Chemical defence responses of Norway spruce to two fungal pathogens

Abstract

Constitutive and inducible terpene production is involved in conifer resistance against insects and fungal infestations. To gain knowledge about local defence responses of Norway spruce bark against pathogens and to find potential chemical markers for resistance breeding, we inoculated the stem of 8‐year‐old Norway spruce (Picea abies) clonal trees with both Endoconidiophora polonica (Ep, a common fungal pathogen associated with the spruce bark beetle Ips typographus) and Heterobasidion parviporum (Hp, a severe pathogen causing root and stem rot disease). Three weeks after inoculation, the fungal‐inoculated and intact bark from each tree was sampled. The terpenes in tree bark were extracted with hexane and characterized by gas chromatography–mass spectrometry (GC‐MS). The two fungi induced varied terpene responses in the four spruce clones used. Three of the clones showed a 2.3‐fold to 5.7‐fold stronger terpene response to Hp relative to Ep inoculation, while one clone responded similarly to inoculation with the two fungal pathogens. The amount of the diterpenes thunbergol and geranyllinalool varied between the clones. The level of thunbergol was higher in both intact and fungal‐inoculated bark from the less susceptible clones compared with the more susceptible clones. Geranyllinalool was present in higher amounts in the susceptible clones and is thus a possible marker for susceptibility. Our observations show that Norway spruce employs a similar chemical mechanism against the two fungal pathogens. Based on the present and earlier published data, we suggest that certain Norway spruce genotypes have a strong defence reaction against these two pathogens. The diterpenes thunbergol and geranyllinalool might be useful markers of susceptibility in tree‐breeding programmes and should be the focus of further detailed investigations.

Variation in Methyl Jasmonate-Induced Defense Among Norway Spruce Clones and Trade-Offs in Resistance Against a Fungal and an Insect Pest

“…Compounds such as (+)-3-carene and geranyllinalool varied more strongly among clones (across the first axis of variation) when MeJA treatment and fungal infection co-occurred. This is consistent with previous studies which have found that (+)-3-carene is often induced following fungal infection ( Croteau et al, 1987 ; Fäldt et al, 2006 ; Zhao et al, 2010 ), and that both compounds appear to play a role in differential susceptibility of Norway spruce to E. polonica and Heterobasidion parviporum infection ( Danielsson et al, 2011 ; Axelsson et al, 2020 ). Moreover, (+)-3-carene has also been associated with conifer resistance against insects.…”

“…This in contrast to the study by López-Goldar et al (2018) where MeJA treatment explained 31% of the variation in defensive compounds, while family explained 2%, in P. pinaster . Yet, it is in line with other studies in which variation in defensive chemistry has been to a large extent explained by among family (or genotype/clone) differences (e.g., Havill and Raffa, 1999 ; Ott et al, 2011 ; Axelsson et al, 2020 ). Our results from terpene chemistry analyses are consistent with our results on insect damage and fungal lesion lengths, for which we also found that differences among clones had a strong effect on the response variables ( Table 1 ).…”

“…The strain was isolated from a Norway spruce log inoculated with the bark beetle Polygraphus poligraphus L. ( Krokene and Solheim, 1996 ). This E. polonica isolate has been used in many of our previous studies (e.g., Zhao et al, 2010 , 2011 ; Axelsson et al, 2020 ), it grows well on Norway spruce and has shown consistent virulence across our studies. The fungal strain was maintained on malt agar (2% malt, 1.5% agar) at 4°C, and transferred to fresh malt agar and cultivated at 25°C in darkness for 7-10 days, before the start of the experiment.…”

“…Further, to also be able to analyze the induced defense caused by fungus inoculation, phloem was collected from directly below the inoculation area. This method of comparing tissue from the “inoculation area” and a “control area” from the opposite side of the stem, is usually used in older trees with thicker trunks (e.g., Axelsson et al, 2020 ). Since we used thinner trees in our experiment, we cannot be completely sure that fungal infection does not affect the opposite side of the stem bark.…”

“…Samples of approx. 1.5 cm 2 in size were chopped into smaller pieces (following standard protocol as described in Persson et al, 1993 ; Axelsson et al, 2020 ), and each was immediately placed (using tweezers) in a 2-ml glass vial with solvent. Phloem samples were extracted in 0.5 mL n-hexane (VWR, Ref no.…”