Resistance responses induced by the plant growth retardant prohexadione‐Ca in apple against scab infections

The efficacy of the dioxygenase inhibitor prohexadione‐Ca (ProCa) as an inducer of resistance responses against apple scab (Venturia inaequalis/Spilocaea pomi) was evaluated in two sets of experiments under glasshouse conditions. ProCa (200 ppm) was applied on seedlings of open‐pollinated Malus domestica L. cv. Golden Delicious, and 10 days after application, a group of seedlings was inoculated with a conidial suspension of S. pomi. Total RNA was extracted from leaf samples collected at different times from both inoculated and noninoculated seedlings, and expression of the pathogenesis‐related proteins PR‐1, PR‐2, PR‐5, PR‐8, PR‐10 and chitinase was studied by reverse transcription–polymerase chain reaction and Northern blot hybridisation. Disease severity was significantly lower on seedlings treated with ProCa than on the water‐treated control seedlings. ProCa triggered a general increase in gene expression of almost all the PR genes; only the level of chitinase and PR‐10 did not show a detectable increase. A strong upregulation of all the assayed genes was observed in leaves of apple seedlings first treated with ProCa and then inoculated with the pathogen, whereas no expression and/or a constitutive signal was found in samples from the control seedlings.