Transcriptional auxin-brassinosteroid crosstalk: who's talking?

The plant hormones auxin and brassinosteroid are both essential regulators of plant growth and known to influence both cell division and cell elongation in various developmental contexts. These physiological effects of auxin and brassinosteroid have been known for many years. Based on observations from external simultaneous application of both hormones to plant tissues, it has been suggested that they act in an interdependent and possibly synergistic manner. Recent work in the model plant Arabidopsis thaliana suggests that, at the molecular level, auxin-brassinosteroid synergism manifests itself in the regulation of the expression of common target genes. However, whether this reflects genuine hormone pathway-dependent crosstalk modulation of the transcription machinery or rather indirect effects of hormone action on other cellular activities, such as hormone biosynthesis or the polar transport of auxin, is not entirely clear. This article reviews the evidence for transcriptional crosstalk between auxin and brassinosteroid and its molecular basis.