Growth and ultrastructural modifications to chestnut calli induced by culture filtrates of virulent and hypovirulent Cryphonectria parasitica strains

Summary Callus cultures of two susceptible Castanea sativa cultivars, ‘Garrone rosso’ and ‘Clone 71’ were grown on culture medium supplemented to 50% with culture filtrates (CFs) from E4 virulent (E4‐V) or E13 hypovirulent (E13‐H) Cryphonectria parasitica strain, respectively. E13‐H CFs caused a reduction in fresh weight and an increase in dry weight on calli of both genotypes. E4‐V CFs did not induce any modification of these parameters compared with the control, but fresh weight of ‘Clone 71’ was reduced. Light and electron microscopy observation showed the presence of large osmiophilic aggregates in the vacuoles and, sometimes, the ruptured tonoplast in the E4‐V treated calli. Calli grown on El 3‐H CFs had a meristematic‐like appearance, with small, scarcely vacuolated cells containing altered mitochondria and plasmalemma proliferations.