Auxin controls seed dormancy through stimulation of abscisic acid signaling by inducing ARF-mediated ABI3 activation in Arabidopsis

Priming of Solanum melongena L. Seeds Enhances Germination, Alters Antioxidant Enzymes, Modulates ROS, and Improves Early Seedling Growth: Indicating Aqueous Garlic Extract as Seed-Priming Bio-Stimulant for Eggplant Production

The current study was aimed to evaluate the seed priming potential of AGE (aqueous garlic extracts) to enhance seed germination and early seedling growth of eggplant. Different concentrations (100, 200, and 300 µg mL−1) of AGE were evaluated along with methyl jasmonate (MeJA) and salicylic acid (SA), plant growth regulators with reported seed priming potential whereas, water was taken as a control treatment. Eggplant seeds were primed for 4-, 8-, and 12-h and seed germination traits such germination rate index, germination percentage, mean germination time, and early seedling growth traits such as fresh and dry weights, root, and shoot lengths were observed. Moreover, plant antioxidant enzymes activities and lipid peroxidation levels, soluble protein contents and reactive oxygen species were monitored to establish the stimulatory/inhibitory effects of the treatments. Our results indicate priming potential of AGE, SA, and MeJA to enhance seed germination and early seedling growth in eggplant and the effects were obvious in various morphological and physiological traits. Seed priming significantly altered the antioxidant enzymes activities such as superoxide dismutase (SOD), and peroxidase (POD) with alteration in the reactive oxygen species (ROS). Interestingly, priming duration also affected the bioactivity of these chemicals because seed priming with 300 µg mL−1 AGE for 4 h had a positive influence, however, prolonged exposure to the same concentration inhibited the seed germination process and induced oxidative stress on the seedlings with elevated levels of malondialdehyde (MDA) content. We propose AGE seed priming as a bio-stimulant to enhance seed germination and early seedling growth in eggplant, and the results hence lay the foundation for the preparation of garlic-based compounds to improve vegetables production under plastic tunnels and greenhouse production units.

Transcriptional reference map of hormone responses in wheat spikes

Background

Phytohormones are key regulators of plant growth, development, and signalling networks involved in responses to diverse biotic and abiotic stresses. Transcriptional reference maps of hormone responses have been reported for several model plant species such as Arabidopsis thaliana, Oryza sativa, and Brachypodium distachyon. However, because of species differences and the complexity of the wheat genome, these transcriptome data are not appropriate reference material for wheat studies.

Results

We comprehensively analysed the transcriptomic responses in wheat spikes to seven phytohormones, including indole acetic acid (IAA), gibberellic acid (GA), abscisic acid (ABA), ethylene (ET), cytokinin (CK), salicylic acid (SA), and methyl jasmonic acid (MeJA). A total of 3386 genes were differentially expressed at 24 h after the hormone treatments. Furthermore, 22.7% of these genes exhibited overlapping transcriptional responses for at least two hormones, implying there is crosstalk among phytohormones. We subsequently identified genes with expression levels that were significantly and differentially induced by a specific phytohormone (i.e., hormone-specific responses). The data for these hormone-responsive genes were then compared with the transcriptome data for wheat spikes exposed to biotic (Fusarium head blight) and abiotic (water deficit) stresses.

Conclusion

Our data were used to develop a transcriptional reference map of hormone responses in wheat spikes.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5726-x) contains supplementary material, which is available to authorized users.

Regulatory-Associated Protein of TOR 1B (RAPTOR1B) regulates hormonal switches during seed germination in Arabidopsis thaliana

Target of Rapamycin (TOR) regulates multiple growth- and metabolic-related processes in Arabidopsis thaliana as in all other eukaryotes. While several of these processes have been investigated in diverse Arabidopsis growth stages, little is known about hormonal and metabolic regulation of TOR during seed germination. This is mainly due to the fact that Arabidopsis knockout lines of TOR are embryo lethal. Here, we utilized the knockout lines of TOR-interacting protein, REGULATORY-ASSOCIATED PROTEIN OF TOR 1B (RAPTOR1B), to perform comprehensive hormone profiling during seed germination. We previously reported that RAPTOR1B positively regulates seed germination by maintaining the nutritional and hormonal balance. In the current analysis, dry and imbibed seeds as well as germinated seeds were subjected to detailed hormone analysis. Accordingly, the abscisic acid content of dry and imbibed raptor1b seeds was higher than that of WT, while the amounts of gibberellins were comparable after stratification. Further analysis showed that raptor1b seeds maintained higher levels of indole-3-acetic acid and jasmonates, namely jasmonic acid (JA) and 12-oxo-phytodienoic acid, even after stratification. The combination of this hormonal perturbation seems to be the driving factor for the observed delayed germination phenotypes in raptor1b seeds.

Phosphatidylinositol-specific phospholipase C2 functions in auxin-modulated root development

Nine phosphatidylinositol-specific phospholipases C (PLCs) have been identified in the Arabidopsis genome; among the importance of PLC2 in reproductive development is significant. However, the role of PLC2 in vegetative development such as in root growth is elusive. Here, we report that plc2 mutants displayed multiple auxin-defective phenotypes in root development, including short primary root, impaired root gravitropism, and inhibited root hair growth. The DR5:GUS expression and the endogenous indole-3-acetic acid (IAA) content, as well as the responses of a set of auxin-related genes to exogenous IAA treatment, were all decreased in plc2 seedlings, suggesting the influence of PLC2 on auxin accumulation and signalling. The root elongation of plc2 mutants was less sensitive to the high concentration of exogenous auxins, and the application of 1-naphthaleneacetic acid or the auxin transport inhibitor N-1-naphthylphthalamic acid could rescue the root hair growth of plc2 mutants. In addition, the PIN2 polarity and cycling in plc2 root epidermis cells were altered. These results demonstrate a critical role of PLC2 in auxin-mediated root development in Arabidopsis, in which PLC2 influences the polar distribution of PIN2.

Plant growth-promoting activity of beta-propeller protein YxaL secreted from Bacillus velezensis strain GH1-13

YxaL is conserved within the Bacillus subtilis species complex associated with plants and soil. The mature YxaL protein contains a repeated beta-propeller domain, but the subcellular location and function of YxaL has not been determined. The gene encoding the mature YxaL protein was PCR amplified from genomic DNA of B. velezensis strain GH1-13 and used for recombinant protein production. A rabbit polyclonal antibody against the purified YxaL was generated and used for western blotting to determine the constitutive expression and secretion of YxaL. During normal culture growth of strain GH1-13, levels of the constitutively secreted YxaL were slowly rising to 100 μg L^-1, and degraded with a half-life of 1.6 h in the culture medium. When the effects of YxaL on plant seed germination and seedling growth were examined, it was shown that seed treatment of Arabidopsis thaliana and rice (Oryza sativa L.) with purified YxaL at the optimal concentration of 1 mg L^-1 was effective at improving the root growth of plants. Seedlings from the treated Arabidopsis seeds markedly increased transcription of a 1-aminocyclopropane-1-carboxylate synthetase marker gene (ACS11) but reduced expression of auxin- and abscisic acid-responsive marker genes (IAA1, GH3.3, and ABF4), especially when provided with exogenous auxin. Horticulture experiments showed that pepper (Capsicum annuum) seeds treated with 1 mg L^-1 YxaL in a soaking solution increased shoot growth and improved tolerance to drought stress. We hypothesize that YxaL secreted from plant growth-promoting Bacillus cells has a significant impact on plant roots, with the potential to improve plant growth and stress tolerance.

Enhancing microRNA167A expression in seed decreases the α-linolenic acid content and increases seed size in Camelina sativa

Despite well established roles of microRNAs in plant development, few aspects have been addressed to understand their effects in seeds especially on lipid metabolism. In this study, we showed that overexpressing microRNA167A (miR167OE) in camelina (Camelina sativa) under a seed-specific promoter changed fatty acid composition and increased seed size. Specifically, the miR167OE seeds had a lower α-linolenic acid with a concomitantly higher linoleic acid content than the wild-type. This decreased level of fatty acid desaturation corresponded to a decreased transcriptional expression of the camelina fatty acid desaturase3 (CsFAD3) in developing seeds. MiR167 targeted the transcription factor auxin response factor (CsARF8) in camelina, as had been reported previously in Arabidopsis. Chromatin immunoprecipitation experiments combined with transcriptome analysis indicated that CsARF8 bound to promoters of camelina bZIP67 and ABI3 genes. These transcription factors directly or through the ABI3-bZIP12 pathway regulate CsFAD3 expression and affect α-linolenic acid accumulation. In addition, to decipher the miR167A-CsARF8 mediated transcriptional cascade for CsFAD3 suppression, transcriptome analysis was conducted to implicate mechanisms that regulate seed size in camelina. Expression levels of many genes were altered in miR167OE, including orthologs that have previously been identified to affect seed size in other plants. Most notably, genes for seed coat development such as suberin and lignin biosynthesis were down-regulated. This study provides valuable insights into the regulatory mechanism of fatty acid metabolism and seed size determination, and suggests possible approaches to improve these important traits in camelina.

Novel loci and a role for nitric oxide for seed dormancy and preharvest sprouting in barley

Barley is used for food and feed, and brewing. Nondormant seeds are required for malting, but the lack of dormancy can lead to preharvest sprouting (PHS), which is also undesired. Here, we report several new loci that modulate barley seed dormancy and PHS. Using genome-wide association mapping of 184 spring barley genotypes, we identified four new, highly significant associations on chromosomes 1H, 3H, and 5H previously not associated with barley seed dormancy or PHS. A total of 71 responsible genes were found mostly related to flowering time and hormone signalling. A homolog of the well-known Arabidopsis Delay of Germination 1 (DOG1) gene was annotated on the barley chromosome 3H. Unexpectedly, DOG1 appears to play only a minor role in barley seed dormancy. However, the gibberellin oxidase gene HvGA20ox1 contributed to dormancy alleviation, and another seven important loci changed significantly during after-ripening. Furthermore, nitric oxide release correlated negatively with dormancy and shared 27 associations. Origin and growth environment affected seed dormancy and PHS more than did agronomic traits. Days to anthesis and maturity were shorter when seeds were produced under drier conditions, seeds were less dormant, and PHS increased, with a heritability of 0.57-0.80. The results are expected to be useful for crop improvement.

Transcriptomics of cytokinin and auxin metabolism and signaling genes during seed maturation in dormant and non-dormant wheat genotypes

To gain insights into the roles of cytokinin (CK) and auxin in regulating dormancy during seed maturation in wheat, we examined changes in the levels of CK and indole-3-acetic acid (IAA) and expression patterns of their metabolism and signaling genes in embryonic and endospermic tissues of dormant and non-dormant genotypes. Seed maturation was associated with a decrease in the levels of isopentenyladenine in both tissues mainly via repression of the CK biosynthetic TaLOG genes. Differential embryonic trans-zeatin content and expression patterns of the CK related genes including TacZOG, TaGLU and TaARR12 between maturing seeds of the two genotypes implicate CK in the control of seed dormancy induction and maintenance. Seed maturation induced a decrease of IAA level in both tissues irrespective of genotype, and this appeared to be mediated by repression of specific IAA biosynthesis, transport and IAA-conjugate hydrolysis genes. The differential embryonic IAA content and expression pattern of the IAA biosynthetic gene TaAO during the early stage of seed maturation between the two genotypes imply the role of IAA in dormancy induction. It appears from our data that the expression of specific auxin signaling genes including TaRUB, TaAXR and TaARF mediate the role of auxin signaling in dormancy induction and maintenance during seed maturation in wheat.

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.

Developmental Roles of AUX1/LAX Auxin Influx Carriers in Plants

Plant hormone auxin regulates several aspects of plant growth and development. Auxin is predominantly synthesized in the shoot apex and developing leaf primordia and from there it is transported to the target tissues e.g. roots. Auxin transport is polar in nature and is carrier-mediated. AUXIN1/LIKE-AUX1 (AUX1/LAX) family members are the major auxin influx carriers whereas PIN-FORMED (PIN) family and some members of the P-GLYCOPROTEIN/ATP-BINDING CASSETTE B4 (PGP/ABCB) family are major auxin efflux carriers. AUX1/LAX auxin influx carriers are multi-membrane spanning transmembrane proteins sharing similarity to amino acid permeases. Mutations in AUX1/LAX genes result in auxin related developmental defects and have been implicated in regulating key plant processes including root and lateral root development, root gravitropism, root hair development, vascular patterning, seed germination, apical hook formation, leaf morphogenesis, phyllotactic patterning, female gametophyte development and embryo development. Recently AUX1 has also been implicated in regulating plant responses to abiotic stresses. This review summarizes our current understanding of the developmental roles of AUX1/LAX gene family and will also briefly discuss the modelling approaches that are providing new insight into the role of auxin transport in plant development.

Alternative Oxidase (AOX) Senses Stress Levels to Coordinate Auxin-Induced Reprogramming From Seed Germination to Somatic Embryogenesis-A Role Relevant for Seed Vigor Prediction and Plant Robustness

Somatic embryogenesis (SE) is the most striking and prominent example of plant plasticity upon severe stress. Inducing immature carrot seeds perform SE as substitute to germination by auxin treatment can be seen as switch between stress levels associated to morphophysiological plasticity. This experimental system is highly powerful to explore stress response factors that mediate the metabolic switch between cell and tissue identities. Developmental plasticity per se is an emerging trait for in vitro systems and crop improvement. It is supposed to underlie multi-stress tolerance. High plasticity can protect plants throughout life cycles against variable abiotic and biotic conditions. We provide proof of concepts for the existing hypothesis that alternative oxidase (AOX) can be relevant for developmental plasticity and be associated to yield stability. Our perspective on AOX as relevant coordinator of cell reprogramming is supported by real-time polymerase chain reaction (PCR) analyses and gross metabolism data from calorespirometry complemented by SHAM-inhibitor studies on primed, elevated partial pressure of oxygen (EPPO)-stressed, and endophyte-treated seeds. In silico studies on public experimental data from diverse species strengthen generality of our insights. Finally, we highlight ready-to-use concepts for plant selection and optimizing in vivo and in vitro propagation that do not require further details on molecular physiology and metabolism. This is demonstrated by applying our research & technology concepts to pea genotypes with differential yield performance in multilocation fields and chickpea types known for differential robustness in the field. By using these concepts and tools appropriately, also other marker candidates than AOX and complex genomics data can be efficiently validated for prebreeding and seed vigor prediction.

VcRR2 regulates chilling-mediated flowering through expression of hormone genes in a transgenic blueberry mutant

The molecular mechanism underlying dormancy release and the induction of flowering remains poorly understood in woody plants. Mu-legacy is a valuable blueberry mutant, in which a transgene insertion caused increased expression of a RESPONSE REGULATOR 2-like gene (VcRR2). Mu-legacy plants, compared with nontransgenic 'Legacy' plants, show dwarfing, promotion of flower bud formation, and can flower under nonchilling conditions. We conducted transcriptomic comparisons in leaves, chilled and nonchilled flowering buds, and late-pink buds, and analyzed a total of 41 metabolites of six groups of hormones in leaf tissues of both Mu-legacy and 'Legacy' plants. These analyses uncovered that increased VcRR2 expression promotes the expression of a homolog of Arabidopsis thaliana ENT-COPALYL DIPHOSPHATE SYNTHETASE 1 (VcGA1), which induces new homeostasis of hormones, including increased gibberellin 4 (GA4) levels in Mu-legacy leaves. Consequently, increased expression of VcRR2 and VcGA1, which function in cytokinin responses and gibberellin synthesis, respectively, initiated the reduction in plant height and the enhancement of flower bud formation of the Mu-legacy plants through interactions of multiple approaches. In nonchilled flower buds, 29 differentially expressed transcripts of 17 genes of five groups of hormones were identified in transcriptome comparisons between Mu-legacy and 'Legacy' plants, of which 22 were chilling responsive. Thus, these analyses suggest that increased expression of VcRR2 was collectively responsible for promoting flower bud formation in highbush blueberry under nonchilling conditions. We report here for the first time the importance of VcRR2 to induce a suite of downstream hormones that promote flowering in woody plants.

Integrated physiologic, proteomic, and metabolomic analyses of Malus halliana adaptation to saline-alkali stress

Saline-alkali stress is a severely adverse abiotic stress limiting plant growth. Malus halliana Koehne is an apple rootstock that is tolerant to saline-alkali stress. To understand the molecular mechanisms underlying the tolerance of M. halliana to saline-alkali stress, an integrated metabolomic and proteomic approach was used to analyze the plant pathways involved in the stress response of the plant and its regulatory mechanisms. A total of 179 differentially expressed proteins (DEPs) and 140 differentially expressed metabolites (DEMs) were identified. We found that two metabolite-related enzymes (PPD and PAO) were associated with senescence and involved in porphyrin and chlorophyll metabolism; six photosynthesis proteins (PSAH2, PSAK, PSBO2, PSBP1, and PSBQ2) were significantly upregulated, especially PSBO2, and could act as regulators of photosystem II (PSII) repair. Sucrose, acting as a signaling molecule, directly mediated the accumulation of D-phenylalanine, tryptophan, and alkaloid (vindoline and ecgonine) and the expression of proteins related to aspartate and glutamate (ASP3, ASN1, NIT4, and GLN1-1). These responses play a central role in maintaining osmotic balance and removing reactive oxygen species (ROS). In addition, sucrose signaling induced flavonoid biosynthesis by activating the expression of CYP75B1 to regulate the homeostasis of ROS and promoted auxin signaling by activating the expression of T31B5_170 to enhance the resistance of M. halliana to saline-alkali stress. The decrease in peroxidase superfamily protein (PER) and ALDH2C4 during lignin synthesis further triggered a plant saline-alkali response. Overall, this study provides an important starting point for improving saline-alkali tolerance in M. halliana via genetic engineering.

Plant defense against virus diseases; growth hormones in highlights

Phytohormones are critical in various aspects of plant biology such as growth regulations and defense strategies against pathogens. Plant-virus interactions retard plant growth through rapid alterations in phytohormones and their signaling pathways. Recent research findings show evidence of how viruses impact upon modulation of various phytohormones affecting plant growth regulations. The opinion is getting stronger that virus-mediated phytohormone disruption and alteration weaken plant defense strategies through enhanced replication and systemic spread of viral particles. These hormones regulate plant-virus interactions in various ways that may involve antagonism and cross talk to modulate small RNA (sRNA) systems. The article aims to highlight the recent research findings elaborating the impact of viruses upon manipulation of phytohormones and virus biology.

TRANSPARENT TESTA 4-mediated flavonoids negatively affect embryonic fatty acid biosynthesis in Arabidopsis

Flavonoids are involved in many physiological processes in plants. TRANSPARENT TESTA 4 (TT4) acts at the first step of flavonoid biosynthesis, and the loss of TT4 function causes a lack of flavonoid. Flavonoid deficiency is reportedly the main cause of increased fatty acid content in pale-coloured oilseeds, but details regarding the relationship between seed flavonoids and fatty acid biosynthesis are elusive. In this work, we applied a genetic strategy combined with biochemical and cytological assays to determine the effect of seed flavonoids on the biosynthesis of fatty acids in Arabidopsis thaliana. We showed that TT4-mediated flavonoids negatively affect embryonic fatty acid biosynthesis. A crossing experiment indicated that seed flavonoid biosynthesis and the impact of this process on fatty acid biosynthesis were controlled in a maternal line-dependent manner. Loss of TT4 function activated glycolysis in seed embryos, thereby enhancing fatty acid biosynthesis, but did not improve seed mucilage production. Moreover, loss of TT4 function reduced PIN-FORMED 4 expression and subsequently increased auxin accumulation in embryos. Pharmacologically and genetically elevated auxin levels enhanced seed fatty acid biosynthesis. These results indicated that flavonoids affect fatty acid biosynthesis by carbon source reallocation via regulation of WRINKLE1 and auxin transport.

An Evolutionarily Conserved Abscisic Acid Signaling Pathway Regulates Dormancy in the Liverwort Marchantia polymorpha

Dormancy is a key process allowing land plants to adapt to changing conditions in the terrestrial habitat, allowing the cessation of growth in response to environmental or physiological cues, entrance into a temporary quiescent state, and subsequent reactivation of growth in more favorable environmental conditions [1-3]. Dormancy may be induced seasonally, sporadically (e.g., in response to drought), or developmentally (e.g., seeds and apical dominance). Asexual propagules, known as gemmae, derived via clonal reproduction in bryophytes, are often dormant until displaced from the parent plant. In the liverwort Marchantia polymorpha, gemmae are produced within specialized receptacles, gemma cups, located on the dorsal side of the vegetative thallus [4]. Mature gemmae are detached from the parent plant but may remain in the cup, with gemma growth suppressed as long as the gemmae remain in the gemma cup and the parental plant is alive [5]. Following dispersal of gemmae from gemma cups by rain, the gemmae germinate in the presence of light and moisture, producing clonal offspring [6]. In land plants, the plant hormone abscisic acid (ABA) regulates many aspects of dormancy and water balance [7]. Here, we demonstrate that ABA plays a central role in the control of gemma dormancy as transgenic M. polymorpha gemmae with reduced sensitivity to ABA fail to establish and/or maintain dormancy. Thus, the common ancestor of land plants used the ABA signaling module to regulate germination of progeny in response to environmental cues, with both gemmae and seeds being derived structures co-opting an ancestral response system.

Reactive Oxygen Species and Gibberellin Acid Mutual Induction to Regulate Tobacco Seed Germination

Seed germination is a complex process controlled by various mechanisms. To examine the potential contribution of reactive oxygen species (ROS) and gibberellin acid (GA) in regulating seed germination, diphenylene iodonium chloride (DPI) and uniconazole (Uni), as hydrogen peroxide (H2O2) and GA synthesis inhibitor, respectively, were exogenously applied on tobacco seeds using the seed priming method. Seed priming with DPI or Uni decreased germination percentage as compared with priming with H2O, especially the DPI + Uni combination. H2O2 and GA completely reversed the inhibition caused by DPI or Uni. The germination percentages with H2O2 + Uni and GA + DPI combinations kept the same level as with H2O. Meanwhile, GA or H2O2 increased GA content and deceased ABA content through corresponding gene expressions involving homeostasis and signal transduction. In addition, the activation of storage reserve mobilization and the enhancement of soluble sugar content and isocitrate lyase (ICL) activity were also induced by GA or H2O2. These results strongly suggested that H2O2 and GA were essential for tobacco seed germination and by downregulating the ABA/GA ratio and inducing reserve composition mobilization mutually promoted seed germination. Meanwhile, ICL activity was jointly enhanced by a lower ABA/GA ratio and a higher ROS concentration.

Identification of Auxin Activity Like 1, a chemical with weak functions in auxin signaling pathway

A new synthetic auxin AAL1 with new structure was identified. Different from known auxins, it has weak effects. By AAL1, we found specific amino acids could restore the effects of auxin with similar structure. Auxin, one of the most important phytohormones, plays crucial roles in plant growth, development and environmental response. Although many critical regulators have been identified in auxin signaling pathway, some factors, especially those with weak fine-tuning roles, are still yet to be discovered. Through chemical genetic screenings, we identified a small molecule, Auxin Activity Like 1 (AAL1), which can effectively inhibit dark-grown Arabidopsis thaliana seedlings. Genetic screening identified AAL1 resistant mutants are also hyposensitive to indole-3-acetic acid (IAA) and 2,4-dichlorophenoxyacetic acid (2,4-D). AAL1 resistant mutants such as shy2-3c and ecr1-2 are well characterized as mutants in auxin signaling pathway. Genetic studies showed that AAL1 functions through auxin receptor Transport Inhibitor Response1 (TIR1) and its functions depend on auxin influx and efflux carriers. Compared with known auxins, AAL1 exhibits relatively weak effects on plant growth, with 20 µM and 50 µM IC50 (half growth inhibition chemical concentration) in root and hypocotyl growth respectively. Interestingly, we found the inhibitory effects of AAL1 and IAA could be partially restored by tyrosine and tryptophan respectively, suggesting some amino acids can also affect auxin signaling pathway in a moderate manner. Taken together, our results demonstrate that AAL1 acts through auxin signaling pathway, and AAL1, as a weak auxin activity analog, provides us a tool to study weak genetic interactions in auxin pathway.

Regulatory Mechanism of ABA and ABI3 on Vegetative Development in the Moss Physcomitrella patens

The moss Physcomitrella patens is a model system for studying plant developmental processes. ABSCISIC ACID INSENSITIVE3 (ABI3), a transcription factor of the ABA signaling pathway, plays an important role in plant growth and development in vascular plant. To understand the regulatory mechanism of ABA and PpABI3 on vegetative development in Physcomitrella patens, we applied physiological, cellular, and RNA-seq analyses in wild type (WT) plants and ∆abi3 mutants. During ABA treatment, the growth of gametophytes was inhibited to a lesser extent ∆abi3 plants compared with WT plants. Microscopic observation indicated that the differentiation of caulonemata from chloronemata was accelerated in ∆abi3 plants when compared with WT plants, with or without 10 μM of ABA treatment. Under normal conditions, auxin concentration in ∆abi3 plants was markedly higher than that in WT plants. The auxin induced later differentiation of caulonemata from chloronemata, and the phenotype of ∆abi3 plants was similar to that of WT plants treated with exogenous indole-3-acetic acid (IAA). RNA-seq analysis showed that the PpABI3-regulated genes overlapped with genes regulated by the ABA treatment, and about 78% of auxin-related genes regulated by the ABA treatment overlapped with those regulated by PpABI3. These results suggested that ABA affected vegetative development partly through PpABI3 regulation in P. patens; PpABI3 is a negative regulator of vegetative development in P. patens, and the vegetative development regulation by ABA and PpABI3 might occur by regulating the expression of auxin-related genes. PpABI3 might be associated with cross-talk between ABA and auxin in P. patens.

The plant pathogen Pseudomonas aeruginosa triggers a DELLA-dependent seed germination arrest in Arabidopsis

To anticipate potential seedling damage, plants block seed germination under unfavorable conditions. Previous studies investigated how seed germination is controlled in response to abiotic stresses through gibberellic and abscisic acid signaling. However, little is known about whether seeds respond to rhizosphere bacterial pathogens. We found that Arabidopsis seed germination is blocked in the vicinity of the plant pathogen Pseudomonas aeruginosa. We identified L-2-amino-4-methoxy-trans-3-butenoic acid (AMB), released by P. aeruginosa, as a biotic compound triggering germination arrest. We provide genetic evidence that in AMB-treated seeds DELLA factors promote the accumulation of the germination repressor ABI5 in a GA-independent manner. AMB production is controlled by the quorum sensing system IQS. In vitro experiments show that the AMB-dependent germination arrest protects seedlings from damage induced by AMB. We discuss the possibility that this could serve as a protective response to avoid severe seedling damage induced by AMB and exposure to a pathogen.

The plant embryo within a seed is well protected. While it cannot stay within the seed forever, the embryo can often wait for the right conditions before it develops into a seedling and continues its life cycle. Indeed, plants have evolved several ways to time this process – which is known as germination – to maximize the chances that their seedlings will survive. For example, if the environment is too hot or too dark, the seed will make a hormone that stops it from germinating.

In addition to environmental factors like light and temperature, a seed in the real word is continuously confronted with soil microbes that may harm or benefit the plant. However, few researchers have asked whether seeds control their germination in response to other living organisms.

The bacterium Pseudomonas aeruginosa lives in a wide spectrum of environments, including the soil, and can cause diseases in both and plants and animals. Chahtane et al. now report that seeds of the model plant Arabidopsis thaliana do indeed repress their germination when this microbe is present. Specifically, the seeds respond to a molecule released from the bacteria called L-2-amino-4-methoxy-trans-3-butenoic acid, or AMB for short. Like the bacteria, AMB is harmful to young seedlings, but Chahtane et al. showed that the embryo within the seed is protected from its toxic effects.

Further experiments revealed that the seed's response to the bacterial molecule requires many of the same signaling components that repress germination when environmental conditions are unfavorable. However, Chahtane et al. note that AMB activates these components in an unusual way that they still do not understand.

The genes that control the production of AMB are known to also control how bacterial populations behave as they accumulate to high densities. It is therefore likely that Pseudomonas aeruginosa would make AMB if it reached a high density in the soil. This raises the possibility that plants have specifically evolved to stop germination if there are enough microbes nearby to pose a risk of disease. This hypothesis, however, is only one of several possible explanations and remains speculative at this stage; further work is now needed to evaluate it. Nevertheless, identifying how AMB interferes with the signaling components that control germination and plant growth may guide the design of new herbicides that could, for example, control weeds in the farming industry.

The Sugar-Signaling Hub: Overview of Regulators and Interaction with the Hormonal and Metabolic Network

Plant growth and development has to be continuously adjusted to the available resources. Their optimization requires the integration of signals conveying the plant metabolic status, its hormonal balance, and its developmental stage. Many investigations have recently been conducted to provide insights into sugar signaling and its interplay with hormones and nitrogen in the fine-tuning of plant growth, development, and survival. The present review emphasizes the diversity of sugar signaling integrators, the main molecular and biochemical mechanisms related to the sugar-signaling dependent regulations, and to the regulatory hubs acting in the interplay of the sugar-hormone and sugar-nitrogen networks. It also contributes to compiling evidence likely to fill a few knowledge gaps, and raises new questions for the future.

The Sugar-Signaling Hub: Overview of Regulators and Interaction with the Hormonal and Metabolic Network

Plant growth and development has to be continuously adjusted to the available resources. Their optimization requires the integration of signals conveying the plant metabolic status, its hormonal balance, and its developmental stage. Many investigations have recently been conducted to provide insights into sugar signaling and its interplay with hormones and nitrogen in the fine-tuning of plant growth, development, and survival. The present review emphasizes the diversity of sugar signaling integrators, the main molecular and biochemical mechanisms related to the sugar-signaling dependent regulations, and to the regulatory hubs acting in the interplay of the sugar-hormone and sugar-nitrogen networks. It also contributes to compiling evidence likely to fill a few knowledge gaps, and raises new questions for the future.

Proteomic analysis of heterotic seed germination in maize using F1 hybrid DHM 117 and its parental inbreds

A number of differentially expressed proteins (DEPs) were identified in comparative two-dimensional gel electrophoresis analysis of dry and 24-h water-imbibed seeds of maize F1 hybrid DHM 117 (BML 6 × BML 7) and its parental inbreds. Of the DEPs, 53.4% (86/161) in dry seeds and 58% (127/219) in water-imbibed seeds exhibited a nonadditive pattern in the F1 hybrid as compared to parental inbreds. A total of 30 DEPs were categorized into different biological processes, most of which were related to metabolism and energy (34%), followed by storage proteins (27%), stress response (23%), transcription and translation (7%), cell cycle (3%), and hormone biosynthesis (3%). The transcript accumulation pattern of 8 selected genes corresponding to DEPs was examined using qRTPCR. Interestingly, LEA protein Rab28 showed higher accumulation in dry seeds at both protein and transcript levels, whereas indole3-acetaldehyde oxidase showed lower accumulation in water-imbibed seeds of the F1 hybrid than the female parent at the protein level. Thus, the DEPs particularly involved in metabolic and energy processes, as well as hormone biosynthesis in the F 1 hybrid, might be responsible for heterotic seed germination in the F1 hybrid. The DEPs identified in this study provide a scope for improving the seed germination trait of agricultural crops.

Endosperm sugar accumulation caused by mutation of PHS8/ISA1 leads to pre-harvest sprouting in rice

Pre-harvest sprouting (PHS) is an unfavorable trait in cereal crops that could seriously decrease grain yield and quality. Although some PHS-associated quantitative trait loci or genes in cereals have been reported, the molecular mechanism underlying PHS remains largely elusive. Here, we characterized a rice mutant, phs8, which exhibits PHS phenotype accompanied by sugary endosperm. Map-based cloning revealed that PHS8 encodes a starch debranching enzyme named isoamylase1. Mutation in PHS8 resulted in the phytoglycogen breakdown and sugar accumulation in the endosperm. Intriguingly, with increase of sugar contents, decreased expression of OsABI3 and OsABI5 as well as reduced sensitivity to abscisic acid (ABA) were found in the phs8 mutant. Using rice suspension cell system, we confirmed that exogenous sugar is sufficient to suppress the expression of both OsABI3 and OsABI5. Furthermore, overexpression of OsABI3 or OsABI5 could partially rescue the PHS phenotype of phs8. Therefore, our study presents important evidence supporting that endosperm sugar not only acts as an essential energy source for seed germination but also determines seed dormancy and germination by affecting ABA signaling.

Carbon monoxide signal regulates light-initiated seed germination by suppressing SOM expression

Light is a critical external signal for seed germination. The photoreceptor phytochrome B (PHYB) perceives light stimulation and promotes seed germination during the early phase after imbibition. SOM is a CCH-type zinc finger protein and negatively regulates PHYB-mediated seed germination by controlling downstream gibberellic acid (GA) and abscisic acid (ABA) metabolism. As a small molecular signal, carbon monoxide (CO) has been reported to regulate seed germination under environmental stress, but the underlying mechanism remains unclear. In this study, we first found that CO enhanced PHYB-dependent seed germination, and red light irradiation increased the transcriptional level of gene encoding Heme oxygenase 1(HY1) for CO production, this process required PHYB. Pharmacological and genetic analyses revealed that CO signals repressed the transcriptional level of SOM to alter downstream GA/ABA metabolism related genes expression, ultimately relieving the inhibitory effect of SOM on seed germination. Furthermore, CO signals possibly recruited histone deacetylase 6 (HDA6) to the promoter region of SOM to decrease its expression by diminishing histone H3 acetylation levels at this locus. Taken together, our results propose a novel mechanism for CO signals in promoting light-initiated seed germination via recruiting HDA6 to epigenetically regulate SOM expression.

Class C ARFs evolved before the origin of land plants and antagonize differentiation and developmental transitions in Marchantia polymorpha

A plethora of developmental and physiological processes in land plants is influenced by auxin, to a large extent via alterations in gene expression by AUXIN RESPONSE FACTORs (ARFs). The canonical auxin transcriptional response system is a land plant innovation, however, charophycean algae possess orthologues of at least some classes of ARF and AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) genes, suggesting that elements of the canonical land plant system existed in an ancestral alga. We reconstructed the phylogenetic relationships between streptophyte ARF and AUX/IAA genes and functionally characterized the solitary class C ARF, MpARF3, in Marchantia polymorpha. Phylogenetic analyses indicate that multiple ARF classes, including class C ARFs, existed in an ancestral alga. Loss- and gain-of-function MpARF3 alleles result in pleiotropic effects in the gametophyte, with MpARF3 inhibiting differentiation and developmental transitions in multiple stages of the life cycle. Although loss-of-function Mparf3 and Mpmir160 alleles respond to exogenous auxin treatments, strong miR-resistant MpARF3 alleles are auxin-insensitive, suggesting that class C ARFs act in a context-dependent fashion. We conclude that two modules independently evolved to regulate a pre-existing ARF transcriptional network. Whereas the auxin-TIR1-AUX/IAA pathway evolved to repress class A/B ARF activity, miR160 evolved to repress class C ARFs in a dynamic fashion.

Evaluation of the plant-growth-promoting abilities of endophytic bacteria from the psammophyteAmmodendron bifolium

The objective of this study was to assess the plant-growth-promoting abilities of 45 endophytic bacterial isolates from Ammodendron bifolium through physiological characteristics detection and endophytic bacteria–plant interaction. Each of these isolates exhibited 1 or more plant-growth-promoting traits, but only 11 isolates belonging to the genera Bacillus, Staphylococcus, and Kocuria were capable of promoting seed germination and radicle growth. These results together with the results of the correlation analysis revealed that the completion of seed germination may not be due to IAA production, phosphate solubilization, pellicle formation, and ACC deaminase, protease and lipase production by endophytic bacteria, but may be closely related to amylase and cellulase production. Further, endophytic bacterial isolates with plant-growth-promoting traits may also provide beneficial effects to host plants at different growth stages. Thus, these results are of value for understanding the ecological roles of endophytic bacteria in host plant habitats and can serve as a foundation for further studies of their potential in plant regeneration.

MicroRNA160 Modulates Plant Development and Heat Shock Protein Gene Expression to Mediate Heat Tolerance in Arabidopsis

Global warming is causing a negative impact on plant growth and adversely impacts on crop yield. MicroRNAs (miRNAs) are critical in regulating the expression of genes involved in plant development as well as defense responses. The effects of miRNAs on heat-stressed Arabidopsis warrants further investigation. Heat stress increased the expression of miR160 and its precursors but considerably reduced that of its targets, ARF10, ARF16, and ARF17. To study the roles of miR160 during heat stress, transgenic Arabidopsis plants overexpressing miR160 precursor a (160OE) and artificial miR160 (MIM160), which mimics an inhibitor of miR160, were created. T-DNA insertion mutants of miR160 targets were also used to examine their tolerances to heat stress. Results presented that overexpressing miR160 improved seed germination and seedling survival under heat stress. The lengths of hypocotyl elongation and rachis were also longer in 160OE than the wild-type (WT) plants under heat stress. Interestingly, MIM160 plants showed worse adaption to heat. In addition, arf10, arf16, and arf17 mutants presented similar phenotypes to 160OE under heat stress to advance abilities of thermotolerance. Moreover, transcriptome and qRT-PCR analyses revealed that HSP17.6A, HSP17.6II, HSP21, and HSP70B expression levels were regulated by heat in 160OE, MIM160, arf10, arf16, and arf17 plants. Hence, miR160 altered the expression of the heat shock proteins and plant development to allow plants to survive heat stress.

Reciprocal Regulation of the TOR Kinase and ABA Receptor Balances Plant Growth and Stress Response

As sessile organisms, plants must adapt to variations in the environment. Environmental stress triggers various responses, including growth inhibition, mediated by the plant hormone abscisic acid (ABA). The mechanisms that integrate stress responses with growth are poorly understood. Here, we discovered that the Target of Rapamycin (TOR) kinase phosphorylates PYL ABA receptors at a conserved serine residue to prevent activation of the stress response in unstressed plants. This phosphorylation disrupts PYL association with ABA and with PP2C phosphatase effectors, leading to inactivation of SnRK2 kinases. Under stress, ABA-activated SnRK2s phosphorylate Raptor, a component of the TOR complex, triggering TOR complex dissociation and inhibition. Thus, TOR signaling represses ABA signaling and stress responses in unstressed conditions, whereas ABA signaling represses TOR signaling and growth during times of stress. Plants utilize this conserved phospho-regulatory feedback mechanism to optimize the balance of growth and stress responses.

Identification and Expression Profiling of the Auxin Response Factors in Capsicum annuum L. under Abiotic Stress and Hormone Treatments

Auxin response factors (ARFs) play important roles in regulating plant growth and development and response to environmental stress. An exhaustive analysis of the CaARF family was performed using the latest publicly available genome for pepper (Capsicum annuum L.). In total, 22 non-redundant CaARF gene family members in six classes were analyzed, including chromosome locations, gene structures, conserved motifs of proteins, phylogenetic relationships and Subcellular localization. Phylogenetic analysis of the ARFs from pepper (Capsicum annuum L.), tomato (Solanum lycopersicum L.), Arabidopsis and rice (Oryza sativa L.) revealed both similarity and divergence between the four ARF families, and aided in predicting biological functions of the CaARFs. Furthermore, expression profiling of CaARFs was obtained in various organs and tissues using quantitative real-time RT-PCR (qRT-PCR). Expression analysis of these genes was also conducted with various hormones and abiotic treatments using qRT-PCR. Most CaARF genes were regulated by exogenous hormone treatments at the transcriptional level, and many CaARF genes were altered by abiotic stress. Systematic analysis of CaARF genes is imperative to elucidate the roles of CaARF family members in mediating auxin signaling in the adaptation of pepper to a challenging environment.

miR160 and miR166/165 Contribute to the LEC2-Mediated Auxin Response Involved in the Somatic Embryogenesis Induction in Arabidopsis

MicroRNAs are non-coding small RNA molecules that are involved in the post-transcriptional regulation of the genes that control various developmental processes in plants, including zygotic embryogenesis (ZE). miRNAs are also believed to regulate somatic embryogenesis (SE), a counterpart of the ZE that is induced in vitro in plant somatic cells. However, the roles of specific miRNAs in the regulation of the genes involved in SE, in particular those encoding transcription factors (TFs) with an essential function during SE including LEAFY COTYLEDON2 (LEC2), remain mostly unknown. The aim of the study was to reveal the function of miR165/166 and miR160 in the LEC2-controlled pathway of SE that is induced in in vitro cultured Arabidopsis explants.In ZE, miR165/166 controls the PHABULOSA/PHAVOLUTA (PHB/PHV) genes, which are the positive regulators of LEC2, while miR160 targets the AUXIN RESPONSE FACTORS (ARF10, ARF16, ARF17) that control the auxin signaling pathway, which plays key role in LEC2-mediated SE. We found that a deregulated expression/function of miR165/166 and miR160 resulted in a significant accumulation of auxin in the cultured explants and the spontaneous formation of somatic embryos. Our results show that miR165/166 might contribute to SE induction via targeting PHB, a positive regulator of LEC2 that controls embryogenic induction via activation of auxin biosynthesis pathway (Wójcikowska et al., 2013). Similar to miR165/166, miR160 was indicated to control SE induction through auxin-related pathways and the negative impact of miR160 on ARF10/ARF16/ARF17 was shown in an embryogenic culture. Altogether, the results suggest that the miR165/166- and miR160-node contribute to the LEC2-mediated auxin-related pathway of embryogenic transition that is induced in the somatic cells of Arabidopsis. A model summarizing the suggested regulatory interactions between the miR165/166-PHB and miR160-ARF10/ARF16/ARF17 nodes that control SE induction in Arabidopsis was proposed.

Inference of Transcription Regulatory Network in Low Phytic Acid Soybean Seeds

A dominant loss of function mutation in myo-inositol phosphate synthase (MIPS) gene and recessive loss of function mutations in two multidrug resistant protein type-ABC transporter genes not only reduce the seed phytic acid levels in soybean, but also affect the pathways associated with seed development, ultimately resulting in low emergence. To understand the regulatory mechanisms and identify key genes that intervene in the seed development process in low phytic acid crops, we performed computational inference of gene regulatory networks in low and normal phytic acid soybeans using a time course transcriptomic data and multiple network inference algorithms. We identified a set of putative candidate transcription factors and their regulatory interactions with genes that have functions in myo-inositol biosynthesis, auxin-ABA signaling, and seed dormancy. We evaluated the performance of our unsupervised network inference method by comparing the predicted regulatory network with published regulatory interactions in Arabidopsis. Some contrasting regulatory interactions were observed in low phytic acid mutants compared to non-mutant lines. These findings provide important hypotheses on expression regulation of myo-inositol metabolism and phytohormone signaling in developing low phytic acid soybeans. The computational pipeline used for unsupervised network learning in this study is provided as open source software and is freely available at https://lilabatvt.github.io/LPANetwork/.

Type B Response Regulators Act As Central Integrators in Transcriptional Control of the Auxin Biosynthesis Enzyme TAA1

During embryogenesis and organ formation, establishing proper gradient is critical for auxin function, which is achieved through coordinated regulation of both auxin metabolism and transport. Expression of auxin biosynthetic genes is often tissue specific and is regulated by environmental signals. However, the underlying regulatory mechanisms remain elusive. Here, we investigated the transcriptional regulation of a key auxin biosynthetic gene, l-Tryptophan aminotransferase of Arabidopsis1 (TAA1). A canonical and a novel Arabidopsis (Arabidopsis thaliana) response regulator (ARR) binding site were identified in the promoter and the second intron of TAA1, which were required for its tissue-specific expression. C-termini of a subset of the type B ARRs selectively bind to one or both cis elements and activate the expression of TAA1 We further demonstrated that the ARRs not only mediate the transcriptional regulation of TAA1 by cytokinins, but also mediate its regulation by ethylene, light, and developmental signals. Through direct protein-protein interactions, the transcriptional activity of ARR1 is enhanced by ARR12, DELLAs, and ethylene-insenstive3 (EIN3). Our study thus revealed the ARR proteins act as key node that mediate the regulation of auxin biosynthesis by various hormonal, environmental, and developmental signals through transcriptional regulation of the key auxin biosynthesis gene TAA1.

Regulatory-associated protein of TOR (RAPTOR) alters the hormonal and metabolic composition of Arabidopsis seeds, controlling seed morphology, viability and germination potential

Target of Rapamycin (TOR) is a positive regulator of growth and development in all eukaryotes, which positively regulates anabolic processes like protein synthesis, while repressing catabolic processes, including autophagy. To better understand TOR function we decided to analyze its role in seed development and germination. We therefore performed a detailed phenotypic analysis using mutants of the REGULATORY-ASSOCIATED PROTEIN OF TOR 1B (RAPTOR1B), a conserved TOR interactor, acting as a scaffold protein, which recruits substrates for the TOR kinase. Our results show that raptor1b plants produced seeds that were delayed in germination and less resistant to stresses, leading to decreased viability. These physiological phenotypes were accompanied by morphological changes including decreased seed-coat pigmentation and reduced production of seed-coat mucilage. A detailed molecular analysis revealed that many of these morphological changes were associated with significant changes of the metabolic content of raptor1b seeds, including elevated levels of free amino acids, as well as reduced levels of protective secondary metabolites and storage proteins. Most of these observed changes were accompanied by significantly altered phytohormone levels in the raptor1b seeds, with increases in abscisic acid, auxin and jasmonic acid, which are known to inhibit germination. Delayed germination and seedling growth, observed in the raptor1b seeds, could be partially restored by the exogenous supply of gibberellic acid, indicating that TOR is at the center of a regulatory hub controlling seed metabolism, maturation and germination.

Global Analysis of Small RNA Dynamics during Seed Development of Picea glauca and Arabidopsis thaliana Populations Reveals Insights on their Evolutionary Trajectories

While DNA methylation carries genetic signals and is instrumental in the evolution of organismal complexity, small RNAs (sRNAs), ~18-24 ribonucleotide (nt) sequences, are crucial mediators of methylation as well as gene silencing. However, scant study deals with sRNA evolution via featuring their expression dynamics coupled with species of different evolutionary time. Here we report an atlas of sRNAs and microRNAs (miRNAs, single-stranded sRNAs) produced over time at seed-set of two major spermatophytes represented by populations of Picea glauca and Arabidopsis thaliana with different seed-set duration. We applied diverse profiling methods to examine sRNA and miRNA features, including size distribution, sequence conservation and reproduction-specific regulation, as well as to predict their putative targets. The top 27 most abundant miRNAs were highly overlapped between the two species (e.g., miR166,-319 and-396), but in P. glauca, they were less abundant and significantly less correlated with seed-set phases. The most abundant sRNAs in libraries were deeply conserved miRNAs in the plant kingdom for Arabidopsis but long sRNAs (24-nt) for P. glauca. We also found significant difference in normalized expression between populations for population-specific sRNAs but not for lineage-specific ones. Moreover, lineage-specific sRNAs were enriched in the 21-nt size class. This pattern is consistent in both species and alludes to a specific type of sRNAs (e.g., miRNA, tasiRNA) being selected for. In addition, we deemed 24 and 9 sRNAs in P. glauca and Arabidopsis, respectively, as sRNA candidates targeting known adaptive genes. Temperature had significant influence on selected gene and miRNA expression at seed development in both species. This study increases our integrated understanding of sRNA evolution and its potential link to genomic architecture (e.g., sRNA derivation from genome and sRNA-mediated genomic events) and organismal complexity (e.g., association between different sRNA expression and their functionality).

Exogenous auxin represses soybean seed germination through decreasing the gibberellin/abscisic acid (GA/ABA) ratio

Auxin is an important phytohormone which mediates diverse development processes in plants. Published research has demonstrated that auxin induces seed dormancy. However, the precise mechanisms underlying the effect of auxin on seed germination need further investigation, especially the relationship between auxins and both abscisic acid (ABA) and gibberellins (GAs), the latter two phytohormones being the key regulators of seed germination. Here we report that exogenous auxin treatment represses soybean seed germination by enhancing ABA biosynthesis, while impairing GA biogenesis, and finally decreasing GA1/ABA and GA4/ABA ratios. Microscope observation showed that auxin treatment delayed rupture of the soybean seed coat and radicle protrusion. qPCR assay revealed that transcription of the genes involved in ABA biosynthetic pathway was up-regulated by application of auxin, while expression of genes involved in GA biosynthetic pathway was down-regulated. Accordingly, further phytohormone quantification shows that auxin significantly increased ABA content, whereas the active GA1 and GA4 levels were decreased, resulting insignificant decreases in the ratiosGA1/ABA and GA4/ABA.Consistent with this, ABA biosynthesis inhibitor fluridone reversed the delayed-germination phenotype associated with auxin treatment, while paclobutrazol, a GA biosynthesis inhibitor, inhibited soybean seed germination. Altogether, exogenous auxin represses soybean seed germination by mediating ABA and GA biosynthesis.

Transcriptome and Differential Expression Profiling Analysis of the Mechanism of Ca2+ Regulation in Peanut (Arachis hypogaea) Pod Development

Calcium not only serves as a necessary nutrient for plant growth but also acts as a ubiquitous central hub in a large number of signaling pathways. Free Ca²⁺ deficiency in the soil may cause early embryo abortion, which eventually led to abnormal development of peanut pod during the harvest season. To understand the mechanisms of Ca²⁺ regulation in pod development, transcriptome analysis of peanut gynophores and pods was performed by comparing the treatments between free Ca²⁺ sufficiency and free Ca²⁺ deficiency using Illumina HiSeq™ 2000. 9,903,082,800 nt bases are generated totally. After assembly, the average length of 102,819 unigenes is 999 nt, N50 is 1,782 nt. RNA-seq based gene expression profilings showed a large number of genes at the transcriptional level changed significantly between the aerial pegs and underground swelling pods under free Ca²⁺ sufficienct or deficiency treatments, respectively. Genes encoding key members of Ca²⁺ signaling transduction pathway, enzymes for hormone metabolism, cell division and growth, transcriptional factor as well as embryo development were highlighted. This information provides useful information for our further study. The results of digital gene expression (DGE) indicated that exogenous calcium might contribute to the development of peanut pod through its signal transduction pathway, meanwhile, promote the normal transition of the gynophores to the reproductive development.

Differentially expressed genes during the imbibition of dormant and after-ripened seeds - a reverse genetics approach

BACKGROUND

Seed dormancy, defined as the incapability of a viable seed to germinate under favourable conditions, is an important trait in nature and agriculture. Despite extensive research on dormancy and germination, many questions about the molecular mechanisms controlling these traits remain unanswered, likely due to its genetic complexity and the large environmental effects which are characteristic of these quantitative traits. To boost research towards revealing mechanisms in the control of seed dormancy and germination we depend on the identification of genes controlling those traits.

METHODS

We used transcriptome analysis combined with a reverse genetics approach to identify genes that are prominent for dormancy maintenance and germination in imbibed seeds of Arabidopsis thaliana. Comparative transcriptomics analysis was employed on freshly harvested (dormant) and after-ripened (AR; non-dormant) 24-h imbibed seeds of four different DELAY OF GERMINATION near isogenic lines (DOGNILs) and the Landsberg erecta (Ler) wild type with varying levels of primary dormancy. T-DNA knock-out lines of the identified genes were phenotypically investigated for their effect on dormancy and AR.

RESULTS

We identified conserved sets of 46 and 25 genes which displayed higher expression in seeds of all dormant and all after-ripened DOGNILs and Ler, respectively. Knock-out mutants in these genes showed dormancy and germination related phenotypes.

CONCLUSIONS

Most of the identified genes had not been implicated in seed dormancy or germination. This research will be useful to further decipher the molecular mechanisms by which these important ecological and commercial traits are regulated.

microRNAs participate in gene expression regulation and phytohormone cross-talk in barley embryo during seed development and germination

BACKGROUND

Small RNA and degradome sequencing have identified a large number of miRNA-target pairs in plant seeds. However, detailed spatial and temporal studies of miRNA-mediated regulation, which can reflect links between seed development and germination are still lacking.

RESULTS

In this study, we extended our investigation on miRNAs-involved gene regulation by a combined analysis of seed maturation and germination in barley. Through bioinformatics analysis of small RNA sequencing data, a total of 1324 known miRNA families and 448 novel miRNA candidates were identified. Of those, 16 known miRNAs with 40 target genes, and three novel miRNAs with four target genes were confirmed based on degradome sequencing data. Conserved miRNA families such as miR156, miR168, miR166, miR167, and miR894 were highly expressed in embryos of developing and germinating seeds. A barley-specific miRNA, miR5071, which was predicted to target an OsMLA10-like gene, accumulated at a high level, suggesting its involvement in defence response during these two developmental stages. Based on target prediction and Kyoto Encyclopedia of Genes and Genomes analysis of putative targets, nine highly expressed miRNAs were found to be related to phytohormone signalling and hormone cross-talk. Northern blot and qRT-PCR analysis showed that these miRNAs displayed differential expression patterns during seed development and germination, indicating their different roles in hormone signalling pathways. In addition, we showed that miR393 affected seed development through targeting two genes encoding the auxin receptors TIR1/AFBs in barley, as over-expression of miR393 led to an increased length-width ratio of seeds, whereas target mimic (MIM393)-mediated inhibition of its activity decreased the 1000-grain weight of seeds. Furthermore, the expression of auxin-responsive genes, abscisic acid- and gibberellic acid-related genes was altered in miR393 misexpression lines during germination and early seedling growth.

CONCLUSIONS

Our work indicates that miRNA-target pairs participate in gene expression regulation and hormone interaction in barley embryo and provides evidence that miR393-mediated auxin response regulation affects grain development and influences gibberellic acid and abscisic acid homeostasis during germination.

Unraveling Key Metabolomic Alterations in Wheat Embryos Derived from Freshly Harvested and Water-Imbibed Seeds of Two Wheat Cultivars with Contrasting Dormancy Status

Untimely rains in wheat fields during harvest season can cause pre-harvest sprouting (PHS), which deteriorates the yield and quality of wheat crop. Metabolic homeostasis of the embryo plays a role in seed dormancy, determining the status of the maturing grains either as dormant (PHS-tolerant) or non-dormant (PHS-susceptible). Very little is known for direct measurements of global metabolites in embryonic tissues of dormant and non-dormant wheat seeds. In this study, physiologically matured and freshly harvested wheat seeds of PHS-tolerant (cv. Sukang, dormant) and PHS-susceptible (cv. Baegjoong, non-dormant) cultivars were water-imbibed, and the isolated embryos were subjected to high-throughput, global non-targeted metabolomic profiling. A careful comparison of identified metabolites between Sukang and Baegjoong embryos at 0 and 48 h after imbibition revealed that several key metabolic pathways [such as: lipids, fatty acids, oxalate, hormones, the raffinose family of oligosaccharides (RFOs), and amino acids] and phytochemicals were differentially regulated between dormant and non-dormant varieties. Most of the membrane lipids were highly reduced in Baegjoong compared to Sukang, which indicates that the cell membrane instability in response to imbibition could also be a key factor in non-dormant wheat varieties for their untimely germination. This study revealed that several key marker metabolites (e.g., RFOs: glucose, fructose, maltose, and verbascose), were highly expressed in Baegjoong after imbibition. Furthermore, the data showed that the key secondary metabolites and phytochemicals (vitexin, chrysoeriol, ferulate, salidroside and gentisic acid), with known antioxidant properties, were comparatively low at basal levels in PHS-susceptible, non-dormant cultivar, Baegjoong. In conclusion, the results of this investigation revealed that after imbibition the metabolic homeostasis of dormant wheat is significantly less affected compared to non-dormant wheat. The inferences from this study combined with proteomic and transcriptomic studies will advance the molecular understanding of the pathways and enzyme regulations during PHS.

Arabidopsis MADS-Box Transcription Factor AGL21 Acts as Environmental Surveillance of Seed Germination by Regulating ABI5 Expression

Seed germination is a crucial checkpoint for plant survival under unfavorable environmental conditions. Abscisic acid (ABA) signaling plays a vital role in integrating environmental information to regulate seed germination. It has been well known that MCM1/AGAMOUS/DEFICIENS/SRF (MADS)-box transcription factors are key regulators of seed and flower development in Arabidopsis. However, little is known about their functions in seed germination. Here we report that MADS-box transcription factor AGL21 is a negative regulator of seed germination and post-germination growth by controlling the expression of ABA-INSENSITIVE 5 (ABI5) in Arabidopsis. The AGL21-overexpressing plants were hypersensitive to ABA, salt, and osmotic stresses during seed germination and early post-germination growth, whereas agl21 mutants were less sensitive. We found that AGL21 positively regulated ABI5 expression in seeds. Consistently, genetic analyses showed that AGL21 is epistatic to ABI5 in controlling seed germination. Chromatin immunoprecipitation assays further demonstrated that AGL21 could directly bind to the ABI5 promoter in plant cells. Moreover, we found that AGL21 responded to multiple environmental stresses and plant hormones during seed germination. Taken together, our results suggest that AGL21 acts as a surveillance integrator that incorporates environmental cues and endogenous hormonal signals into ABA signaling to regulate seed germination and early post-germination growth.

Differences and commonalities of plant responses to single and combined stresses

In natural or agricultural environments, plants are constantly exposed to a wide range of biotic and abiotic stresses. Given the forecasted global climate changes, plants will cope with heat waves, drought periods and pathogens at the same time or consecutively. Heat and drought cause opposing physiological responses, while pathogens may or may not profit from climate changes depending on their lifestyle. Several studies have been conducted to find stress-specific signatures or stress-independent commonalities. Previously this has been done by comparing different single stress treatments. This approach has been proven difficult since most studies, comparing single and combined stress conditions, have come to the conclusion that each stress treatment results in specific transcriptional changes. Although transcriptional changes at the level of individual genes are highly variable and stress-specific, central metabolic and signaling responses seem to be common, often leading to an overall reduced plant growth. Understanding how specific transcriptional changes are linked to stress adaptations and identifying central hubs controlling this interaction will be the challenge for the coming years. In this review, we will summarize current knowledge on plant responses to different individual and combined stresses and try to find a common thread potentially underlying these responses. We will begin with a brief summary of known physiological, metabolic, transcriptional and hormonal responses to individual stresses, elucidate potential commonalities and conflicts and finally we will describe results obtained during combined stress experiments. Here we will concentrate on simultaneous application of stress conditions but we will also touch consequences of sequential stress treatments.

Identification and Expression Profiling of the Auxin Response Factors in Dendrobium officinale under Abiotic Stresses

Auxin response factor (ARF) proteins play roles in plant responses to diverse environmental stresses by binding specifically to the auxin response element in the promoters of target genes. Using our latest public Dendrobium transcriptomes, a comprehensive characterization and analysis of 14 DnARF genes were performed. Three selected DnARFs, including DnARF1, DnARF4, and DnARF6, were confirmed to be nuclear proteins according to their transient expression in epidermal cells of Nicotiana benthamiana leaves. Furthermore, the transcription activation abilities of DnARF1, DnARF4, and DnARF6 were tested in a yeast system. Our data showed that DnARF6 is a transcriptional activator in Dendrobium officinale. To uncover the basic information of DnARF gene responses to abiotic stresses, we analyzed their expression patterns under various hormones and abiotic treatments. Based on our data, several hormones and significant stress responsive DnARF genes have been identified. Since auxin and ARF genes have been identified in many plant species, our data is imperative to reveal the function of ARF mediated auxin signaling in the adaptation to the challenging Dendrobium environment.

Regulatory crosstalk between microRNAs and hormone signalling cascades controls the variation on seed dormancy phenotype at Arabidopsis thaliana seed set

We employed an Illumina sequencing approach to identify candidate microRNA cohorts that may greatly contribute to seed dormancy modulation and to construct a microRNA-gene regulatory network in hormone signalling cascades. MicroRNAs (miRNAs) are important signalling molecules and regulate many developmental programs of plants. Some miRNAs have been integrated into gene regulatory networks (GRNs) and coordinate developmental plasticity, but few study systematically investigated how phenotypical variations are regulated through differential expression of miRNA tags in GRNs during seed set. Using 'top-down' analyses (i.e., identify miRNAs associated with known phenotypical variations), we chose two Arabidopsis ecotypes (Cvi-0 and Col-0) with contrasting seed dormancy and sequenced miRNA reads in the first ten phases at seed set. We computationally predicted target genes of miRNAs and implemented statistical analyses for normalized relative expression of top abundant miRNA cohorts between the two ecotypes. We especially focused on miRNA cohorts targeting mRNAs encoding transcription factors in hormone signalling cascades. We report, with high confidence hits, that a cohort of 14 miRNAs (miR-156b, -159b, -160, -161*, -319a, -390a, -396, -773a, -779, -842, -852, -859, -1886*, and a novel sequence in miR8172 family) may greatly contribute to seed dormancy modulation, of which seven are involved in hormone signalling cascades. Moreover, their expression patterns indicated that 5 ± 1 days after flowering (at embryogenesis-to-maturation transition) is a critical phase at seed set. This study reinforces the notion that miRNAs that regulate seed dormancy modulation and provides a novel paradigm of studying the correlation between genotypes (miRNAs) and phenotypes.

The E3 ligase ABI3-INTERACTING PROTEIN2 negatively regulates FUSCA3 and plays a role in cotyledon development in Arabidopsis thaliana

FUSCA3 (FUS3) is a short-lived B3-domain transcription factor that regulates seed development and phase transitions in Arabidopsis thaliana. The mechanisms controlling FUS3 levels are currently poorly understood. Here we show that FUS3 interacts with the RING E3 ligase ABI3-INTERACTING PROTEIN2 (AIP2). AIP2-green fluorescent protein (GFP) is preferentially expressed in the protoderm during early embryogenesis, similarly to FUS3, suggesting that their interaction is biologically relevant. FUS3 degradation is delayed in the aip2-1 mutant and FUS3-GFP fluorescence is increased in aip2-1, but only during mid-embryogenesis, suggesting that FUS3 is negatively regulated by AIP2 at a specific time during embryogenesis. aip2-1 shows delayed flowering and therefore also functions post-embryonically to regulate developmental phase transitions. Plants overexpressing FUS3 post-embryonically in the L1 layer (ML1p:FUS3) show late flowering and other developmental phenotypes that can be rescued by ML1p:AIP2, further supporting a negative role for AIP2 in FUS3 accumulation. However, additional factors regulate FUS3 levels during embryogenesis, as ML1:AIP2 seeds do not resemble fus3-3. Lastly, targeted expression of a RING-inactive AIP2 variant to the protoderm/L1 layer causes FUS3 and ABI3 overexpression phenotypes and defects in cotyledon development. Taken together, these results indicate that AIP2 targets FUS3 for degradation and plays a role in cotyledon development and flowering time in Arabidopsis.

The AFL subfamily of B3 transcription factors: evolution and function in angiosperm seeds

Seed development follows zygotic embryogenesis; during the maturation phase reserves accumulate and desiccation tolerance is acquired. This is tightly regulated at the transcriptional level and the AFL (ABI3/FUS3/LEC2) subfamily of B3 transcription factors (TFs) play a central role. They alter hormone biosynthesis, mainly in regards to abscisic acid and gibberellins, and also regulate the expression of other TFs and/or modulate their downstream activity via protein-protein interactions. This review deals with the origin of AFL TFs, which can be traced back to non-vascular plants such as Physcomitrella patens and achieves foremost expansion in the angiosperms. In green algae, like the unicellular Chlamydomonas reinhardtii or the pluricellular Klebsormidium flaccidum, a single B3 gene and four B3 paralogous genes are annotated, respectively. However, none of them present with the structural features of the AFL subfamily, with the exception of the B3 DNA-binding domain. Phylogenetic analysis groups the AFL TFs into four Major Clusters of Ortologous Genes (MCOGs). The origin and function of these genes is discussed in view of their expression patterns and in the context of major regulatory interactions in seeds of monocotyledonous and dicotyledonous species.

A Novel RNA-Binding Protein Involves ABA Signaling by Post-transcriptionally Repressing ABI2

The Stress Associated RNA-binding protein 1 (SRP1) repressed by ABA, salt and cold encodes a C2C2-type zinc finger protein in Arabidopsis. The knock-out mutation in srp1 reduced the sensitivity of seed to ABA and salt stress during germination and post-germinative growth stages. In contrast, SRP1-overexpressing seedlings were more sensitive to ABA and salt compared to wild type plants. In the presence of ABA, the transcript levels of ABA signaling and germination-related genes including ABI3. ABI5. EM1 and EM6 were less induced in srp1 compared to WT. Interestingly, expression of ABI2 encoding a protein phosphatase 2C protein were significantly up-regulated in srp1 mutants. By in vitro analysis, SRP1 was identified as a novel RNA-binding protein directly binding to 3'UTR of ABI2 mRNA. Moreover, transient expression assay proved the function of SRP1 in reducing the activity of luciferase whose coding sequence was fused with the ABI2 3'UTR. Together, it is suggested that SRP1 is involved in the ABA signaling by post-transcriptionally repressing ABI2 expression in Arabidopsis.

Karrikins: Regulators Involved in Phytohormone Signaling Networks during Seed Germination and Seedling Development

Seed germination and early seedling establishment are critical stages during a plant's life cycle. These stages are precisely regulated by multiple internal factors, including phytohormones and environmental cues such as light. As a family of small molecules discovered in wildfire smoke, karrikins (KARs) play a key role in various biological processes, including seed dormancy release, germination regulation, and seedling establishment. KARs show a high similarity with strigolactone (SL) in both chemical structure and signaling transduction pathways. Current evidence shows that KARs may regulate seed germination by mediating the biosynthesis and/or signaling transduction of abscisic acid (ABA), gibberellin (GA) and auxin [indoleacetic acid (IAA)]. Interestingly, KARs regulate seed germination differently in different species. Furthermore, the promotion effect on seedling establishment implies that KARs have a great potential application in alleviating shade avoidance response, which attracts more and more attention in plant molecular biology. In these processes, KARs may have complicated interactions with phytohormones, especially with IAA. In this updated review, we summarize the current understanding of the relationship between KARs and SL in the chemical structure, signaling pathway and the regulation of plant growth and development. Further, the crosstalk between KARs and phytohormones in regulating seed germination and seedling development and that between KARs and IAA during shade responses are discussed. Finally, future challenges and research directions for the KAR research field are suggested.