Genetic buffering of cyclic AMP in Arabidopsis thaliana compromises the plant immune response triggered by an avirulent strain of Pseudomonas syringae pv. tomato

Cyclic AMP plays important roles in different physiological processes, including plant defence responses. However, since little is known on plant enzyme responsible for cAMP production/degradation, the study of cAMP functions relied, until now, on non-specific pharmacological approaches. We thus developed a more reliable approach, producing transgenic Arabidopsis thaliana lines overexpressing the "cAMP-sponge" (cAS), a genetic tool that specifically buffers cAMP levels. In response to an avirulent strain of Pseudomonas syringae pv. tomato (PstAvrB), cAS plants showed a higher bacterial growth and a reduced hypersensitive cell death in comparison with WT plants. The low cAMP availability after pathogen infection delayed cytosolic calcium elevation, as well as hydrogen peroxide increase and induction of redox systems. The proteomic analysis, performed 24 hours post-infection, indicated that a core of 49 proteins was modulated in both genotypes, while 16 and 42 proteins were uniquely modulated in WT and cAS lines, respectively. The involvement of these proteins in the impairment of defence response in cAS plants has been discussed. Moreover, an in-silico analysis revealed that the promoter regions of the genes coding for proteins uniquely accumulating in WT plants share the CGCG motif, target of the calcium-calmodulin binding transcription factor AtSR1 (Arabidopsis thaliana signal responsive1). Thus, following pathogen perception, the low free cAMP content, altering timing and levels of defence signals, likely in part through the mis-regulation of AtSR1 activity, affects the speed and strength of the immune response. This article is protected by copyright. All rights reserved.