Dynamic properties of endogenous phytochrome A in Arabidopsis seedlings

The dynamic behavior of phytochrome A (phyA) in seedlings of the model plan t Arabidopsis was examined by in vivo spectroscopy and by western and north ern blotting. Rapid accumulation of phyA was observed, reaching a steady st ate after 3 d. Both red and far-red light initiated a rapid destruction of the far-red-light-absorbing form of phytochrome (Pfr); the apparent half-li fe was only 4-fold longer in far-red than in red light. Furthermore, the Pf r-induced destruction of the red-light-absorbing form of phytochrome (Pr) o f phyA occurred in darkness with a rate identical to that of Pfr destructio n. A 2-fold decrease in mRNA abundance was observed after irradiation, irre spective of the applied light quality. However, reaccumulation occurred rap idly after far-red but slowly after red irradiation, indicating different m odes of regulation of phytochrome expression after light-dark transitions d epending on the light quality of the preceding irradiation. The wavelength dependency of the destruction rates was distinct from that of mustard, a cl ose relative of Arabidopsis, and was explained on the basis of Pfr-induced Pr destruction and a simple kinetic two-step model. No dark reversion was d etectable in the destruction kinetics after a red pulse. From these data we conclude that Arabidopsis phyA differs significantly in several aspects fr om other dicot phytochromes.