Genetic dissection of the tissue‐specific roles of type III effectors and phytotoxins in the pathogenicity of Pseudomonas syringae pv. syringae to cherry

Abstract

When compared with other phylogroups (PGs) of the Pseudomonas syringae species complex, P. syringae pv. syringae (Pss) strains within PG2 have a reduced repertoire of type III effectors (T3Es) but produce several phytotoxins. Effectors within the cherry pathogen Pss 9644 were grouped based on their frequency in strains from Prunus as the conserved effector locus (CEL) common to most P. syringae pathogens; a core of effectors common to PG2; a set of PRUNUS effectors common to cherry pathogens; and a FLEXIBLE set of T3Es. Pss 9644 also contains gene clusters for biosynthesis of toxins syringomycin, syringopeptin and syringolin A. After confirmation of virulence gene expression, mutants with a sequential series of T3E and toxin deletions were pathogenicity tested on wood, leaves and fruits of sweet cherry (Prunus avium) and leaves of ornamental cherry (Prunus incisa). The toxins had a key role in disease development in fruits but were less important in leaves and wood. An effectorless mutant retained some pathogenicity to fruit but not wood or leaves. Striking redundancy was observed amongst effector groups. The CEL effectors have important roles during the early stages of leaf infection and possibly acted synergistically with toxins in all tissues. Deletion of separate groups of T3Es had more effect in P. incisa than in P. avium. Mixed inocula were used to complement the toxin mutations in trans and indicated that strain mixtures may be important in the field. Our results highlight the niche‐specific role of toxins in P. avium tissues and the complexity of effector redundancy in the pathogen Pss 9644.

A Necrotizing Toxin PromotesPseudomonas syringaeInfection Across Evolutionarily Divergent Plant Lineages

“…For example, both lipopeptide toxins underpin P. fluorescens transitions between pathogenic and commensal lifestyles on Arabidopsis roots, such that toxin-deficient mutants phenocopy commensal strains 38 . Moreover, P. syringae (PG2) interaction studies in Prunus avium (cherry) demonstrate a key role for syringomycin in promoting disease in fruit 28,39 , however toxins appear less important for infection in leaves or woody tissues where type-3 effectors play a more dominant role 39 . We similarly observed a limited role for syringomycin during angiosperm (N. benthamiana) leaf infections, yet the toxin was essential for virulence in non-flowering plant tissues.…”