Nitric oxide and hydrogen peroxide: two players in the defence response of tomato plants to root-knot nematodes

Nitric oxide (NO) has been postulated to be required, together with reactive oxygen species (ROS) for activation of disease resistance reaction of plants to pathogen infection. Here, we investigated NO, superoxide (O(*-)2), and hydrogen peroxide (H2O2) in tomato-root-knot nematode interactions to answer the question of whether they are produced during the early stages of nematode infection. NO detection was carried out using diaminofluorescein diacetate (DAF-2DA) by means of confocal laser microscopy and spectrophotometric analyses, and production of NO was estimated by monitoring the conversion of L-[U14C]arginine into L-[U14C]citrulline. O(*-)2 production was determined by using the tetrazolium salt, sodium,3'-{1-[phenylamino-carbonyl]-3,4-tetrazolium}-bis(4-methoxy-6-nitro) benzene-sulfonic acid hydrate (XTT) and H2O2 was measured by using the Amplex Red H2O2/peroxidase assay. Results showed i) the highest NO production in tissues challenged by avr pathotype, 12h after nematode inoculation, ii) NO production by nitric oxide synthase (NOS-like activity), iii) ROSbalance dependent control of NO. Our data evidenced, for the first time, that NO-generated signal, its spatiotemporal expression, and its cross-communication with other pro-oxidants or anti-oxidants critically influence compatible and incompatible tomato-Meloidogyne incognito interactions.