Integrated Use of Aureobasidium pullulans Strain CG163 and Acibenzolar-S-Methyl for Management of Bacterial Canker in Kiwifruit

Community structure of endophytic bacteria of Sargassum thubergii in the intertidal zone of Qingdao in China

Endophytic bacteria are one of the symbiotic microbial groups closely related to host algae. However, less research on the endophytic bacteria of marine algae. In this study, the endophytic bacterial community of Sargassum thunbergii was investigated using the culture method and high-throughput sequencing. Thirty-nine endophytic bacterial strains, belonging to two phyla, five genera and sixteen species, were isolated, and Firmicutes, Bacillus and Metabacillus indicus were the dominant taxa at the phylum, genus and species level, respectively. High-throughput sequencing revealed 39 phyla and 574 genera of endophytic bacteria, and the dominant phylum was Proteobacteria, while the dominant genus was Ralstonia. The results also indicated that the endophytic bacteria of S. thunbergii included various groups with nitrogen fixation, salt tolerance, pollutant degradation, and antibacterial properties but also contained some pathogenic bacteria. Additionally, the endophytic bacterial community shared a large number of groups with the epiphytic bacteria and bacteria in the surrounding seawater, but the three groups of samples could be clustered separately. In conclusion, there are a variety of functional endophytic bacteria living in S. thunbergii, and the internal condition of algae is a selective factor for the formation of endophytic bacterial communities. This study enriched the database of endophytic bacteria in marine macroalgae, paving the way for further understanding of the interrelationships between endophytic bacteria, macroalgae, and the environment.

Scientific and technological advances in the development of sustainable disease management tools: a case study on kiwifruit bacterial canker

Plant disease outbreaks are increasing in a world facing climate change and globalized markets, representing a serious threat to food security. Kiwifruit Bacterial Canker (KBC), caused by the bacterium Pseudomonas syringae pv. actinidiae (Psa), was selected as a case study for being an example of a pandemic disease that severely impacted crop production, leading to huge economic losses, and for the effort that has been made to control this disease. This review provides an in-depth and critical analysis on the scientific progress made for developing alternative tools for sustainable KBC management. Their status in terms of technological maturity is discussed and a set of opportunities and threats are also presented. The gradual replacement of susceptible kiwifruit cultivars, with more tolerant ones, significantly reduced KBC incidence and was a major milestone for Psa containment - which highlights the importance of plant breeding. Nonetheless, this is a very laborious process. Moreover, the potential threat of Psa evolving to more virulent biovars, or resistant lineages to existing control methods, strengthens the need of keep on exploring effective and more environmentally friendly tools for KBC management. Currently, plant elicitors and beneficial fungi and bacteria are already being used in the field with some degree of success. Precision agriculture technologies, for improving early disease detection and preventing pathogen dispersal, are also being developed and optimized. These include hyperspectral technologies and forecast models for Psa risk assessment, with the latter being slightly more advanced in terms of technological maturity. Additionally, plant protection products based on innovative formulations with molecules with antibacterial activity against Psa (e.g., essential oils, phages and antimicrobial peptides) have been validated primarily in laboratory trials and with few compounds already reaching field application. The lessons learned with this pandemic disease, and the acquired scientific and technological knowledge, can be of importance for sustainably managing other plant diseases and handling future pandemic outbreaks.

Kiwifruit Resistance to Sclerotinia sclerotiorum and Pseudomonas syringae pv. actinidiae and Defence Induction by Acibenzolar-S-methyl and Methyl Jasmonate Are Cultivar Dependent

Pathogen susceptibility and defence gene inducibility were compared between the Actinidia arguta cultivar 'Hortgem Tahi' and the two cultivars of A. chinensis 'Hayward' and 'Zesy002'. Plants were treated with acibenzolar-s-methyl (ASM) or methyl jasmonate (MeJA) one week before inoculation with Pseudomonas syringae pv. actinidiae (Psa biovar3) or Sclerotinia sclerotiorum, or secondary induction with chitosan+glucan (Ch-Glu) as a potential pathogen proxy. Defence expression was evaluated by measuring the expression of 18 putative defence genes. 'Hortgem Tahi' was highly susceptible to sclerotinia and very resistant to Psa, whereas 'Zesy002' was highly resistant to both, and 'Hayward' was moderately susceptible to both. Gene expression in 'Hayward' and 'Zesy002' was alike but differed significantly from 'Hortgem Tahi' which had higher basal levels of PR1-i, PR5-i, JIH1, NPR3 and WRKY70 but lower expression of RD22 and PR2-i. Treatment with ASM caused upregulation of NIMIN2, PR1-i, WRKY70, DMR6 and PR5-i in all cultivars and induced resistance to Psa in 'Zesy002' and 'Hayward' but decreased resistance to sclerotinia in 'Zesy002'. MeJA application caused upregulation of LOX2 and downregulation of NIMIN2, DMR6 and PR2-i but did not affect disease susceptibility. The Ch-Glu inducer induced PR-gene families in each cultivar, highlighting its possible effectiveness as an alternative to actual pathogen inoculation. The significance of variations in fundamental and inducible gene expression among the cultivars is explored.

Assessment of the Biocontrol Potential of Bacillus velezensis WL-23 against Kiwifruit Canker Caused by Pseudomonas syringae pv. actinidiae

Kiwifruit canker disease, caused by Pseudomonas syringae pv. actinidiae (Psa), is the main threat to kiwifruit production worldwide. Currently, there is no safe and effective disease prevention method; therefore, biological control technologies are being explored for Psa. In this study, Bacillus velezensis WL-23 was isolated from the leaf microbial community of kiwifruit and used to control kiwifruit cankers. Indoor confrontation experiments showed that both WL-23 and its aseptic filtrate had excellent inhibitory activity against the main fungal and bacterial pathogens of kiwifruit. Changes in OD₆₀₀, relative conductivity, alkaline proteinase, and nucleic acid content were recorded during Psa growth after treatment with the aseptic filtrate, showing that Psa proliferation was inhibited and the integrity of the cell membrane was destroyed; this was further verified using scanning electron microscopy and transmission electron microscopy. In vivo, WL-23 promoted plant growth, increased plant antioxidant enzyme activity, and reduced canker incidence. Therefore, WL-23 is expected to become a biological control agent due to its great potential to contribute to sustainable agriculture.

Potential of Exogenous Treatment with Dehydroascorbate to Control Root-knot Nematode Infection in Rice

Induced resistance (IR) is a unique physiological state characterized by reduced plant susceptibility to (a)biotic stress. Our previous studies showed that exogenous foliar application of dehydroascorbate (DHA), the oxidized form of ascorbic acid, induces systemic resistance against root-knot nematode Meloidogyne graminicola in rice. In the present study, the potential of DHA in protecting rice plants against M. graminicola was evaluated in lab, pot, and field studies. In an experiment where the interval between foliar treatment and inoculation was varied, 20 mM DHA was found to protect rice plants from M. graminicola for at least 14 days. Pot and field studies confirmed that 10 or 20 mM DHA are highly effective in reducing gall formation and led to a significant increase in rice seed yield. A half dose of DHA (10 mM) combined with another IR-stimulus - piperonylic acid (PA) 300 µM - was at par with DHA 20 mM, leading to reductions in gall formation of more than 80%. In in vitro bioassays, DHA was found to be highly nematicidal to the second-stage juveniles of M. graminicola, with more than 90% mortality within 3 h of exposure to 10 or 20 mM concentrations. While seed treatment had no effect, root drenching or root dipping was also effective in reducing rice susceptibility to M. graminicola, next to foliar treatment. As a dual-action compound with extended protection and ease of application, DHA has great potential for effective nematode management in rice.

Effects of Drought and Flooding on Phytohormones and Abscisic Acid Gene Expression in Kiwifruit

Environmental extremes, such as drought and flooding, are becoming more common with global warming, resulting in significant crop losses. Understanding the mechanisms underlying the plant water stress response, regulated by the abscisic acid (ABA) pathway, is crucial to building resilience to climate change. Potted kiwifruit plants (two cultivars) were exposed to contrasting watering regimes (water logging and no water). Root and leaf tissues were sampled during the experiments to measure phytohormone levels and expression of ABA pathway genes. ABA increased significantly under drought conditions compared with the control and waterlogged plants. ABA-related gene responses were significantly greater in roots than leaves. ABA responsive genes, DREB2 and WRKY40, showed the greatest upregulation in roots with flooding, and the ABA biosynthesis gene, NCED3, with drought. Two ABA-catabolic genes, CYP707A i and ii were able to differentiate the water stress responses, with upregulation in flooding and downregulation in drought. This study has identified molecular markers and shown that water stress extremes induced strong phytohormone/ABA gene responses in the roots, which are the key site of water stress perception, supporting the theory kiwifruit plants regulate ABA to combat water stress.

Postharvest Application of Acibenzolar-S-Methyl Activates Salicylic Acid Pathway Genes in Kiwifruit Vines

The plant defence inducer Actigard® (acibenzolar-S-methyl [ASM]) is applied before flowering and after fruit harvest to control bacterial canker in kiwifruit caused by Pseudomonas syringae pv. actinidiae. Pre-flowering application of ASM is known to upregulate defence gene expression; however, the effect of postharvest ASM on defence gene expression in the vine is unknown. In this study, the expression of eight "defence marker" genes was measured in the leaves of Actinidia chinensis var. chinensis, "Zesy002," and Actinidia chinensis var. deliciosa, "Hayward," vines after postharvest treatment with ASM and/or copper. There were two orchards per cultivar with harvest dates approximately three weeks apart for investigating potential changes in responsiveness to ASM during the harvest period. In all trials, postharvest ASM induced the expression of salicylic-acid-pathway defence genes PR1, PR2, PR5, BAD, DMR6, NIMIN2, and WRKY70. Gene upregulation was the greatest at 1 day and 7 days after treatment and declined to the control level after 3 weeks. In "Zesy002", the ASM-induced response was greater at the early harvest site than at the late harvest site. This decline was concomitant with leaf yellowing and a reduction in RNA yield. Effects of postharvest ASM on gene expression did not persist into the following spring, nor were vines conditioned to respond more strongly to pre-flowering ASM application.

A Phage Therapy Model for the Prevention of Pseudomonas syringae pv. actinidiae Infection of Kiwifruit Plants

Great efforts have been made with chemicals and pesticides to contain the spread of Pseudomonas syringae pv. actinidiae (Psa) responsible for kiwifruit canker. Unfortunately, only partial results were obtained for this bacterial pandemic, and alternative remedies were proposed to avoid soil pollution and the onset of antibiotic resistance. Among these, phage therapy represents a possible tool with low environmental impact and high specificity. Several phages have been isolated and tested for the capacity to kill Psa in vitro, but experiments to verify their efficacy in vivo are still lacking. In the present study, we demonstrated that the phage φPSA2 (previously characterized) contains the spread of Psa inside plant tissue and reduces the symptoms of the disease. Our data are a strong indication for the efficiency of this phage and open the possibility of developing a phage therapy based on φPSA2 to counteract the bacterial canker of kiwifruit.

Resistance Inducers for the Protection of Pedunculate Oak (Quercus robur L.) Seedlings against Powdery Mildew Erysiphe alphitoides

Oak powdery mildew caused by Erysiphe alphitoides (Griffon and Maubl.; U. Braun & S. Takam.) is a common disease in European forests. One of the most susceptible species is the pedunculate oak (Quercus robur L.). Presently, a few methods are available to control powdery mildew, e.g., the use of fungicides (e.g., based on citric acid), antagonistic fungi or bacteria, chemical treatments (e.g., sulphur, potassium bicarbonate) or genetic resistance. In our study, we aimed to check the effects of using chitosan derivatives and novel active substances inducing the plants' natural resistance: benzodiathiadiazole (both in neutral and salt form). 84 pedunculate oak seedlings were subjected to the experiment in three treatment variants (plus positive and negative controls). The plants were treated with active substances and inoculated with E. alphitoides. Although the powdery mildew symptoms appeared in all variants, they were manifested mainly by the mycelium in the form of small spots. The experiment indicated that the highest limitation of powdery mildew mycelium was achieved by applying N-methyl-N-methoxyamide-7-carboxybenzo(1,2,3)thiadiazole (BTHWA). The application of BTHWA reduced disease development by 88.9% when compared to the effects of the other variants.

Application of the NanoString nCounter System as an Alternative Method to Investigate Molecular Mechanisms Involved in Host Plant Responses to Plasmodiophora brassicae

Clubroot, caused by the soilborne pathogen Plasmodiophora brassicae, is an important disease of canola (Brassica napus) and other crucifers. The recent application of RNA sequencing (RNA-seq) technologies to study P. brassicae−host interactions has generated large amounts of gene expression data, improving knowledge of the molecular mechanisms of pathogenesis and host resistance. Quantitative PCR (qPCR) analysis has been widely applied to examine the expression of a limited number of genes and to validate the results of RNA-seq studies, but may not be ideal for analyzing larger suites of target genes or increased sample numbers. Moreover, the need for intermediate steps such as cDNA synthesis may introduce variability that could affect the accuracy of the data generated by qPCR. Here, we report the validation of gene expression data from a previous RNA-seq study of clubroot using the NanoString nCounter System, which achieves efficient gene expression quantification in a fast and simple manner. We first confirm the robustness of the NanoString system by comparing the results with those generated by qPCR and RNA-seq and then discuss the importance of some candidate genes for resistance or susceptibility to P. brassicae in the host. The results show that the expression of genes measured using NanoString have a high correlation with the values obtained using the other two technologies, with R > 0.90 and p < 0.01, and the same expression patterns for most genes. The three methods (qPCR, RNA-seq, and NanoString) were also compared in terms of laboratory procedures, time, and cost. We propose that the NanoString nCounter System is a robust, sensitive, highly reproducible, and simple technology for gene expression analysis. NanoString could become a common alternative to qPCR to validate RNA-seq data or to create panels of genes for use as markers of resistance/susceptibility when plants are challenged with different P. brassicae pathotypes.

Advancements in the Use of Bacteriophages to Combat the Kiwifruit Canker Phytopathogen Pseudomonas syringae pv. actinidiae

Over the last several decades, kiwifruit production has been severely damaged by the bacterial plant pathogen Pseudomonas syringae pv. actinidiae (Psa), resulting in severe economic losses worldwide. Currently, copper bactericides and antibiotics are the main tools used to control this bacterial disease. However, their use is becoming increasingly ineffective due to the emergence of antibiotic resistance. In addition, environmental issues and the changes in the composition of soil bacterial communities are also concerning when using these substances. Although biocontrol methods have shown promising antibacterial effects on Psa infection under in vitro conditions, the efficiency of antagonistic bacteria and fungi when deployed under field conditions remains unclear. Therefore, it is crucial to develop a phage-based biocontrol strategy for this bacterial pathogen. Due to the specificity of the target bacteria and for the benefit of the environment, bacteriophages (phages) have been widely regarded as promising biological agents to control plant, animal, and human bacterial diseases. An increasing number of studies focus on the use of phages for the control of plant diseases, including the kiwifruit bacterial canker. In this review, we first introduce the characteristics of the Psa-induced kiwifruit canker, followed by a description of the diversity and virulence of Psa strains. The main focus of the review is the description of recent advances in the isolation of Psa phages and their characterization, including morphology, host range, lytic activity, genome characterization, and lysis mechanism, but we also describe the biocontrol strategies together with potential challenges introduced by abiotic factors, such as high temperature, extreme pH, and UV irradiation in kiwifruit orchards. The information presented in this review highlights the potential role of phages in controlling Psa infection to ensure plant protection.

An Insight into an Olive Scab on the "Istrska Belica" Variety: Host-Pathogen Interactions and Phyllosphere Mycobiome

The olive tree is one of the most important agricultural plants, affected by several pests and diseases that cause a severe decline in health status leading to crop losses. Olive leaf spot disease caused by the fungus Venturia oleaginea can result in complete tree defoliation and consequently lower yield. The aim of the study was to obtain new knowledge related to plant-pathogen interaction, reveal mechanisms of plant defense against the pathogen, and characterize fungal phyllosphere communities on infected and symptomless leaves that could contribute to the development of new plant breeding strategies and identification of novel biocontrol agents. The highly susceptible olive variety "Istrska Belica"' was selected for a detailed evaluation. Microscopy analyses led to the observation of raphides in the mesophyll and parenchyma cells of infected leaves and gave new insight into the complex V. oleaginea pathogenesis. Culturable and total phyllosphere mycobiota, obtained via metabarcoding approach, highlighted Didymella, Aureobasidium, Cladosporium, and Alternaria species as overlapping between infected and symptomless leaves. Only Venturia and Erythrobasidium in infected and Cladosporium in symptomless samples with higher abundance showed statistically significant differences. Based on the ecological role of identified taxa, it can be suggested that Cladosporium species might have potential antagonistic effects on V. oleaginea.

Defence-related pathways, phytohormones and primary metabolism are key players in kiwifruit plant tolerance to Pseudomonas syringae pv. actinidiae

The reasons underlying the differential tolerance of Actinidia spp. to the pandemic pathogen Pseudomonas syringae pv. actinidiae (Psa) have not yet been elucidated. We hypothesized that differential plant-defence strategies linked to transcriptome regulation, phytohormones and primary metabolism might be key and that Actinidia chinensis susceptibility results from an inefficient activation of defensive mechanisms and metabolic impairments shortly following infection. Here, 48 h postinoculation bacterial density was 10-fold higher in A. chinensis var. deliciosa than in Actinidia arguta, accompanied by significant increases in glutamine, ornithine, jasmonic acid (JA) and salicylic acid (SA) (up to 3.2-fold). Actinidia arguta showed decreased abscisic acid (ABA) (0.7-fold), no changes in primary metabolites, and 20 defence-related genes that were only differentially expressed in this species. These include GLOX1, FOX1, SN2 and RBOHA, which may contribute to its higher tolerance. Results suggest that A. chinensis' higher susceptibility to Psa is due to an inefficient activation of plant defences, with the involvement of ABA, JA and SA, leading to impairments in primary metabolism, particularly the ammonia assimilation cycle. A schematic overview on the interaction between Psa and genotypes with distinct tolerance is provided, highlighting the key transcriptomic and metabolomic aspects contributing to the different plant phenotypes after infection.

Defence Responses Associated with Elicitor-Induced, Cultivar-Associated Resistance to Latania Scale in Kiwifruit

Latania scale insect is a pest of global significance affecting kiwifruit. The sessile insect (life stage: settled crawler-mature adult) is covered with a waxy cap that protects it from topical pesticides, so increasingly, a selection of resistant cultivars and application of elicitors are being used in pest control. Thus far, the application of a salicylic acid (SA) phytohormone pathway elicitor, acibenzolar-S-methyl (ASM), has been shown to reduce insect development (as indicated by cap size) on one kiwifruit cultivar ('Hayward'). To investigate how cultivar-associated resistance is affected by the ability to respond to different elicitors, we measured phytohormones (by LCMS) and gene expression (by qPCR and NanoString) on latania scale-tolerant 'Hort16A' and susceptible 'Hayward' kiwifruit over two seasons. Potted plants in the presence/absence of settled latania scales were treated with ASM (0.2 g/L) or methyl jasmonate (MeJA, 0.05% v/v), representing elicitors of the SA and JA signalling pathways, respectively. 'Hort16A' cultivar resistance to latania scale was associated with elevated expression of SA and SA-related defence genes (PR1 and two PR2 family genes) in the ASM treatment. MeJA treatments did not significantly affect insect development in 'Hayward' (latania scale did not survive on 'Hort16A') and did not correlate with phytohormone and gene expression measurements in either cultivar. 'Hayward' had greater concentrations than 'Hort16A' of inert storage forms of both SA and JA across all treatments. This information contributes to the selection of tolerant cultivars and the effective use of elicitors for control of latania scale in kiwifruit.

Acibenzolar-S-Methyl and Resistance Quantitative Trait Loci Complement Each Other to Control Apple Scab and Fire Blight

Diversifying disease control methods is a key strategy to sustainably reduce pesticides. Plant genetic resistance has long been used to create resistant varieties. Plant resistance inducers (PRI) are also considered to promote crop health, but their effectiveness is partial and can vary according to the environment and the plant genotype. We investigated the putative interaction between intrinsic (genetic) and PRI-induced resistance in apple when affected by scab and fire blight diseases. A large F1 mapping population was challenged by each disease after a pre-treatment with acibenzolar-S-methyl (ASM) and compared with the water control. Apple scab and fire blight resistance quantitative trait loci (QTLs) were detected in both conditions and compared. ASM exhibited a strong effectiveness in reducing both diseases. When combined, QTL-controlled and ASM-induced resistance acted complementarily to reduce the symptoms from 85 to 100%, depending on the disease. In our conditions, resistance QTLs were only slightly or rarely affected by ASM treatment, despite their probable implication in various stages of the resistance buildup. Implications of these results are discussed considering already known results, the underlying mechanisms, cross protection of both types of resistance against pathogen adaptation, and practical application in orchard conditions.