Phytochrome degradation

Plants actively modulate the levels of the various phytochrome isoforms during their life cycle to optimize light absorption and perception. For phytochrome A (phyA), one of the most influential methods of control is selective turnover of the photoreceptor upon photoconversion from the red-absorbing form (Pr) to the far-red-absorbing form (Pfr). Whereas the Pr form has a half-life of approximately 1 week, the Pfr form is rapidly degraded with a half-life of 1-2 h. The ubiquitin/26S proteasome pathway has been implicated in phyA breakdown. In this proteolytic pathway, multiple ubiquitins are covalently attached to proteins committed for degradation; these ubiquitin-protein conjugates then serve as intermediates in the breakdown of the target protein by the 26S proteasome, a multi-subunit proteolytic complex. In several plant species, ubiquitin-phyA conjugates have been detected in vivo following Pfr formation that show accumulation and decay kinetics expected for Pfr degradation intermediates. Analyses of phyA mutants and phyA/phyB chimeras expressed in transgenic plants have been particularly useful in mapping domains within the chromoprotein that are necessary for Pfr degradation. Several domains have been identified within both the N- and C-terminal portions of the photoreceptor that presumably serve as recognition and/or acceptor sites for ubiquitination.