Abstract
The C function gene AGAMOUS (AG) encodes for a MADS-box transcription factor required for floral organ identity and floral meristem (FM) determinacy in angiosperms. Unlike Arabidopsis, most legume plants possess two AG homologs arose by an ancient genome duplication event. Recently, two euAGAMOUS genes, MtAGa and MtAGb, were characterized and shown to fulfill the C function activity in the model legume Medicago truncatula. Here, we reported the isolation and characterization of a new mtaga allele by screening the Medicago Tnt1 insertion mutant collection. We found that MtAGa was not only required for controlling the stamen and carpel identity but also affected pod and seed development. Genetic analysis indicated that MtAGa and MtAGb redundantly control Medicago floral organ identity, but have minimal distinct functions in regulating stamen and carpel development in a dose-dependent manner. Interestingly, the stamens and carpels are mostly converted to numerous vexillum-like petals in the double mutant of mtaga mtagb, which is distinguished from Arabidopsis ag. Further qRT-PCR analysis in different mtag mutants revealed that MtAGa and MtAGb can repress the expression of putative A and B function genes as well as MtWUS, but promote putative D function genes expression in M. truncatula. In addition, we found that the abnormal dorsal petal phenotype observed in the mtaga mtagb double mutant is associated with the upregulation of CYCLOIDEA (CYC)-like TCP genes. Taken together, our data suggest that the redundant MtAGa and MtAGb genes of M. truncatula employ a conserved mechanism of action similar to Arabidopsis in determining floral organ identity and FM determinacy but may have evolved distinct function in regulating floral symmetry by coordinating with specific floral dorsoventral identity factors.
Highlights
As the important reproductive organs in flowering plants, flowers show remarkable variation in formation and elaboration, and provide the most trustworthy external characteristics for establishing relationships among different angiosperm species
Five micrograms of total RNA was used for cDNA synthesis by using the SuperScript R III First-Strand Synthesis System for RT-PCR kit (Life Technologies). Quantitative Real-Time PCR (qRT-PCR) was performed using the ROCHE LightCycler R 96 detect system with the TransStart Tip Green qPCR SuperMix (TransGen Biotech). qRT-PCR data were obtained using three biological replicates and transcripts were normalized to MtActin
The defective carpels developed to abnormally spiny pods with reduced whirl numbers
Summary
As the important reproductive organs in flowering plants, flowers show remarkable variation in formation and elaboration, and provide the most trustworthy external characteristics for establishing relationships among different angiosperm species. From outside to Functional Specialization of Medicago AGs the center, these floral organs are sepals in the first whorl, petals in the second whorl, stamens in the third whorl and carpels in the fourth whorl. Understanding how these distinct floral organs are specified has been a long-standing question in plant development and held the fascination of scientists for centuries (Meyerowitz et al, 1989; Schwarz-Sommer et al, 1990; Coen and Meyerowitz, 1991; Theissen et al, 2016). The expanded “ABCDE” model maintains organ identity by a refined combination as that A+E genes control sepals, A+B+E genes specify petals, B+C+E genes determine stamens, C+E genes control carpels and C+D+E genes specify ovules (Theissen et al, 2016)
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