As the world population increases to an estimated 9.7 billion by 2050 (United Nations, 2019), meeting the global demand for food crops whilst reducing reliance on environmentally damaging chemical inputs will be a huge challenge. A sustainable agricultural solution for increased crop productivity includes use of natural soil microbes as ‘biofertilizers’ to deliver essential nutrients to plants.
Arbuscular mycorrhizal (AM) fungi are ubiquitous and ancient soil biofertilizers that colonise most land plants, including crops. Plants benefit nutritionally from AM symbiosis by acquiring essential soil minerals, such as inaccessible Inorganic Phosphate, from a ramified fungal mycelial network that extend far beyond nutrient depletion zones. In return, as obligate biotrophs, AM fungi rely entirely on the plant for organic carbon, which it needs to complete its lifecycle.
A central feature of this mutualistic interaction are fascinating, highly branched fungal feeding structures, arbuscules, that form deep within roots. Arbuscules remain surrounded by a specialized host-derived membrane, the Peri-Arbuscular Membrane, that generates a massive membrane surface area and shared symbiotic interface for nutrient and signal exchange. Arbuscules are surprisingly short-lived (2-3 days) which raises the question of how plant-fungal dialogue is synchronised throughout the dynamic arbuscule lifespan. Moreover, molecular mechanisms that underlie symbiotic crosstalk in arbuscule-containing cells are not fully understood and is the main focus of my lab.
Investigating the role of extracellular vesicles during AM symbiosis
Using innovative imaging approaches and 3D reconstruction my lab builds on our discovery that tiny membrane-enclosed extracellular vesicles (EVs) accumulate in the interstitial apoplastic space separating plant and fungal arbuscule membranes (Roth et al, 2019). This is an exciting finding, as in animals, EVs transport diverse signalling cargoes (including proteins, mRNA, small interfering RNAs, lipids and metabolites) between cells, consistent with their role in inter-cellular communication. Moreover, we postulate that during AM symbiosis EVs courier diverse cargoes across the symbiotic interface to modulate the outcome of the interaction.
In my lab we use a combination of cutting-edge deep tissue imaging and TEM tomography, molecular genetics, biochemical, ‘omics’ and bioinformatic approaches to address the following questions:
- What cargoes are couriered by EVs?
- What is the function of EV cargoes in AM symbiosis?
- How do EV cargoes change over the dynamic lifespan of arbuscules?
Investigating the role of small interfering RNA signals during AM symbiosis
In a parallel genetic approach, my lab recently discovered a class of small interfering RNAs (siRNAs) that modulate AM symbiosis (Roth, unpublished). The identity of siRNAs involved, whether they function endogenously, as systemic signals or are couriered from plant to fungus via extracellular vesicles are currently being investigated in my lab.