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Iron toxicity is a major constraint to rice production, particularly in highly weathered soils of inland valleys in sub‐Saharan Africa where the rice growing area is rapidly expanding. There is a wide variation in tolerance of iron toxicity in the rice germplasm. However, the introgression of tolerance traits into high‐yielding germplasm has been slow owing to the complexity of the tolerance mechanisms and large genotype‐by‐environment effects. We review current understanding of tolerance mechanisms, particularly those involving below‐ground plant–soil interactions. Until now these have been less studied than above‐ground mechanisms. We cover processes in the rhizosphere linked to exclusion of toxic ferrous iron by oxidation, and resulting effects on the mobility of nutrient ions. We also cover the molecular physiology of below‐ground processes controlling iron retention in roots and root‐shoot transport, and also plant iron sensing. We conclude that future breeding programmes should be based on well‐characterized molecular markers for iron toxicity tolerance traits. To successfully identify such markers, the complex tolerance response should be broken down into its components based on understanding of tolerance mechanisms, and tailored screening methods should be developed for individual mechanisms. Progress in identifying markers for Fe toxicity tolerance in rice has been slow because of the complexity of tolerance mechanisms and the importance of below‐ground plant‐soil interactions. We review current understanding of these below‐ground processes, including recent advances in the molecular physiology of tolerance. [ABSTRACT FROM AUTHOR]

Contents Summary 11 I. Introduction 11 II. Phosphate uptake and its regulation 14 III. Phosphate uptake assisted by the arbuscular–mycorrhizal 18 symbiosis IV. Agricultural potential 21 V. Conclusions and perspectives 21 Acknowledgements 22 References 22 Phosphorus (P) is an essential plant nutrient and one of the most limiting in natural habitats as well as in agricultural production world-wide. The control of P acquisition efficiency and its subsequent uptake and translocation in vascular plants is complex. The physiological role of key cellular structures in plant P uptake and underlying molecular mechanisms are discussed in this review, with emphasis on phosphate transport across the cellular membrane at the root and arbuscular–mycorrhizal (AM) interfaces. The tools of molecular genetics have facilitated novel approaches and provided one of the major driving forces in the investigation of the basic transport mechanisms underlying plant P nutrition. Genetic engineering holds the potential to modify the system in a targeted way at the root–soil or AM symbiotic interface. Such approaches should assist in the breeding of crop plants that exhibit improved P acquisition efficiency and thus require lower inputs of P fertilizer for optimal growth. Whether engineering of P transport systems can contribute to enhanced P uptake will be discussed. [ABSTRACT FROM AUTHOR]

Summary For biochemical modification of the root–soil interface, the engineered secretion of stable enzymes from trichoblasts (= root hair bearing rhizodermal cells) is proposed. As a reporter activity, we chose to express a synthetic gene encoding a secretory phytase (PHY) directed by a trichoblast-specific promoter in root hair cells of the crop plant potato. Transgenic plants produced and secreted phytase in sufficient amounts to release phosphate from phytate in liquid medium. When grown in an unsterile substrate containing phytate, transgenic plants accumulated 40% more P in leaves than wild-type plants. The improved P nutrition driven by trichoblast-targeted expression and subsequent secretion of PHY illustrates the potential of using trichoblast-targeted expression of suitable enzymes for future applications in plant nutrition, phytoremediation and molecular farming. [ABSTRACT FROM AUTHOR]

To investigate the extent and size of root-soil air gaps that develop during soil drying, resign casts of roots of the desert succulent Agave deserti Engelm, were made in situ for container-grown plants and in the field. Plants that were droughted in containers for 7 and 14 had 24 and 34% root shrinkage, respectively, leading to root-soil air gaps that would reduce the hydraulic conductivity at the root-soil interface by a factor of about 5. When containers were vibrated during drought, root-soil air gaps were greatly diminished, and the predicted conductivity at the interface was similar to that of the control (moist soil). For plants in the field (4 and 6 wk after the last rainfall), root shrinkage was grater than for container-grown plants, but root-soil contact on the root periphery was greater, which led to a higher predicted hydraulic conductivity at the root-soil contact on the root periphery was greater, which led to a higher predicted hydraulic conductivity at the root-soil interface. To test the hypothesis that root-soil air gaps would help to limit water efflux from roots in drying soil, the water potentials of the soil, root, and shoot of plants from vibrated containers (with gaps eliminated or reduced) and non-vibrated containers were compared. The soil water potential was lower for vibrated containers after 14 d of drought, suggesting more rapid depletion of soil water due to better root-soil contact, and the root water potential was lower as well, suggesting greater water loss by roots in the absence of root-soil air gaps. Thus air gap could benefit A. deserti by helping to maintain a higher root water potential in the early stages of drought and later by limiting root water loss at the root-soil interface when the water potential exceeds that of the soil. [ABSTRACT FROM AUTHOR]

A common assumption in models of water flow from soil to root is that the soil can be described in terms of its representative or effective behavior. Microscale heterogeneity and structure are thereby replaced by effective descriptions, and their role in flow processes at the root‐soil interface is neglected. Here the aim was to explore whether a detailed characterization of the microscale heterogeneity at the scale of a single root impacts the relation between flow rate and pressure gradient. Numerical simulations of water flow toward a root surface were carried out in a two‐dimensional domain with a randomized configuration of spatially variable unsaturated hydraulic conductivities and varying boundary conditions, that is, increasing and decreasing root water uptake rates. By employing Matheron's method, the soil hydraulic properties were varied, while the effective hydraulic conductivity (corresponding to the geometric mean) remained unchanged. Results show that domains with a uniform conductivity could not capture important features of water flow and pressure distribution in spatially variable domains. Specifically, increasing heterogeneity at the root‐soil interface allowed to sustain higher root water uptake rates but caused a slower recovery in xylem suction after transpiration ceased. The significance of this is that, under critical conditions, when pressure gradients and flow rates are high, microscale heterogeneity may become an important determinant and should not be neglected in adequate descriptions of water flow from soil to root in dry soil. Key Points: At the scale of a single root soil heterogeneity impacts the relation between flow rate and pressure headHere the emergent relation between water flow and pressure head gradient cannot be described by a single effective conductivityExplicitly accounting for soil heterogeneity might give new insights on water flow in soil‐plant‐atmosphere systems [ABSTRACT FROM AUTHOR]