The use of and search for drugs and dietary supplements derived from plants have accelerated in recent years. Ethnopharmacologists, botanists, microbiologists, and natural-products chemists are combing the Earth for phytochemicals and “leads” which could be developed for treatment of infectious diseases. While 25 to 50% of current pharmaceuticals are derived from plants, none are used as antimicrobials. Traditional healers have long used plants to prevent or cure infectious conditions; Western medicine is trying to duplicate their successes. Plants are rich in a wide variety of secondary metabolites, such as tannins, terpenoids, alkaloids, and flavonoids, which have been found in vitro to have antimicrobial properties. This review attempts to summarize the current status of botanical screening efforts, as well as in vivo studies of their effectiveness and toxicity. The structure and antimicrobial properties of phytochemicals are also addressed. Since many of these compounds are currently available as unregulated botanical preparations and their use by the public is increasing rapidly, clinicians need to consider the consequences of patients self-medicating with these preparations.

Plant Products as Antimicrobial Agents

Inducible defense-related proteins have been described in many plant species upon infection with oomycetes, fungi, bacteria, or viruses, or insect attack. Several types of proteins are common and have been classified into 17 families of pathogenesis-related proteins (PRs). Others have so far been found to occur more specifically in some plant species. Most PRs and related proteins are induced through the action of the signaling compounds salicylic acid, jasmonic acid, or ethylene, and possess antimicrobial activities in vitro through hydrolytic activities on cell walls, contact toxicity, and perhaps an involvement in defense signaling. However, when expressed in transgenic plants, they reduce only a limited number of diseases, depending on the nature of the protein, plant species, and pathogen involved. As exemplified by the PR-1 proteins in Arabidopsis and rice, many homologous proteins belonging to the same family are regulated developmentally and may serve different functions in specific organs or tissues. Several defense-related proteins are induced during senescence, wounding or cold stress, and some possess antifreeze activity. Many defense-related proteins are present constitutively in floral tissues and a substantial number of PR-like proteins in pollen, fruits, and vegetables can provoke allergy in humans. The evolutionary conservation of similar defense-related proteins in monocots and dicots, but also their divergent occurrence in other conditions, suggest that these proteins serve essential functions in plant life, whether in defense or not.

/pdf/significance-of-inducible-defense-related-proteins-in-1xu2abfq16.pdf

Significance of Inducible Defense-related Proteins in Infected Plants

Antimicrobial host defense peptides are produced by all complex organisms as well as some microbes and have diverse and complex antimicrobial activities. Collectively these peptides demonstrate a broad range of antiviral and antibacterial activities and modes of action, and it is important to distinguish between direct microbicidal and indirect activities against such pathogens. The structural requirements of peptides for antiviral and antibacterial activities are evaluated in light of the diverse set of primary and secondary structures described for host defense peptides. Peptides with antifungal and antiparasitic activities are discussed in less detail, although the broad-spectrum activities of such peptides indicate that they are important host defense molecules. Knowledge regarding the relationship between peptide structure and function as well as their mechanism of action is being applied in the design of antimicrobial peptide variants as potential novel therapeutic agents.

Peptide Antimicrobial Agents

https://dspace.library.uu.nl/bitstream/handle/1874/1590/PMPP-van+Loon-1999.pdf?sequence=1

The families of pathogenesis-related proteins, their activities, and comparative analysis of PR-1 type proteins

Plant gene responses to changing carbohydrate status can vary markedly Some genes are induced, some are repressed, and others are minimally affected As in microorganisms, sugar-sensitive plant genes are part of an ancient system of cellular adjustment to critical nutrient availability However, in multicellular plants, sugar-regulated expression also provides a mechanism for control of resource distribution among tissues and organs Carbohydrate depletion upregulates genes for photosynthesis, remobilization, and export, while decreasing mRNAs for storage and utilization Abundant sugar levels exert opposite effects through a combination of gene repression and induction Long-term changes in metabolic activity, resource partitioning, and plant form result Sensitivity of carbohydrate-responsive gene expression to environmental and developmental signals further enhances its potential to aid acclimation The review addresses the above from molecular to whole-plant levels and considers emerging models for sensing and transducing carbohydrate signals to responsive genes

Carbohydrate-modulated gene expression in plants.

Eight families of antimicrobial peptides, ranging in size from 2 to 9 kD, have been identified in plants. These are thionins, defensins, so-called lipid transfer proteins, hevein- and knottin-like peptides, MBP1, IbAMP, and the recently reported snakins. All of them have compact structures that are stabilized by 2-6 disulfide bridges. They are part of both permanent and inducible defense barriers. Transgenic overexpression of the corresponding genes leads to enhanced tolerance to pathogens, and peptide-sensitive pathogen mutants have reduced virulence.

/pdf/plant-defense-peptides-14pt88alhp.pdf

Plant defense peptides.

https://www.cell.com/trends/microbiology/pdf/S0966-842X(00)88879-4.pdf

The defensive role of nonspecific lipid-transfer proteins in plants

Lipid transfer proteins (nsLTPs) from barley and maize leaves are potent inhibitors of bacterial and fungal plant pathogens

A new type of antimicrobial peptide, snakin-1 (SN1), has been isolated from potato tubers and found to be active, at concentrations <10 μM, against bacterial and fungal pathogens from potato and other plant species. The action of SN1 and potato defensin PTH1 was synergistic against the bacterium Clavibacter michiganensis subsp. sepedonicus and additive against the fungus Botrytis cinerea. Snakin-1 causes aggregation of both gram-positive and gram-negative bacteria. The peptide has 63 amino acid residues (M r 6,922), 12 of which are cysteines, and is unrelated to any previously isolated protein, although it is homologous to amino acid sequences deduced from cloned cDNAs that encode gibberellin-inducible mRNAs and has some sequence motifs in common with kistrin and other hemotoxic snake venoms. A degenerate oligonucleotide probe based on the internal sequence CCEECKC has been used to clone an SN1 cDNA. With the cDNA used as probe, one copy of the StSN1 gene per haploid genome has been estimated and expressi...

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/MPMI.1999.12.1.16

Snakin-1, a Peptide from Potato That Is Active Against Plant Pathogens

Purothionins are basic polypeptides with antimicrobial properties that are present in the endosperm of wheat and other cereal species. Susceptibility to wheat purothionins among phytopathogenic bacteria of the genera Pseudomonas, Xanthomonas, Agrobacterium, Erwinia, and Corynebacterium has been investigated. Sensitive strains have been found in all of these genera except Agrobacterium (the only strain of A. tumefaciens available proved to be resistant). Minimal inhibitory concentrations (MIC) with partially purified crude purothionins ranged from 1 mug/ml for C. sepedonicum (C.5) to 540 mug/ml for E. amylovora (E.3). Minimal bactericidal concentrations (MBC) were not higher than twice the MIC value, except for C. poinsettiae (C.4) (MBC/MIC = 8). Purothionins alpha and beta, obtained by carboxymethyl-cellulose column chromatography, were tested against P. solanacearum (P.2) and X. phaseoli (X.2); alpha purothionin was more active than beta against X.2, and beta more active than alpha against P.2. This suggests a relationship between polypeptide sequence and specificity of action.

https://aem.asm.org/content/aem/23/5/998.full.pdf

Francisco García-Olmedo

Papers

Plant defense peptides.

The defensive role of nonspecific lipid-transfer proteins in plants

Lipid transfer proteins (nsLTPs) from barley and maize leaves are potent inhibitors of bacterial and fungal plant pathogens

Snakin-1, a Peptide from Potato That Is Active Against Plant Pathogens

Susceptibility of Phytopathogenic Bacteria to Wheat Purothionins In Vitro