Protein phosphorylation in the delivery of and response to auxin signals.

The importance of reversible protein phosphorylation in regulation of plant growth and development has been amply demonstrated by decades of research. Here we discuss recent studies that suggest roles for protein phosphorylation in regulation of both auxin responses and polar auxin transport. Specific kinases act at auxin-requiring steps in floral and embryonic development, and at the junction(s) between light and auxin signaling pathways in hypocotyl elongation and phototropism responses. New evidence for rapid mitogen-activated protein kinase (MAPK) activation by auxin treatment suggests that MAPK cascade(s) might mediate cellular responses to auxin. Protein phosphorylation also may play a crucial role in regulating the activity or turnover of auxin-responsive transcription factors. Auxin transport is modulated by phosphorylation, and protein phosphatase activity is involved in regulation of auxin transport streams in roots. Although the regulatory circuits have not been fully elucidated, these studies suggest that protein phosphorylating and dephosphorylating enzymes perform key functions in auxin biology. In some cases, these enzymes act at the intersections between auxin signaling and other signaling pathways.