Ubiquitination and auxin signaling: a degrading story.

Signaling pathways are rarely straightforward, and auxin signal transduction is no exception. The diverse events in which auxin is involved tell of the daunting complexity behind the plant’s response to auxin. This one molecule can cause changes in rates of cell division and cell elongation, alter ion fluxes across membranes, cue changes in patterning and differentiation, and affect the expression of hundreds of genes (Davies, 1995; Berleth and Sachs, 2001). Does auxin act through multiple pathways to these diverse ends, or is there a single pathway that is dependent on the spatial, temporal, and environmental context in which auxin is received? Or is it both? Although the answers to these and other broad questions of auxin biology remain unclear, we are now in a position to connect at least some areas of research on auxin signal transduction into a more cohesive whole. Recent advances in our understanding of the apparent hub of auxin signaling, between perception and downstream auxin-induced gene expression, has developed from the mutational and molecular analysis of three main groups of proteins: the auxin/indoleacetic acids (Aux/IAAs), the auxin response factors, and components of the ubiquitin-mediated proteolytic pathway. It seems that regulated protein degradation is central to most aspects of the auxin response.