Philip J. White

Root responses to cadmium in the rhizosphere: a review

This article reviews the responses of plant roots to elevated rhizosphere cadmium (Cd) concentrations. Cadmium enters plants from the soil solution. It traverses the root through symplasmic or apoplasmic pathways before entering the xylem and being translocated to the shoot. Leaf Cd concentrations in excess of 5-10 μg g(-1) dry matter are toxic to most plants, and plants have evolved mechanisms to limit Cd translocation to the shoot. Cadmium movement through the root symplasm is thought to be restricted by the production of phytochelatins and the sequestration of Cd-chelates in vacuoles. Apoplasmic movement of Cd to the xylem can be restricted by the development of the exodermis, endodermis, and other extracellular barriers. Increasing rhizosphere Cd concentrations increase Cd accumulation in the plant, especially in the root. The presence of Cd in the rhizosphere inhibits root elongation and influences root anatomy. Cadmium concentrations are greater in the root apoplasm than in the root symplasm, and tissue Cd concentrations decrease from peripheral to inner root tissues. This article reviews current knowledge of the proteins involved in the transport of Cd across root cell membranes and its detoxification through sequestration in root vacuoles. It describes the development of apoplastic barriers to Cd movement to the xylem and highlights recent experiments indicating that their maturation is accelerated by high Cd concentrations in their immediate locality. It concludes that accelerated maturation of the endodermis in response to local Cd availability is of functional significance in protecting the shoot from excessive Cd loads.

Calcium channels in higher plants

Calcium channels are involved principally in signal transduction. Their opening results in increased cytoplasmic Ca(2+) concentration, and the spatial and temporal variations in this are thought to elicit specific physiological responses to diverse biotic and abiotic stimuli. Calcium-permeable channels have been recorded in the plasma membrane, tonoplast, endoplasmic reticulum, chloroplast and nuclear membranes of plant cells. This article reviews their electrophysiological properties and discusses their physiological roles. Emphasis is placed on the voltage-dependent and elicitor-activated Ca(2+) channels of the plasma membrane and the depolarisation-activated (SV), hyperpolarisation-activated, IP(3)- and cADPR-dependent Ca(2+) channels of the tonoplast. The closing of stomatal guard cells in the presence of abscisic acid (ABA) is used to illustrate the co-ordination of Ca(2+) channel activities during a physiological response.

Biofortification of UK food crops with selenium

Se is an essential element for animals. In man low dietary Se intakes are associated with health disorders including oxidative stress-related conditions, reduced fertility and immune functions and an increased risk of cancers. Although the reference nutrient intakes for adult females and males in the UK are 60 and 75 microg Se/d respectively, dietary Se intakes in the UK have declined from >60 microg Se/d in the 1970s to 35 microg Se/d in the 1990s, with a concomitant decline in human Se status. This decline in Se intake and status has been attributed primarily to the replacement of milling wheat having high levels of grain Se and grown on high-Se soils in North America with UK-sourced wheat having low levels of grain Se and grown on low-Se soils. An immediate solution to low dietary Se intake and status is to enrich UK-grown food crops using Se fertilisers (agronomic biofortification). Such a strategy has been adopted with success in Finland. It may also be possible to enrich food crops in the longer term by selecting or breeding crop varieties with enhanced Se-accumulation characteristics (genetic biofortification). The present paper will review the potential for biofortification of UK food crops with Se.

Opportunities for improving phosphorus‐use efficiency in crop plants

Limitation of grain crop productivity by phosphorus (P) is widespread and will probably increase in the future. Enhanced P efficiency can be achieved by improved uptake of phosphate from soil (P-acquisition efficiency) and by improved productivity per unit P taken up (P-use efficiency). This review focuses on improved P-use efficiency, which can be achieved by plants that have overall lower P concentrations, and by optimal distribution and redistribution of P in the plant allowing maximum growth and biomass allocation to harvestable plant parts. Significant decreases in plant P pools may be possible, for example, through reductions of superfluous ribosomal RNA and replacement of phospholipids by sulfolipids and galactolipids. Improvements in P distribution within the plant may be possible by increased remobilization from tissues that no longer need it (e.g. senescing leaves) and reduced partitioning of P to developing grains. Such changes would prolong and enhance the productive use of P in photosynthesis and have nutritional and environmental benefits. Research considering physiological, metabolic, molecular biological, genetic and phylogenetic aspects of P-use efficiency is urgently needed to allow significant progress to be made in our understanding of this complex trait.

Sucrose transport in the phloem: integrating root responses to phosphorus starvation

Sugars in plants, derived from photosynthesis, act as substrates for energy metabolism and the biosynthesis of complex carbohydrates, providing sink tissues with the necessary resources to grow and to develop. In addition, sugars can act as secondary messengers, with the ability to regulate plant growth and development in response to biotic and abiotic stresses. Sugar-signalling networks have the ability to regulate directly the expression of genes and to interact with other signalling pathways. Photosynthate is primarily transported to sink tissues as sucrose via the phloem. Under phosphorus (P) starvation, plants accumulate sugars and starch in their leaves. Increased loading of sucrose to the phloem under P starvation not only functions to relocate carbon resources to the roots, which increases their size relative to the shoot, but also has the potential to initiate sugar-signalling cascades that alter the expression of genes involved in optimizing root biochemistry to acquire soil phosphorus through increased expression and activity of inorganic phosphate transporters, the secretion of acid phosphatases and organic acids to release P from the soil, and the optimization of internal P use. This review looks at the evidence for the involvement of phloem sucrose in co-ordinating plant responses to P starvation at both the transcriptional and physiological levels.

Functions of Macronutrients

Publisher Summary This chapter focuses on the role played by various macronutrients such as nitrogen (N), sulfur (S), phosphorus (P), magnesium (Mg), calcium (Ca), and potassium (K) in plant metabolism and growth and describes the symptoms of deficiency and toxicity of these macronutrients. N is the most essential element required after carbon, and it plays a central role in plant metabolism as a constituent of proteins, nucleic acids, chlorophyll, coenzymes, phytohormones, and secondary metabolites. When it is taken as ammonium or nitrate, it is assimilated into amino acids either in the roots or shoots and within the plant, it is translocated as nitrate or amino acids. Sulfur is taken up as sulphate and assimilated into S-containing amino acids such as cysteine that are used to synthesize S-containing enzymes and coenzymes as well as secondary compounds such as phytochelatins (detoxification of metals) or aliins and glucosinolates (feeding deterrents). Phosphorus is a structural element in nucleic acids, and as a component of adenosine phosphates, it plays an important role in energy transfer, and it is also essential for transfer of carbohydrates in leaf cells. Magnesium is a component of chlorophyll, and it is required for photosynthesis and protein synthesis. Calcium is important for cell wall and membrane stabilization, osmoregulation, and as second messenger, thereby allowing plants to regulate developmental processes in response to environmental stimuli. The main role of K is osmoregulation, which is important for cell extension and stomata movement, and it affects loading of sucrose and the rate of mass flow-driven solute movement within the plant.

Shoot yield drives phosphorus use efficiency in Brassica oleracea and correlates with root architecture traits

The environmental and financial costs of using inorganic phosphate fertilizers to maintain crop yield and quality are high. Breeding crops that acquire and use phosphorus (P) more efficiently could reduce these costs. The variation in shoot P concentration (shoot-P) and various measures of P use efficiency (PUE) were quantified among 355 Brassica oleracea L. accessions, 74 current commercial cultivars, and 90 doubled haploid (DH) mapping lines from a reference genetic mapping population. Accessions were grown at two or more external P concentrations in glasshouse experiments; commercial and DH accessions were also grown in replicated field experiments. Within the substantial species-wide diversity observed for shoot-P and various measures of PUE in B. oleracea, current commercial cultivars have greater PUE than would be expected by chance. This may be a consequence of breeding for increased yield, which is a significant component of most measures of PUE, or early establishment. Root development and architecture correlate with PUE; in particular, lateral root number, length, and growth rate. Significant quantitative trait loci associated with shoot-P and PUE occur on chromosomes C3 and C7. These data provide information to initiate breeding programmes to improve PUE in B. oleracea.

The high affinity K+ transporter AtHAK5 plays a physiological role in planta at very low K+ concentrations and provides a caesium uptake pathway in Arabidopsis.

Caesium (Cs(+)) is a potentially toxic mineral element that is released into the environment and taken up by plants. Although Cs(+) is chemically similar to potassium (K(+)), and much is known about K(+) transport mechanisms, it is not clear through which K(+) transport mechanisms Cs(+) is taken up by plant roots. In this study, the role of AtHAK5 in high affinity K(+) and Cs(+) uptake was characterized. It is demonstrated that AtHAK5 is localized to the plasma membrane under conditions of K(+) deprivation, when it is expressed. Growth analysis showed that AtHAK5 plays a role during severe K(+) deprivation. Under K(+)-deficient conditions in the presence of Cs(+), Arabidopsis seedlings lacking AtHAK5 had increased inhibition of root growth and lower Cs(+) accumulation, and significantly higher leaf chlorophyll concentrations than wild type. These data indicate that, in addition to transporting K(+) in planta, AtHAK5 also transports Cs(+). Further experiments showed that AtHAK5 mediated Cs(+) uptake into yeast cells and that, although the K(+) deficiency-induced expression of AtHAK5 was inhibited by low concentrations of NH(4)(+) in planta, Cs(+) uptake by yeast was stimulated by low concentrations of NH(4)(+). Interestingly, the growth of the Arabidopsis atakt1-1 mutant was more sensitive to Cs(+) than the wild type. This may be explained, in part, by increased expression of AtHAK5 in the atakt1-1 mutant. It is concluded that AtHAK5 is a root plasma membrane uptake mechanism for K(+) and Cs(+) under conditions of low K(+) availability.

Genes for calcium-permeable channels in the plasma membrane of plant root cells

In plant cells, Ca(2+) is required for both structural and biophysical roles. In addition, changes in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) orchestrate responses to developmental and environmental signals. In many instances, [Ca(2+)](cyt) is increased by Ca(2+) influx across the plasma membrane through ion channels. Although the electrophysiological and biochemical characteristics of Ca(2+)-permeable channels in the plasma membrane of plant cells are well known, genes encoding putative Ca(2+)-permeable channels have only recently been identified. By comparing the tissue expression patterns and electrophysiology of Ca(2+)-permeable channels in the plasma membrane of root cells with those of genes encoding candidate plasma membrane Ca(2+) channels, the genetic counterparts of specific Ca(2+)-permeable channels can be deduced. Sequence homologies and the physiology of transgenic antisense plants suggest that the Arabidopsis AtTPC1 gene encodes a depolarisation-activated Ca(2+) channel. Members of the annexin gene family are likely to encode hyperpolarisation-activated Ca(2+) channels, based on their corresponding occurrence in secretory or elongating root cells, their inhibition by La(3+) and nifedipine, and their increased activity as [Ca(2+)](cyt) is raised. Based on their electrophysiology and tissue expression patterns, AtSKOR encodes a depolarisation-activated outward-rectifying (Ca(2+)-permeable) K(+) channel (KORC) in stelar cells and AtGORK is likely to encode a KORC in the plasma membrane of other Arabidopsis root cells. Two candidate gene families, of cyclic-nucleotide gated channels (CNGC) and ionotropic glutamate receptor (GLR) homologues, are proposed as the genetic correlates of voltage-independent cation (VIC) channels.

Moving cationic minerals to edible tissues: potassium, magnesium, calcium.

The principal dietary source to humans of the essential cationic mineral elements potassium, magnesium and calcium is through edible plants. The accumulation of these elements in edible portions is the product of selective transport processes catalysing their short-distance and long-distance movement within a plant. In this article we review recent work describing the identification and characterisation of the molecular mechanisms catalysing the uptake and distribution of potassium, magnesium and calcium between organs, cell types and subcellular compartments. Although potassium and magnesium are redistributed effectively within the plant, calcium concentrations in phloem-fed tissues, such as fruits, seeds and tubers, are generally low. However, limitations to the redistribution of mineral elements within the plant, and its consequences for the biofortification of edible crops, can be overcome by appropriate mineral fertilisation and plant breeding strategies. The techniques of ionomics can help identify better genotypes.