Take home message

Many Australian wheat cultivars are heat tolerant. However, new materials developed from extensive diversity using field-based phenotyping and genomic selection show that the heat tolerance of Australian wheat can be significantly improved.

Aim

The work was conducted to improve the heat tolerance of Australian wheat. The research aimed to develop heat tolerant wheat germplasm, protocols for high-throughput field-based screening and molecular tools to assist commercial wheat breeders.

Introduction

Periods of extreme high-temperature, particularly short periods of heat shock, are a major threat to wheat yield and grain quality throughout much of the Australian wheat belt.  Current projections of Australian climate change indicate that heat waves and temperature variability will become more frequent and more intense in the coming decades (CSIRO 2011, Climate Change in Australia. http://climatechangeinaustralia.com.au). It is vital that new wheat germplasm with improved high-temperature tolerance and molecular tags linked to this tolerance are developed and introduced into commercial breeding programs.

Genomic selection is a breeding method that requires a reference population of wheat lines that are phenotyped for the trait of interest and genotyped using many DNA markers distributed across the whole genome.  Statistical methods are then used to estimate the effect of each DNA marker on the phenotype; the collection of all these DNA marker effects provides a prediction of genomic breeding value. This information can then be used to predict new plants that are only genotyped and do not have a phenotype. This allows early selection of plants/lines without phenotyping which decreases the breeding cycle leading to increased genetic gain.

What did we do?

A highly diverse set of agronomically adapted materials were assembled for phenotyping. These included thousands of new lines developed by the University of Sydney, including crosses with synthetic wheat, emmer wheat collected in warm areas, landraces, adapted germplasm with putative tolerance identified in hot wheat growing areas globally and Australian wheat cultivars and other sources of heat tolerance developed by others.

These materials were phenotyped for various traits including yield using a three-tiered strategy. Firstly, thousands of lines were evaluated in the field in replicated yield plots at Narrabri in northwestern NSW at different time of sowing. Later sown materials were exposed to greater heat stress. Subsets of materials, based on performance in the previous year and estimated genetic values, were sown at sites in Western Australia (Merredin and Cadoux) and Victoria (Horsham) at 2-3 times of sowing to assess the transferability of traits. Each year, high performing lines were retained from the previous year, intolerant materials removed, and new materials added. Materials identified as heat tolerant in times of sowing experiments were subsequently evaluated in the field using heat chambers set at 4˚C above the ambient temperature to induce heat shock during reproductive development and grain filling to confirm heat tolerance. Finally, those lines that maintained heat tolerance in the heat chambers were screened in temperature-controlled greenhouses to assess pollen viability under heat stress. Materials surviving all three stages of testing were considered highly heat tolerant.

All materials (>6,000 lines) phenotyped in time of sowing experiments were genotyped using a 90K SNP platform and these formed the reference population for genomic selection from which all DNA marker effects were estimated. A prediction equation was developed and used to calculate genomic estimated breeding values (GEBVs) on selection candidates which were genotyped but not phenotyped. A genomic selection model that incorporated environmental covariates (e.g. temperature, radiation, rainfall) directly was developed and improved. This allowed the prediction of line performance under high temperature conditions. Environmental covariates were defined for each plot and growth development phase (vegetative, flowering, and grain fill).  An in-field validation of GEBV selected lines was then conducted by correlating GEBVs with field trial phenotypes. Various cycles of crosses were made among diverse lines with high GEBVs and progeny subsequently selected for high GEBV. These formed the basis of our new elite heat tolerant materials.

What did we find?

Extensive field-based phenotyping over a 6-year period identified lines with superior adaptation to terminal heat stress. Many of the superior materials had high yield under heat stress, low percentage screenings and high kernel weights. However, stay-green was not an advantage and only an intermediate level of glaucousness was linked to higher yield under stress (Tables 1 and 2). (Glaucous leaves are covered with a grey/blue or whiteish waxy coating that is easily rubbed off). Materials with a wide range of GEBVs were identified and recombined in crosses to produce new heat tolerant lines with higher heat tolerance than current cultivars (Figure 1). The prediction accuracy of genomic selection using models trained at Narrabri was assessed in other environments around Australia (Table 3). The predictions were moderate indicating that phenotyping in Narrabri was relevant nationally.

Table 1. Influence of stay-green on yield in early and late sowing (576 genotypes) at Narrabri

Means in rows followed by different letters are significantly different at the probability indicated

Table 2. Impact of Glaucousness on yield at early and late sowing (576 genotypes) at Narrabri

Means in rows followed by different letters are significantly different at P<0.05

Figure 1. Genomic estimated breeding values (GEBVs) for yield of a subset of the most heat tolerant breeding lines and Australian cultivars (approx. 7,000 genotypes). Main season and late sowing (For PBR status of varieties in graph please refer to Table 5)