Protochlorophyllide forms in etiolated seedlings of photoreceptor mutants of Arabidopsis Thaliana — is Chlorophyll biosynthesis controlled by cooperation between phytochromes and phototropins?

Abstract

The accumulation of protochlorophyllide and the relative content of long-wavelength protochlorophyllide spectral form, i.e. having fluorescence maximum at 655 nm (Pchlide655), were studied in etiolated seedlings of different ecotypes of Arabidopsis as well as in several photoreceptor mutants. The latter parameter provides information on the capability for protochlorophyllide photoreduction upon the first exposure of emergent seedlings to light. The capacity of Pchlide biosynthesis and its subsequent accumulation in the dark has been found to be dependent on the genetic background (ecotype). The Pchlide accumulation and the formation of Pchlide655 have been found to be reduced in seedlings of both phytochrome-deficient (phyA, phyB) and phototropin 2-deficient (phot2) mutants. In contrast, cryptochrome-deficient mutants (cry1, cry2) showed no significant differences of both total Pchlide and Pchlide655 contents in comparison to their respective background ecotypes. The results presented in this work indicate that the equilibrium between Pchlide accumulation and the formation of Pchlide655 in etiolated seedlings of angiosperms, is dependent both on plant ecotype and on light conditions in the environment during seed germination and/or seedlings formation. Mutant analysis points to the role of both phytochromes and phototropin 2 in the regulation of the formation of different Pchlide forms in Arabidopsis.