Do the Members of the Phytochrome Family have Different Roles? Physiological Evidence from Wild-Type, Mutant and Transgenic Plants

Abstract

The realisation that phytochrome represents a family of photoreceptors, encoded by at least three, and probably more, nuclear genes (Sharrock and Quail, 1989), has led to the proposal that the members of the family may have differential roles in the regulation of development and metabolism (see Smith and Whitelam, 1990, for review). In principle, this idea simplifies our understanding of the complexity of photomorphogenesis, by providing a potential resolution of the many conflicts in the physiological data. Investigations over several decades have shown that a number of different response modes, separable on classical photobiological grounds, can be incontrovertibly attributed to phytochrome, but this multiplicity of roles has been extremely difficult to reconcile with the classical concept of phytochrome as a single photoreceptor. Such conflicts may be traced back at least two decades, to the proposal by Hillman (1967) that there must be at least two populations of phytochrome, one that is ‘active’ but present in low concentration, and one that is ‘bulk’, i.e., present in high concentration and responsible for the spectrophotometric observations but not active in the physiological responses. It is, therefore, tempting to speculate that different phytochrome-mediated response modes may be mediated by different phytochromes. This article considers this idea and some of the evidence that currently appears to support it. We conclude that the hypothesis is at least sustainable on present evidence, but the attractiveness of the concept must be weighed in balance with the implications it has for the central dogmas of photomorphogenesis.