Benet Gunsé

A glance into aluminum toxicity and resistance in plants.

Aluminum toxicity is an important stress factor for plants in acidic environments. During the last decade considerable advances have been made in both techniques to assess the potentially toxic Al species in environmental samples, and knowledge about the mechanisms of Al toxicity and resistance in plants. After a short introduction on Al risk assessment, this review aims to give an up-to-date glance into current developments in the field of Al toxicity and resistance in plants, also providing sufficient background information for non-specialists in aluminum research. Special emphasis is paid to root growth and development as primary targets for Al toxicity. Mechanisms of exclusion of Al from sensitive root tips, as well as tolerance of high Al tissue levels are considered.

Cadmium-Induced Decrease of Water Stress Resistance in Bush Bean Plants (Phaseolus vulgaris L. cv. Contender) I. Effects of Cd on Water Potential, Relative Water Content, and Cell Wall Elasticity

Summary The leaf water potential (ψ w ) and the relative water content (RWC) of primary and first trifoliate leaves of bean plants ( Phaseolus vulgaris L. cv. Contender), grown with different Cd concentrations in the medium, were analysed under both normal water supply and water stress conditions. With these data, Richter plots were drawn, from which the maximum turgor pressure, the turgor loss point, the bulk elastic modulus, and the structure coefficient were derived. Cd generally decreased the water stress tolerance of plants, causing turgor loss at higher RWC and ψ w than in non-treated control plants. Cd increased the bulk elastic modulus and therefore decreased the cell wall elasticity. Low cell wall elasticity seems to be an important causes of the low water stress tolerance in Cd-toxic plants.

Calcium‐ and potassium‐permeable plasma membrane transporters are activated by copper in Arabidopsis root tips: linking copper transport with cytosolic hydroxyl radical production

Transition metals such as copper can interact with ascorbate or hydrogen peroxide to form highly reactive hydroxyl radicals (OH(•) ), with numerous implications to membrane transport activity and cell metabolism. So far, such interaction was described for extracellular (apoplastic) space but not cytosol. Here, a range of advanced electrophysiological and imaging techniques were applied to Arabidopsis thaliana plants differing in their copper-transport activity: Col-0, high-affinity copper transporter COPT1-overexpressing (C1(OE) ) seedlings, and T-DNA COPT1 insertion mutant (copt1). Low Cu concentrations (10 µm) stimulated a dose-dependent Gd(3+) and verapamil sensitive net Ca(2+) influx in the root apex but not in mature zone. C1(OE) also showed a fivefold higher Cu-induced K(+) efflux at the root tip level compared with Col-0, and a reduction in basal peroxide accumulation at the root tip after copper exposure. Copper caused membrane disruptions of the root apex in C1(OE) seedlings but not in copt1 plants; this damage was prevented by pretreatment with Gd(3+) . Our results suggest that copper transport into cytosol in root apex results in hydroxyl radical generation at the cytosolic side, with a consequent regulation of plasma membrane OH(•) -sensitive Ca(2+) and K(+) transport systems.

The role of ethylene metabolism in the short-term responses to aluminium by roots of two maize cultivars different in Al-resistance

Abstract The possible role of ethylene in the initial signal transduction of Al-induced root growth responses was investigated in two tropical maize ( Zea mays ) varieties that differ in Al resistance: ATP SR Yellow and HS 701 B. The intensity of Al toxicity effects were evaluated after short (4 and 24 h) exposure to 50 μM Al in complete low ionic strength nutrient solution. Relative root elongation rates (RER) and callose formation in root tips were used as stress indicators. Ethylene production by the root tips and 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxydase activities were analysed. After 24 h exposure to Al, both less callose production and higher RER indicated that ATP SR Yellow was more Al resistant than HS 701 B. The Al resistance of ATP SR Yellow, however, was not expressed after 4 h exposure to Al, when increased callose and decreased RER were observed. In any of the varieties and after any of the time-treatments an Al-induced increase of ethylene production was found. Our results indicate that the Al-resistance genes were not constitutively expressed in the absence of Al in the medium, but activated upon exposure to Al. An efficient protection against Al was achieved after a lag time of more than 4 h. Enhanced ethylene formation does not seem to play a role either in the Al-induced inhibition of root elongation or in the induction of the resistance mechanism.

Aluminium-induced alteration of ion homeostasis in root tip vacuoles of two maize varieties differing in Al tolerance.

Root elongation is a primary target of Al toxicity in plants. The objective of this study was to see whether Al-induced disturbance of ion homeostasis is related to the inhibition of root elongation. For this purpose, root growth rate, free cytoplasmic calcium (Ca²+) and vacuolar content of phosphate (P(i)), potassium (K+), nitrate (NO₃⁻) and malate, as well as malate and citrate exudation and nitrate reductase activity were analysed in tips of two Zea mays L. varieties differing in Al resistance. Aluminium treatment affected root growth and cytoplasmic Ca²+ in the Al sensitive variety Bakero, but not in the Al tolerant variety Sikuani. However, both varieties suffered Al-induced decrease of vacuolar K+, and phosphate concentrations. Vacuolar malate concentrations were more affected by Al in Bakero than in Sikuani. Vacuolar nitrate concentrations increased upon Al exposure in both varieties. Only in Sikuani rhizosphere, pH slightly increased upon Al exposure. Our data are consistent with the hypothesis that disturbance of Ca²+ homeostasis is an early event in the Al toxicity syndrome. However, Al-induced alterations of the root tip homeostasis of major ions seem unrelated to Al-induced inhibition of root elongation.

Study of aluminum toxicity by means of vital staining profiles in four cultivars ofPhaseolus vulgaris L.

Aluminum toxicity is a very important factor limiting crop productivity on acid soils. Early effects of aluminum toxicity comprise inhibition of cell division and effects on root elongation. The plasma membrane can be the primary target of aluminum toxicity and thus, vital staining techniques could be a powerful tool in determining effects of metal stress on the plasma membrane. In this paper. we discuss the effects of Al on growth and membrane integrity by staining root tips with a mixture of fluorescein diacetate and propidium iodide. The results show a good correlation between results from growth measurement and the vital staining. From the comparison of the luminosity patterns generated by vital staining it is easy to determine Al-resistant varieties, revealing this technique as a powerful and fast method for determining tolerance to Al in different varieties.

Differential activation of genes related to aluminium tolerance in two contrasting rice cultivars.

Rice (Oryza sativa) is a highly Al-tolerant crop. Among other mechanisms, a higher expression of STAR1/STAR2 (sensitive to Al rhizotoxicity1/2) genes and of Nrat1 (NRAMP Aluminium Transporter 1), and ALS1 (Aluminium sensitive 1) can at least in part be responsible for the inducible Al tolerance in this species. Here we analysed the responses to Al in two contrasting rice varieties. All analysed toxicity/tolerance markers (root elongation, Evans blue, morin and haematoxylin staining) indicated higher Al-tolerance in variety Nipponbare, than in variety Modan. Nipponbare accumulated much less Al in the roots than Modan. Aluminium supply caused stronger expression of STAR1 in Nipponbare than in Modan. A distinctively higher increase of Al-induced abscisic acid (ABA) accumulation was found in the roots of Nipponbare than in Modan. Highest ABA levels were observed in Nipponbare after 48 h exposure to Al. This ABA peak was coincident in time with the highest expression level of STAR1. It is proposed that ABA may be required for cell wall remodulation facilitated by the enhanced UDP-glucose transport to the walls through STAR1/STAR2. Contrastingly, in the roots of Modan the expression of both Nrat1 coding for a plasma membrane Al-transporter and of ALS1 coding for a tonoplast-localized Al transporter was considerably enhanced. Moreover, Modan had a higher Al-induced expression of ASR1 a gene that has been proposed to code for a reactive oxygen scavenging protein. In conclusion, the Al-exclusion strategy of Nipponbare, at least in part mediated by STAR1 and probably regulated by ABA, provided better protection against Al toxicity than the accumulation and internal detoxification strategy of Modan mediated by Nrat1, ALS1 and ARS1.

Microbial homoserine lactones (AHLs) are effectors of root morphological changes in barley

While colonizing the rhizosphere, bacterial intra- and inter-specific communication is accomplished by N-Acyl-homoserine-lactones (AHLs) in a density-dependent manner. Moreover, plants are naturally exposed to AHLs and respond with tissue-specificity. In the present study, we investigated the influence of N-hexanoyl- (C6-HSL), N-octanoyl- (C8-HSL) and N-dodecanoyl-d/l-homoserine lactone (C12-HSL) on growth and root development in barley (Hordeum vulgare L.), and identified initial reactions in root cells after AHL exposures using physiological, staining, and electrophysiological methods. Treatment with short- and long-chain AHLs modulated plant growth and branched root architecture and induced nitric oxide (NO) accumulation in the calyptra and root elongation zone of excised roots in an AHL derivative-independent way. Additionally, C6- and C8-HSL treatments stimulated K+ uptake in root cells only at certain concentrations, whereas all tested concentrations of C12-HSL induced K+ uptake. In further experiments, C8-HSL promoted membrane hyperpolarization in epidermal root cells. Thus, we conclude AHLs promote plant growth and lateral root formation, and cause NO accumulation as an early response to AHLs. Furthermore, the AHL-mediated membrane hyperpolarization is leading to increased K+ uptake of the root tissue.

A highly versatile and easily configurable system for plant electrophysiology

Graphical abstract

Correlation between extractable chromium, chromium uptake and productivity of beans (Phaseolus vulgaris) grown on tannery sludge-amended soil

The effects of different tannery sludge (0.27% Cr f.w.) doses (0, 8.7, 17.4, 34.8 g/kg soil) on growth and Cr content of Phaseolus vulgaris L. cv. Delinel grown on potted soil in a growth chamber were analyzed. The soil was a vertic Ustochrept (pH 8.05, organic matter 2.26%, CEC 8.42 meq/100 g). Total soil Cr concentrations after treatment were 60, 88, 112 and 160 mg/kg. All sludge treatments significantly decreased plant growth and yield. Chromium toxicity symptoms developed in plants grown on soil with 160 mg Cr/kg. Total soil Cr concentrations were significantly correlated with yield, but not with leaf Cr concentrations. Chromium VI was not detected either in sludge or in sludge-treated soil. Among the different extractants, viz. phosphate buffer, acetic acid, ammonium acetate, DTPA, acetic acid-extractable Cr showed the best correlation with both plant Cr concentration and yield. However, the critical toxicity concentration (CTC) for 10% growth reduction of acetic acid-extractable Cr was not significantly different from the Cr concentration associated with maximum yield. Sequential soil extraction showed that sludge Cr was incorporated mainly into the moderately reduceable fraction, extractable with acid oxalate. Acid oxalate-extractable Cr was significantly correlated with yield. Acid oxalate may be a useful extractant for distinguishing naturally occurring soil Cr from waste Cr, which can cause phytotoxicity under certain soil conditions.