The role of Manganese in tree defenses against pests and pathogens

Manganese (Mn) deficiency is a widespread occurrence across different landscapes, including agricultural systems and managed forests, and causes interruptions in the normal metabolic functioning of plants. The microelement is well-characterized for its role in the oxygen-evolving complex in photosystem II and maintenance of photosynthetic structures. Mn is also required for a variety of enzymatic reactions in secondary metabolism, which play a crucial role in defense strategies for trees. Despite the strong relationship between Mn availability and the biosynthesis of defense-related compounds, there are few studies addressing how Mn deficiency can impact tree defense mechanisms and the ensuing ecological patterns and processes. Understanding this relationship and highlighting the potentially deleterious effects of Mn deficiency in trees can also inform silvicultural and management decisions to build more robust forests. In this review, we address this relationship, focusing on forest trees. We describe Mn availability in forest soils, characterize the known impacts of Mn deficiency in plant susceptibility, and discuss the relationship between Mn and defense-related compounds by secondary metabolite class. In our review, we find several lines of evidence that low Mn availability is linked with lowered or altered secondary metabolite activity. Additionally, we compile documented instances where Mn limitation has altered the defense capabilities of the host plant and propose potential ecological repercussions when studies are not available. Ultimately, this review aims to highlight the importance of untangling the effects of Mn limitation on the ecophysiology of plants, with a focus on forest trees in both managed and natural stands.