Abstract

Branching was a key innovation in plant evolution that allowed plants to colonize larger volumes of space and increase productivity. The molecular control of branching has been well studied in Arabidopsis in which a main shoot apex interacts with further apices in the leaf axils to determine which can grow. The interaction between apices is mediated hormonally by auxin and its polar transport, via plasma membrane canonical PIN proteins, cytokinins and strigolactones.

The axillary branching mode of Arabidopsis and other seed plants is evolutionarily derived and has also arisen by convergence in mosses like Physcomitrella. However, recent work suggests that there is no long range auxin transport in Physcomitrella, that PIN proteins do not localize to the plasma membrane, and that strigolactones do not affect branching in the leafy shoot.

We have used a combination of computational and genetic tools to determine what regulates branching in Physcomitrella. As in Arabidopsis, and in line with the predictions of our model, we find that auxin from an apical source suppresses branching, cytokinin promotes branching, and strigolactone from a basal source suppresses branching. However, our model predicts that a capacity for polar auxin transport is not required to regulate the branching pattern, and mutant analysis reveals that canonical PIN proteins are minor contributors to the branching pattern.

Our work shows that three conserved and ancient hormonal cues have been recruited independently to regulate the branching pattern in moss and Arabidopsis. It also shows that moss canonical PINs are not necessary for branching to occur and predicts that a non-polar auxin transport mechanism is sufficient to effect the auxin transport required to generate the branching pattern, possibly via unknown transporters.