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Abstract
Intercellular distribution of the plant hormone auxin largely depends on the polar subcellular distribution of the plasma membrane PIN-FORMED (PIN) auxin transporters. PIN polarity switches in response to different developmental and environmental signals have been shown to redirect auxin fluxes mediating certain developmental responses. PIN phosphorylation at different sites and by different kinases is crucial for PIN function. Here we investigate the role of PIN phosphorylation during gravitropic response. Loss- and gain-of-function mutants in PINOID and related kinases but not in D6PK kinase as well as mutations mimicking constitutive dephosphorylated or phosphorylated status of two clusters of predicted phosphorylation sites partially disrupted PIN3 phosphorylation and caused defects in gravitropic bending in roots and hypocotyls. In particular, they impacted PIN3 polarity rearrangements in response to gravity and during feed-back regulation by auxin itself. Thus PIN phosphorylation, besides regulating transport activity and apical-basal targeting, is also important for the rapid polarity switches in response to environmental and endogenous signals.
Highlights
The plant hormone auxin, indole-3-acetic acid (IAA), controls plant growth and development by modulating fundamental cellular processes such as cell division, expansion, and differentiation[1]
The observation that gravistimulation induces changes in the polar localization of PIN auxin transporters, which is consistent with auxin flow being aligned with the gravity vector[5,29,35,36,37], provides a possible mechanism for gravity-induced redirection of auxin fluxes
We have demonstrated that PIN phosphorylation and PID/WAGs kinases activity are involved in the gravity-induced relocation of the PIN3 protein in root columella and hypocotyl endodermal cells
Summary
Importance of phosphorylation for gravity-mediated PIN3 relocation and bending. Gravistimulation has been shown to induce changes in polar PIN3 localization in roots where PIN3 relocates towards the bottom side of the columella cells after the gravitropic stimulus[5,29,35,36,37]. After 12 hours of gravitropic treatment we observed no significant differences in any d6pk mutants or overexpression lines (Figure S1a), agreeing with previously reported mild gravitropic defects[28] This argues against an important role of D6PK in root gravitropism, and suggests that the contribution of D6PK for the regulation of PIN activity, either in columella cells for redirection of auxin fluxes, or in epidermis for shootward auxin transport, is not crucial. The results suggest the importance of PIN3 phosphorylation in the root gravitropic response, in particular that mimicking constitutive phosphorylation inhibits gravity-induced PIN3 relocation similar to the effects observed in 35S::PID lines. These data indicate that in both negative gravitropism of the hypocotyl as well as in positive root gravitropism, PIN3 phosphorylation plays a similar role in gravity-induced relocation and gravitropic response in both above and underground organs. The hypocotyl experiments revealed a role of PIN3 phosphorylation in auxin-induced relocation
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