Speed breeding for multiple quantitative traits in durum wheat

Abstract

Background: Plant breeding requires numerous generations to be cycled and evaluated before an improved cultivar is released. This lengthy process is required to introduce and test multiple traits of interest. However, a technology for rapid generation advance named 'speed breeding' was successfully deployed in bread wheat (Triticum aestivum L.) to achieve six generations per year while imposing phenotypic selection for foliar disease resistance and grain dormancy. Here, for the first time the deployment of this methodology is presented in durum wheat (Triticum durum Desf.) by integrating selection for key traits, including above and below ground traits on the same set of plants. This involved phenotyping for seminal root angle (RA), seminal root number (RN), tolerance to crown rot (CR), resistance to leaf rust (LR) and plant height (PH). In durum wheat, these traits are desirable in environments where yield is limited by in-season rainfall with the occurrence of CR and epidemics of LR. To evaluate this multi-trait screening approach, we applied selection to a large segregating F₂ population (n = 1000) derived from a bi-parental cross (Outrob4/Caparoi). A weighted selection index (SI) was developed and applied. The gain for each trait was determined by evaluating F₃ progeny derived from 100 'selected' and 100 'unselected' F₂ individuals.

Results: Transgressive segregation was observed for all assayed traits in the Outrob4/Caparoi F₂ population. Application of the SI successfully shifted the population mean for four traits, as determined by a significant mean difference between 'selected' and 'unselected' F₃ families for CR tolerance, LR resistance, RA and RN. No significant shift for PH was observed.

Conclusions: The novel multi-trait phenotyping method presents a useful tool for rapid selection of early filial generations or for the characterization of fixed lines out-of-season. Further, it offers efficient use of resources by assaying multiple traits on the same set of plants. Results suggest that when performed in parallel with speed breeding in early generations, selection will enrich recombinant inbred lines with desirable alleles and will reduce the length and number of years required to combine these traits in elite breeding populations and therefore cultivars.

Keywords: Crown rot; Drought adaptation; Fusarium; Leaf rust; Root architecture; Segregating populations; Speed breeding; Trait pyramiding.

Fig. 1

The breeding strategy for applying selection in early segregating generations to reach superior inbreds in a period of 11 months. The figure highlights the crossing parents and further generations where a weighted SI was used. The blue colour indicates generations where the phenotyping was conducted. The green coloured generations indicate the generations subject to growth under speed breeding for the entire cycle without selection

Fig. 2

Visual summary of a generation from sowing to harvest using the multi-trait phenotyping procedure: a seeds sown in the clear-pot, b seminal root image analysis, c plants inoculated with leaf rust using airbrush method, d plants inoculated with Fusarium crown rot, e plant height measured using a barcode reader, and f whole-pot view at the time of crown rot assessment during the grain filling stage

Fig. 3

Leaf rust response on the flag leaf for parental genotypes (Outrob4 and Caparoi) and standards (Thatcher and Thatcher + Lr34)

Fig. 4

Frequency distribution for the F₂ segregating population for root angle (a), crown rot (b), root number (c) and leaf rust (d). Calculations of parental line means and confidence intervals (95%) are displayed for each trait to highlight the individuals with higher or lower values in comparison to the parents (bi–directional transgressive segregation)

Fig. 5

Density distribution of the weighted selection index values for selected, unselected and the entire F₂ generation (F2). Selection index values are representative of the sum of all traits simultaneously (RN, RA, LR, CR and PH)

Fig. 6

Density distribution and comparison of population means for selected and unselected F₃ population sets for the following traits: a root angle, b root number, c crown rot severity, d leaf rust severity and e plant height. Each set includes 100 F₃ families