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

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.

Keywords: callose; forest dieback; parasite–host interaction; structural dimorphism; wood.

Figure 1

The scheme of the mistletoe body plan consisted of an external system (marked with a bracket) and an internal system with a haustorium (black arrow), sinkers (white arrowhead), and cortical strands (asterisk). Macroscopic inset view of a mistletoe leaf with parallel veins indicated by a black arrowhead.

Figure 2

Cross-sections of mistletoe internode of shoot growing against the force of gravity. (a,b) Study samples were taken in the winter season of 2022 and (c) in the summer season of 2022. The sections were stained with FCA. Visible is the thick cuticle covering epidermal cells (black arrows), sclereids (white arrowhead), and fibers (white arrows) in the cortical tissue. The anticlinal division in the epidermal cell is highlighted with a black arrowhead. A bracket marks one of the vascular bundles.

Figure 3

SEM. The internode surface of (a) one and (b) six years of age with epidermal cells arranged in longitudinal files and outlined paracytic stomata. White arrows indicate the axial direction and black arrows the transverse direction. The cell dimensions were measured for 10 randomly selected pavement cells. Cross-sectional surface of (c) one-year- and (d) two-year-old internodes. The vascular bundle cap is indicated by arrowheads.

Figure 4

Cross-sections of the internodes of mistletoe aged 1 to 5 years. The shoot grew against the force of gravity. Red autofluorescence of asymmetrically distributed chlorophyll in cortex parenchyma cells is visible. (a–e) Secondary growth is based on the activity of the vascular cambium; (f) epidermis with thick cuticle covering 1-year-old and (g) 5-year-old internodes. Samples were taken in the summer of 2022. The sections were stained with aniline blue and observed under UV light.

Figure 5

Cross-sections of the internodes of mistletoe. The shoot grew upward against gravity (a–c). The shoot grew in the direction of the force of gravity (d–f). The starch grains distributed asymmetrically (purple-black color) are marked with arrows in the internodes, from (a,d) the youngest internode to (c,f) the oldest internode. The black arrowhead indicates a circumferentially stretched epidermal cell. Samples taken in summer 2022. Sections stained with Lugol’s reagent.

Figure 6

Cross-sections of infested tissues of a pine tree. (a) Host branch tissues infested with mistletoe—general view; (b) cross-section through a cortical strand, visible as a circular structure; (c) wood of branch with haustorium composed of vessels (white arrow) and parenchyma cells (asterisk), with chlorophyll showing red autofluorescence under UV light; (d) fragment of haustorium embedded in branch wood showing parenchyma cells with starch grains (black arrow) and vessels (white arrow); (e,g) white arrowheads mark host compression-like tracheids with yellowish green callose fluorescence in the cell wall; and (f,g) mistletoe vessels with spiral thickening of their cell walls placed along the edge of the haustorium, marked with white arrows. Samples taken in summer 2022. Sections stained with aniline blue and observed under UV light (a,c,e–g). (b) FCA and (d) Lugol’s reagent staining were also used.