Photomorphogenic development of the Arabidopsisshy2–1D mutation and its interaction with phytochromes in darkness

Abstract

SummaryWe previously reported a photomorphogenic mutation of Arabidopsis thaliana, shy2-1D, as a dominant suppressor of a hy2 mutation. Here, we report that shy2-1D confers various photo-responsive phenotypes in darkness and the dark phenotypes of the mutant are affected by phytochrome deficiency. Dark-grown seedlings of the mutant developed several photomorphogenic characteristics such as short hypocotyls, cotyledon expansion and opening, and partial differentiation of plastids. When grown further in darkness, the mutant plant underwent most of the developmental stages of a light-grown wild-type plant, including development of foliar leaves, an inflorescence stem with cauline leaves, and floral organs. In addition, two light-inducible genes, the nuclear-encoded CAB and the plastid-encoded PSBA genes, were highly expressed in the dark-grown mutant seedlings. Furthermore, reduced gravitropism, a phytochrome-modulated response, was observed in the mutant hypocotyl in darkness. Thus, shy2-1D is one of the most pleiotropic photomorphogenic mutations identified so far. The results indicate that SHY2 may be a key component regulating photomorphogenesis in Arabidopsis. Surprisingly, double mutants of the shy2-1D mutant with the phytochrome-deficient mutants hy2, hy3 (phyB-1) and fre1-1 (phyA-201) showed reduced photomorphogenic response in darkness with a longer hypocotyl, a longer inflorescence stem, and a lower level expression of the CAB gene than the shy2-1D single mutant. These results showed that phytochromes function in darkness in the shy2-1D mutant background. The implications of these results are discussed.

Phytochromes inhibit hypocotyl negative gravitropism by regulating the development of endodermal amyloplasts through phytochrome-interacting factors

“…Auxin is responsible for the tropic elongation downstream of gravity sensing or photosensing, as shown by altered gravitropism or phototropism of various auxin-related mutants, including nph4, pgp19, shy2-1D, pin2, pin3, and aux1 (28,(37)(38)(39)(40)(41)(42)(43)(44). Phytochromes have been shown to regulate auxin biosynthetic genes, transport genes, and signaling genes either at the transcriptional level or at the protein level (37,(45)(46)(47), suggesting that phytochromes may regulate some of these genes through PIFs.…”

Arabidopsis Mutants Resistant to S(+)-β-Methyl-α, β-Diaminopropionic Acid, a Cycad-Derived Glutamate Receptor Agonist

“…Auxin (Jensen et al, 1998;Kim et al, 1998), gibberellin (Jacobsen and Olszewski, 1993;Steber et al, 1998), and brassinolide (Fujioka et al, 1997;Azpiroz et al, 1998) all act as positive regulators of hypocotyl elongation in Arabidopsis. Ethylene (Kieber et al, 1993) and cytokinins (Chory et al, 1991a), conversely, are believed to act as inhibitors of hypocotyl elongation.…”

Regulation of Arabidopsis SHY2/IAA3 protein turnover

“…Light regulates expression of SHY2/IAA3 and other Aux/ IAA genes (Tepperman et al, 2001;Tian et al, 2002), and shy2-2 mutants make leaves and overexpress several lightregulated genes in the dark (Kim et al, 1996(Kim et al, , 1998Reed et al, 1998;Soh et al, 1999;Tian and Reed, 1999;Tian et al, 2002). Similar mutants in AXR2/IAA7 and AXR3/IAA17 also made leaves in darkness .…”

“…Among these, the short hypocotyl 2 (shy2-1), shy2-2, and shy2-3 mutations in SHY2/IAA3 caused short hypocotyls, slightly auxin resistant hypocotyl and root growth, reduced lateral root formation or outgrowth, slowed gravitropic response, and decreased auxin-regulated gene expression responses (Kim et al, 1996(Kim et al, , 1998Oono et al, 2002;Reed et al, 1998;Soh et al, 1999;Tian and Reed, 1999;Tian et al, 2002). shy2-2 plants have increased level of SHY2/IAA3 protein (Colo  n-Carmona et al, 2000), suggesting that these mutations stabilize SHY2/IAA3, thereby allowing it to inhibit auxin responses.…”