Photodynamic inactivation of Botrytis cinerea by an anionic porphyrin: an alternative pest management of grapevine

Photodynamic Inactivation in agriculture: combating fungal phytopathogens resistant to conventional treatment

Botrytis cinerea is a severe threat in agriculture, as it can infect over 200 different crop species with gray mold affecting food yields and quality. The conventional treatment using fungicides lead to emerging resistance over the past decades. Here, we introduce Photodynamic Inactivation (PDI) as a strategy to combat B. cinerea infections, independent of fungicide resistance. PDI uses photoactive compounds, which upon illumination create reactive oxygen species toxic for killing target organisms. This study focuses on different formulations of sodium-magnesium-chlorophyllin (Chl, food additive E140) as photoactive compound in combination with EDTA disodium salt dihydrate (Na2EDTA) as cell-wall permeabilizer and a surfactant. In an in vitro experiment, three different photosensitizers (PS) with varying Chl and Na2EDTA concentrations were tested against five B. cinerea strains with different resistance mechanisms. We showed that all B. cinerea mycelial spheres of all tested strains were eradicated with concentrations as low as 224 µM Chl and 3.076 mM Na2EDTA (LED illumination with main wavelength of 395 nm, radiant exposure 106 J cm-2). To further test PDI as a Botrytis treatment strategy in agriculture a greenhouse trial was performed on B. cinerea infected bell pepper plants (Capsicum annum L). Two different rates (560 or 1120 g Ha-1) of PS formulation (0.204 M Chl and 1.279 M Na2EDTA) and a combination of PS formulation with 0.05% of the surfactant BRIJ L4 (560 g Ha-1) were applied weekly for 4 weeks by spray application. Foliar lesions, percentage of leaves affected, percentage of leaf area diseased and AUDPC were significantly reduced, while percentage of marketable plants were increased by all treatments compared to a water treated control, however, did not statistically differ from each other. No phytotoxicity was observed in any treatment. These results add to the proposition of employing PDI with the naturally sourced PS Chl in agricultural settings aimed at controlling B. cinerea disease. This approach seems to be effective regardless of the evolving resistance mechanisms observed in response to conventional antifungal treatments.

Light-Driven Tetra- and Octa-β-substituted Cationic Zinc(II) Phthalocyanines for Eradicating Fusarium oxysporum Conidia

Photodynamic inactivation (PDI) is an emerging therapeutic approach that can effectively inactivate diverse microbial forms, including vegetative forms and spores, while preserving host tissues and avoiding the development of resistance to the photosensitization procedure. This study evaluates the antifungal and sporicidal photodynamic activity of two water-soluble amphiphilic tetra- and octa-β-substituted zinc(II) phthalocyanine (ZnPc) dyes with dimethylaminopyridinium groups at the periphery (ZnPcs 1, 2) and their quaternized derivatives (ZnPcs 1a, 2a). Tetra(1, 1a)- and octa(2, 2a)-β-substituted zinc(II) phthalocyanines were prepared and assessed as photosensitizers (PSs) for their effects on Fusarium oxysporum conidia. Antimicrobial photoinactivation experiments were performed with each PS at 0.1, 1, 10, and 20 µM under white light irradiation at an irradiance of 135 mW·cm-2, for 60 min (light dose of 486 J·cm-2). High PDI efficiency was observed for PSs 1a, 2, and 2a (10 µM), corresponding to inactivation until the method's detection limit. PS 1 (20 µM) also achieved a considerable reduction of >5 log10 in the concentration of viable conidia. The quaternized PSs (1a, 2a) showed better PDI performance than the non-quaternized ones (1, 2), even at the low concentration of 1 µM, and a light dose of 486 J·cm-2. These cationic phthalocyanines are potent photodynamic drugs for antifungal applications due to their ability to effectively inactivate resistant forms, like conidia, with low concentrations and reasonable energy doses.

A pectin-based photoactivated bactericide nanosystem for achieving an improved utilization rate, photostability and targeted delivery of hematoporphyrin

Photoactivated pesticides have many advantages, such as high activity, low toxicity, and no drug resistance. However, poor photostability and a low utilization rate limit their practical application. Herein, the photosensitizer hematoporphyrin (HP) was used as a photoactivated pesticide, covalently linked with pectin (PEC) via ester bonds, to prepare an amphiphilic polymer pro-bactericide, and subsequently self-assembled in aqueous solutions to obtain an esterase-triggered nanobactericide delivery system. The fluorescence quenching effect due to the aggregation of HP in nanoparticles (NPs) enabled the inhibition of photodegradation of HP in this system. Esterase stimulation could trigger HP release and increase its photodynamic activity. Antibacterial assays have shown that the NPs had potent antibacterial capacity, almost completely inactivating bacteria after 60 min of exposure to light. The NPs had good adherence to the leaves. Safety assessment indicated that the NPs have no obvious toxic effects on plants. Antibacterial studies on plants have shown that the NPs have excellent antibacterial effects on infected plants. These results provide a new strategy for obtaining a photoactivated bactericide nanosystem with a high utilization rate and good photostability and targeting ability.

In Vitro Photoinactivation of Fusarium oxysporum Conidia with Light-Activated Ammonium Phthalocyanines

Antimicrobial photodynamic therapy (aPDT) has been explored as an innovative therapeutic approach because it can be used to inactivate a variety of microbial forms (vegetative forms and spores) without causing significant damage to host tissues, and without the development of resistance to the photosensitization process. This study assesses the photodynamic antifungal/sporicidal activity of tetra- and octasubstituted phthalocyanine (Pc) dyes with ammonium groups. Tetra- and octasubstituted zinc(II) phthalocyanines (1 and 2) were prepared and tested as photosensitizers (PSs) on Fusarium oxysporum conidia. Photoinactivation (PDI) tests were conducted with photosensitizer (PS) concentrations of 20, 40, and 60 µM under white-light exposure at an irradiance of 135 mW·cm^-2, applied during 30 and 60 min (light doses of 243 and 486 J·cm^-2). High PDI efficiency corresponding to the inactivation process until the detection limit was observed for both PSs. The tetrasubstituted PS was the most effective, requiring the lowest concentration and the shortest irradiation time for the complete inactivation of conidia (40 µM, 30 min, 243 J·cm^-2). Complete inactivation was also achieved with PS 2, but a longer irradiation time and a higher concentration (60 µM, 60 min, 486 J·cm^-2) were necessary. Because of the low concentrations and moderate energy doses required to inactivate resistant biological forms such as fungal conidia, these phthalocyanines can be considered potent antifungal photodynamic drugs.

Photoantimicrobials in agriculture

Classical approaches for controlling plant pathogens may be impaired by the development of pathogen resistance to chemical pesticides and by limited availability of effective antimicrobial agents. Recent increases in consumer awareness of and/or legislation regarding environmental and human health, and the urgent need to improve food security, are driving increased demand for safer antimicrobial strategies. Therefore, there is a need for a step change in the approaches used for controlling pre- and post-harvest diseases and foodborne human pathogens. The use of light-activated antimicrobial substances for the so-called antimicrobial photodynamic treatment is known to be effective not only in a clinical context, but also for use in agriculture to control plant-pathogenic fungi and bacteria, and to eliminate foodborne human pathogens from seeds, sprouted seeds, fruits, and vegetables. Here, we take a holistic approach to review and re-evaluate recent findings on: (i) the ecology of naturally-occurring photoantimicrobials, (ii) photodynamic processes including the light-activated antimicrobial activities of some plant metabolites, and (iii) fungus-induced photosensitization of plants. The inhibitory mechanisms of both natural and synthetic light-activated substances, known as photosensitizers, are discussed in the contexts of microbial stress biology and agricultural biotechnology. Their modes-of-antimicrobial action make them neither stressors nor toxins/toxicants (with specific modes of poisonous activity), but a hybrid/combination of both. We highlight the use of photoantimicrobials for the control of plant-pathogenic fungi and quantify their potential contribution to global food security.

Protoporphyrin IX-Chitosan Oligosaccharide Conjugate with Potent Antifungal Photodynamic Activity

A new chemical conjugate between protoporphyrin IX (PPIX) and chitosan oligosaccharides (CH) was prepared and evaluated in vitro as an antifungal agent against Penicillium digitatum. Chemical characterization and photophysical/photochemical studies were conducted. The antifungal effect of the CH-PPIX conjugate was compared to its components (PPIX and CH) and a physical mixture of both, under dark and illuminated conditions. The CH-PPIX conjugate was photostable and inhibited fungal growth with 100% efficiency at a dose of 0.005% w/v under visible light irradiation, while no antifungal activity was observed in the dark. Under the same conditions, CH and PPIX did not display any fungicidal activity, demonstrating the improved properties of the conjugate. Insights into the mechanism of fungal inactivation revealed an efficient spore uptake and photoinduced membrane damage through singlet oxygen generation. This new bioconjugate, which is based on natural components, represents a promising agent for fungicidal formulations based on antimicrobial photodynamic therapy.

Both Constitutive and Infection-Responsive Secondary Metabolites Linked to Resistance against Austropuccinia psidii (Myrtle Rust) in Melaleuca quinquenervia

Austropuccinia psidii is a fungal plant pathogen that infects species within the Myrtaceae, causing the disease myrtle rust. Myrtle rust is causing declines in populations within natural and managed ecosystems and is expected to result in species extinctions. Despite this, variation in response to A. psidii exist within some species, from complete susceptibility to resistance that prevents or limits infection by the pathogen. Untargeted metabolomics using Ultra Performance Liquid Chromatography with Ion Mobility followed by analysis using MetaboAnalyst 3.0, was used to explore the chemical defence profiles of resistant, hypersensitive and susceptible phenotypes within Melaleuca quinquenervia during the early stages of A. psidii infection. We were able to identify three separate pools of secondary metabolites: (i) metabolites classified structurally as flavonoids that were naturally higher in the leaves of resistant individuals prior to infection, (ii) organoheterocyclic and carbohydrate-related metabolites that varied with the level of host resistance post-infection, and (iii) metabolites from the terpenoid pathways that were responsive to disease progression regardless of resistance phenotype suggesting that these play a minimal role in disease resistance during the early stages of colonization of this species. Based on the classes of these secondary metabolites, our results provide an improved understanding of key pathways that could be linked more generally to rust resistance with particular application within Melaleuca.

Photodynamic Inactivation of plant pathogens part II: fungi

The constantly increasing demand for agricultural produce from organic and conventional farming calls for new, sustainable, and biocompatible solutions for crop protection. The overuse of fungicides leading to contamination of both produce and environment and the emergence of plant pathogenic fungi that are resistant to conventional treatments warrant the need for new methods to combat fungal infections in the field. We here deliver the follow-up study to our research on the Photodynamic Inactivation (PDI) of plant pathogenic bacteria (Glueck et al. in Photochem Photobiol Sci 18(7):1700-1708, 2019) by expanding the scope to fungal pathogens. Both fungal species employed in this study-Alternaria solani and Botrytis cinerea-cause substantial crop and economic losses. Sodium magnesium chlorophyllin (Chl, approved as food additive E140) in combination with Na₂EDTA and the chlorin e6 derivative B17-0024 holding cationic moieties serve as eco-friendly photoactive compounds. Effectiveness of the antifungal PDI was measured by inhibition of growth of mycelial spheres (average diameter 2-3 mm) after incubation with the photosensitizer for 100 min and subsequent illumination using a LED array (395 nm, 106.6 J cm^-2). One hundred micromolar Chl combined with 5 mM Na₂EDTA was able to successfully photokill 94.1% of A. solani and 91.7% of B. cinerea samples. PDI based on B17-0024 can completely inactivate A. solani at 10 times lower concentration (10 µM); however, for B. cinerea, the concentration required for complete eradication was similar to that of Chl with Na₂EDTA (100 µM). Using a plant compatibility assay based on Fragaria vesca, we further demonstrate that both photosensitizers neither affect host plant development nor cause significant leaf damage. The plants were sprayed with 300 µL of treatment solution used for PDI (one or three treatments on consecutive days) and plant growth was monitored for 21 days. Only minor leaf damage was observed in samples exposed to the chelators Na₂EDTA and polyaspartic acid, but overall plant development was unaffected. In conclusion, our results suggest that sodium magnesium chlorophyllin in combination with EDTA and B17-0024 could serve as effective and safe photofungicides.

Photodynamic control of citrus crop diseases

Citrus are economically important fruit crops to which infectious diseases like citrus canker caused by Xanthomonas citri subs. citri, citrus variegated chlorosis caused by Xylella fastidiosa, "huanglongbing" associated with the presence of Candidatus liberibacter species, anthracnose caused by Colletotrichum gloeosporioides and citrus black spot caused by Phyllosticta citricarpa, impose significant losses. Control measures involve chemical treatment of orchards but often, eradication of infected plants is unavoidable. To circumvent the environmental impacts of pesticides and the socio-economic impacts of eradication, innovative antimicrobial approaches like photodynamic inactivation are being tested. There is evidence of the susceptibility of Xanthomonas citri subs. citri and C. gloeosporioides to photodynamic damage. However, the realistic assessment of perspectives for widespread application of photodynamic inactivation in the control of citrus diseases, necessarily implies that other microorganisms are also considered. This review intends to provide a critical summary of the current state of research on photodynamic inactivation of citrus pathogens and to identify some of the current limitations to the widespread use of photodynamic treatments in citrus crops.

Xylan-Based Cross-Linked Hydrogel for Photodynamic Antimicrobial Chemotherapy

Photodynamic antimicrobial chemotherapy or PACT has been shown to be a promising antibacterial treatment that could overcome the challenge of multidrug-resistant bacteria. However, the use of most existing photosensitizers has been severely hampered by their significant self-quenching effect, poor water solubility, lack of selectivity against bacterial cells, and possible damage to the surrounding tissues. The use of hydrogels may overcome some of these limitations. We herein report a simple strategy to synthesize a cross-linked hydrogel from beech xylan. The hydrogel showed a high swelling ratio, up to 62, an interconnected porous structure, and good mechanical integrity, and 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin tetraiodide (TMPyP) was chosen as a model of hydrophilic photosensitizer (PS) and was encapsulated inside the xylan-based hydrogel. TMPyP-loaded hydrogel prolonged release of PS up to 24 h with a cumulative amount that could reach 100%. TMPyP-loaded hydrogel showed a photocytotoxic effect against Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus strains, and Bacillus cereus, while no cytotoxicity was observed in the dark.