Symbiotic associations of plants with beneficial microbes.

The Reinhardt lab investigates symbiotic associations of plants with beneficial microbes. The most prevalent symbiosis of plants is the arbuscular mycorrhiza (AM) symbiosis with soil fungi (Glomeromycota) that colonize the roots and form finely branched intracellular structures, the arbuscules, which serve for nutrient exchange. AM fungi provide the plant with mineral nutrients such as phosphate, nitrate, sulfate, copper and zinc, and in exchange, they receive sugars and lipids. Since most crops including cereals, vegetables, and fruit trees, are potential hosts for AM fungi, AM symbiosis is relevant for agriculture and food production.

Interestingly, some plants (mostly legumes) engage in a second symbiosis, known as legume-rhizobium symbiosis (LRS), with bacteria (rhizobia) that can fix atmospheric nitrogen and supply it to their host. The establishment of AM and LRS requires mutual recognition of the two partners and fundamental reprogramming of their gene expression. In addition, hosts cells are restructured to accommodate the intracellular microbial partners.

We are using a Petunia hybrida line with transposons (“jumping genes“) to isolate mutants affected in AM symbiosis. With this forward genetic approach, we have identified a gene encoding a transcription factor required for the reprogramming of cells for symbiosis (Required for arbuscular mycorrhiza1; RAM1). This transcription factor is required to activate genes that encode nutrient transporters such as Phosphate Transporter4 (PT4), and genes required for lipid delivery to the fungus (e.g. RAM2). A second gene identified in this way is the VAPYRIN gene that is required for the intracellular accommodation of AM fungi. VAPYRIN protein is known to interact with components of the cellular secretion pathway, and it localizes to mobile cellular compartments that are involved in cellular trafficking. The role of VAPYRIN appears be in the transport and delivery of a substrate or an essential protein required for intracellular accommodation of endosymbiotic microbes.

In a project dedicated to the legume-rhizobium symbiosis (LRS), we explore how the mutualistic state is maintained in the interaction between the legume Medicago truncatula and its rhizobial partner Sinorhizobium meliloti. The host plant accommodates the bacteria in a specific symbiotic organ (the nodule), which is initially colonized by a single rhizobial founder cell, that will multiply to give rise to approximately 100’000’000 nitrogen-fixing bacterial cells (bacteroids). Due to the natural error rate in bacterial DNA duplication, mutant clones can spontaneously emerge, which are defective in nitrogen-fixation. Instead, they can use all the energy provided by the plant for proliferation. Such “selfish” bacterial clones could theoretically outcompete the original mutualistic N-fixing clone, thereby leading to degeneration of the symbiotic association.

In addition, there are many inefficient rhizobial strains in the soil that fix only little nitrogen, but they can successfully colonize plants. Such inefficient bacteria are known as cheaters. A large body of evidence suggests that the plant can prevent to be exploited by applying sanctions on bacterial cheaters, however, the involved mechanisms are poorly understood. We explore how the host plant reacts to bacterial cheaters at the level of the metabolome and the proteome in close collaboration with our departmental analytics platform (MAPP). In addition, we use deep sequencing on the bacterial population to follow population dynamics and bacterial genome stability during symbiosis.