Genotypic differences in root traits to design drought-avoiding soybean ideotypes

Root traits involved in drought avoidance
Soybean root traits play a key role for soil resource acquisition and for improved crop performance under abiotic stress including drought or low soil phosphorus availability (Zhao et al., 2004; Ao et al., 2010). The literature (Tabs. 2 and 3) shows that some root traits were modified by the water regime applied but this modification began mainly after early growth period (from V3). An important increase in root tips number, root volume and root length was observed (Fenta et al., 2014; Mwamlima et al., 2019; Dayoub et al., 2021). However, root diameter was decreased under water stress (Mwamlima et al., 2019).
Previous studies found significant correlations in soybean between drought avoidance and root traits such as root biomass, total root length, root volume and number of lateral roots (Liu et al., 2005; Read and Bartlett, 2006). Since soybean yield under drought depends highly on both root depth and root density, deeper rooting system may improve soybean yield. Some soybean cultivars are able to adapt to water stress conditions by developing a deeper taproot and a large and fibrous rooting system and by increasing the root/shoot biomass ratio, which enable to reach deeper soil layers with available water (Manavalan et al., 2009; Boote, 2011). It is well established that root length is one of the main traits that support plants to tolerate the limited water condition during early crop growth stage (Shao et al., 2008).
On the other hand, root nodules are known to be crucial sensors of drought, but their responses and their drought tolerance features remain poorly characterized in the literature for soybean. Root nodule number and size were reduced 60 days after sowing for soybean cultivars under drought (Fenta et al., 2014). Decreased N₂ fixation in response to drought leads to soybean yield losses (King and Purcell, 2001; Sinclair et al., 2010). However, differences exist among cultivars in sensitivity of N₂ fixation to drought (King and Purcell, 2001; Fenta et al., 2014). Some cultivars can maintain more nodules under drought conditions, thus the ability to form and sustain root nodules may also be an important trait underpinning shoot productivity under drought (Fenta et al., 2014). Previous studies showed that a sustained nitrogen fixation is a major trait associated to drought tolerance in some soybean cultivars, which was due mainly to greater nodule size (Pantalone et al., 1996b; King and Purcell, 2001).
Soybean root traits involved in increased water acquisition were defined by:

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  deeper rooting (and faster root growth in depth);
  
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  improved distribution of root length density into deeper soil layers;
  
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  increased length per unit root mass;
  
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  increased assimilate partitioning to roots at the expense of shoot growth (constitutive);
  
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  increased biomass partitioning to roots to increase root length density but only when induced by onset of water deficit (adaptive) and delayed onset of seedling growth to increase assimilate to roots (Boote, 2011).
  

A deeper taproot associated with a high density of lateral roots leads to an increased total root surface area and thus water absorption from soil (Garay and Wilhelm, 1983; Hufstetler et al., 2007; Matsuo et al., 2013). When subject to water stress, soybean cultivars with a shallow root architecture (root angle <60°) tend to decrease the total root length, root surface area and root volume. In contrast, water stress tolerant cultivars show a deep or an intermediate root architecture (root angle >60°) (Fenta et al., 2014). Identifying root traits in soybean cultivars will thus allow to find candidates able to avoid drought that could be an essential step in cultivar adaptations.

Physiological and Molecular Approaches to Improve Drought Resistance in Soybean

doi:10.1093/pcp/pcp082
Drought stress is a major constraint to the production and yield stability of soybean [ Glycine max (L.) Merr.]. For developing high yielding varieties under drought conditions, the most widely employed criterion has traditionally been direct selection for yield stability over multiple locations. However, this approach is time consuming and labor intensive, because yield is a highly quantitative trait with low heritability, and influenced by differences arising from soil heterogeneity and environmental factors. The alternative strategy of indirect selection using secondary traits has succeeded only in a few crops, due to problems with repeatability and lack of phenotyping strategies, especially for root-related traits. Considerable efforts have been directed towards identifying traits associated with drought resistance in soybean. With the availability of the whole genome sequence, physical maps, genetics and functional genomics tools, integrated approaches using molecular breeding and genetic engineering offer new opportunities for improving drought resistance in soybean. Genetic engineering for drought resistance with candidate genes has been reported in the major food crops, and efforts for developing drought-resistant soybean lines are in progress. The objective of this review is to consolidate the current knowledge of physiology, molecular breeding and func- tional genomics which may be influential in integrating breeding and genetic engineering approaches for drought resistance in soybean.
The resilience of legume crops against present-day weather extremes, such as drought, excess water, heat, cool weather during grain filling, and early frost, is considered to predict their adaptation to future climate change (Cutforth et al. 2007). In soybean, drought reduces yield by about 40 % ( Specht et al. 1999). Depending on hybrid characteristics, soybeans use about 450–700 mm of water during the growing season (Dogan et al. 2007). The most critical period for water stress in soybean has been reported to be during the flowering stage and the period following flowering (Meckel et al. 1984).
Plants use various mechanisms to cope with drought stress. These may be classified into three groups: drought escape, drought avoidance and drought tolerance (Turner et al. 2001). Drought escape allows the plant to complete its life cycle during the period of sufficient water supply before the onset of drought. Normally the life cycle is shorter and plants set some seeds instead of complete crop failure. An example of drought escape is the Early Soybean Planting System, now used widely in the southern USA. In this system, short season cultivars are planted during March–April in regions where later maturing cultivars have previously been grown. The early maturing cultivars start flowering in late April to early May and set pods in late May, thus completing the reproductive stage before the period of possible drought in July–August (Heatherly and Elmore 2004). The second mechanism, drought avoidance, involves strategies which help the plant maintain high water status during periods of stress, either by efficient water absorption from roots or by reducing evapotranspiration from aerial parts. The third mechanism, drought tolerance, allows the plant to maintain turgor and continue metabolism even at low water poten- tial, e.g. by protoplasmic tolerance or synthesis of osmopro- tectants, osmolytes or compatible solutes (Nguyen et al. 1997 ). In soybean, a widely accepted equation for grain yield (Y) under water-limited conditions is a function of three components, namely the amount of water transpired (T), water-use efficiency (WUE) and harvest index (HI); Y = T × WUE × HI (Turner et al. 2001). Maintenance of opti- mum transpiration, leading to increased WUE, is one of the strategies to improve yield in soybean. Nine secondary traits have been reported to be associated with the likelihood of increasing or maintaining T during drought. These traits are phenology, photoperiod sensitivity, developmental plastic- ity, leaf area maintenance, heat tolerance, osmotic adjust- ment, early vigor, rooting depth and rooting density. Additionally, transpiration efficiency and leaf reflectance are the other two traits related to WUE (Purcell and Specht 2004 ).