Systemin is a tomato peptide hormone that promotes plant defense against plant-chewing pests. This octadecapeptide is released from the C-terminal region of prosystemin, a 200 amino acid-long cytosolic precursor. Homologues of the tomato prosystemin are present in other Solanaceae but not in tobacco. This species does not respond to the exogenous application of the tomato systemin. Previously, it was shown that the expression of the full-length tomato prosystemin cDNA in tobacco affects the proteomic repertoire and increases plant tolerance against phytopathogenic fungi. In this work, to evaluate the relevance of the systemin sequence, we generated transgenic tobacco plants that express a mutated prosystemin lacking the systemin region. By using proteomics and gene expression analyses, we show that the constitutive expression of the truncated prosystemin altered the proteomic profile of tobacco leaves and increased plant resistance against B. cinerea. The overlap of the modifications caused by the expression of the full-length and the truncated prosystemin indicated that these alterations occur also in the absence of the systemin sequence. Although the cellular mechanisms of prosystemin cleavage are elusive, our work demonstrates that the expression of a heterologous cytosolic peptidehormone precursor, irrespective of the presence of its peptide domain, associated with unpredicted changes at the proteomic and transcriptional level.

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