Abstract
Several transcriptional regulators have been identified and demonstrated to play either positive or negative regulatory roles in seedling development. However, the regulatory coordination between hypocotyl elongation and cotyledon expansion during early seedling development in plants remains unknown. We report the identification of a Z-box binding factor (ZBF2) and its functional characterization in cryptochrome-mediated blue light signaling. ZBF2 encodes a G-box binding factor (GBF1), which is a basic leucine zipper transcription factor. Our DNA-protein interaction studies reveal that ZBF2/GBF1 also interacts with the Z-box light-responsive element of light-regulated promoters. Genetic analyses of gbf1 mutants and overexpression studies suggest that GBF1 acts as a repressor of blue light-mediated inhibition in hypocotyl elongation, however, it acts as a positive regulator of cotyledon expansion during photomorphogenic growth. Furthermore, whereas GBF1 acts as a positive regulator of lateral root formation, it differentially regulates the expression of light-inducible genes. Taken together, these results demonstrate that GBF1 is a unique transcriptional regulator of photomorphogenesis in blue light.
Highlights
Plants are able to perceive various wavelengths of light through photoreceptors
Recent studies have revealed that the Z-box containing synthetic and native promoters are responsive to phyA, phyB, and cry1 photoreceptors and are under the control of downstream regulatory comchromes 1 and 2; GBF1, G-box binding factor 1; ZBF2, Z-box binding factor 2; bZIP, basic leucine zipper; LRE, light-responsive elements; GST, glutathione S-transferase; WL, white light; D, dark/darkness; FR, far-red light; RL, red light; BL, blue light; OE1, -2, over-expressers 1 and 2; Ler, Landsberg erecta; Col, Columbia; RLD, Reschiev; GUS, -glucuronidase
Thereby, the cotyledons of gbf1 mutants were found to be significantly smaller as compared with wild-type seedlings under WL and BL grown conditions (Figs. 3G, 3H, 3L, 4C, and 4D). These results suggest that GBF1 acts as a negative regulator of inhibition of hypocotyl elongation, it functions as a positive regulator of cotyledon expansion in BL
Summary
Plant Materials and Transformations—Surface-sterilized seeds were sown on Murashige and Skoog plates, kept at 4 °C in darkness for 3–5 days, and transferred to light at 22 °C. The T-DNA-tagged mutant lines heterozygous or homozygous for the zbf2/gbf mutations were identified by genomic PCR analyses. A genomic fragment containing full-length GBF1 and ϳ1.3 kb upstream DNA sequence was cloned into the SmaI site of pBI101.2 vector. Several individual lines with a single T-DNA locus, as determined by the segregation of hygromycin- or kanamycin-resistant versus -sensitive ratios (3:1), were selected, and homozygous transgenic plants were generated for further studies. Wild-type and gbf mutant plants containing the same transgene were stained for the same length of time. For. immunoblot analysis, the proteins were transferred to Hybond C-Extra (Amersham Biosciences), blocked with 5% bovine serum albumin in phosphate-buffered saline (10 mM Na2HPO4, 1.8 mM KH2PO4, 140 mM NaCl, 2.7 mM KCl) and probed with affinity-purified GBF1 polyclonal antibodies. Sequence data (GBF1) from this article have been deposited with the EMBL/GenBankTM data libraries under accession number AJ843257
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