Fluorescence analysis of oat phyA deletion mutants expressed in tobacco suggests that the N-terminal domain determines the photochemical and spectroscopic distinctions between phyA′ and phyA″

In vivo low-temperature (85 K) fluorescence spectroscopy has defined two phytochrome A (phyA) subpopulations, designated phyA′ and phyA″, in etiolated seedlings (V. A. Sineshchekov, J. Photochem. Photobiol 28, 53-55, 1995). Phytochrome A′ is the more abundant but light-labile species characterized by longer wavelength emission/absorption maxima (687/673 nm) and by a higher extent of the photoconversion of its red-absorbing form (Pr) into photoproduct (lumi-R) at 85 K (γ₁ ≈ 0.5). Phytochrome A″ is the minor but relatively light-stable species, characterized by shorter wavelength maxima (682/668 nm) and by a lower γ₁ (<0.05). To help define domains within phyA responsible for these differences, the low-temperature spectral properties of transgenic tobacco expressing full-length (FL) oat phyA and C- and N-terminally truncated versions (CD [Δ786-1129] and NA [Δ7-69], respectively) were compared. Oat phytochrome expression was more pronounced than that of tobacco in the basal section of etiolated seedlings following 2 h irradiation with white light. Seedlings expressing FL and CD phyA had spectral properties for phyA′ and phyA″ that were indistinguishable from that of wild-type tobacco. Conversely, expression of NA phyA generated an abundant phy species that behaved like phyA″. From this we conclude that the N-terminal domain of phyA is involved in determining the photochemical and spectroscopic distinctions between the native phyA′ and phyA″ species.