| Flowering is essential in the reproductive process of higher plants.Flowers are arranged orderly on the floral axis,forming inflorescence.Inflorescence structure is an important agronomic trait of crops,closely related with the regional adaptability,yield,and quality of crops.The single flowers of A.thaliana are arranged orderly on the primary inflorescence axis,forming simple inflorescence;The primary inflorescences of legume crops produce secondary inflorescences at the leaf axil,forming compound inflorescences.The inflorescence structure of A.thaliana is regulated by the TFL1,a primary inflorescence determining gene and LFY,AP1 and CAL,the floral meristem specific genes.They participate in the genetic regulatory network underlying compound inflorescence development in legumes.The inflorescence structure of legumes belongs to compound inflorescence,an advanced inflorescence structure,compared with A.thaliana inflorescence structure.More genes and a complex inflorescence regulatory networks is indispensable for the compound inflorescence development.Recent researches have shown that MtTFL1/DET is a key gene that specifically maintains the indeterminate growth of primary inflorescence;MtFULc/VEG1 is a key gene of determining secondary inflorescence meristem;MtPIM/PIM and SGL1/UNI are floral meristem specific genes.However,the genetic relationship between these genes is not clear,and the regulatory relationship between them remains to be further studied.Medicago truncatula is a self-pollinated annual herbage.It has become a legume model plant on account of diploid,small genome,mature genetic transformation system and high natural diversity.In this study,we found a mutant NF12073 with non-flowering extreme phenotype,due to the obstruction of secondary inflorescence development,by screening the Tnt1 insertion mutant library of Medicago truncatula.Genotyping and sequencing results revealed a Tnt1 insertion in the first exon of MtFULc in NF12073.PCR-based genotyping confirmed that the nonflowering phenotype was caused by the disruption of MtFULc.By analyzing the phenotype of the weak allele mtfulc-5 and the expression of genes related to inflorescence development,we found that MtFULc regulated secondary inflorescence development in a dose-dependent manner.After amino acid sequence alignment,MtFULc was found to be a typical MADS-box protein.Analysis of tissue expression localization showed that the expression levels of MtFULc was the highest in the inflorescences apices and flowers.Subcellular localization and luciferase transient expression assays suggested that MtFULc was a transcriptional suppressor.Experiments in vitro showed that MtFULc had CAr G-box DNA binding activity and could inhibit the transcription of MtTFL1.In addition,the results of ChIP and EMSA showed that MtFULc could directly bind to the CAr G-box motifs of the promoter and the downstream region of MtTFL1.Genetic analysis experiments showed that MtTFL1 had an epistatic effect on MtFULc regulating inflorescence development of Medicago truncatula.In a summary,MtFULc was a key factor regulating secondary inflorescence development in a dose-dependent manner.MtFULc was a transcription suppressor belonging to the MADS-box family.And it could inhibit the expression of MtTFL1,by directly binding to the CAr G-box DNA motifs in the transcription regulatory region of MtTFL1 gene.In addition,double mutants and triple mutants of MtFULc,MtTFL1 and MtPIM were constructed to analyze the genetic relationship between MtFULc,MtTFL1 and MtPIM,in regulation of inflorescence development.MtFULc played an irreplaceable role on the inflorescence development model of MtTFL1-MtFULc-MtPIM.MtTFL1 inhibited the expression of MtFULc and MtPIM maintaining the infinite growth of primary inflorescence apices;MtFULc inhibited the expression of MtTFL1 gene in secondary inflorescence development;MtPIM inhibited the expression of MtFULc in the floral development of secondary inflorescence.The three factors together maintained the process of Medicago truncatula compound inflorescence growth and development. |
