Branch formation

Science programme: Plant development

One of the most remarkable and obvious aspects of the world’s estimated 250,000 vascular plant species is their extraordinary diversity in shape and form. A huge part of this diversity of shape is determined by the number of branches a plant has and the position of these branches in the plant. We are interested in understanding what the control processes are in plants that determine plant shape, and how those controls are used to give us plants with such different shapes.

Branching is important in commercial crop species because the number of branches, the timing of their growth and their position in the plant will determine the number of flowers and therefore fruit. For a crop to be commercially viable there must be a balance between vegetative and reproductive growth, and this will differ depending on what tissue is harvested. Excess vegetative growth associated with the production of branches can result in a shifting of resources away from desirable growth (such as wood production or fruit). Additionally, the placement of branches in a plant will determine the method and ease of harvest of the crop.

We are using petunia as a model system to understand the controls of plant branching. Petunia is a good model because this plant has multiple types of branching, and we have mutants with increased branching. These mutants are Dad1, Dad2 and Dad3 (for decreased apical dominance). We are cloning the genes involved in altering the branching in these mutants, and this information allows us to understand what processes plants use to control branch development. We use transposon tagging as well as a candidate gene approach to isolate these genes. We have identified the Dad1 and Dad3 genes, and have tagged plants that will allow us to identify the Dad2 gene.

We also use genetics and plant physiology approaches to understand branching. Graft analysis of our mutants and wild type has allowed us to identify a signal that originates in the roots of plants and moves upwards through the plant to promote branching. Hormone analysis has shown us that this signal is not one of the plant growth substances that have already been identified. Our genetic analysis has allowed us to determine how the elements of the branch control pathway fit together, and the order in which the Dad genes act to produce the branching signal.

Our ultimate aim is to discover the identity of the branching signal, and work out how this signal works in multiple plant species.