Abstract
The timing of both flowering and maturation determine crop adaptability and productivity. Soybean (Glycine max) is cultivated across a wide range of latitudes. The molecular-genetic mechanisms for flowering in soybean have been determined for photoperiodic responses to long days (LDs), but remain only partially determined for the delay of flowering under short-day conditions, an adaptive trait of cultivars grown in lower latitudes. Here, we characterized the late-flowering (LF) habit introduced from the Thai cultivar K3 into a photoperiod-insensitive genetic background under different photo-thermal conditions, and we analyzed the genetic basis using quantitative trait locus (QTL) mapping. The LF habit resulted from a basic difference in the floral induction activity and from the suppression of flowering, which was caused by red light-enriched LD lengths and higher temperatures, during which FLOWERING LOCUS T (FT) orthologs, FT2a and FT5a, were strongly down-regulated. QTL mapping using gene-specific markers for flowering genes E2, FT2a and FT5a and 829 single nucleotide polymorphisms obtained from restriction-site associated DNA sequencing detected three QTLs controlling the LF habit. Of these, a QTL harboring FT2a exhibited large and stable effects under all the conditions tested. A resequencing analysis detected a nonsynonymous substitution in exon 4 of FT2a from K3, which converted the glycine conserved in FT-like proteins to the aspartic acid conserved in TERMINAL FLOWER 1-like proteins (floral repressors), suggesting a functional depression in the FT2a protein from K3. The effects of the remaining two QTLs, likely corresponding to E2 and FT5a, were environment dependent. Thus, the LF habit from K3 may be caused by the functional depression of FT2a and the down-regulation of two FT genes by red light-enriched LD conditions and high temperatures.
Highlights
IntroductionE2 is a soybean ortholog of Arabidopsis GI, which inhibits flowering under long days (LDs) conditions through a pathway distinct from the phytochrome A (PHYA)-regulated E1 pathway [10, 15]
Under outdoor natural day (ND) conditions in Sapporo, AK16 flowered at 61.8 days after sowing (DAS), which was, on average, 22.3 d later than AGS292, which flowered at 39.5 DAS
The flowering delay in AK16 increased under R-long days (LDs) conditions with a 20-h day length; AK16 flowered on average 11.7 d later than AGS292
Summary
E2 is a soybean ortholog of Arabidopsis GI, which inhibits flowering under LD conditions through a pathway distinct from the PHYA-regulated E1 pathway [10, 15] Allelic combinations at these flowering loci produce diverse flowering phenological events under different environmental conditions worldwide [16,17,18,19,20,21,22,23,24]. A reduced or lack of sensitivity to LD lengths is conferred by loss-of-function alleles at the E1, E1Lb, E3 and E4 loci, as well as a transcriptionally upregulated FT5a allele [25,26,27] In addition to these early-flowering alleles, at the E9 locus, which encodes FT2a, the recessive allele e9, having reduced FT2a transcription, contributes to the retention of a sufficient vegetative growth duration by negatively acting on flowering, in early-flowering plants homozygous at the E1 locus for hypomorphic or loss-of-function alleles (e1-as and e1-nl, respectively) [28].
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