Phytochrome degradation and dark reversion

Abstract

The complex dynamics of phytochromes have puzzled plant biologists for decades (for a history of phytochrome research see Sage et al., 1992). Although techniques such as in-vivo spectroscopy, ELISA detection and life monitoring of GFP fusion proteins have revealed fascinating details of light perception and signal transduction, the understanding of phytochrome dynamics in plants is still far from being complete. From the 1950ies to the 1980ies, several ingenious studies have discovered key properties of phytochromes, e.g. light lability, aggregation and dark reversion (Pratt et al., 1974; MacKenzie et al., 1975; Brockmann et al., 1987). The results, however, often were correlative, inconclusive or even contradictory. The advances of molecular genetics during the last decade have enabled experiments under much better controlled conditions, which improved the understanding of phytochrome dynamics considerably. Three achievements were particularly important: (i) It was discovered that phytochrome is a multi gene family. The subsequent identification of mutants in specific phytochrome genes allowed dissecting the function and dynamics of individual phytochromes. (ii) The heterologous expression of phytochromes in yeast and bacteria and their assembly into photoreversible holoproteins enabled detailed kinetic investigations of phytochrome dynamics. And finally (iii) the efficient generation of transgenic plants expressing mutated or truncated phytochromes paved the road for vigorous tests of functional hypothesis. While early research on phytochrome used various model plants including lettuce, squash, cauliflower, oat, mustard and others, recent work followed the raise of Arabidopsis as the dominating model plant. Because this overview will focus on the more recent results it will mainly discuss the Arabidopsis phytochromes but frequently relate to findings from other species as well. Activation of the photoreceptor and the subsequent steps transducing the signal towards the biological response has obviously gained most interest of researchers. An equally important question, however, is what mechanisms allow a signal to be terminated once the inducing stimulus ceases. In a signal transduction chain each molecular step has to be accompanied by a deactivating step, and kinasephosphatase pairs are maybe the most prominent examples of signal activation and termination. In the case of the activated photoreceptor phytochrome, mainly two inactivating mechanisms have been discussed, namely protein degradation (" phytochrome destruction") and dark reversion. The physiological active, far-red light (FR) absorbing Pfr form of many phytochromes has a much shorter half-life than to the biological inactive, red light (R) absorbing Pr form. In etiolated pumpkin seedlings, for instance, the half-life decreases from more than 100 hrs for Pr to less than 1 h for Pfr (Quail et al., 1973a). The loss of photoreversible phytochrome after conversion into the Pfr form was termed " destruction" and subsequently it was found to involve a bona fide protein degradation (see 2.2). However, as not all phytochrome is subject to rapid destruction, the analysis of destruction kinetics lead to the operational distinction of type I phytochrome, which is light-labile, and type II phytochrome, which is more stable in the light. Nonetheless, light stable phytochromes need to be deactivated as well. Spectrometric analysis revealed that most type II Pfr is only meta-stable and undergoes a slow thermal reversion back to inactive Pr. Photoreceptor dynamics, therefore, involve the de-novo synthesis of Pr, which can be approximated by zero order kinetics (Schäfer et al., 1975), photoconversion of Pr and Pfr into each other, destruction for type I phytochrome and dark reversion for probably all type II phytochromes (Figure 1). Importantly, for type I phytochrome of several species dark reversion competes with destruction. In addition to the mentioned reactions, phytochrome dynamics include posttranslational modifications of the protein (e.g. phosphorylation, Hunt and Pratt, 1980) and changes of the intracellular distribution (MacKenzie et al., 1975; Kircher et al., 1999; Yamaguchi et al., 1999).