Propagation through seed or somatic embryogenesis: comparing the effects of two methods and methyl jasmonate treatment on Norway spruce resistance to Heterobasidion infection

The clonal propagation method somatic embryogenesis (SE) has been shown to affect plant resistance to an insect pest. In a field trial, Norway spruce SE-plants (emblings) experienced less damage by the bark-chewing pine weevil compared to seedlings propagated through seeds from the same mother tree. Moreover, if emblings were treated with the defense-inducing hormone methyl jasmonate (MeJA), their resistance became much greater compared to the effects of SE and MeJA alone. Thus, we evaluated in a full factorial design if propagation method (seed or SE) and MeJA treatment (treated or not) can affect Norway spruce resistance to infection by the fungal pathogen Heterobasidion parviporum. Emblings and seedlings were half-sibs originating from four different Norway spruce families. We found that emblings and seedlings exhibited similar fungal lesion lengths (LL) and sapwood growth (SWG) in a constitutive state (no MeJA). Once treated, seedlings exhibited a 26% and 28% reduction in LL and SWG growth respectively, in line with previous studies. For emblings, it was the opposite. MeJA increased LL and SWG by 41% and 16% respectively. This is unexpected given the genetic relatedness of the material, and the previously documented effects of SE. It is possible that SE is causing changes in Norway spruce resistance that are effective against insects, but not pathogens. Also, MeJA treatment may be affecting embling terpene composition in ways that benefit fungal growth, as suggested from separate experiments. Further studies are needed to uncover the mechanisms behind the increased susceptibility of Norway spruce emblings to fungal infection.