With the molecular basis of the central auxin signaling pathway largely uncovered, it is intriguing to note that there is limited evidence for the involvement of canonical pathways, comprising a membrane-bound receptor and a protein kinase cascade, in auxin signaling. The most likely candidate for a membrane-bound auxin receptor is still Auxin Binding Protein 1 (ABP1), a protein that is retained in the ER and is only present in low levels at the cell surface. Several lines of evidence indicate that plasma membrane surface-
localized ABP1 is involved in signaling cascades leading to rapid, TIR1/AFB-independent cell expansion and early electrophysiological auxin responses (Fig. 2) (Napier et al. 2002).
Fig. 2 Hypothetical auxin signaling pathway involving ABP1 and the PDK1/PID pathway regulating PAT. A hypothetical signal transduction pathway for auxin starts with its reception by apoplastic ABP1, which modifies its transmembrane ABP1-docking protein. The latter could transduce the signal in two different ways: through a MAPK cascade or through a hypothetical PDK1 pathway. In the first case, the ABP1-docking protein either activates a G-protein or directly activates a MAPK cascade ultimately leading to the induction of auxin response. In the second case, the ABP1-docking protein modifies PDK1, which starts a signaling cascade passing through PID or other AGC VIII subfamily kinase and ending in the regulation of auxin response. Alternatively, the ABP1-docking protein could, through unidentified factors (dashed arrow), induce transport of PID to the plasma membrane region where PDK1 is anchored; once in this position, PID would be activated by PDK1 and continue to transduce the signal. The acknowledged role of PID as regulator of PINs is also shown: PID activity, which can be repressed by the PBP2-PBK complex, or regulated in response to cytosolic calcium levels by PBP1 (upregulation) or TCH3 (downregula-tion), results in PIN phosphorylation either in endosomes or at the plasma membrane, in which case PINs are respectively induced for exocytosis or to remain at the PM at the apical cellular pole. Other putative pathways are also shown: stress signals activate MAP-KKK NPK1 signal transduction leading to repression of auxin response; phytochromes may phosphorylate Aux/IAA proteins to regulate light and/or auxin induced gene expression
The ultimate data supporting this hypothesis, however, remain to be obtained, since the phosphorylation events occasionally reported to be involved in auxin signaling have not been linked to ABP1. For example, Aux/IAA proteins have been shown to be phosphorylated by phytochromes in vitro, suggesting that light signaling acts on auxin responsive gene expression by influencing the stability of Aux/IAA proteins (Colon-Carmona et al. 2000). Also the Mi-togen Activated Protein Kinase (MAPK) cascade has been implicated in the modulation of auxin response. Roots of Arabidopsis seedlings treated with auxin showed an increase in MAPK activity and this activation was inhibited in the auxin resistant axr4 mutant (Mockaitis and Howell 2000). However, it has been recently shown that AXR4 is involved in the regulation of auxin import by regulating the localization of the putative auxin influx carrier AUX1 at the plasma membrane (Dharmasiri et al. 2006). Therefore, the lack of auxin-induced MAPK activity in axr4 is likely to be the consequence of reduced auxin uptake. This does not exclude, however, that the MAPK signaling pathway is involved in the modulation of auxin response. In fact, it has been shown that the MAP3K NPK1 activates stress responses and represses auxin-induced gene expression (Kovtun et al. 1998). Finally, the serine/threonine kinase PINOID (PID) has been proposed to be a regulator of auxin signaling (Christensen et al. 2000). Although more recent data have clearly related PID activity to the modulation of polar auxin transport (PAT) (Benjamins et al. 2001; Friml et al. 2004), a role for this kinase in auxin signaling can not yet be excluded.
In conclusion, although not part of the central auxin signaling pathway, phosphorylation events seem to modulate auxin responses, and to serve at least to integrate other signals, such as light or stress, with auxin signaling (Fig. 2).
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