Minisymposium 14: Hormones II

Abs # 26002: The Arabidopsis Auxin Response Factor 8 (ARF8) is involved in light and auxin signal transduction

Presenter:	Yamamoto, Kotaro T., kty@ees.hokudai.ac.jp
Authors	Yamamoto, Kotaro T. (A) (B)  Tian, Chang-en  (A) (B)  Muto, Hideki  (A) (B)  Matamura, Tomoyuki  (B)   Tatematsu, Kiyoshi  (B)
Affiliations:	(A): Division of Biological Sciences, Graduate School of Science, Hokkaido University (B): Division of Biological Science, Graduate School of Environmental Earth Science, Hokkaido University

ARFs have been shown to play a central role in multiple auxin-dependent phenomena. To better understand the role of ARFs in auxin action, we have been carrying out phenotypic characterization of disruption lines of ARF genes. Screening of T-DNA insertion lines of Arabidopsis in Biotechnology Center, Univ. Wisconsin, by PCR, we have isolated a mutant line, arf8-1, in which T-DNA inserts into the third exon of ARF8 gene. arf8-1 seedlings (Ws background) showed the long-hypocotyl phenotype when grown in either white, red, far-red or blue light. Introduction of ARF8 ORF driven by ARF8 promoter into arf8-1 mutant restored the mutant phenotype. Transgenic lines that harbored ARF8 cDNA driven by CaMV 35S promoter exhibited shorter hypocotyls than wild type in light condition. These indicate that disruption of ARF8 gene causes the long-hypocotyl phenotype. Growth orientation of hypocotyls in wild type is randomized under red-light condition. However, arf8-1 hypocotyls grew upward in the same condition. These results indicate that light sensitivity is reduced in arf8-1. Mature arf8-1 plants displayed stronger apical dominance than wild type. Etiolated arf8-1 seedlings showed slightly reduced phototropism in hypocotyls, but their gravitropism was not affected significantly. Growth of hypocotyls and roots was inhibited by IAA in arf8-1 as strongly as in wild type. Higher temperature promoted hypocotyl growth in arf8-1 and wild type to a similar extent. These results suggest that sensitivity to auxin is not largely affected in arf8-1. Using ARF8 promoter::GUS, ARF8 gene was found to express in hypocotyls of light-grown seedlings, but it expressed faintly in the dark. It also expressed constitutively in apical shoot and root meristems. The long-hypocotyl phenotype of arf8-1 is a reminiscent of loss-of-function mutants of DFL1 (Nakazawa et al., 2001) and FIN219 (Hsieh et al., 2000), members of GH3 gene family. GH3 may regulate IAA level since it may encode an enzyme involved in synthesis of IAA conjugates (Staswick et al., 2002). In fact, mRNA level of DFL1 decreased significantly in arf8-1. We will discuss a possibility that ARF8 may regulate auxin level depending on light conditions through transcriptional regulation of GH3 genes. Considering that some of dominant Aux/IAA mutants are constitutively photomorphogenetic (Nagpal et al., 2000), ARF and Aux/IAA genes are likely to regulate a complex network integrating auxin and light signals.