1. Plant Nutrients. 2. The Soil as a Plant Nutrient Medium. 3. Nutrient Uptake and Assimilation. 4. Plant Water Relationships. 5. Plant Growth and Crop Production. 6. Fertilizer Application. 7. Nitrogen. 8. Sulphur. 9. Phosphorus. 10. Potassium. 11. Calcium. 12. Magnesium. 13. Iron. 14. Manganese. 15. Zinc. 16. Copper. 17. Molybdenum. 18. Boron. 19. Further Elements of Importance. 20. Elements with More Toxic Effects. General Readings. References. Index.

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Principles of plant nutrition

Several microbes promote plant growth, and many microbial products that stimulate plant growth have been marketed. In this review we restrict ourselves to bacteria that are derived from and exert this effect on the root. Such bacteria are generally designated as PGPR (plant-growth-promoting rhizobacteria). The beneficial effects of these rhizobacteria on plant growth can be direct or indirect. This review begins with describing the conditions under which bacteria live in the rhizosphere. To exert their beneficial effects, bacteria usually must colonize the root surface efficiently. Therefore, bacterial traits required for root colonization are subsequently described. Finally, several mechanisms by which microbes can act beneficially on plant growth are described. Examples of direct plant growth promotion that are discussed include (a) biofertilization, (b) stimulation of root growth, (c) rhizoremediation, and (d) plant stress control. Mechanisms of biological control by which rhizobacteria can promote plant growth indirectly, i.e., by reducing the level of disease, include antibiosis, induction of systemic resistance, and competition for nutrients and niches.

Plant-growth-promoting rhizobacteria.

The rhizosphere encompasses the millimeters of soil surrounding a plant root where complex biological and ecological processes occur. This review describes recent advances in elucidating the role of root exudates in interactions between plant roots and other plants, microbes, and nematodes present in the rhizosphere. Evidence indicating that root exudates may take part in the signaling events that initiate the execution of these interactions is also presented. Various positive and negative plant-plant and plant-microbe interactions are highlighted and described from the molecular to the ecosystem scale. Furthermore, methodologies to address these interactions under laboratory conditions are presented.

The Role of Root Exudates in Rhizosphere Interactions with Plants and Other Organisms

Contents
I. Introduction  424
II. The phosphorus conundrum 424
III. Adaptations to low P 424
IV. Uptake of P  424
V. P deficiency alters root development and function 426
VI. P deficiency modifies carbon metabolism 431
VII. Acid phosphatase 436
VIII. Genetic regulation of P responsive genes 437
IX. Improving P acquisition 439
X. Synopsis  440


Summary
Phosphorus (P) is limiting for crop yield on > 30% of the world's arable land and, by some estimates, world resources of inexpensive P may be depleted by 2050. Improvement of P acquisition and use by plants is critical for economic, humanitarian and environmental reasons. Plants have evolved a diverse array of strategies to obtain adequate P under limiting conditions, including modifications to root architecture, carbon metabolism and membrane structure, exudation of low molecular weight organic acids, protons and enzymes, and enhanced expression of the numerous genes involved in low-P adaptation. These adaptations may be less pronounced in mycorrhizal-associated plants. The formation of cluster roots under P-stress by the nonmycorrhizal species white lupin (Lupinus albus), and the accompanying biochemical changes exemplify many of the plant adaptations that enhance P acquisition and use. Physiological, biochemical, and molecular studies of white lupin and other species response to P-deficiency have identified targets that may be useful for plant improvement. Genomic approaches involving identification of expressed sequence tags (ESTs) found under low-P stress may also yield target sites for plant improvement. Interdisciplinary studies uniting plant breeding, biochemistry, soil science, and genetics under the large umbrella of genomics are prerequisite for rapid progress in improving nutrient acquisition and use in plants.

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Phosphorus acquisition and use: critical adaptations by plants for securing a nonrenewable resource

Organic acids, such as malate, citrate and oxalate, have been proposed to be involved in many processes operating in the rhizosphere, including nutrient acquisition and metal detoxification, alleviation of anaerobic stress in roots, mineral weathering and pathogen attraction. A full assessment of their role in these processes, however, cannot be determined unless the exact mechanisms of plant organic acid release and the fate of these compounds in the soil are more fully understood. This review therefore includes information on organic acid levels in plants (concentrations, compartmentalisation, spatial aspects, synthesis), plant efflux (passive versus active transport, theoretical versus experimental considerations), soil reactions (soil solution concentrations, sorption) and microbial considerations (mineralization). In summary, the release of organic acids from roots can operate by multiple mechanisms in response to a number of well-defined environmental stresses (e.g., Al, P and Fe stress, anoxia): These responses, however, are highly stress- and plant-species specific. In addition, this review indicates that the sorption of organic acids to the mineral phase and mineralisation by the soil's microbial biomass are critical to determining the effectiveness of organic acids in most rhizosphere processes.

Organic acids in the rhizosphere: a critical review

▪ Abstract New and exciting developments in boron research in the past few years greatly contributed to better understanding of the role of boron in plants. Purification and identification of the first boron-polyol transport molecules resolved much of the controversy about boron phloem mobility. Isolation and characterization of the boron-polysaccharide complex from cell walls provided the first direct evidence for boron crosslinking of pectin polymers. Inhibition and recovery of proton release upon boron withdrawal and restitution in plant culture medium demonstrated boron involvement in membrane processes. Rapid boron-induced changes in membrane function could be attributed to boron-complexing membrane constituents. Boron may affect metabolic pathways by binding apoplastic proteins to cis-hydroxyl groups of cell walls and membranes, and by interfering with manganese-dependent enzymatic reactions. In addition, boron has been implicated in counteracting toxic effects of aluminum on root growth of dicotyle...

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Boron in plant structure and function

Under certain stress conditions roots exude organic molecules, which may facilitate the uptake of nutrients. The objective of this research was to identify and measure the effect of low P upon the exudation of organic acids by roots of alfalfa (Medicago sativa L.) seedlings. Surface-sterilized alfalfa seeds were grown aseptically in sterile sand using an apparatus specially designed for the addition of ±P nutrient solutions and for the collection of root exudates. Citric, malic, and succinic acids were detected in the root exudates of 24-day-old alfalfa seedlings. Citrate exudation from the roots of P-stressed alfalfa was 182% that of plants receiving a complete nutrient solution. The increased release of citrate may provide a mechanism by which P-stressed plants enhance the availability of P in the rhizosphere.

Citrate, Malate, and Succinate Concentration in Exudates from P-Sufficient and P-Stressed Medicago sativa L. Seedlings.

The Arabidopsis thaliana AtOPT3 belongs to the oligopeptide transporter (OPT) family, a relatively poorly characterized family of peptide/modified peptide transporters found in archebacteria, bacteria, fungi, and plants. A null mutation in AtOPT3 resulted in embryo lethality, indicating an essential role for AtOPT3 in embryo development. In this article, we report on the isolation and phenotypic characterization of a second AtOPT3 mutant line, opt3-2, harboring a T-DNA insertion in the 5′ untranslated region of AtOPT3. The T-DNA insertion in the AtOPT3 promoter resulted in reduced but sufficient AtOPT3 expression to allow embryo formation in opt3-2 homozygous seeds. Phenotypic analyses of opt3-2 plants revealed three interesting loss-of-function phenotypes associated with iron metabolism. First, reduced AtOPT3 expression in opt3-2 plants resulted in the constitutive expression of root iron deficiency responses regardless of exogenous iron supply. Second, deregulation of root iron uptake processes in opt3-2 roots resulted in the accumulation of very high levels of iron in opt3-2 tissues. Hyperaccumulation of iron in opt3-2 resulted in the formation of brown necrotic areas in opt3-2 leaves and was more pronounced during the seed-filling stage. Third, reduced AtOPT3 expression resulted in decreased accumulation of iron in opt3-2 seeds. The reduced accumulation of iron in opt3-2 seeds is especially noteworthy considering the excessively high levels of accumulated iron in other opt3-2 tissues. AtOPT3, therefore, plays a critical role in two important aspects of iron metabolism, namely, maintenance of whole-plant iron homeostasis and iron nutrition of developing seeds.

The Arabidopsis AtOPT3 Protein Functions in Metal Homeostasis and Movement of Iron to Developing Seeds

To acquire iron, many plant species reduce soil Fe(III) to Fe(II) by Fe(III)-chelate reductases embedded in the plasma membrane of root epidermal cells. The reduced product is then taken up by Fe(II) transporter proteins. These activities are induced under Fe deficiency. We describe here the FRO1 gene from pea (Pisum sativum), which encodes an Fe(III)-chelate reductase. Consistent with this proposed role, FRO1 shows similarity to other oxidoreductase proteins, and expression of FRO1 in yeast conferred increased Fe(III)-chelate reductase activity. Furthermore, FRO1 mRNA levels in plants correlated with Fe(III)-chelate reductase activity. Sites of FRO1 expression in roots, leaves, and nodules were determined. FRO1 mRNA was detected throughout the root, but was most abundant in the outer epidermal cells. Expression was detected in mesophyll cells in leaves. In root nodules, mRNA was detected in the infection zone and nitrogen-fixing region. These results indicate that FRO1 acts in root Fe uptake and they suggest a role in Fe distribution throughout the plant. Characterization of FRO1 has also provided new insights into the regulation of Fe uptake. FRO1 expression and reductase activity was detected only in Fe-deficient roots of Sparkle, whereas both were constitutive in brz and dgl, two mutants with incorrectly regulated Fe accumulation. In contrast, FRO1 expression was responsive to Fe status in shoots of all three plant lines. These results indicate differential regulation of FRO1 in roots and shoots, and improper FRO1 regulation in response to a shoot-derived signal of iron status in the roots of the brz and dgl mutants.

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Characterization of FRO1, a Pea Ferric-Chelate Reductase Involved in Root Iron Acquisition

Although cessation of growth is the most apparent symptom of boron deficiency, the biochemical function of boron in growth processes is not well understood. We propose that the action of boron in root meristems is associated with ascorbate metabolism. Total inhibition of root growth in squash (Cucurbita pepo L.) plants transferred to boron-free medium coincided with a major decrease (up to 98%) in the ascorbate concentration of root apices. Under low-boron conditions, in which root growth was partially inhibited, ascorbate concentration declined in proportion to growth rate. The decline in ascorbate concentration in boron-deficient root tips was not related to ascorbate oxidation. Ascorbate added to the medium improved root growth in plants supplied with insufficient boron. Increasing concentrations of aluminum in the nutrient medium caused progressive inhibition of root growth and a parallel reduction in ascorbate concentration of root apices. Elevated boron levels improved root growth under toxic aluminum conditions and produced root apices with higher ascorbate concentrations. To our knowledge, this is the first report of a correlation between boron nutrition, ascorbate concentration in root apices, and growth. These findings show that root growth inhibition resulting from either boron deficiency or aluminum toxicity may be a consequence of disrupted ascorbate metabolism.

Dale G. Blevins

Papers

Boron in plant structure and function

Citrate, Malate, and Succinate Concentration in Exudates from P-Sufficient and P-Stressed Medicago sativa L. Seedlings.

The Arabidopsis AtOPT3 Protein Functions in Metal Homeostasis and Movement of Iron to Developing Seeds

Characterization of FRO1, a Pea Ferric-Chelate Reductase Involved in Root Iron Acquisition

Root Growth Inhibition in Boron-Deficient or Aluminum-Stressed Squash May Be a Result of Impaired Ascorbate Metabolism.