Jean-Claude Davidian

Sulphate/proton cotransport in plasma-membrane vesicles isolated from roots of Brassica napus L. : increased transport in membranes isolated from sulphur-starved plants

The characteristics of sulphate uptake into right-side-out plasma-membrane vesicles isolated from roots of Brassica napus L., Metzger, cv. Drakkar, and purified by aqueous polymer two-phase partitioning, were investigated. Sulphate uptake into the vesicles was driven by an artificially imposed pH gradient (acid outside), and could be observed for 5–10 min before a plateau was reached and no further net uptake occurred. The uptake was partially inhibited in the presence of depolarizing agents and little uptake was observed in the absence of an imposed pH gradient. Uptake was strongly pH-dependent, being greatest at more acidic pH. After imposition of a pH gradient, the capacity for uptake decreased slowly (t1/2>10 min). The uptake had a high-affinity component which was strongly dependent on the external proton concentration (Km=10μM at pH 5.0, 64 μM at pH 6.5). The Km for protons varied from 0.4–1.9 μM as the sulphate concentration was reduced from 33 to 1 μM. A low-affinity component was observed which could be resolved at low temperatures (0 °C). Microsomal membranes that partitioned into the lower phase of the two-phase system gave no indication of high-affinity sulphate transport. Sulphate uptake into plasma-membrane vesicles isolated from sulphur-starved plant material was approximately twofold greater than that observed in those isolated from sulphate-fed plant material. Isolated vesicles therefore mirror the well-known in-vivo response of roots, indicating an increase in the number of transporters to be, at least in part, the underlying cause of derepression.

Characterization of a Selenate-Resistant Arabidopsis Mutant. Root Growth as a Potential Target for Selenate Toxicity

Screening an Arabidopsis (Arabidopsis thaliana) T-DNA mutant library for selenate resistance enabled us to isolate a selenate-resistant mutant line (sel1-11). Molecular and genetic characterization showed that the mutant contained a lesion in the SULTR1;2 gene that encodes a high affinity root sulfate transporter. We showed that SULTR1;2 is the only gene among 13 mutated genes of the Arabidopsis sulfate transporter family whose mutation conferred selenate resistance to Arabidopsis. The selenate resistance phenotype of the sel1-11 mutant was mirrored by an 8-fold increase of root growth in the presence of selenate as shown by the calculated lethal concentration values. The impairment of SULTR1;2 activity in sel1-11 resulted in a reduced 35S-sulfate uptake capacity by both roots and calli and a reduced sulfate and selenate content in root, shoot, and calli. Comparing sulfate-to-selenate ratios instead of absolute sulfate and selenate contents in roots and shoots enabled us to gain better insight into the mechanism of selenate toxicity in Arabidopsis. Roots of the sel1-11 mutant line showed a higher sulfate to selenate ratio than that of wild-type roots, while there were no significant differences in sulfate to selenate ratios in shoots of wild-type and mutant lines. These results indicated that the mechanism that confers the selenate resistance phenotype to the sel1-11 line takes place rather in the roots. It might be in part the result of a lower selenate uptake and of a protective effect of sulfate against the toxic effects of selenate on root growth. These results revealed in plants a central and specific role of the transporter SULTR1;2 in selenate sensitivity; they further suggested that root growth and potentially the root tip activity might be a specific target of selenate toxicity in Arabidopsis.

Lettuce (Lactuca sativa): a species with a high capacity for cadmium (Cd) accumulation and growth stimulation in the presence of low Cd concentrations

Summary Cadmium (Cd) is a metal pollutant that accumulates in cultivated soils and has detrimental consequences in terms of food safety. Lettuce (Lactuca sativa) can be characterised as having a high capacity to accumulate Cd in its tissues. An analysis of Cd tolerance and Cd accumulation was carried out using two varieties of lettuce (‘Divina’ and ‘Melina’). A wide range of CdCl2 concentrations was used (0.0, 0.1, 0.6, 3.0, and 15.0 µM CdCl2). The lowest concentration (0.1 µM CdCl2) stimulated growth, while the two highest concentrations resulted in a reduction in biomass. Cadmium concentrations were found to be twice as high in roots as in shoots. ‘Divina’ displayed lower concentrations of Cd than ‘Melina’ in nearly all treatments. A strong negative correlation was observed between Cd concentration and Cd tolerance in the roots and shoots (R2 > 0.87) of both ‘Melina’ and ‘Divina’. Lettuce grown in the presence of 15.0 µM CdCl2 had leaf Cd concentrations that were 100-fold higher than the legal maximum level for vegetable products marketed for human consumption, but showed no symptoms of dehydration, chlorosis, or necrosis. This result represents an important alert for lettuce consumers and growers.

Phosphate/Zinc Interaction Analysis in Two Lettuce Varieties Reveals Contrasting Effects on Biomass, Photosynthesis, and Dynamics of Pi Transport

Inorganic phosphate (Pi) and Zinc (Zn) are essential nutrients for normal plant growth. Interaction between these elements has been observed in many crop plants. Despite its agronomic importance, the biological significance and genetic basis of this interaction remain largely unknown. Here we examined the Pi/Zn interaction in two lettuce (Lactuca sativa) varieties, namely, “Paris Island Cos” and “Kordaat.” The effects of variation in Pi and Zn supply were assessed on biomass and photosynthesis for each variety. Paris Island Cos displayed better growth and photosynthesis compared to Kordaat under all the conditions tested. Correlation analysis was performed to determine the interconnectivity between Pi and Zn intracellular contents in both varieties. Paris Island Cos showed a strong negative correlation between the accumulation levels of Pi and Zn in shoots and roots. However, no relation was observed for Kordaat. The increase of Zn concentration in the medium causes a decrease in dynamics of Pi transport in Paris Island Cos, but not in Kordaat plants. Taken together, results revealed a contrasting behavior between the two lettuce varieties in terms of the coregulation of Pi and Zn homeostasis and provided evidence in favor of a genetic basis for the interconnection of these two elements.