Starting strong: development and biomechanics of the seedling–host interaction in European mistletoe (<i>Viscum album</i>)

Attachment to a substrate is fundamental for plant growth and development. This is especially true for species that live either partially or fully off the ground, such as mistletoes, which have developed unique adaptations to anchor themselves securely to host trees from which they draw water and some nutrients. While the mechanical properties of attachment during the adult stages in many plant species have been described, the mechanical principles of the initial developmental stages are rarely investigated. Here, we focus on the parasitic European mistletoe (Viscum album L.) and its attachment to a host plant at the seedling stage. Using a combination of germination experiments, microtomography, histological analysis, and biomechanical tests, this work investigates the role of the three key attachment structures involved in this process: the seed coat, hypocotyl, and holdfast. The viscin layer, a sticky coating on the seed, provides initial adhesion before the growing hypocotyl expands towards the host surface, where it flattens and forms a holdfast that strengthens adhesion and aids tissue penetration. Tensile tests revealed that these three attachment structures withstand similar forces in the early stages, considerably higher than the weight of the seedling. Within a few months, the endophytic system interlocked with the host bark, forming a robust connection that not only transports water but also increased the mechanical strength of the structure. This work highlights the fundamental mechanisms of the initial mistletoe-host interaction, which forms the basis of their decades-long relationship.