The Phytochrome Family and their Roles in the Regulation of Seed Germination

Abstract

Light is an important factor influencing the germination of seeds of many plant species. Phytochrome is well known to be involved in the perception of the light signals that influence seed germination. Higher plants possess multiple, discrete molecular species of phytochrome, the apoproteins of which are encoded by a small family of divergent genes. In Arabidopsis thaliana, five distinct apophytochrome-encoding genes have been identified (PHYA-PHYE). There is increasing evidence that different phytochromes play different roles in photomorphogenesis. The analysis of photomorphogenic mutants, particularly those that are deficient in individual phytochromes or which over-express specific PHY cDNAs, has been especially useful in defining the roles of the members of the phytochrome gene family. Analysis of such mutants has revealed that phytochrome B, a low abundance, light-stable phytochrome, plays the major role in the photocontrol of Arabidopsis seed germination. However, phytochrome A, the light-labile phytochrome species that predominates in etiolated plant tissues, and another, as yet unidentified, phytochrome, also participate in the photocontrol of Arabidopsis seed germination.