Screening for costs of disease resistance caused by stomatal dysfunction

Wheat varieties combining high yield and good resistance against three of the main foliar diseases in the UK (Septoria tritici blotch, yellow rust and brown rust) have proved elusive. There is now significant evidence in the scientific literature that some disease resistance genes, introduced into varieties by conventional plant breeding, impose a yield penalty on the crop. Hence, breeding for disease resistance creates 'yield drag' which slows the rate of yield improvement. This acts as a disincentive for breeders to focus efforts on selection for resistance, so most commercially popular, high yielding wheat varieties are susceptible to foliar diseases. The result of this is that fungicides are routinely used to control important foliar diseases. Dependence on fungicides is associated with high input costs for the grower and strong pressure for the disease-causing pathogens to develop insensitivity to the fungicides used, reducing the number of fungicides that remain effective. The project proposed here will test important disease resistance genes for their effects on attainable yield. This is difficult to achieve in plant breeding programmes currently, because: (i) there are large numbers of genes to test, (ii) without careful experimentation, measurements of the yield loss caused by each gene are hidden by the yield benefit they provide via disease control, and (iii) testing requires production of wheat lines that differ for presence or absence of the resistance gene but are otherwise highly similar. This is important in order to rule out any effects on yield caused by other differences between the resistant and susceptible wheat lines. It would be useful to be able to select resistance genes which provide the benefit of disease control, without an associated yield cost. Recently, evidence has accumulated that the deleterious effects on yield may be caused by disease resistance responses in the cells of the leaf surface disrupting the function of adjacent stomata. Stomata are pores in the leaf surface that normally open during the day (to allow CO2 to enter the leaf for photosynthesis) and close at night (to prevent unnecessary water loss when the leaf is not photosynthesising). As a result of the stomatal dysfunction caused by the resistance response, they may fail to open fully during the day or fail to shut properly at night. The project proposed here will test the idea that measurements of stomatal function can be used to screen resistance genes, to identify those which are, or are not, likely to have deleterious effects on yield. This would allow wheat breeders to focus on introducing genes which are effective against foliar diseases and benign in their effects on the plant.

Recent evidence published from controlled environment (CE) studies demonstrates that host resistance responses to challenge by avirulent fungal plant pathogens can cause dysfunction of stomata adjacent to attempted infection sites. Subsequent field experiments, using near-isogenic lines (NILs) differing for presence/absence of resistance genes, have shown that the effects seen in the CE studies also occur in the field and suggest that stomatal dysfunction occurs with a wider range of cereal pathosystems and resistance genes than previously studied. The work proposed here will test the hypothesis that the yield 'cost' associated with certain resistance genes is caused by stomatal dysfunction. Plant breeders are supporting the project, as they require techniques to allow them to characterise novel resistance genes/QTL for the likelihood of an associated yield penalty, to inform decisions about introgression into their breeding material. If the hypothesis is supported, then stomatal conductance measurements could act as an indicator for physiological cost. Alternatively, such costs may be found to be associated with certain types of resistance response which can be characterised by microscopy (termed, microphenotyping). The specific objectives of the proposed LINK project are to: 1. Screen key disease resistance genes for yield costs. 2. Characterise disease resistance genes which contrast for presence or absence of yield cost for effects on stomatal function. 3. Relate stomatal dysfunction at a leaf level to impacts on radiation use efficiency at a canopy level and grain yield. 4. Test stomatal conductance as an indicator of yield potential in the light-limited environment of the UK. 5. Test improved porometry methods to increase screening throughput. The objectives will be addressed by a combination of CE experiments, field trials and microphenotyping, on NILs and lines from mapping populations of wheat, which contrast for key resistance genes.

Improving crop yield for a given level of crop inputs (principally: land, fertiliser, water and fossil fuel) benefits productivity and reduces environmental impact per tonne of grain. If 'defeated' major genes are found to carry a yield penalty, then selecting against them in plant breeding programmes will increase the rate of yield improvement, without affecting disease control. If currently important resistance genes/QTL are found to carry a yield cost then decisions will need to be made about the trade-off between yield and disease resistance (and hence the degree of dependence on fungicides), until they can be replaced by alternative sources of resistance with lower yield costs. The work will ultimately lead to wheat varieties which combine high yield and good disease resistance. Disease resistance is likely to be more important in future if the availability of effective fungicides is constrained by: (i) revised legislation regulating the approval and use of crop protection products, and (ii) evolution of insensitivity in pathogen populations to the remaining available modes of action. Improving the disease resistance of varieties offers the best prospect for reducing dependence on fungicides and minimising the selective pressure placed on pathogen populations for fungicide insensitivity. Therefore resistant varieties benefit growers by maintaining disease control options, as well as reducing the level of inputs required. High yielding varieties will be crucial to ensure grain production meets projected rising demand and to minimise pressure for land use change and maintain food security. Conversion of grassland and semi-natural vegetation into arable production has adverse consequences for biodiversity and for greenhouse gas emissions from carbon sequestered in soil. Thus, high yielding varieties result in benefits for the wider society.