Breeding wheat for drought adaptation: Development of selection tools for root architectural traits

Abstract

A crop's ability to explore the soil profile and extract available water at different depths is largely determined by root system architecture. For instance in wheat (Triticum aestivum L.), it has been suggested that a narrow and deep root system can provide better access to deep soil moisture. Such root systems are particularly beneficial for rain-fed regions where crops rely heavily on stored soil moisture at depth, as encountered in the eastern Australian wheat belt. Thus, by targeting desirable root architectural traits, wheat breeders could increase genetic gain for yield in response to the growing demand for food. Yet, selection for these below-ground traits is challenging because roots are difficult to measure and are under complex genetic control. The aim of this project was to develop new phenotypic and molecular selection tools to facilitate selection for root architectural traits in Australian wheat breeding programs targeting terminal moisture stress adaptation. This project focuses on narrow seminal root angle and high number of seminal roots in wheat seedlings; two proxy traits for desirable mature root system architecture. Firstly, to overcome the lack of efficient root screening methods, a high-throughput and cost-effective method for phenotyping seminal root angle and number in wheat was developed, using clear pots in a controlled environment growth facility. Compared to pre-existing phenotyping methods, the newly developed method successfully provided higher heritability, greater repeatability, and better efficiency in terms of time, space, and labour. Further, the clear-pot method revealed a high degree of phenotypic variation for both seminal root traits. Subsequently, to test the ability to introgressed allelic variation for seminal root angle into elite Australian wheat cultivars via phenotypic selection, backcross tail populations for both narrow and wide root angle were developed, using the clear-pot method. Rapid shifts in both population distribution and allele frequency were observed after just two rounds of selection. Further, comparison of the tail populations revealed some genomic regions under selection, for which marker-assisted selection appeared successful. Hence, genetic diversity can be exploited via phenotypic and molecular selection to target desired root system architecture in wheat breeding programs.

A robust field-based method to screen heat tolerance in wheat

“…A set of eight CIMMYT genotypes described as heat tolerant under Australian environments was obtained from the University of Sydney (Thistlethwaite et al, 2020). Other genotypes used in these trials included donors of a multi-reference parent nested association mapping (MR-NAM) population developed for screening for heat and drought tolerance in wheat (Christopher et al, 2015(Christopher et al, , 2021Richard, 2017). In total, 35 wheat genotypes were tested in PEM and plot trials, with 32 genotypes at each trial, except for the PEM trial with quadrat harvest in 2020 when only 20 selected genotypes were used (Table S1, Supplementary).…”

Integrating genomic selection and speed breeding to increase genetic gain in spring wheat (Triticum aestivum) breeding

“…A 256-line subset of a spring wheat (Triticum aestivum L.), multi-reference parent NAM population, previously developed by Richard (2017), was used. This subset consisted of ten elite lines, termed 'donors', crossed with the cultivar Suntop, as the female parent, to create ten bi-parental families with a common background, termed the Suntop NAM population.…”

“…In addition to the Suntop NAM, two other multi-reference parent NAM populations were used to enable prediction across families. Scout and Mace were the background parents of these populations, which were also developed by Richard (2017). Details of these population can be found in Table 4-4.…”

“…Parental lines of all three NAM populations were chosen based on several traits considered desirable for the diverse conditions of the Australian wheat belt, including heat tolerance, certain pest and disease resistances, stay-green characteristics and dense root systems. Further detail can be found in Table 4-2, adapted from Richard (2017), which includes the type of line, developing company or country and pedigree of each parent.…”

“…This enables simultaneous analysis of complex traits in a range of genetic backgrounds and consequently, results are applicable to a larger range of breeding programs. The multi-reference parent NAM population developed by Richard (2017) provides such a resource for spring wheat. Additionally, results of recent multi-location field trials were available to be used to determine GS prediction accuracy with multivariate models, both within and across families and trials.…”