Phytochrome regulation of seed germination.

Seed germination of many plant species is influenced by light. Of the various photoreceptor systems, phytochrome plays an especially important role in seed germination. The existence of at least five phytochrome genes has led to the proposal that different members of the family have different roles in the photoregulation of seed germination. Physiological analysis of seed germination ofArabidopsis thaliana (L.) Heynh. with phytochrome-deficient mutants showed for the first time that phytochrome A and phytochrome B modulate the timing of seed germination in distinct actions. Phytochrome A photo-irreversibly triggers the photoinduction of seed germination after irradiation with extremely low fluence light in a wide range of wavelengths, from UV-A, to visible, to far-red. In contrast, phytochrome B mediates the well-characterized photoreversible reaction, responding to red and far-red light of fluences four orders of magnitude higher than those to which PhyA responds. Wild plants, such asA. thaliana, survive under ground as dormant seeds for long periods, and the timing of seed germination is crucial for optimizing growth and reproduction. It therefore seems reasonable for plants to possess at least two different physiological systems for sensing the light environment over a wide spectral range with exquisite sensitivity of different phytochromes. This redundancy seems to enhance plant survival in a fluctuating environment.