NO APICAL MERISTEM (MtNAM) regulates floral organ identity and lateral organ separation in Medicago truncatula

The CUP-SHAPED COTYLEDON (CUC)/NO APICAL MERISTEM (NAM) family of genes control boundary formation and lateral organ separation, which is critical for proper leaf and flower patterning. However, most downstream targets of CUC/NAM genes remain unclear. In a forward screen of the tobacco retrotransposon1 (Tnt1) insertion population in Medicago truncatula, we isolated a weak allele of the no-apical-meristem mutant mtnam-2. Meanwhile, we regenerated a mature plant from the null allele mtnam-1. These materials allowed us to extensively characterize the function of MtNAM and its downstream genes. MtNAM is highly expressed in vegetative shoot buds and inflorescence apices, specifically at boundaries between the shoot apical meristem and leaf/flower primordia. Mature plants of the regenerated null allele and the weak allele display remarkable floral phenotypes: floral whorls and organ numbers are reduced and the floral organ identity is compromised. Microarray and quantitative RT-PCR analyses revealed that all classes of floral homeotic genes are down-regulated in mtnam mutants. Mutations in MtNAM also lead to fused cotyledons and leaflets of the compound leaf as well as a defective shoot apical meristem. Our results revealed that MtNAM shares the role of CUC/NAM family genes in lateral organ separation and compound leaf development, and is also required for floral organ identity and development.