|Jung, Hans Joachim|
Submitted to: Canadian Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/1998
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. Enzymes (protein catalysts) involved in creating lignin, a woody barrier to disease entry, were examined. The amount and type of lignin were also evaluated. Upon inoculation activities for enzymes involved in lignin biosynthesis increased much sooner and to a greater extent in the resistant genotype as compared to the susceptible. Enzyme activities in the resistant genotype remained high throughout the experiment while those of the susceptible did not. Although the total amount of lignin deposited at the inoculation site was similar between the two genotypes, lignin composition in the resistant plants was very different than that in the susceptible plants. These data show that resistance to Hypoxylon canker disease is related to the rapidity of lignin deposition and to the structure of the lignin formed. Efficiency of breeding for resistance to Hypoxylon disease in aspen can be improved by selection of plants that rapidly produce lignin and provide a barrier to fungal growth.
Technical Abstract: Green internodal stem tissues of glasshouse grown Populus tremuloides were either wounded or wound-inoculated with Entoleuca mammata (Hypoxylon mammatum) and assayed for phenylalanine ammonia-lyase (PAL), caffeic acid- O-methyltransferase (CA-OMT) and cinnamyl-alcohol dehydrogenase (CAD) activity over a 96 hr period. Lignin deposited in response to the treatments was analyzed by the Klason and the pyrolysis-GC-MS methodologies. The wound-inoculated treatment resulted in a wound morphology congruent with a typical resistant and susceptible response to E. mammata. Wounding alone resulted in no morphological differences between the two genotypes. In wound-inoculated stem tissue PAL and CAD activity were substantially higher in the resistant relative to the susceptible genotype. Total Klason lignin was similar for both genotypes, however, pyrolysis-GC-MS analysis revealed a difference in the lignin monomeric composition between the two genotypes with the susceptible genotype accumulating higher levels of hydroxyphenyl units relative to the resistant genotype. It is concluded that differences in PAL and CAD activity and the synthesis of distinct phenylpropanoid monomers distinguishes the resistant from the susceptible aspen genotype. Alterations in boundary zone formation due to the differential synthesis of phenylpropanoid monomers and its effect on compartmentalization of the pathogen is discussed.