The role of phytochrome C in gravitropism and phototropism in Arabidopsis thaliana

The phytochrome (phy) photoreceptors, which consist of a small gene family PHYA-E in dicot plants, play important roles in regulating many light-induced responses in plants. Although the best characterised phytochromes are phytochrome A (phyA) and phytochrome (phyB), the functions of phyD and phyE have been increasingly studied. Phytochrome C (phy C) has been the most poorly understood member of the photoreceptor family, since isolation of phyC mutants only has been accomplished within the last few years. Recent reports show that phyC functions in hypocotyl elongation, rosette leaf morphology, and timing of flowering. In the present study, we show that phyC plays a role in tropisms in seedlings and inflorescence stems of light-grown Arabidopsis thaliana (L.) Heynh. (Wassilewskija ecotype). Phytochrome C has a positive effect on gravitropism in hypocotyls and stems, but it has a limited role in root gravitropism. In contrast, phyC attenuates the positive phototropic response to blue light in hypocotyls and the red-light-based positive phototropism in roots. Phytochrome D (phy D) also mediates gravitropism in hypocotyls and inflorescence stems and attenuates positive phototropism in response to blue in hypocotyls and stems. Thus, phyC can be added to the list of the other four phytochromes, which play various roles in both gravitropism and phototropism in plant organs. This report also supports the growing body of evidence demonstrating cross talk between phytochromes and blue-light photoreceptors.