A putative novel role for plant defensins: a defensin from the zinc hyper-accumulating plant, Arabidopsis halleri, confers zinc tolerance

An endophytic fungus interacts with the defensin-like protein OsCAL1 to regulate cadmium allocation in rice

Defensin-like proteins are conserved in multicellular organisms and contribute to innate immune responses against fungal pathogens. In rice, defensins play a novel role in regulating cadmium (Cd) efflux from the cytosol. However, whether the antifungal activity of defensins correlates with Cd-efflux function remains unknown. In this study, we isolated an endophytic Fusarium, designed Fo10, by a comparative microbiome analysis of rice plants grown in a paddy contaminated with Cd. Fo10 is tolerant to high levels of Cd, but is sensitive to the defensin-like protein OsCAL1, which mediates Cd efflux to the apoplast. We found that Fo10 symbiosis in rice is regulated by OsCAL1 dynamics, and Fo10 coordinates multiple plant processes, including Cd uptake, vacuolar sequestration, efflux to the environment, and formation of Fe plaques in the rhizosphere. These processes are dependent on the salicylic acid signaling pathway to keep Cd levels low in the cytosol of rice cells and to decrease Cd levels in rice grains without any yield penalty. Fo10 also plays a role in Cd tolerance in the poaceous crop maize and wheat, but has no observed effects in the eudicot plants Arabidopsis and tomato. Taken together, these findings provide insights into the mechanistic basis underlying how a fungal endophyte and host plant interact to control Cd accumulation in host plants by adapting defense responses to promote the establishment of a symbiosis that permits adaptation to high-Cd environments.

Reduction in PLANT DEFENSIN 1 expression in Arabidopsis thaliana results in increased resistance to pathogens and zinc toxicity

Ectopic expression of defensins in plants correlates with their increased capacity to withstand abiotic and biotic stresses. This applies to Arabidopsis thaliana, where some of the seven members of the PLANT DEFENSIN 1 family (AtPDF1) are recognised to improve plant responses to necrotrophic pathogens and increase seedling tolerance to excess zinc (Zn). However, few studies have explored the effects of decreased endogenous defensin expression on these stress responses. Here, we carried out an extensive physiological and biochemical comparative characterization of (i) novel artificial microRNA (amiRNA) lines silenced for the five most similar AtPDF1s, and (ii) a double null mutant for the two most distant AtPDF1s. Silencing of five AtPDF1 genes was specifically associated with increased aboveground dry mass production in mature plants under excess Zn conditions, and with increased plant tolerance to different pathogens - a fungus, an oomycete and a bacterium, while the double mutant behaved similarly to the wild type. These unexpected results challenge the current paradigm describing the role of PDFs in plant stress responses. Additional roles of endogenous plant defensins are discussed, opening new perspectives for their functions.

Zinc deficiency-induced defensin-like proteins are involved in the inhibition of root growth in Arabidopsis

The depletion of cellular zinc (Zn) adversely affects plant growth. Plants have adaptation mechanisms for Zn-deficient conditions, inhibiting growth through the action of transcription factors and metal transporters. We previously identified three defensin-like (DEFL) proteins (DEFL203, DEFL206 and DEFL208) that were induced in Arabidopsis thaliana roots under Zn-depleted conditions. DEFLs are small cysteine-rich peptides involved in defense responses, development and excess metal stress in plants. However, the functions of DEFLs in the Zn-deficiency response are largely unknown. Here, phylogenetic tree analysis revealed that seven DEFLs (DEFL202-DEFL208) were categorized into one subgroup. Among the seven DEFLs, the transcripts of five (not DEFL204 and DEFL205) were upregulated by Zn deficiency, consistent with the presence of cis-elements for basic-region leucine-zipper 19 (bZIP19) or bZIP23 in their promoter regions. Microscopic observation of GFP-tagged DEFL203 showed that DEFL203-sGFP was localized to the apoplast and plasma membrane. Whereas a single mutation of the DEFL202 or DEFL203 genes only slightly affected root growth, defl202 defl203 double mutants showed enhanced root growth under all growth conditions. We also showed that the size of the root meristem was increased in the double mutants compared with the wild type. Our results suggest that DEFL202 and DEFL203 are redundantly involved in the inhibition of root growth under Zn-deficient conditions through a reduction in root meristem length and cell number.

Dual-function DEFENSIN 8 mediates phloem cadmium unloading and accumulation in rice grains

Grain cadmium (Cd) is translocated from source to sink tissues exclusively via phloem, though the phloem Cd unloading transporter has not been identified yet. Here, we isolated and functionally characterized a defensin-like gene DEFENSIN 8 (DEF8) highly expressed in rice (Oryza sativa) grains and induced by Cd exposure in seedling roots. Histochemical analysis and subcellular localization detected DEF8 expression preferentially in pericycle cells and phloem of seedling roots, as well as in phloem of grain vasculatures. Further analysis demonstrated that DEF8 is secreted into extracellular spaces possibly by vesicle trafficking. DEF8 bound to Cd in vitro, and Cd efflux from protoplasts as well as loading into xylem vessels decreased in the def8 mutant seedlings compared with the wild type. At maturity, significantly less Cd accumulation was observed in the mutant grains. These results suggest that DEF8 is a dual function protein that facilitates Cd loading into xylem and unloading from phloem, thus mediating Cd translocation from roots to shoots and further allocation to grains, representing a phloem Cd unloading regulator. Moreover, essential mineral nutrient accumulation as well as important agronomic traits were not affected in the def8 mutants, suggesting DEF8 is an ideal target for breeding low grain Cd rice.

Characterization of an Arabidopsis Defensin-like Gene Conferring Resistance against Nematodes

Arabidopsis contains 317 genes for defensin-like (DEFL) peptides. DEFLs have been grouped into different families based mainly on cysteine motifs. The DEFL0770 group contains seven genes, of which four are strongly expressed in roots. We found that the expression of these genes is downregulated in syncytia induced by the beet cyst nematode Heterodera schachtii as revealed by RNAseq analysis. We have studied one gene of this group, At3g59930, in detail. A promoter::GUS line revealed that the gene is only expressed in roots but not in other plant organs. Infection of the GUS line with larvae of H. schachtii showed a strong downregulation of GUS expression in infection sites as early as 1 dpi, confirming the RNAseq data. The At3g59930 peptide had only weak antimicrobial activity against Botrytis cinerea. Overexpression lines had no enhanced resistance against this fungus but were more resistant to H. schachtii infection. Our data indicate that At3g59930 is involved in resistance to nematodes which is probably not due to direct nematicidal activity.

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.

Effect of zinc imbalance and salicylic acid co-supply on Arabidopsis response to fungal pathogens with different lifestyles

In higher plants, Zn nutritional imbalance can affect growth, physiology and response to stress, with effect variable depending on host-pathogen interaction. Mechanisms through which Zn operates are not yet well known. The hormone salicylic acid (SA) can affect plant ion uptake, transport and defence responses. Thus, in this study the impact of Zn imbalance and SA co-supply on severity of infection with the necrotrophic fungal pathogen B. cinerea or the biotroph G. cichoracearum was assessed in A. thaliana Col-0. Spectrophotometric assays for pigments and malondialdehyde (MDA) content as a marker of lipid peroxidation, plant defensin 1.2 gene expression by semi-quantitative PCR, callose visualization by fluorescence microscopy and diseases evaluation by macro- and microscopic observations were carried out. Zinc plant concentration varied with the supplied dose. In comparison with the control, Zn-deficit or Zn-excess led to reduced chlorophyll content and PDF 1.2 transcripts induction. In Zn-deficient plants, where MDA increased, also the susceptibility to B. cinerea increased, whereas MDA decreased in G. cichoracearum. Zinc excess increased susceptibility to both pathogens. Co-administration of SA positively affected MDA level, callose deposition, PDF 1.2 transcripts and plant response to the two pathogens. The increased susceptibility to B. cinerea in both Zn-deficient and Zn-excess plants could be related to lack of induction of PDF 1.2 transcripts; oxidative stress could explain higher susceptibility to the necrotroph and lower susceptibility to the biotroph in Zn-deficient plants. This research shows that an appropriate evaluation of Zn supply according to the prevalent stress factor is desirable for plants.

Genome-scale identification of plant defensin (PDF) family genes and molecular characterization of their responses to diverse nutrient stresses in allotetraploid rapeseed

Plant defensins (PDFs), short peptides with strong antibacterial activity, play important roles in plant growth, development, and stress resistance. However, there are few systematic analyses on PDFs in Brassica napus. Here, bioinformatics methods were used to identify genome-wide PDFs in Brassica napus, and systematically analyze physicochemical properties, expansion pattern, phylogeny, and expression profiling of BnaPDFs under diverse nutrient stresses. A total of 37 full-length PDF homologs, divided into two subgroups (PDF1s and PDF2s), were identified in the rapeseed genome. A total of two distinct clades were identified in the BnaPDF phylogeny. Clade specific conserved motifs were identified within each clade respectively. Most BnaPDFs were proved to undergo powerful purified selection. The PDF members had enriched cis-elements related to growth and development, hormone response, environmental stress response in their promoter regions. GO annotations indicate that the functional pathways of BnaPDFs are mainly involved in cells killing and plant defense responses. In addition, bna-miRNA164 and bna-miRNA172 respectively regulate the expression of their targets BnaA2.PDF2.5 and BnaC7.PDF2.6. The expression patterns of BnaPDFs were analyzed in different tissues. BnaPDF1.2bs was mainly expressed in the roots, whereas BnaPDF2.2s and BnaPDF2.3s were both expressed in stamen, pericarp, silique, and stem. However, the other BnaPDF members showed low expression levels in various tissues. Differential expression of BnaPDFs under nitrate limitation, ammonium excess, phosphorus starvation, potassium deficiency, cadmium toxicity, and salt stress indicated that they might participate in different nutrient stress resistance. The genome-wide identification and characterization of BnaPDFs will enrich understanding of their molecular characteristics and provide elite gene resources for genetic improvement of rapeseed resistance to nutrient stresses.

Analysis of a gene family for PDF-like peptides from Arabidopsis

Plant defensins are small, basic peptides that have a characteristic three-dimensional folding pattern which is stabilized by four disulfide bridges. We show here that Arabidopsis contains in addition to the proper plant defensins a group of 9 plant defensin-like (PdfL) genes. They are all expressed at low levels while GUS fusions of the promoters showed expression in most tissues with only minor differences. We produced two of the encoded peptides in E. coli and tested the antimicrobial activity in vitro. Both were highly active against fungi but had lower activity against bacteria. At higher concentrations hyperbranching and swollen tips, which are indicative of antimicrobial activity, were induced in Fusarium graminearum by both peptides. Overexpression lines for most PdfL genes were produced using the 35S CaMV promoter to study their possible in planta function. With the exception of PdfL4.1 these lines had enhanced resistance against F. oxysporum. All PDFL peptides were also transiently expressed in Nicotiana benthamiana leaves with agroinfiltration using the pPZP3425 vector. In case of PDFL1.4 this resulted in complete death of the infiltrated tissues after 7 days. All other PDFLs resulted only in various degrees of small necrotic lesions. In conclusion, our results show that at least some of the PdfL genes could function in plant resistance.

Bioactive Molecules From Native Mexican Avocado Fruit (Persea americana var. drymifolia): A Review

Avocado (Persea americana Mill.) is a tree native from central and eastern México that belongs to the Lauraceae family. Avocado has three botanical varieties known as Mexican (P. americana var. drymifolia), West Indian (P. americana var. americana), and Guatemalan (P. americana var. guatemalensis). It is an oil-rich fruit appreciated worldwide because of its nutritional value and the content of bioactive molecules. Several avocado molecules show attractive activities of interest in medicine. Avocado fatty acids have beneficial effects on cardiovascular disease risk factors. Besides, this fruit possesses a high content of carotenoids and phenolic compounds with possible antifungal, anti-cancer and antioxidant activities. Moreover, several metabolites have been reported with anti-inflammatory effects. Also, an unsaponifiable fraction of avocado in combination with soybean oil is used for the treatment of osteoarthritis. The Mexican variety is native from México and is characterized by the anise aroma in leaves and by small thin-skinned fruits of rich flavor and excellent quality. However, the study of the bioactive molecules of the fruit has not been addressed in detail. In this work, we achieved a literature review on the inflammatory, immunomodulatory and cytotoxic properties of long-chain fatty acids and derivatives from Mexican avocado seed. Also, the antioxidant and anti-inflammatory properties of the oil extracted from the avocado seed are referred. Finally, the antimicrobial, immunomodulatory, and cytotoxic activities of some antimicrobial peptides expressed in the fruit are reviewed.

Zinc toxicity in plants: a review

This review highlights the most recent updated information available about Zn phytotoxicity at physiological, biochemical and molecular levels, uptake mechanisms as well as excess Zn homeostasis in plants. Zinc (Zn) is a natural component of soil in terrestrial environments and is a vital element for plant growth, as it performs imperative functions in numerous metabolic pathways. However, potentially noxious levels of Zn in soils can result in various alterations in plants like reduced growth, photosynthetic and respiratory rate, imbalanced mineral nutrition and enhanced generation of reactive oxygen species. Zn enters into soils through various sources, such as weathering of rocks, forest fires, volcanoes, mining and smelting activities, manure, sewage sludge and phosphatic fertilizers. The rising alarm in environmental facet, as well as, the narrow gap between Zn essentiality and toxicity in plants has drawn the attention of the scientific community to its effects on plants and crucial role in agricultural sustainability. Hence, this review focuses on the most recent updates about various physiological and biochemical functions perturbed by high levels of Zn, its mechanisms of uptake and transport as well as molecular aspects of surplus Zn homeostasis in plants. Moreover, this review attempts to understand the mechanisms of Zn toxicity in plants and to present novel perspectives intended to drive future investigations on the topic. The findings will further throw light on various mechanisms adopted by plants to cope with Zn stress which will be of great significance to breeders for enhancing tolerance to Zn contamination.

Design of improved synthetic antifungal peptides with targeted variations in charge, hydrophobicity and chirality based on a correlation study between biological activity and primary structure of plant defensin γ-cores

Microbial resistance to available drugs is a growing health threat imposing the need for the development of new drugs. The scaffold of plant defensins, including their γ-cores, are particularly good candidates for drug design. This work aimed to improve the antifungal activity of a previous design peptide, named A₃₆,₄₂,₄₄γ_32-46VuDef (for short DD) against yeasts by altering its biochemical parameters. We explore the correlation of the biological activity and structure of plant defensins and compared their primary structures by superimposition with VuDef₁ and DD which indicated us the favorable position and the amino acid to be changed. Three new peptides with modifications in charge, hydrophobicity (RR and WR) and chirality (D-RR) were designed and tested against pathogenic yeasts. Inhibition was determined by absorbance. Viability of mammalian cells was determined by MTT. The three designed peptides had better inhibitory activity against the yeasts with better potency and spectrum of yeast species inhibition, with low toxicity to mammalian cells. WR, the most hydrophobic and cationic, exhibited better antifungal activity and lower toxicity. Our study provides experimental evidence that targeted changes in the primary structure of peptides based on plant defensins γ-core primary structures prove to be a good tool for the synthesis of new compounds that may be useful as alternative antifungal drugs. The method described did not have the drawback of synthesis of several peptides, because alterations are guided. When compared to other methods, the design process described is efficient and viable to those with scarce resources.

Cysteine-rich antimicrobial peptides from plants: The future of antimicrobial therapy

There has been a spurt in the spread of microbial resistance to antibiotics due to indiscriminate use of antimicrobial agents in human medicine, agriculture, and animal husbandry. It has been realized that conventional antibiotic therapy would be less effective in the coming decades and more emphasis should be given for the development of novel antiinfective therapies. Cysteine rich peptides (CRPs) are broad-spectrum antimicrobial agents that modulate the innate immune system of different life forms such as bacteria, protozoans, fungi, plants, insects, and animals. These are also expressed in several plant tissues in response to invasion by pathogens, and play a crucial role in the regulation of plant growth and development. The present work explores the importance of CRPs as potent antimicrobial agents, which can supplement and/or replace the conventional antibiotics. Different plant parts of diverse plant species showed the presence of antimicrobial peptides (AMPs), which had significant structural and functional diversity. The plant-derived AMPs exhibited potent activity toward a range of plant and animal pathogens, protozoans, insects, and even against cancer cells. The cysteine-rich AMPs have opened new avenues for the use of plants as biofactories for the production of antimicrobials and can be considered as promising antimicrobial drugs in biotherapeutics.

Histidine-Rich Defensins from the Solanaceae and Brasicaceae Are Antifungal and Metal Binding Proteins

Plant defensins are best known for their antifungal activity and contribution to the plant immune system. The defining feature of plant defensins is their three-dimensional structure known as the cysteine stabilized alpha-beta motif. This protein fold is remarkably tolerant to sequence variation with only the eight cysteines that contribute to the stabilizing disulfide bonds absolutely conserved across the family. Mature defensins are typically 46–50 amino acids in length and are enriched in lysine and/or arginine residues. Examination of a database of approximately 1200 defensin sequences revealed a subset of defensin sequences that were extended in length and were enriched in histidine residues leading to their classification as histidine-rich defensins (HRDs). Using these initial HRD sequences as a query, a search of the available sequence databases identified over 750 HRDs in solanaceous plants and 20 in brassicas. Histidine residues are known to contribute to metal binding functions in proteins leading to the hypothesis that HRDs would have metal binding properties. A selection of the HRD sequences were recombinantly expressed and purified and their antifungal and metal binding activity was characterized. Of the four HRDs that were successfully expressed all displayed some level of metal binding and two of four had antifungal activity. Structural characterization of the other HRDs identified a novel pattern of disulfide linkages in one of the HRDs that is predicted to also occur in HRDs with similar cysteine spacing. Metal binding by HRDs represents a specialization of the plant defensin fold outside of antifungal activity.

Enhancing of anthracnose disease resistance indicates a potential role of antimicrobial peptide genes in cassava

Cassava (Manihot esculenta Crantz.) is an important economic crop in tropical countries. Demands for using cassava in food, feed and biofuel industries have been increasing worldwide. Cassava anthracnose disease, caused by Colletotrichum gloeosporioides f.sp. manihotis (CAD), is considered a major problem in cassava production. To minimize the effects of such disease, this study investigated the response of cassava to attack by CAD and how the plants defend themselves against this threat. Genome-wide identification of antimicrobial peptide genes (AMPs) and their expression in response to fungal infection was performed in the resistant cassava cultivar (Huay Bong 60; HB60) in comparison with the highly susceptible cultivar (Hanatee; HN). A total of 114 gene members of AMP were identified in the cassava genome database. Fifty-six gene members were selected for phylogenetic tree construction and analysis of putative cis-acting elements in their promoter regions. Differential expression profiles of six candidate genes were observed in response to CAD infection of both cassava cultivars. Upregulation of snakins, MeSN1 and MeSN2 was found in HB60, whereas MeHEL, Me-AMP-D2 and MeLTP2 were highly induced in HN. The MeLTP1 gene was not expressed in either cultivar. HB60 showed a reduced severity rating in comparison to HN after CAD infection. The biomembrane permeability test of fungal CAD was strongly affected after treatment with protein extract derived from CAD-infected HB60. Altogether, these findings suggest that snakins have a potential function in the CAD defense response in cassava. These results could be useful for cassava improvement programs to fight fungal pathogen.

Defensins of Grasses: A Systematic Review

The grass family (Poaceae) is one of the largest families of flowering plants, growing in all climatic zones of all continents, which includes species of exceptional economic importance. The high adaptability of grasses to adverse environmental factors implies the existence of efficient resistance mechanisms that involve the production of antimicrobial peptides (AMPs). Of plant AMPs, defensins represent one of the largest and best-studied families. Although wheat and barley seed γ-thionins were the first defensins isolated from plants, the functional characterization of grass defensins is still in its infancy. In this review, we summarize the current knowledge of the characterized defensins from cultivated and selected wild-growing grasses. For each species, isolation of defensins or production by heterologous expression, peptide structure, biological activity, and structure-function relationship are described, along with the gene expression data. We also provide our results on in silico mining of defensin-like sequences in the genomes of all described grass species and discuss their potential functions. The data presented will form the basis for elucidation of the mode of action of grass defensins and high adaptability of grasses to environmental stress and will provide novel potent molecules for practical use in medicine and agriculture.

Overexpression of PtDefensin enhances resistance to Septotis populiperda in transgenic poplar

Plant defensins have been implicated in the plant defense system, but their role in poplar immunity is still unclear. In the present study, we present evidence that PtDefensin, a putative plant defensin, participates in the defense of poplar plants against Septotis populiperda infection. After the construction of recombinant plasmid PET-32a-PtDefensin, PtDefensin protein was expressed in Escherichia coli strain BL21 (DE3) and purified through Ni-IDA resin affinity chromatography. The Trx-PtDefensin fusion protein displayed no cytotoxic activity against RAW264.7 cells but had cytotoxic activity against E. coli K12D31 cells. Analyses of PtDefensin transcript abundance showed that the expression levels of PtDefensin responded to abiotic and biotic stresses. Overexpression of PtDefensin in 'Nanlin 895' poplars (Populus × euramericana cv 'Nanlin895') increased resistance to Septotis populiperda, coupled with upregulation of MYC2 (basic helix-loop-helix (bHLH) transcription factor) related to jasmonic acid (JA) signal transduction pathways and downregulation of Jasmonate-zim domain (JAZ), an inhibitor in the JA signal transduction pathway. We speculate that systemic acquired resistance (SAR) was activated in non-transgenic poplars after S. populiperda incubation, and that induced systemic resistance (ISR) was activated more obviously in transgenic poplars after S. populiperda incubation. Hence, overexpression of PtDefensin may improve the resistance of poplar plants to pathogens.

Defensins: Transcriptional regulation and function beyond antimicrobial activity

Defensins are one the largest group of antimicrobial peptides and are part of the innate defence. Defensins are produced by animals, plants and fungi. In animals and plants, defensins can be constitutively or differentially expressed both locally or systemically which confer defence before and a stronger response after infection. Immune signalling pathways regulate the gene expression of defensins. These pathways include cellular receptors, which recognise pathogen-associated molecular patterns and are found both in plants and animals. After recognition, signalling pathways and, subsequently, transcriptional factors are activated. There is an increasing number of novel functions in defensins, such as immunomodulators and immune cell attractors. Identification of defensin triggers could help us to elucidate other new functions. The present article reviews the different elicitors of defensins with a main focus on human, fish and marine invertebrate defensins.

Characterization, Expression Profiling, and Functional Analysis of PtDef, a Defensin-Encoding Gene From Populus trichocarpa

PtDef cloned from Populus trichocarpa contained eight cysteine domains specific to defensins. Quantitative reverse-transcription polymerase chain reaction (qRT-PCR) analysis showed that PtDef was expressed in all tissues tested, with lower expression in leaves and higher expression in petioles, stems, and roots. Purified fused PtDef inhibited Aspergillus niger, Alternaria Nees, Mucor corymbifer, Marssonina populi, Rhizopus sp., and Neurospora crassa. PtDef also inhibited the growth of Escherichia coli by triggering autolysis. PtDef overexpression in Nanlin895 poplar (Populus × euramericana cv. Nanlin895) enhanced the level of resistance to Septotinia populiperda. qRT-PCR analysis also showed that the expression of 13 genes related to salicylic acid (SA) and jasmonic acid (JA) signal transduction differed between transgenic and wild-type (WT) poplars before and after inoculation, and that PR1-1 (12–72 h), NPR1-2, TGA1, and MYC2-1 expression was higher in transgenic poplars than in WT. During the hypersensitivity response (HR), large amounts of H₂O₂ were produced by the poplar lines, particularly 12–24 h after inoculation; the rate and magnitude of the H₂O₂ concentration increase were greater in transgenic lines than in WT. Overall, our findings suggest that PtDef, a defensin-encoding gene of P. trichocarpa, could be used for genetic engineering of woody plants for enhanced disease resistance.

Overexpression of a Defensin-Like Gene CAL2 Enhances Cadmium Accumulation in Plants

Accumulation and detoxification of cadmium in rice shoots are of great importance for adaptation to grow in cadmium contaminated soils and for limiting the transport of Cd to grains. However, the molecular mechanisms behind the processes involved in this regulation remain largely unknown. Defensin proteins play important roles in heavy metal tolerance and accumulation in plants. In rice, the cell wall-localized defensin protein (CAL1) is involved in Cd efflux and partitioning to the shoots. In the present study, we functionally characterized the CAL2 defensin protein and determined its contribution to Cd accumulation. CAL2 shared 66% similarity with CAL1, and its mRNA accumulation is mainly observed in roots and is unaffected by Cd stress, but its transcription level was lower than that of CAL1 based on the relative expression of CAL2/Actin1 observed in this study and that reported previously. A promoter-GUS assay revealed that CAL2 is expressed in root tips. Stable expression of the CAL2-mRFP fusion protein indicated that CAL2 is also localized in the cell walls. An in vitro Cd binding experiment revealed that CAL2 has Cd chelation activity. Overexpression of CAL2 increased Cd accumulation in Arabidopsis and rice shoots, but it had no effect on the accumulation of other essential elements. Heterologous expression of CAL2 enhanced Cd sensitivity in Arabidopsis, whereas overexpression of CAL2 had no effect on Cd tolerance in rice. These findings indicate that CAL2 positively regulates Cd accumulation in ectopic overexpression lines of Arabidopsis and rice. We have identified a new gene regulating Cd accumulation in rice grain, which would provide a new genetic resource for molecular breeding.

The plant defensin gene AtPDF2.1 mediates ammonium metabolism by regulating glutamine synthetase activity in Arabidopsis thaliana

BACKGROUND

In plants, ammonium metabolism is particularly important for converting absorbed nitrogen into amino acids. However, the molecular mechanism underlying this conversion remains largely unknown.

RESULTS

Using wild type Arabidopsis thaliana (Col-0) and AtPDF2.1 mutants (pdf2.1-1 and pdf2.1-2), we found that the small cysteine-rich peptide AtPDF2.1, a plant defensin, is involved in regulating ammonium metabolism in the shoot. Ammonium significantly induced the expression of AtPDF2.1 in the shoot and root, particularly in root xylem vascular bundles, as demonstrated by histochemical analysis. Subcellular localization analysis revealed that AtPDF2.1 was localized to the cell wall. Ammonium concentration was higher in the shoot of mutants than in the shoot of Col-0, but no differences were found for total nitrogen content, root ammonium concentration, and the expression of the ammonium transporter gene AtAMT2.1. The activity of glutamine synthetase was significantly decreased in mutants, and the glutamine synthetase family genes GLN1.3 and GLN1.5 were significantly downregulated in mutants compared to Col-0. The activity of nitrate reductase showed no difference between mutants and Col-0.

CONCLUSIONS

Overall, these data suggest that AtPDF2.1 affects ammonium metabolism by regulating the expression of GLN1.3 and GLN1.5 through a yet unidentified mechanism.

The Arabidopsis defensin gene AtPDF2.5 mediates cadmium tolerance and accumulation

Although excess cadmium (Cd) accumulation is harmful to plants, the molecular mechanisms underlying Cd detoxification and accumulation in Arabidopsis thaliana remain largely undetermined. In this study, we demonstrated that the A. thaliana PLANT DEFENSIN 2 gene AtPDF2.5 is involved in Cd tolerance and accumulation. In vitro Cd-binding assays revealed that AtPDF2.5 has Cd-chelating activity. Site-directed mutagenesis of AtPDF2.5 identified eight cysteine residues that were essential for mediating Cd tolerance and chelation. Histochemical analysis demonstrated that AtPDF2.5 was mainly expressed in root xylem vascular bundles, and that AtPDF2.5 was significantly induced by Cd. Subcellular localization analysis revealed that AtPDF2.5 was localized to the cell wall. The overexpression of AtPDF2.5 significantly enhanced Cd tolerance and accumulation in A. thaliana and its heterologous overexpression in rice increased Cd accumulation; however, the functional disruption of AtPDF2.5 decreased Cd tolerance and accumulation. Physiological analysis suggested that AtPDF2.5 promoted Cd efflux from the protoplast and its subsequent accumulation in the cell wall. These data suggest that AtPDF2.5 promotes cytoplasmic Cd efflux via chelation, thereby enhancing Cd detoxification and apoplastic accumulation.

Multi-functionPlantDefensin,AntimicrobialandHeavyMetal Adsorbent Peptide

Background:

Defensin peptide isolated from plants are often heterogeneous in length, sequence and structure, but they are mostly small, cationic and amphipathic. Plant defensins exhibit broad-spectrum antibacterial and antifungal activities against Gram-positive and Gram-negative bacteria, fungi and etc. Plant defensins also play an important role in innate immunity, such as heavy metal and some abiotic stresses tolerance.

Objectives:

In this paper, in vitro broad-spectrum activities, antimicrobial and heavy metal absorption, of a recombinant plant defensin were studied.

Material and Methods:

SDmod gene, a modified plant defensin gene, was cloned in pBISN1-IN (EU886197) plasmid, recombinant protein was produced by transient expression via Agroinfiltration method in common bean. The recombinant protein was tested for antibacterial activity against Gram-negative, Gram-positive bacteria and Fusarium sp. the effects of different treatments on heavy metal zinc absorption by this peptide were tested.

Results:

We confirmed the antibacterial activities of this peptide against Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus and Bacillus cereus) bacteria, and antifungal activities of this peptide against Fusarium spp. (Fusarium oxysporum and Fusarium solani). High metal absorption coefficient for this peptide was also observed.

Results:

Out of six actinobacterial isolates, VITVAMB 1 possessed the most efficient RO-16 decolorization property. It decolorized 85.6% of RO-16 (250 mg L^-1) within 24hrs. Isolate VITVAMB 1 was identified to be Nocardiopsis sp. Maximum dye decolorization occurred at pH 8, temperature 35oC, 3% salt concentration and a dye concentration of 50 mg L^-1.

Conclusions:

Results suggesting that modified defensin peptide facilitates a broader range of defense activities. dedefensins are an important part of the innate immune system in eukaryotes. These molecules have multidimensional properties that making them promising agents for therapeutic drugs.

A non-secreted plant defensin AtPDF2.6 conferred cadmium tolerance via its chelation in Arabidopsis

Plant defensin AtPDF2.6 is not secreted to the apoplast and localized in cytoplasm. AtPDF2.6 is mainly expressed in root vascular bundles of xylem parenchyma cell, and significantly induced by Cd stress. AtPDF2.6 detoxicate cytoplasmic Cd via chelation, thus enhanced Cd tolerance in Arabidopsis. In order to detoxify the heavy metal cadmium (Cd), plants have evolved several mechanisms, among which chelation represents the major Cd-detoxification mechanism. In this study, we aimed to identify a new defensin protein involved in cytoplasmic Cd detoxification by using plant molecular genetics and physiological methods. The results of bioinformatic analysis showed that the Arabidopsis thaliana defensin gene AtPDF2.6 has a signal peptide that may mediate its secretion to the cell wall. Subcellular localization analysis revealed that AtPDF2.6 is localized to the cytoplasm and is not secreted to the apoplast, whereas histochemical analysis indicated that AtPDF2.6 is mainly expressed in the root xylem parenchyma cells and that its expression is significantly induced by Cd. An in vitro Cd-binding assay revealed that AtPDF2.6 has Cd-chelating activity. Heterologous overexpression of AtPDF2.6 increased Cd tolerance in Escherichia coli and yeast, and AtPDF2.6 overexpression significantly enhanced Cd tolerance in Arabidopsis, whereas functional disruption of AtPDF2.6 decreased Cd tolerance. These data suggest that AtPDF2.6 detoxifies cytoplasmic Cd via chelation and thereby enhances Cd tolerance in Arabidopsis. Our findings accordingly challenge the commonly accepted view of defensins as secreted proteins.

Proteomic changes in the xylem sap of Brassica napus under cadmium stress and functional validation

BACKGROUND

The xylem sap of vascular plants primarily transports water and mineral nutrients from the roots to the shoots and also transports heavy metals such as cadmium (Cd). Proteomic changes in xylem sap is an important mechanism for detoxifying Cd by plants. However, it is unclear how proteins in xylem sap respond to Cd. Here, we investigated the effects of Cd stress on the xylem sap proteome of Brassica napus using a label-free shotgun proteomic approach to elucidate plant response mechanisms to Cd toxicity.

RESULTS

We identified and quantified 672 proteins; 67% were predicted to be secretory, and 11% (73 proteins) were unique to Cd-treated samples. Cd stress caused statistically significant and biologically relevant abundance changes in 28 xylem sap proteins. Among these proteins, the metabolic pathways that were most affected were related to cell wall modifications, stress/oxidoreductases, and lipid and protein metabolism. We functionally validated a plant defensin-like protein, BnPDFL, which belongs to the stress/oxidoreductase category, that was unique to the Cd-treated samples and played a positive role in Cd tolerance. Subcellular localization analysis revealed that BnPDFL is cell wall-localized. In vitro Cd-binding assays revealed that BnPDFL has Cd-chelating activity. BnPDFL heterologous overexpression significantly enhanced Cd tolerance in E. coli and Arabidopsis. Functional disruption of Arabidopsis plant defensin genes AtPDF2.3 and AtPDF2.2, which are mainly expressed in root vascular bundles, significantly decreased Cd tolerance.

CONCLUSIONS

Several xylem sap proteins in Brassica napus are differentially induced in response to Cd treatment, and plant defensin plays a positive role in Cd tolerance.

Structural and functional characterization of the membrane-permeabilizing activity of Nicotiana occidentalis defensin NoD173 and protein engineering to enhance oncolysis

Defensins are an extensive family of host defense peptides found ubiquitously across plant and animal species. In addition to protecting against infection by pathogenic microorganisms, some defensins are selectively cytotoxic toward tumor cells. As such, defensins have attracted interest as potential antimicrobial and anticancer therapeutics. The mechanism of defensin action against microbes and tumor cells appears to be conserved and involves the targeting and disruption of cellular membranes. This has been best defined for plant defensins, which upon binding specific phospholipids, such as phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid, form defensin-lipid oligomeric complexes that destabilize membranes, leading to cell lysis. In this study, to further define the anticancer and therapeutic properties of plant defensins, we have characterized a novel plant defensin, Nicotiana occidentalis defensin 173 (NoD173), from N. occidentalis. NoD173 at low micromolar concentrations selectively killed a panel of tumor cell lines over normal primary cells. To improve the anticancer activity of NoD173, we explored increasing cationicity by mutation, with NoD173 with the substitution of Q22 with lysine [NoD173(Q22K)], increasing the antitumor cell activity by 2-fold. NoD173 and the NoD173(Q22K) mutant exhibited only low levels of hemolytic activity, and both maintained activity against tumor cells in serum. The ability of NoD173 to inhibit solid tumor growth in vivo was tested in a mouse B16-F1 model, whereby injection of NoD173 into established subcutaneous tumors significantly inhibited tumor growth. Finally, we showed that NoD173 specifically targets PIP2 and determined by X-ray crystallography that a high-resolution structure of NoD173, which forms a conserved family-defining cysteine-stabilized-αβ motif with a dimeric lipid-binding conformation, configured into an arch-shaped oligomer of 4 dimers. These data provide insights into the mechanism of how defensins target membranes to kill tumor cells and provide proof of concept that defensins are able to inhibit tumor growth in vivo.-Lay, F. T., Ryan, G. F., Caria, S., Phan, T. K., Veneer, P. K., White, J. A., Kvansakul, M., Hulett M. D. Structural and functional characterization of the membrane-permeabilizing activity of Nicotiana occidentalis defensin NoD173 and protein engineering to enhance oncolysis.

A feedback loop between CaWRKY41 and H2O2 coordinates the response to Ralstonia solanacearum and excess cadmium in pepper

WRKY transcription factors have been implicated in both plant immunity and plant responses to cadmium (Cd); however, the mechanism underlying the crosstalk between these processes is unclear. Here, we characterized the roles of CaWRKY41, a group III WRKY transcription factor, in immunity against the pathogenic bacterium Ralstonia solanacearum and Cd stress responses in pepper (Capsicum annuum). CaWRKY41 was transcriptionally up-regulated in response to Cd exposure, R. solanacearum inoculation, and H2O2 treatment. Virus-induced silencing of CaWRKY41 increased Cd tolerance and R. solanacearum susceptibility, while heterologous overexpression of CaWRKY41 in Arabidopsis impaired Cd tolerance, and enhanced Cd and zinc (Zn) uptake and H2O2 accumulation. Genes encoding reactive oxygen species-scavenging enzymes were down-regulated in CaWRKY41-overexpressing Arabidopsis plants, whereas genes encoding Zn transporters and enzymes involved in H2O2 production were up-regulated. Consistent with these findings, the ocp3 (overexpressor of cationic peroxidase 3) mutant, which has elevated H2O2 levels, displayed enhanced sensitivity to Cd stress. These results suggest that a positive feedback loop between H2O2 accumulation and CaWRKY41 up-regulation coordinates the responses of pepper to R. solanacearum inoculation and Cd exposure. This mechanism might reduce Cd tolerance by increasing Cd uptake via Zn transporters, while enhancing resistance to R. solanacearum.

Medicinal Potentialities of Plant Defensins: A Review with Applied Perspectives

Plant-based secondary metabolites with medicinal potentialities such as defensins are small, cysteine-rich peptides that represent an imperative aspect of the inherent defense system. Plant defensins possess broad-spectrum biological activities, e.g., bactericidal and insecticidal actions, as well as antifungal, antiviral, and anticancer activities. The unique structural and functional attributes provide a nonspecific and versatile means of combating a variety of microbial pathogens, i.e., fungi, bacteria, protozoa, and enveloped viruses. Some defensins in plants involved in other functions include the development of metal tolerance and the role in sexual reproduction, while most of the defensins make up the innate immune system of the plants. Defensins are structurally and functionally linked and have been characterized in various eukaryotic microorganisms, mammals, plants, gulls, teleost species of fish, mollusks, insect pests, arachnidan, and crustaceans. This defense mechanism has been improved biotechnologically as it helps to protect plants from fungal attacks in genetically modified organisms (GMO). Herein, we review plant defensins as secondary metabolites with medicinal potentialities. The first half of the review elaborates the origin, structural variations, and mechanism of actions of plant defensins. In the second part, the role of defensins in plant defense, stress response, and reproduction are discussed with suitable examples. Lastly, the biological applications of plant defensins as potential antimicrobial and anticancer agents are also deliberated. In summary, plant defensins may open a new prospect in medicine, human health, and agriculture.

Defensin-like peptides in wheat analyzed by whole-transcriptome sequencing: a focus on structural diversity and role in induced resistance

Antimicrobial peptides (AMPs) are the main components of the plant innate immune system. Defensins represent the most important AMP family involved in defense and non-defense functions. In this work, global RNA sequencing and de novo transcriptome assembly were performed to explore the diversity of defensin-like (DEFL) genes in the wheat Triticum kiharae and to study their role in induced resistance (IR) mediated by the elicitor metabolites of a non-pathogenic strain FS-94 of Fusarium sambucinum. Using a combination of two pipelines for DEFL mining in transcriptome data sets, as many as 143 DEFL genes were identified in T. kiharae, the vast majority of them represent novel genes. According to the number of cysteine residues and the cysteine motif, wheat DEFLs were classified into ten groups. Classical defensins with a characteristic 8-Cys motif assigned to group 1 DEFLs represent the most abundant group comprising 52 family members. DEFLs with a characteristic 4-Cys motif CX{3,5}CX{8,17}CX{4,6}C named group 4 DEFLs previously found only in legumes were discovered in wheat. Within DEFL groups, subgroups of similar sequences originated by duplication events were isolated. Variation among DEFLs within subgroups is due to amino acid substitutions and insertions/deletions of amino acid sequences. To identify IR-related DEFL genes, transcriptional changes in DEFL gene expression during elicitor-mediated IR were monitored. Transcriptional diversity of DEFL genes in wheat seedlings in response to the fungus Fusarium oxysporum, FS-94 elicitors, and the combination of both (elicitors + fungus) was demonstrated, with specific sets of up- and down-regulated DEFL genes. DEFL expression profiling allowed us to gain insight into the mode of action of the elicitors from F. sambucinum. We discovered that the elicitors up-regulated a set of 24 DEFL genes. After challenge inoculation with F. oxysporum, another set of 22 DEFLs showed enhanced expression in IR-displaying seedlings. These DEFLs, in concert with other defense molecules, are suggested to determine enhanced resistance of elicitor-pretreated wheat seedlings. In addition to providing a better understanding of the mode of action of the elicitors from FS-94 in controlling diseases, up-regulated IR-specific DEFL genes represent novel candidates for genetic transformation of plants and development of pathogen-resistant crops.

Overexpression of Chickpea Defensin Gene Confers Tolerance to Water-Deficit Stress in Arabidopsis thaliana

Plant defensins are mainly known for their antifungal activity. However, limited information is available regarding their function in abiotic stresses. In this study, a defensin gene, Ca-AFP, from Cicer arietinum, commonly known as chickpea, was cloned and transformed in Arabidopsis thaliana for its functional characterization under simulated water-deficit conditions. Under simulated water-deficit conditions (mannitol and polyethylene glycol-6000 induced), the transgenic A. thaliana plants had higher accumulation of the Ca-AFP transcript compared to that under non-stress condition and showed higher germination rate, root length, and biomass than the wild-type (WT) plants. To get further insights into the role of Ca-AFP in conferring tolerance to water-deficit stress, we determined various physiological parameters and found significant reduction in the transpiration rate and stomatal conductance whereas the net photosynthesis and water use efficiency was increased in the transgenic plants compared to that in the WT plants under water deficit conditions. The transgenic plants showed enhanced superoxide dismutase, ascorbate peroxidase, and catalase activities, had higher proline, chlorophyll, and relative water content, and exhibited reduced ion leakage and malondialdehyde content under water-deficit conditions. Overall, our results indicate that overexpression of Ca-AFP could be an efficient approach for conferring tolerance to water-deficit stress in plants.

Peanuts that keep aflatoxin at bay: a threshold that matters

Aflatoxin contamination in peanuts poses major challenges for vulnerable populations of sub-Saharan Africa and South Asia. Developing peanut varieties to combat preharvest Aspergillus flavus infection and resulting aflatoxin contamination has thus far remained a major challenge, confounded by highly complex peanut-Aspergilli pathosystem. Our study reports achieving a high level of resistance in peanut by overexpressing (OE) antifungal plant defensins MsDef1 and MtDef4.2, and through host-induced gene silencing (HIGS) of aflM and aflP genes from the aflatoxin biosynthetic pathway. While the former improves genetic resistance to A. flavus infection, the latter inhibits aflatoxin production in the event of infection providing durable resistance against different Aspergillus flavus morphotypes and negligible aflatoxin content in several peanut events/lines well. A strong positive correlation was observed between aflatoxin accumulation and decline in transcription of the aflatoxin biosynthetic pathway genes in both OE-Def and HIGS lines. Transcriptomic signatures in the resistant lines revealed key mechanisms such as regulation of aflatoxin synthesis, its packaging and export control, besides the role of reactive oxygen species-scavenging enzymes that render enhanced protection in the OE and HIGS lines. This is the first study to demonstrate highly effective biotechnological strategies for successfully generating peanuts that are near-immune to aflatoxin contamination, offering a panacea for serious food safety, health and trade issues in the semi-arid regions.

Pathogenesis-related proteins and peptides as promising tools for engineering plants with multiple stress tolerance

Pathogenesis-related (PR) proteins and antimicrobial peptides (AMPs) are a group of diverse molecules that are induced by phytopathogens as well as defense related signaling molecules. They are the key components of plant innate immune system especially systemic acquired resistance (SAR), and are widely used as diagnostic molecular markers of defense signaling pathways. Although, PR proteins and peptides have been isolated much before but their biological function remains largely enigmatic despite the availability of new scientific tools. The earlier studies have demonstrated that PR genes provide enhanced resistance against both biotic and abiotic stresses, which make them one of the most promising candidates for developing multiple stress tolerant crop varieties. In this regard, plant genetic engineering technology is widely accepted as one of the most fascinating approach to develop the disease resistant transgenic crops using different antimicrobial genes like PR genes. Overexpression of PR genes (chitinase, glucanase, thaumatin, defensin and thionin) individually or in combination have greatly uplifted the level of defense response in plants against a wide range of pathogens. However, the detailed knowledge of signaling pathways that regulates the expression of these versatile proteins is critical for improving crop plants to multiple stresses, which is the future theme of plant stress biology. Hence, this review provides an overall overview on the PR proteins like their classification, role in multiple stresses (biotic and abiotic) as well as in various plant defense signaling cascades. We also highlight the success and snags of transgenic plants expressing PR proteins and peptides.

Genome-wide (ChIP-seq) identification of target genes regulated by BdbZIP10 during paraquat-induced oxidative stress

BACKGROUND

bZIP transcription factors play a significant role in many aspects of plant growth and development and also play critical regulatory roles during plant responses to various stresses. Overexpression of the Brachypodium bZIP10 (Bradi1g30140) transcription factor conferred enhanced oxidative stress tolerance and increased viability when plants or cells were exposed to the herbicide paraquat. To gain a better understanding of genes involved in bZIP10 conferred oxidative stress tolerance, chromatin immunoprecipitation followed by high throughput sequencing (ChIP-Seq) was performed on BdbZIP10 overexpressing plants in the presence of oxidative stress.

RESULTS

We identified a transcription factor binding motif, TGDCGACA, different from most known bZIP TF motifs but with strong homology to the Arabidopsis zinc deficiency response element. Analysis of the immunoprecipitated sequences revealed an enrichment of gene ontology groups with metal ion transmembrane transporter, transferase, catalytic and binding activities. Functional categories including kinases and phosphotransferases, cation/ion transmembrane transporters, transferases (phosphorus-containing and glycosyl groups), and some nucleoside/nucleotide binding activities were also enriched.

CONCLUSIONS

Brachypodium bZIP10 is involved in zinc homeostasis, as it relates to oxidative stress.

The evolution, function and mechanisms of action for plant defensins

Plant defensins are an extensive family of small cysteine rich proteins characterised by a conserved cysteine stabilised alpha beta protein fold which resembles the structure of insect and vertebrate defensins. However, secondary structure and disulphide topology indicates two independent superfamilies of defensins with similar structures that have arisen via an extreme case of convergent evolution. Defensins from plants and insects belong to the cis-defensin superfamily whereas mammalian defensins belong to the trans-defensin superfamily. Plant defensins are produced by all species of plants and although the structure is highly conserved, the amino acid sequences are highly variable with the exception of the cysteine residues that form the stabilising disulphide bonds and a few other conserved residues. The majority of plant defensins are components of the plant innate immune system but others have evolved additional functions ranging from roles in sexual reproduction and development to metal tolerance. This review focuses on the antifungal mechanisms of plant defensins. The activity of plant defensins is not limited to plant pathogens and many of the described mechanisms have been elucidated using yeast models. These mechanisms are more complex than simple membrane permeabilisation induced by many small antimicrobial peptides. Common themes that run through the characterised mechanisms are interactions with specific lipids, production of reactive oxygen species and induction of cell wall stress. Links between sequence motifs and functions are highlighted where appropriate. The complexity of the interactions between plant defensins and fungi helps explain why this protein superfamily is ubiquitous in plant innate immunity.

A defensin-like protein drives cadmium efflux and allocation in rice

Pollution by heavy metals limits the area of land available for cultivation of food crops. A potential solution to this problem might lie in the molecular breeding of food crops for phytoremediation that accumulate toxic metals in straw while producing safe and nutritious grains. Here, we identify a rice quantitative trait locus we name cadmium (Cd) accumulation in leaf 1 (CAL1), which encodes a defensin-like protein. CAL1 is expressed preferentially in root exodermis and xylem parenchyma cells. We provide evidence that CAL1 acts by chelating Cd in the cytosol and facilitating Cd secretion to extracellular spaces, hence lowering cytosolic Cd concentration while driving long-distance Cd transport via xylem vessels. CAL1 does not appear to affect Cd accumulation in rice grains or the accumulation of other essential metals, thus providing an efficient molecular tool to breed dual-function rice varieties that produce safe grains while remediating paddy soils.

The Arabidopsis defensin gene, AtPDF1.1, mediates defence against Pectobacterium carotovorum subsp. carotovorum via an iron-withholding defence system

Plant defensins (PDFs) are cysteine-rich peptides that have a range of biological functions, including defence against fungal pathogens. However, little is known about their role in defence against bacteria. In this study, we showed that the protein encoded by ARABIDOPSIS THALIANA PLANT DEFENSIN TYPE 1.1 (AtPDF1.1) is a secreted protein that can chelate apoplastic iron. Transcripts of AtPDF1.1 were induced in both systemic non-infected leaves of Arabidopsis thaliana plants and those infected with the necrotrophic bacterium Pectobacterium carotovorum subsp. carotovorum (Pcc). The expression levels of AtPDF1.1 with correct subcellular localization in transgenic A. thaliana plants were positively correlated with tolerance to Pcc, suggesting its involvement in the defence against this bacterium. Expression analysis of genes associated with iron homeostasis/deficiency and hormone signalling indicated that the increased sequestration of iron by apoplastic AtPDF1.1 overexpression perturbs iron homeostasis in leaves and consequently activates an iron-deficiency-mediated response in roots via the ethylene signalling pathway. This in turn triggers ethylene-mediated signalling in systemic leaves, which is involved in suppressing the infection of necrotrophic pathogens. These findings provide new insight into the key functions of plant defensins in limiting the infection by the necrotrophic bacterium Pcc via an iron-deficiency-mediated defence response.

Experimental and theoretical studies of cadmium ions absorption by a new reduced recombinant defensin

Heavy metal pollutants such as Cd, Hg, Pb, As, and Se are considered as both a global problem and a growing threat to the humanity. Being strongly poisonous to the metal-sensitive enzymes and leading to the growth inhibition and death of organisms, these metals have a toxic impact on the plants and animals. Inducing the metal-binding cysteine-rich peptides such as metallothioneins, phytochelatins, and defensins, higher organisms like plants and animals usually react to the heavy metal stress. In this study, a recombinant defensin protein was expressed in bean and its ability in the cadmium absorption was determined. Experimental studies revealed that this protein was able to absorb cadmium ions in reduced form more than oxide one. Molecular dynamics simulations were carried out in order to evaluation of experimental studies, using a model of Cd²⁺ or Na⁺ and Cl^- ions enclosed in a fully hydrated simulation box with the recombinant defensin. The theoretical results also suggested that the reduced recombinant defensin was more powerful in the absorption of Cd²⁺ than its oxide form. The present study is the first report of Cd²⁺ absorption potential of this new reduced recombinant defensin. The results unraveled that this recombinant defensin can be adopted as a molecular switch in the cadmium pollution of the environment and also the important role of sulfur groups in the absorption of cadmium ions.

The Brassicaceae species Heliophila coronopifolia produces root border-like cells that protect the root tip and secrete defensin peptides

Background and Aims

Root border cells and border-like cells (BLCs), the latter originally described in Arabidopsis thaliana , have been described as cells released at the root tips of the species in which they occur. BLCs are thought to provide protection to root meristems similar to classical root border cells. In addition, four defensin peptides (Hc-AFP1-4) have previously been characterized from Heliophila coronopifolia , a South African semi-desert flower, and found to be strongly antifungal. This provided an opportunity to evaluate if the BLCs of H. coronopifolia indeed produce these defensins, which would provide evidence towards a defence role for BLCs.

Methods

Fluorescence microscopy, using live-cell-imaging technology, was used to characterize the BLCs of H. coronopifolia . Quantitative real-time PCR (qRT-PCR) analysis and immunofluorescence microscopy was used to characterize these defensin peptides.

Key Results

BLCs originated at the root apical meristem and formed a protective sheath at the tip and along the sides as the root elongated in solid medium. BLCs have a cellulose-enriched cell wall, intact nuclei and are embedded in a layer of pectin-rich mucilage. Pectinase treatments led to the dissolution of the sheath and dissociation of the root BLCs. Hc-AFP1-4 genes were all expressed in root tissues, but Hc-AFP3 transcripts were the most abundant in these tissues as measured by qRT-PCR. A polyclonal antibody that was cross-reactive with all four defensins, and probably recognizing a general plant defensin epitope, was used in fluorescence microscopy analysis to examine the presence of the peptides in the root tip and BLCs. Data confirmed the peptides present in the root tip tissues, the mucilage sheath and the BLCs.

Conclusions

This study provides a link between defensin peptides and BLCs, both embedded in a protective pectin mucilage sheath, during normal plant growth and development. The presence of the Hc-AFP3 defensin peptides in the BLCs suggests a role for these cells in root protection.

Convergent evolution of defensin sequence, structure and function

Defensins are a well-characterised group of small, disulphide-rich, cationic peptides that are produced by essentially all eukaryotes and are highly diverse in their sequences and structures. Most display broad range antimicrobial activity at low micromolar concentrations, whereas others have other diverse roles, including cell signalling (e.g. immune cell recruitment, self/non-self-recognition), ion channel perturbation, toxic functions, and enzyme inhibition. The defensins consist of two superfamilies, each derived from an independent evolutionary origin, which have subsequently undergone extensive divergent evolution in their sequence, structure and function. Referred to as the cis- and trans-defensin superfamilies, they are classified based on their secondary structure orientation, cysteine motifs and disulphide bond connectivities, tertiary structure similarities and precursor gene sequence. The utility of displaying loops on a stable, compact, disulphide-rich core has been exploited by evolution on multiple occasions. The defensin superfamilies represent a case where the ensuing convergent evolution of sequence, structure and function has been particularly extreme. Here, we discuss the extent, causes and significance of these convergent features, drawing examples from across the eukaryotes.

Conserved but Attenuated Parental Gene Expression in Allopolyploids: Constitutive Zinc Hyperaccumulation in the Allotetraploid Arabidopsis kamchatica

Allopolyploidization combines parental genomes and often confers broader species distribution. However, little is known about parentally transmitted gene expression underlying quantitative traits following allopolyploidization because of the complexity of polyploid genomes. The allopolyploid species Arabidopsis kamchatica is a natural hybrid of the zinc hyperaccumulator Arabidopsis halleri and of the nonaccumulator Arabidopsis lyrata We found that A. kamchatica retained the ability to hyperaccumulate zinc from A. halleri and grows in soils with both low and high metal content. Hyperaccumulation of zinc by A. kamchatica was reduced to about half of A. halleri, but is 10-fold greater than A. lyrata Homeologs derived from A. halleri had significantly higher levels of expression of genes such as HEAVY METAL ATPASE4 (HMA4), METAL TRANSPORTER PROTEIN1 and other metal ion transporters than those derived from A. lyrata, which suggests cis-regulatory differences. A. kamchatica has on average about half the expression of these genes compared with A. halleri due to fixed heterozygosity inherent in allopolyploids. Zinc treatment significantly changed the ratios of expression of 1% of homeologous pairs, including genes putatively involved in metal homeostasis. Resequencing data showed a significant reduction in genetic diversity over a large genomic region (290 kb) surrounding the HMA4 locus derived from the A. halleri parent compared with the syntenic A. lyrata-derived region, which suggests different evolutionary histories. We also estimated that three A. halleri-derived HMA4 copies are present in A. kamchatica Our findings support a transcriptomic model in which environment-related transcriptional patterns of both parents are conserved but attenuated in the allopolyploids.

NMR structure and conformational dynamics of AtPDFL2.1, a defensin-like peptide from Arabidopsis thaliana

Plant defensins constitute the innate immune response against pathogens such as fungi and bacteria. Typical plant defensins are small, basic peptides that possess a characteristic three-dimensional fold stabilized by three or four disulfide bridges. In addition to known defensin genes, the Arabidopsis genome comprises >300 defensin-like genes coding for small cysteine-rich peptides. One of such genes encodes for AtPDFL2.1, a putative antifungal peptide of 55 amino acids, with six cysteine residues in its primary sequence. To understand the functional role of AtPDFL2.1, we carried out antifungal activity assays and determined its high-resolution three-dimensional structure using multidimensional solution NMR spectroscopy. We found that AtPDFL2.1 displays a strong inhibitory effect against Fusarium graminearum (IC₅₀≈4μM). This peptide folds in the canonical cysteine-stabilized αβ (CSαβ) motif, consisting of one α-helix and one triple-stranded antiparallel β-sheet stabilized by three disulfide bridges and a hydrophobic cluster of residues within its core where the α-helix packs tightly against the β-sheets. Nuclear spin relaxation measurements show that the structure of AtPDFL2.1 is essentially rigid, with the L3 loop located between β-strands 2 and 3 being more flexible and displaying conformational exchange. Interestingly, the dynamic features of loop L3 are conserved among defensins and are probably correlated to the antifungal and receptor binding activities.