CYP72A67 Catalyzes a Key Oxidative Step in Medicago truncatula Hemolytic Saponin Biosynthesis.

In the Medicago genus, triterpenic saponins are bioactive secondary metabolites constitutively synthesized in the aerial and subterranean parts of plants via the isoprenoid pathway. Exploitation of saponins as pharmaceutics, agrochemicals and in the food and cosmetic industries has raised interest in identifying the enzymes involved in their synthesis. We have identified a cytochrome P450 (CYP72A67) involved in hemolytic sapogenin biosynthesis by a reverse genetic TILLING approach in a Medicago truncatula ethylmethanesulfonate (EMS) mutagenized collection. Genetic and biochemical analyses, mutant complementation, and expression of the gene in a microsome yeast system showed that CYP72A67 is responsible for hydroxylation at the C-2 position downstream of oleanolic acid synthesis. The affinity of CYP72A67 for substrates with different substitutions at multiple carbon positions was investigated in the same in vitro yeast system, and in relation to two other CYP450s (CYP72A68) responsible for the production of medicagenic acid, the main sapogenin in M. truncatula leaves and roots. Full sib mutant and wild-type plants were compared for their sapogenin profile, expression patterns of the genes involved in sapogenin synthesis, and response to inoculation with Sinorhizobium meliloti. The results obtained allowed us to revise the hemolytic sapogenin pathway in M. truncatula and contribute to highlighting the tissue specificities (leaves/roots) of sapogenin synthesis.

Sapogenin content variation in Medicago inter-specific hybrid derivatives highlights some aspects of saponin synthesis and control.

In the Medicago genus, saponins are a complex mixture of triterpene glycosides showing a broad spectrum of biological properties. Here we analyzed the variation in the sapogenin content and composition of inter-specific hybrid Medicago sativa × Medicago arborea derivatives to highlight the pattern of this variation in plant organs (leaves/roots) and the possible mechanisms underlying it. In Sativa Arborea Cross (SAC) leaves and roots, saponins and sapogenins were evaluated using chromatographic methods. Phenotypic correlations between sapogenin content and bio-agronomic traits were examined. Expression studies on ß-amyrin synthase and four cytochromes P450 (CYPs) involved in sapogenin biosynthesis and sequence analysis of the key gene of the hemolytic sapogenin pathway (CYP716A12) were performed. Chromatographic analyses revealed a different pattern of among-family variation for hemolytic and nonhemolytic sapogenins and saponins and for the two organs/tissues. Different correlation patterns of gene expression in roots and leaves were found. Diachronic analysis revealed a relationship between sapogenin content and gene transcriptional levels in the early stages of the productive cycle. The results suggest that there are different control mechanisms acting on sapogenin biosynthesis for leaves and roots, which are discussed. A key role for medicagenic acid in the control of sapogenin content in both the tissues is proposed and discussed.

Reverse genetics in Medicago truncatula using a TILLING mutant collection.

Medicago truncatula is one of the model species for legume molecular genetics. In the last decade different types of mutant populations have been created in this species that can be screened by forward and reverse-genetic approaches to identify and functionally characterize genes of interest. TILLING is a reverse-genetic method combining random chemical mutagenesis and a PCR-based screen to identify point mutations in regions of interest. The different steps of the TILLING analysis are described in a mutant collection of ~2,300 M2 individuals for which genomic DNA and M3 seed were obtained. A two-dimensional DNA pooling strategy was adopted to reduce the number of PCR reactions necessary to screen the collection and to unambigously identify the individual M2 plant carrying the mutation. The genotypic and phenotypic analyses of the mutant M3 progeny provide the possibility to study the gene function. In spite of its reduced size, this mutant collection has proved valid in the study of the biosynthetic pathway of a class of secondary metabolites present in the genus Medicago, the triterpenic saponins.