Regulation of Phototropic Signaling in Arabidopsis via Phosphorylation State Changes in the Phototropin 1-interacting Protein NPH3

Abstract

Phototropism, or the directional growth (curvature) of various organs toward or away from incident light, represents a ubiquitous adaptive response within the plant kingdom. This response is initiated through the sensing of directional blue light (BL) by a small family of photoreceptors known as the phototropins. Of the two phototropins present in the model plant Arabidopsis thaliana, phot1 (phototropin 1) is the dominant receptor controlling phototropism. Absorption of BL by the sensory portion of phot1 leads, as in other plant phototropins, to activation of a C-terminal serine/threonine protein kinase domain, which is tightly coupled with phototropic responsiveness. Of the five phot1-interacting proteins identified to date, only one, NPH3 (non-phototropic hypocotyl 3), is essential for all phot1-dependent phototropic responses, yet little is known about how phot1 signals through NPH3. Here, we show that, in dark-grown seedlings, NPH3 exists as a phosphorylated protein and that BL stimulates its dephosphorylation. phot1 is necessary for this response and appears to regulate the activity of a type 1 protein phosphatase that catalyzes the reaction. The abrogation of both BL-dependent dephosphorylation of NPH3 and development of phototropic curvatures by protein phosphatase inhibitors further suggests that this post-translational modification represents a crucial event in phot1-dependent phototropism. Given that NPH3 may represent a core component of a CUL3-based ubiquitin-protein ligase (E3), we hypothesize that the phosphorylation state of NPH3 determines the functional status of such an E3 and that differential regulation of this E3 is required for normal phototropic responsiveness.

The Arabidopsis rcn1-1 Mutation Impairs Dephosphorylation of Phot2, Resulting in Enhanced Blue Light Responses

“…The response in the presence of the inhibitor was very close to that of the phot1-5 rcn1-1 double mutant (Figure 6). A comparable experiment with the phot2-1 mutant showed only inhibition on treatment with cantharidin (see Supplemental Figure 4 online), confirming the results of Pedmale and Liscum (2007) for phot1-mediated phototropism.…”

The Signal Transducer NPH3 Integrates the Phototropin1 Photosensor with PIN2-Based Polar Auxin Transport in Arabidopsis Root Phototropism

“…This result indicated that the scaffold protein NPH3 modifies auxin transport in Arabidopsis roots by transducing the BL signal from phot1 and phot2 BL receptors. Pedmale and Liscum (2007) reported that NPH3 is in a phosphorylated state under dark conditions and that BL illumination causes NPH3 dephosphorylation via a phot1-dependent mechanism. Note that exposure to auxin stimulated NPH3 in the phosphorylated state, but the phosphorylation state of phot1 was not affected by auxin treatment (Chen et al, 2010).…”

Higher plants use LOV to perceive blue light

“…Moreover, even signaling elements acting downstream of a single phototropin appear to be required for a subset of phototropin responses. NPH3, RPT2 and PKS1 directly interact with phot1 suggesting that they act early in the pathway [22][23][24]. However none of them is required for all the phototropin responses that were tested [22][23][24].…”

“…NPH3, RPT2 and PKS1 directly interact with phot1 suggesting that they act early in the pathway [22][23][24]. However none of them is required for all the phototropin responses that were tested [22][23][24]. For example, NPH3 is required for phot1 and phot2-mediated phototropism, phot1-mediated leaf positioning but dispensable for chloroplast positioning and stomatal opening [22,25] (Figure 1).…”

“…Interestingly, NPH3 undergoes a blue-light induced dephosphorylation that depends on phot1 but not phot2 [24]. A second example illustrating rapid branching in phot1 signaling comes from the analysis of Ca 2+ requirements for phot1 responses.…”