Amparo Sanz

Cadmium and nickel accumulation in rice plants. Effects on mineral nutrition and possible interactions of abscisic and gibberellic acids

Rice plants accumulate high quantities of Cd and Ni when grown for 10 days in a medium containing these heavy metals. Accompanying Cd and Ni uptake, a decrease in shoot and root length was observed, though dry matter accumulation was not affected accordingly. Metal treatments also induced a decrease in K, Ca and Mg contents in the plants, particularly in the shoots, indicating that Cd and Ni interfered not only with nutrient uptake but also with nutrient distribution into the different plant parts. Addition of abscisic acid (ABA) or gibberellic acid (GA3) to the external solution could not overcome the depressing effects of the metals on nutrient acquisition, and even induced a further decrease of Ca content in Ni-treated plants. Both hormones also reduced, significantly, heavy metal incorporation into the plants. Additionally, hormonal applications affected the transport of Cd and Ni to the shoots, resulting in a higher percentage of the metals taken up remaining in the roots.

Cd2+ effects on transmembrane electrical potential difference, respiration and membrane permeability of rice (Oryza sativa L) roots.

Among other detrimental effects of the heavy metal Cd2+, a decrease in the plant content of essential mineral nutrients is known. In this study, the effect of Cd2+ on different physiological activities of rice roots involved in nutrient acquisition has been studied. Upon addition of 0.1 or 1 mM Cd2+ to the experimental solution, root cell membranes depolarized in few minutes, reaching very low Em values. This effect was transient and the initial membrane potential recovered totally within 6–8 h. Only the highest concentration used had an inhibitory effect on root respiration. Significant respiratory inhibition appeared after 2 h of exposure to Cd2+ and lasted for at least 4 h. In turn, membrane permeability increased in the presence of Cd2+ for at least 8 h, inducing K+ efflux from the roots. The relationship between these parameters and their possible involvement in lowered nutrient content in Cd2+-treated plants is discussed.

Ni2+ toxicity in rice: Effect on membrane functionality and plant water content

The heavy metal nickel is an essential mineral trace nutrient found at low concentrations in most natural soils. However, it may reach toxic levels in certain areas and affect a number of biochemical and physiological processes in plants. Wilting and leaf necrosis have been described as typical visible symptoms of Ni(2+) toxicity. The plasma membrane (PM) of root cells constitutes the first barrier for the entry of heavy metals but also a target of their toxic action. This work studies the relationship between disturbances of membrane functionality and the development of the typical symptoms of Ni(2+) toxicity. Rice plants (Oryza sativa L. cv. Bahia) grown in nutrient medium containing 0.5mM Ni(2+) showed a significant decrease in water content as a consequence of the stress. Addition of Ni(2+) to the solution bathing the roots induced a concentration-dependent PM depolarization but the activity of the PM-H(+)-ATPase was not inhibited by the presence of Ni(2+) and the initial resting potential recovered in less than 1h. In the short term (hours), membrane permeability of root cells was not significantly affected by Ni(2+) treatments. However, in the long term (days) a drastic loss of K(+) was measured in roots and shoots, which should be responsible for the changes in the water content measured, since stomatal conductance and the transpiration rate remained unaffected by Ni(2+) treatment. The effects induced by Ni(2+) were not permanent and could be reverted, at least in part, by transferring the plants to a medium without Ni(2+).

Common and distinctive responses of rice seedlings to saline- and osmotically-generated stress

Drought and salinity are the major abiotic factors limiting productivityin rice (Oryza sativa L.). Although both generate osmoticstress, ion toxicity is an additional and important component of salinity. Tostudy the morphological and anatomical responses to those types of stress, weused in vitro grown rice seedlings. Based on an initialscreening of several non-penetrating osmotica on seedling growth, we selectedsorbitol to compare its osmotic effects during seedling development with thosegenerated by NaCl stress. At comparable levels of osmolality, the reduction inroot and leaf growth as well as their delayed development were similar for bothsaline- and osmotically-generated stress. Some changes observed in root anatomyand most of the variations in leaf anatomy features caused by the treatmentscould be ascribed to osmotic stress. However, there were evident differences inthe morphology of the root system as well as in chlorophyll levels as afunctionof the stress treatment. Furthermore, the larger size of epidermal andbulliformcells was distinctively related to saline stress. The results obtained providetools for the in vitro identification of either specificorcross-tolerant phenotypes.

Temporal aspects of copper homeostasis and its crosstalk with hormones

To cope with the dual nature of copper as being essential and toxic for cells, plants temporarily adapt the expression of copper homeostasis components to assure its delivery to cuproproteins while avoiding the interference of potential oxidative damage derived from both copper uptake and photosynthetic reactions during light hours. The circadian clock participates in the temporal organization of coordination of plant nutrition adapting metabolic responses to the daily oscillations. This timely control improves plant fitness and reproduction and holds biotechnological potential to drive increased crop yields. Hormonal pathways, including those of abscisic acid, gibberellins, ethylene, auxins, and jasmonates are also under direct clock and light control, both in mono and dicotyledons. In this review, we focus on copper transport in Arabidopsis thaliana and Oryza sativa and the presumable role of hormones in metal homeostasis matching nutrient availability to growth requirements and preventing metal toxicity. The presence of putative hormone-dependent regulatory elements in the promoters of copper transporters genes suggests hormonal regulation to match special copper requirements during plant development. Spatial and temporal processes that can be affected by hormones include the regulation of copper uptake into roots, intracellular trafficking and compartmentalization, and long-distance transport to developing vegetative and reproductive tissues. In turn, hormone biosynthesis and signaling are also influenced by copper availability, which suggests reciprocal regulation subjected to temporal control by the central oscillator of the circadian clock. This transcriptional regulatory network, coordinates environmental and hormonal signaling with developmental pathways to allow enhanced micronutrient acquisition efficiency.

Distinctive phytotoxic effects of Cd and Ni on membrane functionality.

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.

Differential salinity-induced variations in the activity of H⁺-pumps and Na⁺/H⁺ antiporters that are involved in cytoplasm ion homeostasis as a function of genotype and tolerance level in rice cell lines.

The characterisation of cellular responses to salinity in staple crops is necessary for the reliable identification of physiological markers of salinity tolerance. Under saline conditions, variations in proton gradients that are generated by membrane-bound H⁺ pumps are crucial for maintaining cytoplasm homeostasis. We examined short (15 h) and longer term effects (4 days) of NaCl stress on the H⁺ pumping activities that are associated with the plasma membrane (P-ATPase) and the tonoplast (V-ATPase and V-PPase) in rice (Oryza sativa L.) callus lines that displayed different levels of NaCl tolerance and were established from two japonica rice cultivars. The applied stress conditions were based on those that were used in the induction of a stress-responsive polyubiquitin gene promoter (UBI1) in transgenic rice calli. The most remarkable effect of NaCl stress on H⁺ pumping was the rapid activation of tonoplast-bound pumps; this was particularly observed in cv. Bomba, in which the response of the P-ATPase was slower and showed a higher level of activity after 4 days of stress. The responses were cultivar-dependent; however, in general, a stronger activation occurred in the lines that had a higher tolerance (L-T) than in the less-tolerant (L-S) lines. Substrate hydrolysis was less affected than H⁺ pumping, and it yielded higher H⁺/substrate coupling ratios, which is indicative of an enhanced H⁺ pumping efficiency under saline conditions. The Na⁺/H⁺ antiport activity was generally limited to salt-stressed calli, and higher values and stronger activation of the tonoplast antiporter were observed in the L-T lines than in the L-S lines. The results that were obtained with the NaCl-stressed transgenic lines confirmed the close relationship between metabolic activity, H⁺ pumping and the induction of Na⁺/H⁺ exchange activities.

Defective Copper Transport in the copt5 Mutant Affects Cadmium Tolerance

Cadmium toxicity interferes with essential metal homeostasis, which is a problem for both plant nutrition and the consumption of healthy food by humans. Copper uptake is performed by the members of the Arabidopsis high affinity copper transporter (COPT) family. One of the members, COPT5, is involved in copper recycling from the vacuole toward the cytosolic compartment. We show herein that copt5 mutants are more sensitive to cadmium stress than wild-type plants, as indicated by reduced growth. Exacerbated cadmium toxicity in copt5 mutants is due specifically to altered copper traffic through the COPT5 transporter. Three different processes which have been shown to affect cadmium tolerance are altered in copt5 mutants. First, ethylene biosynthesis diminishes under copper deficiency and, in the presence of cadmium, ethylene production diminishes further. Copper deficiency responses are also attenuated under cadmium treatment. Remarkably, while copt5 roots present higher oxidative stress toxicity symptoms than controls, aerial copt5 parts display lower oxidative stress, as seen by reduced cadmium delivery to shoots. Taken together, these results demonstrate that copper transport plays a key role in cadmium resistance, and suggest that oxidative stress triggers an NADPH oxidase-mediated signaling pathway, which contributes to cadmium translocation and basal plant resistance. The slightly lower cadmium levels that reach aerial parts in the copt5 mutants, irrespective of the copper content in the media, suggest a new biotechnological approach to minimize toxic cadmium entry into food chains.

Modulation of copper deficiency responses by diurnal and circadian rhythms in Arabidopsis thaliana

Highlight Cyclic expression of copper transport and the responses to copper deficiency are integrated into the light and circadian–oscillator signalling in plants.

Is ABA involved in tolerance responses to salinity by affecting cytoplasm ion homeostasis in rice cell lines

The ability of plant cells to maintain cytoplasm ion homeostasis under saline stress is among the main mechanisms involved in salt tolerance. To cope with excess Na(+), cells extrude it from the cytoplasm, which requires expenditure of metabolic energy, provided by H(+) gradients generated by membrane-bound H(+)-pumps. ABA is well-known to be involved in physiological processes elicited or enhanced by stresses causing cell dehydration. In this work we studied the possible implication of this plant hormone in the control of salt-induced cellular mechanisms conducting to Na(+) extrusion from the cytoplasm. We used rice (Oryza sativa L.) cell lines selected for their different tolerance to salinity to measure the response to ABA of H(+)-pumps and Na(+)/H(+)-antiporters associated to the plasma membrane and the tonoplast. Our results show that ABA generally enhances H(+)-pumping under salt stress but not under control conditions. This effect occurs to a higher extent across the tonoplast in the more tolerant lines (L-T). Na(+)/H(+) antiport activity is practically undetectable in calli under control conditions, pre-treated or not with ABA, but shows a strong activation under salinity across the tonoplast, particularly in L-T lines (cv Bahia) and also across de plasma membrane in cv Bomba. In these lines, prior treatments with ABA tend to reduce the NaCl enhanced activity of both antiporters. Overall, under saline conditions ABA seems to affect synergistically H(+) pumping and antagonistically Na(+) extrusion. A complex network of positive and negative regulatory signals seems involved in restoring ion cell homeostasis under salt stress.