A plastidic ABC protein involved in intercompartmental communication of light signaling

Plants perceive light via specialized photoreceptors of which the phytochro mes (phyA-E), absorbing far-red (FR) and red light (R) are best understood. Several nuclear and cytoplasmic proteins have been characterized whose def iciencies lead to changes in light-dependent morphological responses and ge ne expression. However, no plastid protein has yet been identified to play a role in phytochrome signal transduction. We have isolated a new Arabidops is mutant, laf (long after FR) 6, with reduced responsiveness preferentiall y toward continuous ER light. The disrupted gene in laf6 encodes a novel pl ant ATP-binding-cassette (atABC1) protein of 557 amino acids with high homo logy to ABC-like proteins from lower eukaryotes. In contrast to lower eukar yotic ABCs, however, atABC1 contains an N-terminal transit peptide, which t argets it to chloroplasts. atABC1 deficiency in Iaf6 results in an accumula tion of the chlorophyll precursor protoporphyrin IX and in attenuation of P R-regulated gene expression. The long hypocotyl phenotype of laf6 and the a ccumulation of protoporphyrin IX in the mutant can be recapitulated by trea ting wild-type (WT) seedlings with flumioxazin, a protoporphyrinogen IX oxi dase (PPO) inhibitor. Moreover, protoporphyrin IX accumulation in flumioxaz in-treated WT seedlings can be reduced by overexpression of atABC1. Consist ent with the notion that ABC proteins are involved in transport, these obse rvations suggest that functional atABC1 is required for the transport and c orrect distribution of protoporphyrin IX, which may act as a light-specific signaling factor involved in coordinating intercompartmental communication between plastids and the nucleus.