Conclusion

The mechanism of action of the phytohormone auxin is unique. The central auxin signaling pathway is triggered by direct binding of auxin to intracel-lular receptors, which activate the auxin-related proteolysis machinery consequently leading to auxin-responsive gene expression. There is only limited data indicating the direct involvement of kinases in this process, although there are suggestions of indirect phosphorylation events, or that ABP1 might be the apoplastic auxin receptor initiating a classical signaling pathway. However, most of the findings tend to suggest that auxin signaling is unique in its mechanism, strongly contrasting with classical routes occurring via membrane-bound hormone-receptors followed by a signaling cascade leading to changes in gene expression. As the tissue/cell specificity of the auxin signal is probably not provided by typical receptors, an alternate and very ingenious system evolved to likely solve this problem: the signal is delivered through the generation of tissue-specific concentration maxima and gradients of auxin, and this is sufficient to promote proper plant development.

The mechanisms behind the generation of such gradients are significantly complex. It involves transporter proteins such as the auxin influx (AUX1) and efflux (PINs) carriers, whose activities depend on their proper asymmetric subcellular localization, and are regulated by a plethora of components. The complex regulatory network of the polar auxin transport system evidently relies on the activity of kinases. Kinases of the AGCVIII subfamily seem to be particularly involved, with PID modulating the direction of PAT, the WAGs being involved in root waving, and the PHOTs controlling phototropism. On the other hand, while many other proteins play a role in the regulation of the activity of these kinases, one kinase, PDK1, seems to be the upstream regulator that links phospholipid signaling with auxin transport.

From the network regulating auxin distribution, only a few links have yet been demonstrated, such as the relationship between PDK1 and PID, PID/PP2A and PINs, and PINs and GNOM. Most connections in this network are, however, still unclear or missing. For example, although we know that the PIN proteins that have been shown to be responsive to PID activity play a role in several tissues, PID expression seems to be restricted to much fewer organs. Considering that PID-related kinases such as WAG1 and WAG2 act in different tissues than PID, it is tempting to speculate that they perform a PID-like function towards the PINs in tissues where PID itself is not present. Furthermore, although the activity of PDK1 towards most of the AGCVIII subfamily kinases has been demonstrated, there is still a gap in our knowledge on the relationship between PDK1 and the phototropins. Our future goal will be to obtain a much more comprehensive understanding of the PAT signaling network, and we expect to answer at least some of the above questions in the near future.

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