PIN proteins and architectural change in plants.

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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.