Do sunflower 2S albumins play a role in resistance to fungi?

Many proteins exerting antifungal activity in vitro have been isolated although in certain cases their contribution in plant resistance to fungal attack is unclear. Screening for antifungal proteins from sunflower seeds, we have detected a basic polypeptide (Ha-AP15) homologous to the previously described storage protein 2S albumin. Results reported here indicate that Ha-AP15 exerts a weak inhibition of hyphal elongation of some fungi but no activity was detected against the seed pathogen Alternaria alternata. Ha-AP15 did not show a synergistic enhancement of the antifungal activity of another antimicrobial protein present in sunflower seeds, and the activity of Ha-AP15 is blocked in the presence of physiological concentrations of CaCl2. In addition, we show that two sunflower genotypes which differ by 10-fold in their content of Ha-AP15 were equally susceptible to A. alternata attack. In conclusion, even if 2S albumins exert antifungal activity in assays developed in vitro, our results indicate that it seems improbable that these storage proteins could also play a role as antimicrobial compounds protecting sunflower seeds from fungal pathogens.