Jonathan R. Howarth

Influence of Iron Status on Cadmium and Zinc Uptake by Different Ecotypes of the Hyperaccumulator Thlaspi caerulescens

We have previously identified an ecotype of the hyperaccumulator Thlaspi caerulescens (Ganges), which is far superior to other ecotypes (including Prayon) in Cd uptake. In this study, we investigated the effect of Fe status on the uptake of Cd and Zn in the Ganges and Prayon ecotypes, and the kinetics of Cd and Zn influx using radioisotopes. Furthermore, the T. caerulescens ZIP (Zn-regulated transporter/Fe-regulated transporter-like protein) genes TcZNT1-G andTcIRT1-G were cloned from the Ganges ecotype and their expression under Fe-sufficient and -deficient conditions was analyzed. Both short- and long-term studies revealed that Cd uptake was significantly enhanced by Fe deficiency only in the Ganges ecotype. The concentration-dependent kinetics of Cd influx showed that theV max of Cd was 3 times greater in Fe-deficient Ganges plants compared with Fe-sufficient plants. In Prayon, Fe deficiency did not induce a significant increase inV max for Cd. Zn uptake was not influenced by the Fe status of the plants in either of the ecotypes. These results are in agreement with the gene expression study. The abundance ofZNT1-G mRNA was similar between the Fe treatments and between the two ecotypes. In contrast, abundance of theTcIRT1-G mRNA was greatly increased only in Ganges root tissue under Fe-deficient conditions. The present results indicate that the stimulatory effect of Fe deficiency on Cd uptake in Ganges may be related to an up-regulation in the expression of genes encoding for Fe2+ uptake, possibly TcIRT1-G.

Co-ordinated expression of amino acid metabolism in response to N and S deficiency during wheat grain filling

Increasing demands for productivity together with environmental concerns about fertilizer use dictate that the future sustainability of agricultural systems will depend on improving fertilizer use efficiency. Characterization of the biological processes responsible for efficient fertilizer use will provide tools for crop improvement under reduced inputs. Transcriptomic and metabolomic approaches were used to study the impact of nitrogen (N) and sulphur (S) deficiency on N and S remobilization from senescing canopy tissues during grain filling in winter wheat (Triticum aestivum). Canopy tissue N was remobilized effectively to the grain after anthesis. S was less readily remobilized. Nuclear magnetic resonance (NMR) metabolite profiling revealed significant effects of suboptimal N or S supply in leaves but not in developing grain. Analysis of amino acid pools in the grain and leaves revealed a strategy whereby amino acid biosynthesis switches to the production of glutamine during grain filling. Glutamine accumulated in the first 7 d of grain development, prior to conversion to other amino acids and protein in the subsequent 21 d. Transcriptome analysis indicated that a down-regulation of the terminal steps in many amino acid biosynthetic pathways occurs to control pools of amino acids during leaf senescence. Grain N and S contents increased in parallel after anthesis and were not significantly affected by S deficiency, despite a suboptimal N:S ratio at final harvest. N deficiency resulted in much slower accumulation of grain N and S and lower final concentrations, indicating that vegetative tissue N has a greater control of the timing and extent of nutrient remobilization than S.

SULPHATE UPTAKE AND TRANSPORT

This chapter describes the physiological and biochemical background of sulphate transport in relation to current molecular approaches. Following identification and isolation of the first plant gene encoding H+/sulphate co-transporter, a large gene family has now been identified. Phylogenetic analysis of this gene family suggests a specialisation of function of subgroups defined by the patterns of gene expression and tissue, cellular and sub-cellular localisation. Functional characteristics of the cloned transporters have been determined by expression in a heterologous yeast expression system, utilising sulphate transporter deficient mutants. A range of affinity constants for sulphate has been determined. An analysis of conserved amino acids together with site-directed mutagenesis indicates residues of probable functional importance.

Mechanical Impedance and Nutrient Acquisition in Rice

The aim of this work was to examine the effect of mechanical impedance on nutrient acquisition and gene expression in rice (Oryza sativa L.). Roots were mechanically impeded in a sand-core apparatus to vary impedance independently of aeration and water status. The effect of impedance on plant growth, anion concentration and expression of genes for anion transporters was compared for six varieties with differences in root penetration ability. Impedance decreased shoot growth more than root growth in all varieties, resulting in increased root/shoot ratios. Impedance substantially increased shoot tissue nitrate concentration in all varieties but only caused a small increase in shoot sulphate and phosphate concentrations. High impedance increased expression of the sulphate transporter OsST1 in five varieties, which was associated with decreased sulphate concentration in root tissues. In contrast, impedance decreased expression of the phosphate transporter OsPT2 expression in all varieties, which was associated with decreased phosphate concentration in root tissues. Localisation of expression of the sulphate transporter by in situ hybridisation indicated high levels of expression in lateral bud primordia. It was suggested that the decreased root phosphate concentrations of impeded roots were caused by low phosphate transporter gene expression, while the increase in sulphate transporter gene expression was due to a derepression mechanism of control.