Gibberellins control various aspects of growth and development. Here, we identified a gene, designated paclobutrazol resistance1 (PRE1), by screening Arabidopsis activation-tagged lines. PRE1 encodes a helix-loop-helix protein and belongs to a small gene family. Physiological and genetic analysis indicated that overexpression of PRE1 altered various aspects of gibberellin-dependent responses such as germination, elongation of hypocotyl/petiole, floral induction and fruit development, and suppressed gibberellin-deficient phenotypes of the ga2 mutant. Expression of some gibberellin-responsive genes was also affected by PRE1. Expression of PRE1 was shown to be early gibberellin inducible in the wild-type plants and under control of SPY and GAI, upstream negative regulators of gibberellin signaling. The shortened hypocotyl length phenotype of the gai-1 mutant was suppressed by PRE1 overexpression. Ectopic overexpression of each of the four PRE1-related genes conferred pleiotropic phenotypes similar to PRE1 overexpression, indicative of overlapping functions among the PRE gene family. Our results of gain-of-function studies suggest that PRE genes may have a regulatory role in gibberellin-dependent development in Arabidopsis thaliana.
The HFR1, a basic helix-loop-helix protein, is required for a subset of phytochrome A-mediated photoresponses in Arabidopsis. Here, we show that overexpression of the HFR1-⌬N105 mutant, which lacks the N-terminal 105 amino acids, confers exaggerated photoresponses even in darkness. Physiological analysis implied that overexpression of HFR1-⌬N105 activated constitutively a branch pathway of light signaling that mediates a subset of photomorphogenic responses, including germination, de-etiolation, gravitropic hypocotyl growth, blocking of greening, and expression of some lightregulated genes such as CAB, DRT112, PSAE, PSBL, PORA, and XTR7, without affecting the light-responsiveness of anthocyanin accumulation and expression of other light-regulated genes such as CHS and PSBS. Although the end-of-day far-red light response and petiole elongation were suppressed in the HFR1-⌬N105-overexpressing plants, flowering time was not affected by HFR1-⌬N105. In addition, the HFR1-⌬N105-overexpressing plants showed hypersensitive photoresponses in the inhibition of hypocotyl elongation, dependently on phytochrome A, FHY1, and FHY3 under FR light or phyB under R light, respectively. Moreover, our double mutant analysis suggested that the hypersensitive photoresponse is due to functional cooperation between HFR1-⌬N105 and other light-signaling components including HY5, a basic leucine zipper protein. Taken together, our results of gain-of-function approach with HFR1-⌬N105 suggest the existence of a complex and important basic helix-loop-helix protein-mediated transcriptional network controlling a branch pathway of light signaling and provide a useful framework for further genetic dissection of light-signaling network in Arabidopsis.Plants sense light not only as an energy source but also to collect information about their surrounding environments, such as seasonal changes and proximity of neighboring plants ( Kendrick and Kronenberg, 1994). Being sessile and photoautotrophs, plants have evolved sophisticated photosensory and regulatory systems to optimize their fitness in response to changing light conditions (Smith, 2000). Light affects various aspects of growth and development in higher plants throughout life cycles, from germination to flowering (Fankhauser and Chory, 1997). The early stage of seedling development clearly illustrates such light-dependent development. Seedlings grown in the dark undergo skotomorphogenesis, characterized by elongated hypocotyls and yellow, closed cotyledons. In response to light, seedlings follow the photomorphogenic developmental program; hypocotyls cease elongating, cotyledons become green and unfolded, and the seedlings become photosynthesis competent. Dramatic changes in gene expression underlie these marked developmental changes. Multiple genes encoding transcription factors are up-or down-regulated by light within 1 h after irradiation (Tepperman et al., 2001). Such changes in transcription factors are presumed to regulate the orchestrated expression of various downstream light-regulated...
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