Comparative transcriptome analysis of the interaction between Actinidia chinensis var. chinensis and Pseudomonas syringae pv. actinidiae in absence and presence of acibenzolar-S-methyl

Functional characterization of AcWRKY94 in response to Pseudomonas syringae pv. actinidiae in kiwifruit

WRKY transcription factors are essential for coping with various biotic stresses. Pseudomonas syringae pv. actinidiae (Psa)-induced kiwifruit canker is a major problem restricting kiwifruit yield. Nevertheless, it's unclear how the kiwifruit WRKY genes respond to Psa. Through genome-wide identification, 112 WRKY members were found in 'Hongyang' genome in this work. Promoter analysis revealed that there were many cis-acting elements associated with stress responses in the AcWRKY gene's promoter region. According to transcriptomic analysis, 90 of the AcWRKY genes were differently expressed following Psa, salicylic acid (SA), or methyl jasmonate (MeJA) treatments. Almost all group III WRKYs were responsive to at least one of these treatments, with tissue-specific expression patterns. Quantitative RT-PCR study provided more evidence that Psa and SA treatments significantly induced the expression of the group III WRKY gene AcWRKY94, whereas MeJA treatment repressed it. AcWRKY94 was a transcriptionally active protein localized in the nucleus. Transient overexpression of AcWRKY94 in the leaves of 'Hongyang' enhanced the resistance of kiwifruit to Psa. Overexpression of AcWRKY94 in kiwifruit callus remarkably promoted the expression of PR and JAZ genes associated with SA and JA signals, respectively. These data imply that AcWRKY94 controls the signaling pathway dependent on SA and JA, thereby enhancing resistance to Psa. Taken together, this study establishes the basis for functional research on WRKY genes and provides important information for elucidating the resistance mechanism of kiwifruit canker disease.

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.

Satureja montana and Mentha pulegium essential oils' antimicrobial properties against Pseudomonas syringae pv. actinidiae and elicitor potential through the modulation of kiwifruit hormonal defenses

Pseudomonas syringae pv. actinidiae (Psa) is responsible for the kiwifruit bacterial canker, the most severe disease of Actinidia spp. The use in agriculture of antibiotics and cooper-based compounds is increasingly being restricted, demanding for new sustainable alternatives to current agrochemicals. We aimed to characterize the anti-Psa potential of essential oils (EOs) of Mentha pulegium and Satureja montana and investigate if they elicit the plant-host hormonal defenses. The EOs were characterized through gas-chromatography with flame ionization detector (GC-FID) and mass spectrometry (MS). Pulegone (78.6%) and carvacrol (43.5%) were the major constituents of M. pulegium and S. montana EO, respectively. Only S. montana EO showed relevant anti-Psa activity in vitro. To evaluate if the EOs also elicited host defenses, in vitro shoots were treated with 2 mg shoot-1 of EO-solution and subsequently inoculated with Psa three days later. Shoots were analyzed 10 min, three days (and 10 min after Psa-inoculation), four and ten days after EO application. The up/down regulation of RNA-transcripts for hormone biosynthesis, Psa biofilm production and virulence genes were quantified by real-time quantitative PCR (RT-qPCR). Phytohormones were quantified by High-Performance Liquid Chromatography (HPLC). S. montana EO showed the most promising results as a defense elicitor, increasing 6-benzylaminopurine (BAP) by 131.07% and reducing indole-3-acetic acid (IAA) levels by 49.19%. Decreases of salicylic acid (SA), and gibberellic acid 3 (GA3) levels by 32.55% and 33.09% respectively and an increase of abscisic acid (ABA) by 85.03%, in M. pulegium EO-treated shoots, revealed some protective post-infection effect. This is the most comprehensive research on the Psa's impact on phytohormones. It also unveils the protective influence of prior EO exposure, clarifying the plant hormonal response to subsequent infections. The results reinforce the hypothesis that carvacrol-rich S. montana EO can be a suitable disease control agent against Psa infection. Its dual action against pathogens and elicitation of host plant defenses make it a promising candidate for incorporation into environmentally friendly disease management approaches. Nonetheless, to fully leverage these promising results, further research is imperative to elucidate the EO mode of action and evaluate the long-term efficacy of this approach.

Metabolome and Transcriptome Analysis of Sulfur-Induced Kiwifruit Stem Laccase Gene Involved in Syringyl Lignin Synthesis against Bacterial Canker

Kiwifruit canker is caused by Pseudomonas syringae pv. actinidiae and is one of the most destructive diseases of kiwifruit worldwide. Sulfur can improve the deposit of lignin in kiwifruit stems and induce disease resistance, but the action mechanism at the molecular level remains unclear. This omics-based study revealed that sulfur-induced S lignin synthesis contributes to disease resistance. Histological staining verified sulfur-enhanced total lignin deposition in kiwifruit stems. High-performance liquid chromatography and confocal Raman microscopy showed that sulfur-activated S lignin was mainly deposited in the cell corner. Metabolome and transcriptome analysis revealed that the levels of phenylpropanoid pathway S lignin precursors sinapic acid and sinapyl alcohol were significantly increased and 16 laccase genes were upregulated. Sulfur-induced resistance defense promoted elevated laccase activity by activating the laccase genes, participating in sinapic acid and sinapyl alcohol substance synthesis, and ultimately polymerizing S lignin at cell corner against kiwifruit canker disease.

Structural characterization, derivatization and antibacterial activity of secondary metabolites produced by termite-associated Streptomyces showdoensis BYF17

BACKGROUND

Insect-associated Streptomyces is a valuable resource for development of compounds with antibacterial potential. However, relatively little is known of the secondary metabolites produced by termite-associated Streptomyces.

RESULTS

Here, seven compounds including o-acetaminophenol (1), phenazine-1,6-dicarboxylic acid (2), phenylacetic acid (3), phenazinolin D (4), izumiphenazine A (5), izumiphenazine B (6) and phenazinolin E (7) were obtained from the fermentation broth of a termite-associated Streptomyces showdoensis BYF17, which was isolated from the body surfaces of Odontotermes formosanus. Two additional novel derivative compounds (6a and 6b) were synthesized via acetylation and methylation, respectively. The structures of these compounds were elucidated by spectroscopic analyses. The antibacterial bioassay showed that compound 6a displayed strong inhibitory effects against Pseudomonas syringae pv. actinidiae (Psa), with a zone of inhibition (ZOI) diameter of 20.6 mm, which was comparable to that of positive gentamicin sulfate with a ZOI value of 25.6 mm. Furthermore, the Day 5 curative activities of both compounds 6 and 6a against kiwifruit bacterial canker were 71.5%, which was higher than those of referred oxine-copper (55.0%) and ethylicin (46.8%) at a concentration of 200 μg mL^-1 . In addition, the mechanism analysis based on scanning electron microscopic observation revealed that both compounds 6 and 6a destroyed the integrity of the Psa cell membrane.

CONCLUSION

The results of biological tests showed that these bioactive compounds exhibit potent antimicrobial activities, which have the potential to be developed into new antibacterial agents. © 2023 Society of Chemical Industry.

The Role of Italy in the Use of Advanced Plant Genomic Techniques on Fruit Trees: State of the Art and Future Perspectives

Climate change is deeply impacting the food chain production, lowering quality and yield. In this context, the international scientific community has dedicated many efforts to enhancing resilience and sustainability in agriculture. Italy is among the main European producers of several fruit trees; therefore, national research centers and universities undertook several initiatives to maintain the specificity of the 'Made in Italy' label. Despite their importance, fruit crops are suffering from difficulties associated with the conventional breeding approaches, especially in terms of financial commitment, land resources availability, and long generation times. The 'new genomic techniques' (NGTs), renamed in Italy as 'technologies for assisted evolution' (TEAs), reduce the time required to obtain genetically improved cultivars while precisely targeting specific DNA sequences. This review aims to illustrate the role of the Italian scientific community in the use of NGTs, with a specific focus on Citrus, grapevine, apple, pear, chestnut, strawberry, peach, and kiwifruit. For each crop, the key genes and traits on which the scientific community is working, as well as the technological improvements and advancements on the regeneration of local varieties, are presented. Lastly, a focus is placed on the legal aspects in the European and in Italian contexts.

Red and Blue Light Differently Influence Actinidia chinensis Performance and Its Interaction with Pseudomonas syringae pv. Actinidiae

Light composition modulates plant growth and defenses, thus influencing plant-pathogen interactions. We investigated the effects of different light-emitting diode (LED) red (R) (665 nm) and blue (B) (470 nm) light combinations on Actinidia chinensis performance by evaluating biometric parameters, chlorophyll a fluorescence, gas exchange and photosynthesis-related gene expression. Moreover, the influence of light on the infection by Pseudomonas syringae pv. actinidiae (Psa), the etiological agent of bacterial canker of kiwifruit, was investigated. Our study shows that 50%R-50%B (50R) and 25%R-75%B (25R) lead to the highest PSII efficiency and photosynthetic rate, but are the least effective in controlling the endophytic colonization of the host by Psa. Monochromatic red light severely reduced ΦPSII, ETR, Pn, TSS and photosynthesis-related genes expression, and both monochromatic lights lead to a reduction of DW and pigments content. Monochromatic blue light was the only treatment significantly reducing disease symptoms but did not reduce bacterial endophytic population. Our results suggest that monochromatic blue light reduces infection primarily by modulating Psa virulence more than host plant defenses.

Genomic Variation and Host Interaction among Pseudomonas syringae pv. actinidiae Strains in Actinidia chinensis ‘Hongyang’

Kiwifruit bacterial canker is a recent epidemic disease caused by Pseudomonas syringae pv. actinidiae (Psa), which has undergone worldwide expansion in a short time and resulted in significant economic losses. ‘Hongyang’ (Actinidia chinensis), a widely grown cultivar because of its health-beneficial nutrients and appreciated red-centered inner pericarp, is highly sensitive to Psa. In this work, ten Psa strains were isolated from ‘Hongyang’ and sequenced for genome analysis. The results indicated divergences in pathogenicity and pathogenic-related genes among the Psa strains. Significantly, the interruption at the 596 bp of HrpR in two low-pathogenicity strains reemphasized this gene, expressing a transcriptional regulator for the effector secretion system, as an important pathogenicity-associated locus of Psa. The transcriptome analysis of ‘Hongyang’ infected with different Psa strains was performed by RNA-seq of stem tissues locally (at the inoculation site) and systemically. Psa infection re-programmed the host genes expression, and the susceptibility to Psa might be attributed to the down-regulation of several genes involved in plant-pathogen interactions, especially calcium signaling transduction, as well as fatty acid elongation. This suppression was found in both low- and high-pathogenicity Psa inoculated tissues, but the effect was stronger with more virulent strains. Taken together, the divergences of P. syringae pv. actinidiae in pathogenicity, genome, and resulting transcriptomic response of A. chinensis provide insights into unraveling the molecular mechanism of Psa-kiwifruit interactions and resistance improvement in the kiwifruit crop.

Genome-wide identification and characterization of the TIFY gene family in kiwifruit

Background

The TIFY gene family is a group of plant-specific transcription factors involved in regulation of plant growth and development and a variety of stress responses. However, the TIFY family has not yet been well characterized in kiwifruit, a popular fruit with important nutritional and economic value.

Results

A total of 27 and 21 TIFY genes were identified in the genomes of Actinidia eriantha and A. chinensis, respectively. Phylogenetic analyses showed that kiwifruit TIFY genes could be classified into four major groups, JAZ, ZML, TIFY and PPD, and the JAZ group could be further clustered into six subgroups (JAZ I to JAZ VI). Members within the same group or subgroup have similar exon-intron structures and conserved motif compositions. The kiwifruit TIFY genes are unevenly distributed on the chromosomes, and the segmental duplication events played a vital role in the expansion of the TIFY genes in kiwifruit. Syntenic analyses of TIFY genes between kiwifruit and other five plant species (including Arabidopsis thaliana, Camellia sinensis, Oryza sativa, Solanum lycopersicum and Vitis vinifera) and between the two kiwifruit species provided valuable clues for understanding the potential evolution of the kiwifruit TIFY family. Molecular evolutionary analysis showed that the evolution of kiwifruit TIFY genes was primarily constrained by intense purifying selection. Promoter cis-element analysis showed that most kiwifruit TIFY genes possess multiple cis-elements related to stress-response, phytohormone signal transduction and plant growth and development. The expression pattern analyses indicated that TIFY genes might play a role in different kiwifruit tissues, including fruit at specific development stages. In addition, several TIFY genes with high expression levels during Psa (Pseudomonas syringae pv. actinidiae) infection were identified, suggesting a role in the process of Pas infection.

Conclusions

In this study, the kiwifruit TIFY genes were identified from two assembled kiwifruit genomes. In addition, their basic physiochemical properties, chromosomal localization, phylogeny, gene structures and conserved motifs, synteny analyses, promoter cis-elements and expression patters were systematically examined. The results laid a foundation for further understanding the function of TIFY genes in kiwifruit, and provided a new potential approach for the prevention and treatment of Psa infection.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08398-8.

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.

In silico identification, characterization expression profile of WUSCHEL-Related Homeobox (WOX) gene family in two species of kiwifruit

The WUSCHEL (WUS)-related homeobox (WOX) gene family is a class of plant-specific transcriptional factors and plays a crucial role in forming the shoot apical meristem and embryonic development, stem cell maintenance, and various other developmental processes. However, systematic identification and characterization of the kiwifruit WOX gene family have not been studied. This study identified 17 and 10 WOX genes in A. chinensis (Ac) and A. eriantha (Ae) genomes, respectively. Phylogenetic analysis classified kiwifruit WOX genes from two species into three clades. Analysis of phylogenetics, synteny patterns, and selection pressure inferred that WOX gene families in Ac and Ae had undergone different evolutionary patterns after whole-genome duplication (WGD) events, causing differences in WOX gene number and distribution. Ten conserved motifs were identified in the kiwifruit WOX genes, and motif architectures of WOXs belonging to different clades highly diverged. The cis-element analysis and expression profiles investigation indicated the functional differentiation of WOX genes and identified the potential WOXs in response to stresses. Our results provided insight into general characters, evolutionary patterns, and functional diversity of kiwifruit WOXs.

Role of methyl jasmonate and salicylic acid in kiwifruit plants further subjected to Psa infection: biochemical and genetic responses

The use of plant elicitors for controlling Pseudomonas syringae pv. actinidiae (Psa), the etiological agent of the kiwifruit bacterial canker (KBC), has been analysed in the past and, while salicylic acid (SA) seems to decrease disease susceptibility, methyl jasmonate (MJ) shows an opposite effect. However, the metabolic and genomic responses of Psa-infected plants following elicitation with these two compounds, as compared with non-elicited Psa-inoculated plants, are poorly understood, being the focus of this study. Micropropagated A. chinensis 'Hayward' plants were elicited with MJ or SA, and further inoculated with Psa. Fifteen days post-inoculation, Psa population in MJ-treated plants was increased by 7.4-fold, whereas SA elicitation led to decreased Psa colonization (0.5-fold), as compared with non-elicited inoculated plants. Additionally, elicitation with MJ or SA generally decreased polyphenols and lignin concentrations (by at least 20%) and increased total proteins (by at least 50%). MJ led to the upregulation of SOD, involved in plant antioxidant system, and reporter genes for the jasmonic acid (JA) (JIH and LOX1), abscisic acid (SnRK), SA (ICS1), and ethylene (ACAS1, ETR1 and SAM) pathways. Moreover, it increased ABA (40%) and decreased carotenoids (30%) concentrations. Contrastingly, comparing with non-elicited Psa-inoculated plants, SA application resulted in the downregulation of antioxidant system-related genes (SOD and APX) and of reporter genes for ethylene (ETR1) and JA (JIH and ETR1). This study contributes to the understanding of potential mechanisms involved in kiwifruit plant defences against Psa, highlighting the role of the JA, ABA and ethylene in plant susceptibility to the pathogen.

A Breach in Plant Defences: Pseudomonas syringae pv. actinidiae Targets Ethylene Signalling to Overcome Actinidia chinensis Pathogen Responses

Ethylene interacts with other plant hormones to modulate many aspects of plant metabolism, including defence and stomata regulation. Therefore, its manipulation may allow plant pathogens to overcome the host’s immune responses. This work investigates the role of ethylene as a virulence factor for Pseudomonas syringae pv. actinidiae (Psa), the aetiological agent of the bacterial canker of kiwifruit. The pandemic, highly virulent biovar of this pathogen produces ethylene, whereas the biovars isolated in Japan and Korea do not. Ethylene production is modulated in planta by light/dark cycle. Exogenous ethylene application stimulates bacterial virulence, and restricts or increases host colonisation if performed before or after inoculation, respectively. The deletion of a gene, unrelated to known bacterial biosynthetic pathways and putatively encoding for an oxidoreductase, abolishes ethylene production and reduces the pathogen growth rate in planta. Ethylene production by Psa may be a recently and independently evolved virulence trait in the arms race against the host. Plant- and pathogen-derived ethylene may concur in the activation/suppression of immune responses, in the chemotaxis toward a suitable entry point, or in the endophytic colonisation.

Role of the Type VI Secretion System in the Pathogenicity of Pseudomonas syringae pv. actinidiae, the Causative Agent of Kiwifruit Bacterial Canker

The type VI secretion system (T6SS), a macromolecular machine, plays an important role in the pathogenicity of many Gram-negative bacteria. However, the role of T6SS in the pathogenicity of Pseudomonas syringae pv. actinidiae (Psa), the pathogen of kiwifruit bacterial canker, is yet to be studied. Here, we found a T6SS gene cluster consisting of 13 core genes (A-J) in the genome of Psa M228 based on a genome-wide analysis. To determine whether the T6SS gene cluster affects the pathogenicity of Psa M228, T6SS and its 13 core gene deletion mutants were constructed and their pathogenicity was determined. The deletion mutants showed different degrees of reduction in pathogenicity compared with the wild-type strain M228; in tssM and tssJ mutants, pathogenicity was significantly reduced by 78.7 and 71.3%, respectively. The pathogenicity results were also confirmed by electron microscopy. To further confirm that the reduction in pathogenicity is related to the function of T6SS, we selected the T6SS gene cluster, comprising tssM and tssJ, for further analyses. Western blot results revealed that tssM and tssJ were necessary for hemolytic co-regulatory protein secretion, indicating that they encode a functional T6SS. Further, we explored the mechanism by which T6SS affects the pathogenicity of Psa M228. The ability of bacterial competition, biofilm formation, hydrogen peroxide tolerance, and proteolytic activity were all weakened in the deletion mutants M228ΔT6SS, M228ΔtssM, and M228ΔtssJ. All these properties of the two gene complementation mutants were restored to the same levels as those of the wild-type strain, M228. Quantitative real-time results showed that during the interaction between the deletion mutant M228ΔT6SS and the host, expression levels of T3SS transcriptional regulatory gene hrpR, structural genes hrpZ, hrcC, hopP1, and effector genes hopH1 and hopM1 were down-regulated at different levels. Taken together, our data provide evidence for the first time that the T6SS plays an important role in the pathogenicity of Psa, probably via effects on bacterial competition, biofilm formation, and environmental adaptability. Moreover, a complicated relationship exists between T6SS and T3SS.

Early Pathogen Recognition and Antioxidant System Activation Contributes to Actinidia arguta Tolerance Against Pseudomonas syringae Pathovars actinidiae and actinidifoliorum

Actinidia chinensis and A. arguta have distinct tolerances to Pseudomonas syringae pv. actinidiae (Psa), but the reasons underlying the inter-specific variation remain unclear. This study aimed to integrate the metabolic and molecular responses of these two kiwifruit species against the highly pathogenic Psa and the less pathogenic P. syringae pv. actinidifoliorum (Pfm) bacterial strains. Disease development was monitored weekly till 21 days post inoculation (dpi), analysing a broad number and variety of parameters including: colony forming units (CFU), foliar symptoms, total chlorophylls, lipid peroxidation, soluble polyphenols, lignin and defense-related gene expression. At the end of the experimental period A. chinensis inoculated with Psa presented the highest endophytic bacterial population, whereas A. arguta inoculated with Pfm showed the lowest values, also resulting in a lower extent of leaf symptoms. Metabolic responses to infection were also more pronounced in A. chinensis with decreased total chlorophylls (up to 55%) and increased lipid peroxidation (up to 53%), compared with non-inoculated plants. Moreover, at 14 dpi soluble polyphenols and lignin concentrations were significantly higher (112 and 26%, respectively) in Psa-inoculated plants than in controls, while in A. arguta no significant changes were observed in those metabolic responses, except for lignin concentration which was, in general, significantly higher in Psa-inoculated plants (by at least 22%), comparing with control and Pfm-inoculated plants. Genes encoding antioxidant enzymes (SOD, APX and CAT) were upregulated at an earlier stage in Psa-inoculated A. arguta than in A. chinensis. In contrast, genes related with phenylpropanoids (LOX1) and ethylene (SAM) pathways were downregulated in A. arguta, but upregulated in A. chinensis in the later phases of infection. Expression of Pto3, responsible for pathogen recognition, occurred 2 dpi in A. arguta, but only 14 dpi in A. chinensis. In conclusion, we found that A. arguta is more tolerant to Psa and Pfm infection than A. chinensis and its primary and secondary metabolism is less impacted. A. arguta higher tolerance seems to be related with early pathogen recognition, the activation of plant antioxidant system, and to the suppression of ET and JA pathways from an earlier moment after infection.

Pseudomonas syringae pv. actinidiae: Ecology, Infection Dynamics and Disease Epidemiology

Since 2008, the kiwifruit industry has been devastated by a pandemic outbreak of Pseudomonas syringae pv. actinidiae (Psa), the causal agent of bacterial canker. This disease has become the most significant limiting factor in kiwifruit production. Psa colonizes different organs of the host plant, causing a specific symptomatology on each of them. In addition, the systemic invasion of the plant may quickly lead to plant death. Despite the massive risk that this disease poses to the kiwifruit industry, studies focusing on Psa ecology have been sporadic, and a comprehensive description of the disease epidemiology is still missing. Optimal environmental conditions for infection, dispersal and survival in the environment, or the mechanisms of penetration and colonization of host tissues have not been fully elucidated yet. The present work aims to provide a synthesis of the current knowledge, and a deeper understanding of the epidemiology of kiwifruit bacterial canker based on new experimental data. The pathogen may survive in the environment or overwinter in dormant tissues and be dispersed by wind or rain. Psa was observed in association with several plant structures (stomata, trichomes, lenticels) and wounds, which could represent entry points for apoplast infection. Environmental conditions also affect the bacterial colonization, with lower optimum values of temperature and humidity for epiphytic than for endophytic growth, and disease incidence requiring a combination of mild temperature and leaf wetness. By providing information on Psa ecology, these data sets may contribute to plan efficient control strategies for kiwifruit bacterial canker.

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

An isolate of Aureobasidium pullulans (strain = CG163) and the plant defence elicitor acibenzolar-S-methyl (ASM) were investigated for their ability to control leaf spot in kiwifruit caused by Pseudomonassyringae pv. actinidiae biovar 3 (Psa). Clonal Actinidia chinensis var. deliciosa plantlets (‘Hayward’) were treated with ASM, CG163 or ASM + CG163 at seven and one day before inoculation with Psa. ASM (0.2 g/L) was applied either as a root or foliar treatments and CG163 was applied as a foliar spray containing 2 × 10⁷ CFU/mL. Leaf spot incidence was significantly reduced by all treatments compared with the control. The combination of ASM + CG163 had greater efficacy (75%) than either ASM (55%) or CG163 (40%) alone. Moreover, treatment efficacy correlated positively with the expression of defence-related genes: pathogenesis-related protein 1 (PR1), β-1,3-glucosidase, Glucan endo 1,3-β-glucosidase (Gluc_PrimerH) and Class IV chitinase (ClassIV_Chit), with greater gene upregulation in plants treated with ASM + CG163 than by the individual treatments. Pathogen population studies indicated that CG163 had significant suppressive activity against epiphytic populations of Psa. Endophytic populations were reduced by ASM + CG163 but not by the individual treatments, and by 96–144 h after inoculation were significantly lower than the control. Together these data suggest that ASM + CG163 have complementary modes of action that contribute to greater control of leaf spotting than either treatment alone.

Plasma activated water as resistance inducer against bacterial leaf spot of tomato

Plant bacterial diseases are routinely managed with scheduled treatments based on heavy metal compounds or on antibiotics; to reduce the negative environmental impact due to the use of such chemical compounds, as pollution or selection of antibiotic resistant pathogens, the integrated control management is required. In the frame of a sustainable agriculture the use of bacterial antagonists, biological agents, plant defence response elicitors or resistant host plant genotypes are the most effective approaches. In this work, cold atmospheric pressure plasma (CAP) was applied to sterile distilled water, inducing the production of a hydrogen peroxide, nitrite and nitrate, and a pH reduction. In particular, an atmospheric pressure dielectric barrier discharge (DBD) has been used to produce plasma activated water (PAW), that was firstly assayed in in vitro experiments and then in planta through application at the root apparatus of tomato plants, against Xanthomonas vesicatoria (Xv), the etiological agent of bacterial leaf spot. Moreover, the transcription abundance of five genes related to the plant defense was investigated in response to PAW treatment. PAW did not show direct antimicrobial activity against Xv in in vitro experiments, but it enhanced the tomato plants defenses. It was effective in reducing the disease severity by giving relative protections of ca. 61, 51 and 38% when applied 1 h, 24 h and 6 days before the experimental inoculation, respectively. In addition, the experiments highlighted the pal gene involvement in response to the PAW treatments and against the pathogen; its transcription levels resulted significantly high from 1 to 48 h until their decrease 192 h after PAW application.

Multiple quantitative trait loci contribute to resistance to bacterial canker incited by Pseudomonas syringae pv. actinidiae in kiwifruit (Actinidia chinensis)

Pseudomonas syringae pv. actinidiae (Psa) biovar 3, a virulent, canker-inducing pathogen is an economic threat to the kiwifruit (Actinidia spp.) industry worldwide. The commercially grown diploid (2×) A. chinensis var. chinensis is more susceptible to Psa than tetraploid and hexaploid kiwifruit. However information on the genetic loci modulating Psa resistance in kiwifruit is not available. Here we report mapping of quantitative trait loci (QTLs) regulating resistance to Psa in a diploid kiwifruit population, derived from a cross between an elite Psa-susceptible 'Hort16A' and a resistant male breeding parent P1. Using high-density genetic maps and intensive phenotyping, we identified a single QTL for Psa resistance on Linkage Group (LG) 27 of 'Hort16A' revealing 16-19% phenotypic variance and candidate alleles for susceptibility and resistance at this loci. In addition, six minor QTLs were identified in P1 on distinct LGs, exerting 4-9% variance. Resistance in the F1 population is improved by additive effects from 'Hort16A' and P1 QTLs providing evidence that divergent genetic pathways interact to combat the virulent Psa strain. Two different bioassays further identified new QTLs for tissue-specific responses to Psa. The genetic marker at LG27 QTL was further verified for association with Psa resistance in diploid Actinidia chinensis populations. Transcriptome analysis of Psa-resistant and susceptible genotypes in field revealed hallmarks of basal defense and provided candidate RNA-biomarkers for screening for Psa resistance in greenhouse conditions.