Contrasting effector profiles between bacterial colonisers of kiwifruit reveal redundant roles converging on PTI-suppression and RIN4

Genomic Biosurveillance of the Kiwifruit Pathogen Pseudomonas syringae pv. actinidiae Biovar 3 Reveals Adaptation to Selective Pressures in New Zealand Orchards

In the late 2000s, a pandemic of Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) devastated kiwifruit orchards growing susceptible, yellow-fleshed cultivars. New Zealand's kiwifruit industry has since recovered, following the deployment of the tolerant cultivar 'Zesy002'. However, little is known about the extent to which the Psa population is evolving since its arrival. Over 500 Psa3 isolates from New Zealand kiwifruit orchards were sequenced between 2010 and 2022, from commercial monocultures and diverse germplasm collections. While effector loss was previously observed on Psa-resistant germplasm vines, effector loss appears to be rare in commercial orchards, where the dominant cultivars lack Psa resistance. However, a new Psa3 variant, which has lost the effector hopF1c, has arisen. The loss of hopF1c appears to have been mediated by the movement of integrative conjugative elements introducing copper resistance into this population. Following this variant's identification, in-planta pathogenicity and competitive fitness assays were performed to better understand the risk and likelihood of its spread. While hopF1c loss variants had similar in-planta growth to wild-type Psa3, a lab-generated ∆hopF1c strain could outcompete the wild type on select hosts. Further surveillance was conducted in commercial orchards where these variants were originally isolated, with 6.6% of surveyed isolates identified as hopF1c loss variants. These findings suggest that the spread of these variants is currently limited, and they are unlikely to cause more severe symptoms than the current population. Ongoing genome biosurveillance of New Zealand's Psa3 population is recommended to enable early detection and management of variants of interest.

Identification and Characterization of Innate Immunity in Actinidia melanandra in Response to Pseudomonas syringae pv. actinidiae

Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) has decimated kiwifruit orchards growing susceptible kiwifruit Actinidia chinensis varieties. Effector loss has occurred recently in Psa3 isolates from resistant kiwifruit germplasm, resulting in strains capable of partially overcoming resistance present in kiwiberry vines (Actinidia arguta, Actinidia polygama, and Actinidia melanandra). Diploid male A. melanandra recognises several effectors, sharing recognition of at least one avirulence effector (HopAW1a) with previously studied tetraploid kiwiberry vines. Sequencing and assembly of the A. melanandra genome enabled the characterisation of the transcriptomic response of this non-host to wild-type and genetic mutants of Psa3. A. melanandra appears to mount a classic effector-triggered immunity (ETI) response to wildtype Psa3 V-13, as expected. Surprisingly, the type III secretion (T3SS) system-lacking Psa3 V-13 ∆hrcC strain did not appear to trigger pattern-triggered immunity (PTI) despite lacking the ability to deliver immunity-suppressing effectors. Contrasting the A. melanandra responses to an effectorless Psa3 V-13 ∆33E strain and to Psa3 V-13 ∆hrcC suggested that PTI triggered by Psa3 V-13 was based on the recognition of the T3SS itself. The characterisation of both ETI and PTI branches of innate immunity responses within A. melanandra further enables breeding for durable resistance in future kiwifruit cultivars.

Pseudomonas cold shock proteins suppress bacterial effector translocation in Nicotiana benthamiana

Introduction

Plants detect the invasion of microbial pathogens through pathogen-associated molecular patterns (PAMPs). Cold shock proteins (CSPs) are a class of PAMPs specifically recognized by Solanales plants. While peptide inoculation studies have revealed the effects of CSPs, their in vivo roles remain poorly understood.

Methods

A model system involving the interactions between Pseudomonas fluorescens and P. syringae pv. tomato DC3000 with Nicotiana benthamiana has been widely used to investigate the molecular mechanism of plant-microbe interactions. Here, we employed this model system to explore the in vivo roles of CSPs in modulating plant immunity by multiple genetic approaches.

Results

Our findings revealed that three highly-conserved CSPs were identified in Pseudomonas strains. Transient expression of these CSPs neither induced reactive oxygen species (ROS) production nor suppressed the hypersensitive response (HR) in N. benthamiana, however, it restricted bacterial effector translocation. Genetic analysis revealed that these CSPs did not contribute to the ROS burst or HR inhibition in vivo but were functionally redundant in suppressing effector translocation in a flagellin (FliC)-independent manner. Furthermore, we demonstrated that the suppression of effector translocation mediated by CSPs was less pronounced compared to that triggered by FliC. Additionally, inoculation with csp15 and csp22 epitopes triggered the pattern-triggered immunity-associated suppression of effector translocations.

Discussion

This study revealed the redundant roles of CSPs in suppressing bacterial effector translocation in vivo, providing deep insights into the PTI elicited by cytoplasmic bacterial proteins.

NbPTR1 confers resistance against Pseudomonas syringae pv. actinidiae in kiwifruit

Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) causes a devastating canker disease in yellow-fleshed kiwifruit (Actinidia chinensis). The effector HopZ5, which is present in all isolates of Psa3 causing global outbreaks of pandemic kiwifruit canker disease, triggers immunity in Nicotiana benthamiana and is not recognised in susceptible A. chinensis cultivars. In a search for N. benthamiana nonhost resistance genes against HopZ5, we found that the nucleotide-binding leucine-rich repeat receptor NbPTR1 recognised HopZ5. RPM1-interacting protein 4 orthologues from N. benthamiana and A. chinensis formed a complex with NbPTR1 and HopZ5 activity was able to disrupt this interaction. No functional orthologues of NbPTR1 were found in A. chinensis. NbPTR1 transformed into Psa3-susceptible A. chinensis var. chinensis 'Hort16A' plants introduced HopZ5-specific resistance against Psa3. Altogether, this study suggested that expressing NbPTR1 in Psa3-susceptible kiwifruit is a viable approach to acquiring resistance to Psa3 and it provides valuable information for engineering resistance in otherwise susceptible kiwifruit genotypes.

Pseudomonas syringae pv. actinidiae Unique Effector HopZ5 Interacts with GF14C to Trigger Plant Immunity

The bacterial canker of kiwifruit caused by Pseudomonas syringae pv. actinidiae (Psa) is the most devastating disease threatening the global kiwifruit production. This pathogen delivers multiple effector proteins into plant cells to resist plant immune responses and facilitate their survival. Here, we focused on the unique effector HopZ5 in Psa, which previously has been reported to have virulence functions. In this study, our results showed that HopZ5 could cause macroscopic cell death and trigger a serious immune response by agroinfiltration in Nicotiana benthamiana, along with upregulated expression of immunity-related genes and significant accumulation of reactive oxygen species and callose. Subsequently, we confirmed that HopZ5 interacted with the phosphoserine-binding protein GF14C in both the nonhost plant N. benthamiana (NbGF14C) and the host plant kiwifruit (AcGF14C), and silencing of NbGF14C compromised HopZ5-mediated cell death, suggesting that GF14C plays a crucial role in the detection of HopZ5. Further studies showed that overexpression of NbGF14C both markedly reduced the infection of Sclerotinia sclerotiorum and Phytophthora capsica in N. benthamiana, and overexpression of AcGF14C significantly enhanced the resistance of kiwifruit against Psa, indicating that GF14C positively regulates plant immunity. Collectively, our results revealed that the virulence effector HopZ5 could be recognized by plants and interact with GF14C to activate plant immunity.

Evolving Archetypes: Learning from Pathogen Emergence on a Nonmodel Host

Research initiatives undertaken in response to disease outbreaks accelerate our understanding of microbial evolution, mechanisms of virulence and resistance, and plant-pathogen coevolutionary interactions. The emergence and global spread of Pseudomonas syringae pv. actinidiae (Psa) on kiwifruit (Actinidia chinensis) showed that there are parallel paths to host adaptation and antimicrobial resistance evolution, accelerated by the movement of mobile elements. Significant progress has been made in identifying type 3 effectors required for virulence and recognition in A. chinensis and Actinidia arguta, broadening our understanding of how host-mediated selection shapes virulence. The rapid development of Actinidia genomics after the Psa3 pandemic began has also generated new insight into molecular mechanisms of immunity and resistance gene evolution in this recently domesticated, nonmodel host. These findings include the presence of close homologs of known resistance genes RPM1 and RPS2 as well as the novel expansion of CCG10-NLRs (nucleotide-binding leucine-rich repeats) in Actinidia spp. The advances and approaches developed during the pandemic response can be applied to new pathosystems and new outbreak events.

Natural product guvermectin inhibits guanosine 5'-monophosphate synthetase and confers broad-spectrum antibacterial activity

Bacterial diseases caused substantial yield losses worldwide, with the rise of antibiotic resistance, there is a critical need for alternative antibacterial compounds. Natural products (NPs) from microorganisms have emerged as promising candidates due to their potential as cost-effective and environmentally friendly bactericides. However, the precise mechanisms underlying the antibacterial activity of many NPs, including Guvermectin (GV), remain poorly understood. Here, we sought to explore how GV interacts with Guanosine 5'-monophosphate synthetase (GMPs), an enzyme crucial in bacterial guanine synthesis. We employed a combination of biochemical and genetic approaches, enzyme activity assays, site-directed mutagenesis, bio-layer interferometry, and molecular docking assays to assess GV's antibacterial activity and its mechanism targeting GMPs. The results showed that GV effectively inhibits GMPs, disrupting bacterial guanine synthesis. This was confirmed through drug-resistant assays and direct enzyme inhibition studies. Bio-layer interferometry assays demonstrated specific binding of GV to GMPs, with dependency on Xanthosine 5'-monophosphate. Site-directed mutagenesis identified key residues crucial for the GV-GMP interaction. This study elucidates the antibacterial mechanism of GV, highlighting its potential as a biocontrol agent in agriculture. These findings contribute to the development of novel antibacterial agents and underscore the importance of exploring natural products for agricultural disease management.

Protein Disorder in Plant Stress Adaptation: From Late Embryogenesis Abundant to Other Intrinsically Disordered Proteins

Global climate change has caused severe abiotic and biotic stresses, affecting plant growth and food security. The mechanical understanding of plant stress responses is critical for achieving sustainable agriculture. Intrinsically disordered proteins (IDPs) are a group of proteins without unique three-dimensional structures. The environmental sensitivity and structural flexibility of IDPs contribute to the growth and developmental plasticity for sessile plants to deal with environmental challenges. This article discusses the roles of various disordered proteins in plant stress tolerance and resistance, describes the current mechanistic insights into unstructured proteins such as the disorder-to-order transition for adopting secondary structures to interact with specific partners (i.e., cellular membranes, membrane proteins, metal ions, and DNA), and elucidates the roles of liquid-liquid phase separation driven by protein disorder in stress responses. By comparing IDP studies in animal systems, this article provides conceptual principles of plant protein disorder in stress adaptation, reveals the current research gaps, and advises on the future research direction. The highlighting of relevant unanswered questions in plant protein disorder research aims to encourage more studies on these emerging topics to understand the mechanisms of action behind their stress resistance phenotypes.