The bacterial Lon protease participates in a variety of biological processes. In Pseudomonas syringae, mutation of lon is known to activate hrpL and a few hrpL-regulated genes in rich medium. The elevated expression of hrpL and hrpL-regulated genes results from increased stability of HrpR, the transcriptional activator of hrpL, in the lon mutant. Here, we conducted a microarray analysis to identify genes that are differentially expressed in a lon- mutant of P. syringae pv. tomato DC3000 grown in the rich medium King's B (KB). Most genes induced in the lon- mutant belong to the HrpL regulon or are related to transcription, protein synthesis, and energy metabolism. A major group of genes reduced in the lon- mutant are related to cell wall biogenesis. The HrpL-regulated genes exhibit different induction patterns in the lon- mutant, suggesting that additional regulators other than HrpL are likely to be involved in regulation of these genes. Compared with the wild-type bacteria, lon- mutants of P. syringae pv. tomato DC3000 and P. syringae pv. phaseolicola NPS3121 strains exhibit elevated hrpL expression in KB medium, but reduced hrpL expression in minimal medium (MM). The reduced hrpL RNA is correlated with reduced hrpR and hrpS RNAs, suggesting that the Lon-mediated regulation of hrpL involves different mechanisms in KB and MM. The lon- mutation also reduced bacterial pathogenicity.