BOTH PHYTOCHROME-A AND PHYTOCHROME-B ARE REQUIRED FOR THE NORMAL EXPRESSION OF PHOTOTROPISM IN ARABIDOPSIS-THALIANA SEEDLINGS

The role of phytochrome A (phyA) and phytochrome B (phyB) in phototrop ism was investigated by using the phytochrome-deficient mutants phyA-1 01, phyB-1 and a phyA/phyB double mutant. The red-light-induced enhanc ement of phototropism, which is normally observed in wild-type seedlin gs, could not be detected in the phyA/phyB mutant at fluences of red l ight between 0.1 and 19 000 mu mol m(-2). The loss of phyB has been sh own to have no apparent effect on enhancement, while the loss of phyA resulted in a loss of enhancement only in the low fluence range (Janou di et al. 1997). The conclusions of the aforementioned study can now b e modified based on the current results which indicate that phototropi c enhancement in the high fluence range is mediated by either phyA or phyB, and that other phytochromes have no role in enhancement. First p ositive phototropism was unaffected in phyA-101 and phyB-1. However, t he magnitude of first positive phototropism in the phyA/phyB mutant wa s significantly lower than that of the wild-type Landsberg parent. Thu s, the presence of either phyA or phyB is required for normal expressi on of first positive phototropism. The time threshold for second posit ive phototropism is unaltered in the phyA-101 and phyB-1 mutants. Howe ver, the time threshold in the phyA/phyB mutant is about 2 h, approxim ately six times that of the wild type. Finally, the magnitude of secon d positive phototropism in both phyA-101 and phyB-1 is diminished in c omparison with the wild-type response. Thus, phyA and phyB, acting ind ependently or in combination, regulate the magnitude of phototropic cu rvature and the time threshold for second positive phototropism. We co nclude that the presence of phyA and phyB is required, but not suffici ent, for the expression of normal phototropism.