A Role for Zinc in Plant Defense Against Pathogens and Herbivores

1Mineral nutrients are directly involved in plant protection as structural components and metabolic regulators. As a first line of defense, the nutritional status can determine plants’ susceptibility to pests and pathogens. Essential and beneficial elements affect plant health both directly, by activating enzymes that produce defense metabolites (callose, glucosinolates, lignin, phenols, and phytoalexins), and indirectly, by altering root exudates, rhizosphere pH, and microbial activity. In addition to chemical and/or biochemical factors, plant protection strategies include physical (shape, surface properties, hairs, color, etc.) and mechanical (fibers, silicon) properties; all these features are also influenced by mineral nutrients.

2The nutritional status of the host plant affects the colonization success by pests and pathogens. However, it is not possible, either globally or individually, to generalize the effects of nutrients for all plant–pest/pathogen systems.

3Many studies have focused on the relationship between macronutrients and plant pests and diseases, and several sound reviews link both topics. Most attention has centered on the effects of N, P, and K due to their reduced availability in many soils and their elevated plant demand. However, inconsistent results of macronutrient fertilization and damage by insect herbivory have been reported. Furthermore, silicon has deserved special attention, primarily on account of its structural role in the reinforcement of cell walls, and more recently as an activator of defense genes and pest protection.

4Micronutrient roles in plant defense are predominantly documented for Mn, Cu, Fe, and Zn. Manganese participates in the production of phenolic compounds and other plant defense mechanisms, while Ni is involved in the plant antioxidant system and in the plant response to stress. Best studied are the pathogenesis-related mechanisms based on the Fe redistribution at the cellular level. Zinc seems to be a major player in both animal and plant immune responses. However, so far, no general model has been proposed for Zn, despite the fact that several processes involving this element have been analyzed in different pathosystems. Furthermore, most of these studies have focused on metal hyperaccumulating species that absorb and accumulate a wide range of elements, including micronutrients like Mn, Fe, Ni, and Zn, or other trace elements like Cd, Se, or As. The high tissue concentrations of these elements may protect plants against pests and diseases (see below).

5This review complies the still scant available information characterizing the Zn-related strategies employed by plants to fight their attackers. The generated overview will hopefully permit the reader to see the state of the art of this topic, as well as the existing gaps, where current knowledge established in plant systems is derived either from Fe research or taken from animal systems.

6Zinc is a catalytic and structural protein cofactor in hundreds of enzymes and has key structural functions in the protein domains that interact with other molecules. The “Zn finger” proteins mediate DNA binding of transcription factors and protein–protein interactions. Zn-binding sites can now be predicted from sequenced metal-binding motifs using bioinformatic approaches. Based on these findings, the Zn proteome may represent about 9% of the entire proteome in eukaryotes and from 5% to 6% in prokaryotes.

7Both Zn excess and Zn deficiency cause Zn to become prooxidant. Therefore, in all organisms, the cellular concentration and compartmentation of mobile Zn is tightly controlled by different sets of proteins responsible for sensing, transporting, buffering, storing, and releasing Zn. An outstanding reason for the requirement of a strict control of Zn homeostasis comes from studies conducted in animal systems demonstrating that Zn is an intracellular second messenger in several signaling pathways; therefore, this element plays key roles in the organism’s metallo-neurochemistry. Zinc acts as a neuromodulator in the central nervous system, and Zn transporter ZnT3 is responsible for Zn concentration in synaptic vesicles of certain glutamatergic neurons. While glutamate has a recognized function in plant signaling, a connection to Zn in this process has not yet been established in plants.

8Zinc plays a pivotal function in the plant response to pests and diseases. Nonetheless, Zn defense-related mechanisms in plants greatly vary. The outcomes of plant–pest/pathogen interactions differ, depending on the effectivity of the Zn-related responses in limiting the invader’s attack as well as on the enemy’s ability to circumvent the plant defenses, in addition to other environmental conditions that can favor either host or invader. Several studies have shown that, in most cases, Zn fertilization decreased plant symptoms. However, a protective Zn concentration against certain pathogens can also induce a higher susceptibility to another pathogen on the same plant (see below).

9Zn proteins play a dual role in plant defense, with potential to simultaneously aid and abet the plant and its invaders. Table 1 provides evidence of some of the described connections of Zn proteins with the plant defense and/or the invader virulence. The most commonly assigned functions of Zn proteins are listed in this table, along with examples relating a specific protein function with their respective defense mechanisms. The table denotes that certain similar Zn protein-based mechanisms have been described for both plant defense factors and the pathogenicity factors of their invaders. In the reported cases, the described mechanism is either exclusively mediated through Zn or Zn is mentioned to be involved. The results of these studies summarized in Table 1 globally address an effective plant defense/invader attack with increased gene expression or enhanced activity of the referred protein function. It should be underscored that the described strategies of defense are not exclusively assigned to Zn proteins but also combined with other responses. Among those, two broad-spectrum responses involved in plant–pest/pathogen interactions are tightly related to Zn: oxidative stress and regulation of Zn finger proteins, both of which are more extensively discussed in the following subsections.

10Zinc affects plant–pathogen interactions via its key role in the activation/stabilization of metalloenzymes. A common component in the plant responses to stress conditions caused by insufficient Zn availability and/or pathogen attack is the plant’s capacity to overcome oxidative stress. Under Zn-deficient conditions, reactive oxygen species (ROS) are considered to be the primary factor responsible for plant growth inhibition. Additionally, changes in the antioxidant capacity and increased ROS formation have also been reported in plants in response to excess Zn. On the other hand, the immune system uses ROS to fight pathogens directly by causing oxidative damage or indirectly by triggering different non-oxidative mechanisms, i.e., pattern recognition and receptor signaling. After plant recognition of pathogen attack, a rapid oxidative burst of oxygen radicals is needed in many cases to trigger plant defense mechanisms, i.e., the hypersensitive response and systemic acquired resistance (SAR). Oxygen radicals are controlled by the activity of antioxidant enzymes, and among them, SODs catalyze the conversion of superoxide radicals to hydrogen peroxide, which is involved in abiotic and biotic stress signaling. Cu/Zn-SOD activity is commonly increased in herbivore/pathogen-challenged plants, yet its activity decreased in Zn-deficient plants (see below).

11In addition to their role in plant growth and development, zinc finger proteins regulate plant responses to biotic stress conditions. With a broad spectrum of structures and functions, these proteins are defined as those with a small, freely folded functional domain that requires one or more zinc ions to stabilize its structure. Zinc finger binding domains are present in the well-known plant resistance proteins NBS-LRRs (nucleotide binding sites-leucine rich) that are involved in the effector-triggered immune response. The authors of this study analyzed 70 plant disease-resistance proteins from different crops. Of these proteins, 37% contain zinc finger domains, which suggests a major role for this protein class in the host’s resistance to pathogens. One of the R-genes with a zinc finger domain studied was Pi54, which confers durable resistance against the fungus Magnaporthe oryzae. In a transgenic line of rice containing the Pi54 gene, the up-regulation of defense response genes (callose, laccase, PAL, and peroxidase) and genes related to transcription factors (NAC6, Dof zinc finger, MAD box, bZIP, and WRKY) was observed. Additionally, RAR1, a zinc binding protein of wheat, confers resistance against the stripe rust pathogen through salicylic acid (SA)-mediated oxidative burst and hypersensitive response. Insect infestation has been related to the up-regulation of two zinc finger transcription factors in potato. Some members of the family of transcription regulators cysteine2/histidine2-type zinc finger proteins (ZAT) play key roles in ROS signaling in the response of plants to biotic and abiotic stresses. A member of this family, AtZAT6, participates in plant development and positively modulates the Arabidopsis response to pathogen infection by activating the expression of SA-related genes such as PATHOGENESIS-RELATED GENE1 [PR1], PR2, and PR5.

12In plant and animal systems, direct roles of Zn to control pests and pathogens include mechanisms to produce either low- or high-Zn scenarios (Figure 1). Host-imposed Zn starvation by sequestering Zn from the infection site in a biologically inactive form is a common strategy deployed by the immune system in animals to fight pathogens. Zinc removal strategies from the media to prevent microbial growth have been less documented in plants, yet most of the research on this topic has focused on Fe. On the other hand, hosts may actively deploy high Zn concentrations to control pathogen proliferation. Such active mechanisms have mostly been reported in animal systems, while few studies have been conducted in plants so far. Most of the research in plants concerning the potential protective role of high Zn concentrations has focused on Zn hyperaccumulating species. High Zn concentrations could protect plants from invaders by direct Zn toxicity and by enhancing Zn-triggered organic defenses.

13In the following sections, plant strategies to fight pests and diseases related to low and high Zn concentrations as well as mechanisms evolved by pathogens and pests to counteract Zn-related plant defenses will be discussed.

14In general, Zn-deficient plants are more susceptible to diseases. Most studies looking for a potential relationship between plant Zn status and disease severity have reported an ameliorated response to diseases caused by fungi in plants supplemented with Zn. For example, a study conducted on the effect of different pathogens in soybeans with different Zn treatments showed that plants grown with either normal or high Zn fertilization had fewer positive counts for bacterial pustules caused by Xanthomonas axonopodis pv. glycines and less lesion area affected by the necrotrophic fungus Sclerotinia sclerotiorum than plants grown with low Zn.

15Along this line, Zn-efficient genotypes have been found to be more disease resistant, showing a positive relationship between increased plant growth and reduced pathogen susceptibility. Although the response of plants to Zn deficiency has been extensively studied, the different components and mechanisms involved in the efficient use of Zn remain unclear. The oxidative damage due to ROS production is reduced in Zn efficient genotypes. Moreover, supplemental Zn has been found to increase SOD activity in plants under abiotic stress.

16Recently, important advances on microRNA (miRNA) functions have been achieved, which would contribute to clarifying the mechanisms involved in the response of plants to pests and pathogens under Zn scarcity. In addition to plant growth and development, miRNAs modulate plant responses to stress conditions. The ROS-quenching metalloenzyme Cu/Zn-SOD is post-transcriptionally regulated by a miRNA; specifically, the miR398 targets isoforms CSD1 and CSD2, which are located at the cytosol and chloroplast, respectively. The down-regulation of miR398 induces the CSD1 gene expression, a mechanism triggered by bacterial infection (Pseudomonas syringae) in Arabidopsis. However, miR398 was up-regulated under low Cu2+ concentrations. Although the effect of Zn starvation on miR398 has not been reported, the up-regulation response found for Cu could also be associated to Zn, as the protein is cofactored by both elements. Nonetheless, further studies are needed to clearly establish the relationship between Zn deficiency and pathogen susceptibility.

17Reduced protein synthesis is one effect of Zn deficiency that leads to a higher accumulation of amino acids. This increase has been related to higher incidences of sucking insects present on the plant. Similar results were obtained for Zn-deficient soybean plants that showed increased aphid colonization compared to plants grown at physiologically sufficient Zn concentrations. Contrastingly, in the same study, the biotrophic obligate pathogen that causes soybean rust, Phakopsora pachyrhizi, had a lesser rate of spread in deficient plants. The authors concluded that one single element can differentially affect the response of a specific host to aphids and pathogens. These results revealed the lack of a Zn-specific requirement to fight pests and pathogens, which is a limiting factor for the use of micronutrient fertilization to manage diseases.

18Some of the mechanisms involved in the relationship between low Zn and disease susceptibility are starting to emerge. Plant innate immune mechanisms against biotrophic pathogens are associated with the buildup of SA concentrations, the ROS burst, and the trigger of SA-dependent SAR in non-infected tissues. Different compounds participate in the long-distance transmission of SAR, among them, the C9 dicarboxylic acid, azelaic acid (AzA). A recent report revealed an evolutionary conserved Zn sensing-mechanism, which connects root growth to the plant’s pathogen responses. The authors found that AZELAIC ACID INDUCED1 [AZI1], a member of the lipid transfer protein family (LTPs) of pathogenesis related (PR) proteins, is triggered by azelaic acid (AzA) during SAR. AZI1 regulated growth and immunity responses depending on Zn availability. The results demonstrated that signaling triggered by low Zn and AzA interact. The low Zn status negatively affected the expression of defense-related genes, among others PR1, a SA-induced marker gene that modulated the response to AzA. During early development, when Zn availability is low, Arabidopsis can prioritize root growth over defense responses, leading to an increase in soil volume mined for available Zn. The authors highlighted the need for more extensive studies on the plant interacting signaling networks between defense and growth. Along this line, it would be very interesting to test these signaling pathways in genotypes with contrasting Zn efficiency. A close connection between biotic stress defense and growth signaling networks becomes especially evident in the view of the fact that AzA is listed among the agricultural chemicals for plant resistance priming, but was found to severely inhibit root growth in Arabidopsis plants supplied with sufficient Zn concentrations.

19The requirement of Zn and other micronutrients by pathogens has impelled the animal immune system to evolve mechanisms to sequestrate them in order to restrict pathogen access to these essential nutrients, a process known as nutritional immunity. There is still little evidence on metal-withholding mechanisms in plants to fight microbes, and most studies focused on Fe. In the case of pathogenic bacteria, the withholding of Fe within a plant storage compartment is considered one of the first mechanisms of defense. A research by suggested that polyphenols in plants may play a similar role to that of ion-binding proteins in animals by withholding iron away from pathogens. The sequestration of Fe might induce the expression of genes associated with Fe homeostasis and activation of immune responses against biotrophic pathogens. Recently, the expression levels of an apoplastic iron chelating protein has been found to correlate with tolerance to the necrotrophic bacterium Pectobacterium carotovorum subsp. carotovorum (Pcc).

20In the view of the essential role of Zn in key metabolic processes in both plants and pathogens, nutritional immunity mechanisms based on competition for Zn are highly plausible. Metal-chelating compounds could restrict the availability of metals to pathogens in plants. A recent study on the ZINC-INDUCED FACILITATOR [ZIF1], a nicotianamine (NA) transporter located at the tonoplast in A. thaliana, showed that the perturbation of the subcellular distribution of the Zn-chelator NA has a great effect in the subcellular and inter-organ partitioning of Zn. As suggested by the authors, such studies, in addition to helping to advance the biofortification of crops, also contribute in providing clues concerning Zn-related mechanisms to ameliorate the plant’s resistance to pathogens. Along this line, a ZIF-1 orthologue in Zea mays, Zm-mfs1, was previously reported to be induced by fungal infection in resistant as well as in susceptible interactions.

21Nutrient deficiencies usually render plants more susceptible to pests and pathogens. Nonetheless, some signaling pathways triggered in response to nutrient scarcity boost the plant immune system.

22Such a defense activation implying altered gene-expression profiles and further metabolite production has specifically been reported for Fe- and Mn-deficient conditions. Information regarding Zn deficiency is still scarce, but some studies indicate that pathways signaling Zn-deficient conditions can trigger plant defense responses.

23Defensins (DFN) are known for their involvement in the innate immune defense system. Recently, an increased expression of three DFN-like proteins in A. thaliana grown under Zn-deficient conditions has been found. These proteins were under the direct control of the basic region leucine-zipper transcription factor gene bZIP19. This transcription factor plays a role in the Zn-depletion responses by regulating ZIP transporter gene expression. The bZIPs regulate key processes in both plants and animals. In plants, bZIPs participate among other processes, in response to abiotic and biotic stress. The bZIP TFs are key players in plant innate immunity regulating genes associated with PAMP-triggered immunity, ET immunity, and hormonal signaling networks. Although plant DFN are also implicated in plant growth and development (Stoltz et al., 2009), exploration and functional characterization of novel bZIP TFs in planta could be helpful tools to improve crop resistance against pathogens and environmental stresses.

24Many studies have reported enhanced plant resistance to diseases and pests after supplemental Zn concentrations, or when a mix of micronutrients including Zn was applied. In fact, several phytosanitary formulae are based on Zn-organic complexes. In this situation, microorganisms and herbivores may be directly affected by the toxic effect of Zn rather than through plant metabolic responses.

25High Zn concentrations are toxic to pests and pathogens. In animals, excess Zn can displace other essential metals from their catalytic center within the protein. Accordingly, the deployment of high Zn concentrations in mucosal surfaces infected with Streptococcus pneumoniae has been found to produce a displacement of Mn from the pathogen’s Mn transporters, inducing Mn starvation and enhancing bacterial sensitivity to oxidative stress, among other effects. Few similar Zn efflux mechanisms have been reported in plants, although found that aphid infestation in Arabidopsis halleri increased phloem Zn concentration as a first line of defense. Phloem Zn content varied and was higher in the phloem exudates of highly valuable tissues needing protection; i.e., young leaves were better defended than older ones, which agrees with the optimal defense theory.

26High Zn concentrations are potentially toxic to all organisms. Zn phytotoxicity usually becomes visible at leaf concentration higher than 300 mg Zn kg−1. Zinc toxicity-associated symptoms in plants include reduced yield and stunted growth, reduced export of photoassimilates from leaves to roots, and Fe-deficiency-induced chlorosis through reductions in chlorophyll synthesis and chloroplast degradation. The use of high Zn in plant defense depends on the relative host and pest/pathogen Zn-tolerance. In this context, the potential use of Zn as a toxic compound for plant defense has mostly been studied in metal hyperaccumulating species, which tolerate extremely high Zn tissue concentrations. Metal hyperaccumulation can provide defense against herbivores and pathogens in plants, an adaptive advantage that might have driven the evolution of this trait. Multiple studies have related metal hyperaccumulation to plant defense against biotic stress. Different mechanisms for such metal-based protection have been proposed. According to the original “Metal Defense Hypothesis,” the metal concentrations reached in shoot tissues of hyperaccumulating species would be high enough to intoxicate the attacking pest or pathogen. Afterwards, it was found that high metal concentration and organic compounds may cooperate in the plant’s defense. This led to the “Joint-Effects Hypothesis” proposing that high metal concentrations in non-hyperaccumulating species in cooperation with organic substances can be effective in plant defense. In fact, high Zn concentrations found in non-hyperaccumulating species can enhance plant defense.

27Direct elemental defense by Zn implies that the high Zn accumulation in the plant tissues is more toxic to the pest/pathogen than the plant. A trade-off between metal-based and organic defenses may further benefit the plant as a potential energy-saving mechanism under biotic stress. However, no hard data are available comparing the energy costs for production of organic defenses and metal accumulation, chelation, and compartmentation to demonstrate such a trade-off. Few studies have shown a direct Zn toxicity mechanism in plants against pathogens. In the hyperaccumulating species Noccaea caerulescens, high tissue Zn concentrations restricted the infection of the biotrophic pathogen P. syringae pv. maculicola. ROS-dependent defense mechanisms commonly associated with plant defense against biotrophs, such as callose deposition and pathogenesis-related (PR) gene expression, were not triggered despite the fact of increased salicylic acid (SA) levels in the infected plants. This suggests that in this model, a direct toxic effect of Zn would be the main factor limiting bacterial colonization.

28A recent study monitoring the effect of high Zn supply in the response of N. caerulescens to Alternaria brassicicola showed that the average Noccaea leaf Zn concentration, after 5 weeks of growth in 25% modified Hoagland solution with 102 µM ZnSO4, was 5,000 µg/g dry weight. This is equivalent to 10 mM Zn in the cell sap, a concentration 20 times higher than the EC50 for A. brassicicola in vitro (Figure 2A). Because Alternaria is a necrotrophic fungus, a direct effect of Zn would be feasible due to the release of toxic Zn concentrations into the extracellular space as a consequence of cell compartmentation destruction during the colonization process (Figure 2B). However, joint effects may also operate, as in this pathosystem, high Zn also triggered organic defenses.

29Nonetheless, most supporting evidence for direct Zn toxicity as a plant defense mechanism in Zn hyperaccumulating species has come from studies with insect herbivores. High Zn exposure induced metal accumulation in insect bodies and eggs, causing lower fecundity, reduced growth, and higher mortality rates. The high Zn concentration in Zn hyperaccumulating species can affect insect herbivores with a chewing feeding mode. High Zn concentrations in the diet resulted in a clear deterrent effect. Fewer studies have focused on insects with a sucking–feeding mode. A Zn-rich diet increased the growth of the generalist aphid Myzus persicae, while enrichment with Cd had no effect. However, the combination of both metals produced a greater negative effect on the phloem-sucking insects with respect to individual metals. The higher phloem concentrations of Cd and Zn found in the metal hyperaccumulator A. halleri growing on metal-rich soils deterred aphid infestation. Furthermore, Zn can additively enhance the toxicity to insects of other metals (Cd, Ni, or Pb). This has been observed both in studies using artificial diets and by adding leaves of hyperaccumulator plants to the insects’ diet. A global overview concerning the influence of Zn hyperaccumulation on the plant–predator interaction is given in Table 2.

30Direct metal toxicity caused by the ingestion of hyperaccumulator plants is difficult to test, as any other potential organic defense may contribute and mask the results. Most studies have been conducted using species belonging to the Brassicaceae family. In these species, glucosinolates (GS) are constitutive or stress-induced compounds that, upon damage, generate hydrolysis products active against insects and pathogens.

31In order to evaluate the role of Zn in plant defense, the presence of organic defense compounds such GS in Brassicaceae must be considered. For example, conducted a study on the effect of Cd and Zn exposure and aphid infestation on the phloem exudate composition of A. halleri and found higher amounts of GS in addition to Zn for Zn-supplemented plants.

32Several studies on the feeding preferences of snails with Thaspi caerulescens and A. halleri and high Zn showed that Zn-hyperaccumulation did not play a key role in leaf palatability, with GS proving more efficient than Zn in deterring these generalist herbivores. Recently, it has been observed that the snail Cantareus aspersus preferentially feeds on leaves of the Zn-Cd-hyperaccumulator Noccaea praecox with low metal concentrations and, better yet, low Zn-Cd leaves with the lowest GS concentrations. However, more prevailing for preferred consumption than low metal and GS was a high leaf sugar concentration.

33Besides its potential direct toxic role in defense, high Zn leaf concentrations may contribute to defense by the priming of defense signaling pathways and enhanced structural defenses. High metal concentrations and pathogens trigger signaling pathways that share common elements. Salicylic acid and jasmonates (JA) play pivotal roles in the systemic defense of plants against biotic stress. reported that a JA-induced enhancement of plant DFN increased Zn tolerance in the non-hyperaccumulating species A. thaliana. Moreover, the Zn-tolerant species, A. halleri, constitutively expressed high DFN and showed an increased tolerance to the necrotrophic fungi Botrytis cinerea.

34It is noteworthy that in plants with normal Zn supply, the xylem-colonizing bacteria Xylella fastidiosa requires the expression of Zn detoxification mechanisms for successful host colonization. A recent study by using different non-hyperaccumulating Arabidopsis genotypes infected with the necrotrophic fungi A. brassicicola highlighted the key importance of Zn in plant defense and supported the joint effect hypothesis, i.e., the cooperation between metal and organic defenses in plants. Camalexin is a phytoalexin that is essential for the A. thaliana resistance against A. brassicicola. Experimental infection with A. brassicicola of the camalexin-deficient mutant of A. thaliana, pad3, revealed that high leaf Zn concentrations could not completely substitute the role of camalexin. However, in the wild type, Zn enhanced the JA-ethylene (ET)-dependent defense signaling pathway and the expression of PAD3, an enzyme that catalyzes the last step in camalexin synthesis (Figure 3).

35Moreover, Zn hyperaccumulation is essential for N. caerulescens resistance to A. brassicicola infection. Regardless of the Zn concentration, Alternaria triggers JA-ET- and SA-dependent defense pathways. However, only Zn hyperaccumulating plants showed incompatible interactions with the pathogen. Nonetheless, a joint effect of Zn and organic defenses could not be discarded, as higher GS concentrations were also found in Alternaria-inoculated Zn hyperaccumulating plants.

36Nitric oxide (NO) is reported to play a role in the plant response to toxic Zn concentrations. Recently, NO has been found in Solanum nigrum to participate in the expression of many Zn-mediated miRNA, with the predicted target genes indicating that excess Zn modulates pathogen tolerance and the transcriptional process by miRNA pathways.

37To counteract the host-imposed Zn scarcity or toxicity, pathogens and pests have evolved efficient Zn management mechanisms and the capability to use Zn to increase virulence.

38In pathogens, high Zn use efficiency implies mechanisms that lead to higher Zn import, export, or/and Zn-buffering capacity. In bacteria, the Zn uptake regulator (Zur), a Zn-responsive transcription factor, member of the Fur family of proteins, is the most common transcription factor that regulates cellular Zn uptake and efflux as well as Zn intracellular trafficking. Several studies have shown that Zur regulates target genes in a graded manner. In the model actinobacteria, Streptomyces coelicolor, Zur activates the high-affinity ABC transporter znuABC, and its homologue, znuB2C2 under Zn scarcity. At sufficient Zn levels, the expression of the Zn exporter zitB is low, and genes for Zn uptake are repressed. At high Zn, zitB induction is also elevated. This fine regulation of Zn activity allows pathogenic bacteria to maintain virulence over a wide range of Zn concentrations in the extracellular media. In addition, under Zn scarcity, pathogenic bacteria and fungi can use host metalloproteins as sources of metals. These mechanisms have been predominately studied in human systems that use Zn-sequestering proteins for defense. These include the neutrophil protein calprotectin (CP) to restrict Zn availability at the infection sites. Host Zn chelation is a key strategy to cope with infectious process. However, different pathogenic bacteria have evolved strategies to use host Zn-chelating proteins, including CP, as a source of Zn.

39Zinc detoxification has been reported to be an important virulence factor in bacteria infecting Zn hyperaccumulator species, as well as non-hyperaccumulating species. In X. fastidiosa-infected plants, the modification of the host leaf ionome correlates with bacterial virulence. This xylem-colonizing bacterium has to detoxify Zn for successful host colonization. The knockout of Zur genes increases the sensitivity of X. fastidiosa to Zn and decreased its virulence. Both pathogenic bacteria and fungi modify Zn bioavailability to enhance their virulence. High Zn concentrations induce the enhanced production of exopolysaccharides (EPS), a pathogenic factor in X. fastidiosa. Enhanced EPS production also indirectly potentiates virulence, as Zn chelation by the negative charges of EPS decreases Zn bioavailability to the bacterial cell. Similarly, high Zn concentrations in a lung secretion-mimicking medium enhanced the biofilm formation and virulence of Pseudomonas aeruginosa.

40Fungal Zn homeostasis is regulated by a major transcription factor, the Zn responsive activator protein 1 (Zap1). Sufficient intracellular Zn levels are sensed by a direct interaction of the metal and the protein, causing the inhibition of the expression of Zap1 target genes. Under low Zn conditions, pathogenic fungi develop complex metal detection and signaling networks distinct to those deployed by bacteria for Zn uptake. Nonetheless, in response to Zn scarcity, fungi increase the expression of high-affinity membrane transporters to enhance Zn uptake as well as tonoplast Zn transporters that allow Zn storage into the vacuole, the zincosome.

41Zinc metalloproteins also play a role in the virulence of pathogenic bacteria and fungi. The oxidative burst is an early event in the plant response to diseases. In some cases, pathogens also use ROS to enlarge their colonization process; therefore, the potential to detoxify ROS is an enhancing trait for the pathogen’s virulence. Accordingly, the reduced virulence of a B. cinerea strain defective in the expression of Cu-Zn-SOD in tomato and Arabidopsis plants was due to an increase in O2− and decreased H2O2, which was associated with an increase in callose. On the other hand, pathogenic bacteria have evolved SOD-related mechanisms to evade the host immune system. For example, in the host cells, the withholding of Zn by CP could be expected to decrease the activity of SOD5 in Candida albicans. However, to circumvent the decrease in Zn availability, C. albicans SOD5 has lost Zn-binding activity and evolved a Cu-only cofactor requirement.

42Metalloproteases that use Zn as a cofactor are further potential virulence factors during bacterial and fungal infection. Metalloproteases have been reported to suppress the host immune responses in animal systems. Zn-metalloproteases, in addition to breaking down the host defensive barriers, could interfere in the host-defense mechanisms. However, their specific role in fighting the plant immune system remains elusive.

43Herbivores have also elaborated protective mechanisms against the presence of toxic Zn concentrations in plant tissues. The ingestion of toxic metal concentrations induces ROS causing oxidative stress in herbivores. In order to survive, herbivores activate an effective antioxidant defense system (enzymes and non-enzyme compounds) that inhibits the metal-induced oxidative damage. Variations of SOD and catalase (CAT) activities have been observed in several herbivores under metal toxicity stress. Apart from antioxidant enzymes, acid phosphatase (ACP) and alkaline phosphatase (AKP) have been associated to the detoxification. An example is the observation of enhanced activity of these metalloenzymes in the lepidopter Lymantria dispar when feeding on Zn-treated Populus.

44Additionally, herbivores might deal with the presence of high levels of plant secondary metabolites in leaves as a plant defense response to metal accumulation. Plant secondary metabolites may induce oxidative stress similarly to that caused by heavy metals. Enhancement of glutathione (GSH) production by herbivores has been proposed as a key mechanism to cope with this stress situation. A GSH-mediated detoxification process in response to high Zn has recently been reported in a study identifying relevant novel genes that confer Zn sensitivity or tolerance in Drosophila melanogaster. The authors found that a gene encoding an ABCC-type transporter family protein, related to yeast Cd factor (YCF1) implicated in the GSH-mediated detoxification of Cd, was up-regulated by excess Zn and conferred tolerance to toxic Zn concentrations in Drosophila.

45The multifunctionality of Zn in all living organisms provides this element key roles in basal metabolism, defense, and virulence, turning Zn into a highly valuable tool to understand the mechanisms underlying the plant–pest/pathogen interactions.

46The presence of highly efficient Zn sensing probes and Zn detoxification systems in herbivores and pathogens indicates the relevance of Zn-related defense mechanisms in plants, which include different stratagems not restricted to Zn-hyperaccumulating species. Nonetheless, some pests and pathogens have evolved mechanisms to circumvent the plant’s Zn-poisoning strategies. Therefore, future studies should tackle this issue in a more integrative manner by contributing to the understanding of the zinc homeostatic mechanisms in plants and pests and pathogens, as well as to their interaction. Keystones for future research advances should further clarify the double role of defensins as Zn-ligand and biotic stress signaling molecules, the ligand exchange processes, and the cellular and subcellular compartmentation mechanisms for both Zn and its ligands. The outcomes of these studies would strongly contribute to visualize potential strategies that might benefit the host immune system and to design specific Zn-related tools to trick plant attackers or/and to ameliorate plant defense.

47All coauthors contributed to search of information and structuring of the review. CC and SM wrote the manuscript with inputs from RT, ML, and CP. BG drew the figures and CP acted as the coordinator and the supervisor.

48This study was funded by the Spanish Government (Ministerio de Ciencia, Innovación y Universidades; projects BFU2013-42839-R and BFU2016-75176-R).

49The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.