Model Legumes: Functional Genomics Tools in Medicago truncatula.

Many researchers have sought along the last two decades a legume species that could serve as a model system for genetic studies to resolve specific developmental or metabolic processes that cannot be studied in other model plants. Nitrogen fixation, nodulation, compound leaf, inflorescence and plant architecture, floral development, pod formation, secondary metabolite biosynthesis, and other developmental and metabolic aspects are legume-specific or show important differences with those described in Arabidopsis thaliana, the most studied model plant. Mainly Medicago truncatula and Lotus japonicus were proposed in the 1990s as model systems due to their key attributes, diploid genome, autogamous nature, short generation times, small genome sizes, and both species can be readily transformed. After more than decade-long, the genome sequences of both species are essentially complete, and a series of functional genomics tools have been successfully developed and applied. Mutagens that cause insertions or deletions are being used in these model systems because these kinds of DNA rearrangements are expected to assist in the isolation of the corresponding genes by Target-Induced Local Lesions IN Genomes (TILLING) approaches. Different M. truncatula mutants have been obtained following γ-irradiation or fast neutron bombardment (FNB), ethyl-nitrosourea (ENU) or ethyl-methanesulfonate (EMS) treatments, T-DNA and activation tagging, use of the tobacco retrotransposon Tnt1 to produce insertional mutants, gene silencing by RNAi, and transient post-transcriptional gene silencing by virus-induced gene silencing (VIGS). Emerging technologies of targeted mutagenesis and gene editing, such as the CRISPR-Cas9 system, could open a new era in this field. Functional genomics tools and phenotypic analyses of several mutants generated in M. truncatula have been essential to better understand differential aspects of legumes development and metabolism.