VAPYRIN attenuates defence by repressing PR gene induction and localized lignin accumulation during arbuscular mycorrhizal symbiosis of Petunia hybrida

The intimate association of host and fungus in arbuscular mycorrhizal (AM) symbiosis can potentially trigger induction of host defence mechanisms against the fungus, implying that successful symbiosis requires suppression of defence. We addressed this phenomenon by using AM-defective vapyrin (vpy) mutants in Petunia hybrida, including a new allele (vpy-3) with a transposon insertion close to the ATG start codon. We explore whether abortion of fungal infection in vpy mutants is associated with the induction of defence markers, such as cell wall alterations, accumulation of reactive oxygen species (ROS), defence hormones and induction of pathogenesis-related (PR) genes. We show that vpy mutants exhibit a strong resistance against intracellular colonization, which is associated with the generation of cell wall appositions (papillae) with lignin impregnation at fungal entry sites, while no accumulation of defence hormones, ROS or callose was observed. Systematic analysis of PR gene expression revealed that several PR genes are induced in mycorrhizal roots of the wild-type, and even more in vpy plants. Some PR genes are induced exclusively in vpy mutants. Our results suggest that VPY is involved in avoiding or suppressing the induction of a cellular defence syndrome that involves localized lignin deposition and PR gene induction.

CERBERUS is critical for stabilization of VAPYRIN during rhizobial infection in Lotus japonicus

CERBERUS (also known as LIN) and VAPYRIN (VPY) are essential for infection of legumes by rhizobia and arbuscular mycorrhizal fungi (AMF). Medicago truncatula LIN (MtLIN) was reported to interact with MtVPY, but the significance of this interaction is unclear and the function of VPY in Lotus japonicus has not been studied. We demonstrate that CERBERUS has auto-ubiquitination activity in vitro and is localized within distinct motile puncta in L. japonicus root hairs and in Nicotiana benthamiana leaves. CERBERUS colocalized with the trans-Golgi network/early endosome markers. In L. japonicus, two VPY orthologs (LjVPY1 and LjVPY2) were identified. CERBERUS interacted with and colocalized with both LjVPY1 and LjVPY2. Co-expression of CERBERUS with LjVPY1 or LjVPY2 in N. benthamiana led to increased protein levels of LjVPY1 and LjVPY2, which accumulated as mobile punctate bodies in the cytoplasm. Conversely, LjVPY2 protein levels decreased in cerberus roots after rhizobial inoculation. Mutant analysis indicates that LjVPY1 and LjVPY2 are required for rhizobial infection and colonization by AMF. Our data suggest that CERBERUS stabilizes LjVPY1 and LjVPY2 within the trans-Golgi network/early endosome, where they might function to regulate endocytic trafficking and/or the formation or recycling of signaling complexes during rhizobial and AMF symbiosis.

VAPYRIN Marks an Endosomal Trafficking Compartment Involved in Arbuscular Mycorrhizal Symbiosis

Arbuscular mycorrhiza (AM) is a symbiosis between plants and AM fungi that requires the intracellular accommodation of the fungal partner in the host. For reciprocal nutrient exchange, AM fungi form intracellular arbuscules that are surrounded by the peri-arbuscular membrane. This membrane, together with the fungal plasma membrane, and the space in between, constitute the symbiotic interface, over which nutrients are exchanged. Intracellular establishment of AM fungi requires the VAPYRIN protein which is induced in colonized cells, and which localizes to numerous small mobile structures of unknown identity (Vapyrin-bodies). In order to characterize the identity and function of the Vapyrin-bodies we pursued a dual strategy. First, we co-expressed fluorescently tagged VAPYRIN with a range of subcellular marker proteins, and secondly, we employed biochemical tools to identify interacting partner proteins of VAPYRIN. As an important tool for the quantitative analysis of confocal microscopic data sets from co-expression of fluorescent proteins, we developed a semi-automated image analysis pipeline that allows for precise spatio-temporal quantification of protein co-localization and of the dynamics of organelle association from movies. Taken together, these experiments revealed that Vapyrin-bodies have an endosomal identity with trans-Golgi features, and that VAPYRIN interacts with a symbiotic R-SNARE of the VAMP721 family, that localizes to the same compartment.

How membranes shape plant symbioses: signaling and transport in nodulation and arbuscular mycorrhiza

As sessile organisms that cannot evade adverse environmental conditions, plants have evolved various adaptive strategies to cope with environmental stresses. One of the most successful adaptations is the formation of symbiotic associations with beneficial microbes. In these mutualistic interactions the partners exchange essential nutrients and improve their resistance to biotic and abiotic stresses. In arbuscular mycorrhiza (AM) and in root nodule symbiosis (RNS), AM fungi and rhizobia, respectively, penetrate roots and accommodate within the cells of the plant host. In these endosymbiotic associations, both partners keep their plasma membranes intact and use them to control the bidirectional exchange of signaling molecules and nutrients. Intracellular accommodation requires the exchange of symbiotic signals and the reprogramming of both interacting partners. This involves fundamental changes at the level of gene expression and of the cytoskeleton, as well as of organelles such as plastids, endoplasmic reticulum (ER), and the central vacuole. Symbiotic cells are highly compartmentalized and have a complex membrane system specialized for the diverse functions in molecular communication and nutrient exchange. Here, we discuss the roles of the different cellular membrane systems and their symbiosis-related proteins in AM and RNS, and we review recent progress in the analysis of membrane proteins involved in endosymbiosis.