Phytochrome degradation

Abstract

ABSTRACTPlants actively modulate the levels of the various phyto‐chrome 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