Negative interference of endogenous phytochrome B with phytochrome A function in Arabidopsis.

To study negative interactions between phytochromes, phytochrome B (phyB) overexpressor lines, the mutants phyA-201, phyB-4, phyB-5, phyD-1, phyA-201 phyB-5, phyA-201 phyD-1, and phyB-5 phyD-1 of Arabidopsis were used. Endogenous phyB, but not phytochrome D (phyD), partly suppressed phytochrome A (phyA)-dependent inhibition of hypocotyl elongation in far-red light (FR). Dichromatic irradiation demonstrated that the negative effect of phyB was largely independent of the photoequilibrium, i.e. far-red light absorbing form of phytochrome formation. Moreover, phyB-4, a mutant impaired in signal transduction, did not show a loss of inhibition of phyA by phyB. Overexpression of phyB, conversely, resulted in an enhanced inhibition of phyA function, even in the absence of supplementary carbohydrates. However, overexpression of a mutated phyB, which cannot incorporate the chromophore, had no detectable effect on phyA action. In addition to seedling growth, accumulation of anthocyanins in FR, another manifestation of the high irradiance response, was strongly influenced by phyB holoprotein. Induction of seed germination by FR, a very low fluence response, was suppressed by both endogenous phyB and phyD. In conclusion, we show that both classical response modes of phyA, high irradiance response, and very low fluence response are subject to an inhibitory action of phyB-like phytochromes. Possible mechanisms of the negative interference are discussed.