Biology subjects

Hemara, L. M.

Publications and source records attributed to Hemara, L. M..

6 recordsLinked to original sources

bioRxiv2025

Diverse Pseudomonas species engage in beneficial and suppressive interactions with the kiwifruit pathogen Psa across Actinidia germplasm

In 2010, a Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) incursion into New Zealand kiwifruit orchards devastated susceptible Actinidia chinensis cultivars. In contrast, many Actinidia species maintained in germplasm collections were resistant to Psa3 and showed limited symptoms. Recent genome biosurveillance revealed the emergence of widespread leaf spot symptoms in Psa3-resistant Actinidia germplasm. Surprisingly, few Psa3 isolates were recovered from symptomatic tissues, despite the frequent isolation of phenotypical Pseudomonas isolates on selective agar. Despite the poor recovery of Psa3 isolates, Psa3 was found in all symptomatic leaf tissue through qPCR and metabarcoding analysis. Metabarcoding revealed stark differences in bacterial community composition from samples taken from lesion-carrying or lesion-free material from the same leaf. Host genotype also appeared to influence community composition but to a lesser extent. Whole genome sequencing of diverse Pseudomonas spp. isolates revealed that many belonged to the P. syringae species complex. Curiously, the kiwifruit-associated P. syringae pv. actinidifoliorum (Pfm), was never recovered, nor were any other phylogroup 1 pathovars. Pathogenicity and competitive assays revealed that while individually diverse Pseudomonas isolates were not more pathogenic than Pfm or Psa3 on resistant Actinidia hosts, they could interact with Psa to sometimes improve their growth, suggesting that these isolates may form pathogenic consortia in these disease-associated phyllosphere communities on typically Psa3-resistant hosts. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=116 SRC="FIGDIR/small/635981v1_ufig1.gif" ALT="Figure 1"> View larger version (23K): org.highwire.dtl.DTLVardef@1e48d68org.highwire.dtl.DTLVardef@1ffa8d7org.highwire.dtl.DTLVardef@1a5c47corg.highwire.dtl.DTLVardef@6bd39_HPS_FORMAT_FIGEXP M_FIG C_FIG

plant biology↗

bioRxiv2024

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 Zealands 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 variants 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 {Delta}hopF1c strain could outcompete 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 Zealands Psa3 population is recommended to enable early detection and management of variants of interest.

genomics↗

bioRxiv2024

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 (A. arguta, A. polygama, and A. 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 (T3S) system-lacking Psa3 V-13 {Delta}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 {Delta}33E strain and to Psa3 V-13 {Delta}hrcC suggested that PTI triggered by Psa3 V-13 was based on the recognition of the T3S 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.

plant biology↗

bioRxiv2022

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

O_LITesting effector-knockout strains of the Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) for reduced in planta growth in their native kiwifruit host revealed a number of non- redundant effectors that contribute to Psa3 pathogenicity. Conversely, complementation in the weak kiwifruit pathogen P. syringae pv. actinidifoliorum (Pfm) for increased growth identified redundant Psa3 effectors. C_LIO_LIPsa3 effectors hopAZ1a and HopS2b and the entire exchangeable effector locus ({Delta}EEL; 10 effectors) were significant contributors to bacterial colonisation of the host and were additive in their effects on pathogenicity. Four of the EEL effectors (HopD1a, AvrB2b, HopAW1a, and HopD2a) redundantly contribute to pathogenicity through suppression of pattern-triggered immunity (PTI). C_LIO_LIImportant Psa3 effectors include several redundantly required effectors early in the infection process (HopZ5a, HopH1a, AvrPto1b, AvrRpm1a, and HopF1e). These largely target the plant immunity hub, RIN4. C_LIO_LIThis comprehensive effector profiling revealed that Psa3 carries robust effector redundancy for a large portion of its effectors, covering a few functions critical to disease. C_LI

plant biology↗

bioRxiv2021

Effector loss drives adaptation of Pseudomonas syringae pv. actinidiae to Actinidia arguta

A pandemic isolate of Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) has devastated kiwifruit orchards growing cultivars of Actinidia chinensis. In contrast, A. arguta (kiwiberry) is resistant to Psa3. This resistance is mediated via effector-triggered immunity, as demonstrated by induction of the hypersensitive response in infected A. arguta leaves, observed by microscopy and quantified by ion-leakage assays. Isolates of Psa3 that cause disease in A. arguta have been isolated and analyzed, revealing a 49 kb deletion in the exchangeable effector locus (EEL). This natural EEL-mutant isolate and strains with synthetic knockouts of the EEL were more virulent in A. arguta plantlets than wild-type Psa3. Screening of a complete library of Psa3 effector knockout strains identified increased growth in planta for knockouts of four effectors - AvrRpm1a, HopF1c, HopZ5a, and the EEL effector HopAW1a - suggesting a resistance response in A. arguta. Hypersensitive response (HR) assays indicate that three of these effectors trigger a host species-specific HR. A Psa3 strain with all four effectors knocked out escaped host recognition, but a cumulative increase in bacterial pathogenicity and virulence was not observed. These avirulence effectors can be used in turn to identify the first cognate resistance genes in Actinidia for breeding durable resistance into future kiwifruit cultivars.

plant biology↗

bioRxiv2020

Variation at the common polysaccharide antigen locus drives lipopolysaccharide diversity within the P. syringae species complex

The common polysaccharide antigen (CPA) from the lipopolysaccharide (LPS) component of cell walls from the species complex Pseudomonas syringae is highly variable both in structure and immunological specificity, but the genetic basis for this is not well understood. We have characterised the CPA locus from P. syringae pv. actinidiae (Psa). This locus has a modular structure with genes for both L- and D- rhamnose (Rha) biosynthesis and that of an unknown sugar. It also contains an operon coding for ABC transporter subunits, a bifunctional glycosyltransferase and an O-methyltransferase. This operon is predicted to have a role in transport, elongation and termination of the Rha backbone of the CPA oligosaccharide and is referred to as the TET operon. This is the first report of the identification of this operon in P. syringae. Two alleles of the TET operon were present amongst the different biovars of Psa and lineages of the closely related pathovar P. syringae pv. actinidifoliorum. This allelic variation was reflected in the electrophoretic properties of purified LPS from the different isolates. Gene knockout of the TET operon allele from biovar 1 and replacement with that from biovar 3, demonstrated the link between the genetic locus and the electrophoretic and immunogenic properties of the LPS molecules in Psa. Sequence analysis of the TET operon from a wide range of P. syringae and P. viridiflava isolates displayed a phylogenetic history which is incongruent with core gene phylogeny, but correlates with previously reported tailocin sensitivity, suggesting a functional relationship between LPS structure and tailocin susceptibility.

microbiology↗