Do Phytochromes and Phytochrome-Interacting Factors Need to Interact?

A new study calls into question whether phytochromeB (phyB)mustdirectly interact with phytochrome-interacting factors (PIFs) to promote light responses. Phytochrome photoreceptors mediate responses to red light in part by inducing the degradation of PIF transcription factors, which generally are repressors of light responses (reviewed in Jeong and Choi, 2013; Wang and Wang, 2015).As thename implies,PIFscandirectly interact with phytochromes, and it has been suggested that interaction between lightactivatedphytochromesandPIFs in focicalled nuclear bodies leads to PIF degradation. TherearefivephytochromesinArabidopsis thaliana, with phyB the main form present in plants grown in the light. Although phyB seemstobeexpressedubiquitously, there is someevidence thatphyBeffectsmaynotbe cell-autonomous, and certain downstream responses to red light perception by phyB must involve signaling between cells and between tissues. New work from Kim et al. (2016) suggests that several effects mediated by phyB can be attributed to phyB in the epidermis. To determine the site(s) of action of phyB, Kim and coworkers characterized transgenic Arabidopsis lines expressing a phyBGFP fusion from tissue-specific promoters. The success of this approach requires exquisitely specific promoters, and the authors found theMERISTEM LAYER1 (ML1) promoter (ML1pro) and the SCARECROW (SCR) promoter (SCRpro) to be suitable for analysis of expression in the epidermis and the endodermis, respectively (see figure). phyB-GFP expressed fromML1pro rescued the phyB mutant in terms of both seed germination and inhibition of hypocotyl elongation in response to red light. Importantly, however, expression driven from SCRpro did not rescue the mutant, indicating that epidermal, but not endodermal, phyB is responsible for these phyB functions. To explore the notion of a non-cellautonomous function for epidermal phyB, Kim et al. used the negative gravitropic response of the hypocotyl as a model system. The inhibition of this response by phyB is likely mediated by endodermal PIFs, as the expression of PIF1 in the endodermis can rescue the response in the pifQ quadruple mutant (Kim et al., 2011). Again, epidermal, but not endodermal, phyB-GFP restored the inhibition of negative gravitropism. Furthermore, epidermal phyB-GFP induced the phosphorylation and degradation of endodermal PIFs, lending more support to the conclusion that epidermal phyB provokes non-cell-autonomous responses. Finally, Kim and coworkers provide evidence that epidermis-specific expression of phyB, despite itsdifference fromtheendogenousphyB expressionpattern, is sufficient to regulate the expression of nearly all phyB-responsive genes in Arabidopsis. Specific phyB expression driven in the epidermis (MILpro) and endodermis (SCRpro). Confocal (upper) and epifluorescence (lower) images show tissue-specific phyB-GFP expression (green; white arrowheads) in the roots of red-light-grown transgenic Arabidopsis seedlings. Propidium iodide (gray) counterstained the cell walls and nuclei. ep, epidermis; co, cortex; en, endodermis; vs, vascular structure. (Adapted from Kim et al. [2016], Figure 1A.)