Abstract
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.
Highlights
Pseudomonas syringae pv. actinidiae (Psa) is a phytopathogen and the causal agent of bacterial canker of both green-fleshed kiwifruit (Actinidia deliciosa) and yellow-fleshed kiwifruit (Actinidia chinensis), resulting in massive damages in orchards [1] and causing severe economic losses worldwide [2,3]
The results show that New Methylene Blue (NMB) at 5.0 μM with the presence of KI at 100 mM (Figure 7c) was the best tested condition to inactivate Psa, allowing the total inactivation of the bacteria (ca. 8 log CFU mL−1 (ANOVA, p < 0.05)) after 90 min of white light irradiation at an irradiance of 50 mW cm−2
Kiwifruit pollen has been recognized as an important source and vector of Psa dissemination, elimination of Psa contamination on kiwifruit pollen is of utmost importance for the kiwifruit production sector
Summary
Pseudomonas syringae pv. actinidiae (Psa) is a phytopathogen and the causal agent of bacterial canker of both green-fleshed kiwifruit (Actinidia deliciosa) and yellow-fleshed kiwifruit (Actinidia chinensis), resulting in massive damages in orchards [1] and causing severe economic losses worldwide [2,3]. Afterwards, until 2008, Psa was found in China, Italy, and South Korea [2,4,5]. More recently, Psa has re-emerged, causing destruction of the main areas of kiwifruit production worldwide, being considered as a pandemic disease [2,3,4]. There are six genetically different biovars of Psa spread all over the world [2], varying in their aggressiveness to the plant. Biovar 3 is highly pathogenic and is affecting kiwifruit orchards in several countries worldwide (New Zealand, Italy, France, Spain, Portugal, and China) [2,6], being called “Psa-V” (from virulent) due to its high aggressiveness [6,7]
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