The molecular basis of quantitative variation in photoperiod response

Plants need to flower when the chances of successful pollination and seed development are highest. To achieve this, most plants flower at a specific time of year. Timing is often achieved by using cues from the environment, particularly temperature and day length (photoperiod). Cereal crops use these cues but the way they use them has been changed by domestication and plant breeding in order to develop varieties that perform well in the diverse farming environments that occur around the world. We know that wheat varieties from different parts of the World, and even from different parts of Europe, differ in how they respond to photoperiod. We have recently made a breakthrough in understanding how they do this by identifying a key gene. In this grant we will study how this gene works and how differences at the DNA level translate into differences in the way the whole plant grows. The photoperiod (Ppd) gene is a member of a family called the pseudo-response regulators. In the model plant Arabidopsis thaliana (thale cress) these genes have been show to be part of, or closely associated with, the circadian clock which is the internal time keeper that is used to measure day length and to control much of the plant's metabolism. Fortunately, many of the other genes that are used to control flowering by day length (the photoperiod pathway) are well conserved in different plant species. This means that we can use information from Arabidopsis to identify the components of the photoperiod pathway in wheat. By studying the photoperiod gene and the other components of the photoperiod pathway we will be able to understand how photoperiod response is altered in wheat. Bread wheat is a hexaploid plant, which means it is a natural hybrid comprising the genomes of three diploid ancestor species. Each of the three genomes can contribute flowering time variation and an important question is how the genes on the three genomes are integrated and regulated. We can now cross-pollinate plants to build up single, double and triple combinations of early or late flowering variants. Understanding how these behave will give valuable new insights into the control of flowering in wheat and this will allow plant breeders can select a range of adaptation suitable for diverse environments. Understanding how genes work in combination is also of relevance to other wheat traits apart from flowering. To explore ideas about how the photoperiod genes work we will test different versions of the genes using transformed barley plants. This provides a simple test system to investigate which regions of the gene are most important. Understanding how crops respond to environmental cues like day length is important because it will allow plant breeders to tailor varieties to particular environments and hence to maximize productivity. It will also assist the development of new varieties tailored to new conditions that are arising from climate change.

Variation in photoperiod response is widespread in wheat and contributes significantly to adaptation and productivity. Early flowering (photoperiod insensitive) types predominate in environments with hotter, drier summers. These conditions are predicted to become prevalent in Northern Europe by climate change models, making it important to understand how photoperiod response can be manipulated. The major photoperiod (Ppd) loci in barley and wheat lie in colinear positions on the group 2 chromosomes, suggesting they are homologous genes. Using the previously cloned barley gene (a member of the pseudo-response regulator family) we isolated the equivalent genes from wheat. Photoperiod insensitive mutants on 2A and 2D had upstream deletions. Tests of the 2D allele showed altered PRR gene expression and activation of the floral regulator FT in short or long days. We will use novel single, double and triple combinations of gain and loss of function alleles to understand their interaction, the coordination of expression from different wheat genomes and the basis of the quantitative variation in flowering time that different combinations of Ppd alleles provide. The results will be important for flowering and of general significance for understanding how wheat and other polyploids behave. We will identify the mutation causing photoperiod insensitivity on 2B. This must be different to 2A and 2D but one possibility is that the 2B mutation is an epiallele with variation in methylation affecting the region deleted on 2A and 2D. We will test hypotheses about gene function using transgenic barley plants, exploiting this genetically simple system and the presence of a loss of function ppd-H1 mutation in the readily transformable variety 'Golden Promise'. We will confirm the effect of synthetic deletions matching those in wheat and test the deletion of smaller regions to establish the key regions affecting flowering time.