Functional characterization of Ostreococcus tauri phototropin

Abstract

SummaryPhototropins (phots) regulate a range of adaptive processes in plants that serve to optimize photosynthetic efficiency and promote growth. Light sensing by Arabidopsis thaliana phots is predominantly mediated by the Light, Oxygen and Voltage sensing 2 (LOV2) flavin-binding motif located within the N-terminus of the photoreceptor. Here we characterize the photochemical and biochemical properties of phot from the marine picoalga Ostreococcus tauri phototropin (Otphot) and examine its ability to replace phot-mediated function in Arabidopsis.Photochemical properties of Otphot rely on both LOV1 and LOV2. Yet, biochemical analysis indicates that light-dependent receptor autophosphorylation is primarily dependent on LOV2. As found for Arabidopsis phots, Otphot associates with the plasma membrane and partially internalizes, albeit to a limited extent, in response to blue-light irradiation.Otphot is able to elicit a number of phot-regulated processes in Arabidopsis, including petiole positioning, leaf expansion, stomatal opening and chloroplast accumulation movement. However, Otphot is unable to restore phototropism and chloroplast avoidance movement. Consistent with its lack of phototropic function in Arabidopsis, Otphot does not associate with or trigger dephosphorylation of the phototropic signalling component Non-Phototropic Hypocotyl 3 (NPH3).Taken together, these findings indicate that the mechanism of action of plant and evolutionarily distant algal phots is less well conserved than previously thought.

Shining Light on the Function of NPH3/RPT2-Like Proteins in Phototropin Signaling

“…However, recent studies centered on phot from the marine picoalga Ostreococcus tauri are not consistent with this conclusion. While O. tauri phot is capable of mediating chloroplast accumulation movement, stomatal opening, as well as leaf positioning and leaf expansion when expressed in the phot1 phot2 double mutant of Arabidopsis, it fails to restore phototropism and chloroplast-avoidance movement (Sullivan et al, 2016a). At least for phototropism, this lack of functionality correlates with the inability of O. tauri phot to complex with NPH3 (Sullivan et al, 2016a).…”

“…While O. tauri phot is capable of mediating chloroplast accumulation movement, stomatal opening, as well as leaf positioning and leaf expansion when expressed in the phot1 phot2 double mutant of Arabidopsis, it fails to restore phototropism and chloroplast-avoidance movement (Sullivan et al, 2016a). At least for phototropism, this lack of functionality correlates with the inability of O. tauri phot to complex with NPH3 (Sullivan et al, 2016a). These findings fall more in line with what is known regarding the evolutionary conservation of NPH3 and other NRL proteins involved in phot signaling.…”

“…Phots are members of the AGCVIII kinase family (Rademacher and Offringa, 2012) but are distinct from transmembrane receptor kinases such as BRI1, as they are hydrophilic and bind to the intracellular side of the plasma membrane (Kong et al, 2013) to initiate signaling (Preuten et al, 2015). Although the ability to associate with the plasma membrane is conserved in algal phots (Sullivan et al, 2016a), the mechanism underlying this attachment is still not known, but it is thought to involve some form of lipid binding/ modification.…”

Ferns, mosses and liverworts as model systems for light‐mediated chloroplast movements

“…Nevertheless, when expressed in Arabidopsis phototropin mutant plants, Crphot could mediate both the accumulation and avoidance responses (Onodera et al . ), similar to Mpphot (Komatsu et al ), and Otphot could mediate the accumulation response (Sullivan et al ) (Fig. ).…”

“…Two chlorophytes, Chlamydomonas reinhardtii and Ostreococcus tauri , have single phototropins, Crphot and Otphot, respectively (Onodera et al . ; Sullivan et al ), but they do not show any chloroplast movement responses. Nevertheless, when expressed in Arabidopsis phototropin mutant plants, Crphot could mediate both the accumulation and avoidance responses (Onodera et al .…”