# Distinctive Traits of European Mistletoe (Viscum album spp. austriacum) and Its Impact on Host Tree Wood (Pinus sylvestris)

## TL;DR

European mistletoe infests pine trees, causing defense responses and anatomical changes that may help the parasite survive.

## Contribution

The study reveals new anatomical features of mistletoe and pine defense mechanisms against parasitism.

## Key findings

- Mistletoe infestation causes callose deposition in pine tracheids, limiting water flow.
- Mistletoe has structural dimorphism with distinct inner and outer systems containing different tissues.
- Starch and chlorophyll in mistletoe parenchyma cells may support internal photosynthesis and water uptake.

## Abstract

European mistletoe is a hemi-parasitic plant increasingly infesting forests in Central Europe, causing premature tree death, and is anticipated to expand its range due to global warming. This study aimed to describe the unique anatomical features of mistletoe and examine the morpho-anatomical response of pine trees to infestation. Anatomical analyses were conducted on mistletoe internodes and the branch wood of affected pines. The findings revealed that mistletoe infestation triggers callose deposition in the cell walls of pine tracheids, a defense mechanism that restricts water flow to the mistletoe. Unique structural features of mistletoe were also identified, including structural dimorphism with the inner system forming only vessels and parenchyma cells, in contrast to the outer system, composed of protective, ground, and conductive tissues, and which displays an uneven distribution of chlorophyll and starch grains along the plant axis. Additionally, starch and chlorophyll were present in the parenchyma cells of the haustorium. Starch presence there may potentially enable internal photosynthesis, and the compounds formed after starch hydrolysis may facilitate water uptake from the host’s xylem sap. These results provide new insights into the anatomical adaptations of mistletoe and the defensive responses of pine trees, contributing to a deeper understanding of host–parasite interactions in forest ecosystems.