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. 2014 Aug 27;9(8):e106042. doi: 10.1371/journal.pone.0106042

Allelic Combinations of Soybean Maturity Loci E1, E2, E3 and E4 Result in Diversity of Maturity and Adaptation to Different Latitudes

Bingjun Jiang 1,#, Haiyang Nan 2,3,#, Youfei Gao 1,#, Lili Tang 2, Yanlei Yue 1, Sijia Lu 2,3, Liming Ma 1, Dong Cao 2, Shi Sun 1, Jialin Wang 2, Cunxiang Wu 1, Xiaohui Yuan 2, Wensheng Hou 1, Fanjiang Kong 2,*, Tianfu Han 1,*, Baohui Liu 2,*
Editor: Fan Chen4
PMCID: PMC4146597  PMID: 25162675

Abstract

Soybean cultivars are extremely diverse in time to flowering and maturation as a result of various photoperiod sensitivities. The underlying molecular genetic mechanism is not fully clear, however, four maturity loci E1, E2, E3 and E4 have been molecularly identified. In this report, cultivars were selected with various photoperiod sensitivities from different ecological zones, which covered almost all maturity groups (MG) from MG 000 to MG VIII and MG X adapted from latitude N 18° to N 53°. They were planted in the field under natural daylength condition (ND) in Beijing, China or in pots under different photoperiod treatments. Maturity-related traits were then investigated. The four E maturity loci were genotyped at the molecular level. Our results suggested that these four E genes have different impacts on maturity and their allelic variations and combinations determine the diversification of soybean maturity and adaptation to different latitudes. The genetic mechanisms underlying photoperiod sensitivity and adaptation in wild soybean seemed unique from those in cultivated soybean. The allelic combinations and functional molecular markers for the four E loci will significantly assist molecular breeding towards high productivity.

Introduction

Soybean (Glycine max (L.) Merrill) is a short-day crop with high protein and oil contents. Many cultivars were bred with different maturity to adapt to various ecological environments. For the convenience of breeding layout, 13 MGs from MG000 to MGX were classified in North America [1]–[3]. Chinese soybean researchers divided cultivars into different maturity groups as well [4]–[5]. Soybean production has increased in America in response to recent increases in global demand and maturity is the key factor determining soybean productivity. Therefore, it is rather important to understand the mechanism of soybean maturity diversity and adaptation.

Flowering and maturity were highly controlled by major genes in soybean. Up to now, nine maturity loci have been identified as E1–E8 and J [1], [6]–[12]. These loci have different roles under different photoperiods. Wang et al [13] found that long daylength condition (LD) might strengthen while short daylength condition (SD) might weaken these maturity loci. More results and progress of maturity genes was reviewed by Xia et al [14]. Furthermore, four loci were identified at molecular level. E1 gene was identified as a transcription factor which functions as a flowering repressor with a putative nuclear localization signal and a B3-related domain [15]. E2 is an orthologue of Arabidopsis flowering gene GIGANTEA [16]. E3 and E4 are phytochrome genes GmPhyA3 [17] and GmPhyA2 [18], respectively. In addition, two homologs of soybean Flowering Locus T (FT) genes, GmFT2A and GmFT5A were identified and coordinately regulate flowering [19]. Four identified maturity genes E1, E2, E3 and E4 delay flowering and maturity under LD through down regulating GmFT2A and GmFT5A [15], [16], [19]. As for other loci, more studies should be done before learning their molecular identities.

Although these four known loci E1, E2, E3 and E4 provide an important key to learn the mechanism of flowering and maturity, we mainly got knowledge based on few cultivars but not on a population level. Population-level knowledge will provide another different view of these four loci’s role on maturity and adaptation. Therefore, in this study we selected a set of soybean cultivars which cover 12 maturity groups from MG000 to MGVIII and MGX plus some cultivars with wide range of latitude from N 18° to N 53°. These cultivars were subjected to different photoperiod treatments. Traits of beginning bloom (R1), physiological maturity (R7) and full maturity (R8) were investigated [20]. Maturity loci E1, E2, E3 and E4 were genotyped in the population. Further association analysis was done. The results showed that allelic combinations of these four E genes significantly determine the ecological-economical adaption of cultivars although they have different impacts on maturity. In addition, the genetic mechanisms underlying photoperiod sensitivity and adaptation in wild soybean seemed unique from those in cultivated soybean.

Materials and Methods

Soybean cultivars were selected from North America (Table 1), China and Russia (Table 2) [21]–[23]. Four wild soybean accessions were also included for genotyping only (Table 2), which are CAAE087 (Heiheyesheng) collected in Heihe (N 50°22′, E 127°53′), Heilongjiang, China; CAAE088 (Bayanyesheng) in Bayan (N 46°08′, E 127°39′), Heilongjiang, China; CAAE089 (Baiyangdianyesheng) in Baoding (N 38°51′, E 115°30′), Hebei, China; and CAAE090 (Guangxiyesheng) in Nanning (N 22°48′, E 108°19′), Guangxi, China. They covered 12 maturity groups from MG 000 to MG VIII and MG X and ranged from N 18° to N 53° indicated in Table 1 and Table 2.

Table 1. Cultivars from North America and their respective maturity group.

Code PI number Variety MG Code PI number Variety MG
CAAE001 PI548594 Maple Presto 000 CAAE021 PI534646 Flyer IV
CAAE002 PI567787 OAC Vision 000 CAAE022 PI598222 TN4-94 IV
CAAE003 PI548648 Canatto 00 CAAE023 PI564849 Nathan V
CAAE005 PI592523 Glacier 00 CAAE024 PI572239 Holladay V
CAAE006 PI629004 MN0201 0 CAAE025 PI633609 Lonoke V
CAAE007 PI596541 Traill 0 CAAE026 PI561400 Rhodes V
CAAE008 PI612764 MN0901 0 CAAE027 PI633610 Desha VI
CAAE009 PI599300 Surge NA CAAE028 PI592756 Dillon VI
CAAE010 PI548641 Haroson I CAAE029 PI617045 NC-Roy VI
CAAE011 PI614833 NE1900 I CAAE030 PI599333 Musen VI
CAAE012 PI608438 Titan I CAAE031 PI531068 Stonewall VII
CAAE013 PI561858 Holt II CAAE032 PI595645 Benning VII
CAAE014 PI567786 OAC Talbot II CAAE033 PI617041 Santee VII
CAAE015 PI533655 Burlison II CAAE034 PI555453 Hagood VII
CAAE016 PI595926 Athow III CAAE035 PI603953 Motte VIII
CAAE017 PI548634 Zane III CAAE036 PI548970 Foster VIII
CAAE018 PI593258 Macon III CAAE037 PI548663 Dowling VIII
CAAE019 PI578057 Saline III CAAE038 Jupiter X
CAAE020 PI614155 NS93-4118 IV
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NA, not available.

Table 2. Cultivars and accessions from China and far-east Russia and their adaption latitudes.

Code Variety Latitude Code Variety Latitude
CAAE039 Dengke 2 N 47°–53° CAAE064 Fengchengzaochadou N 25°–30°
CAAE040 Huajiang 4 N 48°–50° CAAE065 Jin 6606 N 41°–43°
CAAE041 Heihe 27 N 46°–48° CAAE066 Jinzhou 8–14 N 40°–43°
CAAE042 Heihe 3 N 46°–50° CAAE071 Bahong 1 N 39°–42°
CAAE043 Heihe 43 N 46°–48° CAAE072 Mianyanghuangwofeng N 30°
CAAE044 Suinong 14 N 44°–48° CAAE073 Lüpidou N 25°–28°
CAAE045 Hefeng 25 N 39°–43° CAAE074 Liuyuezao N 25°–28°
CAAE047 Jilin 3 N 44°–46° CAAE075 Ruijinxiaohuangdou N 24°–27°
CAAE049 Jiunong 21 N 42°–46° CAAE076 Edou 2 N 30°–35°
CAAE050 Jilin 30 N 41°–43° CAAE077 Yulindahuangdou N 23°
CAAE052 Jindou 19 N 35°–40° CAAE081 Qiudou 1 N 25°–30°
CAAE053 Tiefeng 31 N 35°–40° CAAE082 Jiangledaqingdou N 27°
CAAE054 Jidou 12 N 35°–38° CAAE084 Guixia 1 N 22°–26°
CAAE055 Qihuang 28 N 34°–37° CAAE085 Nandou 12 N 29°–32°
CAAE056 Zhonghuang 13 N 30°–40° CAAE086 Zigongdongdou N 29°
CAAE057 Xudou 9 N 32°–35° CAAE087* Heiheyesheng N 50°
CAAE058 Xudou 1 N 32°–35° CAAE088* Bayanyesheng N 46°
CAAE059 Fengshouhuang N 35°–38° CAAE089* Baiyangdianyesheng N 39°
CAAE061 Dandou 2 N 40°–42° CAAE090* Guangxiyesheng N 23°
CAAE062 Yuejin 4 N 35°–38° CAAE091 Mohe 1 N 53°
CAAE063 Jinda 814 NA CAAE092 Ziweicika 4/75 N 50°
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CAAE092 is a cultivar from far east Russia; *indicates wild soybeans: CAAE087, CAAE088, CAAE089 and CAAE090.

Due to seed availability, 59 cultivars were selected for both field and pot experiments, 12 cultivars only in the field experiment, and 4 cultivars only in the pot experiment. For the field experiment, seeds were sowed on May 14th, 2012 in Beijing (N 39°97′, E 116°34′) and maturity-related traits of R1, R7 and R8 were recorded regularly [20]. For the pot experiment, seeds were sowed in 10-liter pots on May 16th 2012 and grown under ND in Beijing. After emergence, the seedlings were thinned until each pot contained five uniform plants. These uniform plants were grown until the unifoliate expanded then treated with different photoperiods (LD, 16 h light/8 h dark; SD, 12 h light/12 h dark; and ND). For SD, the plants were transferred to dark room to shorten the daylength. For LD, incandescent bulbs (50 µmol m−2 s−2 at the top of plants) with automatic timer controls were used to extend the daylength. Additional details of plant growth and treatments were the same as reported by Wu et al [24]. The days to first flowering of each plant was recorded. Both experiments finished on Oct 15th, 2012. Photoperiod sensitivity (PS) was thus calculated as the following function, where DFFLD is the days to first flowering (R1) from the expansion of the first pair of unifoliates (V1) under LD while DFFSD under SD [25].

graphic file with name pone.0106042.e001.jpg

Genomic DNA was isolated from soybean unifoliate leaves using TianGen New Plant Genomic DNA Isolation Kit (DP320). Maturity loci E1, E2, E3 and E4 were genotyped using functional allele specific molecular markers [26].

Ethics Statement

No specific permissions were required for domestic research of the collections of wild soybean accessions in Heihe (N 50°22′, E 127°53′), Bayan (N 46°08′, E 127°39′), Baoding (N 38°51′, E 115°30′) and Nanning (N 22°48′, E 108°19′), China. All the field studies did not involve endangered or protected species.

Results

Soybean cultivars have diverse flowering and maturity dates

Seventy-one cultivars were planted under ND in the field at Beijing. These cultivars showed rich diversity in maturity (Table 3). Two cultivars CAAE081 and CAAE086 failed to flower until the experiment ended. Nine cultivars CAAE032, CAAE034, CAAE036, CAAE038, CAAE075, CAAE077, CAAE082, CAAE084 and CAAE085, flowered but did not reach R7. Six cultivars CAAE031, CAAE033, CAAE035, CAAE037, CAAE072 and CAAE076 did not reach R8 although they flowered and podded. For other cultivars that regularly flowered and matured, the days to R1 from emergence (VE) ranged from 19.0 (CAAE040) to 75.1 (CAAE030), the days to R7 ranged from 57.2 (CAAE003) to 142.5 (CAAE027), and the days to R8 ranged from 68.0 (CAAE003) to 145.0 (CAAE029 and CAAE030). Thus, the range of time to R1, R7 and R8 showed the maturity diversity of these cultivars. The cultivars that reached R1 less than 50 days after emergence could mature before the frost in Beijing, those that reached R1 in greater than 50 days but less than 70 days could mature partially, and the ones that reached R1 after 70 days could hardly mature (Table 3). In Figure 1 and Figure 2, maturity-related traits of R1, R7 and R8 generally increased from early MG to late MG and from high latitude to low latitude.

Table 3. The days from VE to R1, R7 and R8 of soybean Cultivars under ND condition at Beijing.

Code Phenotype Code Phenotype
VE-R1 VE-R7 VE-R8 VE-R1 VE-R7 VE-R8
CAAE001 20.7±1.7 61.0±3.8 70.9±1.8 CAAE038 122.3±1.0 NA NA
CAAE002 20.0±1.8 63.9±3.4 71.5±1.9 CAAE039 20.4±0.8 61.5±4.8 71.7±2.0
CAAE003 20.1±0.4 57.2±1.0 68.0±0.0 CAAE040 19.0±0.0 67.0±6.7 76.7±3.4
CAAE005 20.3±1.8 68.4±1.6 73.7±1.0 CAAE041 20.1±0.4 69.7±0.5 76.7±1.3
CAAE006 19.9±1.3 69.3±3.0 77.0±2.2 CAAE042 20.4±1.5 71.3±1.8 78.3±2.1
CAAE007 20.3±2.0 70.1±2.2 77±2.2 CAAE043 19.1±0.5 71.0±2.2 81.3±6.5
CAAE008 22.4±1.6 93.4±10.6 108.5±15 CAAE044 21.1±0.7 71.3±2.4 80.7±6.3
CAAE009 21.9±1.2 86.9±2.1 100.7±3.6 CAAE045 20.2±0.4 71.3±1.8 78.3±2.1
CAAE010 22.5±1.4 85.3±2.0 90.9±1.6 CAAE047 23.1±2.3 83.9±1.3 94.4±4.2
CAAE011 25.8±1.9 81.1±11.1 103.9±5.9 CAAE049 23.4±1.2 83.9±9.9 101.4±7.0
CAAE012 21.9±1.4 89.7±1.6 101.3±2.6 CAAE050 24.9±1.1 98.6±5.2 109.9±7.3
CAAE013 22.7±1.4 94.2±1.8 109.9±4.7 CAAE052 24.5±0.8 99.9±2.8 114.2±2.4
CAAE014 21.6±0.5 92.5±3.0 100.3±3.1 CAAE053 23.1±0.3 113.6±3.0 121.7±2.2
CAAE015 25.2±1.9 102.1±7.3 113.9±5.3 CAAE054 37.4±0.6 119.8±6.6 127.3±7.0
CAAE016 24.9±1.0 106.3±7.0 118.5±3.5 CAAE055 42.7±2.3 127.0±8.0 136.8±5.1
CAAE017 26.9±1.8 117.1±2.3 125.5±1.5 CAAE056 36.4±2.0 113.7±6.3 121.1±2.7
CAAE018 24.7±0.6 117.9±1.8 124.9±3.3 CAAE057 31.3±1.0 111.4±2.5 119.9±1.4
CAAE019 28.6±1.2 123.4±2.7 131.6±3.3 CAAE058 24.2±1.0 95.9±2.8 106.2±5.2
CAAE020 24.2±1.2 116.4±2.8 125.7±1.9 CAAE059 25.5±0.5 111.2±3.1 118.7±2.6
CAAE021 26.3±1.3 123.4±3.3 133.4±2.9 CAAE063 41.9±2.1 116.0±2.0 122.0±0.0
CAAE022 32.5±4.4 125.1±4.1 134.6±1.4 CAAE064 41.3±1.0 101.9±3.7 111.3±4.7
CAAE023 56.1±3.9 131.9±7.8 139.3±2.4 CAAE065 27.1±1.2 103.9±9.1 115.7±5.1
CAAE024 50.6±2.6 132.9±6.1 141.0±3.9 CAAE066 42.5±1.5 116.0±3.2 126.8±6.3
CAAE025 53.7±1.1 140.6±4.5 142±0.0† CAAE071 49.1±4.0 132.6±6.4 140.0±5.6
CAAE026 54.9±1.9 141.5±4.4 143.2±1.8 CAAE072 91.3±2.7 141.0±1.4 NA
CAAE027 61.8±4.6 142.5±4.9 142.0±0.0† CAAE073 54.2±1.1 140.0±4.7 144.0±2.8†
CAAE028 63.3±5.5 141.8±4.3 144.0±1.7† CAAE074 69.0±0.0 125.4±4.6 133.5±4.6
CAAE029 59.0±5.2 142.0±3.1 145.0±0.0† CAAE075 94.0±2.1 NA NA
CAAE030 75.1±2.0 141.9±3.0 145.0±0.0† CAAE076 65.5±5.5 143.8±1.6 NA
CAAE031 77.7±3.3 142.8±1.1 NA CAAE077 71.8±9.8 NA NA
CAAE032 70.8±3.2 NA NA CAAE081 NA NA NA
CAAE033 70.4±3.1 145.0±0.0 NA CAAE082 96.8±1.8 NA NA
CAAE034 77.7±1.6 NA NA CAAE084 98.3±2.9 NA NA
CAAE035 75.9±4.4 145.0±0.0 NA CAAE085 95.1±2.6 NA NA
CAAE036 74.3±4.5 NA NA CAAE086 NA NA NA
CAAE037 69.6±4.6 146.1±0.9 NA
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†

Partial plants (<50%) matured. NA, not available.

Figure 1. Maturity-related traits of soybean R1, R7 and R8 in the field at Beijing.

Figure 1

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Left, cultivars of North America, which are approximately sorted by maturity group. Right, cultivars of China, which are sorted roughly by adaption latitude.

Figure 2. Photoperiod sensitivity of soybean cultivars.

Figure 2

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Left, cultivars of North America, which are approximately sorted by maturity group. Right: cultivars of China, which are sorted roughly by adaption latitude.

Soybean cultivars react variously to photoperiod treatments

Sixty-three cultivars were planted in pots. They were treated with different photoperiods after V1. They began flowering in 18.4 to 32.8 days after emergence under SD, in 20.0 to 122.3 days under ND, and in 20.5 to 113.7 under LD, while under LD three cultivars CAAE031 (PI531068, MGVII), CAAE084 (Qiudou 1), and CAAE086 (Zigongdongdou) did not flower (Table 4). The PS was calculated according to the equation [25], which ranged between 10% and 80%. For the three cultivars that did not flower, it could be set at 100%. Thus, CAAE035 (PI603953, MGVIII), CAAE072 (Mianyanghuangwofeng), CAAE085 (Nandou 12), CAAE075 (Ruijinxiaohuangdou) and the former three cultivars are most sensitive to photoperiod (PS>75%). CAAE001 (PI548594, MG000), CAAE002 (PI567787, MG000), CAAE005 (PI592523, MG00), CAAE007 (PI596541, MG0), CAAE042 (Heihe 3) and CAAE091 (Mohe 1) could be classified as photoperiod insensitive because their photoperiod sensitivities are lower than 10%. These data suggested that these soybean cultivars diversify significantly in photoperiod sensitivity. Moreover, as shown in Figure 2, the photoperiod sensitivity of these cultivars generally increased from early MG to late MG and from high to low latitude.

Table 4. The days to R1 from V1 under different photoperiod treatments and the resultant PS at Beijing.

Cultivar Days from V1 to R1 PS (%) Cultivar Days from V1 to R1 PS (%)
SD ND LD SD ND LD
CAAE001 21.4±1.1 20.4±0.7 22.9±2.4 6.3 CAAE035 26.9±1.3 83.2±1.1 108.2±8.9 75.2
CAAE002 22.6±1.1 20.9±1.4 23.9±2.2 5.6 CAAE036 26.7±2.4 83.9±1.4 90.6±0.5 70.6
CAAE003 21.4±1.5 24.7±1.7 26.2±3.3 18.2 CAAE037 27.6±0.7 73.9±3.1 88.4±0.5 68.8
CAAE005 21.4±2.0 20.0±0.6 23.2±0.9 8.1 CAAE050 23.1±0.9 30.2±1.4 37.1±5.0 37.6
CAAE006 21.7±2.1 22.0±2.5 27.0±5.4 19.5 CAAE053 23.1±1.7 28.9±2.2 37.8±6.5 38.8
CAAE007 20.6±1.3 20.1±0.6 22.9±1.7 9.8 CAAE052 22.9±0.8 31.9±1.9 47.2±3.8 51.5
CAAE008 24.1±3.2 25.5±0.9 28.1±4.1 14.2 CAAE054 24.1±1.1 44.1±1.8 54.3±3.0 55.6
CAAE009 22.6±2.7 29.8±1.9 32.9±3.4 31.4 CAAE057 27.0±0.8 45.0±2.7 50.2±1.9 46.2
CAAE010 22.8±1.7 28.5±2.1 33.2±1.6 31.4 CAAE056 28.0±1.7 49.5±1.4 61.3±3.2 54.3
CAAE011 24.8±1.6 32.5±1.5 42.7±3.4 42.0 CAAE061 26.7±3.1 50.3±4.6 54.3±7.1 50.9
CAAE012 25.1±3.0 27.9±2.3 34.6±3.1 27.5 CAAE076 27.3±0.8 73.3±1.5 86.9±0.9 68.5
CAAE013 22.2±2.3 28.6±2.1 33.1±2.5 33.0 CAAE064 32.8±2.0 45.9±2.4 56.5±2.8 41.9
CAAE014 24.8±1.9 28.7±2.6 34.7±4.7 28.5 CAAE059 24.1±1.4 44.8±1.6 77.4±2.0 68.8
CAAE015 26.7±2.1 31.2±2.6 40.1±3.7 33.5 CAAE084 30.8±1.9 106.4±0.7 NA 100.0
CAAE016 22.0±1.9 31.2±3.2 46.9±5.2 53.1 CAAE045 24.3±2.1 27.6±1.5 30.0±2.6 18.9
CAAE017 26.2±2.1 35.6±6.0 46.3±0.5 43.4 CAAE041 21.2±1.7 22.8±0.9 23.6±1.3 10.2
CAAE018 25.1±0.4 31.5±0.8 40.5±1.6 38.0 CAAE042 23.0±1.7 24.3±1.4 23.7±1.7 2.8
CAAE019 25.3±2.1 39.4±1.0 49.9±3.1 49.3 CAAE040 20.2±1.5 20.4±0.5 22.8±1.6 11.5
CAAE020 21.3±1.6 34.1±6.0 46.7±4.2 54.4 CAAE065 26.4±1.4 34.1±2.9 41.3±2.7 36.0
CAAE021 23.7±2.7 36.7±3.1 47.9±3.6 50.5 CAAE066 26.5±0.5 48.6±1.3 56.9±1.9 53.4
CAAE022 23.2±2.2 45.2±2.5 52.5±5.0 55.9 CAAE074 32.2±1.0 62.9±0.3 91.5±0.9 64.8
CAAE024 25.0±0.8 60.2±1.2 83.3±0.6 70.0 CAAE073 26.6±1.2 63.8±5.1 82.0±0.7 67.6
CAAE025 27.9±1.4 67.1±1.4 87.9±1.1 68.3 CAAE072 28.2±1.8 92.7±3.3 124.4±0.8 77.3
CAAE026 26.4±1.3 67.2±0.8 86.9±0.8 69.6 CAAE091 20.4±2.8 22.9±0.7 21.6±1.2 5.2
CAAE027 29.5±1.0 67.0±0.5 84.0±0.0 64.9 CAAE085 29.8±1.7 98.6±1.3 126.8±1.7 76.5
CAAE028 26.3±2.0 68.1±1.3 89.0±2.3 70.5 CAAE075 28.9±1.4 98.3±1.4 124.5±1.2 76.8
CAAE029 24.1±0.8 82.1±3.3 94.7±0.5 74.5 CAAE058 23.9±1.4 30.9±1.9 36.6±7.0 34.7
CAAE030 25.0±0.8 83.8±1.4 94.1±0.6 73.4 CAAE077 29.6±2.8 96.4±0.5 113.7±1.0 74.0
CAAE031 24.7±0.8 83.6±0.5 NA 100.0 CAAE062 27.6±0.9 55.3±2.3 83.0±1.4 66.7
CAAE032 25.1±0.7 72.6±1.3 88.6±1.1 71.7 CAAE092 18.4±2.0 20.9±1.6 20.5±1.8 10.6
CAAE033 24.9±0.4 73.9±2.2 88.9±1.9 72.0 CAAE086 32.8±0.8 122.3±0.8 NA 100.0
CAAE034 24.9±0.6 84.6±1.1 89.0±1.7 72.0
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NA, not available.

Genotyping soybean cultivars of maturity loci E1, E2, E3 and E4

Eighty-five cultivars were genotyped at four maturity loci E1, E2, E3 and E4 (Table 4). Thirty-eight cultivars are from North America, which cover 12 maturity groups from MG000 to MGVIII and MGX. Other cultivars are from China except CAAE092, which is from Russia (Tables 1 and 2). There are ten genotypes in total in this population (Table 5). The genotypes of E1/E2/E3/E4 and E1/e2/E3/E4 are the majority types, which were identified in 28 and 19 cultivars respectively (Table 5). Three genotypes of e1-as/E2/e3/E4, e1-as/e2/e3/e4 and e1/e2/e3/E4 were identified only in one variety each (Table 5). For the E1 locus, allele e1 and el-as are always detected in early-maturing cultivars from MG000 to MGIV or from high latitudes adapted cultivars. Moreover, all of the four wild soybeans were E1/E2/E3/E4 type.

Table 5. Genotype of soybean cultivars at four maturity loci E1, E2, E3 and E4.

Genotype Num Variety
E1 E2 E3 E4
E1 E2 E3 E4 28 CAAE023 (MGV), CAAE024 (MGV), CAAE025 (MGV), CAAE026 (MGV),CAAE027 (MGVI), CAAE028 (MGVI), CAAE029 (MGVI), CAAE030(MGVI), CAAE031 (MGVII), CAAE032 (MGVII), CAAE033 (MGVII),CAAE034 (MGVII), CAAE035 (MG VIII), CAAE036 (MG VIII), CAAE037(MG VIII), CAAE038 (MGX), CAAE071, CAAE072, CAAE075,CAAE081, CAAE082, CAAE084, CAAE085, CAAE086, CAAE087,CAAE088, CAAE089, CAAE090
E1 e2 E3 E4 19 CAAE047, CAAE049, CAAE050, CAAE052, CAAE054, CAAE055,CAAE056, CAAE057, CAAE058, CAAE061, CAAE062, CAAE063,CAAE064, CAAE065, CAAE066, CAAE073, CAAE074, CAAE076,CAAE077
E1 e2 e3 E4 3 CAAE044, CAAE045, CAAE059
e1-as E2 E3 E4 8 CAAE016 (MGIII), CAAE017 (MGIII), CAAE018 (MGIII),CAAE019 (MGIII), CAAE020 (MGIV), CAAE021 (MGIV),CAAE022 (MGIV), CAAE053
e1-as E2 e3 E4 1 CAAE011 (MGI)
e1-as e2 E3 E4 8 CAAE003 (MG00), CAAE008 (MG0), CAAE009, CAAE010(MGI), CAAE012 (MGI), CAAE013 (MGII), CAAE014(MGII), CAAE015 (MGII)
e1-as e2 e3 E4 7 CAAE005 (MG00), CAAE006 (MG0), CAAE007(MG0), CAAE040, CAAE041, CAAE042,CAAE043
e1-as e2 e3 e4 1 CAAE039
e1 e2 e3 E4 1 CAAE092
e1 e2 e3 e4 3 CAAE001 (MG000), CAAE002 (MG000),CAAE091
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Maturity loci E1, E2, E3 and E4 have different impacts on maturity and photoperiod response

In general, recessive alleles e1, el-as, e2, e3 and e4 promoted flowering and maturity but with different impacts (Figure 3). The allele e4 was detected in only four cultivars (CAAE039 with the genotype e1-as/e2/e3/e4, and CAAE001, CAAE002 and CAAE091 with the genotype of e1/e2/e3/e4) in the population. The four cultivars with recessive e4 alleles were adapted to high latitude and showed photoperiod insensitivity suggesting the importance of the e4 allele for high latitude adaptation. The cultivars with the allele e1, e1-as or e2 exhibited a narrower range of the days from VE to R1 (VE-R1) than that of the days from VE to R7 (VE-R7) and that of the days from R1 to R7 (R1–R7) (Figure 3). In contrast, the cultivars with the allele e3 or e4 showed a consistently narrow range although some outliers existed (Figure 3). Moreover, these four recessive alleles promoted flowering under different photoperiod conditions (Figure 4), and the cultivars with more recessive alleles of e1, e1-as, e2, e3 and e4 had a lower PS during photoperiod treatments (Figure 5).

Figure 3. Quartile box plots showing days between the stages of VE, R1 and R7.

Figure 3

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Circles show outliers.

Figure 4. Quartile box plots showing days between the stages of VE and R1 under different photoperiod conditions and PS.

Figure 4

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Circles show outliers.

Figure 5. Photoperiod sensitivity grouped by E genotypes.

Figure 5

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Discussion

Soybean cultivars from different maturity groups show diversity in flowering, maturity and photoperiod sensitivity

The tested soybean cultivars were selected from North America, China and Russia. Some of them covered from MG000 to MGVIII and MGX, almost all of the total 13 MG [2] and the others were collected from N 18° to N 53° to cover the wide range of latitude, which represents the main soybean producing area in China. In addition, some wild soybean accessions were also included. Thus, the population of lines used here should exhibit the diversity of maturity not only in phenotype but also in genotype. The field experiment under ND provided strong evidence. Some cultivars failed to flower (R1), some could not reach pod yellowing (R7) and some could not reach full maturity (R8) (Table 3). Even for those cultivars that flowered and matured, the days to first flowering, physiological maturity and full maturity varied significantly (Table 3). What’s more, in the experiment of photoperiod treatments, the population also showed diversity of photoperiod sensitivity as expected. The days to first flowering varied between different photoperiod treatments and between different cultivars. The photoperiod sensitivity also varied from 10% to 80% (Table 4). Six cultivars were classified as photoperiod insensitive: CAAE001 (PI548594, MG000, e1/e2/e3/e4), CAAE002 (PI567787, MG000, e1/e2/e3/e4), CAAE005 (PI592523, MG00, e1-as/e2/e3/E4), CAAE007 (PI596541, MG0, e1-as/e2/e3/E4), CAAE042 (Heihe 3, e1-as/e2/e3/E4) and CAAE091 (Mohe 1, e1/e2/e3/e4). These results were consistent with previous report that soybean photoperiod insensitivity was at least conditioned by three genetic mechanisms according to allelic combinations of E1, E2 and E4: e3/e4; e1/e3 or e1/e4 and e1-as/e3/E4. In the genetic mechanism of e1-as/e3/E4, novel unidentified gene/genes participated in photoperiod insensitivity [26], [27].

E genes have different impacts on flowering and maturation

Soybean, as a short-day crop, has many cultivars with diversified maturity structure. E1, E2 and E3 are involved with different impacts. In the field experiment under ND, the recessive allele e1, el-as and e2 significantly narrowed the variation of VE-R1 more than that of VE-R7 and R1–R7 (Figure 3), suggesting that E1 and E2 genes have significant impact on pre-flowering development other than post-flowering responses. However, the loci E3 and E4 might function not only in pre-flowering development but also in post-flowering development, indicated by the narrow variation in VE-R1, VE-R7 and R1–R7 of cultivars with e3 and e4 alleles (Figure 3). This result is consistent with Xu et al [26]. that E3 and E4 respond not only to pre-flowering but also to post-flowering by increasing pod filling duration, number of nods and pod numbers by up-regulating the expression of growth habit gene Dt1. This result implies the significance of E3 and E4 loci for molecular genetic breeding to increase soybean productivity. Moreover, the outliers suggested that the disfunction of E3 and E4 might be interrupted by other genes. Similarly, the loci E1 and E3 are related with photoperiod sensitivity (Figure 4).

For the genotype of E1/E2/E3/E4, the photoperiod sensitivity was mostly above 70%. Compared with genotypes E1/E2/E3/E4, e1-as/E2/E3/E4, e1-as/e2/E3/E4 and e1-as/e2/e3/E4 (Figure 5), the photoperiod sensitivity decreased with the numbers of recessive alleles. While comparing E1/e2/E3/E4 and e1-as/E2/E3/E4 with E1E2E3E4, it was suggested that E1 plays a more important role than E2 because e1 decreased the photoperiod sensitivity more significantly and narrowed its range. These results further proved that soybean photoperiod insensitivity was involved by four maturity loci E1, E3, E4 and E7 while E2 locus was not involved [28], [29]. In these tested cultivars of China, E1/e2/E3/E4 is much more abundant. It ranged from N 18° -N 42° while E1/E2/E3/E4 was distributed south of N 39° except for the four wild soybeans (CAAE087, Heiheyesheng; CAAE088, Bayanyesheng; CAAE089, Baiyangdianyesheng; and CAAE090, Guangxiyesheng). For e1 or el-as alleles, the associated cultivars where mostly located above N 45° except for CAAE053 (Tiefeng 31, N 35°–40°). Thus, to improve the ecological adaptability of cultivars, the E1 gene must function less because of its most strong impact on delaying maturity while other E genes might become important in adaptation. Unlike cultivated soybean, two wild soybean accessions, CAAE087 (Heiheyesheng) and CAAE088 (Bayanyesheng) adapted north of N 46° in China where cultivars generally had less photoperiod sensitivity where genotyped as E1/E2/E3/E4. It is greatly important for wild soybean to adapt and survive during season alteration. The genetic mechanisms underlying photoperiod insensitivity and adaptation in wild soybean were therefore unique from those in cultivated soybean.

Allelic combinations of E genes determine maturity groups

In the population analyzed here, MGV to MGX have the same genotype at E1/E2/E3/E4. It suggests that in these maturity groups, other unknown maturity genes should be involved in the determination of Mature Group. MGIII to MGIV are mainly genotyped as e1-as/E2/E3/E4, and MGII is e1-as/e2/E3/E4. For MGII to MGX, each group has one genotype. MGI has the genotypes of e1-as/e2/E3/E4 and e1-as/E2/e3/E4; MG0 and MG00 both have the genotypes of e1-as/e2/E3/E4 and e1-as/e2/e3/E4; and MG000 has e1/e2/e3/e4. From MG000 to MGI, each maturity group has two genotypes, which means that photoperiod is the first key factor in these region to determine maturity group. However, the more recessive alleles at E genes, the earlier cultivars mature. Due to the limited number of cultivars used in each MG, the maturity genotypes for each group may be underestimated and additional genotypes for each MG may be identified with a larger sample of cultivars. Although it is not possible to enumerate all genotypes of a given maturity group, this limited sample of cultivars showed that allelic combinations of E genes determine maturity groups in general.

Conclusions

The E genes (E1, E2, E3 and E4) have different roles in maturity and photoperiod sensitivity and their allelic combinations determine maturity group and adaptation to different latitude.

Acknowledgments

We thank Randall L. Nelson (Soybean/Maize Germplasm, Pathology and Genetics Research Unit, USDA and Department of Crop Sciences, University of Illinois, Urbana, Illinois, USA), Lijuan Qiu and Zhangxiong Liu (Institute of Crop Sciences, Chinese Agricultural Academy of Sciences, China) for providing some soybean germplasm. We also appreciate Dr. Elroy R. Cober (Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Canada) and Dr. Jun Abe (Research Faculty of Agriculture, Hokkaido University) for proofreading of the manuscript.

Data Availability

The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its Supporting Information files.

Funding Statement

This work was supported by the National Natural Science Foundation of China (31201232, 31071445, 31171579, 31201222 and 31371643); Open Foundation of Key Laboratory of Soybean Molecular Design Breeding, Chinese Academy of Sciences; “Hundred Talents” Program of Chinese Academy of Sciences; Strategic Action Plan for Science and Technology Innovation of Chinese Academy of Sciences (XDA08030100); Heilongjiang Natural Science Foundation of China (ZD201001, JC201313); China Agriculture Research System (CARS-04) and Chinese Academy of Agricultural Sciences (CAAS) Innovation Project. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its Supporting Information files.

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