Functional studies of two members of the GH3-like gene family in Arabidopsis

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Light is one of the most important environmental cues as well as signal transduction factors for plants. Arabidopsis has evolved complex photoreceptor systems to perceive different light signals. FIN219 has been shown to be involved in phytochrome A (phyA)-mediated far-red light signaling and belongs to a GH3-like gene family that is composed of 19 members in Arabidopsis. Previous studies by RT-PCR revealed that both GH3-14 and GH3-15 were affected by multiple photoreceptors and signaling components under various light conditions. As well, both genes are very similar at the amino-acid level and belong to the group III of the GH3-like gene family. To further understand the functional relationship of these two genes, the thesis will focus on the functional characterization of them by transgenic approach. RT-PCR results indicated that GH3-14 transcripts were increased in the fin219 mutant compared to that in wild-type seedlings grown under continuous far-red light (cFR), red light (R), blue light (B), dark (D), and white light (WL) for 3 days; whereas GH3-15 transcripts were decreased in the fin219 mutant under red light and blue light compared to wild type, implying that FIN219 as a positive regulator in FR signaling can differentially regulate both GH3-14 and GH3-15 expression. In the light switch test, both the expression levels of GH3-14 and GH3-15 were increased in the fin219 mutant. Furthermore, T-DNA insertion lines of GH3-14 and GH3-15 and the overexpression lines of GH3-14 with increased expression of GH3-14 mRNA levels as well as decreased expression of GH3-15 mRNA levels exhibited a long hypocotyl phenotype under cFR; T-DNA insertion lines of both genes also exhibited delayed flowering phenotypes under long-day and short-day conditions. These results suggested that GH3-14 may be a negative regulator in the far-red light signaling and flowering; GH3-15 may be a positive regulator in these pathways. In addition, subcellular localization studies and promoter activity analysis revealed that both gene products might be located together in cells and in organs. Epistatic analyses revealed that FIN219 was located downstream the GH3-14 and GH3-15 in far-red light signal transduction pathway, implying that the relationship between FIN219 and GH3-14, -15 might be complicated and worth pursuing further.