A Histopathological Study Reveals New Insights Into Responses of Chestnut ( spp.) to Root Infection by .

The European chestnut () is threatened by the hemibiotrophic oomycete , the causal agent of ink disease. Chestnut species have different susceptibility levels to , with the Asian species (; ) exhibiting the highest level of resistance. A histological approach was used to study the responses exhibited by susceptible and resistant chestnut genotypes by characterizing the early stages of infection and the cellular responses it induces in roots. (susceptible) and (resistant) plantlets were inoculated with a virulent isolate with a zoospore suspension or by direct contact with mycelia agar pieces. Root samples were collected at 0.5, 3.5, 24, 48, and 72 h after inoculation (hai) for microscopic observations. Penetration was observed in both species at 0.5 and 3.5 hai with mycelium and zoospore inoculations, respectively. In both inoculation methods, following penetration into the rhizodermis, hyphae grew inter- and intracellularly through the cortex and into the vascular cylinder. cells displayed a delay in the pattern of infection by having fewer cell layers colonized compared with . At 72 hai, the collapse of the first layers of cortical cells was observed, indicating the beginning of necrotrophy. was able to respond more efficiently to than by restricting the pathogen's growth area through the early activation of resistance responses such as callose deposition around some intracellular hyphae, hypersensitive response-like cell death, cell wall thickening, and accumulation of phenolic-like compounds.