Regulation of Plant Phototropic Growth by NPH3/RPT2-like Substrate Phosphorylation and 14-3-3 Binding

Abstract

Polarity underlies all plant physiology and directional growth responses such as phototropism. Yet, our understanding of how plant tropic responses are established is far from complete. The plasma-membrane associated BTB-containing protein, NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3) is a key determinant of phototropic growth which is regulated by AGC kinases known as the phototropins (phots). However, the mechanism by which phots initiate phototropic signalling via NPH3, and other NPH3/RPT2-like (NRL) members, has remained unresolved. Here we demonstrate that NPH3 is directly phosphorylated by phot1 both in vitro and in vivo. Light-dependent phosphorylation within a conserved consensus sequence (RxS) located at the extreme C-terminus of NPH3 is necessary to promote its functionality for phototropism and petiole positioning in Arabidopsis. Phosphorylation of this region by phot1 also triggers 14-3-3 binding combined with changes in NPH3 phosphorylation and localisation status. Seedlings expressing mutants of NPH3 that are unable to bind or constitutively bind 14-3-3s show compromised functionality that is consistent with a model where signalling outputs arising from a gradient in NPH3 RxS phosphorylation/localisation across the stem are a major contributor to phototropic responsiveness. Our current findings provide further evidence that 14-3-3 proteins are instrumental components regulating auxin-dependent growth and show for the first time that NRL proteins are direct phosphorylation targets for plant AGC kinases. Moreover, the C-terminal phosphorylation site/14-3-3-binding motif of NPH3 is conserved in several members of the NRL family, suggesting a common mechanism of regulation.

Air channels create a directional light signal to regulate hypocotyl phototropism

“…This is consistent with the abcg5 phototropic defect being due to a reduced ability to establish a light gradient across the hypocotyl rather than in downstream phot signaling (Extended data Fig. 4) [17][18][19] . Moreover, the defective BL response of abcg5 was specific to phototropism as BL-induced inhibition of hypocotyl elongation was unaltered in abcg5 (Extended data Fig.…”

Phototropin connects blue light perception to starch metabolism in green algae

“…In Arabidopsis, blue light induces the dephosphorylation of NPH3 (NON-PHOTOTROPIC HYPOCOTYL3) in a PHOT1-dependent manner via an unidentified PHOT1-controlled protein phosphatase. The dephosphorylated NPH3 then heterodimerizes with CUL3 (Cullin homolog 3) and serves as an anchor for the PHOT1 substrate, subsequently triggering the ubiquitination of PHOT1 (30). We hypothesize that a protein phosphatase (indicated PPase in Fig.…”