Synergistic enhancement of tolerance mechanisms in response to photoactivation of cationic tetra (N-methylpyridyl) porphyrins in tomato plantlets

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.

Photophysical characterization and in vitro anti-leishmanial effect of 5,10,15,20-tetrakis(4-fluorophenyl) porphyrin and the metal (Zn(II), Sn(IV), Mn(III) and V(IV)) derivatives

In this report 5 compounds were synthesized and structural and their photophysical characterization was performed (Φ_Δ and Φ_f). Furthermore, in this in vitro study, their biological activity against Leishmania panamensis was evaluated. The photophysical behavior of these compounds was measured and high Φ_Δ and low Φ_f was observed. Besides, DFT quantum calculations on the electronic structures were performed. Finally, the biological activity was determined by means of the compounds capacity to inhibit the viability of parasites using the MTT assay. The inclusion of the metal ions substantially modified the photophysical and biological properties in comparison with the free metal porphyrin (1). In fact, Zn²⁺ porphyrin derivative (2) showed a marked decrease of Φ_f and increase of Φ_Δ. In this sense, using TDDFT approaches, a luminescent process for Sn⁴⁺ derivative (3) was described, where emissive states involve the ML-LCT transition. So, this led to a decrease in the singlet oxygen production (0.82-0.67). Biological results showed that all compounds inhibit the viability of L. panamensis with high efficiency; the decrease in the viability was greater as the concentration of exposure increased. Finally, under light irradiation the IC₅₀ of L. panamensis against the Zn(II)-porphyrin (2) and V(IV)-porphyrin (5) was lower than the IC₅₀ of the Glucantime control (IC₅₀ = 2.2 and 6.95 μM Vs IC₅₀ = 12.7 μM, respectively). We showed that the use of porphyrin and metalloporphyrin-type photosensitizers with exceptional photophysical properties can be successful in photodynamic therapy (PDT) against L. panamensis, being the diamagnetic ion Zn²⁺ a candidate for the preparation of metalloporphyrins with high singlet oxygen production.

Photophysical study and in vitro approach against Leishmania panamensis of dicloro-5,10,15,20-tetrakis(4-bromophenyl)porphyrinato Sn(IV)

Background: Photodynamic therapy activity against different biological systems has been reported for porphyrins. Porphyrin modifications through peripheral groups and/or by metal insertion inside the ring are main alternatives for the improvement of its photo-physical properties. In this study, we synthesized and characterized 5,10,15,20-tetrakis(4-bromophenyl)porphyrin and the dicloro-5,10,15,20-tetrakis(4-bromophenyl)porphyrinato Sn(IV).

Methods: Metal-free porphyrin was synthesized using the Alder method, while the Sn(IV)-porphyrin complex was prepared by combining metal-free porphyrin with stannous chloride in DMF; the reaction yields were 47% and 64% respectively. Metal-free porphyrin was characterized by UV-Vis, FT-IR, ESI-mass spectrometry and ¹³C-NMR. Additionally, the Sn(IV) -porphyrin complex was characterized using UV-Vis and FT-IR. Cyclic voltammetry tests in four different solvents. The fluorescence quantum yield (Φ _f) was measured using fluorescein as a standard, the singlet oxygen quantum yield (Φ _D) was estimated using the standard 5,10,15,20-(tetraphenyl)porphyrin (H2TPP) and the quencher of singlet oxygen 1,3-diphenylisobenzofuran (DPBF).

Results: UV-Vis assay showed typical Q and Soret bands for porphyrin and its metallo-porphyrin complex. Compounds showed photoluminescence at the visible range of electromagnetic spectrum. The inclusion of the metal in the porphyrin core changed the Φ _f from 0.15 to 0.05 and the Φ _D increased from 0.55 to 0.59. Finally, the effect of the compounds on the viability of L. panamensis was evaluated by means of the MTT test. The results showed that both compounds decreased the viability of the parasite; this inhibitory activity was greater under light irradiation; the porphyrin compound had IC ₅₀ of 16.5 μM and the Sn(IV)-porphyrin complex had IC ₅₀ of 19.2 μM.

Conclusion: The compounds were synthesized efficiently, their characterization was carried out by different spectroscopy techniques and their own signals were evidenced for both structures, both compounds decreased the cell viability of L. panamensis.

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.

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.

In Vitro Anti-Leishmanial Effect of Metallic Meso-Substituted Porphyrin Derivatives against Leishmania braziliensis and Leishmania panamensis Promastigotes Properties

In this study, a family of porphyrins based on 5,10,15,20-Tetrakis(4-ethylphenyl)porphyrin (1, Ph) and six metallo-derivatives (Zn²⁺(2, Ph-Zn), Sn⁴⁺(3, Ph-Sn), Mn²⁺ (4, Ph-Mn), Ni²⁺ (5, Ph-Ni), Al³⁺ (6, Ph-Al), and V³⁺ (7, Ph-V)) were tested as photosensitizers for photodynamic therapy against Leishmania braziliensis and panamensis. The singlet oxygen quantum yield value (Φ_Δ) for (1–7) was measured using 1,3-diphenylisobenzofuran (DPBF) as a singlet oxygen trapping agent and 5,10,15,20-(tetraphenyl)-porphyrin (H2TPP) as a reference standard; besides, parasite viability was estimated by the MTT assay. After metal insertion into the porphyrin core, the Φ_Δ increased from 0.76–0.90 and cell viability changed considerably. The Φ_Δ and metal type changed the cytotoxic activity. Finally, (2) showed both the highest Φ_Δ (0.90) and the best photodynamic activity against the parasites studied (IC₅₀ of 1.2 μM).

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.

Crossing the First Threshold: New Insights into the Influence of the Chemical Structure of Anionic Porphyrins on Plant Cell Wall Interactions and Photodynamic Cell Death Induction

In this study, our fundamental research interest was to understand how negatively charged porphyrins could interact with a plant cell wall and further act inside cells. Thus, three anionic porphyrins differing in their anionic external groups (carboxylates, sulfonates, and phosphonates) were tested. First, the tobacco cell wall was isolated to monitor in vitro its interactions with the three different anionic porphyrins. Unexpectedly, these negatively charged molecules were able to bind to the negatively charged cell wall probably by weak bonds such as hydrogen bonds and/or electrostatic interactions when the tetrapyrrolic core was protonated. Moreover, we showed that at the pH of spent culture medium (4.5), the neutrality of the carboxylated porphyrin (TPPC) facilitated its cell wall crossing while the diffusion of the two other sulfonated (TPPS) or phosphonated (TPPP) porphyrins that remained anionic was delayed. Once inside Tobacco Bright Yellow-2 (TBY-2) cells, TPPC induced higher levels of production of both H2O2 and malondialdehyde compared to TPPS after illumination. That result correlated well with strong cell death induction by photoactivated TPPC. Furthermore, reactive oxygen species-scavenging enzymes such as catalase, peroxidases, and superoxide dismutase were also strongly downmodulated in response to TPPC, while these enzymes were almost unchanged in response to photoactivated TPPS. To the best of our knowledge, this is the first study that took into account the whole story from interactions of porphyrins with a plant cell wall to their photodynamic activity inside the cells.

Photodynamic inactivation of Bovine herpesvirus type 1 (BoHV-1) by porphyrins

In this work, the photodynamic efficiency of anionic meso-tetrakis sulfonophenyl (TPPS4), cationic meso-tetrakis methylpyridiniumyl (TMPyP) and their zinc complexes (ZnTPPS4 and ZnTMPyP) in the inactivation of Bovine herpesvirus type 1 (BoHV-1) was evaluated. At a non-cytotoxic concentration, all porphyrins showed significant antiviral activity after irradiation using a halogen lamp. The efficiency of the cationic porphyrins was higher than that of the anionic ones. Porphyrin complexation with zinc increases its lipophilicity and the number of absorbed photons, dramatically reducing the time for complete virus inactivation. The high superposition of the compound optical absorption and light source emission spectra played a key role in the virus inactivation efficiency. The results demonstrated the high effectivity of the photodynamic inactivation of BoHV-1. This method can be used as an auxiliary in the treatment of disorders attributed to BoHV-1 infection, and the porphyrins are promising photosensitizers for this application.

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.

Inactivation of plant-pathogenic fungus Colletotrichum acutatum with natural plant-produced photosensitizers under solar radiation

The increasing tolerance to currently used fungicides and the need for environmentally friendly antimicrobial approaches have stimulated the development of novel strategies to control plant-pathogenic fungi such as antimicrobial phototreatment (APT). We investigated the in vitro APT of the plant-pathogenic fungus Colletotrichum acutatum with furocoumarins and coumarins and solar radiation. The compounds used were: furocoumarins 8-methoxypsoralen (8-MOP) and 5,8-dimethoxypsoralen (isopimpinellin), coumarins 2H-chromen-2-one (coumarin), 7-hydroxycoumarin, 5,7-dimethoxycoumarin (citropten) and a mixture (3:1) of 7-methoxycoumarin and 5,7-dimethoxycoumarin. APT of conidia with crude extracts from 'Tahiti' acid lime, red and white grapefruit were also performed. Pure compounds were tested at 50μM concentration and mixtures and extracts at 12.5mgL(-1). The C. acutatum conidia suspension with or without the compounds was exposed to solar radiation for 1h. In addition, the effects of APT on the leaves of the plant host Citrus sinensis were determined. APT with 8-MOP was the most effective treatment, killing 100% of the conidia followed by the mixture of two coumarins and isopimpinellin that killed 99% and 64% of the conidia, respectively. APT with the extracts killed from 20% to 70% of the conidia, and the extract from 'Tahiti' lime was the most effective. No damage to sweet orange leaves was observed after APT with any of the compounds or extracts.