Root cortical aerenchyma (RCA) reduces root respiration in maize by converting living cortical tissue to air volume. We hypothesized that RCA increases drought tolerance by reducing root metabolic costs, permitting greater root growth and water acquisition from drying soil. To test this hypothesis, recombinant inbred lines with high and low RCA were observed under water stress in the field and in soil mesocosms in a greenhouse. In the field, lines with high RCA had 30% more shoot biomass at flowering compared with lines with low RCA under water stress. Root length density in deep soil was significantly greater in the high RCA lines compared with the low RCA lines. Mid-day leaf relative water content in the high RCA lines was 10% greater than in the low RCA lines under water stress. The high RCA lines averaged eight times the yield of the low RCA lines under water stress. In mesocosms, high RCA lines had less seminal root respiration, deeper rooting, and greater shoot biomass compared with low RCA lines under water stress. These results support the hypothesis that RCA is beneficial for drought tolerance in maize by reducing the metabolic cost of soil exploration.

Low phosphorus availability induces the formation of cortical aerenchyma in roots. The adaptive significance of this response is unknown. We hypothesized that aerenchyma may be helpful to low-phosphorus plants by reducing root respiratory and phosphorus requirements, thereby increasing the metabolic efficiency of soil exploration. To test this hypothesis we investigated aerenchyma formation, root respiration and tissue phosphorus concentration in maize and common bean genotypes in response to phosphorus availability and ethylene treatments. Genotypes differed substantially in their ability to form aerenchyma in response to low phosphorus. Aerenchyma formation was disproportionately correlated with reduced root respiration; roots with 30% cross-sectional area as aerenchyma had 70% less respiration than roots without aerenchyma. Aerenchyma formation was also proportionally correlated with reduced root phosphorus concentration. Variation in aerenchyma formation was correlated with root respiration and phosphorus concentration, regardless of whether such variation was caused genetically or by ethylene or phosphorus treatments. Results with isolated roots were confirmed by measurement of whole root respiration of intact maize plants. Our results support the hypothesis that aerenchyma formation reduces the respiratory and phosphorus requirements of soil exploration by roots, and thus, represents a useful adaptation to low phosphorus availability.

Previous work on the adaptation of maize (Zea mays) primary roots to water deficit showed that cell elongation is maintained preferentially toward the apex, and that this response involves modification of cell wall extension properties. To gain a comprehensive understanding of how cell wall protein (CWP) composition changes in association with the differential growth responses to water deficit in different regions of the elongation zone, a proteomics approach was used to examine water soluble and loosely ionically bound CWPs. The results revealed major and predominantly region-specific changes in protein profiles between well-watered and water-stressed roots. In total, 152 water deficit-responsive proteins were identified and categorized into five groups based on their potential function in the cell wall: reactive oxygen species (ROS) metabolism, defense and detoxification, hydrolases, carbohydrate metabolism, and other/unknown. The results indicate that stress-induced changes in CWPs involve multiple processes that are likely to regulate the response of cell elongation. In particular, the changes in protein abundance related to ROS metabolism predicted an increase in apoplastic ROS production in the apical region of the elongation zone of water-stressed roots. This was verified by quantification of hydrogen peroxide content in extracted apoplastic fluid and by in situ imaging of apoplastic ROS levels. This response could contribute directly to the enhancement of wall loosening in this region. This large-scale proteomic analysis provides novel insights into the complexity of mechanisms that regulate root growth under water deficit conditions and highlights the spatial differences in CWP composition in the root elongation zone.Roots often continue to grow under water deficits that completely inhibit shoot and leaf elongation (Sharp and Davies, 1979;Westgate and Boyer, 1985), and this is considered an important mechanism of plant adaptation to water-limited conditions (Sharp and Davies, 1989). Investigation of the mechanisms of root growth adaptation to water deficit is important for improving plant performance under drought, because water resources for agriculture are becoming increasingly limited.The physiology of maize (Zea mays) primary root elongation at low water potentials has been studied extensively (for review, see Sharp et al., 2004), which has provided the foundation for an understanding of the complex network of responses involved. Analysis of the relative elongation rate profile within the root elongation zone showed that under severe water deficit, elongation rates are fully maintained in the apical few millimeters but progressively inhibited as cells are displaced further from the root apex (Sharp et al., 1988;Liang et al., 1997). To help understand the maintenance of elongation in the apical region of roots growing under water deficit conditions, Spollen and Sharp (1991) measured the spatial distribution of turgor pressure and found that values were uniformly decreased by over 50% throughout the e...