In this study, Pseudomonas savastanoi pv. savastanoi isolates were demonstrated to contain two iaaL paralogs, which are both chromosomally located in most strains. Comparative analysis of iaaL nucleotide sequences amplified from these two paralogs revealed that one paralog, iaaL Psn , is 100% identical to iaaL from P. savastanoi pv. nerii, while the other paralog, iaaL Psv , exhibited 93% identity to iaaL from Pseudomonas syringae pv. tomato (iaaL Pto ). A 3-nucleotide motif (TAC) comprised of 3 to 15 repeats, which remained stable after propagation of the strains in olive plants, was found in iaaL Psv . Based on the observed nucleotide sequence variations, a restriction fragment length polymorphism assay was developed that allowed differentiation among iaaL Psn , iaaL Psv , and iaaL Pto. In addition, reverse transcriptase PCR on total RNA from P. savastanoi pv. savastanoi strains demonstrated that both iaaL Psv and iaaL Psn containing 14 or fewer TAC repeats are transcribed. Capillary electrophoresis analysis of PCR-amplified DNA fragments containing the TAC repeats from iaaL Psv allowed the differentiation of P. savastanoi pv. savastanoi isolates.Infections with Pseudomonas savastanoi pv. savastanoi (46), pv. fraxini, and pv. nerii (13) result in knots and galls on members of the Oleaceae family, and P. savastanoi pv. nerii also causes galls on oleander. Symptoms of infected trees include hypertrophy formation on the stems and branches and occasionally on the leaves and fruits. In the olive, the disease is considered to reduce both olive yield and productivity (41, 42); however, quantitative data on the impact of the disease on crop yield or crop quality are not available (19). Nevertheless, losses can be caused directly by localized infections that inhibit flowering and affect fruit development, as well as taste, and can be caused indirectly by weakening immature main leader branches that result in later damage to the tree frame (48).Currently, the only molecular P. savastanoi determinants known to be involved in knot development are the phytohormones indoleacetic acid (IAA) and cytokinins (1,15,33,38,45), as well as biosynthesis of a functional type III secretion system, encoded by the hrp-hrc gene clusters (43,44). Recently, a global genomic analysis of P. savastanoi pv. savastanoi plasmids allowed the identification of several putative virulence factors in the olive pathogen, including several type III secretion system protein effectors and a variety of genes encoding known Pseudomonas syringae virulence factors (32).The genetic determinants of P. savastanoi that are involved in the conversion of tryptophan (Trp) to IAA are Trp monooxygenase (encoded by the iaaM gene), which converts Trp to indoleacetamide (IAM), and IAM hydrolase (encoded by the iaaH gene), which catalyzes transformation of IAM to IAA (28). IAA can be further metabolized in P. savastanoi to an amino acid conjugate, 3-indole-acetyl-Δ-L-lysine (IAA-lysine), through the action of the iaaL gene (14). In P. savastanoi pv. savastanoi and pv. ne...