A novel cell-cell signalling pathway regulating the orientation of cell division in plants

Plant cells are surrounded by a strong cell wall. One consequence of this feature of plant cell development is that the amazingly diverse range of plant shapes are all generated by different parts of the plant growing at different rates. These different growth rates can only be brought about by increasing the size of the cells, or by altering the direction in which the cells divide. In roots for example, the cells nearly always divide at right angles to the outside of the root and this gives the characteristic long thin shape of most roots. Despite the importance of the direction of cell division in plant development, we know very little about how this process is regulated. How does a plant cell know where it is relative to the outside of the plant? The vascular tissue in plants is the tissue that transports water and solutes around the plant. Vascular cells divide in a very ordered and predictable way and consequently are a good target for studying how the process of cell division is organised. We have identified a mutant in which the ordered cell divisions in the vascular tissue are disturbed. Using this mutant we have identified a gene that is an important part of a pathway that tells plant cells where they are, relative to the rest of the plant, and consequently in which direction they must divide. This gene is likely to be only one component of a pathway that is involved in telling cells where they are relative to one another. During the course of this project other components of this pathway will be identified and how they work together to organise the direction of cell division will be determined. It is important to understand these processes for a variety of reasons. Vascular tissue is important since wood is composed of vascular tissue. How vascular cells divide and the cells that they generate are important for determining the properties of the wood, i.e. how strong it is, whether it will make good paper for a printer. Because of the vast amount of rubbish that we now generate there is increasing pressure to find material that is biodegradable. One way of generating biodegradable material is to use natural fibres from plants to replace man-made fibres in materials such as fibre glass. Understanding how cells divide would help to develop fibres that are better suited to the requirements of the material. It is important to understand these processes for a variety of reasons. Vascular tissue is important since wood is composed of vascular tissue. How vascular cells divide and the cells that they generate are important for determining the properties of the wood, i.e. how strong it is, whether it will make good paper for a printer. Because of the vast amount of rubbish that we now generate there is increasing pressure to find material that is biodegradable. One way of generating biodegradable material is to use natural fibres from plants to replace man-made fibres in materials such as fibre glass. Understanding how cells divide would help to develop fibres that are better suited to the requirements of the material.

One of the defining features of plant development is that the amazingly diverse array of plant morphologies are all generated by differential growth. This differential growth can only be generated by changes in cell expansion or by alteration in the orientation of the plane of cell division. Whilst the regulation of both the extent and orientation of cell expansion have been widely studied little is known about what controls the orientation of cell division despite its central role in plant development. During primary vascular development both the xylem and phloem differentiate from the procambium with the xylem located on the inside of the vascular bundle and phloem towards the outside. The developing procambial cells divide periclinally along their long axis to form files of cells. Despite the importance of the vascular cambium in generating wood and fibres, little is known about how this precise orientation of the cell division plane is controlled. Indeed, no components of any cell-cell signalling pathway that controls the orientation of any anticlinal of periclinal division during plant development have been identified to date. This proposal builds on our identification and cloning of the PXY gene. PXY is a receptor-like kinase gene with a predicted to have an extracellular leucine rich repeat domain. It is closely related to CLV1 and is essential for regulating the orientation of periclinal cell divisions that occur during vascular development. A combination of genetic, molecular biology and microarray analysis will be used to identify other components of the signalling pathway. PXY and any other components of the pathway will be characterised to determine exactly where and in which cells they are expressed. This information will be used to determine how this cell-cell signalling pathway works to convey positional information to dividing cells that allows them to divide along the correct division plane.