Histological analysis of Scots pine (Pinus sylvestris L.) seedlings in response to root rot pathogen Heterobasidion annosum inoculation

Heterobasidion root rot is one of the most serious and economically destructive forest diseases in the Northern Hemisphere. Although several studies have explored the genetic and chemical responses of Scots pine to Heterobasidion spp. infection, the histological defense responses of this species remain poorly understood. In this study, we investigated the histological responses of three-year-old Scots pine seedlings to Heterobasidion annosum inoculation and a wounding treatment with no inoculation, focusing on lesion length and traumatic resin-duct characteristics (density, size). Our results showed that H. annosum-inoculated seedlings exhibited significantly more browning necrotic lesions than wounded seedlings. Traumatic resin duct density was significantly higher in H. annosum-inoculated seedlings compared to wounded seedlings, particularly within the first two cm from the inoculation point. However, as the distance from the inoculation point increased, the resin duct density decreased. Notably, there were neither statistically significant differences in the mean size of traumatic and constitutive resin ducts between the two treatments, nor within the H. annosum inoculation treatment itself. In contrast, within the wound treatment, the mean size of traumatic resin ducts was found to be significantly smaller than that of constitutive resin ducts. Furthermore, traumatic resin ducts did not prove to be reliable markers for dating H. annosum infection in Scots pine. Overall, this study advances knowledge about Scots pines histological defense mechanisms against H. annosum invasion, which has significant implications.