T-DNA mutagenesis in the model plant Medicago truncatula: is it efficient enough for legume molecular genetics?

The development of efficient transformation protocols in several legume species has opened the possibility to use T-DNA insertional mutagenesis to investigate legume-specific processes such as symbiosis. A T-DNA mutagenesis approach was initiated in the model legume Medicago truncatula by constructing a small collection of 1000 M. truncatula T-DNA lines, using in vitro-mediated transformation-regeneration protocols. In this T-DNA collection, mutant phenotypes were observed but generally did not result from the disruption of genes by the T-DNA but most probably from somaclonal mutations induced by the in vitro regeneration process or by unsuccess-ful T-DNA integration events resulting in small (undetectable) insertions or deletions in the genome of the regenerated plants. In addition, a promoter trap approach was also not very successful for gene discovery because aberrant gene fusions were observed in this collection. Therefore, the T-DNA mutagenesis approach in M. truncatula seems to be limited by the lack of an efficient in planta transformation protocol that would allow the generation of a large mutant collection suitable for reverse genetic studies. Thus, there is a need for the development of alternative tools for efficient gene disruption in legumes.