Introduction
The accumulation of heavy metals in soils and waters is a global, ever-increasing problem, particularly for agricultural soils, considering the possible transfer of toxic elements into the human food chain [1,2]. Among the various methods of remediation of polluted environments, phytoremediation is probably the most promising, basing its action on the use of plants to extract, sequester, and/or detoxify pollutants [3].
The first reports on the possible use of plants for soil and water decontamination date back to the 1980s, and since then many publications on this subject have appeared (see, e.g., reviews [2,4]). In the last decades, several approaches have been developed, and many herbaceous (with some hyperaccumulators) and woody (mainly fast-growing) species have been indicated [5,6,7] also highlighting the possible different behavior of different genotypes (clones, cultivars) within a species [8].
The vast existing literature has mainly investigated how different species react to the presence of heavy metals in the soil; however, now a new aspect of the topic is gaining interest, which can be summarized as the set of actions that lead plants to mitigate the stress due to the presence of heavy metals and/or improve their absorption. This review focused on the use of in vitro culture for the study of these actions. It was deemed appropriate to better circumscribe an otherwise too vast topic, also considering that in vitro cultures present some peculiar aspects worthy of particular attention.
Tissue culture and in vitro propagation can be considered useful tools for understanding the processes of pollutant uptake by plants and all the mechanisms involved [9], and, in general, for the assessment of tolerance to environmental stresses since stress conditions can be easily circumscribed and controlled in vitro [10]. The complex and variable nature of the different abiotic stresses that can interact under field or greenhouse conditions can make it difficult to study the response of plants to these factors; furthermore, there are constraints on the assessment of tolerance to abiotic stresses under field conditions that may be extremely variable [11]. The use of methods based on in vitro tissue culture, therefore, can be an important tool for a better understanding of the physiology and biochemistry of plants cultured under stress conditions [12,13,14]. The in vitro model systems using cell and organ culture allow for easier sample handling and data retrieval than experiments under field conditions [15,16]. Furthermore, the differences in environmental conditions that we find in vivo, such as variations and interactions among hundreds of substances in the soil, the influence of different simultaneous stresses, interplay among tissues, and competition caused by interorganismic interactions, are drastically reduced in in vitro culture systems due to their defined nutrient media, lighting conditions, temperature, humidity, and homogeneous stressor application. It should also be noted that some substances for the mitigation of the effects of heavy metals on plants have only been tested in vitro, as reported later in this review. On the other hand, it should be taken into account that some aspects of in vitro culture may limit the extensibility of the results obtained with this system. This is because the particular cultivation conditions are a primary source of stress for the plants, which are raised in conditions of nutrition, lighting, humidity, and temperature different from the natural ones. The results of in vitro experiments conducted on this topic are of extreme importance to better understand the mechanisms of stress mitigation due to heavy metals. However, it should also be underlined that these findings require confirmation or modulation based on tests conducted in nature on plants in the field. Balancing the advantages and disadvantages of in vitro culture, however, the large existing literature demonstrates the importance of this technique for carrying out investigations on the aforementioned topic, and therefore it was deemed appropriate to offer a broad overview.
This review offers an overview of studies carried out with in vitro culture techniques on the main substances and organisms used for the mitigation of heavy metal stress in plants and, in some cases, for the consequent better metal uptake. In each paragraph, a brief introduction is presented on the main characteristics and mechanisms of the actions of these substances, with bibliographical references not necessarily linked to the in vitro environment. Subsequently, the main results obtained through in vitro culture research are illustrated.
References
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Journal: Plants Authors: Elazab et al
Introduction The accumulation of heavy metals in soils and waters is a global, ever-increasing problem, particularly for agricultural soils, considering the possible transfer of toxic elements into the human food chain [1,2]. Among the various methods of remediation of polluted environments, phytoremediation is probably the most promising, basing its action on the use of plants to extract, sequester, and/or detoxify pollutants [3]. The first reports on the possible use of plants for soil and water decontamination date back to the 1980s, and since then many publications on this subject have appeared (see, e.g., reviews [2,4]). In the last decades, several approaches have been developed, and many herbaceous (with some hyperaccumulators) and woody (mainly fast-growing) species have been indicated [5,6,7] also highlighting the possible different behavior of different genotypes (clones, cultivars) within a species [8]. The vast existing literature has mainly investigated how different species react to the presence of heavy metals in the soil; however, now a new aspect of the topic is gaining interest, which can be summarized as the set of actions that lead plants to mitigate the stress due to the presence of heavy metals and/or improve their absorption. This review focused on the use of in vitro culture for the study of these actions. It was deemed appropriate to better circumscribe an otherwise too vast topic, also considering that in vitro cultures present some peculiar aspects worthy of particular attention. Tissue culture and in vitro propagation can be considered useful tools for understanding the processes of pollutant uptake by plants and all the mechanisms involved [9], and, in general, for the assessment of tolerance to environmental stresses since stress conditions can be easily circumscribed and controlled in vitro [10]. The complex and variable nature of the different abiotic stresses that can interact under field or greenhouse conditions can make it difficult to study the response of plants to these factors; furthermore, there are constraints on the assessment of tolerance to abiotic stresses under field conditions that may be extremely variable [11]. The use of methods based on in vitro tissue culture, therefore, can be an important tool for a better understanding of the physiology and biochemistry of plants cultured under stress conditions [12,13,14]. The in vitro model systems using cell and organ culture allow for easier sample handling and data retrieval than experiments under field conditions [15,16]. Furthermore, the differences in environmental conditions that we find in vivo, such as variations and interactions among hundreds of substances in the soil, the influence of different simultaneous stresses, interplay among tissues, and competition caused by interorganismic interactions, are drastically reduced in in vitro culture systems due to their defined nutrient media, lighting conditions, temperature, humidity, and homogeneous stressor application. It should also be noted that some substances for the mitigation of the effects of heavy metals on plants have only been tested in vitro, as reported later in this review. On the other hand, it should be taken into account that some aspects of in vitro culture may limit the extensibility of the results obtained with this system. This is because the particular cultivation conditions are a primary source of stress for the plants, which are raised in conditions of nutrition, lighting, humidity, and temperature different from the natural ones. The results of in vitro experiments conducted on this topic are of extreme importance to better understand the mechanisms of stress mitigation due to heavy metals. However, it should also be underlined that these findings require confirmation or modulation based on tests conducted in nature on plants in the field. Balancing the advantages and disadvantages of in vitro culture, however, the large existing literature demonstrates the importance of this technique for carrying out investigations on the aforementioned topic, and therefore it was deemed appropriate to offer a broad overview. This review offers an overview of studies carried out with in vitro culture techniques on the main substances and organisms used for the mitigation of heavy metal stress in plants and, in some cases, for the consequent better metal uptake. In each paragraph, a brief introduction is presented on the main characteristics and mechanisms of the actions of these substances, with bibliographical references not necessarily linked to the in vitro environment. Subsequently, the main results obtained through in vitro culture research are illustrated.
References