Epidermal Phytochrome B Inhibits Hypocotyl Negative Gravitropism Non-Cell-Autonomously

Abstract

Seedling hypocotyls display negative gravitropism in the dark but agravitropism in the light. The Arabidopsis thaliana pif quadruple mutant (pifQ), which lacks four PHYTOCHROME-INTERACTING FACTORS (PIFs), is agravitropic in the dark. Endodermis-specific expression of PIF1 rescues gravitropism in pifQ mutant seedlings. Since phytochromes induce light responses by inhibiting PIFs and the COP1-SPA ubiquitin E3 ligase complex in the nucleus, we asked whether phyB can cell autonomously inhibit hypocotyl negative gravitropism in the endodermis. We found that while epidermis-specific expression of PHYB rescues hypocotyl negative gravitropism and all other phyB mutant phenotypes, endodermis-specific expression of PHYB does not. Epidermal phyB induces the phosphorylation and degradation of endodermal PIFs in response to red light. This induces a global gene expression pattern similar to that induced by red light treatment of seedlings expressing PHYB under the control of its own endogenous promoter. Our results imply that epidermal phyB generates an unidentified mobile signal that travels to the endodermis where it promotes PIF degradation and inhibits hypocotyl negative gravitropism.

Phytochromes and Phytochrome Interacting Factors

“…In general, phytochromes induce the rapid degradation of PIFs in response to light signals via direct physical interactions with PIFs (Al-Sady et al, 2008;Shen et al, 2008), although phyB has been shown to induce PIF1 degradation non cell autonomously (Kim et al, 2016b). DNA binding is not necessary for the light-induced degradation of PIFs (Al-Sady et al, 2008;Shen et al, 2008).…”

“…Interestingly, a recent study performed ChIP followed by microarray (ChIP-chip) to identify in vivo binding sites of PIF1 (Kim et al, 2016b). This study showed that PIF1 DNA-binding sites (PBSs) are enriched with not only G-boxes but also with other hexameric sequence elements named GCEs.…”

Reciprocal proteasome-mediated degradation of PIFs and HFR1 underlies photomorphogenic development in Arabidopsis

“…Conversely, PIFs are phosphorylated, poly-ubiquitylated and subsequently degraded under light (Leivar and Quail, 2011;Xu et al, 2015). However, a recent study has shown that phyB can induce degradation of PIF1 non-cell-autonomously (Kim et al, 2016b). Among the candidate kinases, oat phyA has been shown to directly phosphorylate PIFs and regulate the light-induced degradation of PIF3 under far-red light (Shin et al, 2016).…”

Do Phytochromes and Phytochrome-Interacting Factors Need to Interact?

“…Although phyB seems to be expressed ubiquitously, there is some evidence that phyB effects may not be 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.…”

“…ep, epidermis; co, cortex; en, endodermis; vs, vascular structure. (Adapted from Kim et al [2016], Figure 1A.) OPEN Articles can be viewed without a subscription.…”