Two modes of the light-induced phytochrome A decline--with and without changes in the proportion of its isoforms (phyA' and phyA''): evidence from fluorescence investigations of mutant phyA-3D pea.

Different modes of the phytochrome function are connected with its polymorphism, the major isoforms being phytochromes A and B (phyA and phyB). In its turn, phyA comprises two native species, phyA' and phyA'', whose precise nature and functions remain obscure. With the use of in situ fluorescence spectroscopy, we investigated their properties in a mutant of pea, phyA-3D, characterized by exaggerated photoresponses and impaired photodestruction of phyA. The mutation is a substitution of alanine by valine at the position 194 in phyA. The phyA-3DphyB and phyB mutants were also investigated. In dark-grown plants, all the lines had the content and properties of the two phyA species very similar to the wild type. However, a considerably more intense reduction in [phyA] without changes in the phyA'/phyA'' equilibrium was found in far-red grown mutant plants suggesting a hypersensitivity of phyA-3D with regard to its autoregulation. On the contrary, under red illumination, a higher stability of phyA-3D was observed confirming our earlier findings. This allows a conclusion that the A194V substitution in phyA-3D not only impairs its destruction but also enhances its signaling ability, suggesting a role of this locus in modulation of its activity.