Abstract
The covalent association of phytochromobilin (PΦB) is essential for the diverse photoregulatory activities of phytochromes in vascular plants, while algal phytochromes utilize phycocyanobilin (PCB) as chromophore. To assess the functional consequence of the substitution of PΦB with PCB in plants, we modified the phytochrome chromophore from PΦB to PCB by targeted expression of pcyA with transit peptide in the chromophore-deficient hy2 mutant. Spectroscopic analyses of extracts from etiolated seedlings revealed that PcyA expression restored photoactive phytochrome to wild type levels, albeit with blue-shifted absorption maxima, while also restoring light-lability to phytochrome A. In physiological analysis, PCYA1 showed blue-shifted phytochromeA-mediated far-red high irradiance response and fully rescued phytochrome-mediated red high irradiance responses, low fluence R/FR photoreversible responses and very low fluence responses. These results indicate that PCB chromophore can essentially function in phytochrome-mediated responses.
