Most strains of the Pseudomonas syringae complex isolated from the trunks of woody
plants and belonging to phylogroups (PGs) 1 and 3 share a genomic region, named WHOP
(from woody host and Pseudomonas), which is partially conserved in some PG2 strains.
Annotation of the 14 ORFs encoded within this region, which is absent in the genomes of P.
syringae strains infecting herbaceous hosts, yielded functions possibly involved in the
metabolism of phenolic compounds. The genetic organization of this region was analyzed by
RT-PCR in Pseudomonas savastanoi pv. savastanoi (Psv), the causative agent of olive knot
disease. The WHOP region is organized in four operons (antABC, catBCA, ipoABC and dhoAB)
and three genes transcribed independently (antR, benR and PSA3335_3206). HPLC analyses
confirmed that the antABC and catBCA operons mediate the catabolism of anthranilate and
catechol, respectively, through the β-ketoadipate pathway. In addition, oxygenase activity
on aromatic compounds, tested as the conversion of indole into indigo, was assigned to the
ipoABC operon. Pathogenicity tests revealed that deletion of antABC, catBCA or ipoABC in
Psv caused reduced symptoms in woody olive plants, while such phenotype was not
observed in young micropropagated (non-woody) plants. Similarly, the catBCA and dhoAB
operons and the PSA3335_3206 gene (encoding a putative aerotaxis receptor) are also
required for full bacterial fitness exclusively in woody olive plants. At present, we are
addressing the role in virulence of the orphan benR gene encoded in the WHOP region.
