Objective 9 - John Foulkes (UoN)

Improvement of water use efficiency and drought tolerance traits (UoN and JIC)

Background

Currently about 30% of UK wheat is grown on drought-prone land and drought losses are on average 1-2 t ha-1, which costs >�50M per year (Foulkes et al, 2002). With climate change mean summer rainfall is expected to decrease in southern and eastern UK by 20% by 2020 (Hulme et al, 2002), potentially increasing these losses. In addition to enhancing yield stability, improving water-use efficiency (WUE; above-ground biomass per mm of water extracted from the soil) will decrease crop water consumption in non-drought years. This will increase water returned to the hydrological system for reuse, conserving water resources for use in irrigating other crops, and increasing water flows in rivers and water levels in wetland areas, as well as aquifer recharge. The proposed research will identify key traits underlying improved water uptake, WUE and grain yield under drought. The underlying genetic bases of the key traits will be elucidated, because that information will greatly facilitate the deployment of the traits in improved germplasm. The identification of traits and markers to improve WUE and drought tolerance was not previously addressed in WGIN 1.
Under typical UK drought characterized by a grain yield between 5 and 7 t/ha, most rapid progress will depend on the introduction in high-yielding varieties of traits able to improve drought tolerance without detrimental effects on yield potential. We will test an optimal plant ideotype for durable drought resistance based on a combination of the following traits: (1) deeper rooting, (2) high accumulation and remobilization of stem soluble carbohydrate reserves to grains, (3) delayed senescence with the stay-green trait and (4) high water-use efficiency. Deeper more effective root systems will be important in avoiding the effects of drought. Increasing the root to shoot dry matter ratio could have deleterious effects on yield potential, but a relatively deeper distribution of roots may help, since root length density is often below a critical threshold for potential water capture of ca. 1 cm root cm-3 (Barraclough et al., 1989) at lower depths in the rooting profile (Ford et al., 2006). The improvement of grain-filling capacity under terminal drought stress can be achieved by increasing the mobilization of the vegetative carbohydrate reserves (mainly stored as fructans) from stems to grains. Stay-green plants are characterized by a post-flowering drought tolerance phenotype that gives plants resistance to premature senescence when subjected to drought during grain-filling. In our recent work we have shown a positive correlation between each of stem soluble carbohydrate measured shortly after flowering and the stay-green trait and grain yield under drought amongst UK winter wheat cultivars (Foulkes et al., 2002) and DH lines (Foulkes et al., 2007). Additionally, it will be important to optimize WUE (biomass / crop evapotranspiration) in new drought-tolerant cultivars, particularly to identify sources of high WUE in which growth is unaffected in unstressed conditions. The value of the above traits for contributing to a new drought tolerant ideotype in UK winter wheat will be quantified in the proposed work under activity 9. The proposed activities will be closely co-ordinated with those of LINK project LK0986 �Improving water use efficiency and drought tolerance in UK winter wheats.� Cross-linkages with LK0986 will be established in year 1 to guide the selection of the most appropriate varieties and DH population for study in the experiments and the choice of plant material for the development of new plant resources (one DH mapping population and one drought-tolerance diversity germplasm collection).

Activities

1. To identify the physiological traits explaining improved water-use efficiency and drought tolerance in elite winter wheat varieties.
2. To identify robust QTLs for water-use efficiency and drought-tolerance traits using one existing DH population in an elite background.
3. To develop one new DH population in an elite modern background segregating for drought-tolerance traits.
4. To identify novel genes and alleles controlling water-use efficiency and drought tolerance using the AE Watkins and Gediflux collections.
5. To collate a diverse germplasm collection (cultivars, advanced lines) from worldwide drought-tolerance wheat breeding programmes as a resource for future association genetics studies.

Work plan

i) Trait Identification.
In each of 2009/10 and 2010/11, six varieties (identified according to results of LK0896 as contrasting for drought performance) will be characterized more extensively for underlying physiological traits explaining the differences in drought tolerance. In the field trials carried out in LK0986, WUE (21 varieties) and grain yield and flag leaf CID were quantified for a wide range of varieties (118) in each of 2 years in small plot trials. In the current experiments detailed physiological analysis will be carried for the 6 contrasted varieties to fill in the gaps on underlying traits and inform on the physiological basis of the established differences in drought performance.
In each year, the 6 varieties will be tested under irrigated (trickle irrigation) and unirrigated (rain-fed) conditions in three replicates in one field experiment on a sandy loam drought-susceptible site at University of Nottingham (UN). In each experiment, combine grain yield will be assessed in all plots in at least a 5 m2 area, and yield components determined from analysis of 100 fertile shoots sampled pre-harvest. Dry matter growth and partitioning will be assessed at key developmental stages through the season. The percentage water soluble carbohydrate (WSC) of stems and attached leaf sheaths will be estimated by chemical analysis (anthrone method) at GS61+10d. The percentage of leaf lamina area remaining green will be assessed for the flag-leaf, the penultimate leaf (leaf 2) and leaf 3 from a visual in situ assessment of all fertile shoots per plot at 3-4 day intervals from anthesis to physiological maturity. On each occasion, visual scores ranging from 0-10 will be carried out using a diagnostic key in each replicate. For each leaf (1-3), data for the senescence score will be fitted against thermal time using the equation of Genard et al. (1999) with parameters which allow the estimation of the dates of start and end of senescence as well as the rate of senescence. WUE will be assessed by C-isotope discrimination (CID) of grain samples and through the linear relationship between water uptake (measured using capacitance probes) and biomass at sequential samples. Leaf activity traits (stomatal conductance and photosynthetic rate) will measured using a gas-exchange measuring system to estimate their relative contributions to genetic variation in WUE. Using the approaches described above, useful traits will be identified. Hypotheses will be set out in simple models relating water uptake, WUE and target traits to drought tolerance indices calculated according to yield in stressed and unstressed treatments. This will allow us to confirm the correct traits and the most appropriate DH population for the QTL detection under Activity 2.

Note: Data from LINK Project 0986 shared with activity 9 will be confidential until 2 years after the completion of the LINK project (April 2010). So the initial reporting of the results in activity 9 will use codes for variety names until April 2012, at which point the variety names will be released in the public domain and available to WGIN stakeholders.

ii) QTL detection in one elite DH population.
In previous work on the Beaver x Soissons DH population we have identified QTLs associated with drought resistance including WUE (CID of grain), stem carbohydrate reserves and the stay-green trait (Foulkes et al, 2007; Kumar, 2006; Verma et al, 2004). However, precise mapping of QTLs was complicated due to segregation for Ppd and Rht genes controlling adaptive traits (flowering time and height, respectively). So we will undertake the precise mapping of QTLs for WUE and drought-tolerance traits in one DH mapping population representing elite material but with reduced segregation for background adaptive traits. The DH population will be selected from those currently available with public access (e.g. Avalon x Cadenza, Spark x Rialto, Charger x Badger, Buster x Charger, Buster x Hereward) informed by the trait analysis under Objective 1 and data from the LK0986 project. Precision phenotyping will be carried out in one field experiment in each of 2010/11 and 2011/12 at each of UN (irrigated and unirrigated treatments) and JIC (unirrigated treatment only) for 94 lines and the parents in two replicates. The measurements will come from Objective 1 but will likely include biomass, stay-green, stem carbohydrate reserves and WUE (grain CID). The molecular map for the selected DH population will be extended as necessary by deployment of the latest molecular marker technologies (DArT and SSR). The major QTLs identified will be provided as targets for the backcrossing programme at JIC under activity 2. Finally reference maps for rice and maize will be used to project the detected QTLs to identify candidate genes for traits for further investigation in future work.

iii) Development of one new DH population.
It is unlikely that genetic analysis of all the relevant drought-tolerance traits can be satisfactorily achieved utilizing one DH population. Therefore one new DH population will be developed with the choice of parents to be made as indicated by the steering group of the LK0986 project according to LK project data. The breeders involved in the LK0986 project will make 25 tentative crosses in 2008/9 from promising elite cultivars contrasting for drought performance according to LK0986 data and donate the 25 F1s to activity 9. JIC will then develop in 2009/10 one DH population from the F1 ultimately selected according to ongoing analysis in LK09086. The population will be genotyped with DArT at JIC and be available as a public resource to the reduce lead-in time for satellite projects focused on the genetic analysis of drought-tolerance traits.

Note: Data from LINK Project 0986 shared with activity 9 will be confidential until 2 years after the completion of the LINK project (April 2010). So in the initial reporting of the development of the DH population we will use codes for the parental names until April 2012, at which point the parental names will be released in the public domain and available to WGIN stakeholders.

iv) Identifying novel gene and alleles controlling WUE and drought tolerance using the AE Watkins and the �improved� Gediflux collections.
In addition to identifying traits and QTLs in elite UK germplasm, it will be important to identify novel genes and alleles for WUE and drought tolerance from wider germplasm resources developed in WGIN 1/2. This will be achieved through two approaches.

a. Screening the AE Watkins and the �improved� Gediflux collections:
In 2008/9 and 2009/10, the AE Watkins and the �improved� Gediflux collections will be screened using visual traits (leaf green area and leaf rolling). Association genetics approaches will then be implemented to locate regions of the genome influencing drought-tolerance traits using the Dart genotyping of the Gediflux and AE Watkins collections carried out at JIC under activity 5.

b. Analysis of mapping population(s) to be developed from Watkins collection:
A small number of specific lines from the AE Watkins collection will be identified with high expression of drought-tolerance traits based on the phenotypic screens in years 1 and 2. One or more of these will be crossed with Paragon and mapping population(s) developed at JIC by single seed descent or doubled haploid population(s) of 94 lines under activity 6. The population(s) will be genotyped with DArT at JIC and will be subsequently phenotyped to identify useful novel genes and alleles for WUE and drought-tolerance traits.

v) Creation of diverse germplasm collection for drought tolerance association genetic studies
We will collate a collection of diverse germplasm from worldwide resources (hexaploid wheats and synthetic wheats) from worldwide breeding organisations with drought tolerance programmes. For example: wheat genotypes identified with high WUE and drought-tolerance traits from CIMMYT, Mexico; Marton-v�s�r, Hungary, University of Adelaide, Australia; CAS, China; and those donated by the breeders from their UK and European programmes. Seed stocks of 200+ lines will be available as a resource for future work to explore the natural variation for specific traits and for future association studies at the end of the project.

The activities under objective 9 will be closely co-ordinated with those addressing the integrated trait analysis for optimization for multiple stresses (drought, restricted N availability and take-all disease), as outlined in activity 13 (Interconnections between the three soil-based explored traits).

Barraclough, P.B., Kuhlmann, H., Weir, A.H., 1989. The effects of prolonged drought and nitrogen fertiliser on root and shoot growth and water uptake by winter wheat. J. Agron. Crop Sci. 163, 352-360.
Ford, K. E., Gregory, P. J., Gooding, M. J., Pepler, S. 2006. Genotype and fungicide effects on late-season root growth of winter wheat. Plant Soil 284, 33-44.
Foulkes, M.J., Scott, R.K., Sylvester-Bradley, R., 2002. The ability of wheat cultivars to withstand drought in UK conditions: formation of grain yield. J. Agric. Sci. Camb. 138, 153-169.
Foulkes, M.J., Sylvester-Bradley, R, Weightman, R. & Snape, J.W. (2007). Identifying physiological traits associated with improved drought resistance in winter wheat. Fields Crop Research 103:11-24.
Genard, M., Reich, M., Lobit, P. & Besset, J. 1999. Correlations between sugar and acid content and peach growth. Journal of Horticultural Science & Biotechnology 74, 722-776.
Hulme M, Turnpenny J, Jenkins G (2002) Climate Change Scenarios for the United Kingdom: The UKCIP02 Briefing Report. Tyndall Centre for Climate Change Research, School of Environmental Sciences, University of East Anglia, Norwich, UK. 14 pp.
Kumar A (2006) The physiological and genetic basis of water-use efficiency in winter wheat. PhD Thesis, University of Nottingham, Nottingham, UK, 238 pp.
Verma V, Foulkes MJ, Caligari P, Sylvester-Bradley R, Snape J (2004) Mapping quantitative trait loci for flag leaf senescence as a yield determinant in winter wheat under optimal and drought-stressed environments. Euphytica 135, 255-263.