Conifer pathosystem : physiological and molecular responses to a pathogen infection

Conifers are used for forest plantations and as landscape trees. Norway spruce is the main softwood species in Finland and can occasionally be observed in urban areas. However, root rots of conifer trees caused by Heterobasidion annosum leads to huge economic losses not only in Finland but also in other European countries. Due to the availability of complete genome sequence for both host and pathogen, in this study using Norway spruce and its root rot pathogen (Heterobasidion annosum) as an experimental model, the host-pathogen interaction was investigated. Conifer trees have established a variety of defense mechanisms to repel microbial infections, including constitutive barriers, accumulation of antimicrobial chemicals, activation of signaling pathways and induction of defense-related compounds resulted from an intra-organismic response including cell death. In the conifer pathosystem, cell death associated necrotic browning reactions promote tissue colonization by necrotrophic pathogens such as Heterobasidion annosum. By contrast, in crop plants, cell death associated hypersensitive response (HR) is known to inhibit invasive growth of biotrophic pathogens. However, not much is known about the chemical and molecular characteristics of necrotic cell death responses observed in different developmental stages of conifers from seedlings to mature trees. In addition, the term reaction zone is often used to describe responses of tree tissues bordering the heartwood to pathogenic infections, but nothing is known on whether there are any similarities between the necrosis response and reaction zone. To investigate this, Norway spruce at different developmental stages (seedlings, young trees and mature trees) were challenged with Heterobasidion parviporum. Six major indicators were assayed: necrosis lesions, cell death, pH, reaction zone, reactive oxygen species (ROS) and expressions of defense-related genes (Per1, Per2, LAC, JAZ1, OPR, and STS2). Increased transcript levels of the following genes Per1, Per2, LAC and JAZ1 were documented in seedlings but higher expression of Per2 and LAC was recorded in mature trees. Results revealed that all infected tissues were strongly necrotic or had increased lesion size. This is accompanied by increasing pH units up to 6.0 similar to levels documented in the reaction zone. Furthermore, ROS-generating peroxidase activity increased in infected tissues compared to the control. All these changes were found to be similar to observations in the reaction zone of woody trees. However, cell death was only measured in seedling roots and the method needs to be optimized for woody tissues. Further tests are therefore required for the clarification of relationship between necrotic cell death in seedling plants and reaction zone responses in mature woody tissues