Distribution and Function of Proteoid Roots and other Root Clusters

Proteoid roots are bottlebrush-like clusters of rootlets which form along lateral roots. They are characteristic of most species of the Proteaceae, which are mainly distributed in Australia and South Africa. Homologous root clusters are present in species of the Casuarinaceae, Mimosaceae, Fabaceae, Myricaceae and Moraceae. Many similarities exist between these species in relation to morphology and function of root clusters. Many are non-mycorrhizal and are highly efficient in phosphorus (P) acquisition. In these species, proteoid roots and proteoid-like root clusters are abundant when grown on infertile soils. Their formation is predominantly affected by the P status of the plants, being induced at low P levels and repressed at high P levels. Proteoid roots and proteoid-like root clusters play an important role in acquisition of P and other mineral nutrients. Although increase in root surface area may be a contributing factor, in many species these roots excrete large amounts of organic acids and phenolics. The excretion of these compounds in a small soil volume gives rise to extensive nutrient mobilization by acidification, reduction and chelation of sparingly soluble forms of P and micronutrients such as Fe and Mn.     

Water Transport across Maize Roots : Simultaneous Measurement of Flows at the Cell and Root Level by Double Pressure Probe Technique

A double pressure probe technique was used to measure simultaneously water flows and hydraulic parameters of individual cells and of excised roots of young seedlings of maize (Zea mays L.) in osmotic experiments. By following initial flows of water at the cell and root level and by estimating the profiles of driving forces (water potentials) across the root, the hydraulic conductivity of individual cell layers was evaluated. Since the hydraulic conductivity of the cell-to-cell path was determined separately, the hydraulic conductivity of the cell wall material could be evaluated as well (Lp_cw = 0.3 to 6.10⁻⁹ per meter per second per megapascal). Although, for radial water flow across the cortex and rhizodermis, the apoplasmic path was predominant, the contribution of the hydraulic conductance of the cell-to-cell path to the overall conductance increased significantly from the first layer of the cortex toward the inner layers from 2% to 23%. This change was mainly due to an increase of the hydraulic conductivity of the cell membranes which was Lp = 1.9.10⁻⁷ per meter per second per megapascal in the first layer and Lp = 14 to 9.10⁻⁷ per meter per second per megapascal in the inner layers of the cortex. The hydraulic conductivity of entire roots depended on whether hydrostatic or osmotic forces were used to induce water flows. Hydrostatic Lp_r was 1.2 to 2.3.10⁻⁷ per meter per second per megapascal and osmotic Lp_r = 1.6 to 2.8.10⁻⁸ per meter per second per megapascal. The apparent reflection coefficients of root cells (σ_s) of nonpermeating solutes (KCI, PEG 6000) decreased from values close to unity in the rhizodermis to about 0.7 to 0.8 in the cortex. In all cases, however, σ_s was significantly larger than the reflection coefficient of entire roots (σ_sr). For KCI and PEG 6000, σ_sr was 0.53 and 0.64, respectively. The results are discussed in terms of a composite membrane model of the root.     

Focus Issue on Roots: Root Cortical Aerenchyma Enhances Nitrogen Acquisition from Low-Nitrogen Soils in Maize

Suboptimal nitrogen (N) availability is a primary constraint for crop production in developing nations, while in rich nations, intensive N fertilization carries substantial environmental and economic costs. Therefore, understanding root phenes that enhance N acquisition is of considerable importance. Structural-functional modeling predicts that root cortical aerenchyma (RCA) could improve N acquisition in maize (Zea mays). We evaluated the utility of RCA for N acquisition by physiological comparison of maize recombinant inbred lines contrasting in RCA grown under suboptimal and adequate N availability in greenhouse mesocosms and in the field in the United States and South Africa. N stress increased RCA formation by 200% in mesocosms and by 90% to 100% in the field. RCA formation substantially reduced root respiration and root N content. Under low-N conditions, RCA formation increased rooting depth by 15% to 31%, increased leaf N content by 28% to 81%, increased leaf chlorophyll content by 22%, increased leaf CO₂ assimilation by 22%, increased vegetative biomass by 31% to 66%, and increased grain yield by 58%. Our results are consistent with the hypothesis that RCA improves plant growth under N-limiting conditions by decreasing root metabolic costs, thereby enhancing soil exploration and N acquisition in deep soil strata. Although potential fitness tradeoffs of RCA formation are poorly understood, increased RCA formation appears be a promising breeding target for enhancing crop N acquisition.Nitrogen (N) deficiency is one of the most limiting factors in maize (Zea mays) production worldwide (Ladha et al., 2005). In developing countries such as those in sub-Saharan Africa, less than 20 kg N ha⁻¹ is applied to fields of smallholder farmers due to high fertilizer cost (Azeez et al., 2006; Worku et al., 2007). In developed countries, intensive N fertilization is used to maintain satisfactory yield (Tilman et al., 2002). In the United States, N fertilizers are the greatest economic and energy cost for maize production (Ribaudo et al., 2011). However, less than half of the N applied to crops is actually acquired, and most of the remaining N becomes a source of environmental pollution (Raun and Johnson, 1999; Smil, 1999; Tilman et al., 2002). For example, N and phosphorus (P) effluents into marine systems from agriculture cause eutrophication and hypoxic zones (Diaz and Rosenberg, 2008; Robertson and Vitousek, 2009). Nitrate contamination in surface water and groundwater systems poses serious health risks, such as methemoglobinemia and N-nitroso-induced cancers (UNEP and WHRC, 2007). Emission of nitrous oxides from agricultural activities contributes to ozone damage and global warming (Kulkarni et al., 2008; Sutton et al., 2011). Furthermore, the production of N fertilizers requires considerable energy from fossil fuels, and since energy costs have risen in recent years, farmers face economic pressure from increasing N fertilizer costs, which are linked to higher food prices. It is estimated that a 1% increase in crop N efficiency could save more than $1 billion (U.S.) annually worldwide (Kant et al., 2011). Therefore, even a small improvement in N efficiency would have significant positive impacts on the environment and the economy.Soil N is heterogenous and dynamic. The bioavailability of soil N depends on the balance between the rates of mineralization, nitrification, and denitrification. These processes are determined by several factors, including soil composition, microbial activity, soil temperature, and soil water status (Miller and Cramer, 2004). The predominant form of soil N available to plants in most agricultural systems is nitrate, which is highly soluble in water and thus mobile in the soil (Barber, 1995; Marschner, 1995). Mineralization of organic matter and/or the application of N fertilizer at the beginning of the growing season followed by precipitation and irrigation create a pulse of nitrate that may exceed the N acquisition capacity of seedlings and leach below the root zone. Therefore, it has been proposed that increasing the speed of root exploration of deep soil strata could benefit N acquisition (Lynch, 2013). However, the structural investments and metabolic expenditures of root systems are substantial and can exceed half of daily photosynthesis (Lambers et al., 2002). Therefore, full consideration of the costs and benefits of root systems is crucial for identifying root traits to improve crop production, especially in water- and nutrient-deficient environments (Lynch, 2007). Taking rhizoeconomics and the spatiotemporal availability of soil N into account, Lynch (2013) proposed a root ideotype for enhanced N acquisition in maize called Steep, Cheap, and Deep, in which Steep refers to architectural phenes and Cheap refers to phenes that reduce the metabolic cost of soil exploration. One element of this ideotype is abundant root cortical aerenchyma (RCA).RCA consists of enlarged air spaces in the root cortex (Esau, 1977). RCA is known to form in response to hypoxia, and the role of RCA in improving oxygen transport to roots of many plant species under hypoxic conditions has been well researched (Vartapetian and Jackson, 1997; Jackson and Armstrong, 1999; Mano and Omori, 2007, 2013). Interestingly, RCA can also form in response to drought and edaphic stresses such as N, P, and sulfur deficiencies (Drew et al., 1989; Bouranis et al., 2003; Fan et al., 2003; Zhu et al., 2010a), which suggests that the benefit of RCA extends beyond facilitating oxygen transport. Several lines of evidence suggest that RCA enhances root metabolic efficiency under stress. Fan et al. (2003) found that RCA formation significantly reduced root segment respiration and P content of root tissue, which allowed greater shoot growth in soils with low P availability. Under drought, maize genotypes with high RCA formation had greater root length, deeper rooting, better leaf water status, and 8 times greater yield than closely related genotypes with low RCA (Zhu et al., 2010a). Effects of RCA on root respiration were more pronounced for large-diameter roots compared with small-diameter roots (Jaramillo et al., 2013). Results from the functional-structural plant model SimRoot showed that RCA formation could be an adaptive response to deficiency of N, P, and potassium by decreasing the metabolic cost of soil exploration. By reducing root respiration, RCA decreases the carbon cost of soil exploration, and by decreasing the N and P content of root tissue, RCA permits internal reallocation of nutrients to growing root tissue, which is particularly beneficial under conditions of low N and P availability (Postma and Lynch, 2011a). Under suboptimal P availability, RCA increased the growth of a simulated 40-d-old maize plant by 70% (Postma and Lynch, 2011b). In the case of N, RCA increased the growth of simulated maize plants up to 55% in low-N conditions, and plants benefit from RCA more in high-N-leaching environments than in low-N-leaching environments (Postma and Lynch, 2011a). In addition, the formation of RCA decreases critical soil nutrient levels, defined as the soil fertility below which growth is reduced, suggesting that cultivars with high RCA may require less fertilizer under nonstressed conditions. These in silico results suggest that RCA has potential utility for improving crop nutrient acquisition in both high- and low-input agroecosystems.The overall objective of this research was to assess the utility of RCA for N acquisition in maize under N-limiting conditions. Maize near-isophenic recombinant inbred lines (RILs) sharing a common genetic background (i.e. descending from the same parents) with common root phenotypes but contrasting in RCA formation were grown under N stress to test the hypothesis that RCA formation is associated with reduced root respiration, reduced tissue nutrient content, greater rooting depth, enhanced N acquisition, and therefore greater plant growth and yield under N limitation.     

Focus Issue on Roots: Low Crown Root Number Enhances Nitrogen Acquisition from Low-Nitrogen Soils in Maize

In developing nations, low soil nitrogen (N) availability is a primary limitation to crop production and food security, while in rich nations, intensive N fertilization is a primary economic, energy, and environmental cost to crop production. It has been proposed that genetic variation for root architectural and anatomical traits enhancing the exploitation of deep soil strata could be deployed to develop crops with greater N acquisition. Here, we provide evidence that maize (Zea mays) genotypes with few crown roots (crown root number [CN]) have greater N acquisition from low-N soils. Maize genotypes differed in their CN response to N limitation in greenhouse mesocosms and in the field. Low-CN genotypes had 45% greater rooting depth in low-N soils than high-CN genotypes. Deep injection of ¹⁵N-labeled nitrate showed that low-CN genotypes under low-N conditions acquired more N from deep soil strata than high-CN genotypes, resulting in greater photosynthesis and plant N content. Under low N, low-CN genotypes had greater biomass than high-CN genotypes at flowering (85% in the field study in the United States and 25% in South Africa). In the field in the United States, 1.8× variation in CN was associated with 1.8× variation in yield reduction by N limitation. Our results indicate that CN deserves consideration as a potential trait for genetic improvement of N acquisition from low-N soils.Maize (Zea mays) is one of the world’s most important crops and is a staple food in Latin America and Africa. Maize production requires a large amount of fertilizer, especially nitrogen (N). In the United States, N fertilizers represent the greatest economic and energy costs for maize production (Ribaudo et al., 2011). However, on-farm studies across the northcentral United States revealed that more than half of applied N is not taken up by maize plants and is vulnerable to losses from volatilization, denitrification, and leaching, which pollute air and water resources (Cassman et al., 2002). Conversely, in developing countries, suboptimal N availability is a primary limitation to crop yields and, therefore, food security (Azeez et al., 2006). Increasing yield in these areas is an urgent concern, since chemical fertilizers are not affordable (Worku et al., 2007). Cultivars with greater N acquisition from low-N soils could help alleviate food insecurity in poor nations as well as reduce environmental degradation from excessive fertilizer use in developed countries.The two major soil N forms available to plants are ammonium and nitrate. Nitrate is the main N form in most maize production environments (Miller and Cramer, 2004). Nitrate is highly mobile in soil, and the spatiotemporal availability of soil N is rather complex. In the simplest case, N fertilizers applied to the soil and/or N released from the mineralization of soil organic matter are rapidly converted to nitrate by soil microbes. After irrigation and precipitation events, nitrate moves with water to deeper soil strata. Leaching of nitrate from the root zone has been shown to be a significant cause of low recovery of N fertilizer in commercial agricultural systems (Raun and Johnson, 1999; Cassman et al., 2002). Differences in root depth influence the ability of plants to acquire N. Studies using ¹⁵N-labeled nitrate placed at different soil depths showed that only plants with deep rooting can acquire N sources from deep soil strata, which would otherwise have been lost through leaching (Kristensen and Thorup-Kristensen, 2004a, 2004b). Therefore, selection for root traits enhancing rapid deep soil exploration could be used as a strategy to improve crop N efficiency.The maize root system consists of embryonic and postembryonic components. The embryonic root system consists of two distinct root classes: a primary root and a variable number of seminal roots formed at the scutellar node. The postembryonic root system consists of roots that are formed at consecutive shoot nodes and lateral roots, which are initiated in the pericycle of all root classes. Shoot-borne or nodal roots that are formed belowground are called crown roots, whereas those that are formed aboveground are designated brace roots (Hochholdinger, 2009). While the primary root and seminal roots are essential for the establishment of seedlings after germination, nodal roots and particularly crown roots make up most of the maize root system and are primarily responsible for soil resource acquisition later in development (Hoppe et al., 1986).Lynch (2013) proposed an ideotype for superior N and water acquisition in maize called Steep, Cheap, and Deep (SCD), which integrates root architectural, anatomical, and physiological traits to increase rooting depth and, therefore, the capture of N in leaching environments. One such trait is crown root number (CN). CN is an aggregate trait consisting of the number of belowground nodal whorls and the number of roots per whorl. The crown root system dominates resource acquisition during vegetative growth after the first few weeks and remains important during reproductive development (Hochholdinger et al., 2004). CN in maize ranges from five to 50 under fertile conditions (Trachsel et al., 2011). At the low end of this range, crown roots may be too spatially dispersed to sufficiently explore the soil. There is also a risk of root loss to herbivores and pathogens. If roots are lost in low-N plants, there may be too few crown roots left to support the nutrient, water, and anchorage needs of the plant. At the high end, a large number of crown roots may compete with each other for water and nutrients as well as incur considerable metabolic costs for the plant (Fig. 1). The SCD ideotype proposes that there is an optimal CN for N capture in maize (Lynch, 2013). Under low-N conditions, resources for root growth and maintenance are limiting, and nitrate is a mobile resource that can be captured by a dispersed root system. The optimal CN should tend toward the low end of the phenotypic variation to make resources available for the development of longer, deeper roots rather than more crown roots. According to the SCD ideotype, in low-N soils, maize genotypes with fewer crown roots could explore soils at greater depth, resulting in greater N acquisition, growth, and yield than genotypes with many crown roots.Open in a separate windowFigure 1.Visualization of the maize root system of low- and high-CN genotypes at 40 d after germination. Crown roots are colored in blue, and seminal roots are in red. The CN is eight in the low-CN genotype and 46 in the high-CN genotype. (Image courtesy of Larry M. York.)The objective of this study was to test the hypotheses that (1) low-CN genotypes have greater rooting depth than high-CN genotypes in low-N soils; (2) low-CN genotypes are better at acquiring deep soil N than high-CN genotypes; and (3) low-CN genotypes have greater biomass and yield than high-CN genotypes in low-N conditions.     

Cytochemical Assay for Differential Respiratory Activity in Roots and Root Hairs

The study of the underground parts of plants is often difficult, and as a result roots are often treated as homogeneous physiological entities with respect to root respiration. In this study we demonstrate a partitioning of respiration within root tissues using nitro blue tetrazo-lium staining and an incident light optical system that permits detailed observations of intact roots. The assay is rapid and easy to perform, and reveals that respiratory activity in roots is not uniform in space and time. The results show that root hairs in particular may be regions of enhanced respiratory activity in some species or in certain developmental or physiological states. This fact has important implications for the role of root hairs in the overall respiratory budget of roots and the energetics of nutrient assimilation. The results suggest that root respiration studies should consider differential respiratory activities of root cell types within roots.     

The Oxidising Activity of Roots in Waterlogged Soils

Root oxidising activity has been studied quantitatively in two species, Menyanthes trifoliata and Molinia coerulea, using a non-specific dye technique, and an oxygen-specific “polarographic” technique in conjunction with artificial roots constructed from silicone rubber tubing. Oxidising activity in these two species has been found to be up to nine times greater than can be accounted for by oxygen diffusing from the roots. Enzymatic oxidation is thought to be the cause of such high oxidising activity. The characteristic patterns of iron oxidation found on and around roots are discussed, and oxidation of iron at the rhizophere ‘boundary’ is illustrated. Earlier work on root oxidising activity is briefly reviewed.     

Salt-induced Changes in the Distribution of Amyloplasts in the Root Cap of Excised Pea Roots in Culture

The effects of 120 mM NaCl on the anatomy and ultrastructureof the root tip of cultured excised pea roots was investigatedafter 24 h exposure to salinity. In the meristematic cells mitochondrialdamage was apparent and these cells showed increased vacuolation.The root cap was already severely affected after 24 h exposureto salinity and clumping of the cap amyloplasts around the cellnuclei was apparent. The possibility that salinity may affectroot gravitropic responses is discussed. Pisum sativum L. cv. Alaska, salinity, roots, root culture, amyloplasts, ultrastructure     

A Periscope Method for Observing Root Growth and Distribution in Field Soil

This paper describes a technique for observing root growth inthe field using glass tubes and a periscope. Roots of two crops(winter wheat and millet) were studied in situ with the periscopeand the results compared with those obtained from washed soilsamples. Generally, both techniques gave similar patterns ofgrowth and distributions of roots although the periscope measurements,when compared with washed soil samples, tended to underestimateroot density close to the soil surface and overestimate densitieslower in the profile. Both methods allowed differences in theroot systems of irrigated and unirrigated crops to be distinguishedbut the periscope method was considerably faster     

Nickel Toxicity and Distribution in Maize Roots

A new histochemical method for Ni determination has been developed and employed to study the pattern of Ni distribution in plant tissues. Two-day-old seedlings of maize (Zea mays L.) were transferred onto 15, 20, 25, and 35 M Ni(NO₃)₂ solutions in the presence of 3 mM Ca(NO₃)₂, and Ni localization in shoot and root tissues was investigated at days 2 and 7 of the incubation. Following two days of incubation, Ni was found in all root tissues, and its content increased with the period of exposure and from the tip to the root base. Independent of root region and tissue, Ni content in the protoplasts exceeded that in the cell walls. Ni penetrated the endodermal barrier and accumulated in the endodermis and pericycle to the highest concentration. Ni accumulation in the pericycle restricted root branching. Ni did not affect the final cell length, and the inhibition of root growth resulted from suppressed cell division. In the shoots, Ni content was below the level discerned by the dimethylglyoximine method; we therefore conclude that maize belongs to excluder plants, with their root systems functioning as a barrier limiting heavy metal intake by aboveground organs. The pattern of Ni transport differs from that of Cd and Pb; this difference stands for specific toxic effects of Ni, including an arrest of root branching.     

Colonization of Wheat Root Hairs and Roots by Agrobacteria

Formation of extracellular structures in pure culture and in interaction with wheat root surface was studied by scanning and transmission electron microscopy. The effects of various factors (growth temperature as well as pretreatment of agrobacteria with kalanchoe extract, acetosyringone, and centrifugation) on formation of extracellular structures was tested. The data on Agrobacterium tumefaciens (wild-type strain C58 and mutants LBA2525 (virB2::lacZ) and LBA288 (without the Ti plasmid)) adhesion to wheat root surface and root hairs after pretreatment of agrobacteria with inducer of virulence genes (vir) acetosyringone were obtained. Formation of agrobacterial cell aggregates on wheat root hair tips was demonstrated. The proportion of root hairs with agrobacterial aggregates on the root hair tip insignificantly changed after pretreatment with acetosyringone but considerably increased after treatment of A. tumefaciens C58 and LBA2525 with kalanchoe leaf extract. The most active colonization of root hairs and formation of agrobacterial aggregates on hair root tips was observed at 22°C. The capacity of agrobacteria for adhesion on monocotyledon surface could be changed by pretreatment of bacteria with various surface-active substances. Bacterial cells subjected to centrifugation had a decreased capacity for attachment to both wheat root surface and root hairs. The relationship between the capacity for adhesion and pilus production in agrobacteria was considered.     

Glutamate Dehydrogenase Activity in Roots: Distribution in a Seedling and Storage Root, and the Effects of Red and Far-red Illuminations

Pisum seedling and Pastinaca storage roots contained high glutanrate dehydrogenase (GDH) activity in areas of reported rapid growth and high phytoctrome content. A similar distribution was observed for malate dehydrogenase. Freeze-thawings of mitochondrial preparations from Pisum roots always resulted in increases of GDH specific activity; however, the observed increases were much larger with basal than apical sections. Both intact and freeze-thawed mitochondrial preparations from seedling roots exhibited increases in GDH activity with time after isolation. In intact mitochondrial preparations from roots of etiolated seedlings, an increase in malate dehydrogenase activity was observed similar to that of GDH activity; however, no increased malate dehydrogenase activity was noted in preparations from light-grown seedlings. Illuminating Pisum seedlings with far-red light slowly increased GDH activity in roots over a period of two weeks. Since these observed increases were not due to direct exposure of roots to light, other factors were likely involved.     

Distribution of Lateral Root Primordia in Root Tips of Musa

The distribution of lateral root primordia in the root tipsof four Musa landraces (Grande Naine, Pisang Berlin, Ngok Egomeand Yangambi Km5) grown in the field has been investigated toevaluate the range of genetic variation of lateral root initiation.In banana (Musa sp.), lateral roots are initiated in the roottip, 0.6–4 mm behind the root/cap junction and arise inseveral protoxylem-based longitudinal rows or ‘ranks’.Significant differences were observed among landraces for theposition of the most distal primordium, however the longitudinalspacing between successive primordia along the ranks was similarfor all landraces. All ranks were involved in lateral root initiation.The number of ranks also showed significant variations amonglandraces and was proportional to the stelar diameter. Hencethe density of lateral roots (roots cm^-1) was affected by stelardiameter variations. Finally, root elongation in the root tipwas landrace-specific and not necessarily exponential, unlikesuggested in previous studies. It is concluded that lateralroot initiation in Musa is not involved in the genetic variationsof root architecture in the field. A dissection of root architectureinto components which may account for these variations is proposedin relation to the improvement of root system architecture.Copyright 1999 Annals of Botany Company Lateral root initiation, root architecture, Musa, banana.     

Polyacrylamide Gel Electrophoresis (PAGE) and Densitometric Measurement of Enzyme Activity of the Pea Root Pathogen Aphanomyces euteiches in Pea Roots

A new method to measure enzyme activity of the fungal root pathogen Aphanomyces euteiches in pea roots is described. The specific enzymes of the fungus and the host were separated by polyacrylamide gel electrophoresis (PAGE) and the activity of fungal Glucose-6-phosphate dehydrogenase and Phosphoglucomutase were quantified by densitometry. Fungal activity could be correlated to the percentage infected root length and to the disease symptoms of the plants. The activity of A. euteiches was studied in a time course experiment with increasing levels of zoospore inoculum. The results indicated that an increase in inoculum level resulted in a faster disease development in the plants. The relation between fungal enzyme activity and infection level is discussed.