Photodynamic inactivation of conidia of the fungus Colletotrichum abscissum on Citrus sinensis plants with methylene blue under solar radiation

Fly into the light: eliminating Drosophila melanogaster with chlorophyllin-based Photodynamic Inactivation

Fruit flies spoil crops in agricultural settings. As conventional pesticides may generate negative off-target effects on humans or the environment, existing treatment methods need eco-friendly and safe alternatives. Photodynamic Inactivation (PDI) is based on the photosensitizer-mediated and light-induced overproduction of reactive oxygen species in targets. We here explore the potential of PDI for the control of fruit fly pests. Drosophila melanogaster serves as well-established model organism in this study. Two distinct experimental approaches are presented: the feed assay, in which fruit flies are provided with sodium magnesium chlorophyllin (Chl, approved as food additive E140) along with sucrose (3%) as their food, and the spray assay, where the photosensitizer is sprayed onto the insects. We show that PDI based on Chl can induce moribundity rates of Drosophila melanogaster of more than 99% with 5 mM Chl and LED illumination (395 nm, 8 h incubation in the dark, radiant exposure 78.9 J/cm2) with the feed assay. If the radiant exposure is doubled to 157.8 J/cm2, 88% of insects are killed by PDI based on 1 mM Chl. The photoactive compound is also effective if presented on strawberries without addition of sucrose with somewhat lower moribundity (71% at 5 mM Chl). Spraying Chl onto insects is less effective than feeding the photosensitizer: 5 mM Chl resulted in 79.5% moribundity (drug to light interval 8 h, radiant exposure 78.9 J/cm2), but if 5 h of sun light (532 J/cm2) and overnight (14 h) dark incubation is used for activation of Chl, more than 95% of insects are killed. As conclusion, Chl serves as effective photoinsecticide against Drosophila melanogaster if a drug to light interval of 8 h is maintained. Feeding the photoactive compound together with sucrose is more effective than spraying it onto insects and increasing the radiant exposure allows for lowering the photosensitizer concentration. Photodynamic Inactivation might therefore represent an eco-friendly addition to the farmers armamentarium against (semi-transparent) insects.

Natural Sunlight-Mediated Emodin Photoinactivation of Aeromonas hydrophila

Aeromonas hydrophila can be a substantial concern, as it causes various diseases in aquaculture. An effective and green method for inhibiting A. hydrophila is urgently required. Emodin, a naturally occurring anthraquinone compound, was exploited as a photo-antimicrobial agent against A. hydrophila. At the minimum inhibitory concentration of emodin (256 mg/L) to inactivate A. hydrophilia in 30 min, an 11.32% survival rate was observed under 45 W white compact fluorescent light irradiation. In addition, the antibacterial activity under natural sunlight (0.78%) indicated its potential for practical application. Morphological observations demonstrated that the cell walls and membranes of A. hydrophila were susceptible to damage by emodin when exposed to light irradiation. More importantly, the photoinactivation of A. hydrophila was predominantly attributed to the hydroxyl radicals and superoxide radicals produced by emodin, according to the trapping experiment and electron spin resonance spectroscopy. Finally, a light-dependent reactive oxygen species punching mechanism of emodin to photoinactivate A. hydrophila was proposed. This study highlights the potential use of emodin in sunlight-mediated applications for bacterial control, thereby providing new possibilities for the use of Chinese herbal medicine in aquatic diseases prevention.

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.

Overview of methods and considerations for the photodynamic inactivation of microorganisms for agricultural applications

Antimicrobial resistance in agriculture is a global concern and carries huge financial consequences. Despite that, practical solutions for growers that are sustainable, low cost and environmentally friendly have been sparse. This has created opportunities for the agrochemical industry to develop pesticides with novel modes of action. Recently the use of photodynamic inactivation (PDI), classically used in cancer treatments, has been explored in agriculture as an alternative to traditional chemistries, mainly as a promising new approach for the eradication of pesticide resistant strains. However, applications in the field pose unique challenges and call for new methods of evaluation to adequately address issues specific to PDI applications in plants and challenges faced in the field. The aim of this review is to summarize in vitro, ex vivo, and in vivo/in planta experimental strategies and methods used to test and evaluate photodynamic agents as photo-responsive pesticides for applications in agriculture. The review highlights some of the strategies that have been explored to overcome challenges in the field.

The role of the light source in antimicrobial photodynamic therapy

Antimicrobial photodynamic therapy (APDT) is a promising approach to fight the growing problem of antimicrobial resistance that threatens health care, food security and agriculture. APDT uses light to excite a light-activated chemical (photosensitiser), leading to the generation of reactive oxygen species (ROS). Many APDT studies confirm its efficacy in vitro and in vivo against bacteria, fungi, viruses and parasites. However, the development of the field is focused on exploring potential targets and developing new photosensitisers. The role of light, a crucial element for ROS production, has been neglected. What are the main parameters essential for effective photosensitiser activation? Does an optimal light radiant exposure exist? And finally, which light source is best? Many reports have described the promising antibacterial effects of APDT in vitro, however, its application in vivo, especially in clinical settings remains very limited. The restricted availability may partially be due to a lack of standard conditions or protocols, arising from the diversity of selected photosensitising agents (PS), variable testing conditions including light sources used for PS activation and methods of measuring anti-bacterial activity and their effectiveness in treating bacterial infections. We thus sought to systematically review and examine the evidence from existing studies on APDT associated with the light source used. We show how the reduction of pathogens depends on the light source applied, radiant exposure and irradiance of light used, and type of pathogen, and so critically appraise the current state of development of APDT and areas to be addressed in future studies. We anticipate that further standardisation of the experimental conditions will help the field advance, and suggest key optical and biological parameters that should be reported in all APDT studies. More in vivo and clinical studies are needed and are expected to be facilitated by advances in light sources, leading to APDT becoming a sustainable, alternative therapeutic option for bacterial and other microbial infections in the future.

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.

In vitro and in vivo photodynamic efficacies of novel and conventional phenothiazinium photosensitizers against multidrug-resistant Candida auris

The fast-emerging and multidrug-resistant Candida auris is the first fungal pathogen to be considered a threat to global public health. Thus, there is a high unmet medical need to develop new therapeutic strategies to control this species. Antimicrobial photodynamic therapy (APDT) is a promising alternative that simultaneously targets and damages numerous microbial biomolecules. Here, we investigated the in vitro and in vivo effects of APDT with four phenothiazinium photosensitizers: (i) methylene blue (MB), (ii) toluidine blue (TBO), and two MB derivatives, (iii) new methylene blue (NMBN) and (iv) the pentacyclic derivative S137, against C. auris. To measure the in vitro efficacy of each PS, minimal inhibitory concentrations (MICs) and survival fraction were determined. Also, the efficiency of APDT was evaluated in vivo with the Galleria mellonella insect model for infection and treatment. Although the C. auris strain used in our study was shown to be resistant to the most-commonly used clinical antifungals, it could not withstand the damages imposed by APDT with any of the four photosensitizers. However, for the in vivo model, only APDT performed with S137 allowed survival of infected G. mellonella larvae. Our results show that structural and chemical properties of the photosensitizers play a major role on the outcomes of in vivo APDT and underscore the need to synthesize and develop novel photosensitizing molecules against multidrug-resistant microorganisms.

Thiopyridinium phthalocyanine for improved photodynamic efficiency against pathogenic fungi

The emergence of opportunistic pathogens and the selection of resistant strains have created a grim scenario for conventional antimicrobials. Consequently, there is an ongoing search for alternative techniques to control these microorganisms. One such technique is antimicrobial photodynamic therapy (aPDT), which combines photosensitizers, light, and molecular oxygen to produce reactive oxygen species and kill the target pathogen. Here, the in vitro susceptibilities of three fungal pathogens, namely Candida albicans, Aspergillus nidulans, and Colletotrichum abscissum to aPDT with zinc(II) phthalocyanine (ZnPc) derivative complexes were investigated. Three ZnPc bearing thiopyridinium substituents were synthesized and characterized by several spectroscopic techniques. The Q-band showed sensitivity to the substituent with high absorptivity coefficient in the 680-720 nm region. Derivatization and position of the rings with thiopyridinium units led to high antifungal efficiency of the cationic phthalocyanines, which could be correlated with singlet oxygen quantum yield, subcellular localization, and cellular uptake. The minimum inhibitory concentrations (MIC) of the investigated ZnPc-R complexes against the studied microorganisms were 2.5 μM (C. albicans) and 5 μM (A. nidulans and C. abscissum). One ZnPc derivative achieved complete photokilling of C. albicans and, furthermore, yielded low MIC values when used against the tolerant plant-pathogen C. abscissum. Our results show that chemical modification is an important step in producing better photosensitizers for aPDT against fungal pathogens.

Exploitation of the Antibacterial Properties of Photoactivated Curcumin as ‘Green’ Tool for Food Preservation

In the search for non-chemical and green methods to counteract the bacterial contamination of foods, the use of natural substances with antimicrobial properties and light irradiation at proper light waves has been extensively investigated. In particular, the combination of both techniques, called photodynamic inactivation (PDI), is based on the fact that some natural substances act as photosensitizers, i.e., produce bioactive effects under irradiation. Notably, curcumin is a potent natural antibacterial and effective photosensitizer that is able to induce photodynamic activation in the visible light range (specifically for blue light). Some practical applications have been investigated with particular reference to food preservation from bacterial contaminants.

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.

Phenothiazinium dyes for photodynamic treatment present lower environmental risk compared to a formulation of trifloxystrobin and tebuconazole

The widespread use of conventional chemical antifungal agents has led to worldwide concern regarding the selection of resistant isolates. In this scenario, antimicrobial photodynamic treatment (APDT) has emerged as a promising alternative to overcome this issue. The technique is based on the use of a photosensitizer (PS) and light in the presence of molecular oxygen. Under these conditions, the PS generates reactive oxygen species which damage the biomolecules of the target organism leading to cell death. The great potential of APDT against plant-pathogenic fungi has already been reported both in vitro and in planta, indicating this control measure has the potential to be widely used in crop plants. However, there is a lack of studies on environmental risk with ecotoxicological assessment of PSs used in APDT. Therefore, this study aimed to evaluate the environmental toxicity of four phenothiazinium PSs: i) methylene blue (MB), ii) new methylene blue N (NMBN), iii) toluidine blue O (TBO), and iv) dimethylmethylene blue (DMMB) and also of the commercial antifungal NATIVO®, a mixture of trifloxystrobin and tebuconazole. The experiments were performed with Daphnia similis neonates and zebrafish embryos. Our results showed that the PSs tested had different levels of toxicity, with MB being the less toxic and DMMB being the most. Nonetheless, the environmental toxicity of these PSs were lower when compared to that of NATIVO®. Furthermore, estimates of bioconcentration and of biotransformation half-life indicated that the PSs are environmentally safer than NATIVO®. Taken together, our results show that the toxicity associated with phenothiazinium PSs would not constitute an impediment to their use in APDT. Therefore, APDT is a promising approach to control plant-pathogenic fungi with reduced risk for selecting resistant isolates and lower environmental impacts when compared to commonly used antifungal agents.

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.

Preparation of a Porphyrin Metal-Organic Framework with Desirable Photodynamic Antimicrobial Activity for Sustainable Plant Disease Management

Considering the severity of plant pathogen resistance toward commonly used agricultural microbicides, as well as the potential threats of agrichemicals to the eco-environment, there is a pressing need for antimicrobial approaches that are capable of inactivating pathogens efficiently without the risk of inducing resistances and harm. In this work, a porphyrin metal-organic framework (MOF) nanocomposite was constructed by incorporating 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP) as a photosensitizer (PS) in the cage of a variant MOF (HKUST-1) to efficiently produce singlet oxygen (¹O₂) to inactivate plant pathogens under light irradiation. The results showed that the prepared PS@MOF had a loading rate of PS about 12% (w/w) and excellent and broad-spectrum photodynamic antimicrobial activity in vitro against three plant pathogenic fungi and two pathogenic bacteria. Moreover, PS@MOF showed outstanding control efficacy against Sclerotinia sclerotiorum on cucumber in the pot experiment. Allium cepa chromosome aberration assays and safety evaluation on cucumber and Chinese cabbage indicated that PS@MOF had no genotoxicity and was safe to plants. Thus, porphyrin MOF demonstrated a great potential as an alternative and efficient new microbicide for sustainable plant disease management.

Novel Chitosan-Riboflavin Conjugate with Visible Light-Enhanced Antifungal Properties against Penicillium digitatum

A novel chemical conjugate between chitosan (CH) and riboflavin (RF) has been synthesized and characterized via Fourier transform infrared, NMR, and other spectroscopic methods. Photophysical and photochemical properties such as absorption spectra, fluorescence emission, fluorescence anisotropy, and singlet oxygen generation were characterized as well. This new biopolymer-based conjugate was designed to have an antifungal effect enhanced through antimicrobial photodynamic therapy. The antifungal effect of this conjugate (CH-RF) was compared with CH and RF against Penicillium digitatum in vitro. The conjugate showed the highest fungal growth inhibition of all systems tested at a dose of 0.5% w/v. This new biopolymer-based compound could be a promising alternative to fungicides used in citrus fruits postharvest.

Antimicrobial photodynamic treatment as an alternative approach for Alicyclobacillus acidoterrestris inactivation

Alicyclobacillus acidoterrestris is a cause of major concern for the orange juice industry due to its thermal and chemical resistance, as well as its spoilage potential. A. acidoterrestris spoilage of orange juice is due to off-flavor taints from guaiacol production and some halophenols. The present study aimed to evaluate the effectiveness of antimicrobial Photodynamic Treatment (aPDT) as an emerging technology to inactivate the spores of A. acidoterrestris. The aPDT efficiency towards A. acidoterrestris was evaluated using as photosensitizers the tetracationic porphyrin (Tetra-Py⁺-Me) and the phenothiazinium dye new methylene blue (NMB) in combination with white light-emitting diode (LED; 400-740 nm; 65-140 mW/cm²). The spores of A. acidoterrestris were cultured on YSG agar plates (pH 3.7 ± 0.1) at 45 °C for 28 days and submitted to the aPDT with Tetra-Py⁺-Me and NMB at 10 μM in phosphate-buffered saline (PBS) in combination with white light (140 mW/cm²). The use of Tetra-Py⁺-Me at 10 μM resulted in a 7.3 ± 0.04 log reduction of the viability of A. acidoterrestris spores. No reductions in the viability of this bacterium were observed with NMB at 10 μM. Then, the aPDT with Tetra-Py⁺-Me and NMB at 10 μM in orange juice (UHT; pH 3.9; 11°Brix) alone and combined with potassium iodide (KI) was evaluated. The presence of KI was able to potentiate the aPDT process in orange juice, promoting the inactivation of 5 log CFU/mL of A. acidoterrestris spores after 10 h of white light exposition (140 mW/cm²). However, in the absence of KI, both photosensitizers did not promote a significant reduction in the spore viability. The inactivation of A. acidoterrestris spores artificially inoculated in orange peels (10⁵ spores/mL) was also assessed using Tetra-Py⁺-Me at 10 and 50 μM in the presence and absence of KI in combination with white light (65 mW/cm²). No significant reductions were observed (p < .05) when Tetra-Py⁺-Me was used at 10 μM, however at the highest concentration (50 μM) a significant spore reduction (≈ 2.8 log CFU/mL reductions) in orange peels was observed after 6 h of sunlight exposition (65 mW/cm²). Although the color, total phenolic content (TPC), and antioxidant capacity of orange juice and peel (only color evaluation) seem to have been affected by light exposition, the impact on the visual and nutritional characteristics of the products remains inconclusive so far. Besides that, the results found suggest that aPDT can be a potential method for the reduction of A. acidoterrestris spores on orange groves.

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

Botrytis cinerea is a necrotic plant fungus that causes gray mold disease in over 200 crops, including grapevine. Due to its genetic plasticity, this fungus presents strong resistance to many fungicides. Thus, new strategies against B. cinerea are urgently needed. In this context, antimicrobial photodynamic treatment (APDT) was considered. APDT involves the use of a photosensitizer that generates reactive oxygen species upon illumination with white light. Tetra-4-sulfonatophenyl porphyrin tetra-ammonium (TPPS) was tested on B. cinerea using light. 1.5 µM TPPS completely inhibited mycelial growth. TPPS (12.5 µM) was tested on three grapevine clones from Chardonnay, Merlot and Sauvignon, grown in vitro for 2 months. Treated root apparatus of the three backgrounds increased thiol production as a molecular protection against photoactivated TPPS, leading to a normal phenotype as compared with control plantlets. Finally, 2-month-old grapevine leaves were infected with 4-day-old mycelium of B. cinerea pre-incubated or not with TPPS. The pre-treated mycelium was unable to infect the detached leaves of any of the three grapevine varieties after 72 h growth when subjected to a 16 h photoperiod, contrary to untreated mycelium. These results suggest a strong potential of photo-treatment against B. cinerea mycelium for future agricultural practices in vineyard or other cultures.

Photodynamic inactivation of the phytopathogenic bacterium Xanthomonas citri subsp. citri

The present work intended to evaluate the applicability of photodynamic inactivation (PDI) of Xanthomonas citri subsp. citri with toluidine blue O (TBO), a commercial photosensitizer, as a strategy to control citrus canker. Assays were conducted with cell suspensions and biofilms, constructed either on polypropylene microtubes (in vitro assays) or on the surface of orange leaves (ex vivo assays), in the presence of TBO and under irradiation with artificial white light or natural sunlight. PDI assays using TBO alone caused a maximum 5·8 log10 reduction of X. citri viable cells in suspensions, and a much smaller inactivation (1·5 log10) in biofilms. However, concomitant use of KI potentiated the TBO photosensitization. Biofilms were inactivated down to the detection limit (>6 log10 reduction) with 5·0 µmol l^-1 TBO + 10 mmol l^-1 KI (in vitro) or 5·0 µmol l^-1 TBO + 100 mmol l^-1 KI (ex vivo) after artificial white light irradiation. Under natural sunlight, a reduction down to the detection limit of the Miles-Misra method was achieved with 50 µmol l^-1 TBO and 100 mmol l^-1 KI. PDI has potential to be applied in the control of citrus canker in field conditions although further studies are needed to show that there are no risks to plant physiology or fruit quality. SIGNIFICANCE AND IMPACT OF THE STUDY: Xanthomonas citri subsp. citri is a major cause of disease in citrus orchards. Because of the low efficacy and high environmental toxicity of copper-based treatments, there is growing interest on more sustainable phytosanitary approaches. Photodynamic inactivation (PDI) is being successfully used to control infectious agents and literature reports indicate that it is effective against some fungi and bacteria attacking fruit crops. The results of the present work open the perspective of using a low-cost photosensitizer and sunlight, as energy source, to control of the causative agent of citrus canker.

Antimicrobial Photodynamic Therapy in the Control of Pseudomonas syringae pv. actinidiae Transmission by Kiwifruit Pollen

Pseudomonas syringae pv. actinidiae (Psa) is a phytopathogen responsible for bacterial canker in kiwifruit plants and can be disseminated through pollen. This study aimed to evaluate the effectiveness of antimicrobial photodynamic therapy (aPDT) in the inactivation of Psa on kiwifruit pollen using New Methylene Blue (NMB) and Methylene Blue (MB) in the presence/absence of potassium iodide (KI). Pollen germination assays were also performed to evaluate if it was affected by aPDT. Higher reduction of Psa was achieved using NMB (5.0 μM) combined with KI (100 mM) in vitro (ca. 8 log CFU mL⁻¹ after 90 min of irradiation), while NMB alone promoted a lower reduction (3.7 log CFU mL⁻¹). The most efficient NMB concentration with KI was used to study the photodynamic efficiency of MB (5.0 μM). MB with KI photo-inactivated Psa more efficiently than NMB, causing the same bacterial reduction (ca. 8 log CFU mL⁻¹) in half the irradiation time (45 min). Therefore, MB was selected for the subsequent ex vivo aPDT assays in pollen. Almost all the Psa cells added artificially to the pollen (3.2 log CFU mL⁻¹) were photo-inactivated (3.1 log CFU mL⁻¹), whereas aPDT had a low effect on pollen natural microorganisms. When KI was added, a significant increase in aPDT effectiveness was observed (4.5 log CFU mL⁻¹). No negative effects were observed in the pollen germination after aPDT. The results show aPDT is an effective and safe method to Psa inactivation on kiwifruit pollen, and MB use is a promising alternative in the control of Psa transmission.

Chemical features of the photosensitizers new methylene blue N and S137 influence their subcellular localization and photoinactivation efficiency in Candida albicans

Antimicrobial photodynamic treatment (APDT) has emerged as an effective therapy against pathogenic fungi with both acquired and intrinsic resistance to commonly used antifungal agents. Success of APDT depends on the availability of effective photosensitizers capable of acting on different fungal structures and species. Among the phenothiazinium dyes tested as photoantifungals, new methylene blue N (NMBN) and the novel pentacyclic compound S137 are the most efficient. In the present study we compared the effects of APDT with NMBN and S137 on the survival of Candida albicans and employed a set of fluorescent probes (propidium iodide, FUN-1, JC-1, DHR-123 and DHE) together with confocal microscopy and flow cytometry to evaluate the effects of these two chemically diverse photosensitizers on cell membrane permeability, metabolism and redox status, and mitochondrial activity. Taken together, our results indicate that, due to chemical features resulting in different lipophilicity, NMBN and S137 localize to distinct subcellular structures and hence inactivate C. albicans cells via different mechanisms. S137 localizes mostly to the cell membrane and, upon light exposure, photo-oxidizes membrane lipids. NMBN readily localizes to mitochondria and exerts its photodynamic effects there, which was observed to be a less effective way to achieve cell death at lower light fluences.

The Third International Symposium on Fungal Stress - ISFUS

Stress is a normal part of life for fungi, which can survive in environments considered inhospitable or hostile for other organisms. Due to the ability of fungi to respond to, survive in, and transform the environment, even under severe stresses, many researchers are exploring the mechanisms that enable fungi to adapt to stress. The International Symposium on Fungal Stress (ISFUS) brings together leading scientists from around the world who research fungal stress. This article discusses presentations given at the third ISFUS, held in São José dos Campos, São Paulo, Brazil in 2019, thereby summarizing the state-of-the-art knowledge on fungal stress, a field that includes microbiology, agriculture, ecology, biotechnology, medicine, and astrobiology.

How protoporphyrinogen IX oxidase inhibitors and transgenesis contribute to elucidate plant tetrapyrrole pathway

Several families of herbicides, especially diphenyl ether (DPE) and pyrimidinedione, target the plant tetrapyrrole biosynthesis pathways and in particular one key enzyme, protoporphyrinogen IX oxidase (PPO). When plants are treated with DPE or pyrimidinedione, an accumulation of protoporphyrin IX, the first photosensitizer of this pathway, is observed in cytosol where it becomes very deleterious under light. Indeed these herbicides trigger plant death in two distinct ways: (i) inhibition of chlorophylls and heme syntheses and (ii) a huge accumulation of protoporphyrin IX in cytosol. Recently, a strategy based on plant transgenesis that induces deregulation of the tetrapyrrole pathway by up- or down-regulation of genes encoding enzymes, such as glutamyl-[Formula: see text]RNA reductase, porphobilinogen deaminase and PPO, has been developed. Against all expectations, only transgenic crops overexpressing PPO showed resistance to DPE and pyrimidinedione. This herbicide resistance of transgenic crops leads to the hypothesis that the overall consumption of herbicides will be reduced as previously reported for glyphosate-resistant transgenic crops. In this review, after a rapid presentation of plant tetrapyrrole biosynthesis, we show how only PPO enzyme can be the target of DPE and how transgenic crops can be further resistant not only to herbicide but also to abiotic stress such as drought or chilling. Keeping in mind that this approach is mostly prohibited in Europe, we attempt to discuss it to interest the scientific community, from plant physiologists to chemists, who work on the interface of photosensitizer optimization and agriculture.

Plant Photodynamic Stress: What's New?

In the 1970's, an unconventional stressful photodynamic treatment applied to plants was investigated in two directions. Exogenous photosensitizer treatment underlies direct photodynamic stress while treatment mediating endogenous photosensitizer over-accumulation pinpoints indirect photodynamic stress. For indirect photodynamic treatment, tetrapyrrole biosynthesis pathway was deregulated by 5-aminolevulenic acid or diphenyl ether. Overall, photodynamic stress involves the generation of high amount of reactive oxygen species leading to plant cell death. All these investigations were mainly performed to gain insight into new herbicide development but they were rapidly given up or limited due to the harmfulness of diphenyl ether and the high cost of 5-aminolevulinic acid treatment. Twenty years ago, plant photodynamic stress came back by way of crop transgenesis where for example protoporphyrin oxidases from human or bacteria were overexpressed. Such plants grew without dramatic effects of photodamage suggesting that plants tolerated induced photodynamic stress. In this review, we shed light on the occurrence of plant photodynamic stress and discuss challenging issues in the context of agriculture focusing on direct photodynamic modality. Indeed, we highlighted applications of exogenous PS especially porphyrins on plants, to further develop an emerged antimicrobial photodynamic treatment that could be a new strategy to kill plant pathogens without disturbing plant growth.

Responses of an adventitious fast-growing plant to photodynamic stress: comparative study of anionic and cationic porphyrin effect on Arabidopsis thaliana

Antimicrobial photodynamic treatment (APDT) based on the use of a photosensitizer to produce reactive oxygen species (ROS) that induce cell death could be envisaged to fight against plant pathogens. For setting this strategy, we want to study how plants themselves respond to photodynamic treatment. In previous work we showed that tomato plantlets were able to resist photoactivated tetra (N-methylpyridyl) porphyrin (CP) or the zinc metalated form (CP-Zn). To enlarge our plant expertise related to exogenous porphyrins treatment and to further defend this approach, we studied how a weed like Arabidopsis thaliana responded to exogenous supply of anionic and cationic porphyrins. Both types of photosensitizers had no negative effect on seed germination and did not hamper the development etiolated Arabidopsis plantlet under dark conditions. Thus, post-emergence effects of porphyrin photoactivation on the development of 14 day-old in vitro Arabidopsis plantlet under light were observed. CP-Zn was the most efficient photosensitizer to kill Arabidopsis plantlets while anionic tetra (4-sulfonatophenyl) porphyrin only delayed their growth and development. Indeed only 7% of plantlets could be rescued after CP-Zn treatment. Furthermore, non-enzymatic and enzymatic defense components involved in detoxification of ROS generated by CP-Zn under illumination were downregulated or stable with the exception of sevenfold increase in proline content. As previously demonstrated in the literature for microbial agents and in the present work for Arabidopsis, CP-Zn was efficient enough to eradicate unwanted vegetation and plant pathogens without at the same time killing plants of agronomic interest such as tomato plantlets.

The second International Symposium on Fungal Stress: ISFUS

The topic of 'fungal stress' is central to many important disciplines, including medical mycology, chronobiology, plant and insect pathology, industrial microbiology, material sciences, and astrobiology. The International Symposium on Fungal Stress (ISFUS) brought together researchers, who study fungal stress in a variety of fields. The second ISFUS was held in May 8-11 2017 in Goiania, Goiás, Brazil and hosted by the Instituto de Patologia Tropical e Saúde Pública at the Universidade Federal de Goiás. It was supported by grants from CAPES and FAPEG. Twenty-seven speakers from 15 countries presented their research related to fungal stress biology. The Symposium was divided into seven topics: 1. Fungal biology in extreme environments; 2. Stress mechanisms and responses in fungi: molecular biology, biochemistry, biophysics, and cellular biology; 3. Fungal photobiology in the context of stress; 4. Role of stress in fungal pathogenesis; 5. Fungal stress and bioremediation; 6. Fungal stress in agriculture and forestry; and 7. Fungal stress in industrial applications. This article provides an overview of the science presented and discussed at ISFUS-2017.