Abstract
Classical genetic analysis has revealed that the determinate habit of soybean (Glycine max) is controlled by a recessive allele at the determinate stem (Dt1) locus. To dissect the molecular basis of the determinate habit, we isolated two orthologs of pea (Pisum sativum) TERMINAL FLOWER1a, GmTFL1a and GmTFL1b, from the soybean genome. Mapping analysis indicated that GmTFL1b is a candidate for Dt1. Despite their high amino acid identity, the two genes had different transcriptional profiles. GmTFL1b was expressed in the root and shoot apical meristems (SAMs), whereas GmTFL1a was mainly expressed in immature seed. The GmTFL1b transcript accumulated in the SAMs during early vegetative growth in both the determinate and indeterminate lines but thereafter was abruptly lost in the determinate line. Introduction of the genomic region of GmTFL1b from the indeterminate line complemented the stem growth habit in the determinate line: more nodes were produced, and flowering in the terminal raceme was delayed. The identity between Dt1 and GmTFL1b was also confirmed with a virus-induced gene silencing experiment. Taken together, our data suggest that Dt1 encodes the GmTFL1b protein and that the stem growth habit is determined by the variation of this gene. The dt1 allele may condition the determinate habit via the earlier loss in GmTFL1b expression concomitant with floral induction, although it functions normally under the noninductive phase of flowering. An association test of DNA polymorphisms with the stem growth habit among 16 cultivars suggested that a single amino acid substitution in exon 4 determines the fate of the SAM after floral induction.
Highlights
Classical genetic analysis has revealed that the determinate habit of soybean (Glycine max) is controlled by a recessive allele at the determinate stem (Dt1) locus
These near-isogenic lines (NILs) were developed from the progeny of the residual heterozygous line (RHL) #6-22, which was derived from a cross between determinate cv Misuzudaizu (MI) and indeterminate cv Moshidou Gong 503 (MO)
These two lines differ markedly in their plant architecture at maturity when grown outdoors: the determinate NIL was short with fewer nodes because stem growth terminated after flowering, whereas the indeterminate NIL was tall and produced more nodes (Fig. 1C)
Summary
Classical genetic analysis has revealed that the determinate habit of soybean (Glycine max) is controlled by a recessive allele at the determinate stem (Dt1) locus. Recessive mutations in the CEN gene result in the conversion of the normally indeterminate inflorescence to a determinate condition. The absence of CEN expression in the apex before the floral transition could explain why the cen mutations do not affect flowering time This additional function of Arabidopsis TFL1 correlates with its expression during the vegetative phase, when it delays the commitment of plants to form an inflorescence; the early flowering in the tfl mutants is a result of an earlier commitment to form floral meristems (Bradley et al, 1997). Mutations in PsTFL1a, known as DETERMINATE (DET), cause the determination of the main apex without affecting flowering time, in a manner similar to that in cen mutants of Antirrhinum. In pea, the two functions of the Arabidopsis TFL1 gene, flowering time and apex determinacy, seem to be controlled by two different genes (Benlloch et al, 2007)
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