negative regulators of photomorphogenesis. Here we show that PIF1, the PIF family member with the highest affinity for phys, is degraded after pulses or continuous red, far-red or blue light in a phytochrome dependent manner. In etiolated seedlings, phyA plays a dominant role in regulating the degradation of PIF1 after a pulse of red, far-red or blue light; while phyB, phyD and other phys also influence PIF1 degradation after prolonged illumination. PIF1 interacted with phyA and phyB in a blue light-dependent manner, and the interactions with phys are necessary for the light-induced degradation of PIF1. In response to red, far-red or blue light treatments PIF1 is rapidly phosphorylated, poly-ubiquitinated and degraded via the ubiquitin/26S proteasomal pathway. In addition, we show that PIF1 negatively regulates photomorphogenesis at the seedling stage. The overexpression of a light-stable truncated form of PIF1 causes constitutively photomorphogenic phenotypes in the dark. pif1 seedlings displayed more open cotyledons and slightly reduced hypocotyl length compared to wild type under diurnal (12h light/12h dark) blue light conditions. Double mutant analyses demonstrated that pif1phyA, pif1phyB, pif1cry1 and pif1cry2 have enhanced cotyledon opening compared to the single photoreceptor mutants under diurnal blue light conditions. Taken together, these data suggest that PIF1 functions as a negative regulator of photomorphogenesis and that light-activated phys induce the degradation of PIF1 through the ubi/26S proteasomal pathway to promote photomorphogenesis.