Land plants carry a genetic heritage from the earliest conquerors of land: the ability to form an intimate interaction with beneficial fungi that deliver mineral nutrients to the plant. The earliest land plants did not form roots. During the early colonisation stages of the land, the absence of roots was likely compensated with the help of a far-reaching hyphal network that scavenges nutrients in return for plant produced carbon sources that enable the fungus to grow. Despite the evolution of roots, AM fungi remained essential to bridge depletion zones of mineral nutrients such as phosphate that form around plant roots due to limited diffusion rates. This long distance symbiotic mineral nutrient delivery is likely important for plant growth and survival in natural environments, which explains why the AM symbiosis has been a stable feature throughout the evolution of the land plants and secondary loss of the symbiosis is the exception. Furthermore, AM has the potential to improve plant performance in sustainable agricultural practices with reduced mineral fertilizer input. However, classical crop breeding programs under high fertilizer conditions are excluding the potential of AM from the selection process of high-performance genotypes and may even result in cultivars with reduced AM compatibility, which are less suitable for sustainable agriculture. Thus, understanding the molecular underpinnings of AM development and function represents an important challenge for breeding AM-optimized crops.
For symbiosis to occur, the fungus colonizes the interior of plant roots and forms highly branched, tree-like hyphal structures, the arbuscules, inside root cortical cells. Facilitated by their strongly enlarged membrane surface area, the arbuscules are the key structures, which release mineral nutrients to plant cells, that have been scavenged by the extended extraradical hyphal network from the soil. Arbuscule formation is under the control of the host and involves cellular remodelling of already differentiated cortex cells within the tissue context. Reorganization of plant cells to host arbuscules seems to occur in distinct steps, which can be dissected by plant mutants and are accompanied by expression of distinct marker genes (reviewed by Gutjahr and Parniske, 2013), suggesting that the plant cell has to fulfil distinct tasks in a spatio-temporally coordinated manner to host the arbuscule.
RECEIVE rests on the central hypothesis that the steps of plant cell rearrangement allowing arbuscule formation are accompanied by distinct transcriptional waves, which crucially determine the developmental progress from stage to stage. The characterisation of these waves and the identification of the underlying transcriptional regulons is the main focus of RECEIVE.
