Mistletoe generates non‐trophic and trait‐mediated indirect interactions through a shared host of herbivore consumers

The associational effects of host plant and mistletoe functional traits on leaf herbivory in mistletoe

Associational effects are a phenomenon in which herbivore damage on co-occurring plant species is influenced by neighboring plants. Mistletoes are a group of shrubs that obtain nutrients from host plants through haustoria. Despite the potential for mistletoe herbivory to be affected by associational effects with their hosts, the effects of host and mistletoe functional traits on mistletoe herbivory have been largely overlooked. This study aimed to evaluate the associational effects of host plants and the direct effects of mistletoe functional traits on mistletoe herbivory. To achieve this, we measured leaf herbivory and leaf traits of three mistletoe species (Dendrophthoe pentandra, Scurrula chingii var. yunnanensis, and Helixanthera parasitica) and their associated 11 host species during both dry and wet seasons. Our results showed that leaf herbivory of D. pentandra and S. chingii var. yunnanensis differed significantly on their respective host species, but H. parasitica did not. The relationships between mistletoe and the paired host herbivory differed between seasons, with a stronger positive relationship observed during the dry season. Furthermore, significant relationships were observed between paired leaf carbon, leaf nitrogen, and condensed tannin in mistletoes and their host plants, indicating that host plants can affect mistletoes' leaf functional traits. A group of mistletoe leaf traits provided significant predictions for leaf herbivory: leaves with higher leaf thickness and leaf total nitrogen showed higher herbivory. Overall, our study reveals that mistletoe leaf herbivory is directly affected by its leaf traits and indirectly affected by host associational effects, primarily through changes in mistletoes' leaf traits.

Mitigating the Mistletoe Menace: Biotechnological and Smart Management Approaches

Mistletoes have been considered a keystone resource for biodiversity, as well as a remarkable source of medicinal attributes that attract pharmacologists. Due to their hemiparasitic nature, mistletoes leach water and nutrients, including primary and secondary metabolites, through the vascular systems of their plant hosts, primarily trees. As a result of intense mistletoe infection, the hosts suffer various growth and physiological detriments, which often lead to tree mortality. Because of their easy dispersal and widespread tropism, mistletoes have become serious pests for commercial fruit and timber plantations. A variety of physical and chemical treatment methods, along with silvicultural practices, have shaped conventional mistletoe management. Others, however, have either failed to circumvent the growing range and tropism of these parasitic plants or present significant environmental and public health risks. A biocontrol approach that could sidestep these issues has never achieved full proof of concept in real-field applications. Our review discusses the downsides of conventional mistletoe control techniques and explores the possibilities of biotechnological approaches using biocontrol agents and transgenic technologies. It is possible that smart management options will pave the way for technologically advanced solutions to mitigate mistletoes that are yet to be exploited.

Implications of mistletoe parasitism for the host metabolome: A new plant identity in the forest canopy

Mistletoe-host systems exemplify an intimate and chronic relationship where mistletoes represent protracted stress for hosts, causing long-lasting impact. Although host changes in morphological and reproductive traits due to parasitism are well known, shifts in their physiological system, altering metabolite concentrations, are less known due to the difficulty of quantification. Here, we use ecometabolomic techniques in the plant-plant interaction, comparing the complete metabolome of the leaves from mistletoe (Viscum album) and needles from their host (Pinus nigra), both parasitized and unparasitized, to elucidate host responses to plant parasitism. Our results show that mistletoe acquires metabolites basically from the primary metabolism of its host and synthesizes its own defence compounds. In response to mistletoe parasitism, pines modify a quarter of their metabolome over the year, making the pine canopy metabolome more homogeneous by reducing the seasonal shifts in top-down stratification. Overall, host pines increase antioxidant metabolites, suggesting oxidative stress, and also increase part of the metabolites required by mistletoe, which act as a permanent sink of host resources. In conclusion, by exerting biotic stress and thereby causing permanent systemic change, mistletoe parasitism generates a new host-plant metabolic identity available in forest canopy, which could have notable ecological consequences in the forest ecosystem.