Comparative analysis of membrane-associated signatures at mutualistic and pathogenic plant-microbe interfaces

Abstract

Plants interact with a diverse range of microbes, both above and below ground, spanning a spectrum from mutualistic relationships (where both partners benefit) to parasitic interactions (where the microbe benefits at the plant’s expense). Many interactions between plants and filamentous microbes involve the formation of specialised intracellular structures within living plant cells. These intracellular structures include the highly branched arbuscules formed by mutualistic arbuscular mycorrhizal (AM) fungi and the digit-like haustoria formed by the pathogenic oomycete Phytophthora palmivora. Both haustoria and arbuscules are surrounded by specialised plant interface membranes. My thesis identifies protein and lipid signatures of these interface membranes and explores their roles in interface establishment, maintenance, and the outcomes of plant-microbe interactions. I selected eight candidate genes in Nicotiana benthamiana that were transcriptionally upregulated during both AM colonisation and P. palmivora infection, two of which, SEC and Synaptotagmin 2A (SYT2A), encode proteins which localise around haustoria. My characterisation of upstream regulatory elements suggests that the inducible expression of SYT2A during AM colonisation and P. palmivora infection is controlled by different DNA regions. Functional characterisation of SYT2A through constitutive expression and CRISPR-Cas9 mediated gene inactivation demonstrated a role for SYT2A in P. palmivora leaf infection. This work also investigates the spatial distribution of phosphoinositides in N. benthamiana (endo)membranes using genetically encoded biosensors. I found that PI(4,5)P2 localises to the actively growing tips of intracellular AM fungal structures. I also showed that while PI4P is present around intracellular AM fungal structures and around invasive P. palmivora hyphae, it is excluded from around haustoria. However, PI4P localises around haustoria in AM-colonised roots, suggesting dynamic changes in membrane identity depending on the microbes present. This study underscores the interlinked nature of membrane identity, shaped by both lipid and protein content, each capable of modifying the other while integrating external signals, such as mechanical properties and effector-modulated interference from the microbe. A deeper understanding of interface membrane identity and functionality will be crucial for engineering plant-microbe interactions to enhance crop productivity and disease resistance.