Anatomical Defence Responses of Resistant and Susceptible 12-Years-Old Pine Species Infected with <i>Bursaphelenchus xylophilus</i>

Pine wilt disease (PWD) is caused by the pinewood nematode (PWN), Bursaphelenchus xylophilus , an endoparasite that infects Pinus species. While some pine species are susceptible to PWD, others seem to be resistant or resistant under field conditions, through mechanisms still not understood. In this study, we evaluated the damage and anatomical response of PWN-susceptible ( Pinus sylvestris and Pinus pinaster ) and resistant ( Pinus halepensis and Pinus pinea ) pine species, through histological analysis along five time-periods following nematode inoculations on 12-year-old trees. We observed a time-framed sequence of defensive responses, with the initial formation of wound-periderm, the accumulation of calcium oxalate (CaOx) crystals, and an enhanced activity of polyphenolic parenchyma (PP) cells, particularly in the resistant species. The presence of phenolic compounds, lignification of cell walls, and the formation of traumatic resin canals were also observed in the resistant species. Additionally, the development of callus tissue in response to nematode invasion may represent a potential barrier to nematode progression, emphasising the role of rapid tissue regeneration in nematode-resistant pines. These trees exhibited slower damage progression and more effective nematode containment compared to the susceptible P. pinaster and P. sylvestris , which suffered extensive tissue disruption, apparently due to the absence of critical physical barriers. We did not find evidence of early dissemination of B. xylophilus through the resin canals. Overall, our study highlights the importance of early anatomical and biochemical responses in preventing the dissemination of the PWN, providing a deeper understanding of the defence strategies regulating the susceptibility of pine species to PWD.