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To extend our understanding of flowering time control in eudicots, we screened for mutants in the model legume  Medicago truncatula  (Medicago). We identified an early flowering mutant,  spring1 , in a T-DNA mutant screen, but  spring1  was not tagged and was deemed a somaclonal mutant. We backcrossed the mutant to wild type R108. The F1 plants and the majority of F2 plants were early flowering like  spring1 , strongly indicating that  spring1  conferred monogenic, dominant early flowering. We hypothesized that the  spring1  phenotype resulted from over expression of an activator of flowering. Previously, a major QTL for flowering time in different Medicago accessions was located to an interval on chromosome 7 with six candidate flowering- time activators, including a  CONSTANS  gene,  MtCO , and three  FLOWERING LOCUS T (FT)  genes. Hence we embarked upon linkage mapping using 29 markers from the  MtCO/FT  region on chromosome 7 on two populations developed by crossing  spring1  with Jester.  Spring1  mapped to an interval of ∼0.5 Mb on chromosome 7 that excluded  MtCO,  but contained 78 genes, including the three  FT  genes. Of these  FT  genes, only  FTa1  was up-regulated in  spring1  plants. We then investigated global gene expression in  spring1  and R108 by microarray analysis. Overall, they had highly similar gene expression and apart from  FTa1,  no genes in the mapping interval were differentially expressed. Two MADS transcription factor genes,  FRUITFULLb  ( FULb)  and  SUPPRESSOR OF OVER EXPRESSION OF CONSTANS1a  ( SOC1a),  that were up-regulated in  spring1 , were also up-regulated in transgenic Medicago over-expressing  FTa1 . This suggested that their differential expression in  spring1  resulted from the increased abundance of  FTa1 . A 6255 bp genomic  FTa1  fragment, including the complete 5′ region, was sequenced, but no changes were observed indicating that the  spring1  mutation is not a DNA sequence difference in the  FTa1  promoter or introns.

Fine Mapping Links the FTa1 Flowering Time Regulator to the Dominant Spring1 Locus in Medicago