Insights Into the Mechanisms Implicated in Pinus pinaster Resistance to Pinewood Nematode

Abstract

Pine wilt disease (PWD), caused by the plant–parasitic nematode Bursaphelenchus xylophilus, has become a severe environmental problem in the Iberian Peninsula with devastating effects in Pinus pinaster forests. Despite the high levels of this species' susceptibility, previous studies reported heritable resistance in P. pinaster trees. Understanding the basis of this resistance can be of extreme relevance for future programs aiming at reducing the disease impact on P. pinaster forests. In this study, we highlighted the mechanisms possibly involved in P. pinaster resistance to PWD, by comparing the transcriptional changes between resistant and susceptible plants after infection. Our analysis revealed a higher number of differentially expressed genes (DEGs) in resistant plants (1,916) when compared with susceptible plants (1,226). Resistance to PWN is mediated by the induction of the jasmonic acid (JA) defense pathway, secondary metabolism pathways, lignin synthesis, oxidative stress response genes, and resistance genes. Quantification of the acetyl bromide-soluble lignin confirmed a significant increase of cell wall lignification of stem tissues around the inoculation zone in resistant plants. In addition to less lignified cell walls, susceptibility to the pine wood nematode seems associated with the activation of the salicylic acid (SA) defense pathway at 72 hpi, as revealed by the higher SA levels in the tissues of susceptible plants. Cell wall reinforcement and hormone signaling mechanisms seem therefore essential for a resistance response.

Primary Metabolite Adjustments Associated With Pinewood Nematode Resistance in Pinus pinaster

“…The observed PWN distribution in compartmentalized areas in resistant plants is likely related with highly lignified cell walls surrounding cambial cavities and resin ducts ( Kusumoto et al, 2014 ). In agreement, gene expression analysis, using the same P. pinaster half-sib family, revealed an over-expression of genes involved in lignin synthesis in resistant plants 72 h after infection, which translated to an increase in lignin content and cell wall lignification in stem tissues around the inoculation zone in these plants ( Modesto et al, 2021 ). This modification hinders the PWN horizontal migration and delays damage expansion in the cortex and xylem.…”

“…In trees, a large amount of the fixed carbon is channeled through phenylalanine for the activation of the shikimate pathway and the biosynthesis of phenylpropanoids, including flavonoids, stilbenes, coumarins, SA, tannins, and lignin (Pascual et al, 2016;Abreu et al, 2020). In contrast, studies on P. pinaster responses in the first hours after PWN infection showed an over-expression of genes involved in phenylpropanoid biosynthesis, including defenserelated flavonoid and lignin biosynthesis (Modesto et al, 2021). The observed decrease in phenylalanine at an advanced stage of PWN infection suggests a decrease in the production of these defense-related secondary metabolites in susceptible plants, and a consequent lack of enhanced cell wall lignification as an inducible physical defense strategy.…”

“…Despite the numerous studies on PWN and PWD, the current knowledge on the molecular mechanisms underlying tree resistance to PWD is still very limited. In addition, only few studies have focused on the comparison of transcriptional responses between resistant and susceptible plants, within susceptible species, namely in P. thunbergii ( Hirao et al, 2012 ), P. massoniana ( Liu et al, 2017 ) and more recently P. pinaster ( Modesto et al, 2021 ). These studies showed that resistance and susceptibility to PWD do not rely only on qualitative differences in gene expression, but also on variations in the timing and magnitude of their expression.…”

“…after PWN infection has revealed differentially expressed genes mostly related to secondary metabolism (e.g., phytohormone, phenylpropanoid, and terpenoid biosynthesis), ROS detoxification, and also to primary metabolism, namely carbohydrate metabolism ( Gaspar et al, 2017 , 2020 ; Liu et al, 2017 ; Lee et al, 2018 , 2019 ; Modesto et al, 2021 ). Alterations in secondary metabolism in response PWN infection include an increase in defense-related phytohormones involved in jasmonic acid (JA) and salicylic acid (SA) pathways in PWN-susceptible P. pinaster , but not in resistant phenotypes ( Modesto et al, 2021 ; Rodrigues et al, 2021 ). On the other hand, transcriptomic profiling of P. pinaster plants revealed an enrichment in amino sugar and nucleotide sugar metabolic pathways only in resistant plants ( Modesto et al, 2021 ).…”

“…Alterations in secondary metabolism in response PWN infection include an increase in defense-related phytohormones involved in jasmonic acid (JA) and salicylic acid (SA) pathways in PWN-susceptible P. pinaster , but not in resistant phenotypes ( Modesto et al, 2021 ; Rodrigues et al, 2021 ). On the other hand, transcriptomic profiling of P. pinaster plants revealed an enrichment in amino sugar and nucleotide sugar metabolic pathways only in resistant plants ( Modesto et al, 2021 ). The regulation of primary metabolism in biotic stress responses has gained increasing interest, mainly due to its key role in fulfilling the energetic demands for plant defense mechanisms and as a source of signaling molecules ( Berger et al, 2007 ; Bolton, 2009 ; Steinbrenner et al, 2011 ; Rojas et al, 2014 ).…”