Enhancing wheat field performance and response to abiotic stress with novel growth-regulatory alleles

The 'Green Revolution' that improved worldwide cereal yields from the 1960s was due to a combination of new varieties of wheat and rice and the increased use of nitrogen fertilisers and pesticides. An important feature of the new varieties was reduced height / although originally introduced to allow the plants to tolerate high levels of fertiliser without overgrowing and collapsing, it was discovered that these new semi-dwarf varieties increased yield, as less of the plants' energy was wasted on producing straw and more went into the harvested grain. We now know that these dwarf varieties of wheat carried a gene (called 'Reduced Height' or Rht) that made them unresponsive to the plant's own growth hormone, gibberellin (GA), which normally increases stem height. Similarly, the new semi-dwarf rice varieties contained a defective gene that resulted in reduced levels of the hormone. There are only two types of Rht gene that have been widely used in wheat, and predominantly one in UK varieties. Changes in climate, agricultural practise and possible restrictions in the use of growth-regulating chemicals may mean that wheat varieties containing this gene are no longer capable of producing the highest yield. Part of this programme is aimed at identifying new wheat genes that alter GA signalling and therefore have different effects on height. These genes could then be used to produce new dwarf varieties, either alone or in combination with the existing Rht genes. We have also discovered that dwarfing genes, that confer reduced height through changes in GA signalling, also protect plants against stresses such as drought, heat, or salt. This may become even more important as climate change reduces the amount of rainfall in wheat-producing areas. There is evidence that the existing Rht gene is not ideal for protecting wheat plants from stress. A second aim of this project will therefore be to test a range of genes affected GA signalling for their effectiveness in protecting plants from drought and other stresses. Finally, an important aim of this project is that the new dwarfing genes that we discover should be taken up by plant breeders to develop new commercial varieties. Therefore, we have developed a 'pre-breeding' strategy with the National Institute of Agricultural Botany to bring these new genes into modern, high-yielding varieties that can be passed on to breeders and used in their wheat breeding programmes.

The 'Green Revolution' dwarfing (Rht) alleles that increase wheat yields under high input conditions are orthologues of the Arabidopsis GAI gene and encode mutant DELLA proteins. DELLAs are repressors of plant growth that are degraded in the presence of gibberellin (GA) whereas the gai/Rht mutants are insensitive to GA. Most UK wheat varieties carry the semi-dwarfing Rht2 (Rht-D1b) allele but variation in height between genotypes suggests that other loci play a role in determining stature. We aim to identify these loci through co-localisation of quantitative stature traits identified in UK wheat germplasm with genes in the GA-DELLA pathway. Additionally, TILLING will be used to identify novel alleles of key genes from mutagenised populations of wheat. Based on functional analyses in vitro and performance in the field alleles will be selected for use in wheat breeding. There are reports of Rht mutations affecting the responses of wheat to stress, and a negative correlation between GA content or responsiveness and stress tolerance has been documented. Moreover, our recent work in Arabidopsis implicates the GA-DELLA pathway as a central regulator linking GA, abscisic acid and ethylene in a common stress-related network. It is timely to translate these key discoveries into crop improvement to enhance the tolerance of hexaploid wheat to environmental stresses without compromising productivity. To this end, we will take a knowledge-based approach to compare Arabidopsis and wheat DELLA-mediated stress responses. We will use available genetic stocks to determine whether existing, but relatively untested, Rht alleles affect tolerance to salt, drought and heat stress. Near-isogenic lines will be tested under controlled and field conditions to select alleles that will be taken forward by introgression into elite varieties. Novel alleles of GA-DELLA alleles identified by TILLING will also be assessed for effects on tolerance to drought, heat and other abiotic stresses.