Fluorescence and photochemical characterization of phytochromes A and B in transgenic potato expressing Arabidopsis phytochrome B

Phytochrome in etiolated sprouts of wild type (WT) potato and its transgenic strains (DARA5 and DARA12) expressing Arabidopsis thaliana phytochrome B (phyB) was investigated using low-temperature (85 K) fluorescence spectroscopy and photochemistry. Phytochrome content, [Ptot], position of the Pr emission and excitation spectra, λmax, and extent of the Pr→lumi-R, γ1, and Pr→Pfr, γ2, phototransformations (at 85 and 273 K, respectively) were shown to vary in the transgenic lines and WT depending on tissue used (upper vs. lower parts of etiolated sprouts) and light-induced phytochrome depletion. Differences in the parameters between the transgenic lines and WT were detected which were interpreted in terms of the two phenomenological Pr types: a labile Pr′ with γ1≈0.5 consisting of a major phytochrome A (phyA) fraction (phyA′) and a relatively conserved Pr″ with γ1=0 comprising a minor phyA fraction (phyA″) and phyB. Both DARA lines had higher [Pr″] as compared with WT in the lower parts of etiolated stems, especially after light-induced phytochrome depletion (residual phytochrome in DARA5 and DARA12 made up to one-third of its initial level vs. <5% in WT). These differences were associated with the expression of Arabidopsis phyB in the DARA lines and its higher light stability than that of phyA. Arabidopsis phyB expressed in potato was characterised by λmax=683/669 nm in the emission/excitation (absorption) spectra and γ1=0. PhyB also revealed a relatively low γ2 (approx. 0.5) and its early red drop as compared with the γ2 wavelength dependence for phyA. This is believed to contribute to the lower signalling ability of phyB and to confine the region (red) of its physiological activity.