Histological, chemical and gene expression differences between western redcedar seedlings resistant and susceptible to cedar leaf blight.

Introduction: Western redcedar () is an important species in the Cupressaceae both at economic and cultural levels in the Pacific Northwest of North America. In adult trees, the species produces one of the most weathering-resistant heartwoods among conifers, making it one of the preferred species for outdoor applications. However, young plants are susceptible to infection with cedar leaf blight (), an important foliar pathogen that can be devastating in nurseries and small-spaced plantations. Despite that, variability in the resistance against in has been documented, and such variability can be used to breed for resistance against the pathogen.

Objective: This investigation aimed to discern the phenotypic and gene expression differences between resistant and susceptible seedlings to shed light on the potential constitutive resistance mechanisms against cedar leaf blight in western redcedar.

Methods: The study consisted of two parts. First, the histological differences between four resistant and four susceptible families that were never infected with the pathogen were investigated. And second, the differences between one resistant and one susceptible family that were infected and not infected with the pathogen were analyzed at the chemical (C, N, mineral nutrients, lignin, fiber, starch, and terpenes) and gene expression (RNA-Seq) levels.

Results: The histological part showed that seedlings resistant to had constitutively thicker cuticles and lower stomatal densities than susceptible plants. The chemical analyses revealed that, regardless of their infection status, resistant plants had higher foliar concentrations of sabinene and α-thujene, and higher levels of expression of transcripts that code for leucine-rich repeat receptor-like protein kinases and for bark storage proteins.

Conclusion: The data collected in this study shows that constitutive differences at the phenotypic (histological and chemical) and gene expression level exist between seedlings susceptible and resistant to . Such differences have potential use for marker-assisted selection and breeding for resistance against cedar leaf blight in western redcedar in the future.