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

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.

Ethylene Is Crucial in Abscisic Acid-Mediated Modulation of Seed Vigor, Growth, and Photosynthesis of Salt-Treated Mustard

The current study explored the differential interaction between ethylene (ET) and abscisic acid (ABA) in relation to salt stress in mustard (Brassica juncea L.) plants. Significant reductions in seed germination, growth, and photosynthesis were observed with 100 mmol NaCl. Among the cultivars tested, the Pusa Vijay cultivar was noted as ET-sensitive. Pusa Vijay responded maximally to an application of 2.0 mmol ethephon (Eth; 2-chloethyl phosphonic acid-ethylene source), and exhibited the greatest growth, photosynthesis, activity of 1-aminocyclopropane carboxylic acid (ACC) synthase (ACS), and ET evolution. Notably, Eth (2.0 mmol) more significantly improved the seed germination percentage, germination and vigor index, amylase activity, and reduced H2O2 content under salt stress, while ABA (25 µmol) had negative effects. Moreover, the individual application of Eth and ABA on Pusa Vijay under both optimal and salt-stressed conditions increased the growth and photosynthetic attributes, nitrogen (N) and sulfur (S) assimilation, and antioxidant defense machinery. The addition of aminoethoxyvinylglycine (0.01 µmol AVG, ET biosynthesis inhibitor) to ABA + NaCl-treated plants further added to the effects of ABA on parameters related to seed germination and resulted in less effectiveness of growth and photosynthesis. In contrast, the effects of Eth were seen with the addition of fluoridone (25 µmol Flu, ABA biosynthesis inhibitor) to Eth + NaCl. Thus, it can be suggested that ET is crucial for alleviating salt-induced inhibition in seed germination, growth, and photosynthesis, while ABA collaborated with ET to offer protection by regulating nutrient assimilation and enhancing antioxidant metabolism. These findings provide insight into the complex regulatory processes involved in ET-ABA interaction, enhancing our understanding of plant growth and development and the mitigation of salt stress in mustard. It opens pathways for developing hormonal-based strategies to improve crop productivity and resilience, ultimately benefiting agricultural practices amidst a challenging environment.

Decrypting Molecular Mechanisms Involved in Counteracting Copper and Nickel Toxicity in Jack Pine (Pinus banksiana) Based on Transcriptomic Analysis

The remediation of copper and nickel-afflicted sites is challenged by the different physiological effects imposed by each metal on a given plant system. Pinus banksiana is resilient against copper and nickel, providing an opportunity to build a valuable resource to investigate the responding gene expression toward each metal. The objectives of this study were to (1) extend the analysis of the Pinus banksiana transcriptome exposed to nickel and copper, (2) assess the differential gene expression in nickel-resistant compared to copper-resistant genotypes, and (3) identify mechanisms specific to each metal. The Illumina platform was used to sequence RNA that was extracted from seedlings treated with each of the metals. There were 449 differentially expressed genes (DEGs) between copper-resistant genotypes (RGs) and nickel-resistant genotypes (RGs) at a high stringency cut-off, indicating a distinct pattern of gene expression toward each metal. For biological processes, 19.8% of DEGs were associated with the DNA metabolic process, followed by the response to stress (13.15%) and the response to chemicals (8.59%). For metabolic function, 27.9% of DEGs were associated with nuclease activity, followed by nucleotide binding (27.64%) and kinase activity (10.16%). Overall, 21.49% of DEGs were localized to the plasma membrane, followed by the cytosol (16.26%) and chloroplast (12.43%). Annotation of the top upregulated genes in copper RG compared to nickel RG identified genes and mechanisms that were specific to copper and not to nickel. NtPDR, AtHIPP10, and YSL1 were identified as genes associated with copper resistance. Various genes related to cell wall metabolism were identified, and they included genes encoding for HCT, CslE6, MPG, and polygalacturonase. Annotation of the top downregulated genes in copper RG compared to nickel RG revealed genes and mechanisms that were specific to nickel and not copper. Various regulatory and signaling-related genes associated with the stress response were identified. They included UGT, TIFY, ACC, dirigent protein, peroxidase, and glyoxyalase I. Additional research is needed to determine the specific functions of signaling and stress response mechanisms in nickel-resistant plants.

Molecular mechanisms underlying the signal perception and transduction during seed germination

QuerySeed germination is a vital step in the life cycle of a plant, playing a significant role in seedling establishment and crop yield potential. It is also an important factor in the conservation of plant germplasm resources. This complex process is influenced by a myriad of factors, including environmental conditions, the genetic makeup of the seed, and endogenous hormones. The perception of these environmental signals triggers a cascade of intricate signal transduction events that determine whether a seed germinates or remains dormant. Despite considerable progress in uncovering the molecular mechanisms governing these processes, many questions remain unanswered. In this review, we summarize the current progress in the molecular mechanisms underlying the perception of environmental signals and consequent signal transduction during seed germination, and discuss questions that need to be addressed to better understand the process of seed germination and develop novel strategies for germplasm improvement.

OsNAC120 balances plant growth and drought tolerance by integrating GA and ABA signaling in rice

The crosstalk between gibberellin (GA) and abscisic acid (ABA) signaling is crucial for balancing plant growth and adaption to environmental stress. Nevertheless, the molecular mechanism of their mutual antagonism still remains to be fully clarified. In this study, we found that knockout of the rice NAC (NAM, ATAF1/2, CUC2) transcription factor gene OsNAC120 inhibits plant growth but enhances drought tolerance, whereas OsNAC120 overexpression produces the opposite results. Exogenous GA can rescue the semi-dwarf phenotype of osnac120 mutants, and further study showed that OsNAC120 promotes GA biosynthesis by transcriptionally activating the GA biosynthetic genes OsGA20ox1 and OsGA20ox3. The DELLA protein SLENDER RICE1 (SLR1) interacts with OsNAC120 and impedes its transactivation ability, and GA treatment can remove the inhibition of transactivation activity caused by SLR1. On the other hand, OsNAC120 negatively regulates rice drought tolerance by repressing ABA-induced stomatal closure. Mechanistic investigation revealed that OsNAC120 inhibits ABA biosynthesis via transcriptional repression of the ABA biosynthetic genes OsNCED3 and OsNCED4. Rice OSMOTIC STRESS/ABA-ACTIVATED PROTEIN KINASE 9 (OsSAPK9) physically interacts with OsNAC120 and mediates its phosphorylation, which results in OsNAC120 degradation. ABA treatment accelerates OsNAC120 degradation and reduces its transactivation activity. Together, our findings provide evidence that OsNAC120 plays critical roles in balancing GA-mediated growth and ABA-induced drought tolerance in rice. This research will help us to understand the mechanisms underlying the trade-off between plant growth and stress tolerance and to engineer stress-resistant, high-yielding crops.

Growth and transcriptional response of wheat and rice to the tertiary amine BMVE

Introduction

Seed vigor is largely a product of sound seed development, maturation processes, genetics, and storage conditions. It is a crucial factor impacting plant growth and crop yield and is negatively affected by unfavorable environmental conditions, which can include drought and heat as well as cold wet conditions. The latter leads to slow germination and increased seedling susceptibility to pathogens. Prior research has shown that a class of plant growth regulators called substituted tertiary amines (STAs) can enhance seed germination, seedling growth, and crop productivity. However, inconsistent benefits have limited STA adoption on a commercial scale.

Methods

We developed a novel seed treatment protocol to evaluate the efficacy of 2-(N-methyl benzyl aminoethyl)-3-methyl butanoate (BMVE), which has shown promise as a crop seed treatment in field trials. Transcriptomic analysis of rice seedlings 24 h after BMVE treatment was done to identify the molecular basis for the improved seedling growth. The impact of BMVE on seed development was also evaluated by spraying rice panicles shortly after flower fertilization and subsequently monitoring the impact on seed traits.

Results

BMVE treatment of seeds 24 h after imbibition consistently improved wheat and rice seedling shoot and root growth in lab conditions. Treated wheat seedlings grown to maturity in a greenhouse also resulted in higher biomass than controls, though only under drought conditions. Treated seedlings had increased levels of transcripts involved in reactive oxygen species scavenging and auxin and gibberellic acid signaling. Conversely, several genes associated with increased reactive oxygen species/ROS load, abiotic stress responses, and germination hindering processes were reduced. BMVE spray increased both fresh and mature seed weights relative to the control for plants exposed to 96 h of heat stress. BMVE treatment during seed development also benefited germination and seedling growth in the next generation, under both ambient and heat stress conditions.

Discussion

The optimized experimental conditions we developed provide convincing evidence that BMVE does indeed have efficacy in plant growth enhancement. The results advance our understanding of how STAs work at the molecular level and provide insights for their practical application to improve crop growth.

Transcriptome profiling reveals ethylene formation in rice seeds by trichloroisocyanuric acid

KEY MESSAGE

Ethylene formation via methionine reacting with trichloroisocyanuric acid under FeSO4 condition in a non-enzymatical manner provides one economically and efficiently novel ethylene-forming approach in planta. Rice seed germination can be stimulated by trichloroisocyanuric acid (TCICA). However, the molecular basis of TCICA in stimulating rice seed germination remains unclear. In this study, the molecular mechanism on how TCICA stimulated rice seed germination was examined via comparative transcriptome. Results showed that clustering of transcripts of TCICA-treated seeds, water-treated seeds, and dry seeds was clearly separated. Twenty-two and three hundred differentially expressed genes were identified as TCICA treatment responsive genes and TCICA treatment potentially responsive genes, respectively. Two and one TCICA treatment responsive genes were involved in ethylene signal transduction and iron homeostasis, respectively. Seventeen of the three hundred TCICA treatment potentially responsive genes were significantly annotated to iron ion binding. Meanwhile, level of methionine (ethylene precursor) showed a 73.9% decrease in response to TCICA treatment. Ethylene was then proved to produce via methionine reacting with TCICA under FeSO4 condition in vitro. Revealing ethylene formation by TCICA not only may bring novel insights into crosstalk between ethylene and other phytohormones during rice seed germination, but also may provide one economically and efficiently novel approach to producing ethylene in planta independently of the ethylene biosynthesis in plants and thereby may broaden its applications in investigational and applied purposes.

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.

Exogenously Applied Cyclitols and Biosynthesized Silver Nanoparticles Affect the Soluble Carbohydrate Profiles of Wheat (Triticum aestivum L.) Seedling

Cyclitols, such as myo-inositol and its isomers and methyl derivatives (i.e., d-chiro-inositol and d-pinitol (3-O-methyl-chiro-inositol)), are classified as osmolytes and osmoprotectants and are significantly involved in plant responses to abiotic stresses, such as drought, salinity and cold. Moreover, d-pinitol demonstrates a synergistic effect with glutathione (GSH), increasing its antioxidant properties. However, the role of cyclitols in plant protection against stresses caused by metal nanoparticles is not yet known. Therefore, the present study examined the effects of myo-inositol, d-chiro-inositol and d-pinitol on wheat germination, seedling growth and changes in the profile of soluble carbohydrates in response to biologically synthesized silver nanoparticles ((Bio)Ag NPs). It was found that cyclitols were absorbed by germinating grains and transported within the growing seedlings but this process was disrupted by (Bio)Ag NPs. Cyclitols applied alone induced sucrose and 1-kestose accumulation in seedlings slightly, while (Bio)Ag NP doubled the concentrations of both sugars. This coincided with a decrease in monosaccharides; i.e., fructose and glucose. Cyclitols and (Bio)Ag NPs present in the endosperm resulted in reductions in monosaccharides, maltose and maltotriose, with no effect on sucrose and 1-kestose. Similar changes occurred in seedlings developing from primed grains. Cyclitols that accumulated in grain and seedlings during grain priming with d-pinitol and glutathione did not prevent the phytotoxic effects of (Bio)Ag NPs.

Integrating Transcriptomics and Hormones Dynamics Reveal Seed Germination and Emergence Process in Polygonatum cyrtonema Hua

Polygonatum cyrtonema Hua is a traditional Chinese herb propagated using rhizomes, and excessive demand for seedlings and quality deterioration caused by rhizome propagation has highlighted that seed propagation may be an ideal solution to address these issues. However, the molecular mechanisms involved in P. cyrtonema Hua seed germination and emergence stages are not well understood. Therefore, in the present study, we performed transcriptomics combined with hormone dynamics during different seed germination stages, and 54,178 unigenes with an average length of 1390.38 bp (N50 = 1847 bp) were generated. Significant transcriptomic changes were related to plant hormone signal transduction and the starch and carbohydrate pathways. Genes related to ABA(abscisic acid), IAA(Indole acetic acid), and JA(Jasmonic acid) signaling, were downregulated, whereas genes related to ethylene, BR(brassinolide), CTK(Cytokinin), and SA(salicylic acid) biosynthesis and signaling were activated during the germination process. Interestingly, GA biosynthesis- and signaling-related genes were induced during the germination stage but decreased in the emergence stage. In addition, seed germination significantly upregulated the expression of genes associated with starch and sucrose metabolism. Notably, raffinose biosynthesis-related genes were induced, especially during the emergence stage. In total, 1171 transcription factor (TF) genes were found to be differentially expressed. Our results provide new insights into the mechanisms underlying P. cyrtonema Hua seed germination and emergence processes and further research for molecular breeding.

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.

Physiological and proteomic analyses of γ-aminobutyric acid (GABA)-treated tubers reveals that StPOD42 promotes sprouting in potato

Gamma-aminobutyric acid (GABA) is a nonproteinogenic amino acid that plays vital roles in plant growth and developmental processes. However, its role in regulating potato sprouting is unknown. Therefore, the physiological and molecular mechanisms underlying the sprouting process were assessed, and we found that GABA promoted sprouting after treatment for 50 d. In addition, the GABA and soluble sugar contents increased while the starch content decreased. To study the molecular mechanism by which exogenous GABA accelerates tuber sprouting, comparative proteomic analysis of tuber bud eyes was performed after GABA treatment for 48 h. Further analysis revealed 316 differentially abundant proteins (DAPs) that are mainly involved in fatty acid and sugar metabolism and cutin, suberin and wax biosyntheses. The qRT‒PCR results suggested that the GABA transaminase 2 (GABA-T2) and GABA-T3 expression levels showed the greatest decrease at 30 d of storage. Peroxidase 42 (StPOD42) expression showed the greatest increase at 30 d. Overexpression of StPOD42 in potato was found to promote tuber sprouting. Our results provide new insights into the role of GABA in regulating the sprouting process and indicate that StPOD42 is a target gene for molecular breeding to modulate potato sprouting.

Seed-to-Seedling Transition in Pisum sativum L.: A Transcriptomic Approach

The seed-to-seedling transition is a crucial step in the plant life cycle. The transition occurs at the end of seed germination and corresponds to the initiation of embryonic root growth. To improve our understanding of how a seed transforms into a seedling, we germinated the Pisum sativum L. seeds for 72 h and divided them into samples before and after radicle protrusion. Before radicle protrusion, seeds survived after drying and formed normally developed seedlings upon rehydration. Radicle protrusion increased the moisture content level in seed axes, and the accumulation of ROS first generated in the embryonic root and plumule. The water and oxidative status shift correlated with the desiccation tolerance loss. Then, we compared RNA sequencing-based transcriptomics in the embryonic axes isolated from pea seeds before and after radicle protrusion. We identified 24,184 differentially expressed genes during the transition to the post-germination stage. Among them, 2101 genes showed more prominent expression. They were related to primary and secondary metabolism, photosynthesis, biosynthesis of cell wall components, redox status, and responses to biotic stress. On the other hand, 415 genes showed significantly decreased expression, including the groups related to water deprivation (eight genes) and response to the ABA stimulus (fifteen genes). We assume that the water deprivation group, especially three genes also belonging to ABA stimulus (LTI65, LTP4, and HVA22E), may be crucial for the desiccation tolerance loss during a metabolic switch from seed to seedling. The latter is also accompanied by the suppression of ABA-related transcription factors ABI3, ABI4, and ABI5. Among them, HVA22E, ABI4, and ABI5 were highly conservative in functional domains and showed homologous sequences in different drought-tolerant species. These findings elaborate on the critical biochemical pathways and genes regulating seed-to-seedling transition.

Transcriptome revealing the dual regulatory mechanism of ethylene on the rhynchophylline and isorhynchophylline in Uncaria rhynchophylla

Rhynchophylline (RIN) and isorhynchophylline (IRN) are extracted from Uncaria rhynchophylla, which are used to treat Alzheimer's disease. However, the massive accumulation of RIN and IRN in U. rhynchophylla requires exogenous stimulation. Ethylene is a potential stimulant for RIN and IRN biosynthesis, but there is no study on the role of ethylene in RIN or IRN synthesis. This study investigated the regulation of ethylene in RIN and IRN biosynthesis in U. rhynchophylla. An increase in the content of RIN and IRN was observed that could be attributed to the release of ethylene from 18 mM ethephon, while ethylene released from 36 mM ethephon reduced the content of RIN and IRN. The transcriptome and weighted gene co-expression network analysis indicated the up-regulation of seven key enzyme genes related to the RIN/IRN biosynthesis pathway and starch/sucrose metabolism pathway favored RIN/IRN synthesis. In comparison, the down-regulation of these seven key enzyme genes contributed to the reduction of RIN/IRN. Moreover, the inhibition of photosynthesis is associated with a reduction in RIN/IRN. Photosynthesis was restrained owing to the down-regulation of Lhcb1 and Lhcb6 after 36 mM ethephon treatment and further prevented supply of primary metabolites (such as α-D-glucose) for RIN/IRN synthesis. However, uninterrupted photosynthesis ensured a normal supply of primary metabolites at 18 mM ethephon treatment. AP2/ERF1, bHLH1, and bHLH2 may positively regulate the RIN/IRN accumulation, while NAC1 may play a negative regulatory role. Our results construct the potential bidirectional model for ethylene regulation on RIN/IRN synthesis and provide novel insight into the ethylene-mediated regulation of the metabolism of terpenoid indole alkaloids.

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.

Transcriptome and Metabolite Conjoint Analysis Reveals the Seed Dormancy Release Process in Callery Pear

Seed dormancy transition is a vital developmental process for seedling propagation and agricultural production. The process is precisely regulated by diverse endogenous genetic factors and environmental cues. Callery pear (Pyrus calleryana Decne) is an important rootstock species that requires cold stratification to break seed dormancy, but the mechanisms underlying pear seed dormancy release are not yet fully understood. Here, we analyzed the transcriptome profiles at three different stages of cold stratification in callery pear seeds using RNA sequencing combined with phytohormone and sugar content measurements. Significant alterations in hormone contents and carbohydrate metabolism were observed and reflected the dormancy status of the seeds. The expressions of genes related to plant hormone metabolism and signaling transduction, including indole-3-acetic acid (IAA) biosynthesis (ASAs, TSA, NITs, YUC, and AAO) genes as well as several abscisic acid (ABA) and gibberellic acid (GA) catabolism and signaling transduction genes (CYP707As, GA2ox, and DELLAs), were consistent with endogenous hormone changes. We further found that several genes involved in cytokinin (CTK), ethylene (ETH), brassionolide (BR), and jasmonic acid (JA) metabolism and signaling transduction were differentially expressed and integrated in pear seed dormancy release. In accordance with changes in starch and soluble sugar contents, the genes associated with starch and sucrose metabolism were significantly up-regulated during seed dormancy release progression. Furthermore, the expression levels of genes involved in lipid metabolism pathways were also up-regulated. Finally, 447 transcription factor (TF) genes (including ERF, bHLH, bZIP, NAC, WRKY, and MYB genes) were observed to be differentially expressed during seed cold stratification and might relate to pear seed dormancy release. Our results suggest that the mechanism underlying pear seed dormancy release is a complex, transcriptionally regulated process involving hormones, sugars, lipids, and TFs.

Dynamic Transcriptomic and Metabolomic Analyses of Madhuca pasquieri (Dubard) H. J. Lam During the Post-germination Stages

The wild population of Madhuca pasquieri (Dubard) H. J. Lam is currently dwindling; its understory seedlings are rare, and there is a lack of molecular studies, which impedes the conservation of this species. This study exploited second-generation sequencing and widely targeted metabolomics analysis to uncover the dynamic changes in differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) in five post-germination stages of M. pasquieri whole organism. Notably, the weighted gene co-expression network analysis (WGCNA), transcriptome, and metabolome association analyses all indicated significant enrichment of the flavonoid biosynthesis pathway in stage 4 (two-leaf), and an upregulation of the genes encoding flavonol biosynthesis in this stage. In stage 5 (nine-leaf), the flavonols were significantly accumulated, indicating that the changes in metabolites were driven at the transcript level. According to the significant changes in gene expression encoding auxin transport carriers and their correlation with flavonols during stage 5, the flavonols were speculated to have a direct inhibitory effect on the expression of PIN4 encoding gene, which may inhibit the process of polar auxin transport. The results provided important insights into the molecular network relationships between the transcription and metabolism of this rare and endangered species during the post-germination stages and explained the reasons for the slow growth of its seedlings at the molecular level.

Transition from Seeds to Seedlings: Hormonal and Epigenetic Aspects

Transition from seed to seedling is one of the critical developmental steps, dramatically affecting plant growth and viability. Before plants enter the vegetative phase of their ontogenesis, massive rearrangements of signaling pathways and switching of gene expression programs are required. This results in suppression of the genes controlling seed maturation and activation of those involved in regulation of vegetative growth. At the level of hormonal regulation, these events are controlled by the balance of abscisic acid and gibberellins, although ethylene, auxins, brassinosteroids, cytokinins, and jasmonates are also involved. The key players include the members of the LAFL network-the transcription factors LEAFY COTYLEDON1 and 2 (LEC 1 and 2), ABSCISIC ACID INSENSITIVE3 (ABI3), and FUSCA3 (FUS3), as well as DELAY OF GERMINATION1 (DOG1). They are the negative regulators of seed germination and need to be suppressed before seedling development can be initiated. This repressive signal is mediated by chromatin remodeling complexes-POLYCOMB REPRESSIVE COMPLEX 1 and 2 (PRC1 and PRC2), as well as PICKLE (PKL) and PICKLE-RELATED2 (PKR2) proteins. Finally, epigenetic methylation of cytosine residues in DNA, histone post-translational modifications, and post-transcriptional downregulation of seed maturation genes with miRNA are discussed. Here, we summarize recent updates in the study of hormonal and epigenetic switches involved in regulation of the transition from seed germination to the post-germination 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.

SlERF52 regulates SlTIP1;1 expression to accelerate tomato pedicel abscission

Abscission of plant organs is induced by developmental signals and diverse environmental stimuli and involves multiple regulatory networks, including biotic or abiotic stress-impaired auxin flux in the abscission zone (AZ). Depletion of auxin activates AZ ethylene (ETH) production and triggers acceleration of abscission, a process that requires hydrogen peroxide (H2O2). However, the interaction between these networks and the underlying mechanisms that control abscission are poorly understood. Here, we found that expression of tonoplast intrinsic proteins, which belong to the aquaporin (AQP) family in the AZ was important for tomato (Solanum lycopersicum) pedicel abscission. Liquid chromatography-tandem mass spectrometry and in situ hybridization revealed that SlTIP1;1 was most abundant and specifically present in the tomato pedicel AZ. SlTIP1;1 localized in the plasma membrane and tonoplast. Knockout of SlTIP1;1 resulted in delayed abscission, whereas overexpression of SlTIP1;1 accelerated abscission. Further analysis indicated that SlTIP1;1 mediated abscission via gating of cytoplasmic H2O2 concentrations and osmotic water permeability (Pf). Elevated cytoplasmic levels of H2O2 caused a suppressed auxin signal in the early abscission stage and enhanced ETH production during abscission. Furthermore, we found that increasing Pf was required to enhance the turgor pressure to supply the break force for AZ cell separation. Moreover, we observed that SlERF52 bound directly to the SlTIP1;1 promoter to regulate its expression, demonstrating a positive loop in which cytoplasmic H2O2 activates ETH production, which activates SlERF52. This, in turn, induces SlTIP1;1, which leads to elevated cytoplasmic H2O2 and water influx.

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.

The FLA4-FEI Pathway: A Unique and Mysterious Signaling Module Related to Cell Wall Structure and Stress Signaling

Cell wall integrity control in plants involves multiple signaling modules that are mostly defined by genetic interactions. The putative co-receptors FEI1 and FEI2 and the extracellular glycoprotein FLA4 present the core components of a signaling pathway that acts in response to environmental conditions and insults to cell wall structure to modulate the balance of various growth regulators and, ultimately, to regulate the performance of the primary cell wall. Although the previously established genetic interactions are presently not matched by intermolecular binding studies, numerous receptor-like molecules that were identified in genome-wide interaction studies potentially contribute to the signaling machinery around the FLA4-FEI core. Apart from its function throughout the model plant Arabidopsis thaliana for the homeostasis of growth and stress responses, the FLA4-FEI pathway might support important agronomic traits in crop plants.

From the Outside to the Inside: New Insights on the Main Factors That Guide Seed Dormancy and Germination

The transition from a dormant to a germinating seed represents a crucial developmental switch in the life cycle of a plant. Subsequent transition from a germinating seed to an autotrophic organism also requires a robust and multi-layered control. Seed germination and seedling growth are multistep processes, involving both internal and external signals, which lead to a fine-tuning control network. In recent years, numerous studies have contributed to elucidate the molecular mechanisms underlying these processes: from light signaling and light-hormone crosstalk to the effects of abiotic stresses, from epigenetic regulation to translational control. However, there are still many open questions and molecular elements to be identified. This review will focus on the different aspects of the molecular control of seed dormancy and germination, pointing out new molecular elements and how these integrate in the signaling pathways already known.