Inhibition of ROS-Scavenging Enzyme System Is a Key Event in Tomato Genetic Resistance against Root-Knot Nematodes

Genetic resistance in plants against incompatible pests is expressed by the activation of animmune system; however, the molecular mechanisms of pest recognition and expression of immunity,although long the object of investigation, are far from being fully understood. The immune responsetriggered by the infection of soil-borne parasites, such as root-knot nematodes (RKNs), to incompatibleresistant tomato plants was studied and compared to the compatible response that occurred whenRKNs attacked susceptible plants. In compatible interactions, the invading nematode juveniles wereallowed to fully develop and reproduce, whilst that was impeded in incompatible interactions. Incrude root extracts, a first assay of reactive oxygen species (ROS)-scavenging enzymatic activity wascarried out at the earliest stages of tomato-RKN incompatible interaction. Membrane-bound andsoluble CAT, which is the most active enzyme in hydrogen peroxide (H2O2) scavenging, was foundto be specifically inhibited in roots of inoculated resistant plants until 5 days after inoculation, withrespect to uninoculated plants. The expression of genes encoding for antioxidant enzymes, suchas CAT and glutathione peroxidase (GPX), was not always inhibited in roots of nematode-infectedresistant tomato. Therefore, the biochemical mechanisms of CAT inhibition were further investigated.Two CAT isozymes were characterized by size exclusion HPLC as a tetrameric form with a molecularweight of 220,000 dalton and its subunits (55,000 dalton). Fractions containing such isozymes weretested by their sensitivity to both salicylic acid (SA) and H2O2. It was evidenced that elevatedconcentrations of both chemicals led to a partial inactivation of CAT. Elevated concentrations of H2O2in incompatible interactions have been suggested to be produced by membrane-bound superoxideanion generating, SOD, and isoperoxidase-enhanced activities. Such partial inactivation of CAT hasbeen depicted as one of the earliest key metabolic events, which is specifically associated with tomatoimmunity to RKNs. Enhanced ROS production and the inhibition of ROS-scavenging systems havebeen considered to trigger all the metabolic events leading to cell death and tissue necrosis developedaround the head of the invading juveniles by which this special type of plant resistance is exerted.