Environmental pollution is one of the major problems for human health. Toxic heavy metals are normally present as soil constituents or can also be spread out in the environment by human activity and agricultural techniques. Soil contamination by heavy metals as cadmium, highlights two main aspects: on one side they interfere with the life cycle of plants and therefore reduce crop yields, and on the other hand, once adsorbed and accumulated into the plant tissues, they enter the food chain poisoning animals and humans. Considering this point of view, understanding the mechanism by which plants handle heavy metal exposure, in particular cadmium stress, is a primary goal of plant-biotechnology research or plant breeders whose aim is to create plants that are able to recover high amounts of heavy metals, which can be used for phytoremediation, or identify crop varieties that do not accumulate toxic metal in grains or fruits. In this review we focus on the main symptoms of cadmium toxicity both on root apparatus and shoots. We elucidate the mechanisms that plants activate to prevent absorption or to detoxify toxic metal ions, such as synthesis of phytochelatins, metallothioneins and enzymes involved in stress response. Finally we consider new plant-biotechnology applications that can be applied for phytoremediation.Key words: cadmium; heavy metals; metallothioneins; phytochelatins; phytoremediation; transporters. Available online at www.jipb.net As sessile organisms plants have restricted mechanisms for stress avoidance and are subjected to environmental stresses that change growth conditions and alter (or sometimes disrupt) their metabolic homeostasis. Worldwide, these stresses are the most limiting factors for crop productivity: a large proportion of annual yield is lost due to pathogen attack and to unfavorable abiotic conditions such as drought, salinity and extreme temperatures. The average and record yields of many crops were compared in a classical study (Boyer 1982) and it was found that crop plants were reaching only 20% of their genetic yield potential. Diseases, insects and weeds contributed only in part, with the major yield reduction resulting from abiotic stresses. Therefore, understanding how plants cope with stresses and

During their life, plants have to cope with a variety of abiotic stresses. Cadmium is highly toxic to plants, water soluble and therefore promptly adsorbed in tissues and its presence greatly influences the entire plant metabolism. In this review, we focus on the signal pathways responsible for the sensing and transduction of the "metal signal" inside the cell, ultimately driving the activation of transcription factors and consequent expression of genes that enable plants to counteract the heavy metal stress.

An experimental system has been developed which allows the identification of intermediates in the abscisic acid (ABA) signal transduction pathway leading to desiccation tolerance in plants. Desiccation tolerance in callus of the resurrection plant Craterostigma plantagineum is mediated via the plant hormone ABA, which induces the expression of gene products related to desiccation tolerance. Based on T‐DNA activation tagging, a gene (CDT‐1) was isolated which encodes a signalling molecule in the ABA transduction pathway. Constitutive overexpression of CDT‐1 leads to desiccation tolerance in the absence of ABA and to the constitutive expression of characteristic transcripts. CDT‐1 represents a novel gene with unusual features in its primary sequence.The CDT‐1 gene resembles in several features SINE retrotransposons. Mechanisms by which CDT‐1 activates the pathway could be via a regulatory RNA or via a short polypeptide.

In this study, cDNA-amplified fragment length polymorphism (cDNA-AFLP) analysis was employed to identify genes that exhibited a modulated expression following cadmium (Cd) treatment in Brassica juncea grown in hydroponic culture. Plants were treated for 6 h, 24 h, and 6 weeks with 10 microM Cd(NO3)2 and untreated 6-week-old plants were used as controls. Cd content was measured at these four time points. Long exposure to Cd affected root morphology: roots appeared thinner and sent out side roots. Seventy-three transcript-derived fragments were identified as Cd responsive. Fifty-two of them showed significant homology to genes with known or putative function, 10 transcript-derived fragments were homologous to uncharacterized genes, while 11 transcript-derived fragments did not show significant matches. The expression pattern of several of these genes was confirmed by northern blot analysis. Fifty-two genes of known or putative function were transcriptional factors, expression regulators, and stress responding and transport facilitation genes, as well as genes involved in cellular metabolism and organization and the photosynthetic process, suggesting that a multitude of processes are implicated in Cd stress response. The transcription of drought- and abscisic acid-responsive genes observed in this study also suggested that Cd imposes water stress and that abscisic acid may be involved in the Cd plant response.

Heavy metals are often present naturally in soils, but many human activities (e.g. mining, agriculture, sewage processing, the metal industry and automobiles) increase their prevalence in the environment resulting in concentrations that are toxic to animals and plants. Excess heavy metals affect plant physiology by inducing stress symptoms, but many plants have adapted to avoid the damaging effects of metal toxicity, using strategies such as metal chelation, transport and compartmentalization. Understanding the molecular basis of heavy metal tolerance in plants will facilitate the development of new strategies to create metal-tolerant crops, biofortified foods and plants suitable for the phytoremediation of contaminated sites.

Arabidopsis halleri has the rare ability to colonize heavy metal-polluted sites and is an emerging model for research on adaptation and metal hyperaccumulation. The aim of this study was to analyze the effect of plant-microbe interaction on the accumulation of cadmium (Cd) and zinc (Zn) in shoots of an ecotype of A. halleri grown in heavy metal-contaminated soil and to compare the shoot proteome of plants grown solely in the presence of Cd and Zn or in the presence of these two metals and the autochthonous soil rhizosphere-derived microorganisms. The results of this analysis emphasized the role of plant-microbe interaction in shoot metal accumulation. Differences in protein expression pattern, identified by a proteomic approach involving 2-DE and MS, indicated a general upregulation of photosynthesis-related proteins in plants exposed to metals and to metals plus microorganisms, suggesting that metal accumulation in shoots is an energy-demanding process. The analysis also showed that proteins involved in plant defense mechanisms were downregulated indicating that heavy metals accumulation in leaves supplies a protection system and highlights a cross-talk between heavy metal signaling and defense signaling.

Mineral nutrition of plants greatly depends on both environmental conditions, particularly of soils, and the genetic background of the plant itself. Being sessile, plants adopted a range of strategies for sensing and responding to nutrient availability to optimize development and growth, as well as to protect their metabolisms from heavy metal toxicity. Such mechanisms, together with the soil environment, meaning the soil microorganisms and their interaction with plant roots, have been extensively studied with the goal of exploiting them to reclaim polluted lands; this approach, defined phytoremediation, will be the subject of this review. The main aspects and innovations in this field are considered, in particular with respect to the selection of efficient plant genotypes, the application of improved cultural strategies, and the symbiotic interaction with soil microorganisms, to manage heavy metal polluted soils.

Plants need many different metal elements for growth, development and reproduction, which must be mobilized from the soil matrix and absorbed by the roots as metal ions. Once taken up by the roots, metal ions are allocated to different parts of the plant by the vascular tissues. Metals are naturally present in the soil, but human activities, ranging from mining and agriculture to sewage processing and heavy industry, have increased the amount of metal pollution in the environment. Plants are challenged by environmental metal ion concentrations that fluctuate from low to high toxic levels, and have therefore evolved mechanisms to cope with such phenomena. In this review, we focus on recent data that provide insight into the molecular mechanisms of metal absorption and transport by plants, also considering the effect of metal deficiency and toxicity. We also highlight the positive effects of some non-essential metals on plant fitness.