Periodic Reporting for period 1 - DyCLE (Dynamics of Cadmium concentrations in Leaves in response to a challenging Environment)
Período documentado: 2019-09-01 hasta 2021-08-31
Metal hyperaccumulator plants accumulate extraordinarily high concentrations of metals in their leaves, and very few of these species can be found on both contaminated (metalliferous) and non-contaminated soils. Zinc (Zn) hyperaccumulation (defined as more than 3,000 µg g-1 leaf dry biomass) is a species-wide trait in Arabidopsis halleri (L.) O'Kane & Al-Shehbaz, whereas cadmium (Cd) hyperaccumulation (defined as more than 100 µg g-1 leaf dry biomass) is population-specific. Very little is known about the endogenous and environmental factors determining the exceedingly rare hyperaccumulation of Cd, which in A. halleri is selective for the non-essential Cd(II) in the presence of a large excess of more abundant chemically similar nutrient cations in the soil solution, for example, iron (Fe) and zinc (Zn). Stein et al. (2017) reported Cd hyperaccumulation alongside an unusually large variation in leaf Cd concentrations in two A. halleri populations at Bad Gottleuba (Gott, Germany) and Ukanc (Ukan, Slovenia). The large within-population variation in the extent of Cd hyperaccumulation in the field remains unexplained. Does it reflect a plastic acclimation, and – if so – in response to which environmental factor? Or is it a result of within-population genetic variation? If it reflects an evolutionary adaptation, what selects for it?
An understanding of Cd hyperaccumulation and its variability in the field is important, as it will elucidate a novel aspect of ecology and enhance European scientific excellence necessary for the development of more efficient eco-sustainable strategies for the plant-based clean-up of metal-contaminated soils that are not highly contaminated but unsafe for agriculture (phytoremediation).
Our aim is to disentangle complex plant-environment interactions and genetic variation in order to understand variation in leaf Cd concentrations in plants and its connection to the elemental defence hypothesis i.e. examine whether plants accumulate higher amounts of Cd in leaves when under herbivory attack. The differential hyperaccumulation of Cd within one and the same population could be (i) environmentally governed by abiotic factors; (ii) a plant response to herbivory; (iii) genetically determined. The three hypotheses are not necessarily mutually exclusive. The objectives of the present project are therefore, (i) address in the field the dynamics of Cd concentrations in leaves and soil over one year in the Gott and Ukan populations; (ii) evaluate the response of different genotypes to herbivory (simulated and insect-mediated) under controlled experimental conditions and in an outdoor garden site; (iii) assess the variation in leaf Cd (and Zn) among different genotypes originating from the same population and between treated and untreated plants; (iv) depending on the results of leaf metal concentration, we also wish to test if glucosinolates (organic compounds acting as a plant defensive compounds against insects) are accumulated in leaves, as well (joint effect hypothesis); (v) investigate the segregation of the Cd hyperaccumulation trait in the progeny of parents contrasting in Cd accumulation.
An understanding of Cd hyperaccumulation and its variability in the field is important, as it will elucidate a novel aspect of ecology and enhance European scientific excellence necessary for the development of more efficient eco-sustainable strategies for the plant-based clean-up of metal-contaminated soils that are not highly contaminated but unsafe for agriculture (phytoremediation).
Our aim is to disentangle complex plant-environment interactions and genetic variation in order to understand variation in leaf Cd concentrations in plants and its connection to the elemental defence hypothesis i.e. examine whether plants accumulate higher amounts of Cd in leaves when under herbivory attack. The differential hyperaccumulation of Cd within one and the same population could be (i) environmentally governed by abiotic factors; (ii) a plant response to herbivory; (iii) genetically determined. The three hypotheses are not necessarily mutually exclusive. The objectives of the present project are therefore, (i) address in the field the dynamics of Cd concentrations in leaves and soil over one year in the Gott and Ukan populations; (ii) evaluate the response of different genotypes to herbivory (simulated and insect-mediated) under controlled experimental conditions and in an outdoor garden site; (iii) assess the variation in leaf Cd (and Zn) among different genotypes originating from the same population and between treated and untreated plants; (iv) depending on the results of leaf metal concentration, we also wish to test if glucosinolates (organic compounds acting as a plant defensive compounds against insects) are accumulated in leaves, as well (joint effect hypothesis); (v) investigate the segregation of the Cd hyperaccumulation trait in the progeny of parents contrasting in Cd accumulation.
The focus of the DyCLE project was on describing and analysing within-population variation in Cd hyperaccumulation of A. halleri. A field trip for plant monitoring was planned every 6 months, for a total of three trips. During the first field trip, leaves and rhizosphere soil from approximately 30 individuals were collected on both sites. Each sampled plant was marked with a plastic strap attached to a plastic label in order to be able to find the same individual again during the one-year monitoring. Plants were photographed, and semi-quantitative classification of herbivory damage and density of competing plants were documented. The collected samples were analysed through Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) for their metal content. Based on Cd concentrations in leaves, contrasting genotypes were selected within each of the two populations. Field trips no. 2 and 3 were each delayed by approximately 6 months due to the COVID-19 pandemic. During the second field trip, we sampled the two plants with the highest, two plants with intermediate, and the two plants with the lowest leaf Cd concentrations previously identified (“focus genotypes”).
In the second part of the DyCLE project, we produced vegetative clones from each of the focus genotypes in order to test the elemental defence hypothesis. We also collected approximately 300 kg of natural soil from the Gott field site. After 2 months of pre-cultivation, clones were moved into 1-L pots filled with Gott soil and cultivated for 5 weeks with or without mechanical damage (herbivory simulation). The herbivory simulation was done four times over a 9-day period. At the end of the experiment subsamples of leaves were analysed for metal content by ICP-OES. However, due to the COVID-19 pandemic, these experiments were done with a delay of approximately 8 months, and therefore, leaf glucosinolate concentrations have not been measured yet. In the third part of the project, 5 large clones of each selected genotype grown on Gott soil, were moved from our greenhouse to our outdoor garden site. Plants were left unsupervised and watered as necessary. Plants were monitored each day, and leaves were harvested after a major incident of slug feeding on the leaves. Metal concentrations as quantified by ICP-OES were compared with those of control plants that had been protected from herbivory by a net.
The preliminary results from experiments under controlled conditions support the hypothesis of genotype-herbivory interaction in tuning leaf metal concentrations. Project results dissemination through different communication channels is in preparation. There are currently two manuscripts of review articles in preparation, which will soon be submitted to peer-reviewed journals. An original manuscript containing results from fieldwork and experiments under controlled conditions is also in preparation. Project results were also presented to the scientific community at two international conferences (POPBIO2021 and ICAR 2021). The major objectives of the DyCLE project were presented to researchers and students at the regular meetings of the department and in a dedicated newsletter, YouTube channel, and website. Training in (additional) skills, transferable and complementary skills was attended, as well as the initiation of new collaborations and projects.
In the second part of the DyCLE project, we produced vegetative clones from each of the focus genotypes in order to test the elemental defence hypothesis. We also collected approximately 300 kg of natural soil from the Gott field site. After 2 months of pre-cultivation, clones were moved into 1-L pots filled with Gott soil and cultivated for 5 weeks with or without mechanical damage (herbivory simulation). The herbivory simulation was done four times over a 9-day period. At the end of the experiment subsamples of leaves were analysed for metal content by ICP-OES. However, due to the COVID-19 pandemic, these experiments were done with a delay of approximately 8 months, and therefore, leaf glucosinolate concentrations have not been measured yet. In the third part of the project, 5 large clones of each selected genotype grown on Gott soil, were moved from our greenhouse to our outdoor garden site. Plants were left unsupervised and watered as necessary. Plants were monitored each day, and leaves were harvested after a major incident of slug feeding on the leaves. Metal concentrations as quantified by ICP-OES were compared with those of control plants that had been protected from herbivory by a net.
The preliminary results from experiments under controlled conditions support the hypothesis of genotype-herbivory interaction in tuning leaf metal concentrations. Project results dissemination through different communication channels is in preparation. There are currently two manuscripts of review articles in preparation, which will soon be submitted to peer-reviewed journals. An original manuscript containing results from fieldwork and experiments under controlled conditions is also in preparation. Project results were also presented to the scientific community at two international conferences (POPBIO2021 and ICAR 2021). The major objectives of the DyCLE project were presented to researchers and students at the regular meetings of the department and in a dedicated newsletter, YouTube channel, and website. Training in (additional) skills, transferable and complementary skills was attended, as well as the initiation of new collaborations and projects.
The monitoring of hyperaccumulating plants over one year in their original field is the first of its kind, to the best knowledge of the authors. It allows testing for the correlation of leaf metal levels with environmental factors, such as season, soil metal content, herbivory, soil pH and others that potentially influence leaf metal content in A. halleri. Field-based observations were validated through experiments in controlled conditions and the observed genotype-herbivory interaction could be relevant for the choice of genotypes and growth conditions for phytoremediation of metal-polluted soils.
In addition, data on Cd physiology and distribution in plants, were systematically reviewed and new perspectives were given in understanding Cd hyperaccumulation. The outcomes of this exploratory study will help find better methods of using hyperaccumulators for cleaning up metal-contaminated soils that are unsafe for agriculture.
In addition, data on Cd physiology and distribution in plants, were systematically reviewed and new perspectives were given in understanding Cd hyperaccumulation. The outcomes of this exploratory study will help find better methods of using hyperaccumulators for cleaning up metal-contaminated soils that are unsafe for agriculture.