Breeding wheat for drought adaptation: Development of selection tools for root architectural traits
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
... ble 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. Finally, to dissect the genetic controls of root traits, a multi-reference nested-association mapping wheat population was developed. In order to identify quantitative trait loci (QTL) relevant to Australian breeders, three genetic backgrounds relevant to the western, southern, and eastern production regions of the Australian wheat belt were used as references. Genome wide association mapping successfully identified a large number of QTL for seminal root angle and number, each with small to moderate effect. This improved understanding of the genetics controlling root traits provides opportunities for markerii assisted selection to combine desirable root traits for each of the three Australian megaregions for cereal production. Furthermore, we believe the strategy and outcomes of this project are transferrable to other wheat breeding programs, thus being beneficial not only for Australia, but also for developing countries experiencing similar terminal moisture stress, such as some Indian, South American, and African cropping regions.