THE LACK OF INHIBITION OF CLAVICEPS PURPUREA POLYGALACTURONASES BY PvPGIP2 MAY EXPLAIN THE FAILURE OF TRANSGENIC WHEAT PLANTS TO RESIST TO THE ERGOT PATHOGEN

Claviceps purpurea is a biotrophic fungal pathogen of cereals and grasses, attacking exclusively young ovaries. C. purpurea grows intercellularly in rye (Secale cereale) ovaries by degrading the pectic polymers and the fungal polygalacturonase (PG) has been shown to be a pathogenicity factor. Two pg genes of C. purpurea, cppg1 and cppg2, are responsible for this activity. Transgenic plants expressing a bean polygalacturonase-inhibiting protein (PvPGIP2) proved to be a valuable tool to increase resistance against PG-producing fungi. However, a stable transgenic PvPGIP2 wheat line, previously shown to be more resistant to the fungal pathogens Bipolaris sorokiniana and Fusarium graminearum, exhibited only a very low reduction of symptoms after infection with C. purpurea. To understand whether this reduced protection of PvPGIP2 in wheat transgenic plants was ascribable to a lack of inhibition against the fungal PGs, we tried to perform inhibition experiments against the C. purpurea PG activity. Unfortunately, this fungus does not produce any PG activity in culture, thus it was necessary to express this activity in a Pichia pastoris heterologous system. The two heterologous expressed PGs, when assayed against the PvPGIP2, were poorly affected by this inhibitor, indicating that the lack of resistance in transformed wheat line may be due to the lack of recognition of the PGs of C. purpurea by PvPGIP2. This finding supports a role of PGIP in plant defence only when PG-PGIP interaction occurs. The expressed PGs may be useful to identify more effective PGIPs by a broad screening of plant PGIPs.