Exploiting the Arabidopsis thaliana relative Cardamine hirsuta for understanding dissected leaf development

A key problem 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. 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 to 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. We have recently shown that regulated delivery of auxin which is a small hormone known to trigger leaf formation at the growing tips of the thale cress and other plants, is also required for leaflets to form in hairy bittercress leaves. This is an exciting finding because it suggests that the same mechanism that instructs cells to become leaves may also be used later in development to sculpt the various leaf shapes that distinguish different plants. We now want to understand what aspects of the genetic make up of the hairy bittercress allow auxin responsive leaflet production in this species but not thale cress.

We will study the role of regulated efflux of the plant hormone auxin in compound leaf development. To this end we will use Cardamine hirsuta a dissected leaf relative of the simple leafed model organism A.thaliana as a model system. To identify factors required for compound leaf development we isolated mutants that display reduced leaflets and named these sil (simple leaves) mutants. We have already cloned the SIL1 gene and showed it corresponds to the C.hirsuta orthologue of the A.thaliana PINFORMED1 (PIN1) gene that encodes an auxin efflux transporter. This finding indicates that regulated auxin efflux is required to produce leaflets in C.hirsuta and perhaps other compound leaf plants. Our observations also suggest that the same regulatory module that is used to direct leaf initiation from the pluripotent cell population of the Shoot Apical Meristem (SAM) is reactivated within C.hirsuta leaves to induce leaflet formation from the leaf blade. To understand what aspects of leaf morphogenesis differ between C.hirsuta and A.thaliana such that PIN1 can elicit leaflet formation only in the former species, we will first determine the specific cells and developmental time when C.hirsuta PIN1 (ChPIN1) can trigger leaflet formation. Secondly, we will isolate the SIL3 gene that we have shown is required for the species-specific expression of PIN1 in supporting leaflet formation but not for leaf development from the SAM. Finally, we will investigate whether ChPIN1 leaflet directing action depends on function of the SHOOTMERISTEMELSS gene, which is required for leaflet formation and is expressed in the C.hirsuta but not A.thaliana leaf.