The species Pseudomonas savastanoi, a member of the Pseudomonas syringae complex,
includes four pathovars causing knots or excrescences in woody hosts: P. savastanoi pv.
savastanoi (Psv), pv. fraxini (Psf), pv. nerii (Psn) and pv. retacarpa (Psr), comprising isolates
from olive, ash, oleander and broom plants, respectively. Pathogenicity of P. savastanoi is
dependent, among other factors, on the type III secretion system (T3SS) and its effector
(T3E) repertoire. Furthermore, a putative role in the interaction with woody hosts has been
suggested for several of these T3E. The recent availability of the genome sequences of
several P. savastanoi strains isolated from different hosts has facilitated bioinformatics
predictions of their T3SS genes and T3E pools, the study of their distribution in other strains
of the P. syringae complex isolated from woody hosts and the functional analysis of several
of these secreted proteins. As previously reported for Psv, Psn and Psf, here we show that
pathogenicity of Psr ICMP16945, is also dependent on the T3SS. Psv strains NCPPB 3335,
ICMP4352 and PseNe107 share a core set of at least 22 T3E, 18 of which are also encoded in
Psn ICMP16943, Psf ICMP7711 and Psr ICMP16945. However, these three strains encode
truncated versions of 1-2 of these 18 T3E and, Psr ICMP16945 contains three pathovarspecific
T3E. Our results also show that several T3E, including HopAO1, are phylogenetically
clustered across the P. syringae complex according to the woody/herbaceous nature of their
host of isolation, suggesting host specialization of these effectors in this complex.
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