Spatiotemporal modulation of abscisic acid and gibberellin metabolism and signalling mediates the effects of suboptimal and supraoptimal temperatures on seed germination in wheat (Triticum aestivum L.)

Modulation in the ratio of abscisic acid to gibberellin level determines genetic variation of seed dormancy in barley (Hordeum vulgare L.)

Abscisic acid (ABA) and gibberellin (GA) are major regulators of seed dormancy, an adaptive trait closely associated with preharvest sprouting. This study examined transcriptional regulation of ABA and GA metabolism genes and modulation of ABA and GA levels in seeds of barley genotypes exhibiting a range of dormancy phenotype. We observed a very strong negative correlation between genetic variation in seed germination and embryonic ABA level (r = 0.85), which is regulated by transcriptional modulation of HvNCED1 and/or HvCYP707A genes. A strong positive correlation was evident between variation in seed germination and GA level (r = 0.64), mediated via transcriptional regulation of GA biosynthesis genes, HvGA20ox2 and/or HvGA3oxs, and GA catabolism genes, HvGA2ox3 and/or HvGA3ox6. Modulation of the ABA and GA levels in the genotypes led to the prevalence of ABA to GA level ratio that exhibited a very strong negative correlation (r = 0.84) with seed germination, highlighting the importance of a shift in ABA/GA ratio in determining genetic variation of dormancy in barley seeds. Our results overall show that transcriptional regulation of specific ABA and GA metabolism genes underlies genetic variation in ABA/GA ratio and seed dormancy, reflecting the potential use of these genes as molecular tools for enhancing preharvest sprouting resistance in barley.

Spatial specificity of metabolism regulation of abscisic acid-imposed seed germination inhibition in Korean pine (Pinus koraiensis sieb et zucc)

Introduction

Abscisic acid (ABA) can negatively regulate seed germination, but the mechanisms of ABA-mediated metabolism modulation are not well understood. Moreover, it remains unclear whether metabolic pathways vary with the different tissue parts of the embryo, such as the radicle, hypocotyl and cotyledon.

Methods

In this report, we performed the first comprehensive metabolome analysis of the radicle and hypocotyl + cotyledon in Pinus koraiensis seeds in response to ABA treatment during germination.

Results and discussion

Metabolome profiling showed that following ABA treatment, 67 significantly differentially accumulated metabolites in the embryo were closely associated with pyrimidine metabolism, phenylalanine metabolism, cysteine and methionine metabolism, galactose metabolism, terpenoid backbone biosynthesis, and glutathione metabolism. Meanwhile, 62 metabolites in the hypocotyl + cotyledon were primarily involved in glycerophospholipid metabolism and glycolysis/gluconeogenesis. We can conclude that ABA may inhibit Korean pine seed germination primarily by disrupting the biosynthesis of certain plant hormones mediated by cysteine and methionine metabolism and terpenoid backbone biosynthesis, as well as reducing the reactive oxygen species scavenging ability regulated by glutathione metabolism and shikimate pathway in radicle. ABA may strongly disrupt the structure and function of cellular membranes due to alterations in glycerophospholipid metabolism, and weaken glycolysis/gluconeogenesis in the hypocotyl + cotyledon, both of which are major contributors to ABA-mediated inhibition of seed germination. These results highlight that the spatial modulation of metabolic pathways in Pinus koraiensis seeds underlies the germination response to ABA.

Identification of Novel Loci Precisely Modulating Pre-Harvest Sprouting Resistance and Red Color Components of the Seed Coat in T. aestivum L

The association between pre-harvest sprouting (PHS) and seed coat color has long been recognized. Red-grained wheats generally exhibit greater PHS resistance compared to white-grained wheat, although variability in PHS resistance exists within red-grained varieties. Here, we conducted a genome-wide association study on a panel consisting of red-grained wheat varieties, aimed at uncovering genes that modulate PHS resistance and red color components of seed coat using digital image processing. Twelve loci associated with PHS traits were identified, nine of which were described for the first time. Genetic loci marked by SNPs AX-95172164 (chromosome 1B) and AX-158544327 (chromosome 7D) explained approximately 25% of germination index variance, highlighting their value for breeding PHS-resistant varieties. The most promising candidate gene for PHS resistance was TraesCS6B02G147900, encoding a protein involved in aleurone layer morphogenesis. Twenty-six SNPs were significantly associated with grain color, independently of the known Tamyb10 gene. Most of them were related to multiple color characteristics. Prioritization of genes within the revealed loci identified TraesCS1D03G0758600 and TraesCS7B03G1296800, involved in the regulation of pigment biosynthesis and in controlling pigment accumulation. In conclusion, our study identifies new loci associated with grain color and germination index, providing insights into the genetic mechanisms underlying these traits.

Temporal and spatial transcriptional regulation of phytohormone metabolism during seed development in barley (Hordeum vulgare L.)

Plant hormones play important roles in seed development; however, transcriptional regulation of their metabolism and levels of the respective bioactive forms during barley seed development is poorly understood. To this end, this study performed a comprehensive analysis of changes in the expression patterns phytohormone metabolism genes and levels of the respective bioactive forms in the embryo and endosperm tissues. Our study showed the presence of elevated levels of abscisic acid (ABA), bioactive forms of gibberellins (GAs), jasmonate (JA) and cytokinins (CKs), auxin and salicylic acid (SA) in the endosperm and embryo tissues at early stage of seed filling (SF). The levels of all hormones in both tissues, except that of ABA, decreased to low levels during SF. In contrast, embryonic ABA level increased during SF and peaked at physiological maturity (PM) while the endospermic ABA was maintained at a similar level observed during SF. Although its level decreased high amount of ABA was still present in the embryo during post-PM. We detected low levels of ABA in the endosperm and all the other hormones in both tissues during post-PM phase except the relatively higher levels of jasmonoyl-isoleucine and SA detected at late stage of post-PM. Our data also showed that spatiotemporal changes in the levels of plant hormones during barley seed development are mediated by the expression of specific genes involved in their respective metabolic pathways. These results indicate that seed development in barley is mediated by spatiotemporal modulation in the metabolism and levels of plant hormones.

ZmCIPK32 positively regulates germination of stressed seeds via gibberellin signal

Calcineurin B-like proteins (CBLs) as specific calcium sensors that interact with CBL-interacting protein kinases (CIPKs) play a key role in the regulation of plant development and abiotic stress tolerance. In this study, we isolated and characterized the CIPK32 gene from Zea mays. ZmCIPK32 showed that it comprised 440 amino acids and a conserved NAF motif responsible for the interaction with CBLs localized in the cytoplasm and cell membrane. The interaction of ZmCIPK32 with ZmCBL1 and ZmCBL9 demonstrated using yeast two-hybrid system and bimolecular fluorescence complementation assay required the presence of the NAF domain. Overexpression of ZmCIPK32 promoted early germination in transgenic Arabidopsis seeds relative to that observed in wild-type (WT) plants under mannitol treatment. In addition, ZmCIPK32-overexpressing plants were insensitive to treatments with exogenous abscisic acid and paclobutrazol (PBZ) at seed germination and early seedling stages. Expression levels of the key genes GA20ox and GA3ox involved in the synthesis of gibberellin (GA) were increased, whereas expression levels of genes involved in the conversion of active GA to inactive forms and GA signaling were reduced in ZmCIPK32-overexpressing plants relative to those in WT plants under mannitol and PBZ treatments. Furthermore, overexpression of ZmCIPK32 increased GA level but decreased abscisic acid level in transgenic lines compared to the respective levels in WT plants under PBZ or mannitol treatments. Our results suggest that ZmCIPK32 positively regulates seed germination under stressed conditions by modulating GA signals.

The GATA transcription factor TaGATA1 recruits demethylase TaELF6-A1 and enhances seed dormancy in wheat by directly regulating TaABI5

Seed dormancy is an important agronomic trait in crops, and plants with low dormancy are prone to preharvest sprouting (PHS) under high-temperature and humid conditions. In this study, we report that the GATA transcription factor TaGATA1 is a positive regulator of seed dormancy by regulating TaABI5 expression in wheat. Our results demonstrate that TaGATA1 overexpression significantly enhances seed dormancy and increases resistance to PHS in wheat. Gene expression patterns, abscisic acid (ABA) response assay, and transcriptome analysis all indicate that TaGATA1 functions through the ABA signaling pathway. The transcript abundance of TaABI5, an essential regulator in the ABA signaling pathway, is significantly elevated in plants overexpressing TaGATA1. Chromatin immunoprecipitation assay (ChIP) and transient expression analysis showed that TaGATA1 binds to the GATA motifs at the promoter of TaABI5 and induces its expression. We also demonstrate that TaGATA1 physically interacts with the putative demethylase TaELF6-A1, the wheat orthologue of Arabidopsis ELF6. ChIP-qPCR analysis showed that H3K27me3 levels significantly decline at the TaABI5 promoter in the TaGATA1-overexpression wheat line and that transient expression of TaELF6-A1 reduces methylation levels at the TaABI5 promoter, increasing TaABI5 expression. These findings reveal a new transcription module, including TaGATA1-TaELF6-A1-TaABI5, which contributes to seed dormancy through the ABA signaling pathway and epigenetic reprogramming at the target site. TaGATA1 could be a candidate gene for improving PHS resistance.

Abscisic acid in plants under abiotic stress: crosstalk with major phytohormones

KEY MESSAGE

Extensive crosstalk exists among ABA and different phytohormones that modulate plant tolerance against different abiotic stress. Being sessile, plants are exposed to a wide range of abiotic stress (drought, heat, cold, salinity and metal toxicity) that exert unwarranted threat to plant life and drastically affect growth, development, metabolism, and yield of crops. To cope with such harsh conditions, plants have developed a wide range of protective phytohormones of which abscisic acid plays a pivotal role. It controls various physiological processes of plants such as leaf senescence, seed dormancy, stomatal closure, fruit ripening, and other stress-related functions. Under challenging situations, physiological responses of ABA manifested in the form of morphological, cytological, and anatomical alterations arise as a result of synergistic or antagonistic interaction with multiple phytohormones. This review provides new insight into ABA homeostasis and its perception and signaling crosstalk with other phytohormones at both molecular and physiological level under critical conditions including drought, salinity, heavy metal toxicity, and extreme temperature. The review also reveals the role of ABA in the regulation of various physiological processes via its positive or negative crosstalk with phytohormones, viz., gibberellin, melatonin, cytokinin, auxin, salicylic acid, jasmonic acid, ethylene, brassinosteroids, and strigolactone in response to alteration of environmental conditions. This review forms a basis for designing of plants that will have an enhanced tolerance capability against different abiotic stress.

Heterologous functional analysis and expression patterns of gibberellin 2-oxidase genes of barley (Hordeum vulgare L.)

The level of bioactive gibberellins (GAs) in plants is regulated partly by their inactivation, mainly by the action of GA 2-oxidases (GA2oxs). This study identified three new GA2ox genes in barley: HvGA2ox1, HvGA2ox3 and HvGA2ox6. Analysis of their nucleotide and putative amino acid sequences revealed that they share high sequence identity with other plant GA2oxs and their corresponding proteins. Phylogenetic analysis revealed the HvGA2ox1, HvGA2ox3 and HvGA2ox6 belong to GA2ox structural classes II, I, and III, respectively. Feeding the HvGA2ox1 and HvGA2ox3 recombinant proteins with the C19-GAs, GA₁ and GA₂₀, resulted in the production of GA₈ and GA₂₉, respectively, with no product detected when they were fed with the C20-GA, GA₁₂. Whereas the HvGA2ox6 recombinant protein was able to convert GA₁₂ to GA₁₁₀, and no product was detected when it was fed with GA₁ or GA₂₀. HvGA2ox1 and HvGA2ox3 were highly expressed in internodes and the endosperm of maturing seeds while HvGA2ox6 was predominantly expressed in the embryos. Salinity stress upregulated the expression of all three genes in seedling tissues. Our results indicate that HvGA2ox1, HvGA2ox3 and HvGA2ox6 encode functional GA2oxs that can regulate GA levels, and therefore growth and development of a barley plant, and its interaction with environment.

SiNCED1, a 9-cis-epoxycarotenoid dioxygenase gene in Setaria italica, is involved in drought tolerance and seed germination in transgenic Arabidopsis

Foxtail millet (Setaria italica L.) is a vital cereal food crop with promising development and utilization potential because of its outstanding ability to resist drought stress. However, the molecular mechanisms underlying its drought stress resistance remain unclear. In this study, we aimed to elucidate the molecular function of a 9-cis-epoxycarotenoid dioxygenase gene, SiNCED1, in the drought stress response of foxtail millet. Expression pattern analysis showed that SiNCED1 expression was significantly induced by abscisic acid (ABA), osmotic stress, and salt stress. Furthermore, ectopic overexpression of SiNCED1 could enhance drought stress resistance by elevating endogenous ABA levels and promoting stomatal closure. Transcript analysis indicated that SiNCED1 modulated ABA-related stress responsive gene expression. In addition, we found that ectopic expression of SiNCED1 delayed seed germination under normal and abiotic stress conditions. Taken together, our results show that SiNCED1 plays a positive role in the drought tolerance and seed dormancy of foxtail millet by modulating ABA biosynthesis. In conclusion, this study revealed that SiNCED1 is an important candidate gene for the improvement of drought stress tolerance in foxtail millet and could be beneficial in the breeding and investigation of drought tolerance in other agronomic crops.

Effects of seasonal temperature regimes on embryo growth and endogenous hormones of Taxus chinensis var. mairei seeds

Seed dormancy is a mechanism that prevents seeds from germinating at times of the year when conditions are unfavorable, that is, when the chance of seed survival is low. Determining the seasonal dynamics of seed dormancy is important for exploring how plant regeneration is adapted to the environment. We studied the seed dormancy status of Taxus chinensis var. mairei, an endangered species in China, under simulated seasonal temperature regimes. The embryo length, embryo-to-seed (E : S) ratio, and percentage of seeds with a split seed coat increased when seeds were stratified at spring and autumn temperature regimes. The abscisic acid (ABA) content decreased during stratification at simulated seasonal temperatures, but no obvious pattern in the content of gibberellic acid (GA) and indole acetic acid (IAA) was observed. The GA-ABA and IAA-ABA ratios increased during stratification. These results suggest that T. chinensis var. mairei seeds have morphophysiological dormancy, and that the seasonal dynamics of seed dormancy break are controlled by endogenous hormones and their balances, which was confirmed by the results of a field experiment. Our study provides useful information for understanding the natural population regeneration and propagation of this threatened species.

Low red/far-red ratio can induce cytokinin degradation resulting in the inhibition of tillering in wheat (Triticum aestivum L.)

Shoot branching is inhibited by a low red/far-red ratio (R/FR). Prior studies have shown that the R/FR suppressed Arabidopsis thaliana branching by promotes bud abscisic acid (ABA) accumulation directly. Given that wheat tiller buds are wrapped in leaf sheaths and may not respond rapidly to a R/FR, systemic cytokinin (CTK) may be more critical. Here, systemic hormonal signals including indole-3-acetic acid (IAA), gibberellins (GA) and CTK and bud ABA signals in wheat were tested under a low R/FR. The results showed that a low R/FR reduced the percentage of tiller occurrence of tiller IV and the tiller number per plant. The low R/FR did not rapidly induced ABA accumulation in the tiller IV because of the protection of the leaf sheath and had little effect on IAA content and signaling in the tiller nodes. The significant change in the CTK levels was observed earlier than those of other hormone (ABA, IAA and GA) and exogenous cytokinin restored the CTK levels and tiller number per plant under low R/FR conditions. Further analysis revealed that the decrease in cytokinin levels was mainly associated with upregulation of cytokinin degradation genes (TaCKX5, TaCKX11) in tiller nodes. In addition, exposure to a decreased R/FR upregulated the expression of GA biosynthesis genes (TaGA20ox1, TaGA3ox2), resulting in elevated GA levels, which might further promote CTK degradation in tiller nodes and inhibit tillering. Therefore, our results provide evidence that the enhancement of cytokinin degradation is a novel mechanism underlying the wheat tillering response to a low R/FR.

Comparative transcriptomic analysis provides insights into the molecular basis underlying pre-harvest sprouting in rice

BACKGROUND

Pre-harvest sprouting (PHS) is one of the most serious rice production constraints in areas where prolonged rainfall occurs during harvest. However, the molecular mechanisms of transcriptional regulation underlying PHS remain largely unknown.

RESULTS

In the current study, comparative transcriptome analyses were performed to characterize the similarities and differences between two rice varieties: PHS-sensitive Jiuxiangzhan (JXZ) and PHS-resistant Meixiangxinzhan (MXXZ). The physiological experimental results indicated that PHS causes a significant decrease in starch content and, in contrast, a significant increase in soluble sugar content and amylase activity. The extent of change in these physiological parameters in the sensitive variety JXZ was greater than that in the resistant variety MXXZ. A total of 9,602 DEGs were obtained from the transcriptome sequencing data, and 5,581 and 4,021 DEGs were identified in JXZ and MXXZ under high humidity conditions, respectively. The KEGG pathway enrichment analysis indicated that many DEGs under high humidity treatment were mainly linked to plant hormone signal transduction, carbon metabolism, starch and sucrose metabolism, and phenylpropanoid biosynthesis. Furthermore, the number of upregulated genes involved in these pathways was much higher in JXZ than in MXXZ, while the number of downregulated genes was higher in MXXZ than in JXZ. These results suggest that the physiological and biochemical processes of these pathways are more active in the PHS-sensitive JXZ than in the PHS-resistant MXXZ.

CONCLUSION

Based on these results, we inferred that PHS in rice results from altered phytohormone regulation, more active carbon metabolism and energy production, and enhanced phenylpropanoid biosynthesis. Our study provides a theoretical foundation for further elucidation of the complex regulatory mechanism of PHS in rice and the molecular breeding of PHS-resistant rice varieties.

Network of the transcriptome and metabolomics reveals a novel regulation of drought resistance during germination in wheat

BACKGROUND AND AIMS

The North China Plain, the highest winter-wheat-producing region of China, is seriously threatened by drought. Traditional irrigation wastes a significant amount of water during the sowing season. Therefore, it is necessary to study the drought resistance of wheat during germination to maintain agricultural ecological security. From several main cultivars in the North China Plain, we screened the drought-resistant cultivar JM47 and drought-sensitive cultivar AK58 during germination using the polyethylene glycol (PEG) drought simulation method. An integrated analysis of the transcriptome and metabolomics was performed to understand the regulatory networks related to drought resistance in wheat germination and verify key regulatory genes.

METHODS

Transcriptional and metabolic changes were investigated using statistical analyses and gene-metabolite correlation networks. Transcript and metabolite profiles were obtained through high-throughput RNA-sequencing data analysis and ultra-performance liquid chromatography quadrupole time-of-flight tandem mass spectrometry, respectively.

KEY RESULTS

A total of 8083 and 2911 differentially expressed genes (DEGs) and 173 and 148 differential metabolites were identified in AK58 and JM47, respectively, under drought stress. According to the integrated analysis results, mammalian target of rapamycin (mTOR) signalling was prominently enriched in JM47. A decrease in α-linolenic acid content was consistent with the performance of DEGs involved in jasmonic acid biosynthesis in the two cultivars under drought stress. Abscisic acid (ABA) content decreased more in JM47 than in AK58, and linoleic acid content decreased in AK58 but increased in JM47. α-Tocotrienol was upregulated and strongly correlated with α-linolenic acid metabolism.

CONCLUSIONS

The DEGs that participated in the mTOR and α-linolenic acid metabolism pathways were considered candidate DEGs related to drought resistance and the key metabolites α-tocotrienol, linoleic acid and l-leucine, which could trigger a comprehensive and systemic effect on drought resistance during germination by activating mTOR-ABA signalling and the interaction of various hormones.

Genetic variation of seed dormancy in wheat (Triticum aestivum L.) is mediated by transcriptional regulation of abscisic acid metabolism and signaling

Abscisic acid (ABA) regulates seed dormancy and therefore preharvest sprouting (PHS) in wheat. This study investigated the contribution of transcriptional regulation of ABA metabolism and signaling genes to genetic variation in dormancy of wheat seeds. Our results showed that genetic variation in seed dormancy is highly correlated with ABA content (r > 0.86), which, in turn, was closely associated with the expression levels of ABA biosynthesis genes, TaNCED1 (r = 0.78) and TaNCED2 (r = 0.67). A relatively lower correlation was observed between ABA content and the expression levels of ABA catabolism genes, TaCYP707A1 (r = 0.51) and TaCYP707A2 (r = 0.57). The expression level of TaABI5 exhibited strong associations with the levels of ABA (r = 0.8) and seed dormancy (r > 0.9), indicating the importance of seed ABA sensitivity in mediating genetic variation in dormancy. Furthermore, high positive correlations were prevalent between the expression patterns of TaABI5 and TaNCED1 (r = 0.91) or TaNCED2 (r = 0.82). Overall, our results implicated the significance of TaNCEDs and TaABI5 in regulating genetic variation in ABA level and sensitivity and thereby seed dormancy, highlighting the potential use of these genes to develop molecular markers for incorporating PHS resistance in wheat.

Architectural analysis of root system and phytohormone biosynthetic genes expression in wheat (Triticum aestivum L.) inoculated with Penicillium oxalicum

In this study, a fungal plant growth promoter Penicillium oxalicum T4 isolated from non-rhizosphere soil of Arunachal Pradesh, India, was screened for different plant growth promoting traits in a gnotobiotic study. Though inoculation improved the overall growth of the plants, critical differences were observed in root architecture. Confocal Laser Scanning Microscope, Scanning electron microscope and stereo microscopic study showed that inoculated wheat plants could develop profuse root hairs as compared to control. Root scanning indicated improvement in cumulative root length, root area, root volume, number of forks, links, crossings, and other parameters. Confocal scanning laser microscope indicated signs of endophytic colonization in wheat roots. Gene expression studies revealed that inoculation of T4 modulated the genes affecting root hair development. Significant differences were marked in the expression levels of TaRSL4, TaEXPB1, TaEXPB23, PIN-FORMED protein, kaurene oxidase, lipoxygenase, ACC synthase, ACC oxidase, 9-cis-epoxycarotenoid dioxygenase, and ABA 8'-hydroxylase genes. These genes contribute to early plant development and ultimately to biomass accumulation and yield. The results suggested that P. oxalicum T4 has potential for growth promotion in wheat and perhaps also in other cereals.

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.

Jasmonate regulates seed dormancy in wheat via modulating the balance between gibberellin and abscisic acid

Jasmonate (JA) regulates seed dormancy and germination; however, the underlying mechanisms remain poorly understood. Furthermore, it is unclear if JA is an essential regulator of dormancy and germination. We investigated whether the role of JA in regulating seed dormancy in wheat (Triticum aestivum L.) is mediated by modulation of gibberellin (GA)/abscisic acid (ABA) balance and if the reciprocal modulation of JA level and sensitivity is required for GA-mediated dormancy loss using physiological, pharmacological, and targeted transcriptomic and metabolomic approaches. JA-induced dormancy release in wheat seeds was associated with no change in GA level but up-regulation of GA signaling and ABA catabolism genes, and reduction of the ABA level. Although JA did not affect the expression levels of ABA signaling genes, up-regulation of germination-associated genes indicates a contribution of reduced ABA sensitivity to dormancy release. After-ripening-mediated dormancy loss was also associated with JA-GA synergistic and JA-ABA antagonistic interplays. The prevalence of no effect of GA, which effectively broke dormancy, on the JA-Ile level and expression patterns of JA biosynthesis/signaling and responsive genes reflects that GA-mediated dormancy release occurs independently of JA. Our study concludes that JA induces seed dormancy release in wheat via modulating ABA/GA balance; however, JA is not an essential regulator of dormancy and germination.

Molecular and functional characterization of a jasmonate resistant gene of wheat (Triticum aestivum L.)

Jasmonates play important roles in several plant developmental processes and responses to biotic and abiotic stresses. This study identified a gene encoding jasmonate resistant 1 (JAR1) protein that catalyzes the production of bioactive jasmonoyl-isoleucine (JA-Ile) from hexaploid wheat (Triticum aestivum L), designated as TaJAR1B. The nucleotide sequence of TaJAR1B and amino acid sequence of the corresponding protein exhibited high identity and similarity with other plant JAR1s. Feeding the culture of E. coli cells heterologously expressing TaJAR1B with jasmonic acid (JA) resulted in the production of JA-Ile, indicating the functionality of TaJAR1B in converting JA to JA-Ile. TaJAR1B was highly expressed in the internodes of adult plants and maturing seeds. Salt treatment induced the expression level of TaJAR1B in seedling tissues. Our results indicate that TaJAR1B encodes a functional JAR and is involved in the regulation of plant growth and developmental processes and response to salinity in wheat.

Effect of Temperature on the Germination of Five Coastal Provenances of Nothofagus glauca (Phil.) Krasser, the Most Representative Species of the Mediterranean Forests of South America

Temperature is one of the most important abiotic factors affecting seed germination, and it is strongly influenced by local site conditions. Seeds of Nothofagus glauca, an endemic and vulnerable species of the Mediterranean region of Chile and the most representative of the Mediterranean forests of South America, were collected. In this study, we evaluated the effect of temperature on different germinative attributes of five N. glauca provenances representative of their natural distribution. The seeds were treated at a constant temperature (i.e., 18 °C, 22 °C, 26 °C, or 30 °C) in the absence of light for 40 days. The results show that in all the provenances, the germination ratio and energy increase linearly with temperature until reaching an optimum temperature (i.e., 22 °C), above which they decrease severely. At 22 °C, the response of average germination speed and germination vigor was significantly higher than with the other temperatures (performance of germination start day was not clear). The base temperature was around 18 °C and the maximum, above 30 °C, which may be close to thermo-inhibition. Given the threat of climate change, it is necessary to increase research in terms of the possible adaptation of this species to increased temperatures and prolonged periods of drought

Seed germination and vigor: ensuring crop sustainability in a changing climate

In the coming decades, maintaining a steady food supply for the increasing world population will require high-yielding crop plants which can be productive under increasingly variable conditions. Maintaining high yields will require the successful and uniform establishment of plants in the field under altered environmental conditions. Seed vigor, a complex agronomic trait that includes seed longevity, germination speed, seedling growth, and early stress tolerance, determines the duration and success of this establishment period. Elevated temperature during early seed development can decrease seed size, number, and fertility, delay germination and reduce seed vigor in crops such as cereals, legumes, and vegetable crops. Heat stress in mature seeds can reduce seed vigor in crops such as lettuce, oat, and chickpea. Warming trends and increasing temperature variability can increase seed dormancy and reduce germination rates, especially in crops that require lower temperatures for germination and seedling establishment. To improve seed germination speed and success, much research has focused on selecting quality seeds for replanting, priming seeds before sowing, and breeding varieties with improved seed performance. Recent strides in understanding the genetic basis of variation in seed vigor have used genomics and transcriptomics to identify candidate genes for improving germination, and several studies have explored the potential impact of climate change on the percentage and timing of germination. In this review, we discuss these recent advances in the genetic underpinnings of seed performance as well as how climate change is expected to affect vigor in current varieties of staple, vegetable, and other crops.

The Effectiveness of the Sexual Reproduction in Selected Clonal and Nonclonal Species of the Genus Ranunculus

Simple Summary

The genus Ranunculus (buttercup) includes over 600 species, some of which are endangered, e.g., Illyrian Buttercup. Knowledge of the reproductive biology of such species may be crucial for conservation action. For this purpose, six species with different reproduction modes (nonclonal reproducing sexually by seeds only, clonal propagating by seeds and additionally vegetatively and apomictic) were observed. Selected features related to the efficiency of sexual reproduction were described: pollen viability, number of fruit set, seed viability and germination. It has been shown that in clonal species, which include the Illyrian Buttercup, the efficiency of sexual reproduction is lower compared to nonclonal species. The results will support conservation action taken for this species.

Abstract

Generative processes have been evaluated in six European buttercup species in order to verify the hypothesis that the reproduction efficiency of clonal species is lower than that of nonclonal ones. The study covered common species (Ficaria verna, Ranunculus auricomus, R. bulbosus, R. cassubicus, R. lanuginosus) and the endangered R. illyricus. The following properties have been assessed: pollen viability (staining method), pollen grain germination and the pollen-tube elongation in pistil tissues (fluorescence microscopy), seed formation efficiency, seed viability (tetrazolium test) and germination ability by introducing factors interrupting dormancy (low temperature and gibberellin application). Additionally, the pistil morphology was documented for R. bulbosus, R. illyricus and R. cassubicus using SEM techniques. It was demonstrated that the reproductive efficiency, expressed as the production of viable seeds able to germinate, was significantly higher in the species reproducing sexually (especially in R. lanuginosus) compared to the clonal ones. However, the complexity observed leads to separation of an additional group (cluster) of apomictic species: R. auricomus and R. cassubicus, distinguished by the lowest pollen viability and a low ability of the seeds to germinate. In the vegetatively reproducing R. illyricus, the seed formation efficiency was just 13.2% despite the having highest number of pistils in its flowers. The developed seeds of this species observed in our experiment were viable, but in general effective methods to stimulate their germination have not been proposed yet. Here, the first comparative study concerning the biology of sexual reproduction of R. illyricus is presented in the context of its decreasing distribution in natural habitats.

Integrative signaling of hydrogen peroxide and gibberellin on Zn-mediated alleviation of thermodormancy in sorghum seeds

Increasing global temperatures could result in decreasing crop production by decreasing seed germination in the field due to thermodormancy acquisition. Certain metals appear to modulate seed thermodormancy, although the exact mechanisms of that effect have not yet been elucidated. We report here the effects of Zn on the thermodormancy of sorghum seeds. Seeds treated with 0 or 200 mg Zn L^-1 were germinated at optimal (30°C) and supra-optimal (40°C) temperatures and their germinability and oxidative stress markers were evaluated. The integrative effects of Zn, abscisic acid (ABA), gibberellin (GA), and H₂ O₂ on the physiology of seed thermodormancy were examined. The supra-optimal germination temperature (40°C) induced seed thermodormancy, which was, however, alleviated by treatment with 200 mg Zn L^-1 . Thermodormancy acquired at supra-optimal temperatures in sorghum seeds must reflect de novo synthesis and accumulation of ABA. Although Zn treatment did not prevent ABA accumulation, it increased the activities of mitochondrial ETC enzymes and decreased the antioxidant enzymes' activity, leading to the accumulation of H₂ O₂ . By increasing mitochondria activity and H₂ O₂ production, Zn may induce GA synthesis and alleviate thermodormancy in sorghum seeds. The pretreatment of sorghum seeds with Zn may therefore improve seed germination and assure increased crop performance under normal (30°C) or rising (up to 40°C) temperatures.

Phenotypic Evaluation and Genetic Analysis of Seedling Emergence in a Global Collection of Wheat Genotypes (Triticum aestivum L.) Under Limited Water Availability

The challenge in establishing an early-sown wheat crop in southern Australia is the need for consistently high seedling emergence when sowing deep in subsoil moisture (>10 cm) or into dry top-soil (4 cm). However, the latter is strongly reliant on a minimum soil water availability to ensure successful seedling emergence. This study aimed to: (1) evaluate 233 Australian and selected international wheat genotypes for consistently high seedling emergence under limited soil water availability when sown in 4 cm of top-soil in field and glasshouse (GH) studies; (2) ascertain genetic loci associated with phenotypic variation using a genome-wide association study (GWAS); and (3) compare across loci for traits controlling coleoptile characteristics, germination, dormancy, and pre-harvest sprouting. Despite significant (P < 0.001) environment and genotype-by-environment interactions within and between field and GH experiments, eight genotypes that included five cultivars, two landraces, and one inbred line had consistently high seedling emergence (mean value > 85%) across nine environments. Moreover, 21 environment-specific quantitative trait loci (QTL) were detected in GWAS analysis on chromosomes 1B, 1D, 2B, 3A, 3B, 4A, 4B, 5B, 5D, and 7D, indicating complex genetic inheritance controlling seedling emergence. We aligned QTL for known traits and individual genes onto the reference genome of wheat and identified 16 QTL for seedling emergence in linkage disequilibrium with coleoptile length, width, and cross-sectional area, pre-harvest sprouting and dormancy, germination, seed longevity, and anthocyanin development. Therefore, it appears that seedling emergence is controlled by multifaceted networks of interrelated genes and traits regulated by different environmental cues.

The stereoselectivity of metconazole on wheat grain filling and harvested seeds germination: Implication for the application of triazole chiral pesticides

Plant growth can be influenced by the application of triazole pesticides as these regulate physiological processes such as plant hormonal levels and enzyme activity. Homology modeling and molecular docking studies suggested that inhibition of ADP-glucose pyrophosphorylase activity in two trans-stereoisomers treatments hinders starch accumulation during the grain filling stage. A field experiment investigated the effects of metconazole racemate, cis-1R,5S-stereostereoisomer, and cis-1S,5R-stereoisomer application at the flowering stage on wheat grain ripening and yield. The concentrations of racemate and both cis-stereoisomers were detected in wheat plant and grain samples. Compared with the racemate, both cis-stereoisomers were more persistent in the matrices. Treatment with cis-1R,5S-stereoisomer decreased grain weight and yield of wheat by delaying chlorophyll degradation, increasing the ethylene content, and decreasing the level of abscisic acid. The germination of harvested seeds was adversely affected by racemate treatment as a result of gibberellin and abscisic acid metabolism regulation and the transcription of signaling-related genes. Therefore, cis-1S,5R-stereoisomer was recommended to be used as metconazole pesticide at the flowering stage.

A shift in abscisic acid/gibberellin balance underlies retention of dormancy induced by seed development temperature

Through a combination of physiological, pharmacological, molecular and targeted metabolomics approaches, we showed that retention of wheat (Triticum aestivum L.) seed dormancy levels induced by low and high seed development temperatures during post-desiccation phases is associated with modulation of gibberellin (GA) level and seed responsiveness to abscisic acid (ABA) and GA via expression of TaABI5 and TaGAMYB, respectively. Dormancy retention during imbibition, however, is associated with modulations of both ABA level and responsiveness via expression of specific ABA metabolism (TaNCED2 and TaCYP707A1) and signalling (TaPYL2, TaSnRK2, TaABI3, TaABI4 and TaABI5) genes, and alterations of GA levels and responsiveness through expression of specific GA biosynthesis (TaGA20ox1, TaGA20ox2 and TaGA3ox2) and signalling (TaGID1 and TaGID2) genes, respectively. Expression patterns of GA signalling genes, TaRHT1 and TaGAMYB, lacked positive correlation with that of GA regulated genes and dormancy level observed in seeds developed at the two temperatures, implying their regulation at post-transcriptional level. Our results overall implicate that a shift in ABA/GA balance underlies retention of dormancy levels induced by seed development temperature during post-desiccation and imbibition phases. Consistently, genes regulated by ABA and GA during imbibition overlapped with those differentially expressed between imbibed seeds developed at the two temperatures and mediate different biological functions.

ABA Metabolism and Homeostasis in Seed Dormancy and Germination

Abscisic acid (ABA) is a key hormone that promotes dormancy during seed development on the mother plant and after seed dispersal participates in the control of dormancy release and germination in response to environmental signals. The modulation of ABA endogenous levels is largely achieved by fine-tuning, in the different seed tissues, hormone synthesis by cleavage of carotenoid precursors and inactivation by 8'-hydroxylation. In this review, we provide an overview of the current knowledge on ABA metabolism in developing and germinating seeds; notably, how environmental signals such as light, temperature and nitrate control seed dormancy through the adjustment of hormone levels. A number of regulatory factors have been recently identified which functional relationships with major transcription factors, such as ABA INSENSITIVE3 (ABI3), ABI4 and ABI5, have an essential role in the control of seed ABA levels. The increasing importance of epigenetic mechanisms in the regulation of ABA metabolism gene expression is also described. In the last section, we give an overview of natural variations of ABA metabolism genes and their effects on seed germination, which could be useful both in future studies to better understand the regulation of ABA metabolism and to identify candidates as breeding materials for improving germination properties.

Pre-harvest sprouting in cereals: genetic and biochemical mechanisms

With the growth of the global population and the increasing frequency of natural disasters, crop yields must be steadily increased to enhance human adaptability to risks. Pre-harvest sprouting (PHS), a term mainly used to describe the phenomenon in which grains germinate on the mother plant directly before harvest, is a serious global problem for agricultural production. After domestication, the dormancy level of cultivated crops was generally lower than that of their wild ancestors. Although the shortened dormancy period likely improved the industrial performance of cereals such as wheat, barley, rice, and maize, the excessive germination rate has caused frequent PHS in areas with higher rainfall, resulting in great economic losses. Here, we systematically review the causes of PHS and its consequences, the major indicators and methods for PHS assessment, and emphasize the biological significance of PHS in crop production. Wheat quantitative trait loci functioning in the control of PHS are also comprehensively summarized in a meta-analysis. Finally, we use Arabidopsis as a model plant to develop more complete PHS regulatory networks for wheat. The integration of this information is conducive to the development of custom-made cultivated lines suitable for different demands and regions, and is of great significance for improving crop yields and economic benefits.

Gibberellin Metabolism and Signaling: Targets for Improving Agronomic Performance of Crops

Gibberellins (GAs) are a class of tetracyclic diterpenoid phytohormones that regulate many aspects of plant development, including seed germination, stem elongation, leaf expansion, pollen maturation, and the development of flowers, fruits and seeds. During the past decades, the primary objective of crop breeding programs has been to increase productivity or yields. 'Green Revolution' genes that can produce semidwarf, high-yielding crops were identified as GA synthesis or response genes, confirming the value of research on GAs in improving crop productivity. The manipulation of GA status either by genetic alteration or by exogenous application of GA or GA biosynthesis inhibitors is often used to optimize plant growth and yields. In this review, we summarize the roles of GAs in major aspects of crop growth and development and present the possible targets for the fine-tuning of GA metabolism and signaling as a promising strategy for crop improvement.

Stereoselective Physiological Effects of Metconazole on Seed Germination and Seedling Growth of Wheat

In addition to their fungicidal activity, many triazole fungicides function as plant regulators, which might impose adverse effects on the growth and development of crops. For chiral triazole fungicides, these effects can be alleviated by applying stereoisomers with high fungicidal and low regulator activities. This study investigated the stereoselectivity of four stereoisomers and the racemate of metconazole (2.5 g/100 kg seeds) on emergence and growth of seedlings (BBCH 01-14) in wheat. Wheat seedlings, coated with cis-1S,5R-metconazole, had a significantly lower seedling emergence ratio and shoot length than other metconazole treatments; however, the opposite effects were observed in the trans-1S,5S-metconazole treatment. With regard to the hormonal level, enzyme activity, and gene transcription of gibberellin (GA) and jasmonic acid (JA), cis-1S,5R-metconazole treatment inhibited GA biosynthesis while trans-1S,5S-metconazole treatment promoted GA biosynthesis. Moreover, cis-1S,5R-metconazole, trans-1S,5S-metconazole, trans-1R,5R-metconazole, and racemate treatments increased JA biosynthesis. The oxidative stress responses in trans-1R,5R-metconazole and racemate treatments were more intensive. Therefore, compared with the control, treatment with cis-1R,5S-metcoanzole exhibited minimal influence on wheat seedling growth. The results showed that the application of pure cis-1R,5S-metcoanzole (instead of the racemate) in agricultural management could decrease the risks associated with crop growth and developmental damage.

Ethylene regulates post-germination seedling growth in wheat through spatial and temporal modulation of ABA/GA balance

This study aimed to gain insights into the molecular mechanisms underlying the role of ethylene in regulating germination and seedling growth in wheat by combining pharmacological, molecular, and metabolomics approaches. Our study showed that ethylene does not affect radicle protrusion but controls post-germination endospermic starch degradation through transcriptional regulation of specific α-amylase and α-glucosidase genes, and this effect is mediated by alteration of endospermic bioactive gibberellin (GA) levels, and GA sensitivity via expression of the GA signaling gene, TaGAMYB. Our data implicated ethylene as a positive regulator of embryo axis and coleoptile growth through transcriptional regulation of specific TaEXPA genes. These effects were associated with modulation of GA levels and sensitivity, through expression of GA metabolism (TaGA20ox1, TaGA3ox2, and TaGA2ox6) and signaling (TaGAMYB) genes, respectively, and/or the abscisic acid (ABA) level and sensitivity, via expression of specific ABA metabolism (TaNCED2 or TaCYP707A1) and signaling (TaABI3) genes, respectively. Ethylene appeared to regulate the expression of TaEXPA3 and thereby root growth through its control of coleoptile ABA metabolism, and root ABA signaling via expression of TaABI3 and TaABI5. These results show that spatiotemporal modulation of ABA/GA balance mediates the role of ethylene in regulating post-germination storage starch degradation and seedling growth in wheat.

Seed quality and carbon primary metabolism

Improving seed quality is amongst the most important challenges of contemporary agriculture. In fact, using plant varieties with better germination rates that are more tolerant to stress during seedling establishment may improve crop yield considerably. Therefore, intense efforts are currently being devoted to improve seed quality in many species, mostly using genomics tools. However, despite its considerable importance during seed imbibition and germination processes, primary carbon metabolism in seeds is less studied. Our knowledge of the physiology of seed respiration and energy generation and the impact of these processes on seed performance have made limited progress over the past three decades. In particular, (isotope-assisted) metabolomics of seeds has only been assessed occasionally, and there is limited information on possible quantitative relationships between metabolic fluxes and seed quality. Here, we review the recent literature and provide an overview of potential links between metabolic efficiency, metabolic biomarkers, and seed quality and discuss implications for future research, including a climate change context.

Unraveling Molecular and Genetic Studies of Wheat (Triticum aestivum L.) Resistance against Factors Causing Pre-Harvest Sprouting

Pre-harvest sprouting (PHS) is one of the most important factors having adverse effects on yield and grain quality all over the world, particularly in wet harvest conditions. PHS is controlled by both genetic and environmental factors and the interaction of these factors. Breeding varieties with high PHS resistance have important implications for reducing yield loss and improving grain quality. The rapid advancements in the wheat genomic database along with transcriptomic and proteomic technologies have broadened our knowledge for understanding the regulatory mechanism of PHS resistance at transcriptomic and post-transcriptomic levels. In this review, we have described in detail the recent advancements on factors influencing PHS resistance, including grain color, seed dormancy, α-amylase activity, plant hormones (especially abscisic acid and gibberellin), and QTL/genes, which are useful for mining new PHS-resistant genes and developing new molecular markers for multi-gene pyramiding breeding of wheat PHS resistance, and understanding the complicated regulatory mechanism of PHS resistance.

Hormonal regulation in adventitious roots and during their emergence under waterlogged conditions in wheat

To gain insights into the molecular mechanisms underlying hormonal regulation in adventitious roots and during their emergence under waterlogged conditions in wheat, the present study investigated transcriptional regulation of genes related to hormone metabolism and transport in the root and stem node tissues. Waterlogging-induced inhibition of axile root elongation and lateral root formation, and promotion of surface adventitious and axile root emergence and aerenchyma formation are associated with enhanced expression levels of ethylene biosynthesis genes, ACS7 and ACO2, in both tissues. Inhibition of axile root elongation is also related to increased root indole acetic acid (IAA) and jasmonate (JA) levels that are associated with up-regulation of specific IAA biosynthesis/transport (TDC, YUC1, and PIN9) and JA metabolism (LOX8, AOS1, AOC1, and JAR1) genes, and transcriptional alteration of gibberellin (GA) metabolism genes (GA3ox2 and GA2ox8). Adventitious root emergence from waterlogged stem nodes is associated with increased levels of IAA and GA but decreased levels of cytokinin and abscisic acid (ABA), which are regulated through the expression of specific IAA biosynthesis/transport (TDC, YUC1, and PIN9), cytokinin metabolism (IPT5-2, LOG1, CKX5, and ZOG2), ABA biosynthesis (NCED1 and NCED2), and GA metabolism (GA3ox2 and GA2ox8) genes. These results enhance our understanding of the molecular mechanisms underlying the adaptive response of wheat to waterlogging.

Climate resilient crops for improving global food security and safety

Food security and the protection of the environment are urgent issues for global society, particularly with the uncertainties of climate change. Changing climate is predicted to have a wide range of negative impacts on plant physiology metabolism, soil fertility and carbon sequestration, microbial activity and diversity that will limit plant growth and productivity, and ultimately food production. Ensuring global food security and food safety will require an intensive research effort across the food chain, starting with crop production and the nutritional quality of the food products. Much uncertainty remains concerning the resilience of plants, soils, and associated microbes to climate change. Intensive efforts are currently underway to improve crop yields with lower input requirements and enhance the sustainability of yield through improved biotic and abiotic stress tolerance traits. In addition, significant efforts are focused on gaining a better understanding of the root/soil interface and associated microbiomes, as well as enhancing soil properties.

Molecular Mechanisms Underlying Abscisic Acid/Gibberellin Balance in the Control of Seed Dormancy and Germination in Cereals

Seed dormancy is an adaptive trait that does not allow the germination of an intact viable seed under favorable environmental conditions. Non-dormant seeds or seeds with low level of dormancy can germinate readily under optimal environmental conditions, and such a trait leads to preharvest sprouting, germination of seeds on the mother plant prior to harvest, which significantly reduces the yield and quality of cereal crops. High level of dormancy, on the other hand, may lead to non-uniform germination and seedling establishment. Therefore, intermediate dormancy is considered to be a desirable trait as it prevents the problems of sprouting and allows uniformity of postharvest germination of seeds. Induction, maintenance, and release of seed dormancy are complex physiological processes that are influenced by a wide range of endogenous and environmental factors. Plant hormones, mainly abscisic acid (ABA) and gibberellin (GA), are the major endogenous factors that act antagonistically in the control of seed dormancy and germination; ABA positively regulates the induction and maintenance of dormancy, while GA enhances germination. Significant progress has been made in recent years in the elucidation of molecular mechanisms regulating ABA/GA balance and thereby dormancy and germination in cereal seeds, and this review summarizes the current state of knowledge on the topic.