Two native types of phytochrome A, phyA′ and phyA, differ by the state of phosphorylation at the N-terminus as revealed by fluorescence investigations of the Ser/Ala mutant of rice phyA expressed in transgenic Arabidopsis

Phytochrome A(phyA) mediates different photoresponses what may be connected with the existence of its two types, phyA' and phyA '', differing by spectroscopic, photochemical and functional properties. We investigated a role of phyA phosphorylation in their formation turning to transgenic Arabidopsis thaliana (L. Heynh.) phyA or phyAphyB mutants overexpressing rice wild-type phyA (phyA WT) or mutant phyA (phyA SA) with the first 10 serines substituted by alanines. This prevents phyA phosphorylation at these sites and modifies photoresponses. Etiolated seedlings were employed and phyA parameters were evaluated with the use of low temperature fluorescence spectroscopy and photochemistry. Germination of seeds was induced by white light (WL) pre-treatment for 15 min or 3 h. Emission spectra of rice phyA WT and phyA SA were similar and their total content was comparable. However, the phyA'/phyA '' proportion in phyA WT was high and varied with the duration of the WL pre-treatment, whereas in phyA SA it was substantially shifted towards phyA '' and did not depend on the pre-illumination. This suggests that phyA SA comprises primarily or exclusively the phyA '' pool and supports the notion that the two phyA types differ by the state of serine phosphorylation. phyA '' was also found to be much more effective in the germination induction than phyA'.