Non-host resistance of arabidopsis and apple is associated with callose accumulation and changes in preinfective structures of Colletotrichum species

Non-host resistance (NHR) describes the immunity state of plant species against non-adapted pathogen species and has been considered the most durable and e ffective form of plant resistance in nature. The objective of this work was to study the non-host resistance mechanisms involved in Arabidopsis thaliana and Malus domestica against Colletotrichum fructicola and Colletotrichum higginsianum, respectively. For this, the development of preinfective structures of Colletotrichum species, hypersensitivity responses and callose accumulation were monitored in host and non-host plants. Conidial germination and appressorial formation of Colletotrichum were affected on non-host leaf surfaces. Conidial germination and appressorial melanization of both Colletotrichum species occurred in a faster manner on arabidopsis than on apple leaves. On apple leaves, appressoria of C. higginsianum became more pedicelate, while on its host (arabidopsis) they were typically sessile. Hypersensitive response (HR) and appressorium-associated HR occurred at low frequency in epidermal cells of both plants, and no relationship could be established with NHR. Callose accumulation was signi ficantly higher in inoculated nonhost plants, mainly at attempted entry sites of Colletotrichum . Our results demonstrate that the mechanisms of NHR involved in this heterologous interaction were associated to pre-invasive events as demonstrated by the changes in development of preinfective structures and the accumulation of papillary callose at sites of penetration attempts.