Glutamate Positively Regulates Chitinase Activity and the Biocontrol Efficacy of Pseudomonas protegens.

Broad-spectrum biocontrol by Pseudomonas protegens CHA0 and other fluorescent pseudomonads is achieved through the generation of various secondary metabolites with antibiotic activities against not only other microbes but, also, nematodes and insects present in the rhizosphere. A previous metabolomic study demonstrated that intracellular low-molecular weight effectors, such as guanosine tetraphosphate and γ-aminobutyrate, function as important signals in niche adaptation by strain CHA0 to plant roots. We investigated the role of amino acids in the biocontrol trait of P. protegens Cab57 towards Pythium damping off and root rot in cucumber. Among the 11 amino acids tested, only glutamate markedly enhanced the efficacy of biocontrol. An RNA-Seq analysis revealed that glutamate upregulated the expression of a chitinase gene cluster (c21370-c21380, in which the c21370 gene was annotated as a gene encoding the chitin-binding protein cbp and the c21380 gene encoded chitinase chiC) in strain CHA0. Glutamate upregulated the expression of the regulatory small RNA rsmZ but reduced the production levels of other Gac/Rsm-regulated biocontrol factors, such as 2,4-diacetylphloroglucinol and pyoluteorin. The promoter activity of cbp and chitinase activity were characterized in detail; their activities were up-regulated in response to glutamate and their expression was under the control of GacA. Therefore, glutamate appears to be essential for biocontrol activity in which chitinase production is regulated in response to glutamate. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Phytol, a Constituent of Chlorophyll, Induces Root-Knot Nematode Resistance in Arabidopsis via the Ethylene Signaling Pathway.

Root-knot nematodes (RKNs; Meloidogyne spp.) parasitize the roots or stems of a wide range of plant species, resulting in severe damage to the parasitized plant. The phytohormone ethylene (ET) plays an important role in signal transduction pathways leading to resistance against RKNs. However, little is currently known about the induction mechanisms of ET-dependent RKN resistance. Inoculation of Arabidopsis thaliana roots with RKNs decreased chlorophyll contents in aerial parts of the plant. We observed accumulation of phytol, a constituent of chlorophyll and a precursor of tocopherols, in RKN-parasitized roots. Application of sclareol, a diterpene that has been shown to induce ET-dependent RKN resistance, to the roots of Arabidopsis plants increased phytol contents in roots accompanied by a decrease in chlorophyll in aerial parts. Exogenously applied phytol inhibited RKN penetration of roots without exhibiting nematicidal activity. This phytol-induced inhibition of RKN penetration was attenuated in the ET-insensitive Arabidopsis mutant ein2-1. Exogenously applied phytol enhanced the production of α-tocopherol and expression of VTE5, a gene involved in tocopherol production, in Arabidopsis roots. α-Tocopherol exerted induction of RKN resistance similar to that of phytol and showed increased accumulation in roots inoculated with RKNs. Furthermore, the Arabidopsis vte5 mutant displayed no inhibition of RKN penetration in response to phytol. These results suggest that exogenously applied phytol induces EIN2-dependent RKN resistance, possibly via tocopherol production.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Ultrafine bubble water mitigates plant growth in damaged soil.

Water containing ultrafine/nano bubbles (UFBs) promoted the growth of tomato (Solanum lycopersicum) in soil damaged by cultivation of tomato in the previous year or bacterial wilt-like disease and also promoted the growth of lettuce (Lactuca sativa) when lettuce was grown in the soil damaged by repeated cultivation of lettuce. On the other hand, UFB supply did not affect plant growth in rock wool or healthy soil. Furthermore, the growth of lettuce was not affected by UFB water treatment in the soil damaged by the cultivation of tomato. UFB water partly suppressed the growth of the pathogen of bacteria wilt disease, Ralstonia solanacearum in vitro. These data suggest that UFB water is effective to recover the plant growth from soil damage.

Loliolide, a Carotenoid Metabolite, Is a Potential Endogenous Inducer of Herbivore Resistance.

Jasmonic acid (JA) plays an important role in the induction of herbivore resistance in many plants. However, JA-independent herbivore resistance has been suggested. An herbivore-resistance-inducing substance was isolated from Tobacco mosaic virus-infected tobacco (Nicotiana tabacum) leaves in which a hypersensitive response (HR) was induced and identified as loliolide, which has been identified as a ß-carotene metabolite. When applied to tomato (Solanum lycopersicum) leaves, loliolide decreased the survival rate of the two-spotted spider mite, Tetranychus urticae, egg deposition by the same pest, and the survival rate of larvae of the common cutworm Spodoptera litura without exhibiting toxicity against these herbivores. Endogenous loliolide levels increased not only with an infestation by S litura larvae, but also with the exogenous application of their oral secretions in tomato. A microarray analysis identified cell-wall-associated defense genes as loliolide-responsive tomato genes, and exogenous JA application did not induce the expression of these genes. Suppressor of zeaxanthin-less (szl), an Arabidopsis (Arabidopsis thaliana) mutant with a point mutation in a key gene of the ß-carotene metabolic pathway, exhibited the decreased accumulation of endogenous loliolide and increased susceptibility to infestation by the western flower thrip (Frankliniella occidentalis). A pretreatment with loliolide decreased susceptibility to thrips in the JA-insensitive Arabidopsis mutant coronatine-insensitive1 Exogenous loliolide did not restore reduced electrolyte leakage in szl in response to a HR-inducing bacterial strain. These results suggest that loliolide functions as an endogenous signal that mediates defense responses to herbivores, possibly independently of JA, at least in tomato and Arabidopsis plants.

α-Ionone, an Apocarotenoid, Induces Plant Resistance to Western Flower Thrips, Frankliniella occidentalis, Independently of Jasmonic Acid.

Apocarotenoids, such as ß-cyclocitral, α-ionone, ß-ionone, and loliolide, are derived from carotenes via chemical or enzymatic processes. Recent studies revealed that ß-cyclocitral and loliolide play an important role in various aspects of plant physiology, such as stress responses, plant growth, and herbivore resistance. However, information on the physiological role of α-ionone is limited. We herein investigated the effects of α-ionone on plant protection against herbivore attacks. The pretreatment of whole tomato (Solanum lycopersicum) plants with α-ionone vapor decreased the survival rate of western flower thrips (Frankliniella occidentalis) without exhibiting insecticidal activity. Exogenous α-ionone enhanced the expression of defense-related genes, such as basic ß-1,3-glucanase and basic chitinase genes, in tomato leaves, but not that of jasmonic acid (JA)- or loliolide-responsive genes. The pretreatment with α-ionone markedly decreased egg deposition by western flower thrips in the JA-insensitive Arabidopsis (Arabidopsis thaliana) mutant coi1-1. We also found that common cutworm (Spodoptera litura) larvae fed on α-ionone-treated tomato plants exhibited a reduction in weight. These results suggest that α-ionone induces plant resistance to western flower thrips through a different mode of action from that of JA and loliolide.

The MAMP-Responsive MYB Transcription Factors MYB30, MYB55 and MYB110 Activate the HCAA Synthesis Pathway and Enhance Immunity in Rice.

Phenylpropanoids, including diverse compounds, such as monolignols and hydroxycinnamic acids (HCAAs), are essential for land plants to protect them against abiotic stresses, and create physical and chemical barriers to pathogen infection. However, the control of production of these compounds in response to pathogens has been poorly understood. Previously we showed that a MAMP- (microbe-associated molecular pattern) responsive MAPK (mitogen-activated protein kinase) cascade (MKK4-MPK3/MPK6) comprehensively induced the expression of cinnamate/monolignol synthesis genes in rice cells. Here, we identified three MYB proteins, MYB30, MYB55 and MYB110, which are transcriptionally induced by MAMP treatment, MAPK activation and pathogen inoculation. Induced expression of these MYB genes systematically and specifically induced a large part of the genes encoding enzymes in the cinnamate/monolignol pathway. Furthermore, induced expression of the MYB genes caused accumulation of ferulic acid, one of the HCAAs, and enhanced resistance to both fungal and bacterial pathogens in planta. In conclusion, MYB30, MYB55 and MYB110 are involved in the signal pathway between MAMP perception and cinnamate/monolignol synthesis, and have important roles for plant immunity.

l-Histidine Induces Resistance in Plants to the Bacterial Pathogen Ralstonia solanacearum Partially Through the Activation of Ethylene Signaling.

Wilt disease in plants, which is caused by the soil-borne bacterial pathogen Ralstonia solanacearum, is one of the most devastating plant diseases. We previously detected bacterial wilt disease-inhibiting activity in an extract from yeast cells. In the present study, we purified this activity and identified one of the substances responsible for the activity as the amino acid histidine. The exogenous application of l-histidine, but not d-histidine, inhibited wilt disease in tomato and Arabidopsis plants without exhibiting any antibacterial activity. l-Histidine induced the expression of genes related to ethylene (ET) biosynthesis and signaling as well as the production of ET in tomato and Arabidopsis plants. l-Histidine-induced resistance to R. solanacearum was partially abolished in ein3-1, an ET-insensitive Arabidopsis mutant line. Resistance to the fungal pathogen Botrytis cinerea, which is known to require ET biosynthesis or signaling, was also induced by exogenously applied l-histidine. These results suggest that l-histidine induces resistance to R. solanacearum and B. cinerea partially through the activation of ET signaling in plants.

Mitogen-activated protein kinase 4-like carrying an MEY motif instead of a TXY motif is involved in ozone tolerance and regulation of stomatal closure in tobacco.

The mitogen-activated protein kinases (MAPKs/MPKs) are important factors in the regulation of signal transduction in response to biotic and abiotic stresses. Previously, we characterized a MAPK from tobacco, Nicotiana tabacum MPK4 (NtMPK4). Here, we found a highly homologous gene, NtMPK4-like (NtMPK4L), in tobacco as well as other species in Solanaceae and Gramineae. Deduced amino acid sequences of their translation products carried MEY motifs instead of conserved TXY motifs of the MAPK family. We isolated the full length NtMPK4L gene and examined the physiological functions of NtMPK4L. We revealed that NtMPK4L was activated by wounding, like NtMPK4. However, a constitutively active salicylic acid-induced protein kinase kinase (SIPKK(EE)), which phosphorylates NtMPK4, did not phosphorylate NtMPK4L. Moreover, a tyrosine residue in the MEY motif was not involved in NtMPK4L activation. We also found that NtMPK4L-silenced plants showed rapid transpiration caused by remarkably open stomata. In addition, NtMPK4L-silenced plants completely lost the ability to close stomata upon ozone treatment and were highly sensitive to ozone, suggesting that this atypical MAPK plays a role in ozone tolerance through stomatal regulation.

Lutein, a Natural Carotenoid, Induces α-1,3-Glucan Accumulation on the Cell Wall Surface of Fungal Plant Pathogens.

α-1,3-Glucan, a component of the fungal cell wall, is a refractory polysaccharide for most plants. Previously, we showed that various fungal plant pathogens masked their cell wall surfaces with α-1,3-glucan to evade plant immunity. This surface accumulation of α-1,3-glucan was infection specific, suggesting that plant factors might induce its production in fungi. Through immunofluorescence observations of fungal cell walls, we found that carrot (Daucus carota) extract induced the accumulation of α-1,3-glucan on germlings in Colletotrichum fioriniae, a polyphagous fungal pathogen that causes anthracnose disease in various dicot plants. Bioassay-guided fractionation of carrot leaf extract successfully identified two active substances that caused α-1,3-glucan accumulation in this fungus: lutein, a carotenoid widely distributed in plants, and stigmasterol, a plant-specific membrane component. Lutein, which had a greater effect on C. fioriniae, also induced α-1,3-glucan accumulation in other Colletotrichum species and in the phylogenetically distant rice pathogen Cochliobolus miyabeanus, but not in the rice pathogen Magnaporthe oryzae belonging to the same phylogenetic subclass as Colletotrichum. Our results suggested that fungal plant pathogens reorganize their cell wall components in response to specific plant-derived compounds, which these pathogens may encounter during infection.

Sclareol induces plant resistance to root-knot nematode partially through ethylene-dependent enhancement of lignin accumulation.

The root-knot nematode (RKN) is one of the most devastating parasitic nematodes of plants. Although some secondary metabolites released by the host plant play roles as defense substances against parasitic nematodes, the mechanism underlying the induction of such defense responses is not fully understood. We found that sclareol, a natural diterpene known as an antimicrobial and defense-related molecule, inhibited RKN penetration of tomato and Arabidopsis roots. Sclareol induced genes related to ethylene (ET) biosynthesis and signaling and phenylpropanoid metabolism in Arabidopsis roots. In roots of ein2-1, an ET-insensitive mutant line, both sclareol-induced inhibition of RKN penetration and sclareol-induced enhancement of lignin accumulation were abolished. A mutant defective in lignin accumulation did not exhibit such inhibition. Sclareol also activated MPK3 and MPK6, Arabidopsis mitogen-activated protein kinases whose activation is required for triggering ET biosynthesis. Sclareol-induced inhibition of RKN penetration was exhibited by mutants of neither MPK3 nor MPK6. Treatment with a biosynthetic precursor of ET was insufficient compared with sclareol treatment to inhibit RKN penetration, suggesting the existence of an ET-independent signaling pathway leading to RKN resistance. These results suggested that sclareol induced resistance to RKN penetration partially through ET-dependent accumulation of lignin in roots.

Disarming the jasmonate-dependent plant defense makes nonhost Arabidopsis plants accessible to the American serpentine leafminer.

Here, we analyzed the interaction between Arabidopsis (Arabidopsis thaliana) and the American serpentine leafminer (Liriomyza trifolii), an important and intractable herbivore of many cultivated plants. We examined the role of the immunity-related plant hormone jasmonate (JA) in the plant response and resistance to leafminer feeding to determine whether JA affects host suitability for leafminers. The expression of marker genes for the JA-dependent plant defense was induced by leafminer feeding on Arabidopsis wild-type plants. Analyses of JA-insensitive coi1-1 mutants suggested the importance of JA in the plant response to leafminer feeding. The JA content of wild-type plants significantly increased after leafminer feeding. Moreover, coi1-1 mutants showed lower feeding resistance against leafminer attack than did wild-type plants. The number of feeding scars caused by inoculated adult leafminers in JA-insensitive coi1-1 mutants was higher than that in wild-type plants. In addition, adults of the following generation appeared only from coi1-1 mutants and not from wild-type plants, suggesting that the loss of the JA-dependent plant defense converted nonhost plants to accessible host plants. Interestingly, the glucosinolate-myrosinase defense system may play at most a minor role in this conversion, indicating that this major antiherbivore defense of Brassica species plants probably does not have a major function in plant resistance to leafminer. Application of JA to wild-type plants before leafminer feeding enhanced feeding resistance in Chinese cabbage (Brassica rapa), tomato (Solanum lycopersicum), and garland chrysanthemum (Chrysanthemum coronarium). Our results indicate that JA plays an important role in the plant response and resistance to leafminers and, in so doing, affects host plant suitability for leafminers.

Analysis on blast fungus-responsive characters of a flavonoid phytoalexin sakuranetin; accumulation in infected rice leaves, antifungal activity and detoxification by fungus.

To understand the role of the rice flavonoid phytoalexin (PA) sakuranetin for blast resistance, the fungus-responsive characteristics were studied. Young rice leaves in a resistant line exhibited hypersensitive reaction (HR) within 3 days post inoculation (dpi) of a spore suspension, and an increase in sakuranetin was detected at 3 dpi, increasing to 4-fold at 4 dpi. In the susceptible line, increased sakuranetin was detected at 4 dpi, but not at 3 dpi, by which a large fungus mass has accumulated without HR. Induced expression of a PA biosynthesis gene OsNOMT for naringenin 7-O-methyltransferase was found before accumulation of sakuranetin in both cultivars. The antifungal activity of sakuranetin was considerably higher than that of the major rice diterpenoid PA momilactone A in vitro and in vivo under similar experimental conditions. The decrease and detoxification of sakuranetin were detected in both solid and liquid mycelium cultures, and they took place slower than those of momilactone A. Estimated local concentration of sakuranetin at HR lesions was thought to be effective for fungus restriction, while that at enlarged lesions in susceptible rice was insufficient. These results indicate possible involvement of sakuranetin in blast resistance and its specific relation to blast fungus.

Tobacco MAP kinase phosphatase (NtMKP1) negatively regulates wound response and induced resistance against necrotrophic pathogens and lepidopteran herbivores.

Mitogen-activated protein kinase (MAPK) cascades are universal signal transduction pathways in eukaryotic cells. In tobacco, two MAPK, wound-induced protein kinase (WIPK) and salicylic acid (SA)-induced protein kinase (SIPK), are activated by biotic and abiotic stresses. Both WIPK and SIPK positively regulate the biosynthesis of jasmonic acid (JA) or ethylene (ET) while negatively regulating SA accumulation. We showed previously that recombinant tobacco MAPK phosphatase (NtMKP1) protein dephosphorylates and inactivates SIPK in vitro, and overexpression of NtMKP1 repressed wound-induced activation of both SIPK and WIPK. To elucidate the role of NtMKP1 in response to biotic and abiotic stresses, we generated transgenic tobacco plants in which NtMKP1 expression was suppressed. Suppression of NtMKP1 expression resulted in enhanced activation of WIPK and SIPK and production of both JA and ET upon wounding. Wound-induced expression of JA- or ET-inducible genes, basic PR-1 and PI-II, was also significantly enhanced in these plants. Furthermore, NtMKP1-suppressed plants exhibited enhanced resistance against a necrotrophic pathogen, Botrytis cinerea, and lepidopteran herbivores, Mamestra brassicae and Spodoptera litura. These results suggest that NtMKP1 negatively regulates wound response and resistance against both necrotrophic pathogens and herbivorous insects through suppression of JA or ET pathways via inactivation of MAPK.

Jasmonic acid negatively regulates resistance to Tobacco mosaic virus in tobacco.

Nicotiana tabacum (tobacco) cultivars possessing the N resistance gene to Tobacco mosaic virus (TMV) induce a hypersensitive response, which is accompanied by the production of phytohormones such as salicylic acid (SA) and jasmonic acid (JA), to enclose the invaded virus at the initial site of infection, which inhibits viral multiplication and spread. SA functions as a positive regulator of TMV resistance. However, the role of JA in TMV resistance has not been fully elucidated. Exogenously applied methyl jasmonate, a methyl ester of JA, reduced local resistance to TMV and permitted systemic viral movement. Furthermore, in contrast to a previous finding, we demonstrated that silencing of CORONATINE-INSENSITIVE 1 (COI1), a JA receptor, reduced viral accumulation in a tobacco cultivar possessing the N gene, as did that of allene oxide synthase, a JA biosynthetic enzyme. The reduction in viral accumulation in COI1-silenced tobacco plants was correlated with an increase in SA, and lowering SA levels by introducing an SA hydroxylase gene attenuated this reduction. Viral susceptibility did not change in a COI1-silenced tobacco cultivar lacking the N gene. These results suggest that JA signaling is not directly responsible for susceptibility to TMV, but is indirectly responsible for viral resistance through the partial inhibition of SA-mediated resistance conferred by the N gene, and that a balance between endogenous JA and SA levels is important for determining the degree of resistance.

Transcriptome analysis of WIPK/SIPK-suppressed plants reveals induction by wounding of disease resistance-related genes prior to the accumulation of salicylic acid.

Salicylic acid (SA) plays a key role in plant resistance to pathogens. Accumulation of SA is induced by wounding in tobacco plants in which the expression of WIPK and SIPK, two mitogen-activated protein kinases, is suppressed. Here, the mechanisms underlying the abnormal accumulation of SA in WIPK/SIPK-suppressed plants have been characterized. SA accumulation started around 12 h after wounding and was inhibited by cycloheximide (CHX), a protein synthesis inhibitor. SA accumulation, however, was enhanced several fold when leaf discs were transferred onto CHX after floating on water for ≥6 h. Temporal and spatial analyses of wound-induced and CHX-enhanced SA accumulation suggested that wounding induces activators for SA accumulation followed by the generation of repressors, and late CHX treatment inhibits the production of repressors more efficiently than that of activators. Microarray analysis revealed that the expression of many disease resistance-related genes, including N, a Resistance (R) gene for Tobacco mosaic virus and R gene-like genes, was up-regulated in wounded WIPK/SIPK-suppressed plants. Expression of the N gene and R gene-like genes peaked earlier than that of most other genes as well as SA accumulation, and was mainly induced in those parts of leaf discs where SA was highly accumulated. Moreover, wound-induced SA accumulation was decreased by the treatments which compromise the function of R proteins. These results indicate that signaling leading to the expression of disease resistance-related genes is activated by wounding in WIPK/SIPK-suppressed plants, and induction of R gene and R gene-like genes might lead to the biosynthesis of SA.

Phytosterols Are Involved in Sclareol-Induced Chlorophyll Reductions in Arabidopsis.

Sclareol, a diterpene, has a wide range of physiological effects on plants, such as antimicrobial activity; disease resistance against pathogens; and the expression of genes encoding proteins involved in metabolism, transport, and phytohormone biosynthesis and signaling. Exogenous sclareol reduces the content of chlorophyll in Arabidopsis leaves. However, the endogenous compounds responsible for sclareol-induced chlorophyll reduction remain unknown. The phytosterols campesterol and stigmasterol were identified as compounds that reduce the content of chlorophyll in sclareol-treated Arabidopsis plants. The exogenous application of campesterol or stigmasterol dose-dependently reduced the content of chlorophyll in Arabidopsis leaves. Exogenously-applied sclareol enhanced the endogenous contents of campesterol and stigmasterol and the accumulation of transcripts for phytosterol biosynthetic genes. These results suggest that the phytosterols campesterol and stigmasterol, the production of which is enhanced in response to sclareol, contribute to reductions in chlorophyll content in Arabidopsis leaves.

Antagonistic plant defense system regulated by phytohormones assists interactions among vector insect, thrips and a tospovirus.

The western flower thrips (Frankliniella occidentalis) is a polyphagous herbivore that causes serious damage to many agricultural plants. In addition to causing feeding damage, it is also a vector insect that transmits tospoviruses such as Tomato spotted wilt virus (TSWV). We previously reported that thrips feeding on plants induces a jasmonate (JA)-regulated plant defense, which negatively affects both the performance and preference (i.e. host plant attractiveness) of the thrips. The antagonistic interaction between a JA-regulated plant defense and a salicylic acid (SA)-regulated plant defense is well known. Here we report that TSWV infection allows thrips to feed heavily and multiply on Arabidopsis plants. TSWV infection elevated SA contents and induced SA-regulated gene expression in the plants. On the other hand, TSWV infection decreased the level of JA-regulated gene expression induced by thrips feeding. Importantly, we also demonstrated that thrips significantly preferred TSWV-infected plants to uninfected plants. In JA-insensitive coi1-1 mutants, however, thrips did not show a preference for TSWV-infected plants. In addition, SA application to wild-type plants increased their attractiveness to thrips. Our results suggest the following mechanism: TSWV infection suppresses the anti-herbivore response in plants and attracts its vector, thrips, to virus-infected plants by exploiting the antagonistic SA-JA plant defense systems.

Identification of natural diterpenes that inhibit bacterial wilt disease in tobacco, tomato and Arabidopsis.

The soil-borne bacterial pathogen Ralstonia solanacearum invades a broad range of plants through their roots, resulting in wilting of the plant, but no effective protection against this disease has been developed. Two bacterial wilt disease-inhibiting compounds were biochemically isolated from tobacco and identified as sclareol and cis-abienol, labdane-type diterpenes. When exogenously applied to their roots, sclareol and cis-abienol inhibited wilt disease in tobacco, tomato and Arabidopsis plants without exhibiting any antibacterial activity. Microarray analysis identified many sclareol-responsive genes in Arabidopsis roots, including genes encoding or with a role in ATP-binding cassette (ABC) transporters, and biosynthesis and signaling of defense-related molecules and mitogen-activated protein kinase (MAPK) cascade components. Inhibition of wilt disease by sclareol was attenuated in Arabidopsis mutants defective in the ABC transporter AtPDR12, the MAPK MPK3, and ethylene and abscisic acid signaling pathways, and also in transgenic tobacco plants with reduced expression of NtPDR1, a tobacco homolog of AtPDR12. These results suggest that multiple host factors are involved in the inhibition of bacterial wilt disease by sclareol-related compounds.

Cyanide, a coproduct of plant hormone ethylene biosynthesis, contributes to the resistance of rice to blast fungus.

Rice (Oryza sativa) plants carrying the Pi-i resistance gene to blast fungus Magnaporthe oryzae restrict invaded fungus in infected tissue via hypersensitive reaction or response (HR), which is accompanied by rapid ethylene production and formation of small HR lesions. Ethylene biosynthesis has been implicated to be important for blast resistance; however, the individual roles of ethylene and cyanide, which are produced from the precursor 1-aminocyclopropane-1-carboxylic acid, remain unevaluated. In this study, we found that Pi-i-mediated resistance was compromised in transgenic rice lines, in which ethylene biosynthetic enzyme genes were silenced and then ethylene production was inhibited. The compromised resistance in transgenic lines was recovered by exogenously applying cyanide but not ethephon, an ethylene-releasing chemical in plant tissue. In a susceptible rice cultivar, treatment with cyanide or 1-aminocyclopropane-1-carboxylic acid induced the resistance to blast fungus in a dose-dependent manner, while ethephon did not have the effect. Cyanide inhibited the growth of blast fungus in vitro and in planta, and application of flavonoids, secondary metabolites that exist ubiquitously in the plant kingdom, enhanced the cyanide-induced inhibition of fungal growth. These results suggested that cyanide, whose production is triggered by HR in infected tissue, contributes to the resistance in rice plants via restriction of fungal growth.

Identification of a degradation intermediate of the momilactone A rice phytoalexin by the rice blast fungus.

We have already shown that major rice diterpene phytoalexin, momilactone A, was detoxified by Magnaporthe oryzae. We report here the identification by NMR, MS, and chemical synthesis of 3,6-dioxo-19-nor-9ß-pimara-7,15-diene (1) as the degradation intermediate. Compound 1 exhibited similar antifungal activity to that of momilactone A, indicating 1 to be a precursor of possible detoxified compounds.