No abstract

Summary Suppression of the defence system of plants by some plant pathogens is an important mechanism of their parasitism, and plant-parasitic nematodes use several mechanisms to affect different parts of the plant defence system. This study was designed to investigate the ability of a root-knot nematode, Meloidogyne javanica, to reduce or suppress the plant (tomato) immune system, before or after activation by a strong inducer, salicylic acid (SA). The experimental treatments were tomato plants inoculated with nematodes alone, plants pre-treated with SA and then nematodes (‘SA then nematode’), plants treated with SA after nematode inoculation (‘nematode then SA’), plants treated with SA alone, and plants treated with sterile distilled water. The results showed that treatment of the plants with SA before or after nematode infection reduced nematode disease indices compared to the control (nematode alone). The number of eggs per individual egg mass, in ‘nematode then SA’ treatment was significantly greater than the control, which shows the effect of nematodes on reducing the plant defence mechanism in this treatment. Evaluation of the activity of some defence enzymes such as chitinase, protease, phenylalanine ammonia lyase, catalase and density of hydrogen peroxide also showed that M. javanica is able to suppress these compounds in the ‘SA then nematode’ treatment, and to a greater extent in the ‘nematode then SA’ treatment. Suppression of plant defence responses by phytonematodes is of great importance in their synergistic relationship with secondary pathogens and plants.