Expression profiling studies to specify the symbiotic genetic programme of the model legume Medicago truncatula

Principal Investigator of the IST team: Leonilde Morais Moreira
Contract: PTDC/AGR-GPL/70592/2006 
Start date: 01/10/2007

Duration: 36 months

Legumes constitute the third-largest family of flowering plants and different species from this family have been bred and grown since the beginning of agriculture as a source of plant proteins for feed and food. Nowadays, and only inferior to cereals, legumes (Leguminosae) comprise the most important agricultural taxon on earth. Prominent representatives are the forage legumes Medicago sativa (alfalfa) and Trifolium sp. (clover) as well as the grain legumes Pisum sativum (pea), Vicia faba (broad bean), Cicer arietinum (chickpea), Phaseolus vulgaris (bean) and Glycine max (soybean). From a biological point of view, the most remarkable feature of legume plants is their unique capacity to enter a nitrogen-fixing root nodule endosymbiosis with prokaryotes. This symbiotic interaction, e.g. of Sinorhizobium meliloti with Medicago truncatula results in the conversion of atmospheric N2 gas to ammonia that is subsequently supplied to the plant for incorporation into amino acids. It is this process that allows legumes to grow on nitrogen depleted soils and that facilitates the formation of tissues with high protein content, in particular the seeds of grain legume crops. Whereas the root nodule symbiosis is unique to legumes, more than 80% of higher plants enter an arbuscular mycorrhiza (AM) endosymbiosis with fungi of the phylum Glomeromycota with prominent representatives being, Glomus mosseae and Gigaspora margarita. Comparable to root nodules, nutrient exchange occurs at the intracellular interface between the symbionts, the peri-arbuscular membrane that surrounds the arbuscules. In addition, it is assumed that the intraradical hyphae are a major site for the transfer of nutrients, most importantly for the allocation of carbohydrates to the fungus. In return for the supply with carbohydrates, the fungal microsymbiont transfers minerals, and in particular phosphorus, from the soil to the plant. From a functional point of view, AM essentially enlarges the size and the surface area of a plant root system that way facilitating an optimized uptake of phosphorus and minerals from the soil. This project aims at analysing the transcriptome of wild type and mutant lines of Medicago truncatula in different developmental stages of root endosymbioses. M. truncatula has been selected as a model plant in which to study the biological processes that are unique and pertinent to legumes: nitrogen-fixing root nodules and protein-rich seeds. International efforts to decipher the genome of M. truncatula in conjunction with a large number of EST sequences available for this model legume have led to the recent development of an Affymetrix Medicago truncatula GeneChip array representing more than 48.000 transcripts of M. truncatula plus more than 8000 transcripts of its symbiotic organism Sinorhizobium meliloti. This Medicago truncatula GeneChip array will be employed for the study outlined in this proposal. Unified, MIAME compliant protocols for plant growth/harvesting, and collection of total RNA samples by the project partners will be set up. Samples will be prepared with biological triplicates for analysis on Affymetrix Medicago truncatula GeneChip arrays. The expression data obtained will be used to create the Truncatulix database, which will be tightly linked with existing and future databases that store M. truncatula related information, e.g. about the genome sequence or mutant line collections. In addition, integration with similar large scale genomics/transcriptomics databases for the model plant Arabidopsis thaliana and the model cereal barley will facilitate comparative genomics/transcriptomics efforts. The proposed project will present the first true large-scale analysis of the symbiotic transcriptome of the model legume Medicago truncatula, a key component of the use of legumes in sustainable agricultural systems. The proposed research team consists of national and international experts in the fields required for the success of the project, namely in root nodule and mycorrhizal legume symbiosis, bacterial genetics, DNA array technology and bioinformatics. The in depth analysis of the datasets obtained within this project is expected to foster the development of novel, testable hypotheses for diverse aspects of the genetic basis underlying the symbiotic programme of leguminous plants. Such hypotheses will be addressed in follow-up projects using the reverse genetics tools available for Medicago truncatula. It is expected that the knowledge gained on legume biology within this project will ultimately result in innovative applications, with benefits for agriculture and food industry in general.