Screening of early root growth characteristics in seedlings of Nordic spring barley : utilization of hydroponics and GWAS for identification of QTLs and candidate genes for root traits
Abstract: In the Nordic countries, the cultivation of barley (Hordeum vulgare L.) has a long history, and as one of the major cereals in the world, barley is of global importance for food security. The world population is growing, which naturally will increase demands for agricultural outputs. Simultaneously, there is a need for limiting agricultural caused environmental footprints, including excessive fertilizer usage. It is as well projected that climate change will create challenges for global food security, with higher frequencies of extreme climate events with negative impacts on the agricultural sector. In this light, plant breeding for adapted crop varieties is recognized to be important for maintaining stable yields. Traditionally, breeding efforts have mostly been focusing on improving above-ground plant traits. Although, in later years the advantages of including root architecture traits in breeding programs have been highlighted, e.g. for enhancing resource efficiency, stress tolerance, and plant adaptation to unstable climates. Previous studies have shown that early root growth can correlate with enhanced nutrient uptake and yield formation. In this thesis, a low-tech hydroponic method was used for screening for root dry weight (RootW) and seminal root length (RootL) in seedlings of spring barley. The plant material comprised 259 genotypes of spring barley, from a Nordic PPP–project, including modern lines as well as historic cultivars and landraces, pre-dominantly from the Nordic countries. Moreover, a Genome-Wide Association Study (GWAS) was performed to examine the underlying genetics of the comprised traits. The root traits were successfully phenotyped using the hydroponic screening method, and in total 23 significant QTLs (quantitative trait loci) associated with the two traits were identified with GWAS. When comparing the result with other association-studies comprising root-traits in spring barley, several of the QTLs in this thesis were located close to QTLs located in those studies. Besides, some candidate genes mentioned in related studies, associated with RootW and RootL, were found in the vicinity of obtained QTLs in this thesis. The statistical analyses, considering correlations between the replicates and the Analysis of Variance (ANOVA), gave support for the results regarding RootL, but the correlations were lower for RootW and a significant effect of the replicates was shown. While comparing differences in early root growth between historic cultivars/landraces and modern breeding lines, a trend was observed where the modern lines generally had higher values for RootW and RootL. Possible reasons for this are discussed. Overall, the low-tech hydroponic screening method proved to be useful for screening RootL of many genotypes in a short time, which is a trait that previously has been shown to correlate with nitrogen uptake and yield. This by itself could motivate its usefulness. Moreover, the simplicity of the method compared to other more high-tech solutions might make it useful in situations with a restricted budget, when the screening comprises many genotypes, and/or where modern screening systems are out of reach. Although, regarding RootW, the source of the more variable results must be located for the method to be useful. Other traits of early root growth that could have been interesting to assess are discussed.
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