OsMYC2, an essential factor for JA-inductive sakuranetin production in rice, interacts with MYC2-like proteins that enhance its transactivation ability

Phytoalexin sakuranetin attenuates endocytosis and enhances resistance to rice blast

Phytoalexin sakuranetin functions in resistance against rice blast. However, the mechanisms underlying the effects of sakuranetin remains elusive. Here, we report that rice lines expressing resistance (R) genes were found to contain high levels of sakuranetin, which correlates with attenuated endocytic trafficking of plasma membrane (PM) proteins. Exogenous and endogenous sakuranetin attenuates the endocytosis of various PM proteins and the fungal effector PWL2. Moreover, accumulation of the avirulence protein AvrCO39, resulting from uptake into rice cells by Magnaporthe oryzae, was reduced following treatment with sakuranetin. Pharmacological manipulation of clathrin-mediated endocytic (CME) suggests that this pathway is targeted by sakuranetin. Indeed, attenuation of CME by sakuranetin is sufficient to convey resistance against rice blast. Our data reveals a mechanism of rice against M. oryzae by increasing sakuranetin levels and repressing the CME of pathogen effectors, which is distinct from the action of many R genes that mainly function by modulating transcription.

Biofortified Rice Provides Rich Sakuranetin in Endosperm

Sakuranetin plays a key role as a phytoalexin in plant resistance to biotic and abiotic stresses, and possesses diverse health-promoting benefits. However, mature rice seeds do not contain detectable levels of sakuranetin. In the present study, a transgenic rice plant was developed in which the promoter of an endosperm-specific glutelin gene OsGluD-1 drives the expression of a specific enzyme naringenin 7-O-methyltransferase (NOMT) for sakuranetin biosynthesis. The presence of naringenin, which serves as the biosynthetic precursor of sakuranetin made this modification feasible in theory. Liquid chromatography tandem mass spectrometry (LC-MS/MS) validated that the seeds of transgenic rice accumulated remarkable sakuranetin at the mature stage, and higher at the filling stage. In addition, the panicle blast resistance of transgenic rice was significantly higher than that of the wild type. Specially, the matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) imaging was performed to detect the content and spatial distribution of sakuranetin and other nutritional metabolites in transgenic rice seeds. Notably, this genetic modification also did not change the nutritional and quality indicators such as soluble sugars, total amino acids, total flavonoids, amylose, total protein, and free amino acid content in rice. Meanwhile, the phenotypes of the transgenic plant during the whole growth and developmental periods and agricultural traits such as grain width, grain length, and 1000-grain weight exhibited no significant differences from the wild type. Collectively, the study provides a conceptual advance on cultivating sakuranetin-rich biofortified rice by metabolic engineering. This new breeding idea may not only enhance the disease resistance of cereal crop seeds but also improve the nutritional value of grains for human health benefits.

Jasmonic acid (JA)-mediating MYB transcription factor1, JMTF1, coordinates the balance between JA and auxin signalling in the rice defence response

The plant hormone jasmonic acid (JA) is a signalling compound involved in the regulation of cellular defence and development in plants. In this study, we investigated the roles of a JA-responsive MYB transcription factor, JMTF1, in the JA-regulated defence response against rice bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo). JMTF1 did not interact with any JASMONATE ZIM-domain (JAZ) proteins. Transgenic rice plants overexpressing JMTF1 showed a JA-hypersensitive phenotype and enhanced resistance against Xoo. JMTF1 upregulated the expression of a peroxidase, OsPrx26, and monoterpene synthase, OsTPS24, which are involved in the biosynthesis of lignin and antibacterial monoterpene, γ-terpinene, respectively. OsPrx26 was mainly expressed in the vascular bundle. Transgenic rice plants overexpressing OsPrx26 showed enhanced resistance against Xoo. In addition to the JA-hypersensitive phenotype, the JMTF1-overexpressing rice plants showed a typical auxin-related phenotype. The leaf divergence and shoot gravitropic responses were defective, and the number of lateral roots decreased significantly in the JMTF1-overexpressing rice plants. JMTF1 downregulated the expression of auxin-responsive genes but upregulated the expression of OsIAA13, a suppressor of auxin signalling. The rice gain-of-function mutant Osiaa13 showed high resistance against Xoo. Transgenic rice plants overexpressing OsEXPA4, a JMTF1-downregulated auxin-responsive gene, showed increased susceptibility to Xoo. JMTF1 is selectively bound to the promoter of OsPrx26 in vivo. These results suggest that JMTF1 positively regulates disease resistance against Xoo by coordinating crosstalk between JA- and auxin-signalling in rice.

The OsBZR1-OsSPX1/2 module fine-tunes the growth-immunity trade-off in adaptation to phosphate availability in rice

The growth-promoting hormones brassinosteroids (BRs) and their key signaling component BZR1 play a vital role in balancing normal growth and defense reactions. Here, we discovered that BRs and OsBZR1 upregulated sakuranetin accumulation and conferred basal defense against Magnaporthe oryzae infection under normal conditions. Resource shortages, including phosphate (Pi) deficiency, potentially disrupt this growth-defense balance. OsSPX1 and OsSPX2 have been reported to sense Pi concentration and interact with the Pi signal mediator OsPHR2, thus regulating Pi starvation responses. In this study, we discovered that OsSPX1/2 interacts with OsBZR1 in both Pi-sufficient and Pi-deficient conditions, inhibiting BR-responsive genes. When Pi is sufficient, OsSPX1/2 is captured by OsPHR2, enabling most of OsBZR1 to promote plant growth and maintain basal resistance. In response to Pi starvation, more OsSPX1/2 is released from OsPHR2 to inhibit OsBZR1 activity, resulting in slower growth. Collectively, our study reveals that the OsBZR1-SPX1/2 module balances the plant growth-immunity trade-off in response to Pi availability.

The phenylalanine ammonia-lyase inhibitor AIP induces rice defence against the root-knot nematode Meloidogyne graminicola

The phenylalanine ammonia-lyase (PAL) enzyme catalyses the conversion of l-phenylalanine to trans-cinnamic acid. This conversion is the first step in phenylpropanoid biosynthesis in plants. The phenylpropanoid pathway produces diverse plant metabolites that play essential roles in various processes, including structural support and defence. Previous studies have shown that mutation of the PAL genes enhances disease susceptibility. Here, we investigated the functions of the rice PAL genes using 2-aminoindan-2-phosphonic acid (AIP), a strong competitive inhibitor of PAL enzymes. We show that the application of AIP can significantly reduce the PAL activity of rice crude protein extracts in vitro. However, when AIP was applied to intact rice plants, it reduced infection of the root-knot nematode Meloidogyne graminicola. RNA-seq showed that AIP treatment resulted in a rapid but transient upregulation of defence-related genes in roots. Moreover, targeted metabolomics demonstrated higher levels of jasmonates and antimicrobial flavonoids and diterpenoids accumulating after AIP treatment. Furthermore, chemical inhibition of the jasmonate pathway abolished the effect of AIP on nematode infection. Our results show that disturbance of the phenylpropanoid pathway by the PAL inhibitor AIP induces defence in rice against M. graminicola by activating jasmonate-mediated defence.

Strigolactone deficiency induces jasmonate, sugar and flavonoid phytoalexin accumulation enhancing rice defense against the blast fungus Pyricularia oryzae

Strigolactones (SLs) are carotenoid-derived phytohormones that regulate plant growth and development. While root-secreted SLs are well-known to facilitate plant symbiosis with beneficial microbes, the role of SLs in plant interactions with pathogenic microbes remains largely unexplored. Using genetic and biochemical approaches, we demonstrate a negative role of SLs in rice (Oryza sativa) defense against the blast fungus Pyricularia oryzae (syn. Magnaporthe oryzae). We found that SL biosynthesis and perception mutants, and wild-type (WT) plants after chemical inhibition of SLs, were less susceptible to P. oryzae. Strigolactone deficiency also resulted in a higher accumulation of jasmonates, soluble sugars and flavonoid phytoalexins in rice leaves. Likewise, in response to P. oryzae infection, SL signaling was downregulated, while jasmonate and sugar content increased markedly. The jar1 mutant unable to synthesize jasmonoyl-l-isoleucine, and the coi1-18 RNAi line perturbed in jasmonate signaling, both accumulated lower levels of sugars. However, when WT seedlings were sprayed with glucose or sucrose, jasmonate accumulation increased, suggesting a reciprocal positive interplay between jasmonates and sugars. Finally, we showed that functional jasmonate signaling is necessary for SL deficiency to induce rice defense against P. oryzae. We conclude that a reduction in rice SL content reduces P. oryzae susceptibility by activating jasmonate and sugar signaling pathways, and flavonoid phytoalexin accumulation.

Chemical Structure Diversity and Extensive Biological Functions of Specialized Metabolites in Rice

Rice (Oryza sativa L.) is thought to have been domesticated many times independently in China and India, and many modern cultivars are available. All rice tissues are rich in specialized metabolites (SPMs). To date, a total of 181 terpenoids, 199 phenolics, 41 alkaloids, and 26 other types of compounds have been detected in rice. Some volatile sesquiterpenoids released by rice are known to attract the natural enemies of rice herbivores, and play an indirect role in defense. Momilactone, phytocassane, and oryzalic acid are the most common diterpenoids found in rice, and are found at all growth stages. Indolamides, including serotonin, tryptamine, and N-benzoylserotonin, are the main rice alkaloids. The SPMs mainly exhibit defense functions with direct roles in resisting herbivory and pathogenic infections. In addition, phenolics are also important in indirect defense, and enhance wax deposition in leaves and promote the lignification of stems. Meanwhile, rice SPMs also have allelopathic effects and are crucial in the regulation of the relationships between different plants or between plants and microorganisms. In this study, we reviewed the various structures and functions of rice SPMs. This paper will provide useful information and methodological resources to inform the improvement of rice resistance and the promotion of the rice industry.

Different viral effectors suppress hormone-mediated antiviral immunity of rice coordinated by OsNPR1

Salicylic acid (SA) and jasmonic acid (JA) are plant hormones that typically act antagonistically in dicotyledonous plants and SA and JA signaling is often manipulated by pathogens. However, in monocotyledonous plants, the detailed SA-JA interplay in response to pathogen invasion remains elusive. Here, we show that different types of viral pathogen can disrupt synergistic antiviral immunity mediated by SA and JA via OsNPR1 in the monocot rice. The P2 protein of rice stripe virus, a negative-stranded RNA virus in the genus Tenuivirus, promotes OsNPR1 degradation by enhancing the association of OsNPR1 and OsCUL3a. OsNPR1 activates JA signaling by disrupting the OsJAZ-OsMYC complex and boosting the transcriptional activation activity of OsMYC2 to cooperatively modulate rice antiviral immunity. Unrelated viral proteins from different rice viruses also interfere with the OsNPR1-mediated SA-JA interplay to facilitate viral pathogenicity, suggesting that this may be a more general strategy in monocot plants. Overall, our findings highlight that distinct viral proteins convergently obstruct JA-SA crosstalk to facilitate viral infection in monocot rice.

TcbHLH14 a Jasmonate Associated MYC2-like Transcription Factor Positively Regulates Pyrethrin Biosynthesis in Tanacetum cinerariifolium

Natural pyrethrins have high application value, and are widely used as a green pesticide in crop pest prevention and control. Pyrethrins are mainly extracted from the flower heads of Tanacetum cinerariifolium; however, the natural content is low. Therefore, it is essential to understand the regulatory mechanisms underlying the synthesis of pyrethrins through identification of key transcription factors. We identified a gene encoding a MYC2-like transcription factor named TcbHLH14 from T. cinerariifolium transcriptome, which is induced by methyl jasmonate. In the present study, we evaluated the regulatory effects and mechanisms of TcbHLH14 using expression analysis, a yeast one-hybrid assay, electrophoretic mobility shift assay, and overexpression/virus-induced gene silencing experiments. We found that TcbHLH14 can directly bind to the cis-elements of the pyrethrins synthesis genes TcAOC and TcGLIP to activate their expression. The transient overexpression of TcbHLH14 enhanced expression of the TcAOC and TcGLIP genes. Conversely, transient silencing of TcbHLH14 downregulated the expression of TcAOC and TcGLIP and reduced the content of pyrethrins. In summary, these results indicate that the potential application of TcbHLH14 in improving the germplasm resources and provide a new insight into the regulatory network of pyrethrins biosynthesis of T. cinerariifolium to further inform the development of engineering strategies for increasing pyrethrins contents.

Genome Editing Reveals Both the Crucial Role of OsCOI2 in Jasmonate Signaling and the Functional Diversity of COI1 Homologs in Rice

Jasmonic acid (JA) regulates plant growth, development and stress responses. Coronatine insensitive 1 (COI1) and jasmonate zinc-finger inflorescence meristem-domain (JAZ) proteins form a receptor complex for jasmonoyl-l-isoleucine, a biologically active form of JA. Three COIs (OsCOI1a, OsCOI1b and OsCOI2) are encoded in the rice genome. In the present study, we generated mutants for each rice COI gene using genome editing to reveal the physiological functions of the three rice COIs. The oscoi2 mutants, but not the oscoi1a and oscoi1b mutants, exhibited severely low fertility, indicating the crucial role of OsCOI2 in rice fertility. Transcriptomic analysis revealed that the transcriptional changes after methyl jasmonate (MeJA) treatment were moderate in the leaves of oscoi2 mutants compared to those in the wild type or oscoi1a and oscoi1b mutants. MeJA-induced chlorophyll degradation and accumulation of antimicrobial secondary metabolites were suppressed in oscoi2 mutants. These results indicate that OsCOI2 plays a central role in JA response in rice leaves. In contrast, the assessment of growth inhibition upon exogenous application of JA to seedlings of each mutant revealed that rice COIs are redundantly involved in shoot growth, whereas OsCOI2 plays a primary role in root growth. In addition, a co-immunoprecipitation assay showed that OsJAZ2 and OsJAZ5 containing divergent Jas motifs physically interacted only with OsCOI2, whereas OsJAZ4 with a canonical Jas motif interacts with all three rice COIs. The present study demonstrated the functional diversity of rice COIs, thereby providing clues to the mechanisms regulating the various physiological functions of JA.

ZmEREB92 interacts with ZmMYC2 to activate maize terpenoid phytoalexin biosynthesis upon Fusarium graminearum infection through jasmonic acid/ethylene signaling

Maize (Zea mays) terpenoid phytoalexins (MTPs) induced by multiple fungi display extensive antimicrobial activities, yet how maize precisely regulates MTP accumulation upon pathogen infection remains elusive. In this study, pretreatment with jasmonic acid (JA)/ethylene (ET)-related inhibitors significantly reduced Fusarium graminearum-induced MTP accumulation and resulted in enhanced susceptibility to F. graminearum, indicating the involvement of JA/ET in MTP regulatory network. ZmEREB92 positively regulated MTP biosynthetic gene (MBG) expression by correlation analysis. Knockout of ZmEREB92 significantly compromised maize resistance to F. graminearum with delayed induction of MBGs and attenuated MTP accumulation. The activation of ZmEREB92 on MBGs is dependent on the interaction with ZmMYC2, which directly binds to MBG promoters. ZmJAZ14 interacts both with ZmEREB92 and with ZmMYC2 in a competitive manner to negatively regulate MBG expression. Altogether, our findings illustrate the regulatory mechanism for JA/ET-mediated MTP accumulation upon F. graminearum infection with the involvement of ZmEREB92, ZmMYC2, and ZmJAZ14, which provides new insights into maize disease responses.

Jasmonic acid contributes to rice resistance against Magnaporthe oryzae

BACKGROUND

The annual yield losses caused by the Rice Blast Fungus, Magnaporthe oryzae, range to the equivalent for feeding 60 million people. To ward off infection by this fungus, rice has evolved a generic basal immunity (so called compatible interaction), which acts in concert with strain-specific defence (so-called incompatible interaction). The plant-defence hormone jasmonic acid (JA) promotes the resistance to M. oryzae, but the underlying mechanisms remain elusive. To get more insight into this open question, we employ the JA-deficient mutants, cpm2 and hebiba, and dissect the JA-dependent defence signalling in rice for both, compatible and incompatible interactions.

RESULTS

We observe that both JA-deficient mutants are more susceptible to M. oryzae as compared to their wild-type background, which holds true for both types of interactions as verified by cytological staining. Secondly, we observe that transcripts for JA biosynthesis (OsAOS2 and OsOPR7), JA signalling (OsJAZ8, OsJAZ9, OsJAZ11 and OsJAZ13), JA-dependent phytoalexin synthesis (OsNOMT), and JA-regulated defence-related genes, such as OsBBTI2 and OsPR1a, accumulate after fungal infection in a pattern that correlates with the amplitude of resistance. Thirdly, induction of defence transcripts is weaker during compatible interaction.

CONCLUSION

The study demonstrates the pivotal role of JA in basal immunity of rice in the resistance to M. oryzae in both, compatible and incompatible interactions.

GhMYC2 activates cytochrome P450 gene CYP71BE79 to regulate gossypol biosynthesis in cotton

GhMYC2 regulates the gossypol biosynthesis pathway in cotton through activation of the expression of gossypol synthesis gene CYP71BE79, CDNC, CYP706B1, DH1, and CYP82D113. Cotton is one of the main cash crops globally. Cottonseed contains fiber, fat, protein, and starch, and has important economic value. However, gossypol in cottonseed seriously affects the development and utilization of cottonseed. Nonetheless, gossypol has great application potential in agriculture, medicine, and industry. Therefore, it is very important to study gossypol biosynthesis and its upstream regulatory pathways. It has been reported that the content of gossypol in hairy roots of cotton is regulated through jasmonic acid signaling; however, the specific molecular mechanism has not been revealed yet. We found that the expression of basic helix-loop-helix family transcription factor GhMYC2 was significantly upregulated after exogenous administration of methyl jasmonate to cotton seedlings, and the content of gossypol changed significantly with the variation of GhMYC2 expression. Further studies revealed that GhMYC2 could specifically bind to the G-Box in the promoter region of CDNC, CYP706B1, DH1, CYP82D113, CYP71BE79 to activate its expression and regulate gossypol synthesis, and its activation of CYP71BE79 promoter was inhibited by GhJAZ2. Not only that GhMYC2 could also interact with GoPGF. In this work, the molecular mechanisms of gossypol biosynthesis regulated by GhMYC2 were analyzed. The results provide a theoretical basis for cultivating new varieties of low-gossypol or high-gossypol cotton and creating excellent germplasm resources.

The rice wound-inducible transcription factor RERJ1 sharing same signal transduction pathway with OsMYC2 is necessary for defense response to herbivory and bacterial blight

This study describes biological functions of the bHLH transcription factor RERJ1 involved in the jasmonate response and the related defense-associated metabolic pathways in rice, with particular focus on deciphering the regulatory mechanisms underlying stress-induced volatile emission and herbivory resistance. RERJ1 is rapidly and drastically induced by wounding and jasmonate treatment but its biological function remains unknown as yet. Here we provide evidence of the biological function of RERJ1 in plant defense, specifically in response to herbivory and pathogen attack, and offer insights into the RERJ1-mediated regulation of metabolic pathways of specialized defense compounds, such as monoterpene linalool, in possible collaboration with OsMYC2-a well-known master regulator in jasmonate signaling. In rice (Oryza sativa L.), the basic helix-loop-helix (bHLH) family transcription factor RERJ1 is induced under environmental stresses, such as wounding and drought, which are closely linked to jasmonate (JA) accumulation. Here, we investigated the biological function of RERJ1 in response to biotic stresses, such as herbivory and pathogen infection, using an RERJ1-defective mutant. Transcriptome analysis of the rerj1-Tos17 mutant revealed that RERJ1 regulated the expression of a typical family of conserved JA-responsive genes (e.g., terpene synthases, proteinase inhibitors, and jasmonate ZIM domain proteins). Upon exposure to armyworm attack, the rerj1-Tos17 mutant exhibited more severe damage than the wildtype, and significant weight gain of the larvae fed on the mutant was observed. Upon Xanthomonas oryzae infection, the rerj1-Tos17 mutant developed more severe symptoms than the wildtype. Among RERJ1-regulated terpene synthases, linalool synthase expression was markedly disrupted and linalool emission after wounding was significantly decreased in the rerj1-Tos17 mutant. RERJ1 appears to interact with OsMYC2-a master regulator of JA signaling-and many OsJAZ proteins, although no obvious epistatic interaction was detected between them at the transcriptional level. These results indicate that RERJ1 is involved in the transcriptional induction of JA-mediated stress-responsive genes via physical association with OsMYC2 and mediates defense against herbivory and bacterial infection through JA signaling.

Lack of antagonism between salicylic acid and jasmonate signalling pathways in poplar

Salicylic acid (SA) and jasmonic acid (JA) often play distinct roles in plant defence against pathogens. Research from Arabidopsis thaliana has established that SA- and JA-mediated defences are more effective against biotrophs and necrotrophs, respectively. These two hormones often interact antagonistically in response to particular attackers, with the induction of one leading to suppression of the other. Here, we report a contrasting pattern in the woody perennial Populus: positive SA-JA interplay. Using genetically engineered high SA lines of black poplar and wild-type lines after exogenous hormone application, we quantified SA and JA metabolites, signalling gene transcripts, antifungal flavonoids and resistance to rust (Melampsora larici-populina). Salicylic acid and JA metabolites were induced concurrently upon rust infection in poplar genotypes with varying resistance levels. Analysis of SA-hyperaccumulating transgenic poplar lines showed increased jasmonate levels, elevated flavonoid content and enhanced rust resistance, but no discernible reduction in growth. Exogenous application of either SA or JA triggered the accumulation of the other hormone. Expression of pathogenesis-related (PR) genes, frequently used as markers for SA signalling, was not correlated with SA content, but rather activated in proportion to pathogen infection. We conclude that SA and JA pathways interact positively in poplar resulting in the accumulation of flavonoid phytoalexins.

Combined Analysis of Pharmaceutical Active Ingredients and Transcriptomes of Glycyrrhiza uralensis Under PEG6000-Induced Drought Stress Revealed Glycyrrhizic Acid and Flavonoids Accumulation via JA-Mediated Signaling

Glycyrrhiza uralensis contains many secondary metabolites with a wide range of pharmacological activities. Drought stress acts as a positive regulator to stimulate the production of medicinal active component in G. uralensis, however, the underlying mechanism remains unclear. The aim of this work is to investigate the accumulation and regulatory mechanism of pharmaceutical active ingredients in G. uralensis under drought stress. The materials of the aerial and underground parts of G. uralensis seedlings treated by 10% PEG6000 for 0, 2, 6, 12, and 24 h were used for RNA sequencing and determination of phytohormones and pharmaceutical active ingredients. PEG6000, ibuprofen (IBU), and jasmonic acid (JA) were utilized to treat G. uralensis seedlings for content detection and gene expression analysis. The results showed that, the contents of glycyrrhizic acid, glycyrrhetinic acid, and flavonoids (licochalcone A, glabridin, liquiritigenin, isoliquiritigenin, and liquiritin) were significantly accumulated in G. uralensis underground parts under drought stress. Kyoto Encyclopedia of Genes and Genomes analysis of the transcriptome data of drought-treated G. uralensis indicated that up-regulated differentially expressed genes (UDEGs) involved in glycyrrhizic acid synthesis in the underground parts and flavonoids synthesis in both aerial and underground parts were significantly enriched. Interestingly, the UDEGs participating in jasmonic acid (JA) signal transduction in both aerial and underground parts were discovered. In addition, JA content in both aerial and underground parts under drought stress showed the most significantly accumulated. And drought stress stimulated the contents of JA, glycyrrhizic acid, and flavonoids, coupled with the induced expressions of genes regulating the synthesis and transduction pathway. Moreover, In PEG6000- and JA-treated G. uralensis, significant accumulations of glycyrrhizic acid and flavonoids, and induced expressions of corresponding genes in these pathways, were observed, while, these increases were significantly blocked by JA signaling inhibitor IBU. JA content and expression levels of genes related to JA biosynthesis and signal transduction were also significantly increased by PEG treatment. Our study concludes that drought stress might promote the accumulation of pharmaceutical active ingredients via JA-mediated signaling pathway, and lay a foundation for improving the medicinal component of G. uralensis through genetic engineering technology.

A class of independently evolved transcriptional repressors in plant RNA viruses facilitates viral infection and vector feeding

Plant viruses employ diverse virulence strategies to achieve successful infection, but there are few known general strategies of viral pathogenicity and transmission used by widely different plant viruses. Here, we report a class of independently evolved virulence factors in different plant RNA viruses which possess active transcriptional repressor activity. Rice viruses in the genera Fijivirus, Tenuivirus, and Cytorhabdovirus all have transcriptional repressors that interact in plants with the key components of jasmonic acid (JA) signaling, namely mediator subunit OsMED25, OsJAZ proteins, and OsMYC transcription factors. These transcriptional repressors can directly disassociate the OsMED25-OsMYC complex, inhibit the transcriptional activation of OsMYC, and then combine with OsJAZ proteins to cooperatively attenuate the JA pathway in a way that benefits viral infection. At the same time, these transcriptional repressors efficiently enhanced feeding by the virus insect vectors by repressing JA signaling. Our findings reveal a common strategy in unrelated plant viruses in which viral transcriptional repressors hijack and repress the JA pathway in favor of both viral pathogenicity and vector transmission.

Low Light/Darkness as Stressors of Multifactor-Induced Senescence in Rice Plants

Leaf senescence, as an integral part of the final development stage for plants, primarily remobilizes nutrients from the sources to the sinks in response to different stressors. The premature senescence of leaves is a critical challenge that causes significant economic losses in terms of crop yields. Although low light causes losses of up to 50% and affects rice yield and quality, its regulatory mechanisms remain poorly elucidated. Darkness-mediated premature leaf senescence is a well-studied stressor. It initiates the expression of senescence-associated genes (SAGs), which have been implicated in chlorophyll breakdown and degradation. The molecular and biochemical regulatory mechanisms of premature leaf senescence show significant levels of redundant biomass in complex pathways. Thus, clarifying the regulatory mechanisms of low-light/dark-induced senescence may be conducive to developing strategies for rice crop improvement. This review describes the recent molecular regulatory mechanisms associated with low-light response and dark-induced senescence (DIS), and their effects on plastid signaling and photosynthesis-mediated processes, chloroplast and protein degradation, as well as hormonal and transcriptional regulation in rice.

Structural and functional organization of the MYC transcriptional factors in Camellia sinensis

Genome-wide identification, expression analysis of the MYC family in Camellia sinensis, and potential functional characterization of CsMYC2.1 have laid a solid foundation for further research on CsMYC2.1 in jasmonate (JA)-mediated response. Myelocytomatosis (MYC) of basic helix-loop-helix (bHLH) plays a major role in JA-mediated plant growth and developmental processes through specifically binding to the G-box in the promoters of their target genes. In Camellia sinensis, studies on the MYC gene family are limited. Here, we identified 14 C. sinensis MYC (CsMYC) genes, and further analyzed the evolutionary relationship, gene structure, and motif pattern among them. The expression patterns of these CsMYC genes in different tissues suggested their important roles in diverse function in tea plant. Four MYC transcription factors with the highest homology to MYC2 in Arabidopsis were localized in the nucleus. Two of them, named CsMYC2.1 and CsMYC2.2, exhibited transcriptional self-activating activity, and, therefore, could significantly activate the promoter containing G-box motif, whereas CsJAM1.1 and CsJAM1.2 lack the transcriptional self-activating activity, indirectly mediating the JA pathway through interacting with CsMYC2.1 and CsMYC2.2. Furthermore, Yeast Two-Hybrid (Y2H) and Bimolecular Fluorescent Complimentary (BiFC) assays showed that CsMYC2.1 could interact with CsJAZ3/7/8 proteins. Genetically, the complementation of CsMYC2.1 in myc2 mutants conferred the ability to restore the sensitivity to JA signals. The results provide a comprehensive characterization of the 14 CsMYCs in C. sinensis, establishing a solid foundation for further research on CsMYCs in JA-mediated response.

Identification of methoxylchalcones produced in response to CuCl2 treatment and pathogen infection in barley

Changes in specialized metabolites were analyzed in barley (Hordeum vulgare) leaves treated with CuCl₂ solution as an elicitor. LC-MS analysis of the CuCl₂-treated leaves showed the induced accumulation of three compounds. Among them, two were purified by silica gel and ODS column chromatography and preparative HPLC and were identified as 2',3,4,4',6'-pentamethoxychalcone and 2'-hydroxy-3,4,4',6'-tetramethoxychalcone by spectroscopic analyses. The remaining compound was determined as 12-oxo-phytodienoic acid (OPDA), a major oxylipin in plants, by comparing its spectrum and retention time from LC-MS/MS analysis with those of the authentic compound. The accumulation of these compounds was reproduced in leaves inoculated with Bipolaris sorokiniana, the causal agent of spot blotch of the Poaceae species. This inoculation increased the amounts of other oxylipins, including jasmonic acid (JA), JA-Ile, 9-oxooctadeca-10,12-dienoic acid (9-KODE), and 13-oxooctadeca-9,11-dienoic acid (13-KODE). The treatments of the barley leaves with JA and OPDA induced the accumulation of methoxylchalcones, but treatment with 9-KODE did not. These methoxylchalcones inhibited conidial germination of B. sorokiniana and Fusarium graminearum, thereby indicating that these compounds possessed antifungal activity. Consequently, they are considered to be involved in the chemical defense processes as phytoalexins in barley. Accumulation of methoxylchalcones in response to JA treatment was observed in all seven barley cultivars tested, but was not detected in other wild Hordeum species, wheat, and rice, thus indicating that their production was specific to cultivated barley.

Apple BT2 protein negatively regulates jasmonic acid-triggered leaf senescence by modulating the stability of MYC2 and JAZ2

Jasmonic acid (JA) is shown to induce leaf senescence. However, the underlying molecular mechanism is not well understood, especially in woody plants such as fruit trees. In this study, we are interested in exploring the biological role of MdBT2 in JA-mediated leaf senescence. We found that MdBT2 played an antagonistic role in MdMYC2-promoted leaf senescence. Our results revealed that MdBT2 interacted with MdMYC2 and accelerated its ubiquitination degradation, thus negatively regulated MdMYC2-promoted leaf senescence. In addition, MdBT2 acted as a stabilizing factor to improve the stability of MdJAZ2 through direct interaction, thereby inhibited JA-mediated leaf senescence. Furthermore, our results also showed that MdBT2 interacted with a subset of JAZ proteins in apple, including MdJAZ1, MdJAZ3, MdJAZ4 and MdJAZ8. Our investigations provide new insight into molecular mechanisms of JA-modulated leaf senescence. The dynamic JA-MdBT2-MdJAZ2-MdMYC2 regulatory module plays an important role in JA-modulated leaf senescence.

Coexpression network analysis reveals an MYB transcriptional activator involved in capsaicinoid biosynthesis in hot peppers

Plant biosynthesis involves numerous specialized metabolites with diverse chemical natures and biological activities. The biosynthesis of metabolites often exclusively occurs in response to tissue-specific combinatorial developmental cues that are controlled at the transcriptional level. Capsaicinoids are a group of specialized metabolites that confer a pungent flavor to pepper fruits. Capsaicinoid biosynthesis occurs in the fruit placenta and combines its developmental cues. Although the capsaicinoid biosynthetic pathway has been largely characterized, the regulatory mechanisms that control capsaicinoid metabolism have not been fully elucidated. In this study, we combined fruit placenta transcriptome data with weighted gene coexpression network analysis (WGCNA) to generate coexpression networks. A capsaicinoid-related gene module was identified in which the MYB transcription factor CaMYB48 plays a critical role in regulating capsaicinoid in pepper. Capsaicinoid biosynthetic gene (CBG) and CaMYB48 expression primarily occurs in the placenta and is consistent with capsaicinoid biosynthesis. CaMYB48 encodes a nucleus-localized protein that primarily functions as a transcriptional activator through its C-terminal activation motif. CaMYB48 regulates capsaicinoid biosynthesis by directly regulating the expression of CBGs, including AT3a and KasIa. Taken together, the results of this study indicate ways to generate robust networks optimized for the mining of CBG-related regulators, establishing a foundation for future research elucidating capsaicinoid regulation.

Greenbug (Schizaphis graminum) herbivory significantly impacts protein and phosphorylation abundance in switchgrass (Panicum virgatum)

Switchgrass (Panicum virgatum L.) is an important crop for biofuel production but it also serves as host for greenbugs (Schizaphis graminum Rondani; GB). Although transcriptomic studies have been done to infer the molecular mechanisms of plant defense against GB, little is known about the effect of GB infestation on the switchgrass protein expression and phosphorylation regulation. The global response of the switchgrass cultivar Summer proteome and phosphoproteome was monitored by label-free proteomics shotgun in GB-infested and uninfested control plants at 10 days post infestation. Peptides matching a total of 3,594 proteins were identified and 429 were differentially expressed proteins in GB-infested plants relative to uninfested control plants. Among these, 291 and 138 were up and downregulated by GB infestation, respectively. Phosphoproteome analysis identified 310 differentially phosphorylated proteins (DP) from 350 phosphopeptides with a total of 399 phosphorylated sites. These phosphopeptides had more serine phosphorylated residues (79%), compared to threonine phosphorylated sites (21%). Overall, KEGG pathway analysis revealed that GB feeding led to the enriched accumulation of proteins important for biosynthesis of plant defense secondary metabolites and repressed the accumulation of proteins involved in photosynthesis. Interestingly, defense modulators such as terpene synthase, papain-like cysteine protease, serine carboxypeptidase, and lipoxygenase2 were upregulated at the proteome level, corroborating previously published transcriptomic data.

Overexpression of jasmonate-responsive OsbHLH034 in rice results in the induction of bacterial blight resistance via an increase in lignin biosynthesis

OsbHLH034 acts as a positive regulator in jasmonate signaling in rice. Jasmonic acid (JA) is a plant hormone under strict regulation by various transcription factors (TFs) that acts as a signaling compound in the regulation of plant defense responses and development. Here, we report that a basic helix-loop-helix (bHLH)-type TF, OsbHLH034, plays an important role in the JA-mediated resistance response against rice bacterial blight caused by Xanthomonas oryzae pv. oryzae. The expression of OsbHLH034 was upregulated at a late phase after JA treatment. OsbHLH034 interacted with a Jasmonate ZIM-domain (JAZ) protein, OsJAZ9, in both plant and yeast cells. Transgenic rice plants overexpressing OsbHLH034 exhibited a JA-hypersensitive phenotype and increased resistance against rice bacterial blight. Conversely, OsbHLH034-overexpressing plants exhibited high sensitivity to salt stress. The expression of some JA-responsive secretory-type peroxidase genes was upregulated in the OsbHLH034-overexpressing rice plants. Concomitantly, the lignin content significantly increased in these transgenic plants compared to that in the wild-type. These results indicate that OsbHLH034 acts as a positive regulator of the JA-mediated defense response in rice.

The bHLH gene family and its response to saline stress in Jilin ginseng, Panax ginseng C.A. Meyer

Basic helix-loop-helix (bHLH) gene family is a gene family of transcription factors that plays essential roles in plant growth and development, secondary metabolism and response to biotic and abiotic stresses. Therefore, a comprehensive knowledge of the bHLH gene family is paramount to understand the molecular mechanisms underlying these processes and develop advanced technologies to manipulate the processes efficiently. Ginseng, Panax ginseng C.A. Meyer, is a well-known medicinal herb; however, little is known  about the bHLH genes (PgbHLH) in the species. Here, we identified 137 PgbHLH genes from Jilin ginseng cultivar, Damaya, widely cultivated in Jilin, China, of which 50 are newly identified by pan-genome analysis. These 137 PgbHLH genes were phylogenetically classified into 26 subfamilies, suggesting their sequence diversification. They are alternatively spliced into 366 transcripts in a 4-year-old plant and involved in 11 functional subcategories of the gene ontology, indicating their functional differentiation in ginseng. The expressions of the PgbHLH genes dramatically vary spatio-temporally and across 42 genotypes, but they are still somehow functionally correlated. Moreover, the PgbHLH gene family, at least some of its genes, is shown to have roles in plant response to the abiotic stress of saline. These results provide a new insight into the evolution and functional differentiation of the bHLH gene family in plants, new bHLH genes to the PgbHLH gene family, and saline stress-responsive genes for genetic improvement in ginseng and other plant species.

Natural variation in the expression and catalytic activity of a naringenin 7-O-methyltransferase influences antifungal defenses in diverse rice cultivars

Phytoalexins play a pivotal role in plant-pathogen interactions. Whereas leaves of rice (Oryza sativa) cultivar Nipponbare predominantly accumulated the phytoalexin sakuranetin after jasmonic acid induction, only very low amounts accumulated in the Kasalath cultivar. Sakuranetin is synthesized from naringenin by naringenin 7-O-methyltransferase (NOMT). Analysis of chromosome segment substitution lines and backcrossed inbred lines suggested that NOMT is the underlying cause of differential phytoalexin accumulation between Nipponbare and Kasalath. Indeed, both NOMT expression and NOMT enzymatic activity are lower in Kasalath than in Nipponbare. We identified a proline to threonine substitution in Kasalath relative to Nipponbare NOMT as the main cause of the lower enzymatic activity. Expanding this analysis to rice cultivars with varying amounts of sakuranetin collected from around the world showed that NOMT induction is correlated with sakuranetin accumulation. In bioassays with Pyricularia oryzae, Gibberella fujikuroi, Bipolaris oryzae, Burkholderia glumae, Xanthomonas oryzae, Erwinia chrysanthemi, Pseudomonas syringae, and Acidovorax avenae, naringenin was more effective against bacterial pathogens and sakuranetin was more effective against fungal pathogens. Therefore, the relative amounts of naringenin and sakuranetin may provide protection against specific pathogen profiles in different rice-growing environments. In a dendrogram of NOMT genes, those from low-sakuranetin-accumulating cultivars formed at least two clusters, only one of which involves the proline to threonine mutation, suggesting that the low sakuranetin chemotype was acquired more than once in cultivated rice. Strains of the wild rice species Oryza rufipogon also exhibited differential sakuranetin accumulation, indicating that this metabolic diversity predates rice domestication.

ZmMYC2 exhibits diverse functions and enhances JA signaling in transgenic Arabidopsis

ZmMYC2 was identified as the key regulator of JA signaling in maize and exhibited diverse functions through binding to many gene promoters as well as enhanced JA signaling in transgenic Arabidopsis. The plant hormone jasmonate (JA) extensively coordinates plant growth, development and defensive responses. MYC2 is the master regulator of JA signaling and has been widely studied in many plant species. However, little is known about this transcription factor in maize. Here, we identified one maize transcription factor with amino acid identity of 47% to the well-studied Arabidopsis AtMYC2, named as ZmMYC2. Gene expression analysis demonstrated inducible expression patterns of ZmMYC2 in response to multiple plant hormone treatments, as well as biotic and abiotic stresses. The yeast two-hybrid assay indicated physical interaction among ZmMYC2 and JA signal repressors ZmJAZ14, ZmJAZ17, AtJAZ1 and AtJAZ9. ZmMYC2 overexpression in Arabidopsis myc2myc3myc4 restored the sensitivity to JA treatment, resulting in shorter root growth and inducible anthocyanin accumulation. Furthermore, overexpression of ZmMYC2 in Arabidopsis elevated resistance to Botrytis cinerea. Further ChIP-Seq analysis revealed diverse regulatory roles of ZmMYC2 in maize, especially in the signaling crosstalk between JA and auxin. Hence, we identified ZmMYC2 and characterized its roles in regulating JA-mediated growth, development and defense responses.

Adaptation of the metabolomics profile of rice after Pyricularia oryzae infection

Pyricularia oryzae (P. oryzae), one of the most devastating fungal pathogens, is the cause of blast disease in rice. Infection with a blast fungus induces biological responses in the host plant that lead to its survival through the termination or suppression of pathogen growth, and metabolite compounds play vital roles in plant interactions with a wide variety of other organisms. Numerous studies have indicated that rice has a multi-layered plant immune system that includes pre-developed (e.g., cell wall and phytoanticipins), constitutive and inducible (phytoalexins) defence barriers against stresses. Significant progress towards understanding the basis of the molecular mechanisms underlying the defence responses of rice to P. oryzae has been achieved. Nonetheless, even though the important metabolites in the responses of rice to pathogens have been identified, their exact mechanisms and their contributions to plant immunity against blast fungi have not been elucidated. The purpose of this review is to summarize and discuss recent advances towards the understanding of the integrated metabolite variations in rice after P. oryzae invasion.

Unraveling the Genetic Elements Involved in Shoot and Root Growth Regulation by Jasmonate in Rice Using a Genome-Wide Association Study

BACKGROUND

Due to their sessile life style, plant survival is dependent on the ability to build up fast and highly adapted responses to environmental stresses by modulating defense response and organ growth. The phytohormone jasmonate plays an essential role in regulating these plant responses to stress.

RESULTS

To assess variation of plant growth responses and identify genetic determinants associated to JA treatment, we conducted a genome-wide association study (GWAS) using an original panel of Vietnamese rice accessions. The phenotyping results showed a high natural genetic variability of the 155 tested rice accessions in response to JA for shoot and root growth. The level of growth inhibition by JA is different according to the rice varieties tested. We conducted genome-wide association study and identified 28 significant associations for root length (RTL), shoot length (SHL), root weight (RTW), shoot weight (SHW) and total weight (TTW) in response to JA treatment. Three common QTLs were found for RTL, RTW and SHL. Among a list of 560 candidate genes found to co-locate with the QTLs, a transcriptome analysis from public database for the JA response allows us to identify 232 regulated genes including several JA-responsive transcription factors known to play a role in stress response.

CONCLUSION

Our genome-wide association study shows that common and specific genetic elements are associated with inhibition of shoot and root growth under JA treatment suggesting the involvement of a complex JA-dependent genetic control of rice growth inhibition at the whole plant level. Besides, numerous candidate genes associated to stress and JA response are co-located with the association loci, providing useful information for future studies on genetics and breeding to optimize the growth-defense trade-off in rice.

The NAC family transcription factor GmNAC42-1 regulates biosynthesis of the anticancer and neuroprotective glyceollins in soybean

BACKGROUND

Glyceollins are isoflavonoid-derived pathogen-inducible defense metabolites (phytoalexins) from soybean (Glycine max L. Merr) that have important roles in providing defense against pathogens. They also have impressive anticancer and neuroprotective activities in mammals. Despite their potential usefulness as therapeutics, glyceollins are not economical to synthesize and are biosynthesized only transiently and in low amounts in response to specific stresses. Engineering the regulation of glyceollin biosynthesis may be a promising approach to enhance their bioproduction, yet the transcription factors (TFs) that regulate their biosynthesis have remained elusive. To address this, we first aimed to identify novel abiotic stresses that enhance or suppress the elicitation of glyceollins and then used a comparative transcriptomics approach to search for TF gene candidates that may positively regulate glyceollin biosynthesis.

RESULTS

Acidity stress (pH 3.0 medium) and dehydration exerted prolonged (week-long) inductive or suppressive effects on glyceollin biosynthesis, respectively. RNA-seq found that all known biosynthetic genes were oppositely regulated by acidity stress and dehydration, but known isoflavonoid TFs were not. Systemic acquired resistance (SAR) genes were highly enriched in the geneset. We chose to functionally characterize the NAC (NAM/ATAF1/2/CUC2)-family TF GmNAC42-1 that was annotated as an SAR gene and a homolog of the Arabidopsis thaliana (Arabidopsis) indole alkaloid phytoalexin regulator ANAC042. Overexpressing and silencing GmNAC42-1 in elicited soybean hairy roots dramatically enhanced and suppressed the amounts of glyceollin metabolites and biosynthesis gene mRNAs, respectively. Yet, overexpressing GmNAC42-1 in non-elicited hairy roots failed to stimulate the expressions of all biosynthesis genes. Thus, GmNAC42-1 was necessary but not sufficient to activate all biosynthesis genes on its own, suggesting an important role in the glyceollin gene regulatory network (GRN). The GmNAC42-1 protein directly bound the promoters of biosynthesis genes IFS2 and G4DT in the yeast one-hybrid (Y1H) system.

CONCLUSIONS

Acidity stress is a novel elicitor and dehydration is a suppressor of glyceollin biosynthesis. The TF gene GmNAC42-1 is an essential positive regulator of glyceollin biosynthesis. Overexpressing GmNAC42-1 in hairy roots can be used to increase glyceollin yields > 10-fold upon elicitation. Thus, manipulating the expressions of glyceollin TFs is an effective strategy for enhancing the bioproduction of glyceollins in soybean.

Transcriptomic profiling of the flower scent biosynthesis pathway of Cymbidium faberi Rolfe and functional characterization of its jasmonic acid carboxyl methyltransferase gene

Background

Cymbidium faberi, one of the most famous oriental orchids, has a distinct flower scent, which increases its economic value. However, the molecular mechanism of the flower scent biosynthesis was unclear prior to this study. Methyl jasmonate (MeJA) is one of the main volatile organic compounds (VOC) produced by the flowers of C. faberi. In this study, unigene 79,363 from comparative transcriptome analysis was selected for further investigation.

Results

A transcriptome comparison between blooming and withered flowers of C. faberi yielded a total of 9409 differentially expressed genes (DEGs), 558 of which were assigned to 258 pathways. The top ten pathways included α-linolenic acid metabolism, pyruvate metabolism and fatty acid degradation, which contributed to the conversion of α-linolenic acid to MeJA. One of the DEGs, jasmonic acid carboxyl methyltransferase (CfJMT, Unigene 79,363) was highly expressed in the blooming flower of C. faberi, but was barely detected in leaves and roots. Although the ectopic expression of CfJMT in tomato could not increase the MeJA content, the expression levels of endogenous MeJA biosynthesis genes were influenced, especially in the wound treatment, indicating that CfJMT may participate in the response to abiotic stresses.

Conclusion

This study provides a basis for elucidating the molecular mechanism of flower scent biosynthesis in C. faberi, which is beneficial for the genetically informed breeding of new cultivars of the economically valuable oriental orchids.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5501-z) contains supplementary material, which is available to authorized users.

Overexpression of OsNINJA1 negatively affects a part of OsMYC2-mediated abiotic and biotic responses in rice

The plant hormone jasmonic acid (JA) plays an important role in defense response and plant development. Jasmonate ZIM-domain (JAZ) proteins act as transcriptional repressors of plant responses to JA. In this study, we found that OsNINJA1, which is a JAZ-interacting adaptor protein, plays an important role in JA signaling that is positively regulated by the transcription factor OsMYC2 in rice. The expression of OsNINJA1 was upregulated at an early phase after JA treatment, and OsNINJA1 interacted with several OsJAZ proteins in a C domain-dependent manner. Transgenic rice plants overexpressing OsNINJA1 exhibited a JA-insensitive phenotype and were more susceptible to rice bacterial blight caused by Xanthomonas oryzae pv. oryzae, which is one of the most serious diseases affecting rice. Furthermore, OsNINJA1 negatively affected JA-regulated leaf senescence under dark-induced senescence conditions. Finally, the expression of OsMYC2-responsive pathogenesis-related (PR) genes and senescence-associated genes (SAGs) tended to be downregulated in the OsNINJA1-overexpressing rice plants. These results indicate that OsNINJA1 acts as a negative regulator of OsMYC2-mediated JA signaling in rice.

Rice Secondary Metabolites: Structures, Roles, Biosynthesis, and Metabolic Regulation

Rice (Oryza sativa L.) is an important food crop providing energy and nutrients for more than half of the world population. It produces vast amounts of secondary metabolites. At least 276 secondary metabolites from rice have been identified in the past 50 years. They mainly include phenolic acids, flavonoids, terpenoids, steroids, alkaloids, and their derivatives. These metabolites exhibit many physiological functions, such as regulatory effects on rice growth and development, disease-resistance promotion, anti-insect activity, and allelopathic effects, as well as various kinds of biological activities such as antimicrobial, antioxidant, cytotoxic, and anti-inflammatory properties. This review focuses on our knowledge of the structures, biological functions and activities, biosynthesis, and metabolic regulation of rice secondary metabolites. Some considerations about cheminformatics, metabolomics, genetic transformation, production, and applications related to the secondary metabolites from rice are also discussed.

The root transcriptome of Achyranthes bidentata and the identification of the genes involved in the replanting benefit

The transcriptome profiling in replanting roots revealed that expression pattern changes of key genes promoted important metabolism pathways, antioxidant and pathogen defense systems, adjusted phytohormone signaling and inhibited lignin biosynthesis. The yield of the medicinal plant Achyranthes bidentata could be significantly increased when replanted into a field cultivated previously for the same crop, but the biological basis of this so-called "replanting benefit" is unknown. Here, the RNA-seq technique was used to identify candidate genes responsible for the benefit. The analysis of RNA-seq libraries prepared from mRNA extracted from the roots of first year planting (normal growth, NG) and second year replanting (consecutive monoculture, CM) yielded about 40.22 GB sequencing data. After de novo assembly, 87,256 unigenes were generated with an average length of 1060 bp. Among these unigenes, 55,604 were annotated with public databases, and 52,346 encoding sequences and 2881 transcription factors were identified. A contrast between the NG and CM libraries resulted in a set of 3899 differentially transcribed genes (DTGs). The DTGs related to the replanting benefit and their expression profiles were further analyzed by bioinformatics and qRT-PCR approaches. The major differences between the NG and CM transcriptomes included genes encoding products involved in glycolysis/gluconeogenesis, glutathione metabolism and antioxidant defense, in aspects of the plant/pathogen interaction, phytohormone signaling and phenylpropanoid biosynthesis. The indication was that replanting material enjoyed a stronger level of defense systems, a balance regulation of hormone signals and a suppression of lignin formation, thereby promoting root growth and development. The study provides considerable significant insights for a better understanding of the molecular mechanism of the replanting benefit and suggests their possible application in developing methods to reinforce the effects in medicinal plants.

Identification of new abscisic acid receptor agonists using a wheat cell-free based drug screening system

Abscisic acid (ABA) is the main phytohormone involved in abiotic stress response and its adaptation, and is a candidate agrichemical. Consequently, several agonists of ABA have been developed using the yeast two-hybrid system. Here, we describe a novel cell-free-based drug screening approach for the development and validation of ABA receptor agonists. Biochemical validation of this approach between 14 ABA receptors (PYR/PYL/RCARs) and 7 type 2C-A protein phosphatases (PP2CAs) revealed the same interactions as those of previous proteome data, except for nine new interactions. By chemical screening using this approach, we identified two novel ABA receptor agonists, JFA1 (julolidine and fluorine containing ABA receptor activator 1) and JFA2 as its analog. The results of biochemical validation for this approach and biological analysis suggested that JFA1 and JFA2 inhibit seed germination and cotyledon greening of seedlings by activating PYR1 and PYL1, and that JFA2 enhanced drought tolerance without inhibiting root growth by activating not only PYR1 and PYL1 but also PYL5. Thus, our approach was useful for the development of ABA receptor agonists and their validation.

Expression of RSOsPR10 in rice roots is antagonistically regulated by jasmonate/ethylene and salicylic acid via the activator OsERF87 and the repressor OsWRKY76, respectively

Plant roots play important roles in absorbing water and nutrients, and in tolerance against environmental stresses. Previously, we identified a rice root-specific pathogenesis-related protein (RSOsPR10) induced by drought, salt, and wounding. RSOsPR10 expression is strongly induced by jasmonate (JA)/ethylene (ET), but suppressed by salicylic acid (SA). Here, we analyzed the promoter activity of RSOsPR10. Analyses of transgenic rice lines harboring different-length promoter::β-glucuronidase (GUS) constructs showed that the 3-kb promoter region is indispensable for JA/ET induction, SA repression, and root-specific expression. In the JA-treated 3K-promoter::GUS line, GUS activity was mainly observed at lateral root primordia. Transient expression in roots using a dual luciferase (LUC) assay with different-length promoter::LUC constructs demonstrated that the novel transcription factor OsERF87 induced 3K-promoter::LUC expression through binding to GCC-cis elements. In contrast, the SA-inducible OsWRKY76 transcription factor strongly repressed the JA-inducible and OsERF87-dependent expression of RSOsPR10. RSOsPR10 was expressed at lower levels in OsWRKY76-overexpressing rice, but at higher levels in OsWRKY76-knockout rice, compared with wild type. These results show that two transcription factors, OsERF87 and OsWRKY76, antagonistically regulate RSOsPR10 expression through binding to the same promoter. This mechanism represents a fine-tuning system to sense the balance between JA/ET and SA signaling in plants under environmental stress.

Biochar amendment changes jasmonic acid levels in two rice varieties and alters their resistance to herbivory

Biochar addition to soil not only sequesters carbon for the long-term but enhances agricultural productivity. Several well-known benefits arise from biochar amendment, including constant provision of nutrients, increased soil moisture retention, decreased soil bulk density, and sometimes the induction of systemic resistance against foliar and soil borne plant pathogens. However, no research has investigated the potential of biochar to increase resistance against herbivory. The white-backed plant hopper (WBPH) (Sogatella furcifera Horváth) is a serious agricultural pest that targets rice (Oryza sativa L.), a staple crop that feeds half of the world’s human population. Therefore, we investigated the (1) optimization of biochar amendment levels for two rice varieties (‘Cheongcheong’ and ‘Nagdong’) and (2) subsequent effects of different biochar amendments on resistance and susceptibility of these two varieties to WBPH infestation. Initial screening results for the optimization level revealed that the application of biochar 10% (w/w) to the rooting media significantly improved plant physiological characteristics of both rice varieties. However, levels of biochar amendment, mainly 1, 2, 3, and 20%, resulted in negative effects on plant growth characteristics. Cheongcheong and Nagdong rice plants grown with the optimum biochar level showed contrasting reactions to WBPH infestation. Specifically, biochar application significantly increased plant growth characteristics of Nagdong when exposed to WBPH infestation and significantly decreased these characteristics in Cheongcheong. The amount of WBPH-induced damage to plants was significantly lower and higher in Nagdong and Cheongcheong, respectively, compared to that in the controls. Higher levels of jasmonic acid caused by the biochar priming effect could have accumulated in response to WBPH infestation, resulting in a maladaptive response to stress, negatively affecting growth and resistance to WBPH in Cheongcheong. This study highlights the importance of investigating the effects of biochar on different rice varieties before application on a commercial scale to avoid potential crop losses.

TcMYC2a, a Basic Helix-Loop-Helix Transcription Factor, Transduces JA-Signals and Regulates Taxol Biosynthesis in Taxus chinensis

The multitherapeutic taxol, which can be obtained from Taxus spp., is the most widely used anticancer drug. Taxol biosynthesis is significantly regulated by jasmonate acid (JA), one of the most important endogenous hormones in land plants. Nevertheless, the JA-inducing mechanism remains poorly understood. MYC2 is one of the key regulators of JA signal transfer and the biosynthesis of various secondary metabolites. Here, TcMYC2a was identified to contain a basic helix-loop-helix (bHLH)-leucine zipper domain, a bHLH-MYC_N domain, and a BIF/ACT-like domain. TcMYC2a was also found to bind with TcJAZ3 in yeast, which was a homolog of Arabidopsis JASMONATE ZIM-domain JAZ proteins, indicating that TcMYC2a had a similar function to AtMYC2 of JA signal transduction. TcMYC2a was able to affect the expression of GUS reporter gene by binding with the T/G-box, G-box, and E-box, which were the key cis-elements of TASY and TcERF12/15 promoter. TcMYC2a overexpression also led to significantly increased expression of TASY, tat, dbtnbt, t13h, and t5h genes. Additionally, TcERF15, which played the positive role to regulate tasy gene, was up-regulated by TcMYC2a. All these results revealed that TcMYC2a can regulate taxol biosynthesis either directly or via ERF regulators depending on JA signaling transduction.

In planta functions of cytochrome P450 monooxygenase genes in the phytocassane biosynthetic gene cluster on rice chromosome 2

In response to environmental stressors such as blast fungal infections, rice produces phytoalexins, an antimicrobial diterpenoid compound. Together with momilactones, phytocassanes are among the major diterpenoid phytoalexins. The biosynthetic genes of diterpenoid phytoalexin are organized on the chromosome in functional gene clusters, comprising diterpene cyclase, dehydrogenase, and cytochrome P450 monooxygenase genes. Their functions have been studied extensively using in vitro enzyme assay systems. Specifically, P450 genes (CYP71Z6, Z7; CYP76M5, M6, M7, M8) on rice chromosome 2 have multifunctional activities associated with ent-copalyl diphosphate-related diterpene hydrocarbons, but the in planta contribution of these genes to diterpenoid phytoalexin production remains unknown. Here, we characterized cyp71z7 T-DNA mutant and CYP76M7/M8 RNAi lines to find that potential phytoalexin intermediates accumulated in these P450-suppressed rice plants. The results suggested that in planta, CYP71Z7 is responsible for C2-hydroxylation of phytocassanes and that CYP76M7/M8 is involved in C11α-hydroxylation of 3-hydroxy-cassadiene. Based on these results, we proposed potential routes of phytocassane biosynthesis in planta.

Transcription Factor AsMYC2 Controls the Jasmonate-Responsive Expression of ASS1 Regulating Sesquiterpene Biosynthesis in Aquilaria sinensis (Lour.) Gilg

Sesquiterpenes are one of the most important defensive secondary metabolite components of agarwood. Agarwood, which is a product of the Aquilaria sinensis response to external damage, is a fragrant and resinous wood that is widely used in traditional medicines, incense and perfume. We previously reported that jasmonic acid (JA) plays an important role in promoting agarwood sesquiterpene biosynthesis and induces expression of the sesquiterpene synthase ASS1, which is a key enzyme that is responsible for the biosynthesis of agarwood sesquiterpenes in A. sinensis. However, little is known about this molecular regulation mechanism. Here, we characterized a basic helix-loop-helix transcription factor, AsMYC2, from A. sinensis as an activator of ASS1 expression. AsMYC2 is an immediate-early jasmonate-responsive gene and is co-induced with ASS1. Using a combination of yeast one-hybrid assays and chromatin immunoprecipitation analyses, we showed that AsMYC2 bound the promoter of ASS1 containing a G-box motif. AsMYC2 activated expression of ASS1 in tobacco epidermis cells and up-regulated expression of sesquiterpene synthase genes (TPS21 and TPS11) in Arabidopsis, which was also promoted by methyl jasmonate. Our results suggest that AsMYC2 participates in the regulation of agarwood sesquiterpene biosynthesis in A. sinensis by controlling the expression of ASS1 through the JA signaling pathway.

Genome-wide analysis of basic helix-loop-helix (bHLH) transcription factors in Brachypodium distachyon

BACKGROUND

As a superfamily of transcription factors (TFs), the basic helix-loop-helix (bHLH) proteins have been characterized functionally in many plants with a vital role in the regulation of diverse biological processes including growth, development, response to various stresses, and so on. However, no systemic analysis of the bHLH TFs has been reported in Brachypodium distachyon, an emerging model plant in Poaceae.

RESULTS

A total of 146 bHLH TFs were identified in the Brachypodium distachyon genome and classified into 24 subfamilies. BdbHLHs in the same subfamily share similar protein motifs and gene structures. Gene duplication events showed a close relationship to rice, maize and sorghum, and segment duplications might play a key role in the expansion of this gene family. The amino acid sequence of the bHLH domains were quite conservative, especially Leu-27 and Leu-54. Based on the predicted binding activities, the BdbHLHs were divided into DNA binding and non-DNA binding types. According to the gene ontology (GO) analysis, BdbHLHs were speculated to function in homodimer or heterodimer manner. By integrating the available high throughput data in public database and results of quantitative RT-PCR, we found the expression profiles of BdbHLHs were different, implying their differentiated functions.

CONCLUSION

One hundred fourty-six BdbHLHs were identified and their conserved domains, sequence features, phylogenetic relationship, chromosomal distribution, GO annotations, gene structures, gene duplication and expression profiles were investigated. Our findings lay a foundation for further evolutionary and functional elucidation of BdbHLH genes.

Identification of OsMYC2-regulated senescence-associated genes in rice

The jasmonic acid (JA)-responsive transcription factor OsMYC2 acts as a positive regulator of leaf senescence by direct regulation of some senescence-associated genes in rice. OsMYC2, a transcription factor (TF), acts as a positive regulator of jasmonic acid (JA) signaling involved in development and defense in rice. Here, we report that OsMYC2 plays an important role in leaf senescence under dark-induced senescence (DIS) conditions. Overexpression of OsMYC2 significantly promoted leaf senescence, indicated by reduction of chlorophyll content under DIS conditions in rice. Leaf senescence under the DIS conditions was negatively regulated by OsJAZ8, a rice jasmonate ZIM-domain protein involved in the JA signaling pathway. OsMYC2 upregulated the expression of some senescence-associated genes (SAGs) and selectively bound to the G-box/G-box-like motifs in the promoters of some SAGs in vivo. These results suggest that OsMYC2 acts as a positive regulator of leaf senescence by direct- or indirect-regulation of SAGs in rice.