A Soybean Deletion Mutant That Moderates the Repression of Flowering by Cool Temperatures

Abstract

Ambient growing temperature and photoperiod are major environmental stimuli that summer annual crops use to adjust their reproductive phenology so as to maximize yield. Variation in flowering time among soybean (Glycine max) cultivars results mainly from allelic diversity at loci that control photoperiod sensitivity and FLOWERING LOCUS T (FT) orthologs. However, variation in the thermal regulation of flowering and its underlying mechanisms are poorly understood. In this study, we identified a novel mutant (ef1) that confers altered thermal regulation of flowering in response to cool ambient temperatures. Mapping analysis with simple sequence repeat (SSR) markers located the mutation in the upper part of chromosome 19, where no QTL for flowering has been previously reported. Fine-mapping and re-sequencing revealed that the mutation was caused by deletion of a 214 kbp genomic region that contains 11 annotated genes, including CONSTANS-LIKE 2b (COL2b), a soybean ortholog of Arabidopsis CONSTANS. Comparison of flowering times under different photo-thermal conditions revealed that early flowering in the mutant lines was most distinct under cool ambient temperatures. The expression of two FT orthologs, FT2a and FT5a, was dramatically downregulated by cool temperature, but the magnitude of the downregulation was lower in the mutant lines. Cool temperatures upregulated COL2b expression or delayed peak expression, particularly at the fourth trifoliate-leaf stage. Intriguingly, they also upregulated E1, a soybean-specific repressor of FT orthologs. Our results suggest that the ef1 mutation is involved in thermal regulation of flowering in response to cool ambient temperature, and the lack of COL2b in the mutant likely alleviates the repression of flowering by cool temperature. The ef1 mutant can be used as a novel gene resource in breeding soybean cultivars adapted to cool climate and in research to improve our understanding of thermal regulation of flowering in soybean.

High-Temperature Conditions Promote Soybean Flowering through the Transcriptional Reprograming of Flowering Genes in the Photoperiod Pathway

“…Up-regulation of GmCOL5a/5b and GmCOL6a/6b under high-temperature conditions ( Figure 3 and Figure 4 ) indicated that their expressions showed a positive relationship with the expressions of GmFT2a and GmFT5a , which further suggests that these GmCOLs are involved in high-temperature response. Recently, Zhang et al (2020) reported that cool temperature (18 °C) delays flowering and up-regulates GmCOL2b expression at the fourth trifoliate leaf (V4) stage [ 59 ], suggesting that GmCOL2b works as a flowering repressor upon cool ambient temperatures. However, GmCOL1a/1b and GmCOL2a/2b , which are the best-known GmCOL genes in soybean plants, were, unexpectedly, not changed in response to high temperature ( Figure S5 ).…”

Molecular mechanisms for the photoperiodic regulation of flowering in soybean

“…Intriguingly, the downregulation of COL1a/b by RNAi resulted in the downregulation of E1 (Wu et al, 2019). Furthermore, E1 and E1Lb were induced by chilling at 18°C in LD, but such induction was not observed in an early‐flowering mutant in which a 214 kb segment containing COL2b was deleted (Zhang et al, 2020b). These findings suggest that soybean COL genes could function as activators of E1 expression, an interesting hypothesis to be addressed in the future.…”

Ancient relaxation of an obligate short-day requirement in common bean through loss of CONSTANS-like gene function

“…57 There is even an indication that E1 expression may be weakly promoted by GmCOL2b. 54 We favor a hypothesis in which PvCOL2 and PvE1 act in parallel, and to some extent redundantly, to repress multiple FT genes (Figure 4D). However, the identification of functional variants for PvE1 and PvCOL1 and comprehensive functional analysis of E1-like and group Ia COL genes in soybean will be needed to clarify these relationships and how they may differ between the two species.…”

“…17,18 In soybean, the closest SD comparator of common bean, weak early-flowering phenotypes of COL1b missense and COL2b deletion mutants indicate a modest repressive role for both genes under LD. 53,54 The markedly stronger effect of mutations in the single common bean COL2 gene could simply reflect the lower degree of redundancy relative to the paleotetraploid soybean but may also indicate that COL2 has a more critical functional role in bean than in soybean. Nevertheless, the fact that col2 genotypes are only partially insensitive to photoperiod indicates that other genes also contribute to repression of flowering under LD.…”