Transcriptional regulation is crucial for plant growth and development, and several transcription factors have been identified as the key regulators of seed size. These transcription factors, along with transcriptional co-activators and regulators involved in chromatin modification, play important roles in determining seed size in plants.
3.4. Photoperiod Regulation of Soybean Seed Size
The photoperiod i.e., the duration of light and dark periods in a day, can have an impact on seed size in plants
[41]. Different plant species showed varied responses to photoperiod, and it can influence different aspects of seed development and growth. Plants have developed effective strategies to adapt to environmental changes throughout their evolution. For instance, plants perceive variations in daylight duration, allowing them to undergo the transition to flowering during seasons most conducive to successful reproduction
[42]. Flowering time is a critical trait in domestication, and increased knowledge of flowering regarding its genetic basis and molecular mechanism will greatly enhance crop plants’ adaptation to the environment
[43].
CONSTANS (
CO) acts as a central regulator in the photoperiodic flowering pathway, and
CO orthologs have been identified in various plant species
[44][45][46][47]. Yu et al.
[43] discovered that under favourable conditions of reproductive growth, plants tend to suppress the transcription of
APETALA2 (
AP2) through
CO, thereby regulating the proliferation of seed coat epidermal cells and promoting larger seed production
[48]. Under conditions favouring vegetative growth, CO protein becomes unstable, leading the plants to prioritize nutrient acquisition and generate smaller seeds by attenuating the inhibitory effect of
CO on
AP2. When
CO is mutated, the seed size of the plant becomes unresponsive to photoperiod. Thus providing the first insight into the core regulatory module governing seed size in response to photoperiodic cues.
3.5. Other Regulatory Pathways of Soybean Seed Size
Nguyen et al.
[49] reported and characterized the
GmKIX8-1, a gene that encodes a nuclear protein regulating organ size in soybeans.
GmKIX8-1 encodes a conserved KIX domain protein that is homologous to
AtKIX8 in
A. thaliana, which limits organ growth by modulating cell proliferation in meristematic tissues
[49]. Previous studies have revealed that the
KIX-
PPD-
MYC-
GIF1 module controls seed size in
A. thaliana by inhibiting cell proliferation in the outer integument during the development of embryo sac and early seed
[50], this regulatory mechanism may also exist during seed development in soybean.
Flavonoids viz., anthocyanins, proanthocyanidins, flavonols, and isoflavones, are the most important components affecting seed coat colour
[51]. Zhang et al.
[34] identified a novel gene,
Glyma.08G309000, from the mutant
S006, which was named
Novel Seed Size (
NSS)
[34]. This gene is associated with brown and small-seeded phenotypes. This study indicated a significant increase in anthocyanin accumulation in the
S006 mutant leading to pigmentation in the seed coat. Moreover, the outer integument cell area was significantly reduced in the
S006 mutant, thus negatively impacting seed size in soybeans. This study provides evidence that the brown seed coat colour might be attributed to elevated expression of
chalcone synthase 7/8 genes, while decreased expression of
NSS contributes to reduced seed size. These findings suggest that the
NSS gene represents a novel regulator of seed development. The
NSS gene encodes a protein of unknown function, but it contains a peptidase-c1 domain resembling a potential DNA helicase RuvA subunit, indicating a possible involvement in apoptosis.
The plant cell wall is a highly complex structure composed of structural proteins, enzymes, and various polysaccharides as well as pectin
[52], which plays a crucial role in the primary cell wall of plants
[53]. Li et al.
[29] reported a semi-dominant locus named
ST1 (
Seed Thickness 1)
[29] and successfully identified the underlying major functional gene of
ST1 as
Glyma.08G109100. This gene was documented to regulate soybean seed thickness and encodes a UDP-D-glucuronic acid 4-epimerase, which regulates the production of UDP-rhamnose and promotes pectin biosynthesis. Thus, it may determine seed shape by modulating cell division and expansion patterns in soybeans. Interestingly, this morphological variation simultaneously increases seed oil content. Together with the upregulation of sugar metabolism regulated by
ST1, soybean has become a major oilseed crop, suggesting a strong selection for this gene during domestication. Additionally, Tang et al.
[32] discovered that the soybean transformation inhibitor
GmCIF1 participates in controlling seed maturation by specifically inhibiting the activity of cell wall invertase (CWI)
[32]. Silencing of
GmCIF1 expression elevates the CWI post-translational levels to finely tune sucrose metabolism and sink strength, thereby coordinating the process of seed maturation and increasing seed weight, as well as accumulating hexoses, starch, and proteins in mature seeds.
The 100-seed weight is considered one of the pivotal domestication traits that greatly influences soybean yield. Nevertheless, its elusive genetic foundation remains enigmatic. Zhu et al.
[36] elucidated a
soybean seed size 1 (
sss1) mutant with enlarged seeds in comparison to its wild-type counterpart
[36]. These authors showed that candidate gene
GmSSS1 (
SOYBEAN SEED SIZE 1) encodes a SPINDLY homolog that resides within a well-defined quantitative trait locus (QTL) hotspot on chromosome 19, which underwent intense selection during the cultivation of soybeans. Deletion of
GmSSS1 causes reduced seed size, while its overexpression results the larger seeds, subsequently augmenting the 100-seed weight of a soybean. Further investigations indicate that
GmSSS1 exerts a positive effect on cell expansion and cell division, thus regulating the ultimate size of soybean seeds (
Figure 2).
Figure 2. The major signalling pathways involved in soybean seed size control. The size of seeds is regulated through a complex network of signals, involving genes, hormones, transcription factors, and other intricate mechanisms. Dashed lines represent uncertain genetic relationships. Modulators that regulate seed size by influencing cell proliferation, cell expansion, and other regulatory processes are depicted in red, blue, and green, respectively. Abbreviations: BS, big seed; NIJIA, novel interactors of JAZ; NSS, novel seed size; SSS, soybean seed size; CWI, cell wall invertase; ST, seed thickness.