Waiting for fine times: genetics of flowering time in wheat

To maximise yield potential in any environment, wheat cultivars must have a n appropriate flowering time and life cycle duration which 'fine-tunes' the life cycle to the target environment. This in turn, requires a detailed kn owledge of the genetical control of the key components of the life cycle. T his paper discusses our current knowledge of the genetical control of the t hree key groups of genes controlling life-cycle duration in wheat, namely t hose controlling vernalization response, photoperiod response and developme ntal rate ('earliness per se', Eps genes). It also discusses how our abilit y to carry out comparative mapping of these genes across Triticeae species, and particularly with barley, is indicating new target genes for discovery in wheat. Major genes controlling vernalization response, the Vrn-1 series , have now been located both genetically and physically on the long arms of the homoeologous group five chromosomes. These genes are homoeologous to e ach other and to the vernalization genes on chromosomes 5H of barley and 5R of rye. Comparative analysis with barley also indicates that other series of vernalization response genes may exit on chromosomes of homoeologous gro ups 4 (4B, 4D, 5A) and 1. The major genes controlling photoperiod response in wheat, the Ppd-1 genes, are located on the homoeologous group 2 chromoso mes, and are homoeologous to a gene on barley chromosome 2H. Mapping in bar ley also indicates a photoperiod response locus on barley 1H and 6H, indica ting that a homoeologous series should exist on wheat group 1 and 6 chromos omes. In wheat, only a few `earliness per se loci have been located, such a s on chromosomes of homoeologous group 2. However, in barley, all chromosom es appear to carry such loci, indicating that several series of loci that a ffect developmental rate independent of environment remain to be discovered . Overall, comparative studies indicate that there are probably twenty-five loci controlling the duration of the life-cycle, Vrn, Ppd and Eps genes, t hat remain to be mapped in wheat. There are major gaps in our knowledge of the detailed physiological effects of genes discovered to date on the timin g of the life cycle from different sowing dates. This is being addressed by studying the phenology of isogenic and deletion lines in both field and co ntrolled environmental conditions. This has indicated that the vernalizatio n genes have major effects on the rate of primodia production, whilst the p hotoperiod genes affect the timing of terminal spikelet production and stem elongation, and these effects interact with sowing date.