The role of gibberellin in the regulation of wheat grain development.

Food shortage is a serious problem in many parts of the world: nearly one billion people are undernourished, and the world population of 6.4 billion is increasing. It is therefore essential that we maintain the dramatic increases in global food production that have been achieved over that last half century. Cereal seeds (mainly rice, wheat and maize) account for 50% of the world's food supply and so research into ways to improve cereal yields is very important. Our laboratory is studying the plant hormone gibberellin (GA). GA is a natural compound, produced by all plants, that is involved in controlling many aspects of plant growth, including the germination of seeds, stem and leaf growth, and flowering. Until recently, there was little information on the role of GAs in controlling seed development. However, we have recently shown that increasing the amount of GA in a growing wheat seed (grain) can increase its size. These larger grain might have increased space for storing carbohydrate and protein - in fact, in a preliminary experiment we showed that these plants had a yield increase of up to 20%. In comparison, traditional plant breeding has averaged an increase of about 1% per year over the past few years. This project aims to understand the basis for this increase in size and weight, and to test whether the larger grains can consistently increase yields. We will therefore study the changes in the number and size of cells in the larger grains, to determine the effect of GA. We will also investigate how normal seeds produce GA and how this regulates grain growth. One unfortunate side-effect of increasing GA levels in wheat grain is that this causes a rise in the level of the enzyme alpha-amylase as the seed matures. This is unwelcome as this enzyme breaks down starch in the grain and makes the flour unsuitable for break-making. We will therefore look at other ways of changing GA levels in seeds so that we can achieve the larger grains without affecting grain quality.

The proposed research will investigate the processes regulated by gibberellins (GAs) in cereal grain development, using wheat as the experimental system. The results will indicate whether GAs play a role in determining grain yield and quality and may identify target genes for breeding for these characteristics. Existing transgenic lines with altered GA metabolism in the endosperm will be analysed for effects of transgene expression on grain morphology and in propensity for pre-harvest sprouting and pre-maturity alpha-amylase production, both factors which are potentially under GA regulation and which reduce flour quality. Rht dwarfing genes, which confer insensitivity to GA, will be introgressed into the GA-overproducing lines to confirm that a functional GA-signal transduction pathway is necessary for any phenotypic changes identified. Sites of expression within the developing grain of genes of GA biosynthesis and signal transduction will be determined using reporter genes, in situ hybridisation and laser capture microdissection coupled with real time RT-PCR. Gibberellin 20-oxidases (GA20ox) are highly regulated enzymes of GA biosynthesis in many species, including wheat, and their activity are major determinants of the rate of GA production. At least two GA 20-oxidase (GA20ox) genes are expressed in developing wheat grain in a tissue-specific manner, indicating that they may control different developmental processes. This will be investigated by inactivating the GA20ox gene family members by mutagenesis in the diploid species Triticum monococcum.