40kDa-Binding Protein to Phytochrome and Light-signal Transduction

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Phytochrome is a plant photoreceptor for variety of photomorphogenetic responses that has two UV-visible absorption forms, Pr and Pfr. Pfr is thought to be an active form. Phytochrome B, responsible for low fluence rate responses migrates into nucleus to bind an interacting protein, PIF3, on phototransformation from Pr to Pfr, that initiates transcription of flowering-related genes, however, signaling of phytochrome A is poorly understood. We have detected multiple binding proteins to Pfr of phytochrome A. We determined N-terminal amino acid sequences of several of these binding proteins. One of them, 40kDa species is identified as a homologue of the enzyme that is involved in the important process of IAA, a plant hormone, synthesis. We cloned the gene and expressed it. Enzymatic activities of the expressed products were measured, proposing its involvement in IAA synthesis. Its interaction with phytochrome A and possible photoregulation of its activities are to be investigated.