Phytophthora cinnamomi occurs worldwide and has a host range in excess of 1,000 plant species. Avocados (Persea americana Mill) have been described as highly susceptible to this soilborne pathogen. Here, the regulation of defence responses in avocado root seedlings inoculated with P. cinnamomi mycelia is described. A burst of reactive oxygen species (ROS) was observed 4 days after inoculation. The higher physiological concentration of H 2 O 2 induced by P. cinnamomi on avocado roots had no effect on in vitro growth of the oomycete. Total phenols and epicathecin content showed a significant decrease, but lignin and pyocianidins exhibited no changes after inoculation. Also, increased nitric oxide (NO) production was observed 72 h after treatment. We studied the effects of one NO donor [sodium nitroprusside (SNP)], and one NO scavenger [2-to 4-carboxyphenyl-4,4,5,5-tetramethylimidazole-1-oxyl-3-oxide (CPTIO)] to determine the role of NO during root colonisation by P. cinnamomi mycelia.Pretreatment of the roots with CPTIO, but not with SNP, inhibited root colonisation suggesting an important role for NO production during the avocado-P. cinnamomi interaction. Our data suggest that although defence responses are activated in avocado roots in response to P. cinnamomi infection, these are not sufficient to avoid pathogen invasion.

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