Tiantian Bu,  Sijia Lu, Kai Wang,  Lidong Dong,  Shilin Li,  Qiguang Xie,  Xiaodong Xu,  Qun Cheng, Liyu Chen, Chao Fang, Haiyang Li,  Baohui Liu,  James L. Weller, and Fanjiang Kong

PNAS February 23, 2021 118 (8) e2010241118

Significance

In many plant species, the timing of flowering is sensitive to photoperiod. In many crop species, genetic variation in this sensitivity is critical for adaptation to specific regions and management practices. This study identifies a component of the genetic pathway controlling flowering time in soybean, a legume crop of major global importance. Notably, plants lacking this component flower extremely late. Photoperiod sensitivity in plants, including soybean, was first systematically described in a seminal paper 100 y ago, and the results presented here establish an important new molecular step underlying this response. This step is a critical control point that could be genetically adjusted to engineer photoperiod sensitivity for yield improvement across a broad range of locations and agricultural contexts.

Abstract

Photoperiod sensitivity is a key factor in plant adaptation and crop production. In the short-day plant soybean, adaptation to low latitude environments is provided by mutations at the J locus, which confer extended flowering phase and thereby improve yield. The identity of J as an ortholog of Arabidopsis ELF3, a component of the circadian evening complex (EC), implies that orthologs of other EC components may have similar roles. Here we show that the two soybean homeologs of LUX ARRYTHMO interact with J to form a soybean EC. Characterization of mutants reveals that these genes are highly redundant in function but together are critical for flowering under short day, where the lux1 lux2 double mutant shows extremely late flowering and a massively extended flowering phase. This phenotype exceeds that of any soybean flowering mutant reported to date, and is strongly reminiscent of the “Maryland Mammoth” tobacco mutant that featured in the seminal 1920 study of plant photoperiodism by Garner and Allard [W. W. Garner, H. A. Allard, J. Agric. Res. 18, 553–606 (1920)]. We further demonstrate that the J–LUX complex suppresses transcription of the key flowering repressor E1 and its two homologs via LUX binding sites in their promoters. These results indicate that the EC–E1 interaction has a central role in soybean photoperiod sensitivity, a phenomenon also first described by Garner and Allard. EC and E1 family genes may therefore constitute key targets for customized breeding of soybean varieties with precise flowering time adaptation, either by introgression of natural variation or generation of new mutants by gene editing.

See: https://www.pnas.org/content/118/8/e2010241118

Figure 1: Protein interactions of soybean EC (SEC). (A) J interacts with LUX1 and LUX2 in yeast. Yeast cells transformed with indicated genes were selected on DDO (lacking Leu and Trp) and QDO (lacking Ade, His, Leu, and Trp) media. (B) J interacts with LUX1 and LUX2 in Nicotiana benthamiana leaves in a BiFC assay. LUX1 and LUX2 were fused to the N terminus of YFP and J was fused to the C terminus of YFP. The constructs were coinjected into N. benthamiana leaves, and YFP signals were observed after 48 to 72 h. (Scale bars, 20 μm.) Three biological replicates were performed. (C) LUX1 and LUX2 can pull down J. MBP, MBP-LUX1, and MBP-LUX2 proteins were expressed in Escherichia coli, and J-His protein was expressed using an in vitro translation system. Purified proteins were used for the pull-down assay. MBP, MBP-LUX1, and MBP-LUX2 were detected with anti-MBP antibody, and J-His protein was detected with anti-His antibody. (D) LUX1 and LUX2 interact with each other and themselves in yeast. Yeast cells transformed with indicated genes were selected on DDO and QDO media. (E) LUX1 and LUX2 interact with each other and themselves in N. benthamiana leaves in a BiFC assay. LUX1 and LUX2 were fused to the N and C terminus of YFP. The constructs were coinjected into N. benthamiana leaves, and YFP signals were observed after 48 to 72 h. (Scale bars, 20 μm.) Three biological replicates were performed.