Phytochrome‐mediated inhibition of shade avoidance involves degradation of growth‐promoting bHLH transcription factors

TLDR

Plant growth is highly responsive to light, and shade avoidance is mainly regulated by phytochrome photoreceptors that activate bHLH transcription factors such as PIF4 and PIF5. The study examines whether PIF4 and PIF5 promote shade avoidance by acting early in phytochrome signaling. PIF4 and PIF5 are phosphorylated and then degraded via the proteasome after binding light‑activated phyB through their APB domain. PIF4 and PIF5 accumulate in darkness, are degraded by red light, remain high under shade, and their degradation is required for shade avoidance, as shown by partial reversal of the phyB mutant phenotype when PIF4/5 are absent.

Abstract

Summary Plant growth and development are particularly sensitive to changes in the light environment and especially to vegetational shading. The shade‐avoidance response is mainly controlled by the phytochrome photoreceptors. In Arabidopsis, recent studies have identified several related bHLH class transcription factors (PIF, for phytochrome‐interacting factors) as important components in phytochrome signaling. In addition to a related bHLH domain, most of the PIFs contain an active phytochrome binding (APB) domain that mediates their interaction with light‐activated phytochrome B (phyB). Here we show that PIF4 and PIF5 act early in the phytochrome signaling pathways to promote the shade‐avoidance response. PIF4 and PIF5 accumulate to high levels in the dark, are selectively degraded in response to red light, and remain at high levels under shade‐mimicking conditions. Degradation of these transcription factors is preceded by phosphorylation, requires the APB domain and is sensitive to inhibitors of the proteasome, suggesting that PIF4 and PIF5 are degraded upon interaction with light‐activated phyB. Our data suggest that, in dense vegetation, which is rich in far‐red light, shade avoidance is triggered, at least partially, as a consequence of reduced phytochrome‐mediated degradation of transcription factors such as PIF4 and PIF5. Consistent with this idea, the constitutive shade‐avoidance phenotype of phyB mutants partially reverts in the absence of PIF4 and PIF5.