Salt Stress and Ethylene Antagonistically Regulate Nucleocytoplasmic Partitioning of COP1 to Control Seed Germination

The E3 ubiquitin ligase COP1 regulates salt tolerance via GIGANTEA degradation in roots

Excess soil salinity significantly impairs plant growth and development. Our previous reports demonstrated that the core circadian clock oscillator GIGANTEA (GI) negatively regulates salt stress tolerance by sequestering the SALT OVERLY SENSITIVE (SOS) 2 kinase, an essential component of the SOS pathway. Salt stress induces calcium-dependent cytoplasmic GI degradation, resulting in activation of the SOS pathway; however, the precise molecular mechanism governing GI degradation during salt stress remains enigmatic. Here, we demonstrate that salt-induced calcium signals promote the cytoplasmic partitioning of CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), leading to the 26S proteasome-dependent degradation of GI exclusively in the roots. Salt stress-induced calcium signals accelerate the cytoplasmic localization of COP1 in the root cells, which targets GI for 26S proteasomal degradation. Align with this, the interaction between COP1 and GI is only observed in the roots, not the shoots, under salt-stress conditions. Notably, the gi-201 cop1-4 double mutant shows an enhanced tolerance to salt stress similar to gi-201, indicating that GI is epistatic to COP1 under salt-stress conditions. Taken together, our study provides critical insights into the molecular mechanisms governing the COP1-mediated proteasomal degradation of GI for salt stress tolerance, raising new possibilities for developing salt-tolerant crops.

HY5-dependent light-mediated regulation of galactinol synthase gene, AtGolS1, modulates galactinol biosynthesis in Arabidopsis

The Raffinose Family of Oligosaccharides (RFOs), including Galactinol, Raffinose, and Stachyose, are pivotal carbohydrates with significant roles in abiotic stress tolerance and growth within dynamic environments. Plant development is profoundly influenced by light, a major environmental signal. Despite this, the interconnections between the biosynthesis of secondary sugars and light signaling have remained unexplored. This study reveals that exposure to light induces the expression of Galactinol synthase (AtGolS1), a key enzyme in the RFO biosynthesis pathway. The light-inducible response of AtGolS1 operates downstream of ELONGATED HYPOCOTYL 5 (HY5), a central regulator in light signaling. Mutant seedlings with disrupted HY5 function (hy5-215) exhibit reduced AtGolS1 transcript accumulation compared to wild-type (WT) and HY5 overexpression seedlings. DNA-protein interaction studies demonstrate that HY5 directly binds to light-responsive cis-elements in the promoter region of AtGolS1, thereby mediating its light responsiveness. Quantification of galactinol revealed a diminished accumulation in the hy5-215 mutant compared to wild-type (WT) and HY5 overexpression seedlings. Consequently, these findings shed light on the intricate crosstalk between RFO biosynthesis and light signaling in Arabidopsis.

Functions of Plant Phytochrome Signaling Pathways in Adaptation to Diverse Stresses

Phytochromes are receptors for red light (R)/far-red light (FR), which are not only involved in regulating the growth and development of plants but also in mediated resistance to various stresses. Studies have revealed that phytochrome signaling pathways play a crucial role in enabling plants to cope with abiotic stresses such as high/low temperatures, drought, high-intensity light, and salinity. Phytochromes and their components in light signaling pathways can also respond to biotic stresses caused by insect pests and microbial pathogens, thereby inducing plant resistance against them. Given that, this paper reviews recent advances in understanding the mechanisms of action of phytochromes in plant resistance to adversity and discusses the importance of modulating the genes involved in phytochrome signaling pathways to coordinate plant growth, development, and stress responses.

HY5 inhibits lateral root initiation in Arabidopsis through negative regulation of the microtubule-stabilizing protein TPXL5

Tight control of lateral root (LR) initiation is vital for root system architecture and function. Regulation of cortical microtubule reorganization is involved in the asymmetric radial expansion of founder cells during LR initiation in Arabidopsis (Arabidopsis thaliana). However, critical genetic evidence on the role of microtubules in LR initiation is lacking and the mechanisms underlying this regulation are poorly understood. Here, we found that the previously uncharacterized microtubule-stabilizing protein TPX2-LIKE5 (TPXL5) participates in LR initiation, which is finely regulated by the transcription factor ELONGATED HYPOCOTYL5 (HY5). In tpxl5 mutants, LR density was decreased and more LR primordia (LRPs) remained in stage I, indicating delayed LR initiation. In particular, the cell width in the peripheral domain of LR founder cells after the first asymmetric cell division was larger in tpxl5 mutants than in the wild-type. Consistently, ordered transverse cortical microtubule arrays were not well generated in tpxl5 mutants. In addition, HY5 directly targeted the promoter of TPXL5 and downregulated TPXL5 expression. The hy5 mutant exhibited higher LR density and fewer stage I LRPs, indicating accelerated LR initiation. Such phenotypes were partially suppressed by TPXL5 knockout. Taken together, our data provide genetic evidence supporting the notion that cortical microtubules are essential for LR initiation and unravel a molecular mechanism underlying HY5 regulation of TPXL5-mediated microtubule reorganization and cell remodeling during LR initiation.

COP1 controls salt stress tolerance by modulating sucrose content

The E3 ubiquitin ligase Constitutive Photomorphogenic 1 (COP1) plays evolutionarily conserved and divergent roles. In plants, COP1 regulates a large number of developmental processes including photomorphogenesis, seedling emergence, and gravitropism. Nevertheless, its function in abiotic stress tolerance remains largely unknown. Here, we demonstrate the role of COP1 in salt stress tolerance in Arabidopsis thaliana. In soil, cop1-4 and cop1-6 mutants were more tolerant to high salinity than wild-type (WT) plants during vegetative growth. However, in high salt-containing Murashige and Skoog (MS) medium, cop1-4 and cop1-6 seedlings exhibited significantly impaired growth compared with WT plants. Notably, cop1-4 and cop1-6 seedlings recovered their growth to the WT level upon exogenous sucrose treatment even under high salinity conditions. Compared with WT plants, the sucrose content of cop1-4 mutants was much higher at the vegetative growth stage but similar at the seedling stage. Upon exogenous sucrose supply, root elongation was significantly stimulated in cop1-4 seedlings but only slightly stimulated in WT plants. Thus, no significant difference was observed in root length between the two genotypes. Altogether, our data indicate that cop1 mutants are more tolerant to salt stress than WT plants, and the salt tolerance of cop1 mutants is correlated with their sucrose content.

CONSTITUTIVE PHOTOMORPHOGENIC 1 promotes seed germination by destabilizing RGA-LIKE 2 in Arabidopsis

Under favorable moisture, temperature, and light conditions, gibberellin (GA) biosynthesis is induced and triggers seed germination. A major mechanism by which GA promotes seed germination is by promoting the degradation of the DELLA protein RGA-LIKE 2 (RGL2), a major repressor of germination in Arabidopsis (Arabidopsis thaliana) seeds. Analysis of seed germination phenotypes of constitutive photomorphogenic 1 (cop1) mutants and complemented COP1-OX/cop1-4 lines in response to GA and paclobutrazol (PAC) suggested a positive role for COP1 in seed germination and a relation with GA signaling. cop1-4 mutant seeds showed PAC hypersensitivity, but transformation with a COP1 overexpression construct rendered them PAC insensitive, with a phenotype similar to that of rgl2 mutant (rgl2-SK54) seeds. Furthermore, cop1-4 rgl2-SK54 double mutants showed a PAC-insensitive germination phenotype like that of rgl2-SK54, identifying COP1 as an upstream negative regulator of RGL2. COP1 interacted directly with RGL2, and in vivo this interaction was strongly enhanced by SUPPRESSOR OF PHYA-105 1. COP1 directly ubiquitinated RGL2 to promote its degradation. Moreover, GA stabilized COP1 with consequent RGL2 destabilization. By uncovering this COP1-RGL2 regulatory module, we reveal a mechanism whereby COP1 positively regulates seed germination and controls the expression of germination-promoting genes.

Molecular Mechanisms of Plant Responses to Salt Stress

Saline-alkali soils pose an increasingly serious global threat to plant growth and productivity. Much progress has been made in elucidating how plants adapt to salt stress by modulating ion homeostasis. Understanding the molecular mechanisms that affect salt tolerance and devising strategies to develop/breed salt-resilient crops have been the primary goals of plant salt stress signaling research over the past few decades. In this review, we reflect on recent major advances in our understanding of the cellular and physiological mechanisms underlying plant responses to salt stress, especially those involving temporally and spatially defined changes in signal perception, decoding, and transduction in specific organelles or cells.

Abscisic acid-induced cytoplasmic translocation of constitutive photomorphogenic 1 enhances reactive oxygen species accumulation through the HY5-ABI5 pathway to modulate seed germination

Seed germination is a physiological process regulated by multiple factors. Abscisic acid (ABA) can inhibit seed germination to improve seedling survival under conditions of abiotic stress, and this process is often regulated by light signals. Constitutive photomorphogenic 1 (COP1) is an upstream core repressor of light signals and is involved in several ABA responses. Here, we demonstrate that COP1 is a negative regulator of the ABA-mediated inhibition of seed germination. Disruption of COP1 enhanced Arabidopsis seed sensitivity to ABA and increased reactive oxygen species (ROS) levels. In seeds, ABA induced the translocation of COP1 to the cytoplasm, resulting in enhanced ABA-induced ROS levels. Genetic evidence indicated that HY5 and ABI5 act downstream of COP1 in the ABA-mediated inhibition of seed germination. ABA-induced COP1 cytoplasmic localization increased HY5 and ABI5 protein levels in the nucleus, leading to increased expression of ABI5 target genes and ROS levels in seeds. Together, our results reveal that ABA-induced cytoplasmic translocation of COP1 activates the HY5-ABI5 pathway to promote the expression of ABA-responsive genes and the accumulation of ROS during ABA-mediated inhibition of seed germination. These findings enhance the role of COP1 in the ABA signal transduction pathway.

Exogenous SA Affects Rice Seed Germination under Salt Stress by Regulating Na+/K+ Balance and Endogenous GAs and ABA Homeostasis

Salinity reduces agricultural productivity majorly by inhibiting seed germination. Exogenous salicylic acid (SA) can prevent the harm caused to rice by salinity, but the mechanisms by which it promotes rice seed germination under salt stress are unclear. In this study, the inhibition of germination in salt-sensitive Nipponbare under salt stress was greater than that in salt-tolerant Huaidao 5. Treatment with exogenous SA significantly improved germination of Nipponbare, but had little effect on Huaidao 5. The effects of exogenous SA on ion balance, metabolism of reactive oxygen species (ROS), hormone homeostasis, starch hydrolysis, and other physiological processes involved in seed germination of rice under salt stress were investigated. Under salt stress, Na⁺ content and the Na⁺/K⁺ ratio in rice seeds increased sharply. Seeds were subjected to ion pressure, which led to massive accumulation of H₂O₂, O₂⁻, and malonaldehyde (MDA); imbalanced endogenous hormone homeostasis; decreased gibberellic acid (GA1 and GA4) content; increased abscisic acid (ABA) content; inhibition of α-amylase (EC 3.2.1.1) activity; and slowed starch hydrolysis rate, all which eventually led to the inhibition of the germination of rice seeds. Exogenous SA could effectively enhance the expression of OsHKT1;1, OsHKT1;5, OsHKT2;1 and OsSOS1 to reduce the absorption of Na+ by seeds; reduce the Na⁺/K⁺ ratio; improve the activities of SOD, POD, and CAT; reduce the accumulation of H₂O₂, O₂⁻, and MDA; enhance the expression of the GA biosynthetic genes OsGA20ox1 and OsGA3ox2; inhibit the expression of the ABA biosynthetic gene OsNCED5; increase GA1 and GA4 content; reduce ABA content; improve α-amylase activity, and increase the content of soluble sugars. In summary, exogenous SA can alleviate ion toxicity by reducing Na⁺ content, thereby helping to maintain ROS and hormone homeostasis, promote starch hydrolysis, and provide sufficient energy for seed germination, all of which ultimately improves rice seed germination under salt stress. This study presents a feasible means for improving the germination of direct-seeded rice in saline soil.

Plant salt response: Perception, signaling, and tolerance

Salt stress is one of the significant environmental stressors that severely affects plant growth and development. Plant responses to salt stress involve a series of biological mechanisms, including osmoregulation, redox and ionic homeostasis regulation, as well as hormone or light signaling-mediated growth adjustment, which are regulated by different functional components. Unraveling these adaptive mechanisms and identifying the critical genes involved in salt response and adaption are crucial for developing salt-tolerant cultivars. This review summarizes the current research progress in the regulatory networks for plant salt tolerance, highlighting the mechanisms of salt stress perception, signaling, and tolerance response. Finally, we also discuss the possible contribution of microbiota and nanobiotechnology to plant salt tolerance.

Plants Saline Environment in Perception with Rhizosphere Bacteria Containing 1-Aminocyclopropane-1-Carboxylate Deaminase

Soil salinity stress has become a serious roadblock for food production worldwide since it is one of the key factors affecting agricultural productivity. Salinity and drought are predicted to cause considerable loss of crops. To deal with this difficult situation, a variety of strategies have been developed, including plant breeding, plant genetic engineering, and a wide range of agricultural practices, including the use of plant growth-promoting rhizobacteria (PGPR) and seed biopriming techniques, to improve the plants' defenses against salinity stress, resulting in higher crop yields to meet future human food demand. In the present review, we updated and discussed the negative effects of salinity stress on plant morphological parameters and physio-biochemical attributes via various mechanisms and the beneficial roles of PGPR with 1-Aminocyclopropane-1-Carboxylate(ACC) deaminase activity as green bio-inoculants in reducing the impact of saline conditions. Furthermore, the applications of ACC deaminase-producing PGPR as a beneficial tool in seed biopriming techniques are updated and explored. This strategy shows promise in boosting quick seed germination, seedling vigor and plant growth uniformity. In addition, the contentious findings of the variation of antioxidants and osmolytes in ACC deaminase-producing PGPR treated plants are examined.

Constitutive Photomorphogenic 1 Enhances ER Stress Tolerance in Arabidopsis

Interaction between light signaling and stress response has been recently reported in plants. Here, we investigated the role of CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a key regulator of light signaling, in endoplasmic reticulum (ER) stress response in Arabidopsis. The cop1-4 mutant Arabidopsis plants were highly sensitive to ER stress induced by treatment with tunicarmycin (Tm). Interestingly, the abundance of nuclear-localized COP1 increased under ER stress conditions. Complementation of cop1-4 mutant plants with the wild-type or variant types of COP1 revealed that the nuclear localization and dimerization of COP1 are essential for its function in plant ER stress response. Moreover, the protein amount of ELONGATED HYPOCOTYL 5 (HY5), which inhibits bZIP28 to activate the unfolded protein response (UPR), decreased under ER stress conditions in a COP1-dependent manner. Accordingly, the binding of bZIP28 to the BIP3 promoter was reduced in cop1-4 plants and increased in hy5 plants compared with the wild type. Furthermore, introduction of the hy5 mutant locus into the cop1-4 mutant background rescued its ER stress-sensitive phenotype. Altogether, our results suggest that COP1, a negative regulator of light signaling, positively controls ER stress response by partially degrading HY5 in the nucleus.

Genome-Wide Identification and Functional Investigation of 1-Aminocyclopropane-1-carboxylic Acid Oxidase (ACO) Genes in Cotton

ACO is one of the rate-limiting enzymes in the biosynthesis of ethylene, and it plays a critical role in the regulation of plant growth and development. However, the function of ACO genes in cotton is not well studied. In this study, a total of 332 GhACOs, 187 GaACOs, and 181 GrACOs were identified in G. hirsutum, G. arboretum, and G. raimondii, respectively. Gene duplication analysis showed that whole-genome duplication (WGD) and tandem duplication were the major forces driving the generation of cotton ACO genes. In the promoters of GhACOs, there were cis-acting elements responding to stress, phytohormones, light, and circadian factors, indicating the possible involvement of GhACOs in these processes. Expression and co-expression analyses illustrated that most GhACOs were not only widely expressed in various tissues but also coexpressed with other genes in response to salt and drought stress. GhACO106_At overexpression in Arabidopsis promoted flowering and increased salt tolerance. These results provide a comprehensive overview of the ACO genes of cotton and lay the foundation for subsequent functional studies of these genes.

Comparative transcriptome analysis reveals function of TERF1 in promoting seed germination

Seed germination marks a new life cycle of a plant. Although ethylene promotes seed germination, the underlying molecular mechanism is poorly understood. Ethylene Responsive Factors (ERFs) play an essential role in ethylene signaling. Here we show that overexpression of Tomato Ethylene Responsive Factor 1 (TERF1), an ERF transcription factor isolated from tomato, can promote tobacco seed germination at 23 °C in darkness. Hormones analysis showed that salicylic acid (SA), 3-indoleacetic acid (IAA), abscisic acid (ABA) and gibberellic acids (GAs) were significantly increased by TERF1, while jasmonic acid (JA) was significantly reduced in TERF1 seeds. Transcriptome analysis identified 7,961 differentially expressed genes (DEGs), including 6,213 mRNAs, 25 miRNAs, 1,581 lncRNAs and 141 circRNAs. Gene Ontology (GO) enrichment analysis showed that cell cycles, sugar metabolism, microtubule-based processes were activated by TERF1, while DNA repair, lipid metabolism were repressed by TERF1. We also identified differentially expressed regulatory genes for ABA and GA biosynthesis or signaling in TERF1 seed, including transcription factors, kinases, phosphatases and ubiquitin protein ligases, non-coding RNAs (ncRNAs). At posttranscriptional level TERF1 also regulates gene expression through alternative splicing (AS). Protein-protein interaction (PPI) network analysis revealed three key biological processes regulated by TERF1, including nitrogen metabolism, light related processes and mitosis. Pheynotype and gene expression analysis showed that TERF1 significantly reduced seed sensitivity to ABA and auxin during germination through repressing key components of ABA signaling pathway. Our results unraveled the function of TERF1 in promoting seed germination.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01049-4.

Silicon nanoparticles elicit an increase in lemongrass (Cymbopogon flexuosus (Steud.) Wats) agronomic parameters with a higher essential oil yield

Lemongrass (Cymbopogon flexuosus (Steud.) Wats) is an aromatic grass with great industrial potential. It is cultivated for its essential oil (EO) which has great economical value due to its numerous medicinal, cosmetic and culinary applications. The present study was conducted on silicon nanoparticles (SiNPs) application to lemongrass with the objective of overall agronomic enhancements. Graded concentrations (50-200 mg L^-1) of SiNPs were exogenously applied to lemongrass leaves. The physiological and biochemical analyses revealed that 150 mg L^-1 SiNPs is the optimum concentration for lemongrass plants. This concentration triggered photosynthetic variables, gas exchange modules and activities of enzymes involved in EO (geraniol dehydrogenase) and nitrogen (nitrate reductase) metabolism as well as in the antioxidant system (catalase, peroxidase and superoxide dismutase). These SiNPs-induced metabolic changes altogether significantly (p ≤ 0.05) enhanced overall plant growth and yield. Moreover, SiNPs treatments assisted in palliating lipid peroxidation and H₂O₂ content in lemongrass leaves which added further advantage to plant metabolism. Overall, data indicates SiNPs elicit beneficial effects on lemongrass growth and yield through inducing various physiological and biochemical responses. This renders high possibility that similar objectives could be achieved with SiNPs biotechnological application on further related agronomic crops as well as in diverse industries.

Melatonin promotes seed germination under salt stress by regulating ABA and GA3 in cotton (Gossypium hirsutum L.)

Although previous studies have found that melatonin can promote seed germination, the phytohormone regulation mechanism by which exogenous melatonin mediates salt tolerance during cotton seed germination is still largely unknown. The effects of melatonin on germination traits and physiological parameters of GXM9 cotton seeds (Gossypium hirsutum L.) under three salt stress treatments (CK, germination of seeds pretreated with water alone; S, germination of seeds pretreated in 150 mM NaCl under salt stress; SM, germination of seeds pretreated in 20 μM melatonin under 150 mM NaCl solution) in the laboratory was investigated. The results showed that salt stress (150 mM) inhibited cotton seed germination and endogenous melatonin accumulation, and pretreatment with 20 μM exogenous melatonin enhanced the cotton germination rate and hypocotyl length as well as the content of endogenous melatonin during seed germination. This suggests that exogenous melatonin promotes seed germination from a morphological perspective. The contents of starch, α-amylase (EC3.3.1.1), β-galactosidase (EC3.2.1.23), abscisic acid (ABA), and gibberellin (GA) were determined simultaneously. The results showed that the α-amylase and β-galactosidase contents in the cotton seeds decreased by 56.97% and 20.18%, respectively, under salt stress compared with the control, while the starch content increased by 11.53% compared with the control at day 7. The ABA content increased by 25.18% and GA content decreased by 27.99% under salt stress compared with the control at 24 h. When exogenous melatonin was applied to the cotton seeds, the content of α-amylase and β-galactosidase increased by 121.77% and 32.76%, respectively, whereas the starch contents decreased by 13.55% compared with the S treatment at day 7. Similarly, the ABA content increased by 12.20% and the GA content increased by 4.77% at 24 h. To elucidate the molecular mechanism by which melatonin promotes seed germination under salt stress, the effects of ABA- and GA-related genes on plant hormone signal transduction were analyzed by quantitative real-time PCR and RNA sequencing. The results indicated that melatonin regulated the expression of ABA and GA genes in the plant signal transduction pathway, induced embryo root development and seed germination, and alleviated dormancy. The expression of the ABA signaling gene GhABF2 was up-regulated and GhDPBF2 was down-regulated, and the expression of GA signaling genes (e.g., GhGID1C and GhGID1B) was up-regulated by melatonin. In conclusion, melatonin enhances salt tolerance in cotton seeds by regulating ABA and GA and by mediating the expression of hormone-related genes in plant hormone signal transduction. This should help us to explore the regulatory mechanisms of cotton resistance and provide a foundation for the cultivation of new varieties.

COP1 mediates light-dependent regulation of flavonol biosynthesis through HY5 in Arabidopsis

Flavonols, a class of flavonoids, accumulate as protective agents in response to various stresses. Among various environmental stimuli, light is one of the factors regulating flavonol production. MYB12/11/111, members of the R2R3 MYBs family, regulates spatio-temporal flavonol accumulation in Arabidopsis. Although various studies indicate at the involvement of an E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and ELONGATED HYPOCOTYL 5 (HY5) in flavonoid biosynthesis in response to UV-B, the regulatory roles of these components under visible light are yet to be investigated. Here, we demonstrate that flavonol accumulation in Arabidopsis is light-regulated. Furthermore, our analysis suggests that MYB12 is a HY5-dependent light-inducible gene and plays a key role in the activation of the flavonol biosynthesis in response to light. Our results indicate the involvement of COP1 in the dark-dependent repression of MYB12 expression and flavonol accumulation. In addition, results also suggest that the effect of COP1 on MYB12 is indirect and is mediated through HY5, a direct transcriptional activator of the MYB12. Together these findings indicate that COP1 acts as a master negative regulator of flavonol biosynthesis in the dark.

The apple MdCOP1-interacting protein 1 negatively regulates hypocotyl elongation and anthocyanin biosynthesis

BACKGROUND

In plants, CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) is a key negative regulator in photoperiod response. However, the biological function of COP1-interacting protein 1 (CIP1) and the regulatory mechanism of the CIP1-COP1 interaction are not fully understood.

RESULTS

Here, we identified the apple MdCIP1 gene based on the Arabidopsis AtCIP1 gene. Expression pattern analysis showed that MdCIP1 was constitutively expressed in various tissues of apple, and responded to stress and hormone signals at the transcriptional level. Ectopic expression of MdCIP1 complemented the phenotypes of the Arabidopsis cip1 mutant, and MdCIP1 inhibited anthocyanin biosynthesis in apple calli. In addition, the biochemical assay demonstrated that MdCIP1 could interact with MdCOP1 protein by their coiled-coil domain, and MdCIP1-OX/cop1-4 had a similar phenotype in photomorphogenesis with the cop1-4 mutant, suggesting that COP1 is epistatic to CIP1. Furthermore, the transient transformation assay indicated that MdCIP1 repressed anthocyanin biosynthesis in an MdCOP1-mediated pathway.

CONCLUSION

Take together, this study finds that MdCIP1 acts as a repressor in regulating hypocotyl elongation and anthocyanin biosynthesis through MdCOP1 in apple.

UV RESISTANCE LOCUS8 mediates ultraviolet-B-induced stomatal closure in an ethylene-dependent manner

Although the UV RESISTANCE LOCUS 8 (UVR8)-CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1)-ELONGATED HYPOCOTYL5 (HY5) signaling pathway, ethylene, hydrogen peroxide (H₂O₂), and nitric oxide (NO) all participate in ultraviolet-B (UV-B)-triggered stomatal closing, their interrelationship is not clear. Here, we found that UV-B-induced the expression of ethylene biosynthetic genes, production of ethylene, H₂O₂, and NO, and stomata closing were impaired in uvr8, cop1, and hy5 mutants. UV-B-induced NO production and stomata closing were also defective in mutants for ETHYLENE RESPONSE 1 (ETR1), ETHYLENE INSENSITIVE 2 (EIN2), and EIN3, but UV-B-triggered H₂O₂ generation was only inhibited in etr1. In either the absence or presence of UV-B, ethylene triggered H₂O₂ production but not NO generation and stomatal closure in cop1 and hy5, and stomata closing in cop1 and hy5 was induced by NO but not H₂O₂. Moreover, NO production and stomatal closure were constitutively caused by over-expression of COP1 or HY5 in ein2 and ein3, but not by over-expression of EIN2 or EIN3 in cop1 and hy5. Our data indicate that the UVR8-COP1-HY5 signaling module mediates UV-B-induced ethylene production, ethylene is then perceived by ETR1 to induce H₂O₂ synthesis. H₂O₂ induces NO generation and subsequent stomata closing via an EIN2, EIN3, COP1, and HY5-dependent pathway(s).

CONSTITUTIVELY PHOTOMORPHOGENIC1 promotes ABA-mediated inhibition of post-germination seedling establishment

Under acute stress conditions, precocious seedling development may result in the premature death of young seedlings, before they switch to autotrophic growth. The phytohormone abscisic acid (ABA) inhibits seed germination and post-germination seedling establishment under unfavorable conditions. Various environmental signals interact with the ABA pathway to optimize these early developmental events under stress. Here, we show that light availability critically influences ABA sensitivity during early seedling development. In dark conditions, the ABA-mediated inhibition of post-germination seedling establishment is strongly enhanced. COP1, a central regulator of seedling development in the dark, is necessary for this enhanced post-germination ABA sensitivity in darkness. Despite their slower germination, cop1 seedlings establish faster than wild type in the presence of ABA in both light and dark. PHY and CRY photoreceptors that inhibit COP1 activity in light modulate ABA-mediated inhibition of seedling establishment in light. Genetically, COP1 acts downstream to ABI5, a key transcriptional regulator of ABA signaling, and does not influence the transcriptional and protein levels of ABI5 during the early post-germination stages. COP1 promotes post-germination growth arrest independent of the antagonistic interaction between ABA and cytokinin signaling pathways. COP1 facilitates the binding of ABI5 on its target promoters and the ABA-mediated upregulation of these target genes is reduced in cop1-4. Together, our results suggest that COP1 positively regulates ABA signaling to inhibit post-germination seedling establishment under stress.

Ethylene: A Master Regulator of Salinity Stress Tolerance in Plants

Salinity stress is one of the major threats to agricultural productivity across the globe. Research in the past three decades, therefore, has focused on analyzing the effects of salinity stress on the plants. Evidence gathered over the years supports the role of ethylene as a key regulator of salinity stress tolerance in plants. This gaseous plant hormone regulates many vital cellular processes starting from seed germination to photosynthesis for maintaining the plants' growth and yield under salinity stress. Ethylene modulates salinity stress responses largely via maintaining the homeostasis of Na+/K+, nutrients, and reactive oxygen species (ROS) by inducing antioxidant defense in addition to elevating the assimilation of nitrates and sulfates. Moreover, a cross-talk of ethylene signaling with other phytohormones has also been observed, which collectively regulate the salinity stress responses in plants. The present review provides a comprehensive update on the prospects of ethylene signaling and its cross-talk with other phytohormones to regulate salinity stress tolerance in plants.

Nitrate Promotes Germination Under Inhibition by NaCl or High Concentration of Glucose

Seed germination, one of the most important stages in a plant’s life cycle, can be affected by abiotic stresses, such as salinity. The plant hormone abscisic acid (ABA) and high concentrations of glucose are also known to inhibit germination. In contrast, nitrate is known to stimulate germination in many plants. However, this stimulatory effect has not yet been investigated in the presence of inhibitory effects caused by abiotic stresses, ABA, and glucose. In this study, we show that nitrate can alleviate the inhibitory effects of sodium chloride (NaCl) or high concentrations of glucose on seed germination in Arabidopsis, while it was not able to promote germination that was inhibited by exogenous ABA and mannitol (an inducer of osmotic stress). An analysis of the gene expression involved in the regulation of germination showed that GA20ox1, encoding the gibberellin (GA) synthesis enzyme, SPATULA (SPT), encoding a bHLH transcription factor, and CYP707A2, encoding an ABA catabolic enzyme, were significantly upregulated by the addition of KNO₃ in the presence of NaCl or glucose. Our results suggest the possibility that these genes are involved in the nitrate-mediated control of seed germination in the presence of NaCl or glucose.

Identification of Quantitative Trait Loci Controlling Ethylene Production in Germinating Seeds in Maize (Zea mays L.)

Plant seed germination is a crucial developmental event that has significant effects on seedling establishment and yield production. This process is controlled by multiple intrinsic signals, particularly phytohormones. The gaseous hormone ethylene stimulates seed germination; however, the genetic basis of ethylene production in maize during seed germination remains poorly understood. In this study, we quantified the diversity of germination among 14 inbred lines representing the parental materials corresponding to multiple recombinant inbred line (RIL) mapping populations. Quantitative trait loci (QTLs) controlling ethylene production were then identified in germinating seeds from an RIL population constructed from two parental lines showing differences in both germination speed and ethylene production during germination. To explore the possible genetic correlations of ethylene production with other traits, seed germination and seed weight were evaluated using the same batch of samples. On the basis of high-density single nucleotide polymorphism-based genetic linkage maps, we detected three QTLs for ethylene production in germinating seeds, three QTLs for seed germination, and four QTLs for seed weight, with each QTL explaining 5.8%–13.2% of the phenotypic variation of the trait. No QTLs were observed to be co-localized, suggesting that the genetic bases underlying the three traits are largely different. Our findings reveal three chromosomal regions responsible for ethylene production during seed germination, and provide a valuable reference for the future investigation of the genetic mechanism underlying the role of the stress hormone ethylene in maize germination control under unfavourable external conditions.

Temporal tissue-specific regulation of transcriptomes during barley (Hordeum vulgare) seed germination

The distinct functions of individual cell types require cells to express specific sets of genes. The germinating seed is an excellent model to study genome regulation between cell types since the majority of the transcriptome is differentially expressed in a short period, beginning from a uniform, metabolically inactive state. In this study, we applied laser-capture microdissection RNA-sequencing to small numbers of cells from the plumule, radicle tip and scutellum of germinating barley seeds every 8 h, over a 48 h time course. Tissue-specific gene expression was notably common; 25% (910) of differentially expressed transcripts in plumule, 34% (1876) in radicle tip and 41% (2562) in scutellum were exclusive to that organ. We also determined that tissue-specific storage of transcripts occurs during seed development and maturation. Co-expression of genes had strong spatiotemporal structure, with most co-expression occurring within one organ and at a subset of specific time points during germination. Overlapping and distinct enrichment of functional categories were observed in the tissue-specific profiles. We identified candidate transcription factors amongst these that may be regulators of spatiotemporal gene expression programs. Our findings contribute to the broader goal of generating an integrative model that describes the structure and function of individual cells within seeds during germination.

Golgi-localized cyclophilin 21 proteins negatively regulate ABA signalling via the peptidyl prolyl isomerase activity during early seedling development

Plant possesses particular Golgi-resident cyclophilin 21 proteins (CYP21s) and the catalytic isomerase activities have a negative effect on ABA signalling gene expression during early seedling development. Cyclophilins (CYPs) are essential for diverse cellular process, as these catalyse a rate-limiting step in protein folding. Although Golgi proteomics in Arabidopsis thaliana suggests the existence of several CYPs in the Golgi apparatus, only one putative Golgi-resident CYP protein has been reported in rice (Oryza sativa L.; OsCYP21-4). Here, we identified the Golgi-resident CYP21 family genes and analysed their molecular characteristics in Arabidopsis and rice. The CYP family genes (CYP21-1, CYP21-2, CYP21-3, and CYP21-4) are plant-specific, and their appearance and copy numbers differ among plant species. CYP21-1 and CYP21-4 are common to all angiosperms, whereas CYP21-2 and CYP21-3 evolved in the Malvidae subclass. Furthermore, all CYP21 proteins localize to cis-Golgi, trans-Golgi or both cis- and trans-Golgi membranes in plant cells. Additionally, based on the structure, enzymatic function, and topological orientation in Golgi membranes, CYP21 proteins are divided into two groups. Genetic analysis revealed that Group I proteins (CYP21-1 and CYP21-2) exhibit peptidyl prolyl cis-trans isomerase (PPIase) activity and regulate seed germination and seedling growth and development by affecting the expression levels of abscisic acid signalling genes. Thus, we identified the Golgi-resident CYPs and demonstrated that their PPIase activities are required for early seedling growth and development in higher plants.

An Arabidopsis WD40 repeat-containing protein XIW1 promotes salt inhibition of seed germination

WD40 repeat-containing protein (WD40 protein) acts as versatile scaffolds for protein interactions and participates in a variety of biological processes, such as plant stress and hormone responses. In a recent study, we show that XPO1-Interacting WD40 protein 1 (XIW1) is nuclear accumulated by abscisic acid (ABA), which interacts with ABA insensitive 5 (ABI5) in the nucleus and maintains its stability, thereby affecting seed germination and seedling growth. Here, we show that XIW1 promotes salt inhibition of seed germination (SSG). Mutation of XIW1 reduced the induction of ABI5 and ABA-responsive genes under salt treatment, causing mutant plants with high seed germination and suppression of SSG. In addition, we found that salt treatment facilities XIW1 accumulation in the nucleus. These findings reveal the important role of XIW1 in the regulation of SSG and provide a possible mechanism of XIW1 affecting SSG.

Nucleocytoplasmic Trafficking of the Arabidopsis WD40 Repeat Protein XIW1 Regulates ABI5 Stability and Abscisic Acid Responses

WD40 repeat-containing proteins (WD40 proteins) serve as versatile scaffolds for protein-protein interactions, modulating a variety of cellular processes such as plant stress and hormone responses. Here we report the identification of a WD40 protein, XIW1 (for XPO1-interacting WD40 protein 1), which positively regulates the abscisic acid (ABA) response in Arabidopsis. XIW1 is located in the cytoplasm and nucleus. We found that it interacts with the nuclear transport receptor XPO1 and is exported by XPO1 from the nucleus. Mutation of XIW1 reduces the induction of ABA-responsive genes and the accumulation of ABA Insensitive 5 (ABI5), causing mutant plants with ABA-insensitive phenotypes during seed germination and seedling growth, and decreased drought stress resistance. ABA treatment upregulates the expression of XIW1, and both ABA and abiotic stresses promote XIW1 accumulation in the nucleus, where it interacts with ABI5. Loss of XIW1 function results in rapid proteasomal degradation of ABI5. Taken together, these findings suggest that XIW1 is a nucleocytoplasmic shuttling protein and plays a positive role in ABA responses by interacting with and maintaining the stability of ABI5 in the nucleus.

ERECTA receptor-kinases play a key role in the appropriate timing of seed germination under changing salinity

Appropriate timing of seed germination is crucial for the survival and propagation of plants, and for crop yield, especially in environments prone to salinity or drought. However, the exact mechanisms by which seeds perceive changes in soil conditions and integrate them to trigger germination remain elusive, especially once the seeds are non-dormant. In this study, we determined that the Arabidopsis ERECTA (ER), ERECTA-LIKE1 (ERL1), and ERECTA-LIKE2 (ERL2) leucine-rich-repeat receptor-like kinases regulate seed germination and its sensitivity to changes in salt and osmotic stress levels. Loss of ER alone, or in combination with ERL1 and/or ERL2, slows down the initiation of germination and its progression to completion, or arrests it altogether under saline conditions, until better conditions return. This function is maternally controlled via the tissues surrounding the embryo, with a primary role being played by the properties of the seed coat and its mucilage. These relate to both seed-coat expansion and subsequent differentiation and to salinity-dependent interactions between the mucilage, subtending seed coat layers and seed interior in the germinating seed. Salt-hypersensitive er105, er105 erl1.2, er105 erl2.1 and triple-mutant seeds also exhibit increased sensitivity to exogenous ABA during germination, and under salinity show an enhanced up-regulation of the germination repressors and inducers of dormancy ABA-insensitive-3, ABA-insensitive-5, DELLA-encoding RGL2, and Delay-Of-Germination-1. These findings reveal a novel role of the ERECTA receptor-kinases in the sensing of conditions at the seed surface and the integration of developmental, dormancy and stress signalling pathways in seeds. They also open novel avenues for the genetic improvement of plant adaptation to changing drought and salinity patterns.

GhEIN3, a cotton (Gossypium hirsutum) homologue of AtEIN3, is involved in regulation of plant salinity tolerance

Ethylene insensitive 3 (EIN3), a key transcription factor in ethylene signal transduction, play important roles in plant stress signaling pathways. In this study, we isolated and characterized an EIN3-like gene from cotton (Gossypium hirsutum), designated as GhEIN3. GhEIN3 is highly expressed in vegetative tissues, and its expression is induced by 1-aminocyclopropane-1-carboxylic acid (ACC) and NaCl. Ectopic expression of GhEIN3 in Arabidopsis elevated plants' response to ethylene, which exhibit smaller leaves, more root hairs, shorter roots and hypocotyls. The germination rate, survival rate and root length of GhEIN3 transgenic plants were significantly improved compared to wild type under salt stress. GhEIN3 transgenic plants accumulated less H₂O₂ and malondialdehyde (MDA), while higher superoxide dismutase (SOD) and peroxidase (POD) activities were detected under salt stress. In addition, expression of several genes related to reactive oxygen species (ROS) pathway and ABA signaling pathway was increased in the GhEIN3 transgenic plants under salt stress. In contrast, virus-induced gene silencing (VIGS) of GhEIN3 in cotton enhanced the sensitivity of transgenic plants to salt stress, accumulating higher H₂O₂ and MDA and lower SOD and POD activities compared to control plants. Collectively, our results revealed that GhEIN3 might be involved in the regulation of plant response to salt stress by regulating ABA and ROS pathway during plant growth and development.

Over-expression of an R2R3 MYB Gene, GhMYB73, increases tolerance to salt stress in transgenic Arabidopsis

MYB family genes act as important regulators modulating the response to abiotic stress in plants. However, much less is known about MYB proteins in cotton. Here, we found that a cotton MYB gene, GhMYB73, was induced by NaCl and abscisic acid (ABA). Silencing GhMYB73 expression in cotton increased sensitivity to salt stress. The cotyledon greening rate of Arabidopsis thaliana over-expressing GhMYB73 under NaCl or mannitol treatment was significantly enhanced during the seedling germination stage. What's more, several osmotic stress-induced genes, such as AtNHX1, AtSOS3 and AtP5CS1, were more highly induced in the over-expression lines than in wild type under salt treatment, supporting the hypothesis that GhMYB73 contributes to salinity tolerance by improving osmotic stress resistance. Arabidopsis lines over-expressing GhMYB73 had superior germination and cotyledon greening under ABA treatment, and some abiotic stress-induced genes involved in ABA pathways (AtPYL8, AtABF3, AtRD29B and AtABI5), had increased transcription levels under salt-stress conditions in these lines. Furthermore, we found that GhMYB73 physically interacts with GhPYL8 and AtPYL8, suggesting that GhMYB73 regulates ABA signaling during salinity stress response. Taken together, over-expression of GhMYB73 significantly increases tolerance to salt and ABA stress, indicating that it can potentially be used in transgenic technology approaches to improve cotton salt tolerance.

The hydrogen sulfide signal enhances seed germination tolerance to high temperatures by retaining nuclear COP1 for HY5 degradation

Seed germination is a critical stage during the initiation of the plant lifecycle and is strongly affected by endogenous phytohormones and environmental stress. High temperature (HT) upregulates endogenous abscisic acid (ABA) to suppress seed germination, and ABA-INSENSITIVE 5 (ABI5) is the key positive regulator in the ABA signal-mediated modulation of seed germination. In plants, hydrogen sulfide (H₂S) is a small gas messenger that participates in multiple physiological processes, but its role in seed germination thermotolerance has not been thoroughly elucidated to date. In this study, we found that H₂S enhanced the seed germination rate under HT. Moreover, HT accelerates the efflux of the E3 ligase CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1) from the nucleus to the cytoplasm, which results in increased nuclear accumulation of ELONG HYPCOTYL 5 (HY5) to activate the expression of ABI5 and thereby suppress seed germination. However, the H₂S signal reversed the HT effect, as characterized by increased COP1 in the nucleus, which resulted in increased degradation of HY5 and reduced expression of ABI5 and thereby enhanced the seed germination thermotolerance. Thus, our findings reveal a novel role for the H₂S signal in the modulation of seed germination thermotolerance through the nucleocytoplasmic partitioning of COP1 and the downstream HY5 and ABI5 pathways.

Mulberry EIL3 confers salt and drought tolerances and modulates ethylene biosynthetic gene expression

Ethylene regulates plant abiotic stress responses and tolerances, and ethylene-insensitive3 (EIN3)/EIN3-like (EIL) proteins are the key components of ethylene signal transduction. Although the functions of EIN3/EIL proteins in response to abiotic stresses have been investigated in model plants, little is known in non-model plants, including mulberry (Morus L.), which is an economically important perennial woody plant. We functionally characterized a gene encoding an EIN3-like protein from mulberry, designated as MnEIL3. A quantitative real-time PCR analysis demonstrated that the expression of MnEIL3 could be induced in roots and shoot by salt and drought stresses. Arabidopsis overexpressing MnEIL3 exhibited an enhanced tolerance to salt and drought stresses. MnEIL3 overexpression in Arabidopsis significantly upregulated the transcript abundances of ethylene biosynthetic genes. Furthermore, MnEIL3 enhanced the activities of the MnACO1 and MnACS1 promoters, which respond to salt and drought stresses. Thus, MnEIL3 may play important roles in tolerance to abiotic stresses and the expression of ethylene biosynthetic genes.

Beyond Light: Insights Into the Role of Constitutively Photomorphogenic1 in Plant Hormonal Signaling

Light is an important environmental factor with profound effects in plant growth and development. Constitutively photomorphogenic1 (COP1) is a vital component of the light signaling pathway as a negative regulator of photomorphogenesis. Although the role of COP1 in light signaling has been firmly established for some time, recent studies have proven that COP1 is also a crucial part of multiple plant hormonal regulatory pathways. In this article, we review the available evidence involving COP1 in hormone signaling, its molecular mechanisms, and its contribution to the complicated regulatory network linking light and plant hormone signaling.

The heterologous expression of CmBBX22 delays leaf senescence and improves drought tolerance in Arabidopsis

CmBBX22, a transcription factor of chrysanthemum, was verified to confer drought tolerance in Arabidopsis thaliana. The BBX proteins are known to operate as regulators of plant growth and development, but as yet their contribution to the abiotic stress response has not been well defined. Here, the chrysanthemum BBX family member CmBBX22, an ortholog of AtBBX22, was found to be transcribed throughout the plant, although at varying intensity, and was induced by imposing moisture deficiency via exposure to polyethylene glycol. The heterologous, constitutive expression of this gene in Arabidopsis thaliana compromised germination and seedling growth, but enhanced the plants' ability to tolerate drought stress. In transgenic plants challenged with abscisic acid, leaf senescence was delayed and the senescence-associated genes and chlorophyll catabolic genes SAG29, NYE1, NYE2 and NYC1 were down-regulated. We speculated that CmBBX22 may serves as a regulator in mediating drought stress tolerance and delaying leaf senescence.

Modulation of Ethylene and Ascorbic Acid on Reactive Oxygen Species Scavenging in Plant Salt Response

Salt stress causes retarded plant growth and reduced crop yield. A complicated regulation network to response to salt stress has been evolved in plants under high salinity conditions. Ethylene is one of the most important phytohormones, playing a major role in salt stress response. An increasing number of studies have demonstrated that ethylene modulates salt tolerance through reactive oxygen species (ROS) homeostasis. Ascorbic acid (AsA) is a non-enzymatic antioxidant, contributing to ROS-scavenging and salt tolerance. Here, we mainly focus on the advances in understanding the modulation of ethylene and AsA on ROS-scavenging under salinity stress. We also review the regulators involved in the ethylene signaling pathway and AsA biosynthesis that respond to salt stress. Moreover, the AsA pool is affected by many environmental conditions, and the potential role of ethylene in AsA production is also extensively discussed. Novel insights into the roles and mechanisms of ethylene in AsA-mediated ROS homeostasis will provide critical information for improving crop salt tolerance.

RSM1, an Arabidopsis MYB protein, interacts with HY5/HYH to modulate seed germination and seedling development in response to abscisic acid and salinity

MYB transcription factors are involved in many biological processes, including metabolism, development and responses to biotic and abiotic stresses. RADIALIS-LIKE SANT/MYB 1 (RSM1) belongs to a MYB-related subfamily, and previous transcriptome analysis suggests that RSM1 may play roles in plant development, stress responses and plant hormone signaling. However, the molecular mechanisms of RSM1 action in response to abiotic stresses remain obscure. We show that down-regulation or up-regulation of RSM1 expression alters the sensitivity of seed germination and cotyledon greening to abscisic acid (ABA), NaCl and mannitol in Arabidopsis. The expression of RSM1 is dynamically regulated by ABA and NaCl. Transcription factors ELONGATED HYPOCOTYL 5 (HY5) and HY5 HOMOLOG (HYH) regulate RSM1 expression via binding to the RSM1 promoter. Genetic analyses reveal that RSM1 mediates multiple functions of HY5 in responses of seed germination, post-germination development to ABA and abiotic stresses, and seedling tolerance to salinity. Pull-down and BiFC assays show that RSM1 interacts with HY5/HYH in vitro and in vivo. RSM1 and HY5/HYH may function as a regulatory module in responses to ABA and abiotic stresses. RSM1 binds to the promoter of ABA INSENSITIVE 5 (ABI5), thereby regulating its expression, while RSM1 interaction also stimulates HY5 binding to the ABI5 promoter. However, no evidence was found in the dual-luciferase transient expression assay to support that RSM enhances the activation of ABI5 expression by HY. In summary, HY5/HYH and RSM1 may converge on the ABI5 promoter and independently or somehow dependently regulate ABI5 expression and ABI5-downstream ABA and abiotic stress-responsive genes, thereby improving the adaption of plants to the environment.

The phytohormone abscisic acid (ABA) regulates multiple developmental processes in plants, including seed dormancy and germination, growth, and abiotic stress responses. The transcription factor ELONGATED HYPOCOTYL 5 (HY5), a core regulator of light signaling, is involved in ABA and abiotic stress responses by directly regulating the expression of ABA INSENSITIVE 5 (ABI5). In this study, we show that a MYB-related transcription factor, RADIALIS-LIKE SANT/MYB 1 (RSM1), plays important roles in ABA and salinity signaling in Arabidopsis, and we dissect the relationship between RSM1 and HY5. RSM1 interacts with HY5/HYH in vitro and in vivo and they may function as a regulatory module in responses to ABA and abiotic stresses. RSM1 binds to the promoter of ABI5, thereby regulating its expression; moreover, RSM1 interaction stimulates HY5 binding to the ABI5 promoter, but RSM1 was not found to enhance the activation of ABI5 expression by HY5. Genetic analyses reveal that RSM1 mediates the functions of HY5 in responses of seed germination, post-germination developmental responses to ABA and abiotic stresses, and seedling tolerance to salinity. In summary, our work demonstrates that HY5/HYH and RSM1 may bind with the ABI5 promoter to regulate ABI5 expression independently or somehow dependently, thereby controlling the expression of ABI5-downstream target genes in ABA and salinity signaling.

Phytochrome A and B Negatively Regulate Salt Stress Tolerance of Nicotiana tobacum via ABA-Jasmonic Acid Synergistic Cross-Talk

Light signaling and phytohormones play important roles in plant growth, development, and biotic and abiotic stress responses. However, the roles of phytochromes and cross-talk between these two signaling pathways in response to salt stress in tobacco plants remain underexplored. Here, we explored the defense response in phytochrome-defective mutants under salt stress. We monitored the physiological and molecular changes of these mutants under salt stress conditions. The results showed that phytochrome A (phyA), phytochrome B (phyB) and phyAphyB (phyAB) mutants exhibited improved salt stress tolerance compared with wild-type (WT) plants. The mutant plants had a lower electrolyte leakage (EL) and malondialdehyde (MDA) concentration than WT plants, and the effect was clearly synergistic in the phyAB double mutant plants. Furthermore, the data showed that the transcript levels of defense-associated genes and the activities of some antioxidant enzymes in the mutant plants were much higher than those in WT plants. Additionally, the results indicated that phytochrome signaling strongly modulates the expression of endogenous abscisic acid (ABA) and jasmonic acid (JA) of Nicotiana tobacum in response to salt stress. To illustrate further the relationship between phytochrome and phytohormone, we measured the expression of defense genes and phytochrome. The results displayed that salt stress and application of methyl jasmonate (MeJA) or ABA up-regulated the transcript levels of salt response-associated genes and inhibited the expression of NtphyA and NtphyB. Foliar application of inhibitors of ABA and JA further confirmed that JA co-operated with ABA in phytochrome-mediated salt stress tolerance.

Isolation and identification of Ammodendron bifolium endophytic bacteria and the action mechanism of selected isolates-induced seed germination and their effects on host osmotic-stress tolerance

This study aimed to identify Ammodendron bifolium endophytic bacteria, and to evaluate promoting mechanism of selected isolates on seed germination and their effects on host osmotic-stress tolerance. Forty-five strains were isolated from A. bifolium and were classified into 13 different genera by 16S rDNA gene sequence analysis. AY3, AY9 and AG18, which were identified as Staphylococcus, Kocuria, Bacillus sp., promoted host seed ethylene release during germination. Ethrel and 1-aminocyclopropane-1-carboxylic acid (ACC) imitated the effect of AY3, AY9 and AG18 on seed germination. The data suggest that ethylene mediates AY3-, AY9-, AG18-induced A. bifolium seed germination. In addition, osmotic stress prevented seed germination and radicle elongation. However, the inhibitory effect of osmotic stress on seed germination and radicle elongation were rescued by AY3, AY9 and AG18. The results show that AY3, AY9 and AG18 increased osmotic-stress tolerance in A. bifolium. AY3, AY9, AG18 induced A. bifolium seed germination through promoting ethylene production during endophytic bacteria-plant interaction, and increase osmotic-stress tolerance in A. bifolium. AY3, AY9 and AG18 are potential candidates for the protection of A. bifolium.

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.

Role of Arabidopsis ABF1/3/4 during det1 germination in salt and osmotic stress conditions

Arabidopsis det1 mutants exhibit salt and osmotic stress resistant germination. This phenotype requires HY5, ABF1, ABF3, and ABF4. While DE-ETIOLATED 1 (DET1) is well known as a negative regulator of light development, here we describe how det1 mutants also exhibit altered responses to salt and osmotic stress, specifically salt and mannitol resistant germination. LONG HYPOCOTYL 5 (HY5) positively regulates both light and abscisic acid (ABA) signalling. We found that hy5 suppressed the det1 salt and mannitol resistant germination phenotype, thus, det1 stress resistant germination requires HY5. We then queried publically available microarray datasets to identify genes downstream of HY5 that were differentially expressed in det1 mutants. Our analysis revealed that ABA regulated genes, including ABA RESPONSIVE ELEMENT BINDING FACTOR 3 (ABF3), are downregulated in det1 seedlings. We found that ABF3 is induced by salt in wildtype seeds, while homologues ABF4 and ABF1 are repressed, and all three genes are underexpressed in det1 seeds. We then investigated the role of ABF3, ABF4, and ABF1 in det1 phenotypes. Double mutant analysis showed that abf3, abf4, and abf1 all suppress the det1 salt/osmotic stress resistant germination phenotype. In addition, abf1 suppressed det1 rapid water loss and open stomata phenotypes. Thus interactions between ABF genes contribute to det1 salt/osmotic stress response phenotypes.

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.

FIN219/JAR1 and cryptochrome1 antagonize each other to modulate photomorphogenesis under blue light in Arabidopsis

Plant development is affected by the integration of light and phytohormones, including jasmonates (JAs). To address the molecular mechanisms of possible interactions between blue light and JA signaling in Arabidopsis thaliana, we used molecular and transgenic approaches to understand the regulatory relationships between FAR-RED INSENSITIVE 219 (FIN219)/JASMONATE RESISTANT1 (JAR1) and the blue-light photoreceptor cryptochrome1 (CRY1). FIN219 overexpression in the wild type resulted in a short-hypocotyl phenotype under blue light. However, FIN219 overexpression in cry1, cry2 and cry1cry2 double mutant backgrounds resulted in phenotypes similar to their respective mutant backgrounds, which suggests that FIN219 function may require blue light photoreceptors. Intriguingly, FIN219 overexpression in transgenic plants harboring ectopic expression of the C terminus of CRY1 (GUS-CCT1), which exhibits a hypersensitive short-hypocotyl phenotype in all light conditions including darkness, led to a rescued phenotype under all light conditions except red light. Further expression studies showed mutual suppression between FIN219 and CRY1 under blue light. Strikingly, FIN219 overexpression in GUS-CCT1 transgenic lines (FIN219-OE/GUS-CCT1) abolished GUS-CCT1 fusion protein under blue light, whereas GUS-CCT1 fusion protein was stable in the fin219-2 mutant background (fin219-2/GUS-CCT1). Moreover, FIN219 strongly interacted with COP1 under blue light, and methyl JA (MeJA) treatment enhanced the interaction between FIN219 and GUS-CCT1 under blue light. Furthermore, FIN219 level affected GUS-CCT1 seedling responses such as anthocyanin accumulation and bacterial resistance under various light conditions and MeJA treatment. Thus, FIN219/JAR1 and CRY1 antagonize each other to modulate photomorphogenic development of seedlings and stress responses in Arabidopsis.

COP1 mediates dark-specific degradation of microtubule-associated protein WDL3 in regulating Arabidopsis hypocotyl elongation

CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) is an E3 ubiquitin ligase acting as a central repressor of seedling photomorphogenesis in plants. Many nuclear-localized COP1 substrates have been identified in the last two decades; however, whether COP1 targets cytoplasmic factors for ubiquitination and degradation remains largely unknown. In this study, we show that COP1 interacts with a microtubule-associated protein, WAVE-DAMPENED 2-LIKE 3 (WDL3), in a dark-dependent manner at cortical microtubules. Thus, COP1 targets WDL3 for 26S proteasome-mediated degradation to control hypocotyl elongation in etiolated Arabidopsis seedlings. Collectively, our study uncovers a cytoplasmic substrate of COP1 that functions as a microtubule-associated protein in mediating hypocotyl cell elongation.

CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a well-known E3 ubiquitin ligase, functions as a central regulator of plant growth and photomorphogenic development in plants, including hypocotyl elongation. It has been well-established that, in darkness, COP1 targets many photomorphogenesis-promoting factors for ubiquitination and degradation in the nucleus. However, increasing evidence has shown that a proportion of COP1 is also localized outside the nucleus in dark-grown seedlings, but the physiological function of this localization remains largely unclear. In this study, we demonstrate that COP1 directly targets and mediates the degradation of WAVE-DAMPENED 2-LIKE 3 (WDL3) protein, a member of the microtubule-associated protein (MAP) WVD2/WDL family involved in regulating hypocotyl cell elongation of Arabidopsis seedlings. We show that COP1 interacts with WDL3 in vivo in a dark-dependent manner at cortical microtubules. Moreover, our data indicate that COP1 directly ubiquitinates WDL3 in vitro and that WDL3 protein is degraded in WT seedlings but is abundant in the cop1 mutant in the dark. Consistently, introduction of the wdl3 mutation weakened, whereas overexpression of WDL3 enhanced, the short-hypocotyl phenotype of cop1 mutant in darkness. Together, this study reveals a function of COP1 in regulating the protein turnover of a cytosol-localized MAP in etiolated hypocotyls, thus providing insights into COP1-mediated degradation of downstream factors to control seedling photomorphogenesis.

COP1 conveys warm temperature information to hypocotyl thermomorphogenesis

Plants adjust their architecture to optimize growth and reproductive success under changing climates. Hypocotyl elongation is a pivotal morphogenic trait that is profoundly influenced by light and temperature conditions. While hypocotyl photomorphogenesis has been well characterized at the molecular level, molecular mechanisms underlying hypocotyl thermomorphogenesis remains elusive. Here, we demonstrate that the E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) conveys warm temperature signals to hypocotyl thermomorphogenesis. To investigate the roles of COP1 and its target ELONGATED HYPOCOTYL 5 (HY5) during hypocotyl thermomorphogenesis, we employed Arabidopsis mutants that are defective in their genes. Transgenic plants overexpressing the genes were also produced. We examined hypocotyl growth and thermoresponsive turnover rate of HY5 protein at warm temperatures under both light and dark conditions. Elevated temperatures trigger the nuclear import of COP1, thereby alleviating the suppression of hypocotyl growth by HY5. While the thermal induction of hypocotyl growth is circadian-gated, the degradation of HY5 by COP1 is uncoupled from light responses and timing information. We propose that thermal activation of COP1 enables coincidence between warm temperature signaling and circadian rhythms, which allows plants to gate hypocotyl thermomorphogenesis at the most profitable time at warm temperatures.

The activities of the E3 ubiquitin ligase COP1/SPA, a key repressor in light signaling

Light is a critical signal to integrate plant growth and development with the environment. Downstream of photoreceptors, the E3 ubiquitin ligase COP1/SPA is a key repressor of photomorphogenesis which targets many positive regulators of light signaling, mainly transcription factors, for degradation in darkness. In light-grown plants COP1/SPA activity is repressed, allowing light responses to occur. This review provides an overview on our current knowledge on COP1/SPA repressor function, focusing in particular on the roles of the respective protein domains and the mechanisms of light-induced inactivation of COP1/SPA. Moreover, we summarize how COP1 activity is regulated by other interacting proteins, such as a SUMO E3 ligase and Phytochrome-Interacting Factors (PIFs), as well as by hormones. At last, several novel functions of COP1 that were recently revealed are included.

Transcriptome analysis of bagging-treated red Chinese sand pear peels reveals light-responsive pathway functions in anthocyanin accumulation

Bagging is an efficient method to improve fruit colour development. This work reported a transcriptome analysis using bagging-treated red Chinese sand pear peels. In total, 8,870 differentially expressed genes were further analysed by a weighted gene co-expression network analysis and early-, middle- and late light-responsive genes were identified. An annotation analysis revealed several pathways involved in the different responsive stages. The presence of LONG HYPOCOTLY 5, CRY-DASH and a CONSTANS-like transcription factors among the early light-responsive genes indicated the pivotal role of light, especially blue light, in the biological changes that occurred after bag removal. Other light-responsive transcription factors were also identified from the three light-responsive stages. In addition, the light-responsive pattern of anthocyanin biosynthetic genes differed among the biosynthetic steps. Although yeast-one hybrid assay showed that most of the structural genes were regulated by PpMYB10, their different temporal expressive pattern suggested that besides PpMYB10, other light-responsive transcriptional factors were also involved in the regulation of anthocyanin biosynthesis. In summary, our transcriptome analysis provides knowledge of the transcriptional regulatory network operating during light responses, which results in anthocyanin accumulation and other significant physiological changes in red Chinese sand pear peels after bag removal.

Interplay between Light and Plant Hormones in the Control of Arabidopsis Seedling Chlorophyll Biosynthesis

Chlorophyll biosynthesis is one of the most important cellular processes and is essential for plant photosynthesis. After germination under the soil, dark-grown seedlings are etiolated and accumulate the chlorophyll precursor protochlorophyllide (Pchlide) in cotyledons. Upon exposure to light, Pchlide is rapidly converted to chlorophyll to initiate photoautotrophic growth. In this light-regulated de-etiolation process, multiple endogenous phytohormones are also involved. Although the co-regulation of seedling greening by light and hormones has long been observed, recent studies greatly advanced our understanding of their interplay by identifying the key components connecting these pathways. The integrators, such as PHYTOCHROME-INTERACTING FACTORs, ELONGATED HYPOCOTYL 5, ETHYLENE INSENSTIVE 3 and DELLA proteins, are key transcription regulators in light or hormone signaling pathways. This review focuses on these integrators and illustrates the regulatory networks of light and hormone interactions in chlorophyll biosynthesis.

Integration of Ethylene and Light Signaling Affects Hypocotyl Growth in Arabidopsis

As an ideal model for studying ethylene effects on cell elongation, Arabidopsis hypocotyl growth is widely used due to the unique characteristic that ethylene stimulates hypocotyl elongation in the light but inhibits it in the dark. Although the contrasting effect of ethylene on hypocotyl growth has long been known, the molecular basis of this effect has only gradually been identified in recent years. In the light, ethylene promotes the expression of PHYTOCHROME INTERACTING FACTOR 3 (PIF3) and the degradation of ELONGATED HYPOCOTYL 5 (HY5) protein, thus stimulating hypocotyl growth. In the dark, ETHYLENE RESPONSE FACTOR 1 (ERF1) and WAVE-DAMPENED 5 (WDL5) induced by ethylene are responsible for its inhibitory effect on hypocotyl elongation. Moreover, CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and PHYTOCHROME B (phyB) mediate the light-suppressed ethylene response in different ways. Here, we review several pivotal advances associated with ethylene-regulated hypocotyl elongation, focusing on the integration of ethylene and light signaling during seedling emergence from the soil.

Arabidopsis DDB1-CUL4 E3 ligase complexes in det1 salt/osmotic stress resistant germination

A key regulatory mechanism in plant growth, development, and stress signaling utilizes E3 ubiquitin ligases, which target a variety of substrates for degradation. DE-ETIOLATED 1 (DET1) forms a complex with DDB1 (DAMAGED DNA BINDING protein 1) and CUL4 (CULLIN 4), and negatively regulates light signaling. Another DDB1-CUL4 complex containing DWA1 and DWA2 (DWD hypersensitive to ABA 1 and 2) has been shown to negatively regulate abscisic acid (ABA) signaling. Since distinct DDB1-CUL4 complexes have been shown to influence each other, we analyzed genetic interactions between DET1 and components of DDB1-CUL4 complexes during seed germination under salt and osmotic stress conditions. det1 germination was resistant to salt and osmotic stress and dwa1 and dwa2 enhanced this phenotype. In contrast, ddb1a partially suppressed the det1 germination phenotype on both salt and mannitol, while ddb1b had no effect. Mutations in DDB2, a DDB1-CUL4 complex component involved in DNA repair, also partially suppressed the det1 germination phenotype while mutants in COP1, another light signaling component, completely suppressed the det1 resistant germination phenotypes. Taken together these data suggest that components of E3 ubiquitin ligase complexes have variable but significant effects on det1 salt/osmotic stress responses.

COP1 Controls Abiotic Stress Responses by Modulating AtSIZ1 Function through Its E3 Ubiquitin Ligase Activity

Ubiquitination and sumoylation are essential post-translational modifications that regulate growth and development processes in plants, including control of hormone signaling mechanisms and responses to stress. This study showed that COP1 (Constitutive photomorphogenic 1) regulated the activity of Arabidopsis E3 SUMO (Small ubiquitin-related modifier) ligase AtSIZ1 through its E3 ubiquitin ligase activity. Yeast two hybrid analysis demonstrated that COP1 and AtSIZ1 directly interacted with one another, and subcellular localization assays indicated that COP1 and AtSIZ1 co-localized in nuclear bodies. Analysis of ubiquitination showed that AtSIZ1 was polyubiquitinated by COP1. The AtSIZ1 level was higher in cop1-4 mutants than in wild-type seedlings under light or dark conditions, and overexpression of a dominant-negative (DN)-COP1 mutant led to a substantial increase in AtSIZ1 accumulation. In addition, under drought, cold, and high salt conditions, SUMO-conjugate levels were elevated in DN-COP1-overexpressing plants and cop1-4 mutant plants compared to wild-type plants. Taken together, our results indicate that COP1 controls responses to abiotic stress by modulation of AtSIZ1 levels and activity.