This research project aims to characterize the in planta activity of the PATHOGENESIS-RELATED PROTEIN 1 (PR-1). PR-1 played a pioneering role in discovering plant innate immunity 50 years ago as it was demonstrated that plants accumulate Pathogenes-Related (PR) proteins upon pathogen attack. The most prominent among these proteins was PR-1, which also displayed in vitro antimicrobial properties. PR-1 is a CAP protein (cysteine-rich secretory protein, antigen 5, and pathogenesis-relates) sharing a conserved CAP domain of approximately 150 amino acids. Despite the importance of these proteins, the biochemical function of PR-1, and CAP proteins, remained elusive for decades. However, during this time, PR-1 has become one of the most referred to proteins in many different pathosystems, and its expression is one of the most studied during plant-pathogen interaction. Recently, it was demonstrated that CAP proteins, so does PR-1, are sterol-binding proteins and that the mode of action of PR-1 is to sequester sterols from pathogen’s membranes. The discovery of the in vitro biochemical function and mode of action of PR-1 is the motivation to revisit the in planta activity of PR-1 using updated technology in molecular biology. To this end, we propose to 1) generate gain-of-function and loss-of-function transgenic Arabidopsis lines and 2) test these lines for phenotypic effects of PR-1 manipulation on disease resistance against various notorious plant pathogens. The proposed research will contribute to determining the role of PR-1 in plants and may be the basis for further research. Finally, these results are potentially relevant beyond PR-1 and plant science since some CAP proteins are implicated in mammalian immunity, and future experimentation may be relevant to plant protection and medicine.