The plant hormone auxin plays a crucial role in regulating plant development and plant architecture. The directional auxin distribution within tissues depends on PIN transporters that are polarly localized on the plasma membrane. The PIN polarity and the resulting auxin flow directionality are mediated by the antagonistic actions of PINOID kinase and protein phosphatase 2A. However, the contribution of the PIN phosphorylation to the polar PIN sorting is still unclear. Here, we identified an evolutionarily conserved phosphorylation site within the central hydrophilic loop of PIN proteins that is important for the apical and basal polar PIN localizations. Inactivation of the phosphorylation site in PIN1(Ala) resulted in a predominantly basal targeting and increased the auxin flow to the root tip. In contrast, the outcome of the phosphomimic PIN1(Asp) manipulation was a constitutive, PINOID-independent apical targeting of PIN1 and an increased auxin flow in the opposite direction. Furthermore, the PIN1(Asp) functionally replaced PIN2 in its endogenous expression domain, revealing that the phosphorylation-dependent polarity regulation contributes to functional diversification within the PIN family. Our data suggest that PINOID-independent PIN phosphorylation at one single site is adequate to change the PIN polarity and, consequently, to redirect auxin fluxes between cells and provide the conceptual possibility and means to manipulate auxin-dependent plant development and architecture.

The plant-specific proteins named PIN-FORMED (PIN) efflux carriers facilitate the direction of auxin flow and thus play a vital role in the establishment of local auxin maxima within plant tissues that subsequently guide plant ontogenesis. They are membrane integral proteins with two hydrophobic regions consisting of alpha-helices linked with a hydrophilic loop, which is usually longer for the plasma membrane-localized PINs. The hydrophilic loop harbors molecular cues important for the subcellular localization and thus auxin efflux function of those transporters. The three-dimensional structure of PIN has not been solved yet. However, there are scattered but substantial data concerning the functional characterization of amino acid strings that constitute these carriers. These sequences include motifs vital for vesicular trafficking, residues regulating membrane diffusion, cellular polar localization, and activity of PINs. Here, we summarize those bits of information striving to provide a reference to structural motifs that have been investigated experimentally hoping to stimulate the efforts toward unraveling of PIN structure-function connections.

Auxin is an important regulator of plant ontogenies including embryo development and the exogenous application of this phytohormone has been found to be necessary for the induction of the embryogenic response in plant explants that have been cultured in vitro. However, in the present study, we show that treatment of Arabidopsis explants with trichostatin A (TSA), which is a chemical inhibitor of histone deacetylases, induces somatic embryogenesis (SE) without the exogenous application of auxin. We found that the TSA-treated explants generated somatic embryos that developed efficiently on the adaxial side of the cotyledons, which are the parts of an explant that are involved in auxin-induced SE. A substantial reduction in the activity of histone deacetylase (HDAC) was observed in the TSA-treated explants, thus confirming a histone acetylation-related mechanism of the TSA-promoted embryogenic response. Unexpectedly, the embryogenic effect of TSA was lower on the auxin-supplemented media and this finding further suggests an auxin-related mechanism of TSA-induced SE. Congruently, we found a significantly increased content of indolic compounds, which is indicative of IAA and an enhanced DR5::GUS signal in the TSA-treated explants. In line with these results, two of the YUCCA genes (YUC1 and YUC10), which are involved in auxin biosynthesis, were found to be distinctly up-regulated during TSA-induced SE and their expression was colocalised with the explant sites that are involved in SE. Beside auxin, ROS were extensively accumulated in response to TSA, thereby indicating that a stress-response is involved in TSA-triggered SE. Relevantly, we showed that the genes encoding the transcription factors (TFs) that have a regulatory function in auxin biosynthesis including LEC1, LEC2, BBM, and stress responses (MYB118) were highly up-regulated in the TSA-treated explants. Collectively, the results provide several pieces of evidence about the similarities between the molecular pathways of SE induction that are triggered by TSA and 2,4-D that involve the activation of the auxin-responsive TF genes that have a regulatory function in auxin biosynthesis and stress responses. The study suggests the involvement of histone acetylation in the auxin-mediated release of the embryogenic program of development in the somatic cells of Arabidopsis.