Regulation of polar auxin transport by protein and lipid kinases

Abstract

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.

Keywords: Arabidopsis; auxin; cytokinin; endocytosis; gravitropism; intracellular trafficking; kinase; phosphoinositide; phototropism; polarity; receptor-like kinase.; root.

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Figure 1. Polar auxin transport and PIN localization in the primary root tip.

Schematic representation of a longitudinal root section showing the different root tissues (left). The arrows represent putative auxin fluxes (left) as deduced from PIN protein localization (right). For clarity, PIN3 and PIN7 proteins have been omitted from the stele since their localization is redundant with that of PIN1.

Figure 2. Model for PID and PP2As antagonistic activity on PIN phosphorylation, and the dual action of ABCB1 phosphorylation by PID.

A) Non-phosphorylated PINs undergo continuous endocytic recycling in a GNOM-dependent manner, which mediates their basal localization. The PID kinase phosphorylates PINs at the plasma membrane, which prohibits them from recycling via the GNOM-dependent route. Phosphorylated PINs instead undergo transcytosis to the apical pole of the cell. PINs are dephosphorylated by PP2As phosphatases, presumably at the plasma membrane and endosomes. PID also phosphorylates ABCB1, which promote ABCB1-mediated auxin export in the absence of TWD1, but inhibits it in its presence. B) Schematic representation of PIN1 topology and position of PIN1 phosphorylation site within PIN1 hydrophilic loop (PIN1^HL). The action of the kinases/phosphatases/isomerase on specific residues is highlighted. Note that only D6PK main phosphorylation sites (S4 and S5) are highlighted, but that S1, S2 and S3 are also phosphorylated by this kinase, albeit less potently. Similarly, PID/WAG1/WAG2 are able to phosphorylate S4 and S5 but preferentially act on S1, S2 and S3. Dashed arrows represent direct phosphorylation, dephosphorylation or isomerisation events; arrows with chevron-shaped arrowhead represent trafficking pathways and blunt-ended lines represent inhibition. Yellow-filled circles with the letter ‘P’ represent phosphorylation, while grey-filled circles represent individual amino acid (each residue is numbered according to its position within the PIN1 protein).

Figure 3. Phosphoinositide localization in Arabidopsis root epidermis and function in regulating polar auxin transport.

A) Schematic representation of PI3P, PI4P and PI(4,5)P₂ and example of their respective function in polar auxin transport. Note that they are likely involved in many more pathways and this represents only the studied examples. B) PIP subcellular localization in Arabidopsis root epidermis. The localization of the respective PIP kinases is indicated when known. Arrows with chevron-shaped arrowhead represent trafficking pathways. C) Model for the plasma membrane localization of PID by PI4P-driven electrostatics.

Figure 4. Cytokinin phosphorelay and its role in PIN1 trafficking.

Schematic representation of the ‘classical’ cytokinin phosphorelay pathway from receptor to gene activation (right) and its ‘non-canonical’ role, independent of transcription but relying on AHKs and ARRs, in PIN1 trafficking during lateral root initiation (left). Dashed arrows represent direct phosphorylation events; arrows with chevron-shaped arrowhead represent trafficking pathways and arrows with triangle-shaped arrowhead represent signalling events; blunt-ended lines represent inhibition.

Figure 5. auxin fluxes and PIN trafficking during the gravitropic root response.

A) Auxin fluxes and PIN2/PIN3 localization in the primary root tip before gravistimulation. Note the position of sedimented amyloplasts in the collumela. The arrow with the ‘g’ letter indicates the gravity vector. B) Auxin fluxes and PIN2/PIN3 localization in the primary root tip from 0 to 30 min after gravistimulation. Note the movement of amyloplasts according to the new gravity vector, which triggers PIN3 transcytosis to the lower part of the cell and initiates the establishment of an asymmetric auxin maximum between the upper and lower part of the root. C) Auxin fluxes and PIN2/PIN3 localization in the primary root tip from 30 to 120 min after gravistimulation. High auxin in the lower part of the root inhibits PIN2 endocytosis, which promotes its localization at the plasma membrane and reinforce asymmetric auxin localization. Accumulation of auxin on the lower part of the root locally inhibits cell elongation, which triggers root bending. D) Auxin fluxes and PIN2/PIN3 localization in the primary root tip from 120 to 240 min after gravistimulation. Inhibition of endocytosis by auxin is transient and PIN2 endocytosis is re-established in the lower part of the root. In the meantime, low auxin on the upper part of the root triggers PIN2 degradation in a TIR1/AFBs-dependent manner. Note that the times indicated are compiled between those reported by Band et al., (2012) and Baster et al., (2013), but may vary according to the experimental setup and are only indicative. It is unknown at what point during the gravitropic response PIN3 localization is reset to apolar, hence the purple question mark in C and D.

Figure 6. Missing RLKs in the regulation of PIN2 trafficking in epidermal root cells during gravitropism.

Schematic representation of PIN2 trafficking as regulated by the ‘non-genomic’ auxin pathway and GOLVEN peptides. Note that RLKs are likely involved in both pathways but have yet to be identified. Arrows with chevron-shaped arrowhead represent trafficking pathways and arrows with triangle-shaped arrowhead represent signalling events; blunt-ended lines represent inhibition.

Figure 7. Protein kinases and auxin fluxes during the hypocotyl phototrophic response.

A) Seedling grown in uniform light (left) or under unilateral light (right). B) Schematic representation of a longitudinal section of hypocotyl showing putative auxin maximum (in green) and PIN3 localization (in pink) when seedling are grown in uniform light conditions or in darkness (left) and when they encounter unidirectional light (right). Arrows in dark blue represent PIN1- and ABCB19-mediated rootward auxin flow and the arrow in pink show PIN3-mediated lateralization of auxin flow toward the shaded side of then hypocotyl. C) Schematic representation of PIN3 trafficking in endodermis (illuminated side of the hypocotyl). Phot1 inhibits PID activity by unknown mechanisms, which limits PIN3 phosphorylation by PID and favours its routing by the GNOM-dependent pathway to the inner lateral pole of the cells. Phot1 also directly phosphorylates ABCB19, which inhibits its efflux activity and limits the downward auxin flow. ABCB19 inhibition and PIN3 lateralization stimulates auxin trafficking toward the shaded side of the hypocotyl, which promotes cell elongation and hypocotyl bending toward the light source. Note that phot1 action on ABCB19 and PIN3 trafficking has been represented in the same cell for convenience, but it is not known whether these events happen in the same place of the hypocotyl. Dashed arrows represent direct phosphorylation events; arrows with chevron-shaped arrowhead represent trafficking pathways and blunt-ended line represents inhibition.