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 root nodule symbiosis (RNS), with bacteria (rhizobia) which can fix atmospheric nitrogen and supply it to their host. The establishment of AM and RNS 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. It is known to interact with components of the cellular secretion pathway, and indeed, it localizes to mobile cellular compartments that are involved in cellular trafficking. The role of VAPYRIN may be in the transport and delivery of a substrate required for AM fungi. Interestingly, the moss Physcomitrella patens contains a VAPYRIN protein, although it cannot engage in symbiotic associations. We explore the function of this gene by targeted mutation and by overexpression in the moss.

In addition, we have studied how AM symbiosis improves plant growth and mineral nutrient supply. Conversely, we explored how nutrient status feeds back onto symbiotic development. High levels of phosphate inhibit AM, but only if the plant is well supplied with all other mineral nutrients. If just one other nutrient is limiting, phosphate does not inhibit AM. These results show that plants can modulate their mycorrhizal colonization based on their nutrient status. Currently, the mechanisms by which phosphate inhibits AM is unknown. We are pursuing several hypothesis, in particular the possibility that phosphate triggers a defense response against AM fungi, and that it may inhibit nutrient supply to the fungus.

In a project dedicated to the root nodule symbiosis (RNS) we explore how the mutualism is maintained. Plants form a specific symbiotic organ (the nodule), which is colonized by a single rhizobium cell, that then divides to give rise to ca. 107-109 nitrogen-fixing bacterial cells. Due to the natural error rate in bacterial DNA duplication, mutant clones 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. We explore whether the plant can detect selfish bacteria and sanction them to avoid to become exploited.