PIN proteins and architectural change in plants.

A key challenge in biology is to understand how body parts with complex shapes and specialized functions arise during development. In plants, the overall shape reflects the pattern of branching, the pattern of leaf initiation and the relative growth of leaves initiated from the tip. These traits impact strongly on plant productivity because they affect light interception in photosynthesis. Because many crop species are grasses that have little branching, the shape of leaves and their arrangement around the stem are particularly important. For these reasons, understanding the basic mechanisms that regulate leaf arrangements and growth is of considerable interest to scientists.

To study this problem, we are working on a moss (Physcomitrella patens), which like many plants, has leaves that are arranged in a spiral pattern around the stem. Physcomitrella has many advantages as a model for leaf development. Mosses are an evolutionary ancient group in which most gene families functioning in other plants are represented by fewer family members, making it simpler to pinpoint gene function. The plants are very small and the shoot apex is made up of a single cell. Leaves initiate as a single cell and are a single cell layer thick. This means that we have been able to develop a technique for filming shoot initiation, leaf initiation and leaf development microscopically, and can generate quantitative information about how cell division and growth contribute to overall plant shape.

Analysis and interpretation of this data is difficult without computational input, so we are working with computer scientists to identify key contributors to shape. Such computational analyses have so far abstracted leaf development to the tissue scale, or focussed on a specific aspect of development to minimise computer processing constraints. Because moss leaves have few cells, we have been able to generate a cellular model of leaf development that has made specific predictions about the contribution of cell division and growth to final leaf shape.

A plant hormone, auxin, plays a primary role in modulating leaf initiation patterns and leaf shape in plants like tomato and Arabidopsis as it regulates decisions about cell identity and growth. The regulated distribution of auxin is a key aspect of its activity, and transport is effected by carrier proteins belonging to a small gene family. Current approaches for evaluating how the carrier proteins effect the auxin distribution and impact on overall shape are limited by the contribution of multiple gene family members to transport. Monitoring the auxin distribution in flowering plant leaves has also been challenging as it can only be achieved indirectly by monitoring changes in the activity of auxin responsive genes, and flowering plant leaves have a complex tissue composition that limits tissue penetration in microscopy. The anatomical and genetic simplicity of the moss shoot again brings an advantage.

A further advantage in using moss to understand how plant shape is attained is that moss shoots have an independent evolutionary origin to most shoot systems. This means that if the mechanisms regulating shape are shared with other better studied groups like flowering plants, they are likely to be universal regulators of plant shape. The knowledge that we genenerate will therefore be very broadly applicable.

We aim to understand the basic processes that regulate shoot development and leaf shape, and how they have changed in plant evolution. To this end, we have developed a moss, Physcomitrella, as a model system for shoot development. As moss shoots are gametophytic, and mosses and flowering plants diverged around 430 million years ago, similarities in form arise by convergence. This creates the opportunity to identify mechanisms that are fundamental requirements of shoot development.

Auxin and its transport by PIN proteins are primary regulators of leaf initiation, leaflet initiation, margin growth and vein insertion in generating flowering plant shape, and auxin transport has conserved roles in the vascular plants. Tissue complexity and genetic redundancy make it hard to dissect the contribution of auxin and its transporters to these aspects of leaf development in flowering plants, and basal vascular plants are not amenable to genetics. Our aim is to use the simple leaves of moss to identify links between auxin, growth and shape that would be hard to pull out in flowering plants.

We previously used live-imaging and clonal analysis to determine how cell division and growth contribute to moss leaf development. We instigated a collaboration with computer scientists working in the Prusinkiewicz group (Calgary) to synthesise our data in a computational model of leaf development. This has suggested that the growth rate distribution is a key determinant of shape. As auxin modulates growth, a corollary is that the auxin distribution, and auxin transport, are important regulators of shape.

The aims of this proposal are to:
1. test the function of Physcomitrella PIN homologues.
2. determine the growth rate distribution in leaves.
3. generate a reporter to determine the auxin distribution in moss leaves.
4. identify links between the auxin distribution, auxin transport, the growth rate distribution and shape using genetics and pharmacology.

Who will benefit from this research and how?

Academic beneficiaries
The scientific community will benefit from the outputs and training provided by this research as described in the 'Academic Beneficiaries' section.

Beneficiaries in secondary education
This work is underpinned by a strong grounding in comparative biology, and comparative techniques like phylogeny. We think that phylogeny provides a strong a strong unifying theme for teaching in evolution, genetics, population genetics, health and disease and biodiversity. We are working with the Astra Zeneca Teachers' Trust and Science And Plants for Schools (SAPS) based at the Cambridge Botanic Garden to strengthen understanding and use of phylogeny in secondary teaching. So far this has resulted in the development of a web resource that showcases the use of phylogeny in medicinal plant bio-prospecting in a local 6th form college.

Public sector
The Cambridge Botanic Garden offers an exciting opportunity to showcase the evolutionary aspects of our work in the context of a living collection that manifests the diversity of plant life. We are working with education and outreach officers at the Botanic Garden and SAPS to use the living collection to effectively communicate key ideas relating to the radiation of plants on land. For instance, we are developing interpretation for the 'Life before flowers: a green world greenhouse display', and we will use this as a starting point to weave a storyline about plant evolution as a trail through the rest of the garden. We will work with local artists in the 'fascination of plants' day held at the Botanic Garden to invite visitors to look closely at fossils and basal lineages of land plants in the context of how changes in plant shape have happened through time. We think that this will challenge public views of plant shape in a way that could be constructive in relation to engineering change in crops.

Private sector
We are collaborating with researchers in the Biochemistry Department to exploit moss in generating green energy in bio-photovoltaic fuel cells. A design prototype, the 'moss table' in which many cells act in parallel, has arisen from a collaborative project between biologists and engineers to educate the public about the challenges of generating renewable energy and has been widely exhibited within the UK (http://www.robaid.com/bionics/moss-table-employs-biophotovoltaic-technology-to-generate-power.htm). The electrical output of fuel cells in the 'moss table' is currently low, but a long term aim is to make small self-powering organic-synthetic hybrid objects for use in daily life. We are working with Dr. Bombelli to improve the efficiency and reproducibility of moss fuel cells using our moss growth expertise. We are also exploring possibilities for making self-sustaining, sealed objects powered by moss by building new prototypes, and will explore routes for commercialisation in collaboration with Dr. Bea Schlarb-Ridley, the Cambridge Enterprise Officer, who has an extensive network of private sector contacts.