Root hairs increase rhizosphere extension and carbon input to soil

Unraveling root and rhizosphere traits in temperate maize landraces and modern cultivars: Implications for soil resource acquisition and drought adaptation

A holistic understanding of plant strategies to acquire soil resources is pivotal in achieving sustainable food security. However, we lack knowledge about variety-specific root and rhizosphere traits for resource acquisition, their plasticity and adaptation to drought. We conducted a greenhouse experiment to phenotype root and rhizosphere traits (mean root diameter [Root D], specific root length [SRL], root tissue density, root nitrogen content, specific rhizosheath mass [SRM], arbuscular mycorrhizal fungi [AMF] colonization) of 16 landraces and 22 modern cultivars of temperate maize (Zea mays L.). Our results demonstrate that landraces and modern cultivars diverge in their root and rhizosphere traits. Although landraces follow a 'do-it-yourself' strategy with high SRLs, modern cultivars exhibit an 'outsourcing' strategy with increased mean Root Ds and a tendency towards increased root colonization by AMF. We further identified that SRM indicates an 'outsourcing' strategy. Additionally, landraces were more drought-responsive compared to modern cultivars based on multitrait response indices. We suggest that breeding leads to distinct resource acquisition strategies between temperate maize varieties. Future breeding efforts should increasingly target root and rhizosphere economics, with SRM serving as a valuable proxy for identifying varieties employing an outsourcing resource acquisition strategy.

Visualizing and quantifying 33P uptake and translocation by maize plants grown in soil

Phosphorus (P) availability severely limits plant growth due to its immobility and inaccessibility in soils. Yet, visualization and measurements of P uptake from different root types or regions in soil are methodologically challenging. Here, we explored the potential of phosphor imaging combined with local injection of radioactive 33P to quantitatively visualize P uptake and translocation along roots of maize grown in soils. Rhizoboxes (20 × 40 × 1 cm) were filled with sandy field soil or quartz sand, with one maize plant per box. Soil compartments were created using a gravel layer to restrict P transfer. After 2 weeks, a compartment with the tip region of a seminal root was labeled with a NaH2 33PO4 solution containing 12 MBq of 33P. Phosphor imaging captured root P distribution at 45 min, 90 min, 135 min, 180 min, and 24 h post-labeling. After harvest, 33P levels in roots and shoots were quantified. 33P uptake exhibited a 50% increase in quartz sand compared to sandy soil, likely attributed to higher P adsorption to the sandy soil matrix than to quartz sand. Notably, only 60% of the absorbed 33P was translocated to the shoot, with the remaining 40% directed to growing root tips of lateral or seminal roots. Phosphor imaging unveiled a continuous rise in 33P signal in the labeled seminal root from immediate post-labeling until 24 h after labeling. The highest 33P activities were concentrated just above the labeled compartment, diminishing in locations farther away. Emerging laterals from the labeled root served as strong sinks for 33P, while a portion was also transported to other seminal roots. Our study quantitatively visualized 33P uptake and translocation dynamics, facilitating future investigations into diverse root regions/types and varying plant growth conditions. This improves our understanding of the significance of different P sources for plant nutrition and potentially enhances models of plant P uptake.

Harnessing root-soil-microbiota interactions for drought-resilient cereals

Cereal plants form complex networks with their associated microbiome in the soil environment. A complex system including variations of numerous parameters of soil properties and host traits shapes the dynamics of cereal microbiota under drought. These multifaceted interactions can greatly affect carbon and nutrient cycling in soil and offer the potential to increase plant growth and fitness under drought conditions. Despite growing recognition of the importance of plant microbiota to agroecosystem functioning, harnessing the cereal root microbiota remains a significant challenge due to interacting and synergistic effects between root traits, soil properties, agricultural practices, and drought-related features. A better mechanistic understanding of root-soil-microbiota associations could lead to the development of novel strategies to improve cereal production under drought. In this review, we discuss the root-soil-microbiota interactions for improving the soil environment and host fitness under drought and suggest a roadmap for harnessing the benefits of these interactions for drought-resilient cereals. These methods include conservative trait-based approaches for the selection and breeding of plant genetic resources and manipulation of the soil environments.

Rhizosphere engineering for soil carbon sequestration

The rhizosphere is the central hotspot of water and nutrient uptake by plants, rhizodeposition, microbial activities, and plant-soil-microbial interactions. The plasticity of plants offers possibilities to engineer the rhizosphere to mitigate climate change. We define rhizosphere engineering as targeted manipulation of plants, soil, microorganisms, and management to shift rhizosphere processes for specific aims [e.g., carbon (C) sequestration]. The rhizosphere components can be engineered by agronomic, physical, chemical, biological, and genomic approaches. These approaches increase plant productivity with a special focus on C inputs belowground, increase microbial necromass production, protect organic compounds and necromass by aggregation, and decrease C losses. Finally, we outline multifunctional options for rhizosphere engineering: how to boost C sequestration, increase soil health, and mitigate global change effects.

Comparative Transcriptome Analysis Reveals the Genes and Pathways Related to Wheat Root Hair Length

Tube-like outgrowths from root epidermal cells, known as root hairs, enhance water and nutrient absorption, facilitate microbial interactions, and contribute to plant anchorage by expanding the root surface area. Genetically regulated and strongly influenced by environmental conditions, longer root hairs generally enhance water and nutrient absorption, correlating with increased stress resistance. Wheat, a globally predominant crop pivotal for human nutrition, necessitates the identification of long root hair genotypes and their regulatory genes to enhance nutrient capture and yield potential. This study focused on 261 wheat samples of diverse genotypes during germination, revealing noticeable disparities in the length of the root hair among the genotypes. Notably, two long root hair genotypes (W106 and W136) and two short root hair genotypes (W90 and W100) were identified. Transcriptome sequencing resulted in the development of 12 root cDNA libraries, unveiling 1180 shared differentially expressed genes (DEGs). Further analyses, including GO function annotation, KEGG enrichment, MapMan metabolic pathway analysis, and protein-protein interaction (PPI) network prediction, underscored the upregulation of root hair length regulatory genes in the long root hair genotypes. These included genes are associated with GA and BA hormone signaling pathways, FRS/FRF and bHLH transcription factors, phenylpropanoid, lignin, lignan secondary metabolic pathways, the peroxidase gene for maintaining ROS steady state, and the ankyrin gene with diverse biological functions. This study contributes valuable insights into modulating the length of wheat root hair and identifies candidate genes for the genetic improvement of wheat root traits.

The hidden roots of wetland methane emissions

Wetlands are the largest natural source of methane (CH4 ) globally. Climate and land use change are expected to alter CH4 emissions but current and future wetland CH4 budgets remain uncertain. One important predictor of wetland CH4 flux, plants, play an important role in providing substrates for CH4 -producing microbes, increasing CH4 consumption by oxygenating the rhizosphere, and transporting CH4 from soils to the atmosphere. Yet, there remain various mechanistic knowledge gaps regarding the extent to which plant root systems and their traits influence wetland CH4 emissions. Here, we present a novel conceptual framework of the relationships between a range of root traits and CH4 processes in wetlands. Based on a literature review, we propose four main CH4 -relevant categories of root function: gas transport, carbon substrate provision, physicochemical influences and root system architecture. Within these categories, we discuss how individual root traits influence CH4 production, consumption, and transport (PCT). Our findings reveal knowledge gaps concerning trait functions in physicochemical influences, and the role of mycorrhizae and temporal root dynamics in PCT. We also identify priority research needs such as integrating trait measurements from different root function categories, measuring root-CH4 linkages along environmental gradients, and following standardized root ecology protocols and vocabularies. Thus, our conceptual framework identifies relevant belowground plant traits that will help improve wetland CH4 predictions and reduce uncertainties in current and future wetland CH4 budgets.

Interactions between root hairs and the soil microbial community affect the growth of maize seedlings

Root hairs are considered important for rhizosphere formation, which affects root system functioning. Through interactions with soil microorganisms mediated by root exudation, root hairs may affect the phenotypes and growth of young plants. We tested this hypothesis by integrating results from two experiments: (1) a factorial greenhouse seedling experiment with Zea mays B73-wt and its root-hairless mutant, B73-rth3, grown in live and autoclaved soil, quantifying 15 phenotypic traits, seven growth rates, and soil microbiomes and (2) a semi-hydroponic system quantifying root exudation of maize genotypes. Possibly as compensation for lacking root hairs, B73-rth3 seedlings allocated more biomass to roots and grew slower than B73-wt seedlings in live soil, whereas B73-wt seedlings grew slowest in autoclaved soil, suggesting root hairs can be costly and their benefits were realized with more complete soil microbial assemblages. There were substantial differences in root exudation between genotypes and in rhizosphere versus non-rhizosphere microbiomes. The microbial taxa enriched in the presence of root hairs generally enhanced growth compared to taxa enriched in their absence. Our findings suggest the root hairs' adaptive value extends to plant-microbe interactions mediated by root exudates, affecting plant phenotypes, and ultimately, growth.

Root colonization by beneficial rhizobacteria

Rhizosphere microbes play critical roles for plant's growth and health. Among them, the beneficial rhizobacteria have the potential to be developed as the biofertilizer or bioinoculants for sustaining the agricultural development. The efficient rhizosphere colonization of these rhizobacteria is a prerequisite for exerting their plant beneficial functions, but the colonizing process and underlying mechanisms have not been thoroughly reviewed, especially for the nonsymbiotic beneficial rhizobacteria. This review systematically analyzed the root colonizing process of the nonsymbiotic rhizobacteria and compared it with that of the symbiotic and pathogenic bacteria. This review also highlighted the approaches to improve the root colonization efficiency and proposed to study the rhizobacterial colonization from a holistic perspective of the rhizosphere microbiome under more natural conditions.

Maize (Zea mays L.) root exudation profiles change in quality and quantity during plant development - A field study

Deciphering root exudate composition of soil-grown plants is considered a crucial step to better understand plant-soil-microbe interactions affecting plant growth performance. In this study, two genotypes of Zea mays L. (WT, rth3) differing in root hair elongation were grown in the field in two substrates (sand, loam) in custom-made, perforated columns inserted into the field plots. Root exudates were collected at different plant developmental stages (BBCH 14, 19, 59, 83) using a soil-hydroponic-hybrid exudation sampling approach. Exudates were characterized by LC-MS based non-targeted metabolomics, as well as by photometric assays targeting total dissolved organic carbon, soluble carbohydrates, proteins, amino acids, and phenolics. Results showed that plant developmental stage was the main driver shaping both the composition and quantity of exuded compounds. Carbon (C) exudation per plant increased with increasing biomass production over time, while C exudation rate per cm² root surface area h-1 decreased with plant maturity. Furthermore, exudation rates were higher in the substrate with lower nutrient mobility (i.e., loam). Surprisingly, we observed higher exudation rates in the root hairless rth3 mutant compared to the root hair-forming WT sibling, though exudate metabolite composition remained similar. Our results highlight the impact of plant developmental stage on the plant-soil-microbe interplay.

Magnetite nanoparticle coating chemistry regulates root uptake pathways and iron chlorosis in plants

Understanding the fundamental interaction of nanoparticles at plant interfaces is critical for reaching field-scale applications of nanotechnology-enabled plant agriculture, as the processes between nanoparticles and root interfaces such as root compartments and root exudates remain largely unclear. Here, using iron deficiency-induced plant chlorosis as an indicator phenotype, we evaluated the iron transport capacity of Fe3O4 nanoparticles coated with citrate (CA) or polyacrylic acid (PAA) in the plant rhizosphere. Both nanoparticles can be used as a regulator of plant hormones to promote root elongation, but they regulate iron deficiency in plant in distinctive ways. In acidic root exudates secreted by iron-deficient Arabidopsis thaliana, CA-coated particles released fivefold more soluble iron by binding to acidic exudates mainly through hydrogen bonds and van der Waals forces and thus, prevented iron chlorosis more effectively than PAA-coated particles. We demonstrate through roots of mutants and visualization of pH changes that acidification of root exudates primarily originates from root tips and the synergistic mode of nanoparticle uptake and transformation in different root compartments. The nanoparticles entered the roots mainly through the epidermis but were not affected by lateral roots or root hairs. Our results show that magnetic nanoparticles can be a sustainable source of iron for preventing leaf chlorosis and that nanoparticle surface coating regulates this process in distinctive ways. This information also serves as an urgently needed theoretical basis for guiding the application of nanomaterials in agriculture.

Heterogeneous nutrient supply modulates root exudation and accumulation of medicinally valuable compounds in Artemisia annua and Hypericum perforatum

Plants have evolved complex mechanisms to adapt to nutrient-deficient environments, including stimulating lateral root proliferation into local soil patches with high nutrient content in response to heterogeneous nutrient distribution. Despite the widespread occurrence of this phenomenon in soil, the effect of heterogeneous nutrient distribution on the accumulation of secondary compounds in plant biomass and their exudation by roots remains largely unknown. This study aims to fill this critical knowledge gap by investigating how deficiency and unequal distributions of nitrogen (N), phosphorus (P), and iron (Fe) affect plant growth and accumulation of the antimalarial drug artemisinin (AN) in leaves and roots of Artemisia annua, as well as AN exudation by roots. Heterogeneous N and P supplies strongly increased root exudation of AN in half of a split-root system exposed to nutrient deficiency. By contrast, exposure to a homogeneous nitrate and phosphate deficiency did not modulate root exudation of AN. This indicates that a combination of local and systemic signals, reflecting low and high nutritional statuses, respectively, were required to enhance AN exudation. This exudation response was independent of the regulation of root hair formation, which was predominantly modulated by the local signal. In contrast to the heterogeneous supply of N and P, heterogeneous Fe supply did not modulate AN root exudation but increased AN accumulation in locally Fe-deficient roots. No modulation of nutrient supply significantly changed the accumulation of AN in A. annua leaves. The impact of a heterogeneous nitrate supply on growth and phytochemical composition was also investigated in Hypericum perforatum plants. Unlike in A. annue, the uneven N supply did not significantly influence the exudation of secondary compounds in the roots of H. perforatum. However, it did enhance the accumulation of several biologically active compounds, such as hypericin, catechin, and rutin isomers, in the leaves of H. perforatum. We propose that the capacity of plants to induce the accumulation and/or differential exudation of secondary compounds under heterogeneous nutrient supply is both species- and compound-specific. The ability to differentially exude AN may contribute to A. annua's adaptation to nutrient disturbances and modulate allelopathic and symbiotic interactions in the rhizosphere.

Phyllosphere symbiont promotes plant growth through ACC deaminase production

Plant growth promoting bacteria can confer resistance to various types of stress and increase agricultural yields. The mechanisms they employ are diverse. One of the most important genes associated with the increase in plant biomass and stress resistance is acdS, which encodes a 1-aminocyclopropane-1-carboxylate- or ACC-deaminase. The non-proteinogenic amino acid ACC is the precursor and means of long-distance transport of ethylene, a plant hormone associated with growth arrest. Expression of acdS reduces stress induced ethylene levels and the enzyme is abundant in rhizosphere colonizers. Whether ACC hydrolysis plays a role in the phyllosphere, both as assembly cue and in growth promotion, remains unclear. Here we show that Paraburkholderia dioscoreae Msb3, a yam phyllosphere symbiont, colonizes the tomato phyllosphere and promotes plant growth by action of its ACC deaminase. We found that acdS is required for improved plant growth but not for efficient leaf colonization. Strain Msb3 readily proliferates on the leaf surface of tomato, only occasionally spreading to the leaf endosphere through stomata. The strain can also colonize the soil or medium around the roots but only spreads into the root if the plant is wounded. Our results indicate that the degradation of ACC is not just an important trait of plant growth promoting rhizobacteria but also one of leaf dwelling phyllosphere bacteria. Manipulation of the leaf microbiota by means of spray inoculation may be more easily achieved than that of the soil. Therefore, the application of ACC deaminase containing bacteria to the phyllosphere may be a promising strategy to increasing plant stress resistance, pathogen control, and harvest yields.

The effect of root hairs on exudate composition: a comparative non-targeted metabolomics approach

Root exudation is a major pathway of organic carbon input into soils. It affects soil physical properties, element solubility as well as speciation, and impacts the microbial community in the rhizosphere. Root exudates contain a large number of primary and secondary plant metabolites, and the amount and composition are highly variable depending on plant species and developmental stage. Detailed information about exudate composition will allow for a better understanding of exudate-driven rhizosphere processes and their feedback loops. Although non-targeted metabolomics by high-resolution mass spectrometry is an established tool to characterize root exudate composition, the extent and depth of the information obtained depends strongly on the analytical approach applied. Here, two genotypes of Zea mays L., differing in root hair development, were used to compare six mass spectrometric approaches for the analysis of root exudates. Reversed-phase liquid chromatography and hydrophilic interaction liquid chromatography combined with time-of-flight mass spectrometry (LC-TOF-MS), as well as direct infusion Fourier-transform ion cyclotron resonance mass spectrometry (DI-FT-ICR-MS), were applied with positive and negative ionization mode. By using the same statistical workflow, the six approaches resulted in different numbers of detected molecular features, ranging from 176 to 889, with a fraction of 48 to 69% of significant features (fold change between the two genotypes of > 2 and p-value < 0.05). All approaches revealed the same trend between genotypes, namely up-regulation of most metabolites in the root hair defective mutant (rth3). These results were in agreement with the higher total carbon and nitrogen exudation rate of the rth3-mutant as compared to the corresponding wild-type maize (WT). However, only a small fraction of features were commonly found across the different analytical approaches (20-79 features, 13-31% of the rth3-mutant up-regulated molecular formulas), highlighting the need for different mass spectrometric approaches to obtain a more comprehensive view into the composition of root exudates. In summary, 111 rth3-mutant up-regulated compounds (92 different molecular formulas) were detected with at least two different analytical approaches, while no WT up-regulated compound was found by both, LC-TOF-MS and DI-FT-ICR-MS. Zea mays L. exudate features obtained with multiple analytical approaches in our study were matched against the metabolome database of Zea mays L. (KEGG) and revealed 49 putative metabolites based on their molecular formula.

The spatial distribution of rhizosphere microbial activities under drought: water availability is more important than root-hair-controlled exudation

Root hairs and soil water content are crucial in controlling the release and diffusion of root exudates and shaping profiles of biochemical properties in the rhizosphere. But whether root hairs can offset the negative impacts of drought on microbial activity remains unknown. Soil zymography, ¹⁴ C imaging and neutron radiography were combined to identify how root hairs and soil moisture affect rhizosphere biochemical properties. To achieve this, we cultivated two maize genotypes (wild-type and root-hair-defective rth3 mutant) under ambient and drought conditions. Root hairs and optimal soil moisture increased hotspot area, rhizosphere extent and kinetic parameters (V_max and K_m ) of β-glucosidase activities. Drought enlarged the rhizosphere extent of root exudates and water content. Colocalization analysis showed that enzymatic hotspots were more colocalized with root exudate hotspots under optimal moisture, whereas they showed higher dependency on water hotspots when soil water and carbon were scarce. We conclude that root hairs are essential in adapting rhizosphere properties under drought to maintain plant nutrition when a continuous mass flow of water transporting nutrients to the root is interrupted. In the rhizosphere, soil water was more important than root exudates for hydrolytic enzyme activities under water and carbon colimitation.

An Experimental Rhizobox System for the Integrative Analysis of Root Development and Abiotic Stress Responses Under Water-Deficit Conditions

The study of root growth and plasticity in situ is rendered difficult by the opacity and mechanical barrier of soil substrates. Therefore, for the analysis of developmental processes and abiotic stress and development relationships, it is essential to set up cultivation systems that overcome these hindrances in a non-invasive and non-destructive manner. For this purpose, we have developed a useful and powerful rhizobox culture system, where the roots are separated from the soil substrate by a porous membrane with a mesh of such width that allows the exchange of water and solutes without allowing the roots to penetrate the soil. This system provides direct, easy, and quick access to the roots and allows to follow root growth and development, root system architecture, and root system plasticity at different stages of plant development and under various environmental conditions. Moreover, these rhizoboxes provide clean and intact roots that can be easily harvested to perform further physiological, biochemical, and molecular analyses at different stages of development and in response to various environmental constraints. This rhizobox method was validated by assessing root response plasticity of drought-stressed Arabidopsis and pea plants grown in soil displaying water content alterations. This rhizobox system is suitable for many types of abiotic stress-development studies, including the comparison of different stress intensities or of various mutants and genotypes.

Beneficial shift of rhizosphere soil nutrients and metabolites under a sugarcane/peanut intercropping system

Intercropping systems have been studied as a sustainable agricultural planting pattern to increase soil quality and crop yields. However, the relationships between metabolites and soil physicochemical properties remain poorly understood under sugarcane/peanut intercropping system. Thus, we determined the rhizosphere soil physicochemical properties, and analyzed rhizosphere soil metabolites and root metabolites by metabolomics method under monoculture and intercropping patterns of sugarcane and peanut. The results showed that pH, the contents of total phosphorus (P), total potassium (K), available nitrogen (N), available phosphorus (P), and available potassium (K) were higher in rhizosphere soil of intercropping peanut than monoculture peanut, and the content of total P was higher in rhizosphere soil of intercropping sugarcane than monoculture sugarcane. Sugarcane/peanut intercropping also significantly increased the activities of acid phosphatase and urease in rhizosphere soil. The metabolomics results showed that 32 metabolites, mainly organic acids and their derivatives (25.00%), nucleotides and their metabolites (18.75%), were detected in root and rhizosphere soil samples. In the MP-S (rhizosphere soil of monoculture peanut) vs. IP-S (rhizosphere soil of intercropping peanut) comparison, 47 differential metabolites (42 upregulated) were screened, including glycerolipids (19.15%), organic acids and their derivatives (17.89%), and amino acids and their metabolites (12.77%). In the MS-S (rhizosphere soil of monoculture sugarcane) vs. IS-S (rhizosphere soil of intercropping sugarcane) comparison, 51 differential metabolites (26 upregulated) were screened, including heterocyclic compounds (15.69%), glycerolipids (11.76%), and organic acids and their derivatives (9.80%). The metabolite species from MP-S, MS-S, IP-S, and IS-S were similar, but some metabolite contents were significantly different, such as adenine, adenosine, maltotriose, thermozeaxanthin-13 and PE-NMe (20:0/24:0). Adenine and adenosine were detected in root and rhizosphere soils, and their levels were increased in the intercropping treatment, which were mainly related to enhanced purine metabolism in root and rhizosphere soils under the sugarcane/peanut intercropping system. Importantly, adenine and adenosine were significantly positively correlated with total P and total K contents, acid phosphatase and urease activities, and pH. This study clarified that the sugarcane/peanut intercropping system could improve soil nutrients and enzymes and was related to purine metabolism.

Contrasting distribution of enzyme activities in the rhizosphere of European beech and Norway spruce

Recent policies and silvicultural management call for forest regeneration that involve the selection of tree species able to cope with low soil nutrient availability in forest ecosystems. Understanding the impact of different tree species on the rhizosphere processes (e.g., enzyme activities) involved in nutrient mobilisation is critical in selecting suitable species to adapt forests to environmental change. Here, we visualised and investigated the rhizosphere distribution of enzyme activities (cellobiohydrolase, leucine-aminopeptidase, and acid phosphomonoesterase) using zymography. We related the distribution of enzyme activities to the seedling root morphological traits of European beech (Fagus sylvatica) and Norway spruce (Picea abies), the two most cultivated temperate tree species that employ contrasting strategies in soil nutrient acquisition. We found that spruce showed a higher morphological heterogeneity along the roots than beech, resulting in a more robust relationship between rhizoplane-associated enzyme activities and the longitudinal distance from the root apex. The rhizoplane enzyme activities decreased in spruce and increased in beech with the distance from the root apex over a power-law equation. Spruce revealed broader rhizosphere extents of all three enzymes, but only acid phosphomonoesterase activity was higher compared with beech. This latter result was determined by a larger root system found in beech compared with spruce that enhanced cellobiohydrolase and leucine-aminopeptidase activities. The root hair zone and hair lengths were significant variables determining the distribution of enzyme activities in the rhizosphere. Our findings indicate that spruce has a more substantial influence on rhizosphere enzyme production and diffusion than beech, enabling spruce to better mobilise nutrients from organic sources in heterogeneous forest soils.

Lysinibacillus macroides-mediated control of cellulose-producing morphotype of Salmonella

BACKGROUND

Soil-dwelling human pathogens like Salmonella are transmitted by fresh produce such as tomato, spinach, onion and cabbage. With >2600 serovars, it is difficult to classify the good plant colonizers from the non-colonizers. Generally, soil microbiota are classified as autochthonous or zymogenous organisms, based on their ability to survive in soil. However, such information for soil-dwelling human pathogens is not available Thus there is a need to classify these organisms for designing a strategy to prevent their outbreak. Moreover, soil harbours a plethora of microbes, which can be screened for competitive organisms to control such human pathogens.

RESULTS

In this study, we examined whether the morphotype based on the attachment factors (e.g., cellulose and curli fimbri) of Salmonella was important for its colonization of roots. Secondly, we tracked the location of the bacteria in the plant cell. Interestingly, most of the epidermal cells occupied by Salmonella showed propidium iodide-positive nuclei. As an extension of the study, a screening of competitive rhizospheric bacteria was performed. One isolate, identified as Lysinibacillus macroides, was able to inhibit the biofilm of Salmonella and subsequently reduced its colonization on roots.

CONCLUSION

Based on this study, we classified the Rdar (red, dry and rough) morphotypes as good plant colonists. The ability to colonize and subsequent kill the live plant cell throws light on the zymogenous life cycle of soil-dwelling Salmonella. Additionally, Lysinibacillus macroides served as a biocontrol agent by reducing the burden of Salmonella in various vegetables. Such organisms can further be explored to prevent contamination of the food chain. © 2022 Society of Chemical Industry.

Rhizosheath: An adaptive root trait to improve plant tolerance to phosphorus and water deficits?

Drought and nutrient limitations adversely affect crop yields, with below-ground traits enhancing crop production in these resource-poor environments. This review explores the interacting biological, chemical and physical factors that determine rhizosheath (soil adhering to the root system) development, and its influence on plant water uptake and phosphorus acquisition in dry soils. Identification of quantitative trait loci for rhizosheath development indicate it is genetically determined, but the microbial community also directly (polysaccharide exudation) and indirectly (altered root hair development) affect its extent. Plants with longer and denser root hairs had greater rhizosheath development and increased P uptake efficiency. Moreover, enhanced rhizosheath formation maintains contact at the root-soil interface thereby assisting water uptake from drying soil, consequently improving plant survival in droughted environments. Nevertheless, it can be difficult to determine if rhizosheath development is a cause or consequence of improved plant adaptation to dry and nutrient-depleted soils. Does rhizosheath development directly enhance plant water and phosphorus use, or do other tolerance mechanisms allow plants to invest more resources in rhizosheath development? Much more work is required on the interacting genetic, physical, biochemical and microbial mechanisms that determine rhizosheath development, to demonstrate that selection for rhizosheath development is a viable crop improvement strategy.

Altered properties and structures of root exudate polysaccharides in a root hairless mutant of barley

Root exudates and rhizosheaths of attached soil are important features of growing roots. To elucidate factors involved in rhizosheath formation, wild-type (WT) barley (Hordeum vulgare L. cv. Pallas) and a root hairless mutant, bald root barley (brb), were investigated with a combination of physiological, biochemical, and immunochemical assays. When grown in soil, WT barley roots bound ∼5-fold more soil than brb per unit root length. High molecular weight (HMW) polysaccharide exudates of brb roots had less soil-binding capacity than those of WT root exudates. Carbohydrate and glycan monoclonal antibody analyses of HMW polysaccharide exudates indicated differing glycan profiles. Relative to WT plants, root exudates of brb had reduced signals for arabinogalactan-protein (AGP), extensin, and heteroxylan epitopes. In contrast, the root exudate of 2-week-old brb plants contained ∼25-fold more detectable xyloglucan epitope relative to WT. Root system immunoprints confirmed the higher levels of release of the xyloglucan epitope from brb root apices and root axes relative to WT. Epitope detection with anion-exchange chromatography indicated that the increased detection of xyloglucan in brb exudates was due to enhanced abundance of a neutral polymer. Conversely, brb root exudates contained decreased amounts of an acidic polymer, with soil-binding properties, containing the xyloglucan epitope and glycoprotein and heteroxylan epitopes relative to WT. We, therefore, propose that, in addition to physically structuring soil particles, root hairs facilitate rhizosheath formation by releasing a soil-binding polysaccharide complex.

Root phenotypes as modulators of microbial microhabitats

Plant roots are colonized by a multitude of microbial taxa that dynamically influence plant health. Plant-microbe interactions at the root-soil interface occur at the micro-scale and are affected by variation in root phenotypes. Different root phenotypes can have distinct impacts on physical and chemical gradients at the root-soil interface, leading to heterogeneous microhabitats for microbial colonization. Microbes that influence plant physiology will establish across these heterogeneous microhabitats, and, therefore, exploiting variation in root phenotypes can allow for targeted manipulation of plant-associated microbes. In this mini-review, we discuss how changes in root anatomy and architecture can influence resource availability and the spatial configuration of microbial microhabitats. We then propose research priorities that integrate root phenotypes and microbial microhabitats for advancing the manipulation of root-associated microbiomes. We foresee the yet-unexplored potential to harness diverse root phenotypes as a new level of precision in microbiome management in plant-root systems.

The response of wheat and its microbiome to contemporary and historical water stress in a field experiment

In a field experiment, we evaluated the impact of 37 years of contrasting water stress history on the microbial response in various plant compartments at two distinct developmental stages when four wheat genotypes were exposed to contemporary water stress. Seeds were collected and sampled at the end of the experiment to characterize endophytic and epiphytic microbial communities. Amplicon sequencing data revealed that plant development stage and water stress history were the main factors shaping the microbiome of the major plant parts in response to contemporary water limitation. Our results indicate that seeds can become colonized by divergent microbial communities within a single generation based on the initial pool of microbes as determined by historical contingencies, which was modulated by the contemporary environmental conditions and the plant genotype. Such information is essential to incorporate microbial-based strategies into conventional plant breeding to enhance plant resistance to stress.

Microbial Inoculation Improves Growth, Nutritional and Physiological Aspects of Glycine max (L.) Merr

Considering a scenario where there is a low availability and increasing costs of fertilizers in the global agricultural market, as well as a finitude of important natural resources, such as phosphorus (P), this study tested the effect of the inoculation of rhizospheric or endophytic microorganisms isolated from Hymenaea courbaril and Butia purpurascens on the growth promotion of Glycine max (L.) Merr. The tests were conducted in a controlled greenhouse system, and the effects of biofertilization were evaluated using the following parameters: dry biomass, nutritional content, and photochemical and photosynthetic performance of plants. Seed biopriming was performed with four bacterial and four fungal isolates, and the results were compared to those of seeds treated with the commercial product Biomaphos^®. Overall, microbial inoculation had a positive effect on biomass accumulation in G. max, especially in strains PA12 (Paenibacillus alvei), SC5 (Bacillus cereus), and SC15 (Penicillium sheari). The non-inoculated control plants accumulated less nutrients, both in the whole plant and aerial part, and had reduced chlorophyll index and low photosynthetic rate (A) and photochemical efficiency. Strains PA12 (P. alvei), SC5 (B. cereus), and 328EF (Codinaeopsis sp.) stood out in the optimization of nutrient concentration, transpiration rate, and stomatal conductance. Plants inoculated with the bacterial strains PA12 (P. alvei) and SC5 (B. cereus) and with the fungal strains 328EF (Codinaeopsis sp.) and SC15 (P. sheari) showed the closest pattern to that observed in plants treated with Biomaphos^®, with the same trend of direction of the means associated with chlorophyll index, (A), dry mass, and concentration of important nutrients such as N, P, and Mg. We recommend the use of these isolates in field tests to validate these strains for the production of biological inoculants as part of the portfolio of bioinputs available for G. max.

Building soil sustainability from root-soil interface traits

Great potential exists to harness plant traits at the root-soil interface, mainly rhizodeposition and root hairs, to 'build' soils with better structure that can trap more carbon and resources, resist climate stresses, and promote a healthy microbiome. These traits appear to have been preserved in modern crop varieties, but scope exists to improve them further because they vary considerably between genotypes and respond to environmental conditions. From emerging evidence, rhizodeposition can act as a disperser, aggregator, and/or hydrogel in soil, and root hairs expand rhizosheath size. Future research should explore impacts of selecting these traits on plants and soils concurrently, expanding from model plants to commercial genotypes, and observing whether impacts currently limited to glasshouse studies occur in the field.

Nutrients in the rhizosphere: A meta-analysis of content, availability, and influencing factors

Nutrient deficiency in most terrestrial ecosystems constrains global primary productivity. Rhizosphere nutrient availability directly regulates plant growth and is influenced by many factors, including soil properties, plant characteristics and climate. A quantitatively comprehensive understanding of the role of these factors in modulating rhizosphere nutrient availability remains largely unknown. We reviewed 123 studies to assess nutrient availability in the rhizosphere compared to bulk soil depending on various factors. The increase in microbial nitrogen (N) content and N-cycling related enzyme activities in the rhizosphere led to a 10% increase in available N relative to bulk soil. The available phosphorus (P) in the rhizosphere decreased by 12% with a corresponding increase in phosphatase activities, indicating extreme demand and competition between plants and microorganisms for P. Greater organic carbon (C) content around taproots (+17%) confirmed their stronger ability to store more organic compounds than the fibrous roots. This corresponds to higher bacterial and fungal contents and slightly higher available nutrients in the rhizosphere of taproots. The maximal rhizosphere nutrient accumulation was common for low-fertile soils, which is confirmed by the negative correlation between most soil chemical properties and the effect sizes of available nutrients. Increases in rhizosphere bacterial and fungal population densities (205-254%) were much higher than microbial biomass increases (indicated as microbial C: +19%). Consequently, despite the higher microbial population densities in the rhizosphere, the biomass of individual microbial cells decreased, pointing on their younger age and faster turnover. This meta-analysis shows that, contrary to the common view, most nutrients are more available in the rhizosphere than in bulk soil because of higher microbial activities around roots.

Selection for early shoot vigour in wheat increases root hair length but reduces epidermal cell size of roots and leaves

Six cycles of recurrent selection for early shoot vigour in wheat resulted in significant increases in leaf width and shoot biomass. Here, in replicated controlled-environment studies, the effect of early shoot vigour on root biomass, rhizosheath size, root hair length, and cell size in the roots and leaves was examined across different cycles of selection. Increased shoot vigour was associated with greater root biomass, larger rhizosheath size, and longer root hairs. Our findings demonstrate that rhizosheath size was a reliable surrogate for root hair length in this germplasm. Examination of the root epidermis revealed that the 'cell body' of the trichoblasts (hair-forming cells) and the atrichoblasts (non-hair-forming cells) decreased in size as shoot vigour increased. Therefore, in higher vigour germplasm, longer root hairs emerged from smaller trichoblasts so that total trichoblast volume (root hair plus cell body) was generally similar regardless of shoot vigour. Similarly, the sizes of the four main cell types on the leaf epidermis became progressively smaller as shoot vigour increased, which also increased stomatal density. The relationship between shoot vigour and root traits is considered, and the potential contribution of below-ground root traits to performance and competitiveness of high vigour germplasm is discussed.

A glass bead semi-hydroponic system for intact maize root exudate analysis and phenotyping

BACKGROUND

Although there have been numerous studies describing plant growth systems for root exudate collection, a common limitation is that these systems require disruption of the plant root system to facilitate exudate collection. Here, we present a newly designed semi-hydroponic system that uses glass beads as solid support to simulate soil impedance, which combined with drip irrigation, facilitates growth of healthy maize plants, collection and analysis of root exudates, and phenotyping of the roots with minimal growth disturbance or root damage.

RESULTS

This system was used to collect root exudates from seven maize genotypes using water or 1 mM CaCl₂, and to measure root phenotype data using standard methods and the Digital imaging of root traits (DIRT) software. LC-MS/MS (Liquid Chromatography-Tandem Mass Spectrometry) and GC-MS (Gas Chromatography-Mass Spectrometry) targeted metabolomics platforms were used to detect and quantify metabolites in the root exudates. Phytohormones, some of which are reported in maize root exudates for the first time, the benzoxazinoid DIMBOA (2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one), amino acids, and sugars were detected and quantified. After validating the methodology using known concentrations of standards for the targeted compounds, we found that the choice of the exudate collection solution affected the exudation and analysis of a subset of analyzed metabolites. No differences between collection in water or CaCl₂ were found for phytohormones and sugars. In contrast, the amino acids were more concentrated when water was used as the exudate collection solution. The collection in CaCl₂ required a clean-up step before MS analysis which was found to interfere with the detection of a subset of the amino acids. Finally, using the phenotypic measurements and the metabolite data, significant differences between genotypes were found and correlations between metabolites and phenotypic traits were identified.

CONCLUSIONS

A new plant growth system combining glass beads supported hydroponics with semi-automated drip irrigation of sterile solutions was implemented to grow maize plants and collect root exudates without disturbing or damaging the roots. The validated targeted exudate metabolomics platform combined with root phenotyping provides a powerful tool to link plant root and exudate phenotypes to genotype and study the natural variation of plant populations.

Significance of root hairs in developing stress-resilient plants for sustainable crop production

Root hairs represent a beneficial agronomic trait to potentially reduce fertilizer and irrigation inputs. Over the past decades, research in the plant model Arabidopsis thaliana has provided insights into root hair development, the underlying genetic framework and the integration of environmental cues within this framework. Recent years have seen a paradigm shift, where studies are now highlighting conservation and diversification of root hair developmental programs in other plant species and the agronomic relevance of root hairs in a wider ecological context. In this review, we specifically discuss the molecular evolution of the RSL (RHD Six-Like) pathway that controls root hair development and growth in land plants. We also discuss how root hairs contribute to plant performance as an active physiological rooting structure by performing resource acquisition, providing anchorage and constructing the rhizosphere with desirable physical, chemical and biological properties. Finally, we outline future research directions that can help achieve the potential of root hairs in developing sustainable agroecosystems.

Breeding rice for a changing climate by improving adaptations to water saving technologies

Climate change is expected to increasingly affect rice production through rising temperatures and decreasing water availability. Unlike other crops, rice is a main contributor to greenhouse gas emissions due to methane emissions from flooded paddy fields. Climate change can therefore be addressed in two ways in rice: through making the crop more climate resilient and through changes in management practices that reduce methane emissions and thereby slow global warming. In this review, we focus on two water saving technologies that reduce the periods lowland rice will be grown under fully flooded conditions, thereby improving water use efficiency and reducing methane emissions. Rice breeding over the past decades has mostly focused on developing high-yielding varieties adapted to continuously flooded conditions where seedlings were raised in a nursery and transplanted into a puddled flooded soil. Shifting cultivation to direct-seeded rice or to introducing non-flooded periods as in alternate wetting and drying gives rise to new challenges which need to be addressed in rice breeding. New adaptive traits such as rapid uniform germination even under anaerobic conditions, seedling vigor, weed competitiveness, root plasticity, and moderate drought tolerance need to be bred into the current elite germplasm and to what extent this is being addressed through trait discovery, marker-assisted selection and population improvement are reviewed.

Lead speciation and availability affected by plants in a contaminated soil

Changes in lead (Pb) speciation in the rhizosphere may be plant species-dependent and dictate Pb fate and behavior in the soil-plant system. X-ray absorption near edge structure (XANES) spectroscopy can explain how these changes affect Pb availability in soils and its uptake by plants. We investigated the changes in Pb speciation and availability in the rhizosphere of eucalypt (Eucalyptus urophylla x Eucalyptus grandis), palisade grass (Urochloa brizantha cv. Marandu), and Indian mustard (Brassica juncea L.) using XANES spectroscopy. A greenhouse experiment was performed in a complete randomized design, with three plant species and a no plant control treatment. After three months, rhizosphere and bulk soil samples were collected, Pb speciation was assessed by Pb L₃-edge XANES spectroscopy, and Pb concentration was determined in plant tissue. In bulk soil, we found Pb primarily as Pb-Il (Pb sorbed to illite; 48%) and lead monoxide (PbO; 36%). In the rhizosphere, Pb-Ka (Pb sorbed to kaolinite; 33-56%) and (CH₃COO)₂Pb (25-41%) were the main Pb forms, the latter suggesting Pb complexation by low molecular weight organic acids (LMWOAs). Palisade grass rhizospheric soil had a lower abundance of Pb-Ka and a higher abundance of (CH₃COO)₂Pb than eucalypt and mustard, which led to low Pb concentration in plant tissue. LMWOAs exudation followed by Pb²⁺ complexation is the apparent mechanism used by palisade grass to detoxify the rhizosphere and control Pb uptake. Given its low Pb uptake and potential to complex Pb in organic forms, palisade grass may be a promising species for Pb phytostabilization in contaminated soils.

Root hairs vs. trichomes: Not everyone is straight!

Trichomes show 47 morphological phenotypes, while literature reports only two root hair phenotypes in all plants. However, could hair-like structures exist below-ground in a similar wide range of morphologies like trichomes? Genetic mutants and root hair stress phenotypes point to the possibility of uncharacterized morphological variation existing belowground. For example, such root hairs in Arabidopsis (Arabidopsis thaliana) can be wavy, curled, or branched. We found hints in the literature about hair-like structures that emerge before root hairs belowground. As such, these early emerging hair structures can be potential exceptions to the contrasting morphological variation between trichomes and root hairs. Here, we show a previously unreported 'hooked' hair structure growing below-ground in common bean. The unique 'hooking' shape distinguishes the 'hooked hair' morphologically from root hairs. Currently, we cannot fully characterize the phenotype of our observation due to the lack of automated methods for phenotyping root hairs. This phenotyping bottleneck also handicaps the discovery of more morphology types that might exist below-ground as manual screening across species is slower than computer-assisted high-throughput screening.

Comparison of methods for mapping rhizosphere processes in the context of their surrounding root and soil environments

The rhizosphere embodies a complex biogeochemical zone with enhanced rates of nutrient exchange between plants, soil, and microbial communities. Understanding controls on rhizosphere dynamics is critical to support emerging concepts including rhizosphere engineering and reduced dependence on chemical fertilizers which have direct application to food production, increased biofuel generation, and habitat restoration efforts. Yet, its fine spatial scale and complex interactions between geochemical and microbial processes within complex spatiotemporal gradients make the rhizosphere notoriously difficult to study. Emerging instrumentation and methodologies, however, are providing improved resolution to rhizosphere measurements and helping to address critical knowledge gaps in rhizosphere function, ecology, and establishment. Here, we examine recent advances in analysis techniques and the resulting potential for improved understanding of rhizosphere function.

A starting guide to root ecology: strengthening ecological concepts and standardising root classification, sampling, processing and trait measurements

In the context of a recent massive increase in research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on generating cutting-edge, meaningful and integrated knowledge. Consideration of the below-ground components in plant and ecosystem studies has been consistently called for in recent decades, but methodology is disparate and sometimes inappropriate. This handbook, based on the collective effort of a large team of experts, will improve trait comparisons across studies and integration of information across databases by providing standardised methods and controlled vocabularies. It is meant to be used not only as starting point by students and scientists who desire working on below-ground ecosystems, but also by experts for consolidating and broadening their views on multiple aspects of root ecology. Beyond the classical compilation of measurement protocols, we have synthesised recommendations from the literature to provide key background knowledge useful for: (1) defining below-ground plant entities and giving keys for their meaningful dissection, classification and naming beyond the classical fine-root vs coarse-root approach; (2) considering the specificity of root research to produce sound laboratory and field data; (3) describing typical, but overlooked steps for studying roots (e.g. root handling, cleaning and storage); and (4) gathering metadata necessary for the interpretation of results and their reuse. Most importantly, all root traits have been introduced with some degree of ecological context that will be a foundation for understanding their ecological meaning, their typical use and uncertainties, and some methodological and conceptual perspectives for future research. Considering all of this, we urge readers not to solely extract protocol recommendations for trait measurements from this work, but to take a moment to read and reflect on the extensive information contained in this broader guide to root ecology, including sections I-VII and the many introductions to each section and root trait description. Finally, it is critical to understand that a major aim of this guide is to help break down barriers between the many subdisciplines of root ecology and ecophysiology, broaden researchers' views on the multiple aspects of root study and create favourable conditions for the inception of comprehensive experiments on the role of roots in plant and ecosystem functioning.

Root traits as drivers of plant and ecosystem functioning: current understanding, pitfalls and future research needs

The effects of plants on the biosphere, atmosphere and geosphere are key determinants of terrestrial ecosystem functioning. However, despite substantial progress made regarding plant belowground components, we are still only beginning to explore the complex relationships between root traits and functions. Drawing on the literature in plant physiology, ecophysiology, ecology, agronomy and soil science, we reviewed 24 aspects of plant and ecosystem functioning and their relationships with a number of root system traits, including aspects of architecture, physiology, morphology, anatomy, chemistry, biomechanics and biotic interactions. Based on this assessment, we critically evaluated the current strengths and gaps in our knowledge, and identify future research challenges in the field of root ecology. Most importantly, we found that belowground traits with the broadest importance in plant and ecosystem functioning are not those most commonly measured. Also, the estimation of trait relative importance for functioning requires us to consider a more comprehensive range of functionally relevant traits from a diverse range of species, across environments and over time series. We also advocate that establishing causal hierarchical links among root traits will provide a hypothesis-based framework to identify the most parsimonious sets of traits with the strongest links on functions, and to link genotypes to plant and ecosystem functioning.

A starting guide to root ecology: strengthening ecological concepts and standardising root classification, sampling, processing and trait measurements

In the context of a recent massive increase in research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on generating cutting-edge, meaningful and integrated knowledge. Consideration of the below-ground components in plant and ecosystem studies has been consistently called for in recent decades, but methodology is disparate and sometimes inappropriate. This handbook, based on the collective effort of a large team of experts, will improve trait comparisons across studies and integration of information across databases by providing standardised methods and controlled vocabularies. It is meant to be used not only as starting point by students and scientists who desire working on below-ground ecosystems, but also by experts for consolidating and broadening their views on multiple aspects of root ecology. Beyond the classical compilation of measurement protocols, we have synthesised recommendations from the literature to provide key background knowledge useful for: (1) defining below-ground plant entities and giving keys for their meaningful dissection, classification and naming beyond the classical fine-root vs coarse-root approach; (2) considering the specificity of root research to produce sound laboratory and field data; (3) describing typical, but overlooked steps for studying roots (e.g. root handling, cleaning and storage); and (4) gathering metadata necessary for the interpretation of results and their reuse. Most importantly, all root traits have been introduced with some degree of ecological context that will be a foundation for understanding their ecological meaning, their typical use and uncertainties, and some methodological and conceptual perspectives for future research. Considering all of this, we urge readers not to solely extract protocol recommendations for trait measurements from this work, but to take a moment to read and reflect on the extensive information contained in this broader guide to root ecology, including sections I-VII and the many introductions to each section and root trait description. Finally, it is critical to understand that a major aim of this guide is to help break down barriers between the many subdisciplines of root ecology and ecophysiology, broaden researchers' views on the multiple aspects of root study and create favourable conditions for the inception of comprehensive experiments on the role of roots in plant and ecosystem functioning.

Heterogeneous nutrient supply promotes maize growth and phosphorus acquisition: additive and compensatory effects of lateral roots and root hairs

BACKGROUND AND AIMS

Root proliferation is a response to a heterogeneous nutrient distribution. However, the growth of root hairs in response to heterogeneous nutrients and the relationship between root hairs and lateral roots remain unclear. This study aims to understand the effects of heterogeneous nutrients on root hair growth and the trade-off between root hairs and lateral roots in phosphorus (P) acquisition.

METHODS

Near-isogenic maize lines, the B73 wild type (WT) and the rth3 root hairless mutant, were grown in rhizoboxes with uniform or localized supply of 40 (low) or 140 (high) mg P kg-1 soil.

RESULTS

Both WT and rth3 had nearly two-fold greater shoot biomass and P content under local than uniform treatment at low P. Significant root proliferation was observed in both WT and rth3 in the nutrient patch, with the WT accompanied by an obvious increase (from 0.7 to 1.2 mm) in root hair length. The root response ratio of rth3 was greater than that of WT at low P, but could not completely compensate for the loss of root hairs. This suggests that plants enhanced P acquisition through complementarity between lateral roots and root hairs, and thus regulated nutrient foraging and shoot growth. The disappearance of WT and rth3 root response differences at high P indicated that the P application reduced the dependence of the plants on specific root traits to obtain nutrients.

CONCLUSIONS

In addition to root proliferation, the root response to a nutrient-rich patch was also accompanied by root hair elongation. The genotypes without root hairs increased their investment in lateral roots in a nutrient-rich patch to compensate for the absence of root hairs, suggesting that plants enhanced nutrient acquisition by regulating the trade-off of complementary root traits.

Significance of root hairs for plant performance under contrasting field conditions and water deficit

BACKGROUND AND AIMS

Previous laboratory studies have suggested selection for root hair traits in future crop breeding to improve resource use efficiency and stress tolerance. However, data on the interplay between root hairs and open-field systems, under contrasting soils and climate conditions, are limited. As such, this study aims to experimentally elucidate some of the impacts that root hairs have on plant performance on a field scale.

METHODS

A field experiment was set up in Scotland for two consecutive years, under contrasting climate conditions and different soil textures (i.e. clay loam vs. sandy loam). Five barley (Hordeum vulgare) genotypes exhibiting variation in root hair length and density were used in the study. Root hair length, density and rhizosheath weight were measured at several growth stages, as well as shoot biomass, plant water status, shoot phosphorus (P) accumulation and grain yield.

KEY RESULTS

Measurements of root hair density, length and its correlation with rhizosheath weight highlighted trait robustness in the field under variable environmental conditions, although significant variations were found between soil textures as the growing season progressed. Root hairs did not confer a notable advantage to barley under optimal conditions, but under soil water deficit root hairs enhanced plant water status and stress tolerance resulting in a less negative leaf water potential and lower leaf abscisic acid concentration, while promoting shoot P accumulation. Furthermore, the presence of root hairs did not decrease yield under optimal conditions, while root hairs enhanced yield stability under drought.

CONCLUSIONS

Selecting for beneficial root hair traits can enhance yield stability without diminishing yield potential, overcoming the breeder's dilemma of trying to simultaneously enhance both productivity and resilience. Therefore, the maintenance or enhancement of root hairs can represent a key trait for breeding the next generation of crops for improved drought tolerance in relation to climate change.

Plant phosphorus-acquisition and -use strategies affect soil carbon cycling

Increased anthropogenic nitrogen (N) deposition is driving N-limited ecosystems towards phosphorus (P) limitation. Plants have evolved strategies to respond to P limitation which affect N cycling in plant-soil systems. A comprehensive understanding of how plants with efficient P-acquisition or -use strategies influence carbon (C) and N cycling remains elusive. We highlight how P-acquisition/-use strategies, particularly the release of carboxylates into the rhizosphere, accelerate soil organic matter (SOM) decomposition and soil N mineralisation by destabilising aggregates and organic-mineral associations. We advocate studying the effects of P-acquisition/-use strategies on SOM formation, directly or through microbial turnover.

Characterization of contrasting rice (Oryza sativa L.) genotypes reveals the Pi-efficient schema for phosphate starvation tolerance

BACKGROUND

Phosphorus (P), being one of the essential components of nucleic acids, cell membranes and enzymes, indispensable for diverse cellular processes like photosynthesis/carbohydrate metabolism, energy production, redox homeostasis and signaling. Crop yield is severely affected due to Phosphate (Pi) deficiency; and to cope with Pi-deficiency, plants have evolved several strategies. Some rice genotypes are compatible with low Pi availability, whereas others are sensitive to Pi deficiency. However, the underlying molecular mechanism for low Pi tolerance remains largely unexplored.

RESULT

Several studies were carried out to understand Pi-deficiency responses in rice at seedling stage, but few of them targeted molecular aspects/responses of Pi-starvation at the advanced stage of growth. To delineate the molecular mechanisms for low Pi tolerance, a pair of contrasting rice (Oryza sativa L.) genotypes [viz. Pusa-44 (Pi-deficiency sensitive) and its near isogenic line (NIL-23, Pi-deficiency tolerant) harboring Phosphorus uptake 1 (Pup1) QTL from an aus landrace Kasalath] were used. Comparative morphological, physiological, and biochemical analyses confirmed some of the well-known findings. Transcriptome analysis of shoot and root tissues from 45-day-old rice plants grown hydroponically under P-sufficient (16 ppm Pi) or P-starved (0 ppm Pi) medium revealed that Pi-starvation stress causes global transcriptional reprogramming affecting several transcription factors, signaling pathways and other regulatory genes. We could identify several significantly up-regulated genes in roots of NIL-23 under Pi-starvation which might be responsible for the Pi starvation tolerance. Pathway enrichment analysis indicated significant role of certain phosphatases, transporters, transcription factors, carbohydrate metabolism, hormone-signaling, and epigenetic processes in improving P-starvation stress tolerance in NIL-23.

CONCLUSION

We report the important candidate mechanisms for Pi acquisition/solubilization, recycling, remobilization/transport, sensing/signalling, genetic/epigenetic regulation, and cell wall structural changes to be responsible for P-starvation tolerance in NIL-23. The study provides some of the novel information useful for improving phosphorus-use efficiency in rice cultivars.

Deciphering the genetic basis of wheat seminal root anatomy uncovers ancestral axial conductance alleles

Root axial conductance, which describes the ability of water to move through the xylem, contributes to the rate of water uptake from the soil throughout the whole plant lifecycle. Under the rainfed wheat agro-system, grain-filling is typically occurring during declining water availability (i.e., terminal drought). Therefore, preserving soil water moisture during grain filling could serve as a key adaptive trait. We hypothesized that lower wheat root axial conductance can promote higher yields under terminal drought. A segregating population derived from a cross between durum wheat and its direct progenitor wild emmer wheat was used to underpin the genetic basis of seminal root architectural and functional traits. We detected 75 QTL associated with seminal roots morphological, anatomical and physiological traits, with several hotspots harbouring co-localized QTL. We further validated the axial conductance and central metaxylem QTL using wild introgression lines. Field-based characterization of genotypes with contrasting axial conductance suggested the contribution of low axial conductance as a mechanism for water conservation during grain filling and consequent increase in grain size and yield. Our findings underscore the potential of harnessing wild alleles to reshape the wheat root system architecture and associated hydraulic properties for greater adaptability under changing climate.

Arbuscular mycorrhizal fungi-induced mitigation of heavy metal phytotoxicity in metal contaminated soils: A critical review

The heavy metal pollution is a worldwide problem and has received a serious concern for the ecosystem and human health. In the last decade, remediation of the agricultural polluted soil has attracted great attention. Phytoremediation is one of the technologies that effectively alleviate heavy metal toxicity, however, this technique is limited to many factors contributing to low plant growth rate and nature of metal toxicities. Arbuscular mycorrhizal fungi (AMF) assisted alleviation of heavy metal phytotoxicity is a cost-effective and environment-friendly strategy. AMF have a symbiotic relationship with the host plant. The bidirectional exchange of resources is a hallmark and also a functional necessity in mycorrhizal symbiosis. During the last few years, a significant progress in both physiological and molecular mechanisms regarding roles of AMF in the alleviation of heavy metals (HMs) toxicities in plants, acquisition of nutrients, and improving plant performance under toxic conditions of HMs has been well studied. This review summarized the current knowledge regarding AMF assisted remediation of heavy metals and some of the strategies used by mycorrhizal fungi to cope with stressful environments. Moreover, this review provides the information of both molecular and physiological responses of mycorrhizal plants as well as AMF to heavy metal stress which could be helpful for exploring new insight into the mechanisms of HMs remediation by utilizing AMF.

Response of Plant Rhizosphere Microenvironment to Water Management in Soil- and Substrate-Based Controlled Environment Agriculture (CEA) Systems: A Review

As natural agroecology deteriorates, controlled environment agriculture (CEA) systems become the backup support for coping with future resource consumption and potential food crises. Compared with natural agroecology, most of the environmental parameters of the CEA system rely on manual management. Such a system is dependent and fragile and prone to degradation, which includes harmful bacteria proliferation and productivity decline. Proper water management is significant for constructing a stabilized rhizosphere microenvironment. It has been proved that water is an efficient tool for changing the availability of nutrients, plant physiological processes, and microbial communities within. However, for CEA issues, relevant research is lacking at present. The article reviews the interactive mechanism between water management and rhizosphere microenvironments from the perspectives of physicochemical properties, physiological processes, and microbiology in CEA systems. We presented a synthesis of relevant research on water-root-microbes interplay, which aimed to provide detailed references to the conceptualization, research, diagnosis, and troubleshooting for CEA systems, and attempted to give suggestions for the construction of a high-tech artificial agricultural ecology.

Looking for Root Hairs to Overcome Poor Soils

Breeding new cultivars allowing reduced fertilization and irrigation is a major challenge. International efforts towards this goal focus on noninvasive methodologies, platforms for high-throughput phenotyping of large plant populations, and quantitative description of root traits as predictors of crop performance in environments with limited water and nutrient availability. However, these high-throughput analyses ignore one crucial component of the root system: root hairs (RHs). Here, we review current knowledge on RH functions, mainly in the context of plant hydromineral nutrition, and take stock of quantitative genetics data pointing at correlations between RH traits and plant biomass production and yield components.

Genome-wide association study for phosphate deficiency responsive root hair elongation in chickpea

Root hairs (RHs) are single-celled elongated epidermal cells and play a vital role in nutrient absorption, particularly for immobile minerals like phosphorus (P). As an adaptive response to P deficiency, an increase in RH length enhances root-soil contact and absorptive area for P absorption. Genetic variations have been reported for RH length and its response to P deficiency in plants. However, only a few association studies have been conducted to identify genes and genetic loci associated with RH length. Here, we screened desi chickpea accessions for RH length and its plasticity under P deficiency. Further, the genome-wide association study (GWAS) was conducted to identify the genetic loci associated with RH length in P deficient and sufficient conditions. Although high variability was observed in terms of RH length in diverse genotypes, majority of the accessions showed typical response of increase in RH length in low P. Genome-wide association mapping identified many SNPs with significant associations with RH length in P-sufficient and P-deficient conditions. A few candidate genes for RH length in P deficient (SIZ1-like and HAD superfamily protein) and sufficient (RSL2-like and SMAP1-like) conditions were identified which have known roles in RH development and P deficiency response or both. Highly associated loci and candidate genes identified in this study would be useful for genomic-assisted breeding to develop P-efficient chickpea.

Visualising spatio-temporal distributions of assimilated carbon translocation and release in root systems of leguminous plants

The release of rhizodeposits differs depending on the root position and is closely related to the assimilated carbon (C) supply. Therefore, quantifying the C partitioning over a short period may provide crucial information for clarifying root-soil carbon metabolism. A non-invasive method for visualising the translocation of recently assimilated C into the root system inside the rhizobox was established using 11CO2 labelling and the positron-emitting tracer imaging system. The spatial distribution of recent 11C-photoassimilates translocated and released in the root system and soil were visualised for white lupin (Lupinus albus) and soybean (Glycine max). The inputs of the recently assimilated C in the entire root that were released into the soil were approximately 0.3%-2.9% for white lupin within 90 min and 0.9%-2.3% for soybean within 65 min, with no significant differences between the two plant species; however, the recently assimilated C of lupin was released at high concentrations in specific areas (hotspots), whereas that of soybean was released uniformly in the soil. Our method enabled the quantification of the spatial C allocations in roots and soil, which may help to elucidate the relationship between C metabolism and nutrient cycling at specific locations of the root-soil system in response to environmental conditions over relatively short periods.

Optical coherence tomography imaging of plant root growth in soil

Complex interactions between roots and soil provide the nutrients and physical support required for robust plant growth. Yet, visualizing the root-soil interface is challenged by soil's opaque scattering characteristics. Herein, we describe methods for using optical coherence tomography (OCT) to provide non-destructive 3D and cross-sectional root imaging not available with traditional bright-field microscopy. OCT is regularly used for bioimaging, especially in ophthalmology, where it can detect retinal abnormalities. Prior use of OCT in plant biology has focused on surface defects of above-ground tissues, predominantly in food crops. Our results show OCT is also viable for detailed, in situ study of living plant roots. Using OCT for direct observations of root growth in soil can help elucidate key interactions between root morphology and various components of the soil environment including soil structure, microbial communities, and nutrient patches. Better understanding of these interactions can guide efforts to improve plant nutrient acquisition from soil to increase agricultural efficiency as well as better understand drivers of plant growth in natural systems.

Estimating the importance of maize root hairs in low phosphorus conditions and under drought

BACKGROUND AND AIMS

Root hairs are single-cell extensions of the epidermis that face into the soil and increase the root-soil contact surface. Root hairs enlarge the rhizosphere radially and are very important for taking up water and sparingly soluble nutrients, such as the poorly soil-mobile phosphate. In order to quantify the importance of root hairs for maize, a mutant and the corresponding wild type were compared.

METHODS

The rth2 maize mutant with very short root hairs was assayed for growth and phosphorus (P) acquisition in a slightly alkaline soil with low P and limited water supply in the absence of mycorrhization and with ample P supply.

KEY RESULTS

Root and shoot growth was additively impaired under P deficiency and drought. Internal P concentrations declined with reduced water and P supply, whereas micronutrients (iron, zinc) were little affected. The very short root hairs in rth2 did not affect internal P concentrations, but the P content of juvenile plants was halved under combined stress. The rth2 plants had more fine roots and increased specific root length, but P mobilization traits (root organic carbon and phosphatase exudation) differed little.

CONCLUSIONS

The results confirm the importance of root hairs for maize P uptake and content, but not for internal P concentrations. Furthermore, the performance of root hair mutants may be biased by secondary effects, such as altered root growth.

Root cortical anatomy is associated with differential pathogenic and symbiotic fungal colonization in maize

Root anatomical phenotypes vary among maize (Zea mays) cultivars and may have adaptive value by modifying the metabolic cost of soil exploration. However, the microbial trade-offs of these phenotypes are unknown. We hypothesized that nodal roots of maize with contrasting cortical anatomy have different patterns of mutualistic and pathogenic fungal colonization. Arbuscular mycorrhizal colonization in the field and mesocosms, root rots in the field, and Fusarium verticillioides colonization in mesocosms were evaluated in maize genotypes with contrasting root cortical anatomy. Increased aerenchyma and decreased living cortical area were associated with decreased mycorrhizal colonization in mesocosm and field experiments with inbred genotypes. In contrast, mycorrhizal colonization of hybrids increased with larger aerenchyma lacunae; this increase coincided with larger root diameters of hybrid roots. F. verticillioides colonization was inversely correlated with living cortical area in mesocosm-grown inbreds, and no relation was found between root rots and living cortical area or aerenchyma in field-grown hybrids. Root rots were positively correlated with cortical cell file number and inversely correlated with cortical cell size. Mycorrhizae and root rots were inversely correlated in field-grown hybrids. We conclude that root anatomy is associated with differential effects on pathogens and mycorrhizal colonization of nodal roots in maize.