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Metal ions essential for plant growth, such as Fe, Mn, Ni, Cu or Zn, are taken up by plants from the soil solution through metal transporters at the plasma membrane, mainly of the ZIP and Nramp families. These transport systems, however, can also give entry to other metals (Al, Cd, Hg, Pb). Non-nutritive elements, as well as the essential nutrients at higher than metabolic concentrations, can cause phytotoxicity. We have studied previously the effects of an essential (Ni) and a non essential (Cd) heavy metal on root cell plasma membranes, the first selective barrier encountered when entering the plant, using rice as model plant. Distinctive effects of Cd and Ni on membrane function (i.e., Em and membrane permeability) were observed in the short term. We have now confirmed the pattern of Em changes caused by Cd and Ni using barley roots and have also followed the effects of both metals in longer term in rice. Our data indicate that the distinct effects caused by Cd and Ni are due to differences in cellular responses, triggered when entering the cytoplasm (i.e., an efficient detoxifying mechanism for Cd), more than to different direct effects on membranes.
Nickel and cadmium are transition metals naturally found at trace concentrations in most soils except for serpentine soils where Ni, among other metals, is abundant.1 There is no evidence for Cd acting as a mineral nutrient, whereas Ni stimulates seed germination and plant growth,2 and also plays an important role in nitrogen metabolism. Therefore it is considered to be an essential nutrient for plants.3,4 Though beneficial (Ni) or harmless (Cd) at low concentrations, both elements induce phytotoxic effects at concentrations higher than the tolerance threshold of the different plant species, since they are taken up through metal transporters with low specificity.5,6 Many physiological processes are impaired by Cd and by excess Ni resulting not only in common but also in specific symptoms of metal toxicity.7–11
Previous work with rice in our laboratory was focussed on the effects of Cd and Ni on membranes of root cells, as the first point of contact of ions with the plant body inside the cell wall. Our results showed that their effects on membrane electrical potential (Em) and permeability are different. Thus, addition of 0.5 mM Ni to the perfusion solution in root experiments induced a transient membrane depolarization with a recovery of the resting potential within some minutes.12 In contrast, perfusion of rice roots with a five-fold lower Cd concentration resulted in a strong depolarization of cortical cells, down to values within the range of the diffusion potential. After 2 h in the presence of Cd, cells repolarized, but attaining the initial Em values only within 6–8 h.13 In shoot tissues membranes did not repolarize spontaneously.14,15 In maize roots, a similar response to Cd as in rice roots was reported, with a total repolarization in about 12 h but subsequently membranes depolarized again and after 24 h the cells still remained in a depolarized state.16 We have now confirmed the results obtained with rice in barley roots, to which different Cd concentrations were applied (Fig. 1). Under our experimental conditions the resting Em of barley roots is generally more negative than that of rice roots. The extent of root cell depolarization upon Cd addition was concentration dependent, with an almost total repolarization within around 6 h. As in rice, at a higher concentration (1 mM), Ni induced a smaller and transient depolarization lasting only a few minutes (Fig. 1, inset). We also followed the response of Em to Cd and Ni in rice roots in the longer term. Thus, in the interval of 10–20 h after addition of 0.1 mM Cd, the mean Em value measured did not differ from that obtained in the interval of 5–8 h after treatment (Table 1). Moreover, roots of plants grown in the presence of Cd for 10 days showed an Em similar to that of control plants (−107 ± 12 mV) and was scarcely affected by addition of Cd to the perfusion solution (Fig. 1). In contrast, the mean Em value obtained from plants treated with 0.5 mM Ni for 5–8 h was more negative than the initial potential and remained at similar values in the 10–20 h interval (Table 1).
In addition to the strong depolarization phase after Cd application, membrane permeability increased during the first hours; however the rate of K+ efflux decreased gradually thereafter.13 In turn, Ni did not induce immediate effects on membrane permeability but the K+ loss continued with time.12 Thus, the different response to Cd and Ni treatments is reflected in a lower K+ content in the roots of plants grown in Ni-supplemented medium for more than two days, while no differences with respect to controls were detected in plants treated with Cd for up to ten days (Fig. 2).
Heavy metals can bind to membranes through oxygen atoms or histidine, tryptophan and tyrosine groups of polypeptides.17 Though both metals are borderline elements, their ligand affinities differ,18 and the distinct response of root cell membranes could be due to a different degree of association with the plasma membrane, thus altering membrane lipids and enzyme proteins to a different extent. However, according to previous results on Ni and Cd accumulation in rice,19 the degree of association to root cell plasma membrane vesicles is similar for both metals, when related to their concentration in the tissue (5 × 10−3%). Therefore, their distinctive effects should rather be due to the different responses elicited once being inside the cell. Since the inhibitory effect of Cd on the active component of Em cannot be explained by changes in root respiration, we suggested a direct effect of this metal on PM-ATPases, possibly by forming a complex with ATP, thus decreasing the availability of the substrate of the ATPase. The subsequent recovery of Em would then be due to induction of a Cd detoxification system.13 Different detoxifying mechanisms have been described to maintain cellular metal homeostasis,5,20–22 and some of them have been reported for rice.23–28 In this context, it is interesting to note that the effect of 0.5 mM Ni on rice root PM-H+/ATPase is similar when applied in vitro as in vivo, while the strong inhibitory effect of 0.1–0.5 mM Cd measured in vitro is not longer present in plants treated with the metal for 5-10 days.19 Thus, under our experimental conditions, a detoxifying mechanism seems to be induced in rice root cells soon after Cd uptake, reverting the immediate negative effects caused by this metal. In contrast, Ni, an essential nutrient, does not elicit immediate responses, even at higher external concentration, but its continuous influx leads to disturbances in K+ content and water balance, along with other disorders of cell metabolism.12,29
Previously published online: www.landesbioscience.com/journals/psb/article/9668