The Sulfate Supply Maximizing Arabidopsis Shoot Growth Is Higher under Long- than Short-Term Exposure to Cadmium.

The processes involved in cadmium detoxification in plants deeply affect sulfate uptake and thiol homeostasis and generate increases in the plant nutritional request for sulfur. Here, we present an analysis of the dependence of Arabidopsis growth on the concentration of sulfate in the growing medium with the aim of providing evidence on how plants optimize growth at a given sulfate availability. Results revealed that short-term (72 h) exposure to a broad range of Cd concentrations (0.1, 1, and 10 µM) inhibited plant growth but did not produce any significant effects on the growth pattern of both shoots and roots in relation to the external sulfate. Conversely, long-term (22 days) exposure to 0.1 µM Cd significantly changed the pattern of fresh weight accumulation of the shoots in relation to the external sulfate, without affecting that of the roots, although their growth was severely inhibited by Cd. Moreover, under long-term exposure to Cd, increasing the sulfate external concentration up to the critical value progressively reduced the inhibitory effects exerted by Cd on shoot growth, indicating the existence of sulfate-dependent adaptive responses protecting the shoot tissues against Cd injury. Transcriptional induction of the high-affinity sulfate transporter genes (SULTR1; 1 and SULTR1; 2) involved in sulfate uptake by roots was a common adaptive response to both short- and long-term exposure to Cd. Such a response was closely related to the total amount of non-protein thiols accumulated by a single plant under short-term exposure to Cd, but did not showed any clear relation with thiols under long-term exposure to Cd. In this last condition, Cd exposure did not change the level of non-protein thiols per plant and thus did not alter the nutritional need for sulfur. In conclusion, our results indicate that long term-exposure to Cd, although it induces sulfate uptake, decreases the capacity of the Arabidopsis roots to efficiently absorb the sulfate ions available in the growing medium making the adaptive response of SULTR1; 1 and SULTR1; 2 "per se" not enough to optimize the growth at sulfate external concentrations lower than the critical value.

Kinetic Analysis of Zinc/Cadmium Reciprocal Competitions Suggests a Possible Zn-Insensitive Pathway for Root-to-Shoot Cadmium Translocation in Rice.

BACKGROUND: Among cereals, rice has a genetic propensity to accumulate high levels of cadmium (Cd) in grains. Xylem-mediated root-to-shoot translocation rather than root uptake has been suggested as the main physiological factor accounting for the genotypic variation observed in Cd accumulation in shoots and grains. Several evidence indicate OsHMA2 - a putative zinc (Zn) transporter - as the main candidate protein that could be involved in mediating Cd- and Zn-xylem loading in rice. However, the specific interactions between Zn and Cd in rice often appear anomalous if compared to those observed in other staple crops, suggesting that root-to-shoot Cd translocation process could be more complex than previously thought. In this study we performed a complete set of competition experiments with Zn and Cd in order to analyze their possible interactions and reciprocal effects at the root-to-shoot translocation level. RESULTS: The competition analysis revealed the lack of a full reciprocity when considering the effect of Cd on Zn accumulation, and vice versa, since the accumulation of Zn in the shoots was progressively inhibited by Cd increases, whereas that of Cd was only partially impaired by Zn. Such behaviors were probably dependent on Cd-xylem loading mechanisms, as suggested by: i) the analysis of Zn and Cd content in the xylem sap performed in relation to the concentration of the two metals in the mobile fractions of the roots; ii) the analysis of the systemic movement of (107)Cd in short term experiments performed using a positron-emitting tracer imaging system (PETIS). CONCLUSIONS: Our results suggest that at least two pathways may mediate root-to-shoot Cd translocation in rice. The former could involve OsHMA2 as Zn(2+)/Cd(2+) xylem loader, whereas the latter appears to involve a Zn-insensitive system that still needs to be identified.

Cadmium exposure and sulfate limitation reveal differences in the transcriptional control of three sulfate transporter (Sultr1;2) genes in Brassica juncea.

BACKGROUND: Cadmium (Cd) exposure and sulfate limitation induce root sulfate uptake to meet the metabolic demand for reduced sulfur. Although these responses are well studied, some aspects are still an object of debate, since little is known about the molecular mechanisms by which changes in sulfate availability and sulfur metabolic demand are perceived and transduced into changes in the expression of the high-affinity sulfate transporters of the roots. The analysis of the natural variation occurring in species with complex and highly redundant genome could provide precious information to better understand the topic, because of the possible retention of mutations in the sulfate transporter genes. RESULTS: The analysis of plant sulfur nutritional status and root sulfate uptake performed on plants of Brassica juncea - a naturally occurring allotetraploid species - grown either under Cd exposure or sulfate limitation showed that both these conditions increased root sulfate uptake capacity but they caused quite dissimilar nutritional states, as indicated by changes in the levels of nonprotein thiols, glutathione and sulfate of both roots and shoots. Such behaviors were related to the general accumulation of the transcripts of the transporters involved in root sulfate uptake (BjSultr1;1 and BjSultr1;2). However, a deeper analysis of the expression patterns of three redundant, fully functional, and simultaneously expressed Sultr1;2 forms (BjSultr1;2a, BjSultr1;2b, BjSultr1;2c) revealed that sulfate limitation induced the expression of all the variants, whilst BjSultr1;2b and BjSultr1;2c only seemed to have the capacity to respond to Cd. CONCLUSIONS: A novel method to estimate the apparent kM for sulfate, avoiding the use of radiotracers, revealed that BjSultr1;1 and BjSultr1;2a/b/c are fully functional high-affinity sulfate transporters. The different behavior of the three BjSultr1;2 variants following Cd exposure or sulfate limitation suggests the existence of at least two distinct signal transduction pathways controlling root sulfate uptake in dissimilar nutritional and metabolic states.

Cadmium retention in rice roots is influenced by cadmium availability, chelation and translocation.

Analysis of rice plants exposed to a broad range of relatively low and environmentally realistic Cd concentrations showed that the root capacity to retain Cd ions rose from 49 to 79%, corresponding to increases in the external Cd²+ concentration in the 0.01-1 µM range. Fractioning of Cd ions retained by roots revealed that different events along the metal sequestration pathway (i.e. chelation by thiols, vacuolar compartmentalization, adsorption) contributed to Cd immobilization in the roots. However, large amounts of Cd ions (around 24% of the total amount) predictable as potentially mobile were still found in all conditions, while the amount of Cd ions loaded in the xylem seemed to have already reached saturation at 0.1 µM Cd²+, suggesting that Cd translocation may also play an indirect role in determining Cd root retention, especially at the highest external concentrations. In silico search and preliminary analyses in yeast suggest OsHMA2 as a good candidate for the control of Cd xylem loading in rice. Taken as a whole, data indicate Cd chelation, compartmentalization, adsorption and translocation processes as components of a complex 'firewall system' which acts in limiting Cd translocation from the root to the shoot and which reaches different equilibrium positions depending on Cd external concentration.

Oxidative stress and senescence-like status of pear calli co-cultured on suspensions of incompatible quince microcalli.

This work presents a simple in vitro system to study physiological, biochemical and molecular changes occurring in a pear callus (Pyrus communis L., cv. Beurré Bosc) grown in close proximity to spatially separated undifferentiated homologous (pear) or heterologous (quince; Cydonia oblonga Mill., East Malling clone C) cells in its neighboring environment. After a 7-day co-culture period, the presence of heterologous cells produced negative effects on the pear callus, whose relative weight increase and adenylate energy charge decreased by 30 and 24%, respectively. Such behavior was associated with a higher O(2) consumption rate (+125%) which did not seem to be coupled to adenosine triphosphate synthesis. Analyses of alternative oxidase and enzymatic activities involved in reactive oxygen species (ROS) detoxification strongly suggested that the higher O(2) consumption rate, measured in the pear callus grown in the heterologous combination, may probably be ascribed to extra-respiratory activities. These, in turn, might contribute to generate metabolic scenarios where ROS-induced oxidative stresses may have the upper hand. The increase in the levels of 2-thiobarbituric acid reactive metabolites, considered as diagnostic indicators of ROS-induced lipid peroxidation, seemed to confirm this hypothesis. Moreover, reverse transcription polymerase chain reaction analysis revealed that the expression levels of a few senescence-associated genes were higher in the pear callus grown in the heterologous combination than in the homologous one. Taken as a whole, physiological and molecular data strongly suggest that undifferentiated cells belonging to a pear graft-incompatible quince clone may induce an early senescence-like status in a closely co-cultured pear callus.

Heavy metal stress and sulfate uptake in maize roots.

ZmST1;1, a putative high-affinity sulfate transporter gene expressed in maize (Zea mays) roots, was functionally characterized and its expression patterns were analyzed in roots of plants exposed to different heavy metals (Cd, Zn, and Cu) interfering with thiol metabolism. The ZmST1;1 cDNA was expressed in the yeast (Saccharomyces cerevisiae) sulfate transporter mutant CP154-7A. Kinetic analysis of sulfate uptake isotherm, determined on complemented yeast cells, revealed that ZmST1;1 has a high affinity for sulfate (Km value of 14.6 +/- 0.4 microm). Cd, Zn, and Cu exposure increased both ZmST1;1 expression and root sulfate uptake capacity. The metal-induced sulfate uptakes were accompanied by deep alterations in both thiol metabolism and levels of compounds such as reduced glutathione (GSH), probably involved as signals in sulfate uptake modulation. Cd and Zn exposure strongly increased the level of nonprotein thiols of the roots, indicating the induction of additional sinks for reduced sulfur, but differently affected root GSH contents that decreased or increased following Cd or Zn stress, respectively. Moreover, during Cd stress a clear relation between the ZmST1;1 mRNA abundance increment and the entity of the GSH decrement was impossible to evince. Conversely, Cu stress did not affect nonprotein thiol levels, but resulted in a deep contraction of GSH pools. Our data suggest that during heavy metal stress sulfate uptake by roots may be controlled by both GSH-dependent or -independent signaling pathways. Finally, some evidence suggesting that root sulfate availability in Cd-stressed plants may limit GSH biosynthesis and thus Cd tolerance are discussed.