Submitted to: Canadian Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/1998
Publication Date: N/A
Citation: Interpretive Summary: Aspen is the predominant forest tree in the Great Lakes States. Hypoxylon canker is the most important fungus disease affecting aspen, causing death of 1 to 2% of these trees each year. Isolation of resistant varieties and understanding the biochemical mechanisms leading to resistance are crucial to maintaining aspen production. In this study, a resistant line of aspen was identified and biochemical mechanisms leading to resistance were evaluated. Resistant and susceptible trees were wounded and then either inoculated with Hypoxylon fungus or water. The tissue at the inoculation and/or wound site was evaluated with light microscope for changes in cell growth, alterations in cell wall chemistry, and rapidity of cell wall deposition. The resistant genotype differed from the susceptible in rapidity of response and in cell wall chemistry. The resistant plants showed increased cell division near the inoculation site and had more plant tdefense compounds (phenolics) deposited in the cell walls surrounding the fungus than did the susceptible plants. These data show that resistance to Hypoxylon canker disease is related to the rapidity of the response and the composition of the newly formed cell walls. Efficiency in breeding for resistance to Hypoxylon disease in aspen can be improved by selecting plants that rapidly produce a chemical barrier to fungal growth.
Technical Abstract: Internodal stem tissue of resistant and susceptible genotypes of Populus tremuloides wounded or wound-inoculated with Entoleuca mammata was prepared for histochemical staining, UV illumination, and microspectrophotometric analysis. Stem samples were collected throughout a 96 h period. Results show that parenchyma cell walls associated with the response zone of E. mammata-infected resistant and susceptible genotypes accumulated phenolic substances having lignin-like properties. Developmental features of the lignified response zone distinguished resistant from susceptible genotypes. The response zone of the resistant genotype developed adjacent to the wound margin and was discontinuous. Development of wound callus was another feature of the resistant E. mammata-infected genotype not observed in the susceptible genotype. In the resistant genotype, wound callus developed immediately internal to the lignified response zone, contained phenolic substances, and was visible 48 h after inoculation. In contrast, wound callus in the susceptible genotype failed to develop. Wounded tissue of both resistant and susceptible genotypes displayed no distinguishing response characteristics. Both produced equivalent amounts of wound callus, accumulated similar levels of the lignin-like substance, and deposited it in identical locations. It is concluded that in the presence of E. mammata the resistant genotype is capable of developing two distinct barriers that restrict infection by E. mammata, an intact lignified barrier and wound callus rich in phenolic substances. These two defense mechanisms seal the wound site and create a barrier not easily penetrable by the pathogen. Contrarily, the susceptible genotype is ineffective at developing either of these barriers, thereby leaving the area prone to infection.