OsGLP3-7 positively regulates rice immune response by activating hydrogen peroxide,jasmonic acid,and phytoalexin metabolic pathways

Although germin-like proteins (GLPs) have been demonstrated to participate in plant biotic stress responses, their specific functions in rice disease resistance are still largely unknown. Here, we report the identification and characterization of OsGLP3-7, a member of the GLP family in rice. Expression of OsGLP3-7 was significantly induced by pathogen infection, jasmonic acid (JA) treatment, and hydrogen peroxide (H₂O₂) treatment. OsGLP3-7 was highly expressed in leaves and sublocalized in the cytoplasm. Overexpression of OsGLP3-7 increased plant resistance to leaf blast, panicle blast, and bacterial blight, whereas disease resistance in OsGLP3-7 RNAi silenced plants was remarkably compromised, suggesting this gene is a positive regulator of disease resistance in rice. Further analysis showed that OsGLP3-7 has superoxide dismutase (SOD) activity and can influence the accumulation of H₂O₂ in transgenic plants. Many genes involved in JA and phytoalexin biosynthesis were strongly induced, accompanied with elevated levels of JA and phytoalexins in OsGLP3-7-overexpressing plants, while expression of these genes was significantly suppressed and the levels of JA and phytoalexins were reduced in OsGLP3-7 RNAi plants compared with control plants, both before and after pathogen inoculation. Moreover, we showed that OsGLP3-7-dependent phytoalexin accumulation may, at least partially, be attributed to the elevated JA levels observed after pathogen infection. Taken together, our results indicate that OsGLP3-7 positively regulates rice disease resistance by activating JA and phytoalexin metabolic pathways, thus providing novel insights into the disease resistance mechanisms conferred by GLPs in rice.     

Jasmonic acid and salicylic acid activate a common defense system in rice

Jasmonic acid (JA) and salicylic acid (SA) play important roles in plant defense systems. JA and SA signaling pathways interact antagonistically in dicotyledonous plants, but, the status of crosstalk between JA and SA signaling is unknown in monocots. Our rice microarray analysis showed that more than half of the genes upregulated by the SA analog BTH are also upregulated by JA, suggesting that a major portion of the SA-upregulated genes are regulated by JA-dependent signaling in rice. A common defense system that is activated by both JA and SA is thus proposed which plays an important role in pathogen defense responses in rice.     

Molecular cloning and characterization of a cDNA encoding ent-cassa-12,15-diene synthase, a putative diterpenoid phytoalexin biosynthetic enzyme, from suspension-cultured rice cells treated with a chitin elicitor

We have isolated and characterized a cDNA encoding a novel diterpene cyclase, OsDTC1, from suspension-cultured rice cells treated with a chitin elicitor. OsDTC1 functions as ent-cassa-12,15-diene synthase, which is considered to play a key role in the biosynthesis of (-)-phytocassanes recently isolated as rice diterpenoid phytoalexins. The expression of OsDTC1 mRNA was also confirmed in ultraviolet (UV)-irradiated rice leaves. In addition, we identified ent-cassa-12,15-diene, a putative diterpene hydrocarbon precursor of (-)-phytocassanes, as an endogenous compound in the chitin-elicited suspension-cultured rice cells and the UV-irradiated rice leaves. The OsDTC1 cDNA isolated here will be a useful tool to investigate the regulatory mechanisms of the biosynthesis of (-)-phytocassanes in rice.     

Phosphatidylinositol 3-phosphate is required for abscisic acid-induced hydrogen peroxide production in rice leaves

Rice leaves produce H₂O₂ in response to abscisic acid (ABA), which results in induction of senescence and accumulation of NH₄⁺. The upstream steps of the ABA-induced H₂O₂ production pathway in rice leaves remain largely unclear. In animal cells, H₂O₂ production in neutrophils is activated by phosphatidylinositol 3-phosphate (PI3P), a product of phosphatidylinositol 3-knase (PI3K). In the present study, we examined whether PI3P plays a role in H₂O₂ production in rice leaves exposed to ABA. We found that PI3K inhibitors LY 294002 (LY) or wortmannin (WM) inhibited ABA-induced H₂O₂ production, senescence and NH₄⁺ accumulation. Hydrogen peroxide almost completely rescued the inhibitory effect of LY or WM. It appears that PI3P plays a role in ABA-induced H₂O₂ production, senescence, and NH₄⁺ accumulation in rice leaves.     

Jasmonic Acid is Induced in a Biphasic Manner in Response of Pea Seedlings to Wounding

The role of jasmonic acid (JA) in plant wounding response has been demonstrated. However, the source of JA in wound signaling remains unclear. In the present study, pea seedlings were used as material to investigate the systemic induction of JA and the activation of lipoxygenase (LOX)-dependent octadecanoid pathway upon wounding. The results showed that endogenous JA could induce two peaks in the wounded leaves and the stalks, while only one peak in the systemic leaves.LOX activity and its protein amount were also induced and the stimulation mainly occurred in the late phase, while one peak of induction was present after pretreatment with JA. Applied nordihydroguaiaretic acid (NDGA), an inhibitor of LOX activity, only inhibited the induction of JA in the late phase, and the resistance of pea was impaired. Furthermore, 13(S)-hydroperoxy-9(Z), 11 (E)-octadecadienoic acid (13(S)-H(P)ODE) was confirmed to be the main product of LOX throughout the experimental time. In addition, immunocytochemical analysis also revealed the occurrence of JA biosynthesis and transport upon wounding. These results demonstrated that wound-induced JA in wounded leaves resulted from Its biosynthesis and conversion from its conjugates, while in systemic leaves resulted from its transport and biosynthesis; and proved that the LOX pathway was vital to the wound-induced defense response involved in JA biosynthesis.     

Ethylene production is associated with germination but not seed dormancy in red rice

BACKGROUND AND AIMS: The relationship between ethylene production and both seed dormancy and germination was investigated using red rice (weedy rice) as a model species. METHODS: Both fully dormant and after-ripened (non-dormant) naked caryopses were incubated with or without inhibitors of ethylene synthesis [aminoethoxyvinylglycine (AVG)] and perception [silver thiosulfate (STS)], or in the presence of the natural ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). The kinetics of ethylene emissions were measured with a sensitive laser-photoacoustic system. KEY RESULTS: Dormant red rice caryopses did not produce ethylene. In non-dormant caryopses, ethylene evolution never preceded the first visible stage of germination (pericarp splitting), and ethylene inhibitors completely blocked ethylene production, but not pericarp splitting. Accordingly, endogenous ACC appeared to be lacking before pericarp splitting. However, early seedling growth (radicle or coleoptile attaining the length of 1 mm) followed ethylene evolution and was delayed by the inhibitors. Wounding the dormant caryopses induced them to germinate and produce ethylene, but their germination was slow and pericarp splitting could be speeded up by ethylene. CONCLUSIONS: The findings suggest that, in red rice, endogenous ethylene stimulates the growth of the nascent seedling, but does not affect seed dormancy or germination inception. Correspondingly, this phytohormone does not play a role in the dormancy breakage induced by wounding, but accelerates germination after such breakage has occurred.     

Isopentylamine is a novel defence compound induced by insect feeding in rice

Plants produce a broad variety of defensive metabolites to protect themselves against herbivorous insects. Although polyamines have been implicated in various responses to abiotic and biotic stress, there have been no studies focused on amines in response to insect herbivory. By screening for bioactive amines, we identified isopentylamine as a novel type of herbivory‐induced compound in rice leaves, which was derived from the amino acid leucine in stable isotope labelling experiments. Accumulation of isopentylamine increased during herbivory by the brown planthopper (Nilaparvata lugens, BPH) and the rice‐feeding armyworm (Mythimna loreyi), as well as in response to treatment with the plant hormone, jasmonic acid. Likewise, isopentylamine accumulation was compromised in rice jasmonate biosynthesis mutants, hebiba and Osjar1. In bio‐assays, BPH insects feeding on rice seedlings submerged in 50 mg/L isopentylamine solution had a higher mortality compared with BPH feeding on seedlings submerged in water. Notably, the rice leaves submerged in 50 mg/L solution showed the endogenous concentrations of isopentylamine similar to that induced by BPHs. These results suggest that isopentylamine functions as a new type of plant defence metabolite that is rapidly induced by herbivore attack and deters insect herbivores in rice.     

The tomato mutant spr1 is defective in systemin perception and the production of a systemic wound signal for defense gene expression

Wound-induced systemic expression of defensive proteinase inhibitor (PI) genes in tomato plants requires the action of systemin and its precursor protein prosystemin. Although it is well established that systemin induces PI expression through the octadecanoid pathway for jasmonic acid (JA) biosynthesis, relatively little is known about how systemin and JA interact to promote long-distance signaling between damaged and undamaged leaves. Here, this question was addressed by characterizing a systemin-insensitive mutant (spr1) that was previously identified as a suppressor of prosystemin-mediated responses. In contrast to JA biosynthetic or JA signaling mutants that lack both local and systemic PI expression in response to wounding, spr1 plants were deficient mainly in the systemic response. Consistent with this phenotype, spr1 plants exhibited normal PI induction in response to oligosaccharide signals that are thought to play a role in the local wound response. Moreover, spr1 abolished JA accumulation in response to exogenous systemin, and reduced JA accumulation in wounded leaves to approximately 57% of wild-type (WT) levels. Analysis of reciprocal grafts between spr1 and WT plants showed that spr1 impedes systemic PI expression by blocking the production of the long-distance wound signal in damaged leaves, rather than inhibiting the recognition of that signal in systemic undamaged leaves. These experiments suggest that Spr1 is involved in a signaling step that couples systemin perception to activation of the octadecanoid pathway, and that systemin acts at or near the site of wounding (i.e. in rootstock tissues) to increase JA synthesis to a level that is required for the systemic response. It was also demonstrated that spr1 plants are not affected in the local or systemic expression of a subset of rapidly induced wound-response genes, indicating the existence of a systemin-independent pathway for wound signaling.     

Jasmonic Acid is Induced in a Biphasic Manner in Response of Pea Seedlings to Wounding

The role of jasmonic acid （JA） in plant wounding response has been demonstrated. However, the source of JA in wound signaling remains unclear. In the present study, pea seedlings were used as material to investigate the systemic induction of JA and the activation of lipoxygenase （LOX）-dependent octadecanoid pathway upon wounding. The results showed that endogenous JA could induce two peaks in the wounded leaves and the stalks, while only one peak in the systemic leaves. LOX activity and its protein amount were also induced and the stimulation mainly occurred in the late phase, while one peak of induction was present after pretreatment with JA. Applied nordihydroguaiaretic acid （NDGA）, an inhibitor of LOX activity, only inhibited the induction of JA in the late phase, and the resistance of pea was impaired. Furthermore, 13（S）- hydroperoxy-9（Z）, 11 （E）-octadecadienoic acid （13（S）-H（P）ODE） was confirmed to be the main product of LOX throughout the experimental time. in addition, immunocytochemical analysis also revealed the occurrence of JA biosynthesis and transport upon wounding. These results demonstrated that wound-induced JA in wounded leaves resulted from its biosynthesis and conversion from its conjugates, while in systemic leaves resulted from its transport and biosynthesis; and proved that the LOX pathway was vital to the wound-induced defense response involved in JA biosynthesis.     

Induction of ASCORBATE PEROXIDASE 2 expression in wounded Arabidopsis leaves does not involve known wound-signalling pathways but is associated with changes in photosynthesis

ASCORBATE PEROXIDASE 2 (APX2) encodes a key enzyme of the antioxidant network. In excess light-stressed Arabidopsis leaves, photosynthetic electron transport (PET), hydrogen peroxide (H(2)O(2)) and abscisic acid (ABA) regulate APX2 expression. Wounded leaves showed low induction of APX2 expression, and when exposed to excess light, APX2 expression was increased synergistically. Signalling pathways dependent upon jasmonic acid (JA), chitosan and ABA were not involved in the wound-induced expression of APX2, but were shown to require PET and were preceded by a depressed rate of CO(2) fixation. This led to an accumulation of H(2)O(2) in veinal tissue. Diphenyl iodonium (DPI), which has been shown previously to be a potent inhibitor of H(2)O(2) accumulation in the veins of wounded leaves, prevented induction of APX2 expression probably by inhibition of PET. Thus, the weak induction of APX2 expression in wounded leaves may require H(2)O(2) and PET only. As in other environmental stresses, wounding of leaves resulted in decreased photosynthesis leading to increased reactive oxygen species (ROS) production. This may signal the induction of many 'wound-responsive' genes not regulated by JA-dependent or other known JA-independent pathways.     

Jasmonic acid carboxyl methyltransferase regulates development and herbivory‐induced defense response in rice

Jasmonic acid(JA) and related metabolites play a key role in plant defense and growth. JA carboxyl methyltransferase(JMT) may be involved in plant defense and development by methylating JA to methyl jasmonate(Me JA) and thus influencing the concentrations of JA and related metabolites. However, no JMT gene has been well characterized in monocotyledon defense and development at the molecular level. After we cloned a rice JMT gene,Os JMT1, whose encoding protein was localized in the cytosol, we found that the recombinant Os JMT1 protein catalyzed JA to Me JA. Os JMT1 is up-regulated in response to infestation with the brown planthopper(BPH; Nilaparvata lugens). Plants in which Os JMT1 had been overexpressed(oeJMT plants) showed reduced height and yield. These oe-JMT plants also exhibited increased Me JA levels but reduced levels of herbivore-induced JA and jasmonoyl-isoleucine(JAIle). The oe-JMT plants were more attractive to BPH female adults but showed increased resistance to BPH nymphs,probably owing to the different responses of BPH female adults and nymphs to the changes in levels of H_2O_2 and Me JA in oe-JMT plants. These results indicate that Os JMT1,by altering levels of JA and related metabolites, plays a role in regulating plant development and herbivore-induced defense responses in rice.     

Fatty acid elongase is required for shoot development in rice

Organisms are covered extracellularly with cuticular waxes that consist of various fatty acids. In higher plants, extracellular waxes act as indispensable barriers to protect the plants from physical and biological stresses such as drought and pathogen attacks. However, the effect of fatty acid composition on plant development under normal growth conditions is not well understood. Here we show that the ONION1 (ONI1) gene, which encodes a fatty acid elongase (β-ketoacyl CoA synthase) involved in the synthesis of very-long-chain fatty acids, is required for correct fatty acid composition and normal shoot development in rice. oni1 mutants containing a reduced amount of very-long-chain fatty acids produced very small shoots, with an aberrant outermost epidermal cell layer, and ceased to grow soon after germination. These mutants also showed abnormal expression of a KNOX family homeobox gene. ONI1 was specifically expressed in the outermost cell layer of the shoot apical meristem and developing lateral organs. These results show that fatty acid elongase is required for formation of the outermost cell layer, and this layer is indispensable for entire shoot development in rice.     

p53 is required for chloroquine-induced atheroprotection but not insulin sensitization

An intact genotoxic stress response appears to be atheroprotective and insulin sensitizing. ATM, mutated in ataxia telangiectasia, is critical for the genotoxic stress response, and its deficiency is associated with accelerated atherosclerosis and insulin resistance in humans and mice. The antimalarial drug chloroquine activates ATM signaling and improves metabolic phenotypes in mice. p53 is a major effector of ATM signaling, but it is unknown if p53 is required for the beneficial effects of chloroquine. We tested the hypothesis that the cardiometabolic effects of chloroquine are p53-dependent. ApoE-null mice with or without p53 were treated with low-dose chloroquine or saline in the setting of a Western diet. After 8 weeks, there was no p53-dependent or chloroquine-specific effect on serum lipids or body weight. Chloroquine reduced plaque burden in mice wild-type for p53, but it did not decrease lesion extent in p53-null mice. However, chloroquine improved glucose tolerance, enhanced insulin sensitivity, and increased hepatic Akt signaling regardless of the p53 genotype. These results indicate that atheroprotection induced by chloroquine is p53-dependent but the insulin-sensitizing effects of this agent are not. Discrete components of the genotoxic stress response might be targeted to treat lipid-driven disorders, such as diabetes and atherosclerosis.