The ecological genetics of Pseudomonas syringae in the kiwifruit phyllosphere : a thesis submitted in partial fulfilment of the requirements for the degree of Ph.D. in Evolutionary Genetics, New Zealand Institute for Advanced Study at Massey University, Auckland, New Zealand
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Date
2017
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Massey University
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Abstract
The impact of disease-causing bacteria on their hosts is shaped by
interactions with co-occurring microbes, but such interactions are rarely
studied. Pseudomonas syringae is a ubiquitous and significant plant
pathogen infecting a wide range of plants, often of agricultural importance.
The community context of P. syringae in infected plant hosts has been
little explored. I determined the population structure and genetic diversity
of Pseudomonas syringae strains collected from infected and uninfected
orchards over the course of a growing season during the current outbreak of
bacterial canker of kiwifruit (P. syringae pv. actinidiae, Psa) in New
Zealand. A total of 148 strains comprising Phylogroups 1, 2, 3 and 5 were
characterised by Multi Locus Sequence Typing (MLST). The overall
population structure was clonal, but with a low level of recombination for
single housekeeping genes within phylogroups. More than half of the
isolates belonged to a new Phylogroup 3 clade (PG3a) that was also
commonly found on kiwifruit leaves in China and previously reported from
kiwifruit leaves in Japan. To understand the ecological basis of the
co-occurrence of PG3a and PG1 (Psa) I looked for evidence of niche
specialisation by performing reciprocal invasion from rare assays of a
selected representative from each lineage both in vitro and in planta. P.
syringae G33C (PG3a) demonstrates antagonistic behaviour towards Psa
NZ54, whereas Psa NZ54 exhibits a beneficial effect on growth of P.
syringae G33C; an effect that could not be attributed to virulence activity
encoded by the Type 3 Secretion System. Given this antagonistic
behaviour, I explored the virulence repertoire in these commensal strains to
determine their potential in the emergence of future more virulent types of
Psa. In addition, I used comparative genomics to unravel the phylogenetic
resolution of the novel P. syringae clade in context with known
representatives of P. syringae PG3. Together my data draw attention to
the community context of disease and demonstrate the value of
incorporating an ecological dimension into the study of the genetic
structure of pathogen populations.
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Keywords
Pseudomonas syringae|, Genetics, Evolutionary genetics, Kiwifruit, Diseases and pests, Research Subject Categories::NATURAL SCIENCES::Biology::Cell and molecular biology::Genetics