Nitric oxide modulates ozone-induced cell death, hormone biosynthesis and gene expression in Arabidopsis thaliana

Nitric oxide (NO) is involved together with reactive oxygen species (ROS) in the activation of various stress responses in plants. We have used ozone (O-3) as a tool to elicit ROS-activated stress responses, and to activate cell death in plant leaves. Here, we have investigated the roles and interactions of ROS and NO in the induction and regulation of O-3-induced cell death. Treatment with O-3 induced a rapid accumulation of NO, which started from guard cells, spread to adjacent epidermal cells and eventually moved to mesophyll cells. During the later time points, NO production coincided with the formation of hypersensitive response (HR)-like lesions. The NO donor sodium nitroprusside (SNP) and O-3 individually induced a large set of defence-related genes; however, in a combined treatment SNP attenuated the O-3 induction of salicylic acid (SA) biosynthesis and other defence-related genes. Consistent with this, SNP treatment also decreased O-3-induced SA accumulation. The O-3-sensitive mutant rcd1 was found to be an NO overproducer; in contrast, Atnoa1/rif1 (Arabidopsis nitric oxide associated 1/resistant to inhibition by FSM1), a mutant with decreased production of NO, was also O-3 sensitive. This, together with experiments combining O-3 and the NO donor SNP suggested that NO can modify signalling, hormone biosynthesis and gene expression in plants during O-3 exposure, and that a functional NO production is needed for a proper O-3 response. In summary, NO is an important signalling molecule in the response to O-3.