A hormone for every occasion. Auxin puts polarity in organ patterning

Speaker: Lars Østergaard, John Innes Centre
Host: John Mundy, Section for Functional genomics

Abstract
Multicellular organisms including plants and animals develop specialised organs, which are composed of different types of tissues. The structure – or pattern – of organs is determined by the polarity within tissues along axes of symmetry and coordinated by mobile substances such as hormones. Auxin was one of the first hormones ever to be studied and the effect of auxin on light-regulated plant growth (phototropism) was investigated by Charles Darwin and his son Francis in the 1880s. Studies into the molecular mechanism of auxin has intensified in the last two decades and it has become clear that auxin is involved in essentially all aspects of plant growth and development. Regulation of auxin dynamics is emerging as an immediate downstream output of transcription factor networks involved in patterning of organs as diverse as roots, leaves and the female reproductive organ, the gynoecium. For example, precise auxin-concentration maxima initiate lateral root formation, whereas auxin is recruited at the gynoecium apex to promote a bilateral-to-radial symmetry transition that rarely occurs in nature. In the established auxin-signalling pathway, auxin mediates degradation of transcriptional repressor proteins leading to de-repression of auxin response factors. Given auxin’s role in highly diverse developmental mechanisms, it is unclear how such a single signalling module with a limited number of components can capture all the auxin effects. Experiments described in this presentation are aimed at providing a mechanistic and developmental understanding of a newly discovered auxin-signalling module, which may be particularly well suited to facilitate precise switches in polarity throughout plant development.