Pathways that limit leaflet formation in Cardamine hirsuta.

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.

We will study leaflet formation in Cardamine hirsuta - a genetically tractable compound leaf relative of the simple leafed model organism Arabidopsis thaliana. We will investigate pathways that direct leaflet positioning in space and time thus giving rise to the specific leaflet number and arrangement that characterizes this species. Through forward genetics we have identified three repressors of leaflet formation. These are microRNA164A, a post transcriptional repressor of CUC2 (CUP SHAPED COTYLEDON 2) which encodes a transcription factor that promotes leaflet formation;TCP4 (Teosinte branched1, Cycloidea, and PCF) which encodes a transcription factor that restricts growth; and an uncloned gene termed MOL1 (MORE LEAFLETS 1). Here we propose to clone MOL1 and understand how regulatory relationships between mir164A, TCP4 and MOL1 define correct leaflet number. We will investigate whether, when and where the respective gene products act and how they influence cell division associated with leaflet formation. We also propose to examine whether activity of these three repressors of leaflet formation converges on regulating the activities of KNOX (KNOTTED1 like homeobox) and CUC2 genes and the small indolic hormone auxin, all of which promote leaflet formation. To elucidate the regulatory logic of these pathways that regulate leaflet number we will construct computational models of C.hirsuta leaf development in collaboration with P. Prusinkiewitz. We will then test to what degree such models capture the developmental processes via which leaflet formation is defined.

Potential beneficiaries of this research project and plans to engage effectively with these groups are indicated as follows: i. General public ii. Schools iii. Policymakers iv. Brassica community including breeders i. General public engagement will center on discussing methodology and societal implications of scientific enquiry in the area of plant development and diversity. We will highlight the necessity to build on strong fundamental research to underpin high quality translational research in plant science. Forums for such events are given in impact plan and include local reading groups organized by the lead PI and occasional public lectures. ii. Co-PI Hay works together with the Secondary Education Officer for the Oxford University Museum of Natural History and Botanic Gardens to engage on these research topics with schools through lectures and practical workshops. iii. Specific policymaker engagement will take place in appropriate forums such as GARNET where the lead PI is a committee member, and the 2020 Joint BBSRC/DFG workshop where the lead PI participated. Policymakers or future policymakers may also be engaged at public events outlined in (i) and ultimately bring this knowledge into better informed discussions and science policy decision making. iv. The information developed here should contribute to building a sound knowledge-base for crucifer development that will inform Brassica crop improvement. Furthermore, we will generate a set of lasting resources (transgenic lines and mutants strains) that in the future should be useful for the comparative study in C.hirsuta and Arabidopsis of growth and development traits that are of agronomic interest. SNP markers and ESTs will be made public and could also be useful to brassica breeders. Elucidation of processes that regulate leaflet growth will empower efforts to modify tissue growth for agricultural purposes. In this respect computational models will also be of value to identify the most sensitive control points in complex homeostatic systems such as plant lateral organ development. These issues will be flagged in presentations, for example at the annual Brassica Research Community and Oilseed Rape Genetic Improvement Network meetings, on the lab website and via ISIS innovation: the University technology transfer company.