SAR induction in tomato plants is not effective against root-knot nematode infection

Systemic Acquired Resistance (SAR) is one of the defence mechanisms plants employee against pathogen attack, resulting in resistance and protection of distal tissues. It is characterized by the accumulation of salicylic acid (SA) and the expression of a group of pathogenesis-related (PR) genes; synthetic analogues of SA, such as INA and BTH, can induce SAR. The effects of INA and BTH on tomato plant development and on the tomato-Meloidogyne javanica interaction were investigated, and the response of both SAR and nematode pathogenesis to changes in the expression of several SA-responsive PR genes, PR-1b, PR-2 and PR-5, was determined. In addition, the response of calmodulin and PR-3 transcripts - two SA-independent genes - to SAR elicitor chemicals was also studied. All elicitor-treated plants exhibited successful infestation by M. javanica, although a significant reduction of gall numbers was observed. INA was more efficient than BTH at inducing the SAR markers PR-1b and PR-2, but the root-knot nematode apparently suppressed the expression of these genes. This downregulation of gene expression following nematode infestation was also detected with regard to PR-3 expression. On the other hand, expression of PR-5 was not affected by elicitor treatment, though Meloidogyne infestation induced its expression.