Variation in defence strategies in the metal hyperaccumulator plant Noccaea caerulescens is indicative of synergies and trade-offs between forms of defence

Effects of metal amendment and metalloid supplementation on foliar defences are plant accession-specific in the hyperaccumulator Arabidopsis halleri

Soil pollution by metals and metalloids as a consequence of anthropogenic industrialisation exerts a seriously damaging impact on ecosystems. However, certain plant species, termed hyperaccumulators, are able to accumulate extraordinarily high concentrations of these metal(loid)s in their aboveground tissues. Such hyperaccumulation of metal(loid)s is known to act as a defence against various antagonists, such as herbivores and pathogens. We investigated the influences of metal(loid)s on potential defence traits, such as foliar elemental, organic and mechanical defences, in the hyperaccumulator plant species Arabidopsis halleri (Brassicaceae) by artificially amending the soil with common metallic pollutants, namely cadmium (Cd) and zinc (Zn). Additionally, unamended and metal-amended soils were supplemented with the metalloid silicon (Si) to study whether Si could alleviate metal excess. Individuals originating from one non-/low- and two moderately to highly metal-contaminated sites with different metal concentrations (hereafter called accessions) were grown for eight weeks in a full-factorial design under standardised conditions. There were significant interactive effects of metal amendment and Si supplementation on foliar concentrations of certain elements (Zn, Si, aluminium (Al), iron (Fe), potassium (K) and sulfur (S), but these were accession-specific. Profiles of glucosinolates, characteristic organic defences of Brassicaceae, were distinct among accessions, and the composition was affected by soil metal amendment. Moreover, plants grown on metal-amended soil contained lower concentrations of total glucosinolates in one of the accessions, which suggests a potential trade-off between inorganic defence acquisition and biosynthesis of organic defence. The density of foliar trichomes, as a proxy for the first layer of mechanical defence, was also influenced by metal amendment and/or Si supplementation in an accession-dependent manner. Our study highlights the importance of examining the effects of co-occurring metal(loid)s in soil on various foliar defence traits in different accessions of a hyperaccumulating species.

Sequential removal of cation/H+ exchangers reveals their additive role in elemental distribution, calcium depletion and anoxia tolerance

Multiple Arabidopsis H+ /Cation exchangers (CAXs) participate in high-capacity transport into the vacuole. Previous studies have analysed single and double mutants that marginally reduced transport; however, assessing phenotypes caused by transport loss has proven enigmatic. Here, we generated quadruple mutants (cax1-4: qKO) that exhibited growth inhibition, an 85% reduction in tonoplast-localised H+ /Ca transport, and enhanced tolerance to anoxic conditions compared to CAX1 mutants. Leveraging inductively coupled plasma mass spectrometry (ICP-MS) and synchrotron X-ray fluorescence (SXRF), we demonstrate CAX transporters work together to regulate leaf elemental content: ICP-MS analysis showed that the elemental concentrations in leaves strongly correlated with the number of CAX mutations; SXRF imaging showed changes in element partitioning not present in single CAX mutants and qKO had a 40% reduction in calcium (Ca) abundance. Reduced endogenous Ca may promote anoxia tolerance; wild-type plants grown in Ca-limited conditions were anoxia tolerant. Sequential reduction of CAXs increased mRNA expression and protein abundance changes associated with reactive oxygen species and stress signalling pathways. Multiple CAXs participate in postanoxia recovery as their concerted removal heightened changes in postanoxia Ca signalling. This work showcases the integrated and diverse function of H+ /Cation transporters and demonstrates the ability to improve anoxia tolerance through diminishing endogenous Ca levels.

Impairment of the Zn/Cd detoxification systems affects the ability of Salmonella to colonize Arabidopsis thaliana

Salmonella capacity to colonize different environments depends on its ability to respond efficiently to fluctuations in micronutrient availability. Among micronutrients, Zn, besides playing an essential role in bacterial physiology, is a key element whose concentration can influence bacterial survival in a particular niche. Plant colonization by Salmonella enterica was described for several years, and some molecular determinants involved in this host-pathogen interaction have started to be characterized. However, it is still unclear if Zn plays a role in the outcome of this interaction, as well established for animal hosts that employ nutritional immunity strategies to counteract pathogens infections. In this study, we have investigated the involvement of Salmonella Typhimurium main effectors of zinc homeostasis in plant colonization, using Arabidopsis thaliana as a model host. The results show that to colonize plant tissues, Salmonella takes advantage of its ability to export excess metal through the efflux pumps ZntA and ZitB. In fact, the deletion of these Zn/Cd detoxification systems can affect bacterial persistence in the shoots, depending on metal availability in the plant tissues. The importance of Salmonella ability to export excess metal was enhanced in the colonization of plants grown in high Zn conditions. On the contrary, the bacterial disadvantage related to Zn detoxification impairment can be abrogated if the plant cannot efficiently translocate Zn to the shoots. Overall, our work highlights the role of Zn in Salmonella-plant interaction and suggests that modulation of plant metal content through biofortification may be an efficient strategy to control pathogen colonization.

Effects of Plant Hormones, Metal Ions, Salinity, Sugar, and Chemicals Pollution on Glucosinolate Biosynthesis in Cruciferous Plant

Cruciferous vegetable crops are grown widely around the world, which supply a multitude of health-related micronutrients, phytochemicals, and antioxidant compounds. Glucosinolates (GSLs) are specialized metabolites found widely in cruciferous vegetables, which are not only related to flavor formation but also have anti-cancer, disease-resistance, and insect-resistance properties. The content and components of GSLs in the Cruciferae are not only related to genotypes and environmental factors but also are influenced by hormones, plant growth regulators, and mineral elements. This review discusses the effects of different exogenous substances on the GSL content and composition, and analyzes the molecular mechanism by which these substances regulate the biosynthesis of GSLs. Based on the current research status, future research directions are also proposed.

The elemental defense effect of cadmium on Alternaria brassicicola in Brassica juncea

BACKGROUND

The elemental defense hypothesis states a new defensive strategy that hyperaccumulators defense against herbivores or pathogens attacks by accumulating heavy metals. Brassica juncea has an excellent ability of cadmium (Cd) accumulation. However, the elemental defense effect and its regulation mechanism in B. juncea remain unclear.

RESULTS

In this study, we profiled the elemental defense effect and the molecular regulatory mechanism in Cd-accumulated B. juncea after Alternaria brassicicola infection. B. juncea treated with 180 mg Kg^- 1 DW CdCl₂ 2.5H₂O exhibited obvious elemental defense effect after 72 h of infection with A. brassicicola. The expression of some defense-related genes including BjNPR1, BjPR12, BjPR2, and stress-related miRNAs (miR156, miR397, miR398a, miR398b/c, miR408, miR395a, miR395b, miR396a, and miR396b) were remarkably elevated during elemental defense in B. juncea.

CONCLUSIONS

The results indicate that Cd-accumulated B. juncea may defend against pathogens by coordinating salicylic acid (SA) and jasmonic acid (JA) mediated systemic acquired resistance (SAR) and elemental defense in a synergistic joint effect. Furthermore, the expression of miRNAs related to heavy metal stress response and disease resistance may regulate the balance between pathogen defense and heavy metal stress-responsive in B. juncea. The findings provide experimental evidence for the elemental defense hypothesis in plants from the perspectives of phytohormones, defense-related genes, and miRNAs.

An update on redox signals in plant responses to biotic and abiotic stress crosstalk: insights from cadmium and fungal pathogen interactions

Complex signalling pathways are involved in plant protection against single and combined stresses. Plants are able to coordinate genome-wide transcriptional reprogramming and display a unique programme of transcriptional responses to a combination of stresses that differs from the response to single stresses. However, a significant overlap between pathways and some defence genes in the form of shared and general stress-responsive genes appears to be commonly involved in responses to multiple biotic and abiotic stresses. Reactive oxygen and nitrogen species, as well as redox signals, are key molecules involved at the crossroads of the perception of different stress factors and the regulation of both specific and general plant responses to biotic and abiotic stresses. In this review, we focus on crosstalk between plant responses to biotic and abiotic stresses, in addition to possible plant protection against pathogens caused by previous abiotic stress. Bioinformatic analyses of transcriptome data from cadmium- and fungal pathogen-treated plants focusing on redox gene ontology categories were carried out to gain a better understanding of common plant responses to abiotic and biotic stresses. The role of reactive oxygen and nitrogen species in the complex network involved in plant responses to changes in their environment is also discussed.

Intraspecific Variation in Nickel Tolerance and Hyperaccumulation among Serpentine and Limestone Populations of Odontarrhena serpyllifolia (Brassicaceae: Alysseae) from the Iberian Peninsula

Odontarrhena serpyllifolia (Desf.) Jord. & Fourr. (=Alyssum serpyllifolium Desf.) occurs in the Iberian Peninsula and adjacent areas on a variety of soils including both limestone and serpentine (ultramafic) substrates. Populations endemic to serpentine are known to hyperaccumulate nickel, and on account of this remarkable phenotype have, at times, been proposed for recognition as taxonomically distinct subspecies or even species. It remains unclear, however, to what extent variation in nickel hyperaccumulation within this taxon merely reflects differences in the substrate, or whether the different populations show local adaptation to their particular habitats. To help clarify the physiological basis of variation in nickel hyperaccumulation among these populations, 3 serpentine accessions and 3 limestone accessions were cultivated hydroponically under common-garden conditions incorporating a range of Ni concentrations, along with 2 closely related non-accumulator species, Clypeola jonthlaspi L. and Alyssum montanum L. As a group, serpentine accessions of O. serpyllifolia were able to tolerate Ni concentrations approximately 10-fold higher than limestone accessions, but a continuous spectrum of Ni tolerance was observed among populations, with the least tolerant serpentine accession not being significantly different from the most tolerant limestone accession. Serpentine accessions maintained relatively constant tissue concentrations of Ca, Mg, K, and Fe across the whole range of Ni exposures, whereas in the limestone accessions, these elements fluctuated widely in response to Ni toxicity. Hyperaccumulation of Ni, defined here as foliar Ni concentrations exceeding 1g kg^-1 of dry biomass in plants not showing significant growth reduction, occurred in all accessions of O. serpyllifolia, but the higher Ni tolerance of serpentine accessions allowed them to hyperaccumulate more strongly. Of the reference species, C. jonthlaspi responded similarly to the limestone accessions of O. serpyllifolia, whereas A. montanum displayed by far the lowest degree of Ni tolerance and exhibited low foliar Ni concentrations, which only exceeded 1 g kg^-1 in plants showing severe Ni toxicity. The continuous spectrum of physiological responses among these accessions does not lend support to segregation of the serpentine populations of O. serpyllifolia as distinct species. However, the pronounced differences in degrees of Ni tolerance, hyperaccumulation, and elemental homeostasis observed among these accessions under common-garden conditions argues for the existence of population-level adaptation to their local substrates.

Root and Shoot Response to Nickel in Hyperaccumulator and Non-Hyperaccumulator Species

The soil-root interface is the micro-ecosystem where roots uptake metals. However, less than 10% of hyperaccumulators' rhizosphere has been examined. The present study evaluated the root and shoot response to nickel in hyperaccumulator and non-hyperaccumulator species, through the analysis of root surface and biomass and the ecophysiological response of the related aboveground biomass. Ni-hyperaccumulators Alyssoides utriculata (L.) Medik. and Noccaea caerulescens (J. Presl and C. Presl) F.K. Mey. and non-hyperaccumulators Alyssum montanum L. and Thlaspi arvense L. were grown in pot on Ni-spiked soil (0-1000 mg Ni kg^-1, total). Development of root surfaces was analysed with ImageJ; fresh and dry root biomass was determined. Photosynthetic efficiency was performed by analysing the fluorescence of chlorophyll a to estimate the plants' physiological conditions at the end of the treatment. Hyperaccumulators did not show a Ni-dependent decrease in root surfaces and biomass (except Ni 1000 mg kg^-1 for N. caerulescens). The non-hyperaccumulator A. montanum suffers metal stress which threatens plant development, while the excluder T. arvense exhibits a positive ecophysiological response to Ni. The analysis of the root system, as a component of the rhizosphere, help to clarify the response to soil nickel and plant development under metal stress for bioremediation purposes.

Trace elements in Athyrium distentifolium from alpine vegetation in the Karkonosze, SW Poland

The Karkonosze National Park is affected by long-distance metal air transport of anthropogenic contamination as well as by tourist activity. Therefore, concentrations of Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn were evaluated in soil as well as in vital and non-vital fronds of Athyrium distentifolium Opiz collected in the middle of the growing season from glacial cirques in the Karkonosze. Additionally, fronds of the same species turning brown in autumn were collected from the same sampling sites. The health of ferns was impacted by the contents of Co, Cu, Fe, Mn, Ni, Pb and Zn in fronds, which may indicate that A. distentifolium uses elements accumulated in its tissues as defence against pathogens. Individuals from higher altitudes contained higher concentrations of Cd, Cu, Ni, Fe and Pb than those from lower altitudes. Autumn fronds of A. distentifolium contained a significantly higher concentration of Cr, Cu, Fe, Mn and Pb than vital summer fronds, which may indicate sequestration of these elements in senescing fronds probably to remove potentially harmful metals. Non-vital ferns were a better accumulator of Pb than vital ones, while both types of ferns accumulated Mn in a similar way.