Topic 18.8 Phytochrome and Cryptochromes Contribute to Phototropism
Phototropin is not the only blue light photoreceptor. Phytochrome also absorbs blue light and the cryptochromes are another set of blue light photoreceptors. Several findings suggest that phytochromes and phototropins act together in the cytosol to regulate phototropism. For example, PHYA mutants have severe phototropic defects in response to low blue light. PHYA appears to enhance phototropism by facilitating the movement of PHOT1 from the plasma membrane to the cytosol in response to blue light.
According to the current model, the molecular link between PHYA and PHOT1 is PHYTOCHROME KINASE SUBSTRATE 1 (PKS1). PKS1 was originally identified as a substrate for light-regulated phytochrome kinase. PKS1, 2, and 4 are important for PHOT1-mediated phototropism. PKS1 interacts with both phot1 and NPH3, and PKS4 is a PHOT1 substrate. Since PHYA is translocated to the nucleus after blue light irradiation, PHYA appears to initiate transcriptional responses required for phototropism via PIF4 and PIF5 (see textbook Chapter 16). At the plasma membrane, PHOT1 and PHYA have been shown to interact with each other and with PKS, which may provide a model how PHOT1 and PHYA act together to modulate phototropism.
Similar to phytochrome, cryptochrome is important for transcriptional responses and cell elongation in PHOT-dependent phototropism. Therefore, there is cross-regulation among the photoreceptor activities modulating phototropism (see textbook Chapter 15).