OsDOG1L-3 regulates seed dormancy through the abscisic acid pathway in rice

StNF-YA8-YB20-YC5 module regulates potato tuber dormancy by modulating gibberellin and abscisic acid pathways

The molecular mechanisms involved in the regulation of potato tuber dormancy are complex, involving a variety of related genes and enzymes, which modulate multiple signaling pathways. Nuclear factor-Y (NF-Y) transcription factors (TFs) are widely found in eukaryotes and are involved in the regulation of plant embryonic development, seed germination, fruit ripening, and in response to biotic and abiotic stress. Previously, we found that StNF-YA8 gene expression was increasing with the release of potato tuber dormancy. In this study, it was found that StNF-YA8 overexpressed tubers broke dormancy earlier than non-transgenic (NT) and StNF-YA8 downregulated tubers. Changes in abscisic acid (ABA) and gibberellin (GA) content of different types of tubers at different dormancy periods confirmed that both GA and ABA hormones influenced the differences in dormancy time. This was confirmed by the expression of GA pathway genes StGA3ox1 and StGA20ox1 genes and ABA pathway genes StCYP707A2 and StPP2CA1 genes in different tubers. The four genes described above were further shown to be target genes of the StNF-YA8 TF, which transcriptionally activates the expression of these genes. In addition, we verified the involvement of StNF-YA8 in the tuber dormancy release process by the interacting proteins StNF-YB20 and StNF-YC5, which are able to bind to the StNF-YA8-B20-C5 module to activate the transcription of GA and ABA pathway genes. Our study reveals the StNF-YA8-C5 module activates the transcription of the StCYP707A2, StPP2CA1, StGA3ox1, and StGA20ox1 genes and alters GA and ABA content, accelerating the release of dormancy in potato tubers.

Integrated analyses provide insights into the seed dormancy mechanisms of the endangered plant Sinojackia sarcocarpa

Sinojackia sarcocarpa, an endangered ornamental plant endemic to China, faces germination challenges that contribute to its endangered status. The mechanisms of its seed dormancy are not well understood. This study used morphological, physiological, transcriptomic, and gene function analyses to investigate these mechanisms. Our research shows that seed dormancy in Sinojackia sarcocarpa involves both physical and physiological factors. We found that removing the hard endocarp and applying gibberellic acid can effectively break dormancy. Transcriptomic analysis identified 2218 up-regulated and 374 down-regulated genes during germination. Notably, DOG1-domain genes SsDOGL4, SsTGA9, and SsTGA10 were significantly downregulated, while SsDOG1 was not. Additionally, overexpression of SsDOGL4 in Arabidopsis endosperm was found to enhance seed dormancy. Collectively, these findings offer significant insights into the mechanisms underlying seed dormancy in this endangered plant species.

Utilizing CRISPR-based genetic modification for precise control of seed dormancy: progress, obstacles, and potential directions

Seed dormancy, a complex trait that is influenced by both nuclear and cytoplasmic factors, poses a significant challenge to agricultural productivity. Conventional dormancy-breaking techniques, including mechanical, physiological, and chemical methods, often yield inconsistent results, impair seed quality, and lack precision. This has necessitated exploration of more targeted and efficient approaches. CRISPR-based gene editing has emerged as a promising tool for the precise regulation of seed dormancy without compromising seed viability or sustainability. Although CRISPR has been successfully applied to modify genes that govern physiological traits in various crops, its use in dormancy regulation remains in the early stages. This review examines recent advancements in CRISPR-based approaches for modulating seed dormancy and discusses key gene targets, modification techniques, and the resulting effects. We also consider the future potential of CRISPR to enhance dormancy control across diverse crop species.

Dissecting the Genetic Basis of Preharvest Sprouting in Rice Using a Genome-Wide Association Study

Preharvest sprouting (PHS) is an unfavorable trait in cereal crops that significantly reduces grain yield and quality. However, the regulatory mechanisms underlying this complex trait are still largely unknown. Here, 276 rice accessions from the 3000 Rice Genomes Project were used to perform a genome-wide association study. In total, seven PHS-associated quantitative trait loci (QTLs) were identified, including two novel QTLs and five previously reported QTLs. Among them, four QTLs were identified in 2020 and 2021, and rice accessions carrying at least two favorable alleles exhibited significantly improved PHS resistance. Within these four stable QTLs, five candidate genes were identified based on haplotype and gene expression analyses, including two genes in qPhs-1.1, one gene in qPhs-3, one gene in qPhs-6, and one gene in qPhs-7. Notably, the novel QTL qPhs-6 was found to contain the candidate gene Pi starvation-induced transcription factor 1 (OsPTF1). We discovered that OsPTF1 plays a novel role in negatively regulating PHS in rice and identified two elite haplotypes of OsPTF1 associated with low PHS. Our results provide future insight into the genetic basis of PHS and will prove useful in the future studies on the role of OsPTF1 in PHS in rice.

The transcription factor CCT30 promotes rice preharvest sprouting by regulating sugar signalling to inhibit the ABA-mediated pathway

Seed dormancy is an important adaptive trait in plants. Proper seed dormancy enables the avoidance of preharvest sprouting in the undesirable conditions like rainfall frequently. In this study, qPSR8, a major QTL for preharvest sprouting, was isolated, and a previously reported heading-date gene, CCT30, was verified as the candidate gene. The CCT30 knockout mutants (CCT30-CR) enhanced seed dormancy and ABA sensitivity as compared with the wild-type ZH11. Conversely, CCT30 overexpressing plants had opposite phenotype changes and had a decreased ABA content. The expression of ABA synthesis genes such as OsNCEDs and ABA signalling genes such as ABI3 and ABI5 were upregulated and sugar metabolism-related genes such as amylase genes were downregulated in CCT30-CR. Correspondingly, fewer free sugars, such as monosaccharides and oligosaccharides, accumulated in CCT30-CR. The freshly harvested seeds from CCT30-CR had no ability to transmit sugar signals when treated with 1% exogenous glucose. In addition, CCT30 interacted with the transcription factor OsbZIP37, which negatively regulates seed dormancy. Overall, CCT30 promotes preharvest sprouting by enhancing sugar signals that inhibit the ABA-mediated pathway, and CCT30 is a good gene for breeding rice varieties resistant to preharvest sprouting.

J, Kaushik M, Mulani E, S. M, Roy J, Phogat S, Sareen B, Madhavan J, Sevanthi AM, Solanke AU, Kumar P, Mandal PK. Low titre of agroinoculum with prolonged incubation period and low auxin concentration in the regeneration media are the key to high frequency of transformation in climate-resilient Aus-type rice genotype Nagina 22

Developing an efficient and reproducible regeneration protocol holds paramount significance for advancing genetic transformation technologies in rice, facilitating their utilisation in crop improvement. Nagina 22 (N22), a climate-resilient Aus-type rice genotype known for its tolerance against multiple stresses, lacks a standardised transformation protocol, limiting its utilisation as a background for genetic transformation. This study reports, for the first time, a highly efficient transformation and regeneration protocol for N22 using a CRISPR/Cas9 vector. Mature seeds were used to induce embryogenic calli on CHU(N6)-based callus induction media (CIM) with varying concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D). The highest callus induction efficiency (~ 94%) was achieved using 3 mgL-1 2,4-D. For regeneration, calli were transferred to different regeneration media-I (RM-Ia to RM-Ie), where a combination of 5 mgL-1 6-benzylaminopurine (BAP) and 0.02 mgL-1 naphthalene acetic acid (NAA) resulted in ~ 44% regeneration frequency. Subsequent optimisation of regeneration media-II (RM-II) with low NAA concentration enhanced shoot elongation and root development. Furthermore, reducing basal salt concentration in the resuspension media significantly enhanced transformation efficiency to 44%, achieved, by only using sterile distilled water (SDW) with 150 mM acetosyringone for calli infection. The optimised protocol was successfully validated using CRISPR/Cas9 vector, facilitating targeted gene knockouts for functional genomic studies. This approach addresses a critical gap in N22 genetic transformation, providing a reliable protocol for advancing rice improvement through gene editing. It offers valuable insights for future research and practical applications in genetic transformation of this elite rice genotype for various agronomic and scientific purposes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-025-04210-y.

Epitranscriptome profiles reveal participation of the RNA methyltransferase gene OsMTA1 in rice seed germination and salt stress response

BACKGROUND

RNA m6A methylation installed by RNA methyltransferases plays a crucial role in regulating plant growth and development and environmental stress responses. However, the underlying molecular mechanisms of m6A methylation involved in seed germination and stress responses are largely unknown. In the present study, we surveyed global m6A methylation in rice seed germination under salt stress and the control (no stress) using an osmta1 mutant and its wild type.

RESULTS

The knockout of OsMTA1 resulted in a decreased level of m6A methylation and delayed seed germination, together with increased oxidative damage in the osmta1-1 mutant, especially under salt stress, indicating that OsMTA1 performs a crucial function in rice seed germination and salt stress response. Comparative analysis of m6A profiling using methylated RNA immunoprecipitation sequencing revealed that a unique set of genes that functioned in seed germination, cell growth, and development, including OsbZIP78 and OsA8, were hypomethylated in osmta1-1 embryos and germinating seeds. Numerous genes involved in plant growth and stress response were hypomethylated in the osmta1-1 mutant during seed germination under salt stress. Further combined analysis of the m6A methylome and transcriptome revealed that the loss of function of OsMTA1 had a more complex impact on gene expression in osmta1-1. Several hypomethylated genes with a negative role in growth and development, such as OsHsfA7 and OsHDAC3, were highly up-regulated in the osmta1-1 mutant under the control condition. In contrast, several hypomethylated genes positively associated with stress response were down-regulated, whereas a different set of hypomethylated genes that functioned as negative regulators of growth and stress response were up-regulated in the osmta1-1 mutant under salt stress. These results further demonstrated that OsMTA1-mediated m6A methylation modulated rice seed germination and salt stress response by regulating transcription of a unique set of genes with diverse functions.

CONCLUSION

Our results reveal a crucial role for the m6A methyltransferase gene OsMTA1 in regulating rice seed germination and salt stress response, and provide candidate genes to assist in breeding new stress-tolerant rice varieties.

The phosphatidylethanolamine-binding proteins OsMFT1 and OsMFT2 regulate seed dormancy in rice

Seed dormancy is crucial for optimal plant life-cycle timing. However, domestication has largely diminished seed dormancy in modern cereal cultivars, leading to challenges such as preharvest sprouting (PHS) and subsequent declines in yield and quality. Therefore, it is imperative to unravel the molecular mechanisms governing seed dormancy for the development of PHS-resistant varieties. In this study, we screened a mutant of BASIC HELIX-LOOP-HELIX TRANSCRIPTION FACTOR4 (OsbHLH004) with decreased seed dormancy and revealed that OsbHLH004 directly regulates the expression of 9-CIS-EPOXYCAROTENOID DIOXYGENASE3 (OsNCED3) and GIBBERELLIN 2-OXIDASE6 (OsGA2ox6) in rice (Oryza sativa). Additionally, we determined that two phosphatidylethanolamine-binding proteins, MOTHER OF FT AND TFL1 and 2 (OsMFT1 and OsMFT2; hereafter OsMFT1/2) interact with OsbHLH004 and Ideal Plant Architecture 1 (IPA1) to regulate their binding capacities on OsNCED3 and OsGA2ox6, thereby promoting seed dormancy. Intriguingly, FT-INTERACTING PROTEIN1 (OsFTIP1) interacts with OsMFT1/2 and affects their nucleocytoplasmic translocation into the nucleus, where OsMFT1/2-OsbHLH004 and OsMFT1/2-IPA1 antagonistically modulate the expression of OsNCED3 and OsGA2ox6. Our findings reveal that OsFTIP1-mediated inhibition of nuclear translocation of OsMFT1/2 and the dynamic transcriptional modulation of OsNCED3 and OsGA2ox6 by OsMFT1/2-OsbHLH004 and OsMFT1/2-IPA1 complexes in seed dormancy in rice.

OsSCYL2 is Involved in Regulating ABA Signaling-Mediated Seed Germination in Rice

Seed germination represents a multifaceted biological process influenced by various intrinsic and extrinsic factors. In the present study, our investigation unveiled the regulatory role of OsSCYL2, a gene identified as a facilitator of seed germination in rice. Notably, the germination kinetics of OsSCYL2-overexpressing seeds surpassed those of their wild-type counterparts, indicating the potency of OsSCYL2 in enhancing this developmental process. Moreover, qRT-PCR results showed that OsSCYL2 was consistently expressed throughout the germination process in rice. Exogenous application of ABA on seeds and seedlings underscored the sensitivity of OsSCYL2 to ABA during both seed germination initiation and post-germination growth phases. Transcriptomic profiling following OsSCYL2 overexpression revealed profound alterations in metabolic pathways, MAPK signaling cascades, and phytohormone-mediated signal transduction pathways, with 15 genes related to the ABA pathways exhibiting significant expression changes. Complementary in vivo and in vitro assays unveiled the physical interaction between OsSCYL2 and TOR, thereby implicating OsSCYL2 in the negative modulation of ABA-responsive genes and its consequential impact on seed germination dynamics. This study elucidated novel insights into the function of OsSCYL2 in regulating the germination process of rice seeds through the modulation of ABA signaling pathways, thereby enhancing the understanding of the functional significance of the SCYL protein family in plant physiological processes.

Analysis of the global transcriptome and miRNAome associated with seed dormancy during seed maturation in rice (Oryza sativa L. cv. Nipponbare)

BACKGROUND

Seed dormancy is a biological mechanism that prevents germination until favorable conditions for the subsequent generation of plants are encountered. Therefore, this mechanism must be effectively established during seed maturation. Studies investigating the transcriptome and miRNAome of rice embryos and endosperms at various maturation stages to evaluate seed dormancy are limited. This study aimed to compare the transcriptome and miRNAome of rice seeds during seed maturation.

RESULTS

Oryza sativa L. cv. Nipponbare seeds were sampled for embryos and endosperms at three maturation stages: 30, 45, and 60 days after heading (DAH). The pre-harvest sprouting (PHS) assay was conducted to assess the level of dormancy in the seeds at each maturation stage. At 60 DAH, the PHS rate was significantly increased compared to those at 30 and 45 DAH, indicating that the dormancy is broken during the later maturation stage (45 DAH to 60 DAH). However, the largest number of differentially expressed genes (DEGs) and differentially expressed miRNAs (DEmiRs) were identified between 30 and 60 DAH in the embryo and endosperm, implying that the gradual changes in genes and miRNAs from 30 to 60 DAH may play a significant role in breaking seed dormancy. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses confirmed that DEGs related to plant hormones were most abundant in the embryo during 45 DAH to 60 DAH and 30 DAH to 60 DAH transitions. Alternatively, most of the DEGs in the endosperm were related to energy and abiotic stress. MapMan analysis and quantitative real-time polymerase chain reaction identified four newly profiled auxin-related genes (OsSAUR6/12/23/25) and one ethylene-related gene (OsERF087), which may be involved in seed dormancy during maturation. Additionally, miRNA target prediction (psRNATarget) and degradome dataset (TarDB) indicated a potential association between osa-miR531b and ethylene biosynthesis gene (OsACO4), along with osa-miR390-5p and the abscisic acid (ABA) exporter-related gene (OsMATE19) as factors involved in seed dormancy.

CONCLUSIONS

Analysis of the transcriptome and miRNAome of rice embryos and endosperms during seed maturation provided new insights into seed dormancy, particularly its relationship with plant hormones such as ABA, auxin, and ethylene.

Overexpression of NtDOGL4 improves cadmium tolerance through abscisic acid signaling pathway in tobacco

The DELAY OF GERMINATION1-LIKE (DOGL) genes play an essential role in diverse biological processes in plants. However, their exact involvement in the response to cadmium (Cd) stress via the ABA pathway remains unclear. Here, we focused on NtDOGL4, a tobacco DOGL gene whose expression is highly induced upon exposure to Cd. Overexpression of NtDOGL4 in tobacco resulted in elevated endogenous ABA levels, reduced Cd accumulation, and increased tolerance to Cd. Moreover, NtDOGL4 overexpression led to decreased accumulation of reactive oxygen species (ROS) and improved ROS scavenging capacity under Cd stress. Further analyses revealed the direct binding of the transcription factor ABSCISIC ACID-INSENSITIVE 5 (ABI5) to the NtDOGL4 promoter, positively regulating its expression in tobacco. Notably, NtDOGL4 overexpression promoted suberin formation and deposition, while suppressing the expression of Cd transporter genes in tobacco roots, as evidenced by histochemical staining, suberin fraction determination, and qRT-PCR assays. Collectively, our results demonstrate that NtDOGL4 overexpression reduces Cd accumulation, thereby improving Cd stress tolerance through the modulation of antioxidant system, transcription of Cd transporters, and suberin deposition. Notably, the NtABI5-NtDOGL4 module functions as a positive regulator in tobacco's Cd tolerance, underscoring its potential as a molecular target for developing low-Cd crops to ensure environmental safety.

Phenotypic characterization of pre-harvest sprouting resistance mutants generated by the CRISPR/Cas9-geminiviral replicon system in rice

Pre-harvest sprouting is a critical phenomenon involving germination of seeds in the mother plant before harvest under relative humid conditions and reduced dormancy. In this paper, we generated HDR mutant lines with one region SNP (C/T) and an insertion of 6 bp (GGT/GGTGGCGGC) in OsERF1 genes for pre-harvest sprouting (PHS) resistance using CRISPR/Cas9 and a geminiviral replicon system. The incidence of HDR was 2.6% in transformed calli. T1 seeds were harvested from 12 HDR-induced calli and named ERF1-hdr line. Molecular stability, key agronomic properties, physiological properties, and biochemical properties of target genes in the ERF1-hdr line were investigated for three years. The ERF1-hdr line showed significantly enhanced seed dormancy and pre-harvest sprouting resistance. qRT-PCR analysis suggested that enhanced ABA signaling resulted in a stronger phenotype of PHS resistance. These results indicate that efficient HDR can be achieved through SNP/InDel replacement using a single and modular configuration applicable to different rice targets and other crops. This work demonstrates the potential to replace all genes with elite alleles within one generation and greatly expands our ability to improve agriculturally important traits. [BMB Reports 2024; 57(2): 79-85].

WEAK SEED DORMANCY 1, an aminotransferase protein, regulates seed dormancy in rice through the GA and ABA pathways

Seed dormancy is a critical trait that enhances plant survival by preventing seed germination at the wrong time or under unsuitable conditions. Lack of seed dormancy in rice can lead to pre-harvest sprouting on mother plants leading to reduced yield and seed quality. Although some genes have been identified, knowledge of regulation of seed dormancy is limited. Here, we characterized a weak seed dormancy mutant named weak seed dormancy 1 (wsd1) that showed a higher seed germination percentage than the wild-type following the harvest ripeness. We cloned the WSD1 encoding an aminotransferase protein using a MutMap approach. WSD1 was stably expressed after imbibition and its protein was localized in the endoplasm reticulum. A widely targeted metabolomics assay and amino acid analysis showed that WSD1 had a role in regulating homeostasis of amino acids. PAC treatment and RNA-seq analysis showed that WSD1 regulates seed dormancy by involvement in the GA biosynthesis pathway. GA1 content and expression of GA biosynthesis-related genes were increased in the wsd1 mutant compared with the wild-type. The wsd1 mutant had reduced sensitivity to ABA. Our overall results indicated that WSD1 regulates seed dormancy by balancing the ABA and GA pathways.

Dry side of the core: a meta-analysis addressing the original nature of the ABA signalosome at the onset of seed imbibition

The timing of seedling emergence is a major agricultural and ecological fitness trait, and seed germination is controlled by a complex molecular network including phytohormone signalling. One such phytohormone, abscisic acid (ABA), controls a large array of stress and developmental processes, and researchers have long known it plays a crucial role in repressing germination. Although the main molecular components of the ABA signalling pathway have now been identified, the molecular mechanisms through which ABA elicits specific responses in distinct organs is still enigmatic. To address the fundamental characteristics of ABA signalling during germination, we performed a meta-analysis focusing on the Arabidopsis dry seed proteome as a reflexion basis. We combined cutting-edge proteome studies, comparative functional analyses, and protein interaction information with genetic and physiological data to redefine the singular composition and operation of the ABA core signalosome from the onset of seed imbibition. In addition, we performed a literature survey to integrate peripheral regulators present in seeds that directly regulate core component function. Although this may only be the tip of the iceberg, this extended model of ABA signalling in seeds already depicts a highly flexible system able to integrate a multitude of information to fine-tune the progression of germination.

A rice seed-specific glycine-rich protein OsDOR1 interacts with GID1 to repress GA signaling and regulates seed dormancy

Seed dormancy is an important agronomic trait under the control of complex genetic and environmental interactions, which have not been yet comprehensively understood. From the field screening of rice mutant library generated by a Ds transposable element, we identified a pre-harvest sprouting (PHS) mutant dor1. This mutant has a single insertion of Ds element at the second exon of OsDOR1 (LOC_Os03g20770), which encodes a novel seed-specific glycine-rich protein. This gene successfully complemented the PHS phenotype of dor1 mutant and its ectopic expression enhanced seed dormancy. Here, we demonstrated that OsDOR1 protein binds to the GA receptor protein, OsGID1 in rice protoplasts, and interrupts with the formation OsGID1-OsSLR1 complex in yeast cells. Co-expression of OsDOR1 with OsGID1 in rice protoplasts attenuated the GA-dependent degradation of OsSLR1, the key repressor of GA signaling. We showed the endogenous OsSLR1 protein level in the dor1 mutant seeds is significantly lower than that of wild type. The dor1 mutant featured a hypersensitive GA-response of α-amylase gene expression during seed germination. Based on these findings, we suggest that OsDOR1 is a novel negative player of GA signaling operated in the maintenance of seed dormancy. Our findings provide a novel source of PHS resistance.

Genome-Wide Association Study for Seed Dormancy Using Re-Sequenced Germplasm under Multiple Conditions in Rice

Seed dormancy is a key factor used to determine seed germination in rice production. So far, only a few genes controlling seed dormancy have been reported, and the genetic mechanism of rice seed dormancy is still elusive. In this study, a population of 195 diverse re-sequenced accessions from 40 countries was evaluated for the seed germination rate (GR) without dormancy breaking (WDB) as a control and under dry heating (DH) and gibberellic acid (GA) treatments, as dormancy breaking agents to identify QTLs for seed dormancy. Phenotypic assessment revealed that these accessions had abundant variations in seed dormancy. GWAS using 1,120,223 high-quality single nucleotide polymorphisms (SNPs) and a mixed linear model (MLM) incorporating both principal components (PCs) and kinship (K) identified 30 QTLs on 10 chromosomes, accounting for 7.3-20.4% of the phenotypic variance in GR. Ten of the QTLs were located in the regions of previously reported QTLs, while the rest were novel ones. Thirteen high-confidence candidate genes were predicted for the four QTLs detected in two or three conditions (qGR4-4, qGR4-5, qGR8 and qGR11-4) and one QTL with a large effect (qGR3). These genes were highly expressed during seed development and were significantly regulated by various hormone treatments. This study provides new insights into the genetic and molecular basis of rice seed dormancy/germination. The accessions with moderate and strong dormancy and markers for the QTLs and candidate genes are useful for attaining a proper level of seed dormancy.

The SlDOG1 Affect Biosynthesis of Steroidal Glycoalkaloids by Regulating GAME Expression in Tomato

Steroidal alkaloids (SAs) and steroidal glycoalkaloids (SGAs) are common constituents of plant species belonging to the Solanaceae family. However, the molecular mechanism regulating the formation of SAs and SGAs remains unknown. Here, genome-wide association mapping was used to elucidate SA and SGA regulation in tomatoes: a SlGAME5-like glycosyltransferase (Solyc10g085240) and the transcription factor SlDOG1 (Solyc10g085210) were significantly associated with steroidal alkaloid composition. In this study, it was found that rSlGAME5-like can catalyze a variety of substrates for glycosidation and can catalyze SA and flavonol pathways to form O-glucoside and O-galactoside in vitro. The overexpression of SlGAME5-like promoted α-tomatine, hydroxytomatine, and flavonol glycoside accumulation in tomatoes. Furthermore, assessments of natural variation combined with functional analyses identified SlDOG1 as a major determinant of tomato SGA content, which also promoted SA and SGA accumulation via the regulation of GAME gene expression. This study provides new insights into the regulatory mechanisms underlying SGA production in tomatoes.

Antagonistic control of seed dormancy in rice by two bHLH transcription factors

Preharvest sprouting (PHS) due to lack of seed dormancy seriously threatens crop production worldwide. As a complex quantitative trait, breeding of crop cultivars with suitable seed dormancy is hindered by limited useful regulatory genes. Here by repeatable phenotypic characterization of fixed recombinant individuals, we report a quantitative genetic locus, Seed Dormancy 6 (SD6), from aus-type rice, encoding a basic helix-loop-helix (bHLH) transcription factor, which underlies the natural variation of seed dormancy. SD6 and another bHLH factor inducer of C-repeat binding factors expression 2 (ICE2) function antagonistically in controlling seed dormancy by directly regulating the ABA catabolism gene ABA8OX3, and indirectly regulating the ABA biosynthesis gene NCED2 via OsbHLH048, in a temperature-dependent manner. The weak-dormancy allele of SD6 is common in cultivated rice but undergoes negative selection in wild rice. Notably, by genome editing SD6 and its wheat homologs, we demonstrated that SD6 is a useful breeding target for alleviating PHS in cereals under field conditions.

Understanding of Hormonal Regulation in Rice Seed Germination

Seed germination is a critical stage during the life cycle of plants. It is well known that germination is regulated by a series of internal and external factors, especially plant hormones. In Arabidopsis, many germination-related factors have been identified, while in rice, the important crop and monocot model species and the further molecular mechanisms and regulatory networks controlling germination still need to be elucidated. Hormonal signals, especially those of abscisic acid (ABA) and gibberellin (GA), play a dominant role in determining whether a seed germinates or not. The balance between the content and sensitivity of these two hormones is the key to the regulation of germination. In this review, we present the foundational knowledge of ABA and GA pathways obtained from germination research in Arabidopsis. Then, we highlight the current advances in the identification of the regulatory genes involved in ABA- or GA-mediated germination in rice. Furthermore, other plant hormones regulate seed germination, most likely by participating in the ABA or GA pathways. Finally, the results from some regulatory layers, including transcription factors, post-transcriptional regulations, and reactive oxygen species, are also discussed. This review aims to summarize our current understanding of the complex molecular networks involving the key roles of plant hormones in regulating the seed germination of rice.

Transcriptomics View over the Germination Landscape in Biofortified Rice

Hidden hunger, or micronutrient deficiency, is a worldwide problem. Several approaches are employed to alleviate its effects (e.g., promoting diet diversity, use of dietary supplements, chemical fortification of processed food), and among these, biofortification is considered as one of the most cost-effective and highly sustainable. Rice is one of the best targets for biofortification since it is a staple food for almost half of the world’s population as a high-energy source but with low nutritional value. Multiple biofortified rice lines have been produced during the past decades, while few studies also reported modifications in germination behavior (in terms of enhanced or decreased germination percentage or speed). It is important to underline that rapid, uniform germination, and seedling establishment are essential prerequisites for crop productivity. Combining the two traits, biofortified, highly-nutritious seeds with improved germination behavior can be envisaged as a highly-desired target for rice breeding. To this purpose, information gathered from transcriptomics studies can reveal useful insights to unveil the molecular players governing both traits. The present review aims to provide an overview of transcriptomics studies applied at the crossroad between biofortification and seed germination, pointing out potential candidates for trait pyramiding.

Identification of Genes and MicroRNAs Affecting Pre-harvest Sprouting in Rice (Oryza sativa L.) by Transcriptome and Small RNAome Analyses

Pre-harvest sprouting (PHS) is one of the primary problems associated with seed dormancy in rice (Oryza sativa L.). It causes yield loss and reduces grain quality under unpredictable humid conditions at the ripening stage, thus affecting the economic value of the rice crop. To resolve this issue, understanding the molecular mechanism underlying seed dormancy in rice is important. Recent studies have shown that seed dormancy is affected by a large number of genes associated with plant hormone regulation. However, understanding regarding the effect of heat stress on seed dormancy and plant hormones is limited. This study compared the transcriptome and small RNAome of the seed embryo and endosperm of two contrasting japonica rice accessions, PHS susceptible (with low seed dormancy) and PHS resistant (with high seed dormancy), at three different maturation stages. We found that 9,068 genes and 35 microRNAs (miRNAs) were differentially expressed in the embryo, whereas 360 genes were differentially expressed in the endosperm. Furthermore, we identified and verified the candidate genes associated with seed dormancy and heat stress-related responses in rice using quantitative real-time PCR. We newly discovered eight hormone-related genes, four heat shock protein-related genes, and two miRNAs potentially involved in PHS. These findings provide a strong foundation for understanding the dynamics of transcriptome and small RNAome of hormone- and heat stress-related genes, which affect PHS during seed maturation.

Genome-Wide Identification and Expression Analysis of OsbZIP09 Target Genes in Rice Reveal Its Mechanism of Controlling Seed Germination

Seed dormancy and germination are key events in plant development and are critical for crop production, and defects in seed germination or the inappropriate release of seed dormancy cause substantial losses in crop yields. Rice is the staple food for more than half of the world's population, and preharvest sprouting (PHS) is one of the most severe problems in rice production, due to a low level of seed dormancy, especially under warm and damp conditions. Therefore, PHS leads to yield loss and a decrease in rice quality and vitality. We reveal that mutation of OsbZIP09 inhibited rice PHS. Analysis of the expression of OsbZIP09 and its encoded protein sequence and structure indicated that OsbZIP09 is a typical bZIP transcription factor that contains conserved bZIP domains, and its expression is induced by ABA. Moreover, RNA sequencing (RNA-seq) and DNA affinity purification sequencing (DAP-seq) analyses were performed and 52 key direct targets of OsbZIP09 were identified, including OsLOX2 and Late Embryogenesis Abundant (LEA) family genes, which are involved in controlling seed germination. Most of these key targets showed consistent changes in expression in response to abscisic acid (ABA) treatment and OsbZIP09 mutation. The data characterize a number of key target genes that are directly regulated by OsbZIP09 and contribute to revealing the molecular mechanism that underlies how OsbZIP09 controls rice seed germination.