The Plant Family Brassicaceae

This chapter introduces the plant family Brassicaceae (Cruciferae or mustard family) and also summarizes significant roles of some representative plant species from this family for metals and metalloids phytoremediation. Brassicaceae family is one of the largest dicot families of flowering (angiospermic) plant kingdom which comprises 10–19 tribes with a total of 338–360 genera and nearly 3,709 species. The Brassicaceae are easily recognized by having unique flowers [with four petals, forming a cross or sometimes reduced or lacking; six stamens, the outer two being shorter than the inner four (however, sometimes only two or four stamens are present) and capsule (having two valves capsule with a septum dividing it into two chambers)]. The plant family Brassicaceae includes several plant species of great scientific, economic and agronomic importance including model species (Arabidopsis and Brassica), developing model generic systems (Boechera, Brassica, and Cardamine), as well as many widely cultivated species. The well-known model plants from the family Brassicaceae viz., Arabidopsis (Arabidopsis thaliana) and Brassica species have revolutionized our knowledge in almost every field of modern plant biology. In addition, several representatives of the family Brassicaceae are equally playing significant roles for achieving environmental sustainability.

“Heavy metals” are the chemical elements which, in their standard state, have a specific gravity of more than about 5 g cm⁻³ i.e. their densities are five times greater than water. These constitute a very heterogeneous group of elements greatly varied in their chemical properties and biological functions. Heavy metals are kept under environmental pollutant category due to their toxic effects in plants, human and food particularly in areas with high anthropogenic pressure. Heavy metal pollution is one of the most important environmental problems today. Various industries produce and discharge wastes containing different heavy metals into the environment, such as mining and smelting of metalliferous, surface finishing industry, energy and fuel production, fertilizer and pesticide industry and application, metallurgy, iron and steel, electroplating, electrolysis, electro-osmosis, leatherworking, photography, electric appliance manufacturing, metal surface treating, aerospace and atomic energy installation etc. They are widely used in all fields of life i.e. batteries, dyes, alloys, chemical compounds, pharmaceutical and cosmetic products thus suggesting that the risk of pollution is very high. Thus, metal as a kind of resource is becoming shortage and also brings about serious environmental pollution, threatening human health and ecosystem. Three kinds of heavy metals are of concern, including toxic metals (such as Hg, Cr, Pb, Cd, As, etc.), precious metals (such as Pd, Pt, Ag, Au, Ru etc.) and radionuclides (such as U, Th, Ra, Am, etc.). The presence of heavy metal in atmosphere, soil and water, even in traces represent a severe risk to all organisms for their long term toxicological effects. Heavy metal bioaccumulation and biomagnifications in the food chain can be extremely dangerous to human health.

The mustard family – Brassicaceae – is well known as family of plants, metallophytes, which are able to accumulate wide range of heavy metals and metalloids, especially zinc and cadmium, but also nickel, thallium, chromium and selenium. Ecological importance of this process consists partially in plants themselves to survive negative environmental conditions. There are two basic different strategies, how to survive these conditions – accumulation of heavy metals in plants tissues with different intensity in individual cell types, but also organs, which is partially given by chemical composition of cell walls, and ability to synthesize special defensive – detoxification compounds rich on thiol groups – glutathione and phytochelatins, which are able to bind heavy metals and transport them to the “secure” cell compartment – vacuole. The second principle is based on ability to exclude heavy metals. Role of secondary metabolites rich on sulphur in detoxification of heavy metals is still discussed with unclear conclusions. Members of Brassicaceae family, especially genera Thlaspi and Brassica, are well-known hyperaccumulators of heavy metals with possible utilization in phytoremediation technologies. In this review chapter, mechanisms of cadmium uptake and transport and its deposition in various plant cells and tissues are discussed with respect with possible utilization in phytoremediation. In addition, role of special sulphur metabolites, which are typical for plants of Brassicaceae family – glucosinolates – in detoxification of heavy metals is discussed.

Toxic metals (TMs) and metalloids are natural components of environments, but elevated toxic levels and high persistence of TMs and metalloids in major compartments of the biosphere has posed various uncompromising and fatal effects on flora and fauna, and thus, has threatened the stability of the ecosystems as well. In addition, with the rapid increase in anthropological practices, a large number of TMs and metalloids ions are being added to the natural environment disrupting the ecosystem. A plethora of plant species have been identified so far to have potential for the remediation of TMs and metalloids-contaminated sites. Although, a large number of natural metal hyperaccumulator plant species from 34 different plant families including Asteraceace, Brassicaceae, Caryophyllaceae, Poaceae, Violaceae and Fabaceae has evolved the ability to take up, tolerate and accumulate exceptionally high concentrations of metals and metalloids present in the soil (and water) and, more importantly, in their aboveground biomass without visible toxicity symptoms but with 87 species classified as metal hyperaccumulators, the family Brassicaceae best represents amongst these metal-hyperaccumulator families. Of these 87 different metal-hyperaccumulator plant species in the family Brassicaceae, plant species in particular model metal hyperaccumutaor plant species Alyssum, Thlaspi and Arabidopsis have been studied extensively for their ability to hyperaccumulate, remove, destroy, degrade, sequester, transform, assimilate, metabolize or detoxify majority of TMs and metalloids in varied environmental compartments. Additionally, significant technological advancements in varied scientific fields have now deciphered important physiological and molecular mechanisms of TMs- and metalloids-remediation processes/intricacies in metal hyper accumulating plant species. Based on the plethora of recent published reports the current chapter critically discusses important strategies adopted by Alyssum, Arabidopsis and Thlaspi for TMs- and metalloids-hyperaccumulation/remediation and tolerance.

Numerous toxic metals such as cadmium, selenium, lead, zinc etc. released from industrial production, mining, smelting and traffic contaminate the agricultural soils. This has raised concerns not only for crop quality but also for human health. Engineering and/or microbial based technologies are used to remove the toxic metals from contaminated soils. But these approaches are costly and less efficient in comparison to phytoremediation technique which has emerged as more efficient and cost effective method for decontamination of the toxic metal affected sites. There are highly specialized plants that have the ability to accumulate and tolerate high concentrations of toxic metals from soils and may provide the basis for remediation of heavy metal contaminated sites. Members of the family Brassicaceae have a key role in phytoremediation technology. Metal uptake, sensitivity and sequestration have been extensively investigated in Arabidopsis thaliana. There are a number of Brassica and related crop species that have been reported as the potential candidates for phytoremediation. Brassica spp. display a great diversity of morphological form and many are of economic value as oilseeds, vegetables and forages. They are well suited to genetic manipulation and in vitro culture techniques and are attractive candidates for the introduction of genes aimed at phytoremediation. Brassica oilseeds are adaptable to a range of environmental conditions. Moreover, rhizosphere of Brassica spp. is colonized by several beneficial microbes that can be isolated, mass cultured and utilized to maximize the biomass of this crop plant for use in phytoremediation technology. These microbial inoculants are cheaper as compared to the chemical fertilizers, and are not hazardous to the environment.

Plant-based environmental remediation has been widely pursued by academic and industrial scientists as a favorable low-cost clean-up technology. Phytoremediation is being developed as an alternative technology for removing or, more accurately, reducing the concentration of toxic pollutants to clean up the environment. In the present research, potential of green plants have been screened for phytoremediation of heavy metals both from aquatic and terrestrial environment. Indian mustard (Brassica juncea) has been found as a potential candidate for phytoremediation of heavy metals. B. juncea has been used for remediation of Cd, Pb and Zn at varying concentrations, viz., 0, 5, 10, 20 and 50 ppm. The depletion of heavy metals was observed at the intervals of 0, 1, 3, 7, 14 and 21 days and metal uptake was studied in the roots/shoots of the plants. The percentage removal of Cd, Pb and Zn was found 88.9%, 80% and 89.8%, respectively at the higher exposure concentration (50 ppm). Similarly B. juncea has also been used for phytoremediation of heavy metals (Cd, Pb and Zn) at varying concentrations, viz., 0, 5, 10, 20 and 50 mg/kg from mycorrhizal soil in pot culture technique and uptake was studied in the roots/shoots; after harvesting the plants. The uptake of metals in roots was found 25,000 μg g⁻¹ – Cd, 32,750 μg g⁻¹ -Pb and 30,550 μg g⁻¹ –Zn; whereas uptake in shoots was found 4,596 μg g⁻¹ Cd, 3,469 μg g⁻¹ Pb and 15,878 μg g⁻¹ Zn at higher exposure concentration (50 ppm). The research study has proved effective remediation of heavy metals (Cd, Pb and Zn) by B. juncea in water-soil environment.

Brassicaoilseed and related cruciferous crop species of economic importance are identified as metal hyper accumulator with high biomass. These species possess genetically inherited traits of metal hyper accumulation and tolerance. They have been reported to store metal in their upper ground part with the character of metal tolerance. These species has been adapted with various mechanisms to counter metal toxicity. This adaption attracted everyone to understand range of mechanism in these plants with relation to accumulation of metal ion and tolerance to nullify metal ion mediated toxicity. Toxic metal influenced the various physiological processes such as growth, photosynthesis, ion and water uptake and nitrate assimilation in plant. At the cellular level, they have been reported to cause damage including blocking functional groups of enzymes, denaturing or inactivating enzymes, disturbance in the function of polynucleotide, transport mechanism for nutrient ions and disrupting cell and organelle membrane integrity. These symptoms might occur due to interaction of biomolecules with excessive amount of toxic metals. In addition, toxic metal excess stimulates the formation of free radical and reactive oxygen species. Brassica species and related cruciferous crop evolved to survive and thrive in metal toxicity and adapt a range of mechanisms that may be involved in the detoxification and tolerance. Plant antioxidant system scavenged free radicals ion induced by toxic metal exposure. Such tolerance has also been related to increased level of antioxidant molecules and detoxifying enzymes in response to toxic metal ions. Plants are synthesizing a variety of metal chelating legends including phytochelatins, metallothioneins and organic acids. These legends ensure metal detoxification by complexation and vacuolar sequestration. A whole range of metal transporter families have been identified in plant that could play a key role in tolerance and metal homeostasis. These plants possess genes for resistance to toxic effects of a wide range of metals. Apart from tolerance to metal toxicity, these plants also have fine balance of metals that are regulated either by preventing or reducing the entry into the cell or through efflux mechanisms. So, Brassica utilized the mechanism of accumulation, translocation and uptake of toxic metal more efficiently for tolerance. Brassica has been proposed as a natural environmentally safe option to clean contaminated sites. These species are well adapted to a range of environmental conditions and suitable for phytoremediation due to adequate accumulation with highly regulated translocation and uptake of toxic metal. These species are likely source of genes for phytoremediation. These plants will be playing a key role in phytoremediation technology and can be used for remediation of polluted areas. The adaptation ability of Brassica to toxic metals can be utilized to understand the mechanism of tolerance to toxic metals and development of toxic metal restricted plants in metalliferous soil.

Brassicaceae are scattered all over the world, where they exclusively grow on serpentine rocks in Western Australia, New Zealand, South Africa, Japan, Philippines, Brazil, Portugal, Italy, Turkey, Cuba, eastern Canada, and western north America. Although serpentine rocks cover only less than 1% of the earth’s surface their worldwide distribution has recently attracted many researchers in exploring their distinctive potential for phytoremediation plant communities, mainly members of Brassicaceae plant family inhabiting on serpentine rocks of these countries. On the other hand, the majority of Brassicaceae plant family are slow-growing plants producing little biomass and their use for phytoextraction purposes may not be practical, especially when bioavailable metal concentration is high in the contaminated conditions. Therefore, recently emerging practices in the field of phytoremediation have pointed out various focuses such as the utility of high-biomass crops such as maize, peas, oats and Indian mustard and associated soil practices including application of synthetic chelators such as ethylenediaminetetraacetic acid and nitrilotriacetate and elemental sulphur to enhance metal uptake by these plants. These approaches may meet the conditions required for the phytoremediation. However, one of the most critical components of phytoextraction process is the bioavailability of heavy metals meaning the portion of the metals that is available for absorption into living organisms such as plants. It has been known that various plant growth-promoting rhizobacteria (PGPR) associated with plant roots may provide some beneficial effects on plant growth and nutrition through a series of well known mechanisms, namely, nitrogen fixation, production of phytohormones and siderophores, and transformation of nutrients once they are either applied to seeds or incorporated into the soil. Similarly, heavy metal mobility and availability can substantially be driven by PGPR populations through their release of chelating agents, acidification, and phosphate solubilization in rhizosphere. Miscellaneous PGPR were also shown to tolerate heavy metals in different ways including the mechanisms of exclusion, active removal, biosorption, precipitation, and extra- or intracellular bioaccumulation. Since these processes may affect the solubility and the bioavailability of heavy metals to the plant and hence modifying their toxic effects, interactions between hyperaccumulator plants such as Brassicaceae spp., and metal tolerant or resistant PGPR are considered to have an increasing biotechnological potential in the remediation of anthropogenically polluted soils. Present chapter/review considers the role of PGPR on soil-heavy metal-plant interactions and more specifically bioaccumulation of toxic metals by Brassicaceae plant family.

Phytoremediation has been accepted advantageous over commonly used civil engineering remediation methods in costs, practice and the scale at which the processes operate. Understanding the metabolic answer and the adaptation of plants towards toxic metal exposure opens the way to future phytoremediation of contaminated sites. The majority of these metals get accumulated in plants and may either directly or indirectly find their way into the food chain causing severe secondary consequences. In particular, excess cadmium (Cd), copper (Cu) and zinc (Zn) are known to induce stress effects in all plant species. However, while Cu and Zn are normally present in different soils, and are part of or act as cofactors of many cell macromolecules, plants have no metabolic requirement for Cd. Arabidopsis thaliana L. is considered a model plant for many studies as its genomic sequence was completely identified and its mechanisms in genomic, transcriptomic and proteomic regulation are often similar to other plant species. The molecular, biochemical, physiological and morphological characteristics of this species are strongly affected by the exposure to Cd, Cu and Zn. The aim of this work is to give an up-to-date overview on the recent breakthroughs in the area of responses and adaptation of A. thaliana to Cd, Cu and Zn, three of the most common metals found in polluted soils, both alone and in combination. This chapter aims to contribute to a better understanding of the fundamental aspects of detoxification of metals and general responses in phytoremediation. The numerous and easily available genetic resources developed in A. thaliana should be extended to fast growing plant species of high biomass having significant tolerance to metals and suitable for phytoremediation purposes.

A survey undertaken in Turkey revealed that there are six species of Brassica genus distributed in the wild in Turkey. Brassica elongata, B. nigra, B. tournefortii, B. cretica, B. deflexa and B. campestris. Cultivated forms found in Turkey are kale, cauliflower, cabbage, brussel sprouts, kohlrabi and broccoli. Mediterranean region is most probably concerned with their domestication and cultivated forms of the genus were probably introduced into Asia during ancient times. Studies conducted on the germination and growth behaviour of some of these species revealed that the seeds of B. nigra show a dormancy period of 6 months, whereas other species germinate immediately. Growth regulators not only stimulate the germination under salinity stress, but also overcome the delaying effects of salt. Ethephon sprayed on plants using different solutions showed that at higher concentrations there is a stimulation in vegetative and reproductive growth. Molecular studies of the genomes of species of the Brassicaceae such as B. napus has revealed that there is an extensive genome duplication, indicative of multiple polyploidy events during evolution. Extensive genetic and molecular analysis has been done on six cultivated Brassica species. The four closely related crop species B. rapa, B. juncea, B. napus, and B. carinata provide about 12% of the worldwide edible oil supply. The other two species B. nigra and B. oleracea provide many vegetables for healthy human diet having a valuable source of dietary fiber, vitamin C and other anticancer compounds. B. nigra has the second smallest genome size among the six cultivated species of Brassica. Brassica species are well known as metal accumulators and some of them are being used for phytoremediation in contaminated soils. Approximately 25% of the documented metal hyper accumulating species are members of the Brassicaceae. Because of their slow growth and low biomass, other fast-growing and high biomass Brassica crop plants, for example B. juncea and B. nigra have been evaluated for their ability to hyper accumulate metals from contaminated soils. The Diyarbakir ecotype of B. nigra distributed in the southeastern part of Turkey is a good hyperaccumulator of Cu. Microarray analysis undertaken during the comparative transcriptome analysis in order to find out the expression level of metal induced genes and transcriptome changes both in low and high Cu treated plants showed that some of the genes were highly expressed (several hundred fold) with Cu treated plants compared to control. Microarray data using Affymetrix GeneChip Arabidopsis Genome Array (ATH1-121501 Genechip) indicated that possibly several genes including the genes in glutathione pathway, metal ATPase and ABC transporters are involved in metal tolerances in this ecotype.

Members of the family Brassicaceae have a special ability to absorb such large amounts of metals as are often beyond the tolerance range of other plants. Among the oilseed Brassicas, work on phytoextraction has been centered on Brassica juncea, a well known metal hyperaccumulating species. The oilseed Brassicas mainly include B. carinata (Ethiopian mustard), B. elongata (elongated mustard), B. juncea (Indian mustard), B. napus (oilseed rape/canola), B. narinosa (broad-beaked mustard), B. nigra (black mustard) and B. rapa (turnip mustard). Although, there has been a considerable research on the phytoextraction abilities of these plants from heavy metal contaminated soil, lesser work has been done with reference to chelate-assisted phytoextraction. Research on chelate assisted phytoextraction has mainly been centred on B. juncea and B. napus on account of a better performance of these plants in metal uptake. Chelating agents like EDTA are capable of improving translocation of metals from roots to shoots and then into leaves. Higher bioaccumulation factors have been observed in stems and leaves of plants under the influence of chelating agents. As many of these oilseed crops yield edible oil, the high heavy metal content translocated to the oil bearing seeds is important. Research shows evidence that the seeds contain a considerable amount of hazardous toxic metals if grown on metal contaminated sites. However, the translocation into seeds is checked under lower doses of chelating agents like EDTA. Among the heavy metals, most of the research work on chelate assisted phytoextraction has been on Pb contaminated soil and application of chelating agents like EDTA and EDDS have shown significantly higher metal uptake in plants. Work has also been done on heavy metals like Cd, Cu, Cr and Zn. Chelate assisted phytoextraction has two main drawbacks. Firstly, the phytotoxic effect of the chelate itself with a potentially long residence time in soil and secondly, the leaching hazard of biolabile heavy metals to cause ground water pollution. Several measures have been suggested to overcome these hazards.

The current chapter reviews in detail significant physiological mechanisms of metal accumulating Brassica species and discusses rhizospheric processes and soil management, including the role of soil amendments such as chelators in enhancing the uptake of toxic metals, focusing on their roles in phytoremediation of contaminated sites worldwide, in addition to presenting an overview of the field of phytoremediation, including its merits and shortcomings. Recent progress towards the use of oilseed Brassica species in field-based studies is also discussed.