Root system architecture screening in sorghum differs from other cereal crops
Individual root growth patterns, including tropism, branching and elongation rates are determined by underlying genetics, contributing to the architecture of the overall system (Rich et al., 2020). The passage of roots through the soil is also constrained by soil physical and chemical conditions, with many biotic and abiotic factors contributing to the final architecture, including gravity, temperature, water, oxygen and nutrient availability, and soil biota (Rich and Watt, 2013). Changes to the mature (or near-mature) architecture will also be driven by the timing and initiation of lateral and adventitious roots under the control of nutrient and hormonal signals (Atkinson et al., 2014). The efficiency of soil water uptake is determined by the root system because roots are usually the site of the greatest resistance in the pathway for liquid phase movement of water through the soil-plant-atmosphere continuum (Kramer and Boyer, 1995).
High-throughput phenotyping of RSA traits is of major importance to elucidate the underlying genetic control and to incorporate such traits in breeding programs through marker-assisted selection. Recently designed high-throughput root phenotyping platforms are an opportunity for further improvement of crop productivity, particularly in moisture-limited environments. Various high-throughput RSA phenotyping systems have been designed to screen seminal roots at the seedling stage i.e. clear pot and growth pouch methods in wheat (Richard et al., 2015) and clear-pot phenotyping method in barley (Robinson et al., 2016). However, these methods are less relevant for sorghum since seminal roots do not constitute the major proportion of the mature root system. Hence, Singh et al. (2010) developed a new root chamber-based method to screen nodal root angle in sorghum. While the assumption is that seedling screens for root architecture will be correlated with mature root systems, this is not always the case (Rich et al., 2020). However, it was reported that narrow primary root angle was found to be associated with deep field roots, providing some evidence that screening for narrow root angle at the seedling stage is worthwhile (Rich et al., 2020).
Correlations between seedling and mature root traits in the field have been shown to be inconsistent from site to site and year to year in wheat (Rich et al., 2020). This could be due to the influence of soil physical, chemical, and edaphic factors on root traits. However, Singh et al. (2012) reported that root angle of young sorghum plants can be a useful selection criterion for specific drought adaptation. The Singh et al. (2012) study also reported that relationships between root angle screening and mature field traits were best achieved using soil rather than germination paper and agar in the initial screening. Hence, exploiting RSA variability and identifying genomic regions controlling the variability could open avenues for sorghum breeders to develop more drought adapted varieties through manipulating root architecture. In particular, the extent to which screening for root angle will help sorghum breeders to develop more drought adapted varieties will depend on the strength of the correlation between RA at seedling stage (6th fully expanded leaf stage) and the mature root system architecture. RSA may play a role in stay-green during post-flowering moisture-stress conditions. For example, narrow root angle increases access to water stored deep in the soil during the post-flowering period compared to wider root angles in wheat (Manschadi et al., 2006) and sorghum (Singh et al., 2012). Therefore, narrow root angle may contribute to a stay-green phenotype, although this will depend to some extent on genetics (e.g. canopy size), management (e.g. row spacing, fertilization) and environment (e.g. soil type, water availability).
Genetic Variation For Diverse Rsa Traits Identified In Ethiopian Sorghum
In this study, genetic materials were collected from all sorghum producing regions in Ethiopia to capture the genetic diversity. Hence, significant variability for RSA traits was identified. The study also revealed high repeatability and a broad range of variation for individual root traits, in particular, for root angle. Nodal root angle showed wider variation and higher repeatability than other root-related traits, suggesting that the variation observed in this trait was under genetic control. The high repeatability of root angle in this study (78%) is consistent with previous studies in sorghum (Mace et al. 2012; Joshi et al. 2017), which reported high repeatability in root angle measurements (73.7% and 96%, respectively), while Singh et al. (2011) also observed moderate repeatability (46.6%) in nodal root angle in sorghum. The presence of genetic variation with moderate to high repeatability suggests that nodal root angle could respond to selection in sorghum-improvement programs.
Root Angle Is Poorly Associated With Other Traits
Drought adaptation is of particular interest because it depends upon components associated with both water supply (e.g. root traits) and water demand (e.g. canopy traits). Root structural traits are important sub-component traits for extracting water and nutrients from soil. These traits might contribute positively or negatively to drought adaptation in association with other traits. Therefore, understanding the interaction between component traits could be helpful for breeders to decide whether to make direct or indirect selection for better yield under drought conditions.
In this study, a poor association between root angle and other RSA traits was observed. Similar results have been observed previously in sorghum (Singh et al. 2011; Mace et al. 2012), wheat (Sanguineti et al. 2007) and barley (Robinson et al. 2018). The finding of weak correlations between root angle and plant size traits also agrees with the results reported by Singh et al. (2011), which found that traits associated with plant size (root mass, shoot mass, and leaf area) had a very low correlation with root angle in sorghum. These weak associations suggest that root angle and plant size traits might be controlled by different sets of genes. Furthermore, the incorporation of root angle screening into a sorghum breeding program would be helped if root angle was not associated with plant size because such an association might inadvertently result in selection for early vigour (Singh et al., 2011). Therefore, nodal root angle selection could be one strategy for selection in the improvement of drought adaptation.
Genetic Control Of RSA Traits
GWAS has proven to be a powerful tool to understand the genetic control of quantitative traits in sorghum and other species (Huang et al. 2012; Morris et al. 2013; Tao et al., 2020; Tao et al. 2021). Using this approach, this study identified 22 QTLs for key RSA traits (root angle, length, dry mass and number) in Ethiopian sorghum germplasm. Among RSA QTLs, 15 were co-located with RSA QTL reported in previous sorghum studies using different genetic materials. These QTL were repeatedly reported with different genetic backgrounds, possibly indicating hotspot genomic regions for root development. The seven unique QTLs potentially represent novel genetic loci for RSA from Ethiopian germplasm.
The root traits investigated in this study have previously been associated with grain yield under water stressed conditions in different cereal crops (Mace et al. 2012; Ali et al. 2016; Robinson et al. 2018). Ali et al (2016) reported that total root mass and root length in maize were significantly positively correlated with grain yield under water deficit conditions in the field. A positive significant association between narrow root angle and grain yield was reported in sorghum by Mace et al (2012).
Root Angle Qtls Co-located With Previously Reported Stay-green Qtls
The majority of root angle QTLs (6/7) in this study co-located with previously reported stay-green QTLs under water-stressed conditions. For example, QERA7.6 co-located with a previously studied QTL (green leaf area 15 days after flowering) by Haussmann et al. 2002 who conducted the experiment during the post-rainy season at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT). These authors reported that the season was ideal for evaluating stay-green under terminal moisture-stress conditions and the crop was dependent almost entirely on stored soil moisture. Similarly, QERA6.4 co-located with a previously studied ‘green leaf area at maturity’ QTL under post-flowering moisture-stress conditions (Srinivas et al. 2009). Additionally, QERA6.5 and QERA8.7 co-located with previously studied green leaf area QTLs (QGLFA6.1 and QGLFA8.1, respectively) at 45 days after flowering under post-flowering moisture-stress conditions (Sabadin et al. 2012). The authors reported that one of these QTL (QGLFA8.1) explained the green leaf area variation by 14.46% and co-located with grain yield under water-stressed conditions. This is consistent with reports that stay-green QTL co-located with QTL for nodal root angle in sorghum (Mace et al. 2012) and seminal root angle in wheat (Christopher et al. 2018). These previous studies indicate that genotypes with narrow root angle can extract water from greater depth, thereby increasing access to water under drought, resulting in a stay-green phenotype. This suggests the importance of nodal root angle during post-flowering moisture-stress conditions.
Stay-green is an important drought-adaptation feature in cereal crops, especially when they are subjected to moisture stress after flowering. Stay-green (Stg) loci reduce canopy growth at flowering by modifying tillering, leaf number, and leaf size (Borrell et al. 2014ab). The decreased canopy area at flowering reduces pre-anthesis water demand, moving it from pre- to post-anthesis and therefore boosting water availability during grain filling and thus grain production. However, the increased post-anthesis water use of Stg QTLs compared to RTx7000 was not solely a consequence of changes in temporal water-use patterns, as it more than compensated for the reduced pre-anthesis water use, resulting in increased total water use of 19 mm and 10 mm in the Stg1 and Stg3 NILs, respectively (Borrell et al., 2014ab). As water uptake under drought stress is supply-limited, these results indicate that the Stg QTLs could access more water than RTx7000, achieved either by better water extraction from the soil by roots or increasing the soil volume explored by the roots via deeper rooting or greater lateral spread.
Importantly, the majority of root angle QTLs identified in this study were co-located with previously identified QTLs for stay-green. This suggests that the stay-green phenotype is a consequence of not only shifting water from pre- to post-anthesis (due to water savings from reduced pre-anthesis canopy size), but of increased water uptake throughout crop growth due to modification of root angle. This finding has significant implications for crop improvement programs because it highlights that root system architecture contributes to an enhanced stay-green phenotype in sorghum, presumably by impacting elements of water supply in water-limited scenarios. This aligns with the findings of Liedtke et al. (2020) which suggest that factors other than canopy size (e.g. roots) have an important role in the expression of a stay-green phenotype in grain sorghum.