Pathways that limit leaflet formation in Cardamine hirsuta.

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A key challenge in biology is to understand how different organisms come to have different forms. In plants this variation in form is obvious in the many different leaf shapes one sees during a walk in the park or when eating a salad. Leaves are also interesting to study because they play a key role in the food chain being the main photosynthetic organs of land plants and thus responsible for CO2 fixation in terrestrial ecosystems. For these reasons, understanding how diversity in leaf form is generated is of considerable interest to scientists. To study this problem we work with hairy bittercress (Cardamine hirsuta), which is a plant that has leaves fully subdivided into smaller leaflets. The presence of leaflets makes this plant very different from its close relative thale cress (Arabidopsis thaliana), which has entire, undivided leaves. We already know a lot about how an entire leaf shape is produced in thale cress because it is easy to grow and do experiments with. Hairy bittercress is also very easy to work with in the lab, so we use it to understand how leaflets are produced and what controls their formation. Here we want to understand processes that restrict leaflet growth during bitter cress development to result in the correct leaflet number and position that characterizes this plant. Our results will also help understand the developmental mechanisms that direct repeated processes, such as leaflet formation, to take place a limited number of times such that the precise form typical of any given species is attained.