The Serine-Rich N-Terminal Domain of Oat Phytochrome A Helps Regulate Light Responses and Subnuclear Localization of the Photoreceptor

Jorge J. Casal ^*, Seth J. Davis , Daniel Kirchenbauer , Andras Viczian , Marcelo J. Yanovsky , Richard C. Clough , Stefan Kircher , Emily T. Jordan-Beebe , Eberhard Schäfer , Ferenc Nagy , and Richard D. Vierstra

IFEVA, Facultad de Agronomía, Universidad de Buenos Aires, Avenida San Martín 4453, 1417 Buenos Aires, Argentina (J.J.C., M.J.Y.); Cellular and Molecular Biology Program and the Department of Horticulture, University of Wisconsin, Madison, Wisconsin 53706 (S.J.D., R.C.C., E.T.J.-B., R.D.V.); Institut für Biologie II, Universität Freiburg, Schänzlestrasse 1, D--79104 Freiburg, Germany (D.K., S.K., E.S.); and Institute of Plant Biology, Biological Research Center, P.O. Box 521, H--6701, Hungary (A.V., F.N.)

^* Corresponding author; email: casal{at}ifeva.edu.ar.

Phytochrome (phy) A mediates two distinct photobiological responses in plants: the very-low-fluence response (VLFR), which can be saturated by short pulses of very-low-fluence light, and the high-irradiance response (HIR), which requires prolonged irradiation with higher fluences of far-red light (FR). To investigate whether the VLFR and HIR involve different domains within the phyA molecule, transgenic tobacco (Nicotiana tabacum cv Xanthi) and Arabidopsis seedlings expressing full-length (FL) and various deletion mutants of oat (Avena sativa) phyA were examined for their light sensitivity. Although most mutants were either partially active or inactive, a strong differential effect was observed for the ${Delta}$ 6-12 phyA mutant missing the serine-rich domain between amino acids 6 and 12. ${Delta}$ 6-12 phyA was as active as FL phyA for the VLFR of hypocotyl growth and cotyledon unfolding in Arabidopsis, and was hyperactive in the VLFR of hypocotyl growth and cotyledon unfolding in tobacco, and the VLFR blocking subsequent greening under white light in Arabidopsis. In contrast, ${Delta}$ 6-12 phyA showed a dominant-negative suppression of HIR in both species. In hypocotyl cells of Arabidopsis irradiated with FR phyA:green fluorescent protein (GFP) and ${Delta}$ 6-12 phyA:GFP fusions localized to the nucleus and coalesced into foci. The proportion of nuclei with abundant foci was enhanced by continuous compared with hourly FR provided at equal total fluence in FL phyA:GFP, and by ${Delta}$ 6-12 phyA mutation under hourly FR. We propose that the N-terminal serine-rich domain of phyA is involved in channeling downstream signaling via the VLFR or HIR pathways in different cellular contexts.

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