Xyloglucans xyloglucan endotransglucosylase (XET) activity and arabinogalactan-protein (AGP)-like molecules: a new inter-relationship

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Each cell in a plant has an outer cell wall, which may be either stretchable (allowing the cell to grow, e.g. in a young beanstalk) or not (e.g. in an old leaf). Our experiments aim to discover how the plant can change its cell walls (making them more, or less, stretchable) and thus how the plant is able to increase and decrease cell growth. This ability is important so that, for instance, a seedling developing in bright light can produce short stocky stems and large leaves, efficient for photosynthesis, whereas one developing in the dark can grow a long thin stem (with a better chance of reaching any light) but only small leaves. Growing big leaves in the dark would be wasteful because they wouldn't be able to photosynthesise. Cell walls also glue neighbouring cells together, and so changes in the walls can dictate whether or not a plant tissue softens, e.g. in ripening fruit. Walls of young plant cells (those potentially able to grow) are built of several kinds of string-like structural molecules called polysaccharides. Two important ones are cellulose and xyloglucan. Our lab has been at the forefront of discovering, and understanding the role of, enzymes called xyloglucan endotransglucosylases (XETs). These catalyse a reaction in which the string-like xyloglucan molecules are cut and then re-joined. The cutting/re-joining process may be necessary (1) for new xyloglucan molecules to become bonded into the wall while it is being built, and (2) for an existing cell wall to loosen its structure (enabling cell growth). Work in other labs had highlighted another interesting class of polymers present in and around the plant cell wall. These are the arabinogalactan-proteins (AGPs), complex molecules made of carbohydrate and protein. They can bind to certain other carbohydrates, but any benefit of this binding so far as the plant is concerned is unknown. AGPs make up only a small proportion of the weight of the cell wall. They are not true structural wall components and are not enzymes; however, various experiments show that they somehow increase the ability of the wall to stretch and are thus important in enabling cell growth. We do not know how AGPs affect wall stretchability. However, our HYPOTHESIS is that AGPs act by increasing the rate at which XETs catalyse their cutting/re-joining reaction. We recently showed that some AGPs and related molecules can indeed promote XET action in the test tube. We will try to strengthen and extend the evidence for this hypothesis. Starting with cauliflowers and related plant tissues, we will purify and work out more details of the chemical structures of AGPs. We will also purify several different XETs from these tissues. We will then measure how fast each XET can catalyse its cutting/re-joining reaction in the presence of each different AGP. We will further test the new hypothesis by seeing whether Yariv antigens (chemicals that inactivate AGPs) stop XETs working in living plant tissues. We will also test whether, and how, AGP molecules bind to XETs and/or to xyloglucans. The knowledge gained would open the way to identifying genes that regulate the plant cell wall's properties, and thus in the future could give us the ability to control genetically or biotechnologically the growth and development of crops.