A 2910-bp full-length cDNA encoding a lipoxygenase (RlemLOX) was identified from rough lemon (Citrus jambhiri Lush.). The deduced amino acid sequence of the gene is 54 to 60% identical to other plant LOXs, such as PotLOX2(Hl) of potato, TomLOXC of tomato and AtLOX2 of Arabidopsis. The amino acid sequence possesses a putative N-terminal chloroplast transit peptide. The RlemLOX transcript was not constitutively present in leaves, stems and roots but accumulated in leaves within 30 min after wounding or inoculation with nonpathogenic isolates of Alternaria alternata. The RlemLOX was also induced, but the signal declined rapidly after inoculation with the rough lemon pathotype of A. alternata, which produces the host-selective ACR-toxin. Transcripts of another stress-inducible gene, chalcone synthase (CHS), were also induced but declined rapidly in response to this pathogen. An increase in enzymatic activity of LOX after either wounding or inoculation with a nonpathogen was almost completely suppressed in leaves infected with the pathogen. These reductions in LOX transcript accumulation and its enzymatic activity after inoculation with the pathogenic but not the nonpathogenic A. alternata indicate that LOX may play a role in citrus-microbe interactions.