Subcellular Sites of Phytochrome A Signal Transduction and Light-Dependent Degradation in Arabidopsis

Abstract

Phytochrome A (phyA), which mediates the far-red high-irradiance and very low fluence responses, accumulates in the nucleus upon light activation by far-red light. Phytochrome A is rapidly degraded mainly in the cytoplasm after red light illumination. We investigated the subcellular sites of phyA signal transduction and degradation. PHYA-GFP (PAG) was fused to either nuclear localization signal (PAGL) or nuclear export signal (PAGE) and expressed under the control of the PHYA authentic promoter in the phyA deficient mutant. Microscopic observation confirmed that PAGL and PAGE were localized to the intended sites. Hypocotyl analysis revealed that PAGL was more active than PAG whereas PAGE failed to elicit the response. Hence, phyA appears to transduce the signal in the nucleus for the far-red high-irradiance responses. Interestingly, both PAGL and PAGE were degraded rapidly with similar kinetics after irradiation with red light. Hence, light-dependent degradation can take place in both the sites.