Encyclopedia
Please note this is a comparison between Version 1 by Junming Sun and Version 3 by Catherine Yang.
Tocopherols are natural antioxidants that increase the stability of fat-containing foods and are well known for their health benefits. The inclination of tocopherol concentrations with various MGs provided further evidence of the significance of MG in soybean breeding for seed tocopherol components. Furthermore, the correlation between the seed tocopherol components and geographical factors revealed that α-, γ-, and total tocopherols had significant positive correlations with latitude, while δ-tocopherol showed an opposite trend. The elite accessions with high and stable tocopherol concentrations determined could be used to develop functional foods, industrial materials, and breeding lines to improve tocopherol composition in soybean seeds.
- tocopherols
- soybean [Glycine max (L.) Merrill]
1. Introduction
Soybean [Glycine max (L.) Merr.] seeds are consumed worldwide, owing to their nutritional values and health benefits. Soybean seeds are the principal source of natural tocopherols. Tocopherols are natural antioxidants that improve the stability of fat-containing foods and have vital biological functions as well as protect food from oxidation processes induced by free radicals [1]. They aid in preventing atherosclerosis, cancer, heart diseases, and neurological and neurodegenerative diseases such as Alzheimer’s and Parkinson’s and strengthen the immune system [2][3][4][5][2,3,4,5]. Tocopherols are classified into four types of isomers (α, β, γ, and δ), differing in their molecular structures and antioxidant activities [6]. The antioxidative activities of tocopherols in biological systems against lipid peroxidation are α > β > γ > δ (100, 50, 10, and 3% relative activity, respectively) [7]. Alpha- and γ-tocopherols are significant for their nutritional and functional effects on human health and food system stability. Delta-tocopherol shows a strong antioxidant potency in vitro [8]. Because of its nutritional value and importance for oil stability, increasing tocopherol content in soybean seeds through breeding programs has become a new and important objective. However, in order to improve soybean seed tocopherol content through conventional breeding, it is important to assess the phytochemical diversity of soybean accessions so that they can be mined for the desired traits.
Tocopherol content and composition in soybean seeds are affected by both genetic and environmental factors. High temperature and drought stress during seed maturation enhance the content of α-tocopherol in soybean seeds by two to three times, with a decrease in δ-tocopherol content [9][10][9,10]. On the other hand, low temperature (16–20 °C) during seed maturation facilitates δ-tocopherol accumulation [11] and decreases total tocopherol content in soybean seed, which is attributed to loss of γ-tocopherol [12]. The impact of environmental factors on seed tocopherol content has gotten much attention [13][14][15][13,14,15]. However, genotypic variability, which is the foundation of any breeding program, is seldom reported [13][16][17][13,16,17] for tocopherol content and profile in soybean seeds. Maturity group (MG) is an important genotypic factor influencing the quality and composition of soybean seed that has not been considered as a variable for seed tocopherol composition in previous studies. The variation in tocopherol composition across MGs and the underlying mechanisms has not yet been reported. MG, in the case of soybean, is a significant parameter that determines its geographical adaptation and facilitates the possibility of introducing new varieties, but such a crucial role in soybean breeding is still neglected. Therefore, the present investigation determined the geographical distribution of MGs and the effects of the different MGs on tocopherol content and composition of diverse world soybean accessions.
It is widely acknowledged that introducing new germplasms is essential for increasing genetic diversity and strengthening breeding stock resources. Traditional breeding techniques have been using a diverse set of germplasm to produce novel soybean lines with desirable tocopherol composition. The availability of germplasm resources plays a crucial role in the success of breeding techniques. This will allow breeders to develop new cultivars with improved tocopherol profiles and broaden soybean cultivar genetic diversity, which has been narrowed down due to the overuse of a few superior cultivars in breeding programs [18].
2. Geographical Distribution of Soybean Seed Tocopherol Components
Length of growing period or maturity is a vital feature of crops since it influences the geographical adaptation of a variety [19][41]. Soybean cultivars’ growth cycles (from sowing/emergence to the reproductive stage) vary in different ecological regions. As a result, the content of tocopherols varies greatly among soybean varieties worldwide. Tocopherol components are affected by geographic variables (latitude, longitude, and altitude) in different soybean accessions, indicating that the origin of the accessions has a significant impact on the composition of tocopherols. Therefore, the mean individual and total tocopherol contents of accessions across locations and years are significantly associated with geographical factors such as the latitude, longitude, and altitude of their corresponding region of origins . Longitude and latitude have a direct impact on the environment and climate. However, latitude has a more significant impact on climate than longitude due to the apparent rise in temperatures near the world’s equator.
In this study, α-, γ-, and total tocopherols showed significant positive associations with latitude (r = 0.66***, 0.49***, and 0.13**, respectively), indicating that the contents of α-, γ-, and total tocopherols were relatively higher in accessions from higher latitude regions, which is consistent with the ouresearchers' previous report [20][21]. This may be due to the growth period of soybean accessions of high latitude regions being shorter than those planted at low latitude regions, which introduces the grain filling period to long day length and higher solar radiation conditions, therefore, facilitating the accumulation of more α- and γ-tocopherols in the seeds of high latitude accessions. In addition, higher-latitude regions are the zones of adaptation for soybean MGs ranging from 0 to II [21][22][23][32,42,43], which characterizes the situation of early-maturing accessions that contain higher levels of α-, γ-, and total tocopherols. The geographical distribution map of MGs also revealed a similar pattern.
On the other hand, δ-tocopherol showed significant and negative correlations with latitude (r = −0.56***), reflecting that lower latitude accessions produce higher levels of δ-tocopherol due to exposure of late-maturing accessions to cooler environments during the grain filling period [11]. Lower-latitude regions are the areas of adaptation for the late-maturing accessions (MG IV to VI), which was also evident from the geographical distribution map. These findings are consistent with our earlier research [20][21]. However, the geographical distribution map of MGs clearly showed that early MGs were distributed in the higher-latitude regions and late MGs were distributed in the lower-latitude regions, which also indirectly illustrated the association between tocopherol content and geographical factors.
3. Stability Performance of Soybean Accessions for Tocopherols across Environments
Soybeans are cultivated across a broad latitude range (about 20 to 53° N) under a wide range of environmental circumstances in China [24][44]. The stability of soybean accessions varies significantly under different environmental conditions [25][45]. The CV of each accession was used to reflect the stability of accession, as a lower CV indicates higher stability for a cultivar in various environments. Soybean accessions that maintain high levels of desired tocopherols with high stability for various ecoregions would be useful for genetic improvement, varietal introduction, and soybean breeding programs aimed at improving soybean tocopherol components. Since the accumulation of tocopherols is significantly affected by planting locations [26][27]; therefore, it is important to identify stable and high tocopherol-containing soybean genotypes among different environments. Based on CV, the top five high tocopherol containing stable genotypes from different origins are listed in Table 12. These genotypes can be recommended due to their overall performance. The means of the seed tocopherols were plotted against their CVs to show how much the optimal level of each tocopherol was stable within each germplasm origin (Figure 14A–D).
Figure 14. Scatter plots showing the relationship between means of tocopherol components and coefficient of variation (CV) for 493 soybean accessions grown in four different environments: (A) scatter plot of α-tocopherol; (B) scatter plot of γ-tocopherol; (C) scatter plot of δ-tocopherol; (D) scatter plot of total tocopherol. Horizontal and vertical dashed lines, in blue, represent the average CV and mean of the tocopherols, respectively.
Table 12.
List of soybean accessions that exhibit desired contents of tocopherols with higher stability (lower CV).
| Tocopherol Isomers | ID Number | Name | Mean (μg g | −1 | ) | CV (%) | Origin |
|---|
| Higher α-Toc | ZDD00041 | Heihe1 | 33.38 | 18.14 | China |
| ZDD23615 | Hefeng47 | 31.35 | 7.42 | China | |
| ZDD22657 | Hefeng35 | 30.77 | 19.59 | China | |
| ZDD24346 | Kenfeng22 | 28.74 | 15.01 | China | |
| ZDD22798 | Dongnong42E | 25.99 | 22.89 | China | |
| Higher γ-Toc | ZDD06815 | Heihe4 | 186.69 | 5.08 | China |
| WDD02019 | 9234 | 185.16 | 10.32 | USA | |
| ZDD24376 | Hefeng52 | 184.33 | 11.14 | China | |
| WDD02599 | C∏691 | 184.33 | 6.52 | Russia | |
| ZDD24342 | Kenfeng13 | 183.86 | 7.37 | China | |
| Higher δ-Toc | ZDD06638 | Baishuidou | 136.67 | 9.29 | China |
| ZDD06595 | Maojihui | 129.54 | 9.30 | China | |
| ZDD06361 | Dapudou | 118.63 | 5.14 | China | |
| ZDD13149 | Qingpidou | 111.35 | 4.95 | China | |
| ZDD14232 | Huangdou | 108.15 | 9.02 | China | |
| Higher Tot-Toc | ZDD24336 | Huajiang4 | 291.24 | 8.80 | China |
| ZDD24157 | Quxian1 | 289.75 | 7.61 | China | |
| ZDD01629 | Liaodou26 | 286.65 | 3.08 | China | |
| ZDD06815 | Heihe4 | 285.45 | 6.43 | China | |
| ZDD24685 | Jidou17 | 279.22 | 6.73 | China |
Here, Tot-Toc represents total tocopherol. CV = coefficient of variation.
Accessions having a higher content of α-tocopherol and lower CV values (Figure 14A) are more preferred because α-tocopherol is the most active form of vitamin E in the biological system of humans [8] because of its strong affinity for the hepatic tocopherol transfer protein [3][27][3,39]. Therefore, the main interest among tocopherols remains in α-tocopherol. Since soybean seeds contain a low concentration of α-tocopherol, hence, vitamin E activity in soybean is lower than that of other oilseed crops [3]. Thus, increasing the α-tocopherol content and enhancing the vitamin E activity in soybean seed is an important breeding objective, which could contribute in reducing the high production cost of synthetic α-tocopherol. In the present study, thwe researchers identified novel soybean varieties with high α-tocopherol contents. Among all soybean accessions, ZDD00041 from China had higher α-tocopherol content (33.38 μg g−1) with lower CV (18.14%), followed by another Chinese accession ZDD23615, as its α-tocopherol content was 31.35 μg g−1 with a lower CV of 7.42%. Other accessions with higher and stable α-tocopherol content are presented in Table 12. Therefore, these accessions could be used as a breeding parent to produce soybean cultivars with high levels of α-tocopherol that open up new market opportunities for soybeans.
A collection of the USA, Chinese, Japanese, and Russian accessions with high γ-tocopherol content are shown in Figure 14B. The accessions that revealed higher stability and higher γ-tocopherol levels are critical for soybean crops to inhibit oil peroxidation and enhance oil quality [28][46]. The USA accession PI592523 showed higher γ-tocopherol content (220.91 μg g−1) followed by another USA accession WDD01715 (199.44 μg g−1), but their CV values (13.45% and 12.72%, respectively) were higher than the average CV values (11.96%) of γ-tocopherol, indicating that these genotypes were less stable (Figure 14B). The results showed that among soybean accessions, ZDD06815 from China had higher γ-tocopherol content (186.69 μg g−1) with lower CV (5.08%), followed by WDD02019 from the USA, which had a higher γ-tocopherol content (185.16 μg g−1) with a lower CV of 10.32%. Other stable accessions with higher γ-tocopherol levels are shown in Table 12.
Accessions with a higher level of δ-tocopherol and lower CV values (Figure 14C) are also one of the desired forms, since δ-tocopherol is more effective than α- or γ-tocopherol in suppressing tumor growth [29][47]. The Chinese accession ZDD06638 showed the highest δ-tocopherol content of 136.67 μg g−1 with a CV of 9.29%, followed by ZDD06595 from China, which had a higher δ-tocopherol content (129.54 μg g−1) with a lower CV of 9.30%. Other accessions with higher δ-tocopherol levels demonstrating greater stability are listed in Table 12.
Total tocopherol content was calculated by summing up the contents of individual isomers; hence, increasing the total tocopherol content is one of the most effective ways to enhance the amount of a particular type of tocopherol isomer. Therefore, accessions that reveal higher stability and higher total tocopherol levels are also vital for soybean breeders. For total tocopherol, accession PI592523 from the USA showed the highest mean total tocopherol content (344.03 μg g−1), but it seems unstable due to a higher CV value (14.25%) (Figure 14D). Based on the current results, the Chinese accession, ZDD24336, was the most suitable and desirable genotype, which showed a stable (CV: 8.80%) total tocopherol level (291.24 μg g−1) at different sites (Table 12). Other accessions with higher total tocopherol levels exhibiting greater stability are shown in Table 12. This implied that soybean accessions mentioned in Table 1 2 are suited for the production of high and stable concentrations of tocopherols across environments that could be used for industrial and pharmaceutical applications and genetic improvement of soybean tocopherols. Our results (Table 12) showed that low latitude (SR of China) cultivars were more susceptible to light and temperature than high latitude (NR of China) cultivars, suggesting that high latitude cultivars have broader regional adaptability than low latitude cultivars, which is associated with the large CV values of the Southern Region (SR) soybean accessions. This finding was in agreement with earlier studies [30][31][32][48,49,50]. Taken together, it is concluded that introducing new germplasms to China will provide promising germplasm resources with desired tocopherol profiles and excellent stability over a wide range of environments [33][51].
Soybean is a short-day plant that is extremely sensitive to photoperiods [34][52]. This high sensitivity to photoperiods severely hampers the improvement of soybean. Moreover, tocopherol content in soybean seed is substantially affected by photoperiods. Thus, soybean accessions from the four origins with stable tocopherol content in different environments are thought to be insensitive to the day length or photoperiod, which will aid in the genetic improvement of soybean tocopherol composition with wide adaptability. The findings of this research will contribute greatly to the selection of accessions with optimum tocopherol levels, which are less affected by varying environmental conditions. The identified soybean accessions could be exploited as elite genetic materials for soybean breeding and tocopherol industries.
