The effect of mycorrhizal cheaters on plant:fungal symbiosis
In temperate and boreal forests, nitrogen is a key limiting factor, as dominant trees produce leaf litter that is slow to decompose, causing nitrogen to remain locked up in organic complexes. Thus, dominant trees in these forests heavily depend on ectomycorrhizal (ECM) fungi, which have a specialised ability to break down organic matter and release nitrogen. These fungi colonise the roots of the trees and form a symbiotic relationship, whereby the fungi provide the trees with nitrogen and the tree transports carbon sugars to the fungi in return.
Generally, ECM fungi cooperate in this relationship. However, recent molecular work on the Pisolithus microcarpus-Eucalyptus grandis system has shown that some fungi “cheat”, by providing less nitrogen than others while still receiving as much or more carbon. It is not known whether cheating is “inducible” (i.e. caused by environmental factors) or “constitutive” (i.e. genetically determined), nor what effect cheating has on host trees.
My aims are to determine the effects of various environmental factors on fungal cooperation, to identify any common genomic traits of cheater isolates, and to compare the effects of colonisation by cheater and cooperative fungi on host plant health. This will be the first known project focussing on the genetic mechanisms behind ECM fungal cheating: while there is research emerging on nutrient exchange mechanisms within ECM symbioses, the concept of cheating has not been paid much attention. The effects of cheating on plant health are also unknown as nutrient exchange and host fitness outcomes are rarely looked at together, and most studies on host fitness outcomes look at changes in community composition, instead of changes in the activities of individual isolates.
My project will allow for a better understanding of what causes ectomycorrhizal fungi to provide more or less nitrogen to their hosts, start to reveal why cheating has persisted through evolution, and finally, contribute to the understanding of how ECM symbioses affect plant health and carbon sequestration in forest ecosystems.
Dr Jonathan Plett, Professor Ian Anderson, Dr Krista Plett