PHYTOCHROMES AND SEED-GERMINATION

The control of seed germination by red and far-red light is one of the earliest documented phytochrome-mediated processes. Phytochrome is no w known to be a small family of photoreceptors whose apoproteins are e ncoded by different genes. Phytochrome B (phyB) is present in dry seed s and affects germination of dark imbibed seeds but other phytochromes could also be involved. Phytochrome A (phyA) appears after several ho urs of imbibition and mediates very-low-fluence responses. PhyB and ot her phytochromes different from phyA mediate the classical low-fluence responses. The phytochrome involved in high-irradiance responses of s eed germination (inhibition of germination under continuous far-red) h as not been unequivocally established, although phyA is the most likel y candidate. Phytochrome can affect embryo growth capacity and/or the constraint imposed by the tissues surrounding the embryo. At least in some species, gibberellins participate in the signalling process. In t he field, phyA has teen implicated in the perception of light during s oil cultivations, and phyB would be involved in the perception of red/ far-red ratios associated with the presence of gaps in the canopy. Thi s review describes recent advances in phytochrome research, particular ly those derived from the analysis of germination in specific mutants, and their connection with traditional observations on phytochrome con trol of seed germination.