Biochemical and molecular analysis of signalling components

Abstract

Phototransformation of phytochrome initiates a series of events that ultimately leads to physiological adaptation to new environmental conditions. The chain of events occurring in between is complex, with multiple branches and interactions that are context dependent so that they should probably be described as a signalling web rather than a signalling chain. Recent progress in the phytochrome-signalling field clearly indicates that key transduction events also occur in distinct subcellular compartments. The light regulated localisation of the photoreceptors themselves is a striking illustration of this concept (see Chapter 9). The aim of this chapter is to review the biochemical and molecular events of phytochrome signalling with an emphasis on events occurring outside of the nucleus. The large recent progress in the field prompted us to split phytochrome signalling into several chapters. Please consult chapter 17 for nuclear events and chapters 16, 18, 20 and 21 for additional strongly recommend reading the chapter by S. Roux in the 2nd edition of photomorphogenesis in plants (Roux, 1994). Phototransformation occurs in the time scale of milliseconds, but the manifestation of this initial event varies largely depending on the response that is studied. This can take as little as a few seconds, as in light regulated cytoplasmic motility (Takagi et al., 2003), or up to many days for photoperiodic induction of flowering (see Chapter 27). Many of these events presumably require a transcriptional cascade, although very quick responses probably do not. Such early branching in signalling events has been well characterised in other systems, e.g., for the response to pheromone in yeast (Leberer et al., 1997). The presence of multiple phytochromes within any plant species (phyA-phyE in Arabidopsis thaliana) adds an additional level of complexity since it is well known that despite similar phototransformation reactions the different phytochromes will trigger distinct responses when exited by light (see Chapter 7). A detailed description of the different phytochrome signalling modes can be found in chapter 16. Additional levels of complexity are due to the distinct tissues and to the timing of the light cue. Although the initial phytochrome phototransformation events are believed to be the same in all tissues, the outcome of the subsequent chain of events will depend upon the organ and the timing of the event. For example, light enhances cell growth in the cotyledons but inhibits it in the hypocotyl. Moreover the effect of a saturating pulse of red light on flower induction very much depends on the timing of delivery (see Chapter 27). These examples illustrate the complexity of the signalling events occurring after photoperception. Most of the genetic screens that have so far been performed to identify phytochrome-signalling components rely on phenotypes that take several days before they can be scored. This presumably explains the current lack of temporal resolution for phytochrome signalling events. We still know very little about the order of action of a fast growing list of phytochrome signalling-components. In this chapter we will concentrate on the components that are either constitutively cytoplasmic or that shuttle between cytoplasm and nucleus. Signalling in animals often requires second messengers such as Ca2+, cyclic nucleotides, phosphorylation events or inositol phospholipids. The requirement for such molecules for phytochrome signalling has been carefully investigated, although the " jury is still out" for several of them. The involvement of phosphorylation events has been revived with the discovery of bacteriophytochromes, many of which are light regulated histidine kinases (see Chapter 6).