Characterization of RCI-1, a chloroplastic rice lipoxygenase whose synthesis is induced by chemical plant resistance activators

ARGONAUTE4 and the DNA demethylase REPRESSOR OF SILENCING 1C mediate dehydroascorbate-induced intergenerational nematode resistance in rice

Plants can transmit information to the next generation and modulate the phenotype of their offspring through epigenetic mechanisms. In this study, we demonstrate the activation of "intergenerational acquired resistance" (IAR) in the progeny of rice (Oryza sativa) plants exogenously treated with dehydroascorbate (DHA). The offspring of lifelong DHA-treated plants (DHA-IAR) were significantly less susceptible to the root-knot nematode Meloidogyne graminicola and partially inherited the DHA-induced transcriptional response found in the parental plants. Phytohormone analyses on the DHA-IAR plants unveiled higher basal abscisic acid levels and a primed induction of the jasmonic acid pathway. RNA-seq analysis on the embryonic tissues of immature seeds of DHA-treated plants revealed major shifts in the expression of genes associated with epigenetic pathways. We confirmed that DHA treatment leads to a significant but transient pattern of global DNA hypomethylation in the parental plants 12 to 24 h after treatment. The induction of resistance in the parental plants requires the DNA demethylase REPRESSOR OF SILENCING 1C (ROS1c) and ARGONAUTE 4, suggesting a role for DNA demethylation and subsequent remethylation in establishment of this phenotype. Confirming the transience of global hypomethylation upon DHA treatment, no significant change in global DNA methylation levels was observed in DHA-IAR versus naïve plants. Finally, DHA could not induce IAR in the ros1c mutant line and the ARGONAUTE 4 (ago4ab)-RNAi line. These data indicate that a controlled collaboration between transient DNA demethylation and remethylation underlies the induced resistance and IAR phenotypes upon DHA treatment.

OsRCI-1-Mediated GLVs Enhance Rice Resistance to Brown Planthoppers

Green leaf volatiles (GLVs) play pivotal roles in plant anti-herbivore defense. This study investigated whether the rice 13-lipoxygense gene OsRCI-1 is involved in GLV production and plant defense in rice. The overexpression of OsRCI-1 (oeRCI lines) in rice resulted in increased wound-induced levels of two prominent GLVs, cis-3-hexen-1-ol and cis-3-hexenal. In a previous study, we found that the overexpression of OsRCI-1 reduced the colonization by the rice brown planthopper (BPH, Nilaparvata lugens) but increased the attractiveness to the egg parasitoid Anagrus nilaparvatae compared to wild-type (WT) plants. This study found that when cis-3-hexen-1-ol, but not cis-3-hexenal, was added to WT plants, it could change the BPH's colonization preference, i.e., more BPHs preferred to colonize the oeRCI lines. The exogenous application of cis-3-hexen-1-ol or cis-3-hexenal to BPH-infested WT plants could weaken or overturn the preference of A. nilaparvatae for oeRCI lines. However, field experiments revealed that only cis-3-hexenal was attractive to the parasitoid and increased the parasitism rates of BPH eggs. These results indicate that OsRCI-1 is involved in rice GLV production and therefore modulates both direct and indirect defense in rice.

Chemopriming for induction of disease resistance against pathogens in rice

Rice is an important grain crop of Asian population. Different fungal, bacterial and viral pathogens cause large reduction in rice grain production. Use of chemical pesticides, to provide protection against pathogens, has become incomplete due to pathogens resistance and is cause of environmental concerns. Therefore, induction of resistance in rice against pathogens via biopriming and chemopriming with safe and novel agents has emerged on a global level as ecofriendly alternatives that provide protection against broad spectrum of rice pathogens without any significant yield penalty. In the past three decades, a number of chemicals such as silicon, salicylic acid, vitamins, plant extract, phytohormones, nutrients etc. have been used to induce defense against bacterial, fungal and viral rice pathogens. From the detailed analysis of abiotic agents used, it has been observed that silicon and salicylic acid are two potential chemicals for inducing resistance against fungal and bacterial diseases in rice, respectively. However, an inclusive evaluation of the potential of different abiotic agents to induce resistance against rice pathogens is lacking due to which the studies on induction of defense against rice pathogens via chemopriming has become disproportionate and discontinuous. The present review deals with a comprehensive analysis of different abiotic agents used to induce defense against rice pathogens, their mode of application, mechanism of defense induction and the effect of defense induction on grain yield. It also provides an account of unexplored areas, which might be taken into attention to efficiently manage rice diseases. DATA AVAILABILITY STATEMENT: Data sharing not applicable to this article as no datasets were generated or analysed during the current study.

The lipoxygenase gene OsRCI-1 is involved in the biosynthesis of herbivore-induced JAs and regulates plant defense and growth in rice

The pathway mediated by jasmonic acid (JA), biosynthesized via 13-lipoxygenases (LOX), plays a central role in both plant development and defense. In rice, there are at least fourteen 13-LOXs. Yet, only two 13-LOXs have been known to be involved in the biosynthesis of JA and plant defenses in rice. Here we cloned a chloroplast-localized 13-LOX gene from rice, OsRCI-1, whose transcripts were upregulated following infestation by brown planthopper (BPH, Nilaparvata lugens), one of the most important pests in rice. Overexpression of OsRCI-1 (oeRCI lines) increased levels of BPH-induced JA, jasmonate-isoleucine, trypsin protease inhibitors and three volatile compounds, 2-heptanone, 2-heptanol and α-thujene. BPHs showed a decreased colonization, fecundity and mass, and developed slowly on oeRCI plants compared with wild-type (WT) plants. Moreover, BPH-infested oeRCI plants were more attractive to the egg parasitoid of BPH, Anagrus nilaparvatae than equally treated WT plants. The decreased attractiveness to BPH and enhanced attractiveness to the parasitoid of oeRCI plants correlated with higher levels of BPH-induced 2-heptanone and 2-heptanol, and 2-heptanone, respectively. Compared with oeRCI plants, WT plants had higher plant height and 1000-grain weight. These results indicate that OsRCI-1 is involved in herbivore-induced JA bursts and plays a role in plant defense and growth.

Biochemical Characterization of 13-Lipoxygenases of Arabidopsis thaliana

13-lipoxygenases (13-LOX) catalyze the dioxygenation of various polyunsaturated fatty acids (PUFAs), of which α-linolenic acid (LeA) is converted to 13-S-hydroperoxyoctadeca-9, 11, 15-trienoic acid (13-HPOT), the precursor for the prostaglandin-like plant hormones cis-(+)-12-oxophytodienoic acid (12-OPDA) and methyl jasmonate (MJ). This study aimed for characterizing the four annotated A. thaliana 13-LOX enzymes (LOX2, LOX3, LOX4, and LOX6) focusing on synthesis of 12-OPDA and 4Z,7Z,10Z)-12-[[-(1S,5S)-4-oxo-5-(2Z)-pent-2-en-1yl] cyclopent-2-en-1yl] dodeca-4,7,10-trienoic acid (OCPD). In addition, we performed interaction studies of 13-LOXs with ions and molecules to advance our understanding of 13-LOX. Cell imaging indicated plastid targeting of fluorescent proteins fused to 13-LOXs-N-terminal extensions, supporting the prediction of 13-LOX localization to plastids. The apparent maximal velocity (V_max app) values for LOX-catalyzed LeA oxidation were highest for LOX4 (128 nmol·s⁻¹·mg protein⁻¹), with a K_m value of 5.8 µM. A. thaliana 13-LOXs, in cascade with 12-OPDA pathway enzymes, synthesized 12-OPDA and OCPD from LeA and docosahexaenoic acid, previously shown only for LOX6. The activities of the four isoforms were differently affected by physiologically relevant chemicals, such as Mg²⁺, Ca²⁺, Cu²⁺ and Cd²⁺, and by 12-OPDA and MJ. As demonstrated for LOX4, 12-OPDA inhibited enzymatic LeA hydroperoxidation, with half-maximal enzyme inhibition at 48 µM. Biochemical interactions, such as the sensitivity of LOX toward thiol-reactive agents belonging to cyclopentenone prostaglandins, are suggested to occur in human LOX homologs. Furthermore, we conclude that 13-LOXs are isoforms with rather specific functional and regulatory enzymatic features.

Mobilization of storage lipid reserve and expression analysis of lipase and lipoxygenase genes in rice (Oryza sativa var. Pusa Basmati 1) bran during germination

Storage lipid mobilization by lipases and lipoxygenases (LOXs) in response to developmental cues take place during seed germination. After rice grain milling, the endogenous lipases and LOXs present in the bran fraction come in contact with the storage lipid reserve or triacylglycerol (TAG). Lipases catalyze the hydrolysis of TAGs to non-esterified fatty acids (NEFAs) and glycerol. The NEFAs, especially linoleic acid (18:2) produced, are further subjected to oxidative rancidity via peroxidation reaction catalyzed by LOXs. This results in the production of conjugated hydroperoxides of 18:2 that influence the off-flavors in rice bran lipids. The aim of this study is to understand how lipid mobilization and expression of lipase and LOX genes occur in the bran of germinating rice grains (Oryza sativavar. Pusa Basmati 1). Our results show that the primary source of storage lipids in bran is TAG, and its mobilization starts at 4 days after imbibition (4 DAI). Using publically available RNA-seq data and phylogeny analyses, we selected a total of 18 lipase and 16 LOX genes in rice for their expression profiles during onset of lipid mobilization. Gene expression analyses revealed OsLip1, OsLip9, and OsLip13; and OsLOX3 and OsLOX14 as the predominantly expressed genes in bran of germinating rice grains. This study explores two important events in the germinating rice grains, namely, mobilization of storage lipids and expression pattern of lipase and LOX genes. The information generated in this study can be used to efficiently manipulate the genes to enhance the shelf-stability of bran lipid reserve.

Recombinant Lipoxygenases and Hydroperoxide Lyases for the Synthesis of Green Leaf Volatiles

Green leaf volatiles (GLVs) are mainly C₆- and in rare cases also C₉-aldehydes, -alcohols, and -esters, which are released by plants in response to biotic or abiotic stresses. These compounds are named for their characteristic smell reminiscent of freshly mowed grass. This review focuses on GLVs and the two major pathway enzymes responsible for their formation: lipoxygenases (LOXs) and fatty acid hydroperoxide lyases (HPLs). LOXs catalyze the peroxidation of unsaturated fatty acids, such as linoleic and α-linolenic acids. Hydroperoxy fatty acids are further converted by HPLs into aldehydes and oxo-acids. In many industrial applications, plant extracts have been used as LOX and HPL sources. However, these processes are limited by low enzyme concentration, stability, and specificity. Alternatively, recombinant enzymes can be used as biocatalysts for GLV synthesis. The increasing number of well-characterized enzymes efficiently expressed by microbial hosts will foster the development of innovative biocatalytic processes for GLV production.

Alternative Splicing of Key Genes in LOX Pathway Involves Biosynthesis of Volatile Fatty Acid Derivatives in Tea Plant (Camellia sinensis)

Volatile fatty acid derivatives (VFADs) produced in tea plants (Camellia sinensis) not only have been shown to function as defense compounds but also impart a "fresh green" odor to green tea products; however, little is known about alternative splicing (AS) of genes in regulating the production of VFADs in plants. In this study, the contents of VFADs and corresponding transcriptome profiles were obtained in five different months (April, June, August, September, and October). Correlation analysis identified seven unique transcripts of enzyme-coding genes (CsLOX2, CsLOX4, CsADH4, CsADH8, and CsADH10), which are responsible for regulating VFAD biosynthesis; four AS transcripts of these genes (CsLOX2, CsLOX4, CsADH4, and CsADH8) were validated by RT-PCR. By employing the gene-specific antisense oligodeoxynucleotide-mediated reduction method, we found the expression levels of alternatively spliced transcripts of CsLOX4-iso1, CsLOX4-iso2, and CsADH4-iso3 were lower, and the contents of cis-3-hexenol were correspondingly reduced in the leaves of tea plant; this result suggested that the AS play important roles in regulating biosynthesis of VFADs in C. sinensis. Our results provide new insights into the important contribution of AS events in regulating the VFAD biosynthesis in tea plant.

Transcriptome sequencing of Festulolium accessions under salt stress

Objectives

The objective of this study was to establish transcriptome assemblies of Festulolium hybrids under salt stress, and identify genes regulated across the hybrids in response to salt stress. The development of transcriptome assemblies for Festulolium hybrids and cataloguing of genes regulated under salt stress will facilitate further downstream studies.

Results

Plants were grown at three salt concentrations (0.5%, 1% and 1.5%) and phenotypic and transcriptomic data was collected. Salt stress was confirmed by progressive loss of green leaves as salt concentration increased from 0 to 1.5%. We generated de-novo transcriptome assemblies for two Festulolium pabulare festucoid genotypes, for a single Festulolium braunii genotype, and a single F. pabulare loloid genotype. We also identified 1555 transcripts that were up regulated and 1264 transcripts that were down regulated in response to salt stress in the Festulolium hybrids. Some of the identified transcripts showed significant sequence similarity with genes known to be regulated during salt and other abiotic stresses.

Electronic supplementary material

The online version of this article (10.1186/s13104-019-4349-2) contains supplementary material, which is available to authorized users.

Characterization and Alternative Splicing Profiles of the Lipoxygenase Gene Family in Tea Plant (Camellia sinensis)

Oxylipins, including jasmonic acid (JA) and volatiles, are important for signaling in plants, and these are formed by the lipoxygenase (LOX) enzyme family. There is a large gap in understanding of the underlying molecular basis of their roles in tea plants. Here, we identified 11 CsLOX genes from the tea plant (Camellia sinensis), and characterized their phylogeny, gene structure and protein features into three subclasses. We then examined their enzymatic activities, LOX expression and alternative splicing of transcripts during development and in response to abiotic or biotic stresses in tea plants. In vitro expressed protein assays showed that the CsLOX2, 3 and 9 enzymatically function to produce 9/13-HPOT, 13-HPOT and 9-HPOT, respectively. CsLOX2 and CsLOX9 green fluorescent protein (GFP) fusion proteins localized to chloroplasts and the cytoplasm, respectively. RNA sequencing, quantitative reverse transcription-PCR and Northern blot analysis suggested that CsLOX5, 6 and 9 were predominantly expressed in seeds, flowers and roots, respectively. CsLOX2, 3, 4, 6 and 7 were up-regulated after attack by the insect Ectropis oblique, while CsLOX1 was induced after infection with the pathogen Glomerella cingulata. CsLOX3, 7 and 10 were up-regulated by JA but not ABA or salicylic acid. Long-term cold stress down-regulated CsLOX expression while a short duration of cold induced the expression of CsLOX1, 6 and 7. Alternatively spliced transcripts of six CsLOX genes were dynamically regulated through time and varied in relative abundances under the investigated stresses; we propose a mechanism of competing or compensating regulation between isoforms. This study improves our understanding of evolution of LOXs and regulation of their diverse functions in plants.

Frameshift Mutation Confers Function as Virulence Factor to Leucine-Rich Repeat Protein from Acidovorax avenae

Many plant pathogens inject type III (T3SS) effectors into host cells to suppress host immunity and promote successful infection. The bacterial pathogen Acidovorax avenae causes brown stripe symptom in many species of monocotyledonous plants; however, individual strains of each pathogen infect only one host species. T3SS-deleted mutants of A. avenae K1 (virulent to rice) or N1141 (virulent to finger millet) caused no symptom in each host plant, suggesting that T3SS effectors are involved in the symptom formation. To identify T3SS effectors as virulence factors, we performed whole-genome and predictive analyses. Although the nucleotide sequence of the novel leucine-rich repeat protein (Lrp) gene of N1141 had high sequence identity with K1 Lrp, the amino acid sequences of the encoded proteins were quite different due to a 1-bp insertion within the K1 Lrp gene. An Lrp-deleted K1 strain (KΔLrp) did not cause brown stripe symptom in rice (host plant for K1); by contrast, the analogous mutation in N1141 (NΔLrp) did not interfere with infection of finger millet. In addition, NΔLrp retained the ability to induce effector-triggered immunity (ETI), including hypersensitive response cell death and expression of ETI-related genes. These data indicated that K1 Lrp functions as a virulence factor in rice, whereas N1141 Lrp does not play a similar role in finger millet. Yeast two-hybrid screening revealed that K1 Lrp interacts with oryzain α, a pathogenesis-related protein of the cysteine protease family, whereas N1141 Lrp, which contains LRR domains, does not. This specific interaction between K1 Lrp and oryzain α was confirmed by Bimolecular fluorescence complementation assay in rice cells. Thus, K1 Lrp protein may have acquired its function as virulence factor in rice due to a frameshift mutation.

Lessons from the Far End: Caterpillar FRASS-Induced Defenses in Maize, Rice, Cabbage, and Tomato

Plant defenses to insect herbivores have been studied in response to several insect behaviors on plants such as feeding, crawling, and oviposition. However, we have only scratched the surface about how insect feces induce plant defenses. In this study, we measured frass-induced plant defenses in maize, rice, cabbage, and tomato by chewing herbivores such as European corn borer (ECB), fall armyworm (FAW), cabbage looper (CL), and tomato fruit worm (TFW). We observed that caterpillar frass induced plant defenses are specific to each host-herbivore system, and they may induce herbivore or pathogen defense responses in the host plant depending on the composition of the frass deposited on the plant, the plant organ where it is deposited, and the species of insect. This study adds another layer of complexity in plant-insect interactions where analysis of frass-induced defenses has been neglected even in host-herbivore systems where naturally frass accumulates in enclosed feeding sites over extended periods of time.

Functional Characterization of 9-/13-LOXs in Rice and Silencing Their Expressions to Improve Grain Qualities

Lipoxygenases (LOXs) are involved in oxidative rancidity and render rice unsuitable for human consumption. Here, RNA interference- (RNAi-) induced gene expression inhibition was used to analyze the functions of the bran/seed-specific LOXs in rice. r9-LOX1 and L-2 (9-LOX category) were the candidate genes expressing a bran/seed-specific LOX, while RCI-1 was (13-LOX category) a plastid-specific LOX. Real-time PCR showed that three LOXs were cultivar/tissue specific expression on a certain level. r9-LOX1 and L-2 were generally much higher in active bran/seed than in stabilized bran, mature seed, and regenerated plant. RCI-1 was barely expressed in seed. In transgenic lines, r9-LOX1, as well as L-2, expression was dramatically downregulated, compared to the nontransgenic controls. SPME/GC-MS analysis of r9-LOX1 RNAi transgenic lines showed 74.33% decrease in nonanal content (formed during oxidation of linoleic acid by lipoxygenase), but 388.24% increase in acetic acid and 184.84% hexanal (direct products of 13-LOX). These results indicate that r9-LOX1 positively regulates the amount of nonanal but negatively regulates acetic acid and hexanal. The negative regulation may be due to a mechanism of negative feedback between LOX family members. The information will help comprehensively understand the function of the bran/seed-specific LOXs, r9-LOX1, and improve the storage quality in the future.

The 9-lipoxygenase Osr9-LOX1 interacts with the 13-lipoxygenase-mediated pathway to regulate resistance to chewing and piercing-sucking herbivores in rice

Oxylipins produced by the 13-lipoxygenase (LOX) have been reported to play an important role in plant defense responses to herbivores. Yet, the role of oxylipins produced by the 9-LOX pathway in this process remains largely unknown. Here we cloned a gene encoding a chloroplast-localized 9-LOX, Osr9-LOX1, from rice. Transcriptional analysis revealed that herbivore infestation, mechanical wounding and jasmonic acid (JA) treatment either repressed or did not enhance the level of Osr9-LOX1 transcripts at early stages but did at later stages, whereas salicylic acid (SA) treatment quickly increased the transcript level of Osr9-LOX1. Antisense expression of Osr9-lox1 (as-r9lox1) decreased the amount of wound-induced (Z)-3-hexenal but increased levels of striped stem borer (SSB)-induced linolenic acid, JA, SA and trypsin protease inhibitors. These changes were associated with increased resistance in rice to the larvae of the SSB Chilo suppressalis. In contrast, although no significant differences were observed in the duration of the nymph stage or the number of eggs laid by female adults between the brown planthopper (BPH) Nilaparvata lugens that fed on as-r9lox1 lines and BPH that fed on wild-type (WT) rice plants, the survival rate of BPH nymphs that fed on as-r9lox1 lines was higher than that of nymphs that fed on WT plants, possibly because of a higher JA level. The results demonstrate that Osr9-LOX1 plays an important role in regulating an herbivore-induced JA burst and cross-talk between JA and SA, and in controlling resistance in rice to chewing and phloem-feeding herbivores.

Modulating rice stress tolerance by transcription factors

Plants are non-mobile organisms and have to adapt to environmental stresses mostly by modulating their growth and development in addition to physiological and biochemical changes. Transcription factors (TFs) regulate genome expression in response to environmental and physiological signals, and some of them switch on plant adaptive developmental and physiological pathways. One TF is encoded by a single gene but regulates the expression of several other genes leading to the activation of complex adaptive mechanisms and hence represents major molecular targets to genetically improve the tolerance of crop plants against different stresses. In this review an updated account of the discovery of TFs involved in biotic and abiotic stress tolerance in the model monocotyledonous plant, rice (Oryza sativa L.) is presented. We illustrate how the elucidation of the function of these TFs can be used to set up genetic engineering strategies and to rationalize molecular breeding using molecular assisted selection towards enhancement of rice tolerance to various stresses. Attempts have also been made to provide information on the molecular mechanisms involved in stress resistance or tolerance processes. We discuss how the comparison of the action of TFs isolated from the dicotyledonous model plant Arabidopsis thaliana in rice and vice versa can contribute to determine whether common or divergent mechanisms underlie stress tolerance in the two plant species. Lastly, we discuss the necessity to discover TFs controlling specifically the root adaptive development which constitutes a major way for the plant to escape to several stresses such as water deficit or mineral nutrient deficiency.

Preformed expression of defense is a hallmark of partial resistance to rice blast fungal pathogen Magnaporthe oryzae

BACKGROUND

Partial resistance to plant pathogens is extensively used in breeding programs since it could contribute to resistance durability. Partial resistance often builds up during plant development and confers quantitative and usually broad-spectrum resistance. However, very little is known on the mechanisms underlying partial resistance. Partial resistance is often explained by poorly effective induction of plant defense systems. By exploring rice natural diversity, we asked whether expression of defense systems before infection could explain partial resistance towards the major fungal pathogen Magnaporthe oryzae. The constitutive expression of 21 defense-related genes belonging to the defense system was monitored in 23 randomly sampled rice cultivars for which partial resistance was measured.

RESULTS

We identified a strong correlation between the expression of defense-related genes before infection and partial resistance. Only a weak correlation was found between the induction of defense genes and partial resistance. Increasing constitutive expression of defense-related genes also correlated with the establishment of partial resistance during plant development. Some rice genetic sub-groups displayed a particular pattern of constitutive expression, suggesting a strong natural polymorphism for constitutive expression of defense. Constitutive levels of hormones like salicylic acid and ethylene cannot explain constitutive expression of defense. We could identify an area of the genome that contributes to explain both preformed defense and partial resistance.

CONCLUSION

These results indicate that constitutive expression of defense-related genes is likely responsible for a large part of partial resistance in rice. The finding of this preformed defense system should help guide future breeding programs and open the possibility to identify the molecular mechanisms behind partial resistance.

Gain of deleterious function causes an autoimmune response and Bateson-Dobzhansky-Muller incompatibility in rice

Reproductive isolation plays an important role in speciation as it restricts gene flow and accelerates genetic divergence between formerly interbreeding population. In rice, hybrid breakdown is a common reproductive isolation observed in both intra and inter-specific crosses. It is a type of post-zygotic reproductive isolation in which sterility and weakness are manifested in the F(2) and later generations. In this study, the physiological and molecular basis of hybrid breakdown caused by two recessive genes, hbd2 and hbd3, in a cross between japonica variety, Koshihikari, and indica variety, Habataki, were investigated. Fine mapping of hbd2 resulted in the identification of the causal gene as casein kinase I (CKI1). Further analysis revealed that hbd2-CKI1 allele gains its deleterious function that causes the weakness phenotype by a change of one amino acid. As for the other gene, hbd3 was mapped to the NBS-LRR gene cluster region. It is the most common class of R-gene that triggers the immune signal in response to pathogen attack. Expression analysis of pathogen response marker genes suggested that weakness phenotype in this hybrid breakdown can be attributed to an autoimmune response. So far, this is the first evidence linking autoimmune response to post-zygotic isolation in rice. This finding provides a new insight in understanding the molecular and evolutionary mechanisms establishing post-zygotic isolation in plants.

Role of hydroperoxide lyase in white-backed planthopper (Sogatella furcifera Horváth)-induced resistance to bacterial blight in rice, Oryza sativa L

A pre-infestation of the white-backed planthopper (WBPH), Sogatella furcifera Horváth, conferred resistance to bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) in rice (Oryza sativa L.) under both laboratory and field conditions. The infestation of another planthopper species, the brown planthopper (BPH) Nilaparvata lugens Stål, did not significantly reduce the incidence of bacterial blight symptoms. A large-scale screening using a rice DNA microarray and quantitative RT-PCR revealed that WBPH infestation caused the upregulation of more defence-related genes than did BPH infestation. Hydroperoxide lyase 2 (OsHPL2), an enzyme for producing C(6) volatiles, was upregulated by WBPH infestation, but not by BPH infestation. One C(6) volatile, (E)-2-hexenal, accumulated in rice after WBPH infestation, but not after BPH infestation. A direct application of (E)-2-hexenal to a liquid culture of Xoo inhibited the growth of the bacterium. Furthermore, a vapour treatment of rice plants with (E)-2-hexenal induced resistance to bacterial blight. OsHPL2-overexpressing transgenic rice plants exhibited increased resistance to bacterial blight. Based on these data, we conclude that OsHPL2 and its derived (E)-2-hexenal play some role in WBPH-induced resistance in rice.

A rare sugar, d-allose, confers resistance to rice bacterial blight with upregulation of defense-related genes in Oryza sativa

We investigated responses of rice plant to three rare sugars, d-altrose, d-sorbose, and d-allose, due to establishment of mass production methods for these rare sugars. Root growth and shoot growth were significantly inhibited by d-allose but not by the other rare sugars. A large-scale gene expression analysis using a rice microarray revealed that d-allose treatment causes a high upregulation of many defense-related, pathogenesis-related (PR) protein genes in rice. The PR protein genes were not upregulated by other rare sugars. Furthermore, d-allose treatment of rice plants conferred limited resistance of the rice against the pathogen Xanthomonas oryzae pv. oryzae but the other tested sugars did not. These results indicate that d-allose has a growth inhibitory effect but might prove to be a candidate elicitor for reducing disease development in rice.

Silencing OsHI-LOX makes rice more susceptible to chewing herbivores, but enhances resistance to a phloem feeder

The jasmonic acid (JA) pathway plays a central role in plant defense responses against insects. Some phloem-feeding insects also induce the salicylic acid (SA) pathway, thereby suppressing the plant's JA response. These phenomena have been well studied in dicotyledonous plants, but little is known about them in monocotyledons. We cloned a chloroplast-localized type 2 13-lipoxygenase gene of rice, OsHI-LOX, whose transcripts were up-regulated in response to feeding by the rice striped stem borer (SSB) Chilo suppressalis and the rice brown planthopper (BPH) Niaparvata lugens, as well as by mechanical wounding and treatment with JA. Antisense expression of OsHI-LOX (as-lox) reduced SSB- or BPH-induced JA and trypsin protease inhibitor (TrypPI) levels, improved the larval performance of SBB as well as that of the rice leaf folder (LF) Cnaphalocrocis medinalis, and increased the damage caused by SSB and LF larvae. In contrast, BPH, a phloem-feeding herbivore, showed a preference for settling and ovipositing on WT plants, on which they consumed more and survived better than on as-lox plants. The enhanced resistance of as-lox plants to BPH infestation correlated with higher levels of BPH-induced H(2)O(2) and SA, as well as with increased hypersensitive response-like cell death. These results imply that OsHI-LOX is involved in herbivore-induced JA biosynthesis, and plays contrasting roles in controlling rice resistance to chewing and phloem-feeding herbivores. The observation that suppression of JA activity results in increased resistance to an insect indicates that revision of the generalized plant defense models in monocotyledons is required, and may help develop novel strategies to protect rice against insect pests.

Molecular tagging of the Bph1 locus for resistance to brown planthopper (Nilaparvata lugens Stål) through representational difference analysis

During brown planthopper (BPH) feeding on rice plants, we employed a modified representational difference analysis (RDA) method to detect rare transcripts among those differentially expressed in SNBC61, a BPH resistant near-isogenic line (NIL) carrying the Bph1 resistance gene. This identified 3 RDA clones: OsBphi237, OsBphi252 and OsBphi262. DNA gel-blot analysis revealed that the loci of the RDA clones in SNBC61 corresponded to the alleles of the BPH resistant donor Samgangbyeo. Expression analysis indicated that the RDA genes were up-regulated in SNBC61 during BPH feeding. Interestingly, analysis of 64 SNBC NILs, derived from backcrosses of Samgangbyeo with a BPH susceptible Nagdongbyeo, using a cleaved amplified polymorphic sequence (CAPS) marker indicated that OsBphi252, which encodes a putative lipoxygenase (LOX), co-segregates with BPH resistance. Our results suggest that OsBphi252 is tightly linked to Bph1, and may be useful in marker-assisted selection (MAS) for resistance to BPH.

A novel lipoxygenase gene from developing rice seeds confers dual position specificity and responds to wounding and insect attack

OsLOX1 is a novel full-length cDNA isolated from developing rice seeds. We have examined its biochemical properties and expression patterns. The protein has dual positional specificity, as it releases both C-9 and C-13 oxidized products in a 4:3 ratio. OsLOX1 transcripts were detected at low abundance in immature seeds and newly germinated seedlings, but accumulate rapidly and transiently in response to wounding or brown planthopper (BPH) attack, reaching a peak 3 h after wounding and 6 h after insect feeding. We produced transgenic rice lines carrying either sense or antisense constructs under the control of a cauliflower mosaic virus 35S promoter, and these rice lines showed altered OsLOX1 activity. In all of the antisense lines and more than half of the sense lines the expression levels of OsLOX1, the levels of enzyme activity, and the levels of the endogenous OsLOX1 products (jasmonic acid, (Z)-3-hexenal and colneleic acid) at 6, 48, and 48 h after BPH feeding respectively, were below the levels found in non-transgenic control plants; yet, the levels in the remaining sense transformants were enhanced relative to controls. Transformants with a lower level of OsLOX1 expression were less able to tolerate BPH attack, while those with enhanced OsLOX1 expression were more resistant. Our data suggest that the OsLOX1 product is involved in tolerance of the rice plant to wounding and BPH attack.

Proteomic analysis of rice defense response induced by probenazole

Here, we report the first proteomic analysis of rice defense response induced by probenazole (PBZ), an agricultural chemical that has been widely used to protect rice plants from rice blast and the bacterial blight pathogen. Two-dimensional gel electrophoresis (2-DE) was utilized to identify a total of 40 protein spots including 9 protein spots that are up-regulated by PBZ and 31 abundant protein spots. A total of 11 unique proteins from these 9 spots were identified by LC-MS/MS, and the majority of them were classified and/or possessed orthologs in defense-related functions. Five protein spots with only one protein species identified in each spot appear to be PBZ-regulated proteins. They are a putative glutathione S-transferase GSTU17, a putative phenylalanine ammonia-lyase (PAL, XP_466843), a putative caffeic acid 3-O-methyltransferase (COMT), a putative NADH-ubiquinone oxidoreductase, and a putative glucose-1-phosphate adenyltransferase. However, the other six protein species identified from the remaining four protein spots could not be conclusively described as PBZ-regulated proteins due to either the co-migration of two protein species in one spot or the presence of one protein species in two spots. Through real-time reverse transcription polymerase chain reaction (RT-PCR), it was determined that PAL (XP_466843) is likely regulated at the protein level, whereas GSTU17 and COMT were regulated at the mRNA level after PBZ application. Interestingly, the mRNA transcripts of two PAL paralogs were found to be up-regulated by PBZ. We propose that PAL, COMT, and GSTU17 are likely to confer PBZ-induced disease resistance via such functions as biosynthesis and transport of flavonoid-type phytoalexin and/or lignin biogenesis.

Wounding and pathogen infection induce a chloroplast-targeted lipoxygenase in the common bean (Phaseolus vulgaris L.)

Chloroplastic LOXs are implicated in the biosynthesis of oxylipins like jasmonic acid and C6 volatiles among others. In this study, we isolated the cDNA of a novel chloroplast-targeted Phaseolus vulgaris LOX, (PvLOX6). This gene is highly induced after wounding, non-host pathogen infection, and by signaling molecules as H2O2, SA, ethylene and MeJA. The phylogenetic analysis of PvLOX6 showed that it is closely related to chloroplast-targeted LOX from potato (H1) and tomato (TomLOXC); both of them are implicated in the biosynthesis of C6 volatiles. Induction of PvLOX6 mRNA by wounding ethylene and jasmonic acid on the one side, and non-host pathogen, salicylic acid on the other indicates that common bean uses the same LOX to synthesize oxylipins in response to different stresses.

Rice WRKY45 plays a crucial role in benzothiadiazole-inducible blast resistance

Benzothiadiazole (BTH) is a so-called plant activator and protects plants from diseases by activating the salicylic acid (SA) signaling pathway. By microarray screening, we identified BTH- and SA-inducible WRKY transcription factor (TF) genes that were upregulated within 3 h after BTH treatment. Overexpression of one of them, WRKY45, in rice (Oryza sativa) markedly enhanced resistance to rice blast fungus. RNA interference-mediated knockdown of WRKY45 compromised BTH-inducible resistance to blast disease, indicating that it is essential for BTH-induced defense responses. In a transient expression system, WRKY45 activated reporter gene transcription through W-boxes. Epistasis analysis suggested that WRKY45 acts in the SA signaling pathway independently of NH1, a rice ortholog of Arabidopsis thaliana NPR1, which distinguishes WRKY45 from known Arabidopsis WRKY TFs. Two defense-related genes, encoding a glutathione S-transferase and a cytochrome P450, were found to be regulated downstream of WRKY45 but were not regulated by NH1, consistent with the apparent independence of the WRKY45- and NH1-dependent pathways. Defense gene expression in WRKY45-overexpressed rice plants varied with growth conditions, suggesting that some environmental factor(s) acts downstream of WRKY45 transcription. We propose a role for WRKY45 in BTH-induced and SA-mediated defense signaling in rice and its potential utility in improving disease resistance of rice, an importance food resource worldwide.

Functional analysis of rice NPR1-like genes reveals that OsNPR1/NH1 is the rice orthologue conferring disease resistance with enhanced herbivore susceptibility

The key regulator of salicylic acid (SA)-mediated resistance, NPR1, is functionally conserved in diverse plant species, including rice (Oryza sativa L.). Investigation in depth is needed to provide an understanding of NPR1-mediated resistance and a practical strategy for the improvement of disease resistance in the model crop rice. The rice genome contains five NPR1-like genes. In our study, three rice homologous genes, OsNPR1/NH1, OsNPR2/NH2 and OsNPR3, were found to be induced by rice bacterial blight Xanthomonas oryzae pv. oryzae and rice blast Magnaporthe grisea, and the defence molecules benzothiadiazole, methyl jasmonate and ethylene. We confirmed that OsNPR1 is the rice orthologue by complementing the Arabidopsis npr1 mutant. Over-expression of OsNPR1 conferred disease resistance to bacterial blight, but also enhanced herbivore susceptibility in transgenic plants. The OsNPR1-green fluorescent protein (GFP) fusion protein was localized in the cytoplasm and moved into the nucleus after redox change. Mutations in its conserved cysteine residues led to the constitutive localization of OsNPR1(2CA)-GFP in the nucleus and also abolished herbivore hypersensitivity in transgenic rice. Different subcellular localizations of OsNPR1 antagonistically regulated SA- and jasmonic acid (JA)-responsive genes, but not SA and JA levels, indicating that OsNPR1 might mediate antagonistic cross-talk between the SA- and JA-dependent pathways in rice. This study demonstrates that rice has evolved an SA-mediated systemic acquired resistance similar to that in Arabidopsis, and also provides a practical approach for the improvement of disease resistance without the penalty of decreased herbivore resistance in rice.

Competence of roots for race-specific resistance and the induction of acquired resistance against Magnaporthe oryzae

SUMMARY Generally, Magnaporthe oryzae, the causal agent of rice blast disease, is considered to be a typical leaf-infecting plant pathogenic fungus. However, it was recently reported that M. oryzae shares many characteristics in common with root-infecting pathogens and indeed was able to infect roots. Here, we report on studies testing for the capacity of roots of rice and barley to resist infections with M. oryzae. We established that roots of rice plants were colonized by M. oryzae in a manner which is different from the gene-for-gene specificity seen in leaves for the same genotypes. Furthermore, treatment of rice seedlings with benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), a chemical that protects leaves effectively against blast by conditioning acquired resistance, was not able to prevent colonization of roots by M. oryzae although a reduction in disease levels was observed. Moreover, BTH was not able to protect barley roots against infection with M. oryzae. Taken together, our results suggest that although roots show intrinsic variation in their ability to resist colonization by M. oryzae, neither gene-for-gene incompatibility nor aquired resistance are as effective at blocking the pathogen as they are in leaves.

Duplicate maize 13-lipoxygenase genes are differentially regulated by circadian rhythm, cold stress, wounding, pathogen infection, and hormonal treatments

Most plant oxylipins, a large class of diverse oxygenated polyunsaturated fatty acids and their derivatives, are produced through the lipoxygenase (LOX) pathway. Recent progress in dicots has highlighted the biological roles of oxylipins in plant defence responses to pathogens and pests. By contrast, the physiological function of LOXs and their metabolites in monocots is poorly understood. Two maize LOXs, ZmLOX10 and ZmLOX11 that share >90% amino acid sequence identity but are localized on different chromosomes, were cloned and characterized. Phylogenetic analysis revealed that ZmLOX10 and ZmLOX11 cluster together with well-characterized plastidic type 2 linoleate 13-LOXs from diverse plant species. Regio-specificity analysis of recombinant ZmLOX10 protein overexpressed in Escherichia coli proved it to be a linoleate 13-LOX with a pH optimum at approximately pH 8.0. Both predicted proteins contain putative transit peptides for chloroplast import. ZmLOX10 was preferentially expressed in leaves and was induced in response to wounding, cold stress, defence-related hormones jasmonic acid (JA), salicylic acid (SA), and abscisic acid (ABA), and inoculation with an avirulent strain of Cochliobolus carbonum. These data suggested a role for this gene in maize adaptation to abiotic stresses and defence responses against pathogens and pests. ZmLOX11 was preferentially expressed in silks and was induced in leaves only by ABA, indicating its possible involvement in responses to osmotic stress. In leaves, mRNA accumulation of ZmLOX10 is strictly regulated by a circadian rhythm, with maximal expression coinciding temporally with the highest photosynthetic activity. This study reveals the evolutionary divergence of physiological roles for relatively recently duplicated genes. Possible physiological functions of these 13-LOXs are suggested.

Induced disease resistance and gene expression in cereals

Disease resistance strategies reduce chemical input into the environment and are therefore powerful approaches to sustainable agriculture. Induced resistance (IR) has emerged as a potential alternative, or a complementary strategy, for crop protection. IR signifies the control of pathogens and pests by prior activation of plant defence pathways. A molecular understanding of IR in cereals, including the most important global crops wheat and rice, has been largely missing. Evidence indicating that central elements of IR pathways are conserved among Di- and Monocotyledoneae has only recently been presented, although their regulation and interaction with other plant pathways may be quite divergent. We present here a synopsis of current molecular knowledge of cereal IR mechanisms.

Elicitation of defense related enzymes and resistance by L-methionine in pearl millet against downy mildew disease caused by Sclerospora graminicola

Induction of resistance to downy mildew in pearl millet was studied after treatment with L-methionine. The disease severity was drastically reduced in the seedlings after treatment with L-methionine. Northern hybridization was used to study the transcript (mRNA) accumulation of defense response genes upon treatment with L-methionine in pearl millet seedlings and the mechanism during the induction of resistance. The gene activation patterns in the highly resistant cultivar IP18294 were compared with those exhibited by the susceptible cultivar 7042S at various time intervals to determine if differences in timing or levels of transcript accumulation could be correlated with the differences in the susceptibility of pearl millet to the downy mildew pathogen, Sclerospora graminicola. The mRNA levels of genes for Pr-1a, beta-1,3-glucanase, chitinase, peroxidase, lipoxygenases and chalcone synthase showed an increase after inoculation with S. graminicola. Significant gene activation was observed in the susceptible cultivar 7042S after treatment with L-methionine followed by challenge inoculation with the downy mildew pathogen which was comparable to the resistant cultivar.

Association between molecular markers and blast resistance in an advanced backcross population of rice

An advanced backcross population consisting of 80 BC(3)F(3) lines derived from rice vars. Vandana/ Moroberekan was analysed for blast resistance and genotyped with 50 candidate genes and 23 simple sequence repeat (SSR) markers. Six candidate defence response genes [thaumatin, three nucleotide-binding site-leucine-rich repeat sequences from maize and two resistance gene analogue (RGA) markers] and one SSR marker (RM21) were significantly associated with partial blast resistance in rice ( P=0.01). These markers accounted for phenotypic variation ranging from 9.6% to 29.4% and contributed to 76% of the total variation of percentage diseased leaf area (DLA) observed under natural infection. Four candidate genes (oxalate oxidase, 14-3-3 protein and two RGA markers) and four SSR markers (RM21, RM168, RM215 and RM250) were significantly associated with resistance to a single pathogen isolate, PO6-6. Among these, two markers were for DLA, five for lesion number and one for lesion size. These markers accounted for 9.1-28.7% of the phenotypic variation. A moderate correlation ( r=0.48, P<0.01) was found between the level of partial resistance measured in the greenhouse and that measured under natural conditions. Analysis of BC(3)F(4) progeny using genotypes of BC(3)F(3) confirmed the phenotypic contribution of these markers. Cluster analysis of DNA profiles showed that the BC(3) population was genetically similar (>85%) to the recurrent parent Vandana. Although no obvious relationship between DNA profiles and resistant phenotypes was observed, three lines (VM19, VM46 and VM76) in a cluster with high similarity to Vandana (89-96%) expressed a high level of partial blast resistance in the field. Analysis of disease progress in the field confirmed the performance of selected lines based on greenhouse and nursery analyses. The advanced backcross progeny with resistance phenotypes tagged by markers will be useful for accumulating blast resistance in upland rice.

A new 9-lipoxygenase cDNA from developing rice seeds

We isolated a novel C9 position specific lipoxygenase (r9-LOX1) cDNA from developing rice seeds. The enzymatic features of r9-LOX1 resembled those of rice LOX-L3 known to be contained in rice germ and to have C9-specific LOX activity. However, the expression level of the r9-LOX1 gene was higher in imbibed seeds rather than developing seeds. A homology search against the rice nucleotide database revealed the r9-LOX1 gene to be on rice chromosome 3 (accession number AC093017). The restriction enzyme map of the reported genomic sequence agreed with the result of the Southern blot analysis for the r9-LOX1. The enzyme could be useful for in vitro synthesis of 9,10-ketol-octadecadienoic acid.

Global transcript analysis of rice leaf and seed using SAGE technology

We have compiled two comprehensive gene expression profiles from mature leaf and immature seed tissue of rice (Oryza sativa ssp. japonica cultivar Nipponbare) using Serial Analysis of Gene Expression (SAGE) technology. Analysis revealed a total of 50 519 SAGE tags, corresponding to 15 131 unique transcripts. Of these, the large majority (approximately 70%) occur only once in both libraries. Unexpectedly, the most abundant transcript (approximately 3% of the total) in the leaf library was derived from a type 3 metallothionein gene. The overall frequency profiles of the abundant tag species from both tissues differ greatly and reveal seed tissue as exhibiting a non-typical pattern of gene expression characterized by an over abundance of a small number of transcripts coding for storage proteins. A high proportion ( approximately 80%) of the abundant tags (> or = 9) matched entries in our reference rice EST database, with many fewer matches for low abundant tags. Singleton transcripts that are common to both tissues were collated to generate a summary of low abundant transcripts that are expressed constitutively in rice tissues. Finally and most surprisingly, a significant number of tags were found to code for antisense transcripts, a finding that suggests a novel mechanism of gene regulation, and may have implications for the use of antisense constructs in transgenic technology.

Flagellin from an incompatible strain of Acidovorax avenae mediates H2O2 generation accompanying hypersensitive cell death and expression of PAL, Cht-1, and PBZ1, but not of Lox in rice

Acidovorax avenae causes a brown stripe disease in monocot plants. We recently reported that a rice-incompatible strain of A. avenae caused hypersensitive cell death in rice and that the flagellin of the incompatible strain was involved in this response. The incompatible strain induced the rapid generation of H2O2 accompanying hypersensitive cell death and the expression of defense genes such as PAL, Cht-1, PBZ1, and LOX, whereas the compatible strain did not. The purified incompatible flagellin also induced the expression of PAL, Cht-1, and PBZ1, but LOX expression was not induced by the incompatible flagellin. PAL and LOX enzymatic activities were increased by inoculation with the incompatible strain, whereas only PAL activity was increased by the incompatible flagellin. Interestingly, the flagellin-deficient incompatible strain lost the ability to generate H2O2 and induce hypersensitive cell death, but PAL, Cht-1, and PBZ1 expression still were induced by inoculation with the deficient strain, suggesting that induction of these genes is regulated not only by flagellin but also by some other signal. Thus, the incompatible flagellin of A. avenae is a specific elicitor in rice, but it is not the only factor capable of inducing the rice defense system.

Molecular characterization of the gene encoding rice allene oxide synthase and its expression

The gene encoding rice allene oxide synthase, OsAOS, was intronless and had nucleotide sequences with the high GC content of 67%. Deduced amino acid sequences had very high similarity with other AOS proteins, in particular 74% similarity to barley, characterized by the conserved motifs of P450 cytochrome of the CYP74A family. Purified recombinant rice AOS protein expressed in Escherichia coli converted 13-hydroperoxylinolenic acid to allene oxide. Several restriction enzyme digestions and Southern analysis showed that OsAOS was likely to have two copies in its genome. The basal level of OsAOS expression was detected in various tissues and the transcription level was increased by jasmonate treatment.

Jasmonates and octadecanoids: signals in plant stress responses and development

Plants are sessile organisms. Consequently they have to adapt constantly to fluctuations in the environment. Some of these changes involve essential factors such as nutrients, light, and water. Plants have evolved independent systems to sense nutrients such as phosphate and nitrogen. However, many of the environmental factors may reach levels which represent stress for the plant. The fluctuations can range between moderate and unfavorable, and the factors can be of biotic or abiotic origin. Among the biotic factors influencing plant life are pathogens and herbivores. In case of bacteria and fungi, symbiotic interactions such as nitrogen-fixating nodules and mycorrhiza, respectively, may be established. In case of insects, a tritrophic interaction of herbivores, carnivores, and plants may occur mutualistically or parasitically. Among the numerous abiotic factors are low temperature, frost, heat, high light conditions, ultraviolet light, darkness, oxidation stress, hypoxia, wind, touch, nutrient imbalance, salt stress, osmotic adjustment, water deficit, and desiccation. In the last decade jasmonates were recognized as being signals in plant responses to most of these biotic and abiotic factors. Signaling via jasmonates was found to occur intracellularly, intercellularly, and systemically as well as interorganismically. Jasmonates are a group of ubiquitously occurring plant growth regulators originally found as the major constituents in the etheric oil of jasmine, and were first suggested to play a role in senescence due to a strong senescence-promoting effect. Subsequently, numerous developmental processes were described in which jasmonates exhibited hormone-like properties. Recent knowledge is reviewed here on jasmonates and their precursors, the octadecanoids. After discussing occurrence and biosynthesis, emphasis is placed upon the signal transduction pathways in plant stress responses in which jasmonates act as a signal. Finally, examples are described on the role of jasmonates in developmental processes.