Sublethal concentrations of salicylic acid decrease the formation of reactive oxygen species but maintain an increased nitric oxide production in the root apex of the ethylene-insensitive never ripe tomato mutants.

The pattern of salicylic acid (SA)-induced production of reactive oxygen species (ROS) and nitric oxide (NO) were different in the apex of adventitious roots in wild-type and in the ethylene-insensitive never ripe (Nr) mutants of tomato (Solanum lycopersicum L. cv Ailsa Craig). ROS were upregulated, while NO remained at the control level in apical root tissues of wildtype plants exposed to sublethal concentrations of SA. In contrast, Nr plants expressing a defective ethylene receptor displayed a reduced level of RO S and a higher NO content in the apical root cells. In wild-type plants NO production seems to be RO S(H2O2)-dependent at cell death-inducing concentrations of SA, indicating that ROS and NO may interact to trigger oxidative cell death. In the absence of significant RO S accumulation, the increased NO production caused moderate reduction in cell viability in root apex of Nr plants exposed to 10(-3) M SA. This suggests that a functional ethylene signaling pathway is necessary for the control of ROS and NO production induced by SA.