Methionine-induced phytoalexin production in rice leaves

Defensive Molecules Momilactones A and B: Function, Biosynthesis, Induction and Occurrence

Labdane-related diterpenoids, momilactones A and B were isolated and identified in rice husks in 1973 and later found in rice leaves, straws, roots, root exudate, other several Poaceae species and the moss species Calohypnum plumiforme. The functions of momilactones in rice are well documented. Momilactones in rice plants suppressed the growth of fungal pathogens, indicating the defense function against pathogen attacks. Rice plants also inhibited the growth of adjacent competitive plants through the root secretion of momilactones into their rhizosphere due to the potent growth-inhibitory activity of momilactones, indicating a function in allelopathy. Momilactone-deficient mutants of rice lost their tolerance to pathogens and allelopathic activity, which verifies the involvement of momilactones in both functions. Momilactones also showed pharmacological functions such as anti-leukemia and anti-diabetic activities. Momilactones are synthesized from geranylgeranyl diphosphate through cyclization steps, and the biosynthetic gene cluster is located on chromosome 4 of the rice genome. Pathogen attacks, biotic elicitors such as chitosan and cantharidin, and abiotic elicitors such as UV irradiation and CuCl2 elevated momilactone production through jasmonic acid-dependent and independent signaling pathways. Rice allelopathy was also elevated by jasmonic acid, UV irradiation and nutrient deficiency due to nutrient competition with neighboring plants with the increased production and secretion of momilactones. Rice allelopathic activity and the secretion of momilactones into the rice rhizosphere were also induced by either nearby Echinochloa crus-galli plants or their root exudates. Certain compounds from Echinochloa crus-galli may stimulate the production and secretion of momilactones. This article focuses on the functions, biosynthesis and induction of momilactones and their occurrence in plant species.

Sakuranetin and its therapeutic potentials - a comprehensive review

Sakuranetin (SKN), a naturally derived 7-O-methylated flavonoid, was first identified in the bark of the cherry tree (Prunus spp.) as an aglycone of sakuranin and then purified from the bark of Prunus puddum. It was later reported in many other plants including Artemisia campestris, Boesenbergia pandurata, Baccharis spp., Betula spp., Juglans spp., and Rhus spp. In plants, it functions as a phytoalexin synthesized from its precursor naringenin and is the only known phenolic phytoalexin in rice, which is released in response to different abiotic and biotic stresses such as UV-irradiation, jasmonic acid, cupric chloride, L-methionine, and the phytotoxin coronatine. Till date, SKN has been widely reported for its diverse pharmacological benefits including antioxidant, anti-inflammatory, antimycobacterial, antiviral, antifungal, antileishmanial, antitrypanosomal, glucose uptake stimulation, neuroprotective, antimelanogenic, and antitumor properties. Its pharmacokinetics and toxicological properties have been poorly understood, thus warranting further evaluation together with exploring other pharmacological properties such as antidiabetic, neuroprotective, and antinociceptive effects. Besides, in vivo studies or clinical investigations can be done for proving its effects as antioxidant and anti-inflammatory, antimelanogenic, and antitumor agent. This review summarizes all the reported investigations with SKN for its health-beneficial roles and can be used as a guideline for future studies.

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.

Jasmonic Acid, Abscisic Acid, and Salicylic Acid Are Involved in the Phytoalexin Responses of Rice to Fusarium fujikuroi, a High Gibberellin Producer Pathogen

Fusarium fujikuroi, the causal agent of bakanae disease, is the main seedborne pathogen on rice. To understand the basis of rice resistance, a quantitative method to simultaneously detect phytohormones and phytoalexins was developed by using HPLC-MS/MS. With this method dynamic profiles and possible interactions of defense-related phytohormones and phytoalexins were investigated on two rice cultivars, inoculated or not with F. fujikuroi. In the resistant cultivar Selenio, the presence of pathogen induced high production of phytoalexins, mainly sakuranetin, and symptoms of bakanae were not observed. On the contrary, in the susceptible genotype Dorella, the pathogen induced the production of gibberellin and abscisic acid and inhibited jasmonic acid production, phytoalexins were very low, and bakanae symptoms were observed. The results suggested that a wide range of secondary metabolites are involved in plant defense against pathogens and phytoalexin synthesis could be an important factor for rice resistance against bakanae disease.

Identification of UV-Induced Diterpenes Including a New Diterpene Phytoalexin, Phytocassane F, from Rice Leaves by Complementary GC/MS and LC/MS Approaches

Rice phytoalexins are regarded as one of the most important weapons against pathogenic microorganisms. We attempted to identify novel phytoalexins and their derivatives using GC/MS and LC/MS analyses. Diterpene derivatives, 9β-pimara-7,15-diene-3β,6β,19-triol, 1, stemar-13-en-2α-ol, 2, and 1α,2α-dihydroxy-ent-12,15-cassadiene-3,11-dione, 3, were isolated from UV-irradiated rice leaves by chromatographic methods. These structures were confirmed by 1D- and 2D-NMR and MS analyses. Interestingly, all three compounds were accumulated following an infection by the rice blast pathogen Magnaporthe oryzae. Compounds 1 and 2 exhibited weak antifungal activity and may be the biosynthetic intermediates of rice phytoalexins momilactones and oryzalexin S, respectively. Compound 3 exhibited relatively high inhibitory activity against the fungal mycelial growth of M. oryzae to the same extent as the known phytoalexin phytocassane A. We conclude that 3 is a member of the cassane-type phytoalexin family and propose the name phytocassane F.

Nucleoporin 75 is involved in the ethylene-mediated production of phytoalexin for the resistance of Nicotiana benthamiana to Phytophthora infestans

Mature Nicotiana benthamiana shows stable resistance to the oomycete pathogen Phytophthora infestans. Induction of phytoalexin (capsidiol) production is essential for the resistance, which is upregulated via a mitogen-activated protein kinase (MAPK) cascade (NbMEK2-WIPK/SIPK) followed by ethylene signaling. In this study, NbNup75 (encodes a nuclear pore protein Nucleoporin75) was identified as an essential gene for resistance of N. benthamiana to P. infestans. In NbNup75-silenced plants, initial events of elicitor-induced responses such as phosphorylation of MAPK and expression of defense-related genes were not affected, whereas induction of later defense responses such as capsidiol production and cell death induction was suppressed or delayed. Ethylene production induced by either INF1 or NbMEK2 was reduced in NbNup75-silenced plants, whereas the expression of NbEAS (a gene for capsidiol biosynthesis) induced by ethylene was not affected, indicating that Nup75 is required for the induction of ethylene production but not for ethylene signaling. Given that nuclear accumulation of polyA RNA was increased in NbNup75-silenced plants, efficient export of mRNA from nuclei via nuclear pores would be important for the timely upregulation of defense responses. Collectively, Nup75 is involved in the induction of a later stage of defense responses, including the ethylene-mediated production of phytoalexin for the resistance of N. benthamiana to P. infestans.

Biosynthesis, elicitation and roles of monocot terpenoid phytoalexins

A long-standing goal in plant research is to optimize the protective function of biochemical agents that impede pest and pathogen attack. Nearly 40 years ago, pathogen-inducible diterpenoid production was described in rice, and these compounds were shown to function as antimicrobial phytoalexins. Using rice and maize as examples, we discuss recent advances in the discovery, biosynthesis, elicitation and functional characterization of monocot terpenoid phytoalexins. The recent expansion of known terpenoid phytoalexins now includes not only the labdane-related diterpenoid superfamily but also casbane-type diterpenoids and β-macrocarpene-derived sequiterpenoids. Biochemical approaches have been used to pair pathway precursors and end products with cognate biosynthetic genes. The number of predicted terpenoid phytoalexins is expanding through advances in cereal genome annotation and terpene synthase characterization that likewise enable discoveries outside the Poaceae. At the cellular level, conclusive evidence now exists for multiple plant receptors of fungal-derived chitin elicitors, phosphorylation of membrane-associated signaling complexes, activation of mitogen-activated protein kinase, involvement of phytohormone signals, and the existence of transcription factors that mediate the expression of phytoalexin biosynthetic genes and subsequent accumulation of pathway end products. Elicited production of terpenoid phytoalexins exhibit additional biological functions, including root exudate-mediated allelopathy and insect antifeedant activity. Such findings have encouraged consideration of additional interactions that blur traditionally discrete phytoalexin classifications. The establishment of mutant collections and increasing ease of genetic transformation assists critical examination of further biological roles. Future research directions include examination of terpenoid phytoalexin precursors and end products as potential signals mediating plant physiological processes.

Ethylene signaling pathway modulates attractiveness of host roots to the root-knot nematode Meloidogyne hapla

Infective juveniles of the root-knot nematode Meloidogyne hapla are attracted to the zone of elongation of roots where they invade the host but little is known about what directs the nematode to this region of the root. We found that Arabidopsis roots exposed to an ethylene (ET)-synthesis inhibitor attracted significantly more nematodes than control roots and that ET-overproducing mutants were less attractive. Arabidopsis seedlings with ET-insensitive mutations were generally more attractive whereas mutations resulting in constitutive signaling were less attractive. Roots of the ET-insensitive tomato mutant Never ripe (Nr) were also more attractive, indicating that ET signaling also modulated attraction of root-knot nematodes to this host. ET-insensitive mutants have longer roots due to reduced basipetal auxin transport. However, assessments of Arabidopsis mutants that differ in various aspects of the ET response suggest that components of the ET-signaling pathway directly affecting root length are not responsible for modulating root attractiveness and that other components of downstream signaling result in changes in levels of attractants or repellents for M. hapla. These signals may aid in directing this pathogen to an appropriate host and invasion site for completing its life cycle.

Allelochemical tricin in rice hull and its aurone isomer against rice seedling rot disease

BACKGROUND

One promising area of rice disease management is the potential of exploiting biological control agents. Rice seedling rot disease caused by soil-borne pathogenic fungi has become a dominant disease problem because of greater use of direct seeding. Rice hull has been potentially used to control paddy weeds, but little information is available on rice disease. This study was conducted to investigate the relationships between disease incidence and soil amended with tricin-releasing rice hull, and to assess fungicidal activity of tricin and its synthesised aurone isomer, with an attempt to develop an allelochemical-based fungicide against rice seedling rot disease.

RESULTS

Tricin was detected in all hulls of 12 rice cultivars tested, but its contents in rice hulls varied greatly with the cultivar and genotype. Tricin in rice hulls was released into the soil once amended. Disease incidence was significantly reduced by soil amended with rice hull. Tricin-rich rice hull amendment greatly suppressed soil-borne pathogenic fungi Fusarium oxysporum Schlecht. and Rhizoctonia solani Kühn which cause rice seedling rot disease. In attempting to obtain enough tricin for further experiments, the aurone isomer (5,7,4'-trihydroxy-3',5'-dimethoxyaurone) of tricin rather than tricin itself was unexpectedly synthesised. This aurone isomer had much stronger fungicidal activity on both F. oxysporum and R. solani than tricin itself.

CONCLUSION

Soil amended with tricin-rich rice hull was associated with reduced risk of developing seedling rot disease. The tricin isomer, aurone, is more effective against rice seedling rot disease than tricin itself, making it an ideal lead compound for new fungicide discovery.

Biological elicitors of plant secondary metabolites: mode of action and use in the production of nutraceutics

Many secondary metabolites of interest for human health and nutrition are produced by plants when they are under attack of microbial pathogens or insects. Treatment with elicitors derived from phytopathogens can be an effective strategy to increase the yield of specific metabolites obtained from plant cell cultures. Understanding how plant cells perceive microbial elicitors and how this perception leads to the accumulation of secondary metabolites, may help us improve the production of nutraceutics in terms of quantity and of quality of the compounds. The knowledge gathered in the past decades on elicitor perception and transduction is now being combined to high-throughput methodologies, such as transcriptomics and metabolomics, to engineer plant cells that produce compounds of interest at industrial scale.

Three flavonoids targeting the beta-hydroxyacyl-acyl carrier protein dehydratase from Helicobacter pylori: crystal structure characterization with enzymatic inhibition assay

Flavonoids are the major functional components of many herbal and insect preparations and demonstrate varied pharmacological functions including antibacterial activity. Here by enzymatic assay and crystal structure analysis, we studied the inhibition of three flavonoids (quercetin, apigenin, and (S)-sakuranetin) against the beta-hydroxyacyl-acyl carrier protein dehydratase from Helicobacter pylori (HpFabZ). These three flavonoids are all competitive inhibitors against HpFabZ by either binding to the entrance of substrate tunnel B (binding model A) or plugging into the tunnel C near the catalytic residues (binding model B) mainly by hydrophobic interaction and hydrogen-bond pattern. Surrounded by hydrophobic residues of HpFabZ at both positions of models A and B, the methoxy group at C-7 of (S)-sakuranetin seems to play an important role for the inhibitor's binding to HpFabZ, partly responsible for the higher inhibitory activity of (S)-sakuranetin than those of quercetin and apigenin against HpFabZ (IC(50) in microM: (S)-sakuranetin, 2.0 +/- 0.1; quercetin: 39.3 +/- 2.7; apigenin, 11.0 +/- 2.5). Our work is expected to supply useful information for understanding the potential antibacterial mechanism of flavonoids.

Fate and impact on microorganisms of rice allelochemicals in paddy soil

Allelopathic rice can release allelochemicals from roots to inhibit neighboring plant species, but little is currently known about their fate and impact on microorganisms in paddy soil. This study showed that allelopathic rice PI312777 released much higher concentrations of allelochemical (5,7,4'-trihydroxy-3',5'-dimethoxyflavone) than non-allelopathic rice Liaojing-9 in field. When quantitative 5,7,4'-trihydroxy-3',5'-dimethoxyflavone was added into soil, flavone gave a short half-life of 18.27 +/- 2.32 h (r(2) = 0.94) and could easily be degraded into benzoic acid. Benzoic acid with a half-life of 29.99 +/- 2.19 h (r(2) = 0.96) was more resistant toward degradation in paddy soil. Furthermore, both the culturable microbial population and the entire microbial community structure of soil incubated with flavone and benzoic acid were evaluated using the soil dilution plate method and phospholipid fatty acid (PLFA) analysis, respectively. It appeared from the results that flavone could reduce microorganisms especially for fungi present in paddy soil, while benzoic acid could induce a higher response for soil microorganisms especially for bacteria. Consequently, flavone would be responsible for the dynamics of soil microorganisms during the early period, and any observed effect during the late period would be very likely due to its degradation product benzoic acid rather than flavone itself. These results suggested that allelopathic rice varieties could modify soil microorganisms to their advantage through the release of allelochemicals. The concentration and fate of discriminating 5,7,4'-trihydroxy-3',5'-dimethoxyflavone between allelopathic and non-alleloparhic varieties tested in rice soil would result in the different patterns of microbial population and community structure in paddy ecosystems.

Suppressive components in rice husk against mutagens-induced SOS response usingSalmonella typhimuriumTA1535/pSK1002umutest

The EtOAc extract from rice (Oriza sativa cv. Hinohikari) husk showed a suppressive effect on umu gene expression of the SOS response in Salmonella typhimurium TA1535/pSK1002 against the mutagen, Trp-P-1, which requires liver metabolizing enzyme. To obtain the suppressive compound, the EtOAc extract was fractionated by SiO(2) column chromatography using umu test as a bioassay guide. Suppressive compound was isolated and identified as momilactone A (1) by EIMS, IR, (1)H- and (13)C-NMR spectroscopy. Compound 1 inhibited of the SOS-inducing activity of Trp-P-1 in the umu test. Gene expression was suppressed by 32.6% at less than 0.60 mM. Compound 1 was assayed with activated Trp-P-1. The suppressive effect of Compound 1 was decreased compared with that of Trp-P-1. Furthermore, 1 was assayed with another mutagens, such as MeIQ, activated MeIQ, furylfuramide (AF-2), MNNG, and UV-irradiation. Compound 1 showed greater suppressive effect on AF-2-inducing SOS response than other mutagens.

The role of ethylene in host-pathogen interactions

The phytohormone ethylene is a principal modulator in many aspects of plant life, including various mechanisms by which plants react to pathogen attack. Induced ethylene biosynthesis and subsequent intracellular signaling through a single conserved pathway have been well characterized. This leads to a cascade of transcription factors consisting of primary EIN3-like regulators and downstream ERF-like transcription factors. The latter control the expression of various effector genes involved in various aspects of systemic induced defense responses. Moreover, at this level significant cross-talk occurs with other defense response pathways controlled by salicylic acid and jasmonate, eventually resulting in a differentiated disease response.

Biosynthesis of rice phytoalexin: enzymatic conversion of 3beta-hydroxy-9beta-pimara-7,15-dien-19,6beta-olide to momilactone A

Momilactone A, a major rice diterpene phytoalexin, could be synthesized by dehydrogenation at the 3-position of 3beta-hydroxy-9beta-pimara-7,15-dien-19,6beta-olide in rice leaves. The presence of 3beta-hydroxy-9beta-pimara-7,15-dien-19,6beta-olide in UV-irradiated rice leaves was confirmed by comparing the mass spectra and retention times after a GC/MS analysis of the natural and synthetic compounds. The soluble protein fraction from UV-irradiated rice leaves showed dehydrogenase activity to convert 3beta-hydroxy-9beta-pimara-7,15-dien-19,6beta-olide into momilactone A. The enzyme required NAD+ or NADP+ as a hydrogen acceptor. The optimum pH for the reaction was 8. The Km value to 3beta-hydroxy-9beta-pimara-7,15-dien-19,6beta-olide was 36 microM when NAD+ was supplied as a cofactor at a concentration of 1 mM. 3fl-Hydroxy-9beta-pimara-7,15-dien-19,6beta-olide and its dehydrogenase activity were induced in a time-dependent manner by UV irradiation.