Cellular and regulatory basis of the early stages of stem development.

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Virtually all of the aerial parts of the plant originate from specialised structures called shoot apical meristems, where reserves of actively dividing cells are maintained. Decades of study have revealed much about how new leaves or flowers are initiated in the periphery of the meristems. In contrast, little is known about how new stem tissues are produced by the basal region of the SAM, called the rib meristem. This has been in part due to the inaccessibility of the rib meristem to imaging methods that had a key role in revealing other aspects of meristem function. The origin of the stem is not only a major plant developmental process that has been relatively neglected, but is also of great importance in crop improvement: the height and sturdiness of the stem affect the likelihood that plants fall over in bad weather, how much of the plant's resources can be directed to making fruits and seeds, and how easy it is to harvest them. In some plants, including wheat, the stem also stores starch that provides energy reserves for grain filling. Here, we propose to take advantage of recent developments in imaging techniques to reveal the cell division and growth patterns that underpin stem development and how this cell behaviour is influenced by regulatory genes that function in the rib meristem. Using Arabidopsis as the model, we will focus on genes that also control stem development even in distantly related crop plants, such as rice. Specifically, we aim to answer the following questions: 1. What patterns of cell division and growth underpin the early stages of stem formation? We will use high-resolution imaging and computer-based 3D reconstruction to obtain quantitative data on cell growth and division in the developing stem. We will also use methods that genetically mark cells and allow us to follow how their descendants contribute to the formation of new tissues. 2. How do regulatory genes influence the cell behaviour studied in 1? We will use the same methods to compare normal plants and plants in which stem development is altered by mutation of regulatory genes. 3. How is growth and tissue formation co-ordinated across the developing stem? This question will be addressed using plants in which regulatory genes are activated in only a subset of the cells of the developing stem; this will allow us to determine whether specific cells and tissues produce signals that control the behaviour of adjacent cells and tissues. 4. What changes in gene expression underlie the effects of regulatory genes on cell behaviour and signalling during stem development? Regulatory genes typically activate or repress whole sets of other genes to cause changes in cell behaviour. We will compare changes in gene activity caused by activation of different stem regulatory to reveal what genes mediate their effects on cell division, growth and signalling. In addition to giving us insight into a major but poorly understood aspect of how plants grow, the knowledge and methods produced are expected to aid future crop breeding. By knowing the cellular basis for early stem development, it will be possible to design more precise ways to manipulate stem architecture during crop breeding.