Improving nitrogen use and yield with stay-green phenotypes in wheat

Wheat grain yield is strongly related to nitrogen (N) fertiliser input, a major cost factor and potential environmental pollutant. Much of the grain N requirement is met by N remobilisation from the canopy. Unfortunately, a consequence is canopy senescence and decreased photosynthetic capacity, reducing carbon available for grain-filling. One approach to achieve both higher N use efficiency and grain yield would be to extend the duration of photosynthesis using delayed leaf senescence “stay-green” phenotypes. Three stay-green and two fast-senescing EMS mutants of wheat (cv. Paragon) were characterised. A fast-senescing line, a stay-green line and the wild-type were grown to characterise the interaction between senescence and N availability. Stay-green line SG3 was able to allocate similar proportions of N to the grain under N-limiting and N-sufficient conditions. The accelerated senescence of line FS2 reduced grain yield and N allocation to the grain. Candidate regulatory genes of leaf senescence genes were characterised by correlating their expression with leaf senescence by screening wheat genotypes with varied senescence characteristics in the field. Among the genes were members of the WRKY and NAC transcription factor families that have been related to senescence. Overexpression of the NAC gene resulted in a stay-green phenotype and increased grain N concentrations, but had no effects on shoot biomass or grain yield. Expression of a WRKY-RNAi construct did not reduce WRKY mRNA levels, but led to accelerated leaf senescence and increases in plant height, the number of fertile tillers and grain yield. These results show that the relationships between senescence, nitrogen remobilisation and grain yield are complex and not easily manipulated. The phenotypes and genes identified could contribute to wheat improvement