Interplays between nitric oxide and reactive oxygen species in cryptogein signalling.

Abstract

The cellular messenger nitric oxide (NO) has many functions in plants. In this study, we investigated its interplays with Reactive Oxygen Species (ROS) in the defense responses triggered by the elicitin cryptogein produced by the oomycete Phytophthora cryptogea. The production of NO induced by cryptogein in tobacco cell suspensions was partly regulated through a ROS-dependent pathway involving the NADPH oxidase NtRBOHD. In turn, NO down-regulated the level of H2O2 derived from NtRBOHD activity. Both NO and ROS synthesis appeared to be under the control of two redundant isoforms of histone deacetylases of type 2 acting as negative regulators of cell death. Occurrence of an interplay between NO and ROS was further supported by the finding that cryptogein triggered a fast production of peroxynitrite (ONOO─) resulting from the coupling reaction of superoxide (O2•─) with NO. We provided evidence that ROS, but not NO, negatively regulate the intensity of activity of the protein kinase NtOSAK, a member of the SnRK2 protein kinase family. Furthermore, using a micro-array approach, we next identified fifteen genes early induced by cryptogein via NO. Interestingly, only a part of these genes was also modulated by ROS derived from NtRBOHD activity and encoded proteins showing sequence identity to ubiquitin ligases. Expression of those genes appeared to be negatively regulated by ONOO─, suggesting that ONOO─ mitigate the effects of NO and ROS in cell response to cryptogein. Finally, we provided evidence that NO required NtRBOHD activity for inducing cell death, thus confirming previous assumption that ROS channel NO through cell death pathways.