Large Root Cortical Cell Size Improves Drought Tolerance in Maize

The objective of this study was to test the hypothesis that large cortical cell size (CCS) would improve drought tolerance by reducing root metabolic costs. Maize (Zea mays) lines contrasting in root CCS measured as cross-sectional area were grown under well-watered and water-stressed conditions in greenhouse mesocosms and in the field in the United States and Malawi. CCS varied among genotypes, ranging from 101 to 533 µm². In mesocosms, large CCS reduced respiration per unit of root length by 59%. Under water stress in mesocosms, lines with large CCS had between 21% and 27% deeper rooting (depth above which 95% of total root length is located in the soil profile), 50% greater stomatal conductance, 59% greater leaf CO₂ assimilation, and between 34% and 44% greater shoot biomass than lines with small CCS. Under water stress in the field, lines with large CCS had between 32% and 41% deeper rooting (depth above which 95% of total root length is located in the soil profile), 32% lighter stem water isotopic ratio of ¹⁸O to ¹⁶O signature, signifying deeper water capture, between 22% and 30% greater leaf relative water content, between 51% and 100% greater shoot biomass at flowering, and between 99% and 145% greater yield than lines with small cells. Our results are consistent with the hypothesis that large CCS improves drought tolerance by reducing the metabolic cost of soil exploration, enabling deeper soil exploration, greater water acquisition, and improved growth and yield under water stress. These results, coupled with the substantial genetic variation for CCS in diverse maize germplasm, suggest that CCS merits attention as a potential breeding target to improve the drought tolerance of maize and possibly other cereal crops.Suboptimal water availability is a primary constraint for terrestrial plants and a primary limitation to crop production. In developing countries, the problem of yield loss due to drought is most severe (Edmeades, 2008, 2013), and the problem will be further exacerbated in the future due to climate change (Burke et al., 2009; Schlenker and Lobell, 2010; Lobell et al., 2011a; IPCC, 2014; St. Clair and Lynch, 2010). The development of drought-tolerant crops is therefore an important goal for global agriculture. Breeding for drought adaptation using yield as a selection criterion is generally not efficient, since yield is an integration of complex mechanisms at different levels of organization affected by many elements of the phenotype and the environment interacting in complex and often unknown ways. Trait-based selection or ideotype breeding is generally a more efficient selection strategy, permitting the identification of useful sources of variation among lines that have poor agronomic adaptation, elucidation of genotype-environment interactions, and informed trait stacking (Lynch, 2007; Araus et al., 2008; Richards et al., 2010; Wasson et al., 2012; York et al., 2013; Lynch, 2014).Under drought stress, plants allocate more resources to root growth relative to shoot growth, which can enhance water acquisition (Sharp and Davies, 1979; Palta and Gregory, 1997; Lynch and Ho, 2005). The metabolic costs of soil exploration by root systems are significant and can exceed 50% of daily photosynthesis (Lambers et al., 2002). With a large root system, each unit of leaf area has more nonphotosynthetic tissue to sustain, which may reduce productivity by diverting resources from shoot and reproductive growth (Smucker, 1993; Nielsen et al., 2001; Boyer and Westgate, 2004). Genotypes with less costly root tissue could develop the extensive, deep root systems required to fully utilize soil water resources in drying soil without as much yield penalty. Therefore, root phenes that reduce the metabolic costs of soil exploration, thereby improving water acquisition, are likely to be valuable for improving drought tolerance (Lynch and Ho, 2005; Zhu et al., 2010; Lynch, 2011; Richardson et al., 2011; Jaramillo et al., 2013; Lynch 2014).Maize (Zea mays) is the principal global cereal. Maize production is facing major challenges as a result of the increasing frequency and intensity of drought (Tuberosa and Salvi, 2006), and this problem will likely be exacerbated by climate change (Lobell et al., 2011b). The Steep, Cheap, and Deep ideotype has been proposed for improving water and nitrogen acquisition by maize when these resources are limited (Lynch, 2013). This ideotype consists of root architectural, anatomical, and physiological traits that may increase rooting depth and thereby improve water acquisition from drying soils. Anatomical phenes could influence the metabolic cost of soil exploration by changing the proportion of respiring and nonrespiring root tissue and affecting the metabolic cost of tissue construction and maintenance, which is an important limitation to root growth and plant development under edaphic stress. Specific anatomical phenes that may contribute to rooting depth by reducing root metabolic costs include components of living cortical area (LCA; Jaramillo et al., 2013), including root cortical aerenchyma (RCA), cortical cell size (CCS), and cortical cell file number (Lynch, 2013).RCA consists of large air-filled lacunae that replace living cortical cells as a result of programmed cell death (Evans, 2004). Previous studies have demonstrated that RCA improves crop adaptation to edaphic stress by reducing the metabolic cost of soil exploration and exploitation (Fan et al., 2003; Zhu et al., 2010; Postma and Lynch, 2011a, Saengwilai et al., 2014a). RCA is associated with a disproportionate reduction of root respiration, thereby permitting greater root growth and acquisition of soil resources (Fan et al., 2003; Zhu et al., 2010). SimRoot modeling indicated that RCA can substantially increase the acquisition of nitrogen, phosphorus, and potassium in maize by reducing respiration and the nutrient content of root tissue (Postma and Lynch, 2011b). Under water stress in the field, maize genotypes with more RCA had deeper roots, better leaf water status, and 800% greater yield than genotypes with less RCA (Zhu et al., 2010). Under nitrogen stress in the field and in greenhouse mesocosms, maize genotypes with more RCA had greater rooting depth, greater nitrogen capture from deep soil strata, greater nitrogen content, greater leaf photosynthesis, greater biomass, and greater yield (Saengwilai et al., 2014a).LCA refers to the living portion of the cortex that remains after the formation of aerenchyma (Jaramillo et al., 2013). Recently, we reported that LCA is an important determinant of root metabolic cost and a better predictor of root respiration than RCA (Jaramillo et al., 2013). In that study, maize lines contrasting in LCA were grown under well-watered or water-stressed conditions in soil mesocosms, and LCA was associated with a reduction of specific root respiration. These results provided the impetus to investigate the relative contribution of each component of LCA to metabolic cost. Our focus here is on root CCS.Plant cell size varies substantially both among and within species (Sugimoto-Shirasu and Roberts, 2003). Cell size in a given species and tissue is under genetic control and results from the coordinated control of cell growth and cell division (Sablowski and Carnier Dornelas, 2014). The increased volume of individual cells is attributable to cytoplasmic growth and cell expansion (Marshall et al., 2012; Chevalier et al., 2014). Cytoplasmic growth is the net accumulation of macromolecules and cellular organelles, while cell expansion refers to increased cell volume caused by enlargement of the vacuole (Taiz, 1992; Sablowski and Carnier Dornelas, 2014). Lynch (2013) proposed that large CCS would decrease the metabolic costs of root growth and maintenance, both in terms of the carbon cost of root respiration and the nutrient content of living tissue, by increasing the ratio of vacuolar to cytoplasmic volume.The objective of this study was to test the hypothesis that large CCS would reduce specific root respiration (i.e. respiration per unit of root length), which under water stress would result in greater root growth, greater acquisition of subsoil water, better plant water status, and improved plant growth and yield. Diverse sets of genotypes (including landraces and recombinant inbred lines [RILs]) contrasting for CCS were evaluated under water stress and well-watered conditions in soil mesocosms in controlled environments, in the field in the United States using automated rainout shelters, and in the field in Malawi. Our results demonstrate that substantial variation for CCS exists in maize and that this variation has substantial effects on the metabolic cost of soil exploration and thereby water acquisition under drought.