Romola J. Davenport

Control of Sodium Transport in Durum Wheat

In many species, salt sensitivity is associated with the accumulation of sodium (Na+) in photosynthetic tissues. Na+ uptake to leaves involves a series of transport steps and so far very few candidate genes have been implicated in the control of these processes. In this study, Na+ transport was compared in two varieties of durum wheat (Triticum turgidum) L. subsp. durum known to differ in salt tolerance and Na+ accumulation; the relatively salt tolerant landrace line 149 and the salt sensitive cultivar Tamaroi. Genetic studies indicated that these genotypes differed at two major loci controlling leaf blade Na+ accumulation (R. Munns, G.J. Rebetzke, S. Husain, R.A. James, R.A. Hare [2003] Aust J Agric Res 54: 627–635). The physiological traits determined by these genetic differences were investigated using measurements of unidirectional 22Na+ transport and net Na+ accumulation. The major differences in Na+ transport between the genotypes were (1) the rate of transfer from the root to the shoot (xylem loading), which was much lower in the salt tolerant genotype, and (2) the capacity of the leaf sheath to extract and sequester Na+ as it entered the leaf. The genotypes did not differ significantly in unidirectional root uptake of Na+ and there was no evidence for recirculation of Na+ from shoots to roots. It is likely that xylem loading and leaf sheath sequestration are separate genetic traits that interact to control leaf blade Na+.

Physiological Characterization of Two Genes for Na+ Exclusion in Durum Wheat, Nax1 and Nax2

Durum wheat (Triticum turgidum L. subsp. durum Desf.) Line 149 contains two novel major genes for excluding Na+ from leaf blades, named Nax1 and Nax2. The genes were separated into families containing a single gene and near-isogenic homozygous lines were selected. Lines containing either Nax1 or Nax2 had lower rates of Na+ transport from roots to shoots than their near-isogenic pairs due to lower rates of net loading of the xylem, not to lower rates of net uptake from the soil or higher rates of retranslocation in the phloem. Nax1 and Nax2 lines also had higher rates of K+ transport from root to shoot, resulting in an enhanced discrimination of K+ over Na+. Lines containing Nax1 differed from those containing Nax2 by unloading Na+ from the xylem as it entered the shoot so that Na+ was retained in the base of the leaf, leading to a high sheath to blade ratio of Na+ concentration. Gradients in tissue concentrations of Na+ along the leaf suggested that Na+ was continually removed from the xylem. The Nax2 line did not retain Na+ in the base of the leaf, suggesting that it functioned only in the root. The Nax2 gene therefore has a similar function to Kna1 in bread wheat (Triticum aestivum).

Sodium Influx and Accumulation in Arabidopsis

Arabidopsis is frequently used as a genetic model in plant salt tolerance studies, however, its physiological responses to salinity remain poorly characterized. This study presents a characterization of initial Na+ entry and the effects of Ca2+ on plant growth and net Na+ accumulation in saline conditions. Unidirectional Na+ influx was measured carefully using very short influx times in roots of 12-d-old seedlings. Influx showed three components with distinct sensitivities to Ca2+, diethylpyrocarbonate, and osmotic pretreatment. Pharmacological agents and known mutants were used to test the contribution of different transport pathways to Na+ uptake. Influx was stimulated by 4-aminobutyric acid and glutamic acid; was inhibited by flufenamate, quinine, and cGMP; and was insensitive to modulators of K+ and Ca2+ channels. Influx did not differ from wild type in akt1 and hkt1 insertional mutants. These data suggested that influx was mediated by several different types of nonselective cation channels. Na+ accumulation in plants grown in 50 mm NaCl was strongly reduced by increasing Ca2+ activity (from 0.05-3.0 mm), and plant survival was improved. However, plant biomass was not affected by shoot Na+ concentration, suggesting that in Arabidopsis Na+ toxicity is not dependent on shoot Na+ accumulation. These data suggest that Arabidopsis is a good model for investigation of Na+ transport, but may be of limited utility as a model for the study of Na+ toxicity.

Ammonium toxicity and the real cost of transport

Abstract Recently, it has been proposed that ammonium is toxic to barley because of the energetic cost of pumping ammonium that has leaked into root cells back into the soil. This does not occur in rice because high levels of ammonium reduce the potential difference across the plasma membrane of rice – whereas the potential difference in barley appears to be ammonium insensitive. These results highlight the potentially high costs of membrane transport, and thus the central importance of transport processes in plants.

Investigating glutamate receptor-like gene co-expression in Arabidopsis thaliana

There is increasing evidence of the important roles of glutamate receptors (GLRs) in plant development and in adaptation to stresses. However, the studies of these putative ion channels, both in planta and in Xenopus oocytes, may have been limited by our lack of knowledge of possible GLR heteromer formation in plants. We have developed a modification of the single-cell sampling technique to investigate GLR co-expression, and thus potential heteromer formation, in single cells of Arabidopsis thaliana leaves. Micro-EXpression amplification (MEX) has allowed us to amplify gene transcripts from a single cell, enabling expression of up to 100 gene transcripts to be assayed. We measured, on average, the transcripts of five to six different AtGLRs in a single cell. However, no consistent patterns of co-expression or cell-type-specific expression were detected, except that cells sampled from the same plant showed similar expression profiles. The only discernible feature was the detection of AtGLR3.7 in every cell examined, an observation supported by GUS staining patterns in plants stably expressing promoter::uidA fusions. In addition, we found AtGLR3.7 expression in oocytes induces a Ba2+-, Ca2+- and Na+-permeable plasma membrane conductance.

The Voltage-Independent Cation Channel in the Plasma Membrane of Wheat Roots Is Permeable to Divalent Cations and May Be Involved in Cytosolic Ca2+ Homeostasis

A voltage-independent cation (VIC) channel has been identified in the plasma membrane of wheat (Triticum aestivum) root cells (P.J. White [1999] Trends Plant Sci 4: 245–246). Several physiological functions have been proposed for this channel, including roles in cation nutrition, osmotic adjustment, and charge compensation. Here, we observe that Ca2+ permeates this VIC channel when assayed in artificial, planar lipid bilayers, and, using an energy barrier model to describe cation fluxes, predict that it catalyzes Ca2+ influx under physiological ionic conditions. Thus, this channel could participate in Ca2+ signaling or cytosolic Ca2+ homeostasis. The pharmacology of45Ca2+ influx to excised wheat roots and inward cation currents through the VIC channel are similar: Both are insensitive to 20 μm verapamil or 1 mmtetraethylammonium, but inhibited by 0.5 mmBa2+ or 0.5 mm Gd3+. The weak voltage dependency of the VIC channel (and its lack of modulation by physiological effectors) suggest that it will provide perpetual Ca2+ influx to root cells. Thus, it may effect cytosolic Ca2+ homeostasis by contributing to the basal Ca2+ influx required to balance Ca2+ efflux from the cytoplasm through ATP- and proton-coupled Ca2+transporters under steady-state conditions.

The Decline of Adult Smallpox in Eighteenth‐Century London

Smallpox was probably the single most lethal disease in eighteenth-century Britain, but was a minor cause of death by the mid-nineteenth century. Although vaccination was crucial to the decline of smallpox, especially in urban areas, from the beginning of the nineteenth century, it remains disputed the extent to which smallpox mortality declined before vaccination. Analysis of age-specific changes in smallpox burials within the large west London parish of St Martin-in-the-Fields revealed a precipitous reduction in adult smallpox risk from the 1770s, and this pattern was duplicated in the east London parish of St Dunstans. Most adult smallpox victims were rural migrants, and such a drop in their susceptibility is consistent with a sudden increase in exposure to smallpox in rural areas. We investigated whether this was due to the spread of inoculation, or an increase in smallpox transmission, using changes in the age patterns of child smallpox burials. Smallpox mortality rose among infants, and smallpox burials became concentrated at the youngest ages, suggesting a sudden increase in infectiousness of the smallpox virus. Such a change intensified the process of smallpox endemicization in the English population, but also made cities substantially safer for young adult migrants.

The Arabidopsis thaliana Glutamate-like Receptor Family (AtGLR)

The 20 genes that encode the Arabidopsis thaliana glutamate-like receptor family (AtGLR) share significant similarity in amino acid coding sequence and predicted secondary structure with animal ionotropic glutamate receptor (iGluR) subunits. In animals, iGluR subunits form glutamate-gated non-selective cation channels (NSCCs) catalysing Na+ and/or Ca2+ influx into cells; in one iGluR subfamily glycine also is required as a coagonist. In Arabidopsis, both glutamate and glycine have been demonstrated to depolarise the plasma membrane and increase [Ca2+]cyt, and iGluR antagonists blocked these effects. AtGLRs are therefore predicted to function in an analogous manner to iGluR. Attempts to functionally characterise AtGLRs in heterologous expression systems have proved inconclusive with no ligand-gated activity detected. Research into the glutamate receptor-like family has been hindered by the lack of phenotypes associated with the AtGLR genes but several phenotypes associated with AtGLR overexpression and knockout have recently given hints as to their function. AtGLR have been implicated in light and C:N signalling, hypocotyl detiolation, root growth, abscisic acid (ABA) metabolism, stress responses, and general ion transport. This review will concentrate on recent developments in the AtGLR field, including the roles and effects of glutamate and glycine and related metabolites in plant physiology relative to potential roles for AtGLRs. It will examine progress made toward defining the functions of particular AtGLRs and will conclude by recommending potentially fruitful avenues of future research.

Year of Birth Effects in the Historical Decline of Tuberculosis Mortality: A Reconsideration

Birth cohort patterns in mortality are often used to infer long-lasting impacts of early life conditions. One of the most widely accepted examples of a birth cohort effect is that of tuberculosis mortality before the late 1940s. However the evidential basis for claims of cohort-specific declines in tuberculosis mortality is very slight. Reanalysis of original or enhanced versions of datasets used previously to support claims of cohort effects in tuberculosis mortality indicated that: 1. where the initial decline in tuberculosis mortality occurred within the period of observation, onset of decline occurred simultaneously in many age groups, in a pattern indicative of ‘period’ not cohort-dependent effects. 2. there was little evidence of ‘proportional hazard’-type cohort patterns in tuberculosis mortality for any female population studied. Therefore any mechanisms proposed to underlie this type of cohort pattern in male mortality must be sex-specific. 3. sex ratios of tuberculosis mortality at older ages peaked in cohorts born around 1900, and resembled cohort sex ratios of lung cancer mortality. This analysis indicates that age-specific patterns in the decline in tuberculosis mortality before 1950 are unlikely to reflect improvements in early life conditions. The patterns observed are generally more consistent with the influence of factors that reduced mortality simultaneously in most age groups. Additional influences, possibly smoking habits, impeded the decline of tuberculosis in older adult males, and produced the sex-specific shifts in age distributions of mortality that were previously interpreted as evidence of cohort-dependent mortality decline.

Urban Inoculation and the Decline of Smallpox Mortality in Eighteenth‐Century Cities - A Reply to Razzell

Smallpox was probably the single most lethal disease in eighteenth‐century Britain but was reduced to a minor cause of death by the mid‐nineteenth century due to vaccination programmes post‐1798. While the success of vaccination is unquestionable, it remains disputed to what extent the prophylactic precursor of vaccination, inoculation, reduced smallpox mortality in the eighteenth century. Smallpox was most lethal in urban populations, but most researchers have judged inoculation to have been unpopular in large towns. Recently, however, Razzell argued that inoculation significantly reduced smallpox mortality of adults and older children in London in the last third of the eighteenth century. This article uses demographic evidence from London and Manchester to confirm previous findings of a sudden fall in adult smallpox mortality and a rise in the importance of smallpox in early childhood c. 1770. The nature of these changes is consistent with an increase in smallpox transmission in London and Manchester after 1770 and indicates that smallpox inoculation was insufficient to reduce smallpox mortality in large towns. It remains unclear whether inoculation could have operated to enhance smallpox transmission or whether changes in the properties of the smallpox virus drove the intensification of smallpox mortality among young children post‐1770.