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Cultivars with increased efficiency of uptake and utilization of soil nutrients are likely to have positive environmental effects through reduced usage of chemicals in agriculture. This review assesses the available literature on differential uptake and utilization efficiency of K in farming systems. Large areas of agricultural land in the world are deficient in K (e.g. 3/4 of paddy soils in China, 2/3 of the wheatbelt in Southern Australia), with export in agricultural produce (especially hay) and leaching (especially in sandy soils) contributing to lowering of K content in the soil. The capacity of a genotype to grow and yield well in soils low in available K is K efficiency. Genotypic differences in efficiency of K uptake and utilization have been reported for all major economically important plants. The K‐efficient phenotype is a complex one comprising a mixture of uptake and utilization efficiency mechanisms. Differential exudation of organic compounds to facilitate release of non‐exchangeable K is one of the mechanisms of differential K uptake efficiency. Genotypes efficient in K uptake may have a larger surface area of contact between roots and soil and increased uptake at the root–soil interface to maintain a larger diffusive gradient towards roots. Better translocation of K into different organs, greater capacity to maintain cytosolic K+ concentration within optimal ranges and increased capacity to substitute Na+ for K+ are the main mechanisms underlying K utilization efficiency. Further breeding for increased K efficiency will be dependent on identification of suitable markers and compounding of efficiency mechanisms into locally adapted germplasm.

Melatonin has been detected in plants in 1995; however, the function and signaling pathway of this putative phytohormone are largely undetermined due to a lack of knowledge about its receptor. Here, we discovered the first phytomelatonin receptor (CAND2/PMTR1) in Arabidopsis thaliana and found that melatonin governs the receptor-dependent stomatal closure. The application of melatonin induced stomatal closure through the heterotrimeric G protein α subunit-regulated H O and Ca signals. The Arabidopsis mutant lines lacking AtCand2 that encodes a candidate G protein-coupled receptor were insensitive to melatonin-induced stomatal closure. Accordingly, the melatonin-induced H O production and Ca influx were completely abolished in cand2. CAND2 is a membrane protein that interacts with GPA1 and the expression of AtCand2 was tightly regulated by melatonin in various organs and guard cells. CAND2 showed saturable and specific I-melatonin binding, with apparent K (dissociation constant) of 0.73 ± 0.10 nmol/L (r = .99), demonstrating this protein is a phytomelatonin receptor (PMTR1). Our results suggest that the phytomelatonin regulation of stomatal closure is dependent on its receptor CAND2/PMTR1-mediated H O and Ca signaling transduction cascade.

Summary Crop nutrition is frequently inadequate as a result of the expansion of cropping into marginal lands, elevated crop yields placing increasing demands on soil nutrient reserves, and environmental and economic concerns about applying fertilizers. Plants exposed to nutrient deficiency activate a range of mechanisms that result in increased nutrient availability in the rhizosphere compared with the bulk soil. Plants may change their root morphology, increase the affinity of nutrient transporters in the plasma membrane and exude organic compounds (carboxylates, phenolics, carbohydrates, enzymes, etc.) and protons. Chemical changes in the rhizosphere result in altered abundance and composition of microbial communities. Nutrient‐efficient genotypes are adapted to environments with low nutrient availability. Nutrient efficiency can be enhanced by targeted breeding through pyramiding efficiency mechanisms in a desirable genotype as well as by gene transfer and manipulation. Rhizosphere microorganisms influence nutrient availability; adding beneficial microorganisms may result in enhanced availability of nutrients to crops. Understanding the role of plant–microbe–soil interactions in governing nutrient availability in the rhizosphere will enhance the economic and environmental sustainability of crop production.

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This review paper critically assesses the literature on soil-microbe-plant interactions influencing availability of micronutrients in the rhizosphere. The emphasis is placed on Zn and Mn, but Fe is also covered to some extent.Micronutrient availability in the rhizosphere is controlled by soil and plant properties, and interactions of roots with microorganisms and the surrounding soil. Plants exude a variety of organic compounds (carboxylate anions, phenolics, carbohydrates, amino acids, enzymes, etc.) and inorganic ions (protons, phosphate, etc.) to change chemistry and biology of the rhizosphere and increase micronutrient availability. Increased availability may result from solubilization and mobilization by short-chain organic acid anions, amino acids and other low-molecular-weight organic compounds. Acidification of the rhizosphere soil increases mobilization of micronutrients (eg. for Zn, 100-fold increase in solubility for each unit of pH decrease).For diffusion-supplied micronutrients, the uptake rate is governed by the soil nutrient supply. Fertilisation with micronutrients (more so in case of Zn than Fe) can be effective in increasing the concentration of micronutrients at the soil-root interface. In addition, micronutrient-efficient crops and genotypes can increase an available nutrient fraction and hence increase micronutrient uptake.Our understanding of the physiological processes governing exudation and the soil-plant-microbe interactions in the rhizosphere is currently inadequate, especially in terms of spatial and temporal variability in root exudation as well as the fate and effectiveness of organic and inorganic compounds in increasing availability of soil micronutrients and undesirable trace elements. The interactions between microorganisms and plants at the soilroot interface are particularly important as well obscure.

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