New findings on resistance to root rot can be used in spruce breeding

Markers for resistance to more than one species of Heterobasidion

In his recently published doctoral dissertation, he shows that there is large variation in the levels of resistance to both H. parviporum and H. annosum s.s. among Norway spruces trees in the Swedish breeding program and that the resistance has a high heritability, ie the Norway spruce genotype has a large impact on how resistant it is to root rot. However, the resistance to different species of the fungus is not correlated. 

– We found that the offspring from a specific tree could have very good resistance to one species of Heterobasidion but not to the other. We also saw that the environment the spruces grew in affects how much the measured resistance varied, says Hernán. 

Hernán and his co-authors then analyzed whether they could identify new genetic markers in Norway spruce that could be used to find trees that have good resistance to H. parviporum and H. annosum s.s. or against both fungi simultaneously.

Genes that help slow down the infecting fungus

Geneticists and breeders call genetic markers associated with more than one trait pleiotrophic markers. The researchers found 20 pleiotrophic markers in this study, most of them were positive pleiotrophic markers, i.e., the same marker was associated with better resistance to both fungal species at the same time. 

– An exciting positive pleiotrophic marker is the laccase PaLAC5, the gene is active in the bark right next to the infection with Heterobasidion. Together with results of other researchers, we know that PaLAC5 is activated during stress, and we think it helps to produce defence substances and lignin in the tissue that slow down the infecting fungus, says Hernán.

Spruces in Europe and North America have common markers of resistance

In his third paper, Hernán studied the known resistance marker PaLAR3, which controls how fast H. parviporum can spread in Norway spruce wood. Hernán compared the DNA sequence of PaLAR3 in Norway spruce with the DNA sequences of North American spruce species to understand how it changed during evolution. In Norway spruce, the variation in is likely to have been maintained through what is known as balancing selection. The fact that a gene is under balancing selection means that it has relatively few variants with relatively even (balanced) frequencies, which are also stable for many generations. 

– Our results indicate that PaLAR3 was under balancing selection before Norway spruce and the North American white spruce separated from each other and became different species, says Hernán.

Robust gene expression patterns in trees with higher resistance to Heterobasidion

Another method that Hernán has used in his Ph. D. work is gene expression studies. In these studies, he and his colleagues examined which genes are active in Norway spruce and H. annosum s.s.  before and after infection, respectively, according to the hypothesis that genes that control resistance or infectivity are turned on or off when the fungus infects spruce tissues. The study used Norway spruces with different levels of resistance and H. annosum s.s. individuals with different virulence. By virulence, researchers mean how far fungal individuals can spread in the tree and how many disease symptoms they cause. 

The experiment showed that Norway spruce individuals with relatively high resistance to Heterobasidion reacted in a robust manner. Regardless of which fungal individual they were infected with, approximately the same group of genes were active after the infection.

– Among the active genes, there were several genes involved in recognition of Pathogenic microorganisms. In contrast, the gene expression in the more susceptible Norway spruce individual was more variable. The activity of genes in the susceptible individual depended on the severity of the disease symptoms caused by the fungus, concluced Hernán.

Written by Malin Elfstrand