Constitutive auxin response in Physcomitrella reveals complex interactions between Aux/IAA and ARF proteins

Auxin is a plant hormone that regulates many aspects of growth and development. It does so by promoting the degradation of proteins called the Aux/IAAs. These proteins normally act to keep genes switched off by interacting with another family of proteins called ARFs that can bind directly to the genes. Some ARFs activate genes, while others repress gene activity. In most plants, large families of genes encode Aux/IAA and ARF proteins and individual members often have overlapping roles, which makes it more difficult to study how they work. To avoid this problem, Lavy et al. chose to study how these proteins work in a moss called Physcomitrella patens, which only has three genes that encode Aux/IAA proteins. The experiments show that the loss of all three Aux/IAA proteins results in the plants becoming completely insensitive to auxin. Furthermore, over a third of known moss genes had altered activity in the mutant plants compared to normal moss. These findings suggest that the Aux/IAA proteins have a bigger role in regulating the activities of genes than previously thought. Further experiments show that repressive ARFs act by directly competing with the activating ARFs for binding to auxin-regulated genes. However, repression of gene activity by the ARFs is weaker than the effect of the Aux/IAAs. Unexpectedly, the loss of repressive ARFs causes moss plants to become less sensitive to auxin, which Lavy et al. suggest is due to the recruitment of additional Aux/IAA proteins to the genes. Thus these findings demonstrate that control of gene activity by auxin involves the coordinated action of both types of ARFs and the Aux/IAAs. Future challenges are to find out which genes directly bind ARFs and the Aux/IAAs, and to use computational approaches to create models of how auxin regulates gene activity in the moss.