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.

Transcriptomic and Proteomic Profiling Reveal the Key Role of AcMYB16 in the Response of Pseudomonas syringae pv. actinidiae in Kiwifruit

Kiwifruit bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa), is an important disease of kiwifruit (Actinidia Lind.). Plant hormones may induce various secondary metabolites to resist pathogens via modulation of hormone-responsive transcription factors (TFs), as reported in past studies. In this study, we showed that JA accumulated in the susceptible cultivar Actinidia chinensis 'Hongyang' but decreased in the resistant cultivar of A. chinensis var. deliciosa 'Jinkui' in response to Psa. Integrated transcriptomic and proteomic analyses were carried out using the resistant cultivar 'Jinkui'. A total of 5,045 differentially expressed genes (DEGs) and 1,681 differentially expressed proteins (DEPs) were identified after Psa infection. Two pathways, 'plant hormone signal transduction' and 'phenylpropanoid biosynthesis,' were activated at the protein and transcript levels. In addition, a total of 27 R2R3-MYB transcription factors (TFs) were involved in the response to Psa of 'Jinkui,' including the R2R3-MYB TF subgroup 4 gene AcMYB16, which was downregulated in 'Jinkui' but upregulated in 'Hongyang.' The promoter region of AcMYB16 has a MeJA responsiveness cis-acting regulatory element (CRE). Transient expression of the AcMYB16 gene in the leaves of 'Jinkui' induced Psa infection. Together, these data suggest that AcMYB16 acts as a repressor to regulate the response of kiwifruit to Psa infection. Our work will help to unravel the processes of kiwifruit resistance to pathogens and will facilitate the development of varieties with resistance against bacterial pathogens.

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.

Genetic Diversity of Pseudomonas syringae pv. actinidiae: Seasonal and Spatial Population Dynamics

Pseudomonassyringae pv. actinidiae (Psa) is a gram-negative bacterium responsible for the bacterial canker in Actinidia chinensis var. deliciosa and A. chinensis var. chinensis, a quarantine organism threatening the kiwifruit industry sustainability. The present study aimed to determine the genetic structure of the endophytic and epiphytic populations of Psa isolated from four different Portuguese orchards with distinct abiotic conditions in two consecutive seasons. The results identified several coexisting and highly heterogeneous Psa populations. Moreover, evident changes in population structure occurred between the epiphytic and endophytic populations, and between seasons with a notable decrease in Psa diversity in autumn. This work provided solid evidence that the initial clonal expansion of Psa in Europe was followed by a wide genomic diversification. This perspective is important for the understanding of kiwifruit bacterial canker disease occurrence and Psa evolution, namely when adopting strategies for management of epidemics.

NMR-based metabolic study of leaves of three species of Actinidia with different degrees of susceptibility to Pseudomonas syringae pv. actinidiae

Bacterial canker of Actinidia, caused by the bacterium Pseudomonas syringae pv. actinidiae (Psa), is the most serious disease of these plants worldwide. Leaves of three species of Actinidia, namely A. chinensis var. chinensis, A. chinensis var. deliciosa and A. arguta, having different degrees of tolerance to Psa, were analyzed by Nuclear Magnetic Resonance spectroscopy. Aqueous extracts of leaves were studied and several metabolites, classified as organic acids, amino acids, carbohydrates, phenols and other metabolites, were identified by 1D and 2D NMR experiments and quantified. The metabolic profiles of these species were compared through univariate statistical analysis ANOVA and multivariate PCA. Levels of metabolites with known antibacterial activity, such as caffeic and chlorogenic acids, were observed to be higher in the A. arguta samples. Moreover, these metabolites have different Pearson correlation patterns among the three Actinidia species, suggesting a difference at the phenylpropanoid biosynthetic pathway.

The Scientific, Economic, and Social Impacts of the New Zealand Outbreak of Bacterial Canker of Kiwifruit (Pseudomonas syringae pv. actinidiae)

The introduction of Pseudomonas syringae pv. actinidiae (Psa) severely damaged the New Zealand kiwifruit industry, which in 2010 was based on only two cultivars. Despite an extraordinarily quick and strong response by industry, government, and scientists to minimize the economic and social impacts, the economic consequences of this outbreak were severe. Although our understanding of Psa epidemiology and control methods increased substantively over the past six years, the kiwifruit industry largely recovered because of the introduction of a less-susceptible yellow-fleshed cultivar. The New Zealand population of Psa is clonal but has evolved rapidly since its introduction by exchanging mobile genetic elements, including integrative conjugative elements (ICEs), with the local bacterial populations. In some cases, this has led to copper resistance. It is currently believed that the center of origin of the pathogen is Japan or Korea, but biovar 3, which is responsible for the global outbreak, originated in China.

Programmed cell death in kiwifruit stigmatic arms and its relationship to the effective pollination period and the progamic phase

BACKGROUND AND AIMS

Kiwifruit is a crop with a highly successful reproductive performance, which is impaired by the short effective pollination period of female flowers. This study investigates whether the degenerative processes observed in both pollinated and non-pollinated flowers after anthesis may be considered to be programmed cell death (PCD).

METHODS

Features of PCD in kiwifruit, Actinidia chinensis var. deliciosa, were studied in both non-pollinated and pollinated stigmatic arms using transmission electron microscopy, DAPI (4',6-diamidino-2-phenylindole) staining, TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling) assays, DNA gel electrophoresis and caspase-like activity assays.

KEY RESULTS

In the secretory tissues of the stigmatic arms, cell organelles disintegrated sequentially while progressive vacuolization was detected. At the same time, chromatin condensation, nuclear deformation, and DNA fragmentation and degradation were observed. These features were detected in both non-pollinated and pollinated stigmatic arms; they were evident in the stigmas of pollinated flowers by the second day after anthesis but only by 4 d after anthesis in non-pollinated flowers. In addition, in pollinated stigmatic arms, these features were first initiated in the stigma and gradually progressed through the style, consistent with pollen tube growth. This timing of events was also observed in both non-pollinated and pollinated stigmatic arms for caspase-3-like activity.

CONCLUSIONS

The data provide evidence to support the hypothesis that PCD processes occurring in the secretory tissue of non-pollinated kiwifruit stigmatic arms could be the origin for the observed short effective pollination period. The results obtained in the secretory tissue of pollinated kiwifruit stigmatic arms upon pollination support the idea that PCD might be accelerated by pollination, pointing to the involvement of PCD during the progamic phase.