Regulation of polar auxin transport by protein and lipid kinases.

The directional transport of auxin, known as polar auxin transport (PAT), allows asymmetric distribution of this hormone in different cells and tissues. This system creates local auxin maxima, minima, and gradients that are instrumental in both organ initiation and shape determination. As such, PAT is crucial for all aspects of plant development but also for environmental interaction, notably in shaping plant architecture to its environment. Cell to cell auxin transport is mediated by a network of auxin carriers that are regulated at the transcriptional and post-translational levels. Here we review our current knowledge on some aspects of the 'non-genomic' regulation of auxin transport, placing an emphasis on how phosphorylation by protein and lipid kinases controls the polarity, intracellular trafficking, stability, and activity of auxin carriers. We describe the role of several AGC kinases, including PINOID, D6PK, and the blue light photoreceptor phot1, in phosphorylating auxin carriers from the PIN and ABCB families. We also highlight the function of some receptor-like kinases (RLKs) and two-component histidine kinase receptors in PAT, noting that there are probably RLKs involved in co-ordinating auxin distribution yet to be discovered. In addition, we describe the emerging role of phospholipid phosphorylation in polarity establishment and intracellular trafficking of PIN proteins. We outline these various phosphorylation mechanisms in the context of primary and lateral root development, leaf cell shape acquisition, as well as root gravitropism and shoot phototropism.

Functional interplay between protein kinase CK2 and salicylic acid sustains PIN transcriptional expression and root development.

We have previously reported that CK2-defective Arabidopsis thaliana plants (CK2mut plants) were impaired severely in root development and auxin polar transport, and exhibited transcriptional misregulation of auxin-efflux transporters (Plant J., 67, 2011a, 169). In this work we show that CK2mut roots accumulate high levels of salicylic acid (SA) and that the gene that encodes isochorismate synthase (SID2) is overexpressed, strongly suggesting that CK2 activity is required for SA biosynthesis via the shikimate pathway. Moreover, SA activates transcription of CK2-encoding genes and, thus, SA and CK2 appear to be part of an autoregulatory feed-back loop to fine-tune each other's activities. We also show that exogenous SA and constitutive high SA levels in cpr mutants reproduce the CK2mut root phenotypes (decrease of root length and of number of lateral roots), whereas inhibition of CK2 activity in SA-defective and SA-signalling mutants lead to less severe phenotypes, suggesting that the CK2mut root phenotypes are SA-mediated effects. Moreover, exogenous SA mediates transcriptional repression of most of PIN-FORMED (PIN) genes, which is the opposite effect observed in CK2mut roots. These results prompted us to propose a model in which CK2 acts as a link between SA homeostasis and transcriptional regulation of auxin-efflux transporters. We also show that CK2 overexpression in Arabidopsis has neither impact on SA biosynthesis nor on auxin transport, but it improves the Arabidopsis root system. Thus, unlike the outcome in mammals, an excess of CK2 in plant cells does not produce neoplasia, but it might be advantageous for plant fitness.

A dominant negative mutant of protein kinase CK2 exhibits altered auxin responses in Arabidopsis.

Protein kinase CK2 is a pleiotropic Ser/Thr kinase, evolutionary conserved in eukaryotes. Studies performed in different organisms, from yeast to humans, have highlighted the importance of CK2 in cell growth and cell-cycle control. However, the signalling pathways in which CK2 is involved have not been fully identified. In plants, the phytohormone auxin is a major regulator of cell growth. Recent discoveries have demonstrated that differential distribution of within auxin plant tissues is essential for developmental processes, and that this distribution is dependent on polar auxin transport. We report here that a dominant-negative mutant of CK2 (CK2mut) in Arabidopsis thaliana shows phenotypic traits that are typically linked to alterations in auxin-dependent processes. However, CK2mut plants exhibit normal responses to exogenous indole-3-acetic acid (IAA) indicating that they are not affected in the perception of the hormone but upstream in the pathway. We demonstrate that mutant plants are not deficient in IAA but are impaired in its transport. Using genetic and pharmacological tools we show that CK2 activity depletion hinders correct formation of auxin gradients and leads to widespread changes in the expression of auxin-related genes. In particular, members of the auxin efflux carrier family (PINs), and the protein kinase PINOID, both key regulators of auxin fluxes, were misexpressed. PIN4 and PIN7 were also found mislocalized, with accumulation in endosomal bodies. We propose that CK2 functions in the regulation of auxin-signalling pathways, particularly in auxin transport.

Auxin-Regulated Reversible Inhibition of TMK1 Signaling by MAKR2 Modulates the Dynamics of Root Gravitropism.

Plants are able to orient their growth according to gravity, which ultimately controls both shoot and root architecture.1 Gravitropism is a dynamic process whereby gravistimulation induces the asymmetric distribution of the plant hormone auxin, leading to asymmetric growth, organ bending, and subsequent reset of auxin distribution back to the original pre-gravistimulation situation.1-3 Differential auxin accumulation during the gravitropic response depends on the activity of polarly localized PIN-FORMED (PIN) auxin-efflux carriers.1-4 In particular, the timing of this dynamic response is regulated by PIN2,5,6 but the underlying molecular mechanisms are poorly understood. Here, we show that MEMBRANE ASSOCIATED KINASE REGULATOR2 (MAKR2) controls the pace of the root gravitropic response. We found that MAKR2 is required for the PIN2 asymmetry during gravitropism by acting as a negative regulator of the cell-surface signaling mediated by the receptor-like kinase TRANSMEMBRANE KINASE1 (TMK1).2,7-10 Furthermore, we show that the MAKR2 inhibitory effect on TMK1 signaling is antagonized by auxin itself, which triggers rapid MAKR2 membrane dissociation in a TMK1-dependent manner. Our findings suggest that the timing of the root gravitropic response is orchestrated by the reversible inhibition of the TMK1 signaling pathway at the cell surface.

The Protein Kinase CK2 Mediates Cross-Talk between Auxin- and Salicylic Acid-Signaling Pathways in the Regulation of PINOID Transcription.

The protein kinase CK2 is a ubiquitous and highly conserved enzyme, the activity of which is vital for eukaryotic cells. We recently demonstrated that CK2 modulates salicylic acid (SA) homeostasis in Arabidopsis thaliana, and that functional interplay between CK2 and SA sustains transcriptional expression of PIN-FORMED (PIN) genes. In this work, we show that CK2 also plays a key role in the transcriptional regulation of PINOID (PID), an AGC protein kinase that modulates the apical/basal localization of auxin-efflux transporters. We show that PID transcription is up-regulated by auxin and by SA and that CK2 is involved in both pathways. On the one hand, CK2 activity is required for proteosome-dependent degradation of AXR3, a member of the AUX/IAA family of auxin transcriptional repressors that must be degraded to activate auxin-responsive gene expression. On the other hand, the role of CK2 in SA homeostasis and, indirectly, in SA-driven PID transcription, was confirmed by using Arabidopsis NahG transgenic plants, which cannot accumulate SA. In conclusion, our results evidence a role for CK2 as a functional link in the negative cross-talk between auxin- and SA-signaling.

A PtdIns(4)P-driven electrostatic field controls cell membrane identity and signalling in plants.

Many signalling proteins permanently or transiently localize to specific organelles. It is well established that certain lipids act as biochemical landmarks to specify compartment identity. However, they also influence membrane biophysical properties, which emerge as important features in specifying cellular territories. Such parameters include the membrane inner surface potential, which varies according to the lipid composition of each organelle. Here, we found that the plant plasma membrane (PM) and the cell plate of dividing cells have a unique electrostatic signature controlled by phosphatidylinositol-4-phosphate (PtdIns(4)P). Our results further reveal that, contrarily to other eukaryotes, PtdIns(4)P massively accumulates at the PM, establishing it as a critical hallmark of this membrane in plants. Membrane surface charges control the PM localization and function of the polar auxin transport regulator PINOID as well as proteins from the BRI1 KINASE INHIBITOR1 (BKI1)/MEMBRANE ASSOCIATED KINASE REGULATOR (MAKR) family, which are involved in brassinosteroid and receptor-like kinase signalling. We anticipate that this PtdIns(4)P-driven physical membrane property will control the localization and function of many proteins involved in development, reproduction, immunity and nutrition.

Linking protein kinase CK2 and auxin transport.

Studies performed in different organisms have highlighted the importance of protein kinase CK2 in cell growth and cell viability. However, the plant signaling pathways in which CK2 is involved are largely unknown. We have reported that a dominant-negative mutant of CK2 in Arabidopsis thaliana shows phenotypic traits that are typically linked to alterations in auxin-dependent processes. We demonstrated that auxin transport is, indeed, impaired in these mutant plants, and that this correlates with misexpression and mislocalization of PIN efflux transporters and of PINOID. Our data establishes a link between CK2 activity and the regulation of auxin homeostasis in plants, strongly suggesting that CK2 might be required at multiple points of the pathways regulating auxin fluxes.