The Hypersensitive Response (HR) is a defense program stimulated upon recognition of avirulent pathogens that triggers the apoptosis of infected cells. We have recently found that tissues developing HR accumulate high levels of L-proline, although we do not know if the response contributes to activate defenses. We here evaluate two possible consequences of proline increase in HR developing tissues: a) avoidance of pathogen proliferation by rise of extracellular proline levels and consequent reduction of apoplastic water potential; b) enhancement of proline degradation and accumulation of pro-apoptotic derivatives. Both effects were analyzed in Arabidopsis thaliana leaf tissues infiltrated with virulent/ avirulent races of Pseudomonas syringae. We found that the levels of apoplastic proline accumulated in HR were not sufficient to inhibit bacterial growth in vitro. On the other hand, we detected transcriptional alterations of genes controlling proline degradation that suggest the accumulation of pro-apoptotic compounds at initial steps of HR. Moreover, treatment with salicylic acid (SA), an inducer of defenses, stimulates similar behaviour of these catabolic genes. These results indicated that proline degradation may contribute to signal cell death in HR involving SA-dependent responses.
