Jasmonic Acid-Mediated-Induced Resistance in Groundnut (Arachis hypogaea L.) Against Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae)

Jasmonic acid (JA) acts as a signal molecule to induce resistance in plants against herbivores and its levels are elevated in plants after wounding or insect damage. Groundnut is an important crop in many tropical and subtropical regions worldwide, but there is surprisingly little knowledge on its induced defenses against herbivores. The effect of JA as a spray on induced resistance in three groundnut genotypes, namely, ICGV 86699 (resistant), NCAc 343 (resistant), and TMV 2 (susceptible), against Helicoverpa armigera was studied. The activity of oxidative enzymes [peroxidase (POD) and polyphenol oxidase (PPO)] and the amounts of other host plant defense components [total phenols, hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and protein content] were recorded at 24, 48, 72, and 96 h after pretreatment (1 day) with JA followed by infestation with H. armigera (PJA + HIN) and H. armigera infestation with simultaneous JA application (HIN + JA) to understand the consequences of induced resistance in groundnut. The plant damage, larval survival, and larval weights were also recorded. There was a significant increase in POD and PPO activities and in the amounts of total phenols, H₂O₂, MDA, and proteins in PJA + HIN- and JA + HIN-treated plants as compared to the plants treated with JA and infested with H. armigera individually and to untreated control plants. Among all the genotypes, the strongest induction of defense was observed in the ICGV 86699 genotype. It is concluded that pretreatment with JA and its application during low levels of insect infestation can increase the levels of host plant resistance against herbivorous insects and reduce the pest-associated losses in groundnut.     

Herbivore- and Elicitor- Induced Resistance in Groundnut to Asian armyworm,Spodoptera litura (Fab.) (Lepidoptera: Noctuidae)

Induced defense was studied in three groundnut genotypes ICGV 86699 (resistant), NCAc 343 (resistant) and TMV 2 (susceptible) in response to Spodoptera litura infestation and jasmonic acid (JA) application. The activity of the oxidative enzymes [peroxidase (POD) and polyphenol oxidase (PPO)] and the amounts other host plant defense components [total phenols, hydrogen peroxide (H2O2), malondialdehyde (MDA), and protein content] were recorded at 24, 48, 72 and 96 h in JA pretreated (one day before) plants and infested with S. litura, and JA application and simultaneous infestation with S. litura to understand the defense response of groundnut genotypes against S. litura damage. Data on plant damage, larval survival and larval weights were also recorded. There was a rapid increase in the activities of POD and PPO and in the quantities of total phenols, H2O2, MDA and protein content in the JA pretreated + S. litura infested plants. All the three genotypes showed quick response to JA application and S. litura infestation by increasing the defensive compounds. Among all the genotypes, higher induction was recorded in ICGV 86699 in most of the parameters. Reduced plant damage, low larval survival and larval weights were observed in JA pretreated plants. It suggests that pretreatment with elicitors, such as JA could provide more opportunity for plant defense against herbivores.     

Direct and Indirect Impacts of Infestation of Tomato Plant by Myzus persicae (Hemiptera: Aphididae) on Bemisia tabaci (Hemiptera: Aleyrodidae)

The impacts of infestation by the green peach aphid (Myzus persicae) on sweetpotato whitefly (Bemisia tabaci) settling on tomato were determined in seven separate experiments with whole plants and with detached leaves through manipulation of four factors: durations of aphid infestation, density of aphids, intervals between aphid removal after different durations of infestation and the time of whitefly release, and leaf positions on the plants. The results demonstrated that B. tabaci preferred to settle on the plant leaves that had not been infested by aphids when they had a choice. The plant leaves on which aphids were still present (direct effect) had fewer whiteflies than those previously infested by aphids (indirect effect). The whiteflies were able to settle on the plant which aphids had previously infested, and also could settle on leaves with aphids if no uninfested plants were available. Tests of direct factors revealed that duration of aphid infestation had a stronger effect on whitefly landing preference than aphid density; whitefly preference was the least when 20 aphids fed on the leaves for 72 h. Tests of indirect effects revealed that the major factor that affected whitefly preference for a host plant was the interval between the time of aphid removal after infestation and the time of whitefly release. The importance of the four factors that affected the induced plant defense against whiteflies can be arranged in the following order: time intervals between aphid removal and whitefly release > durations of aphid infestation > density of aphids > leaf positions on the plants. In conclusion, the density of aphid infestation and time for which they were feeding influenced the production of induced compounds by tomatoes, the whitefly responses to the plants, and reduced interspecific competition.     

Effect of geminivirus infection and Bemisia infestation on accumulation of pathogenesis-related proteins in tomato

The whitefly, Bemisia tabaci biotype B, has been shown to cause pathogenesis-related (PR) proteins to accumulate in plants as a result of direct feeding, but their specific role in plant defensive systems is unclear. Our objective was to compare accumulation of tomato PR proteins (beta-1,3-glucanase, chitinase, peroxidase, P2 and P4) in response to whitefly, with or without tomato mottle virus (ToMoV) infection. Tomato PR protein response was measured over time in plants divided into three treatments: uninfected controls (with or without whiteflies) and plants infested with viruliferous (ToMoV) whiteflies. Five- to six-leaf plants were infested with approximately 5 adult whitefly per leaf. Plants were sampled prior to whitefly infestation and at 14, 28, 42, and 56 days. By 56 days, plants infested with viruliferous whiteflies had significantly more eggs (2.5-fold) and nymphs (4.5-fold) than plants with nonviruliferous whiteflies. A significant increase in the enzymatic activity of all measured PR proteins, as compared to control plants, was only seen in viruliferous whitefly-infested plants. No significant difference was observed in enzyme activities between the uninfected control plants either with or without whiteflies. The greatest differences for all PR proteins assayed were observed 42 days after treatment initiation. Protein blot analyses showed that the differences in PR protein activities among the treatments were due to changes in specific enzyme levels within the plant and were associated with concomitant increases in levels of P2 and P4 PR proteins. Under our experimental conditions, it is clear that PR protein response is much more intense when it is attacked by whiteflies carrying ToMoV than by whitefly alone.     

A gain-of-function mutation in IAA8 alters Arabidopsis floral organ development by change of jasmonic acid level

Auxin regulates a variety of physiological processes via its downstream factors included Aux/IAAs. In this study, one of these Aux/IAAs, IAA8 is shown to play its role in Arabidopsis development with transgenic plants expressing GFP-mIAA8 under the control of IAA8 promoter, in which IAA8 protein was mutated by changing Pro₁₇₀ to Leu₁₇₀ in its conserved domain II. These transgenic dwarfed plants had more lateral branches, short primary inflorescence stems, decreased shoot apical dominance, curled leaves and abnormal flower organs (short petal and stamen, and bent stigmas). Further experiments revealed that IAA8::GFP-mIAA8 plants functioned as gain-of-function mutation to increase GFP-mIAA8 amount probably by stabilizing IAA8 protein against proteasome-mediated protein degradation with IAA8::GFP-IAA8 plants as control. The searching for its downstream factors indicated its interaction with both ARF6 and ARF8, suggesting that IAA8 may involve in flower organ development. This was further evidenced by analyzing the expression of jasmonic acid (JA) biosynthetic genes and JA levels because ARF6 and ARF8 are required for normal JA production. These results indicated that in IAA8::GFP-mIAA8 plants, JA biosynthetic genes including DAD1 (AT2G44810), AOS (AT5G42650) and ORP3 (AT2G06050) were dramatically down-regulated and JA level in the flowers was reduced to 70 % of that in wild-type. Furthermore, exogenous JA application can partially rescue short petal and stamen observed IAA8::GFP-mIAA8 plants. Thus, IAA8 plays its role in floral organ development by changes in JA levels probably via its interaction with ARF6/8 proteins.     

Exogenous jasmonic acid modulates the physiology,antioxidant defense and glyoxalase systems in imparting drought stress tolerance in different Brassica species

This study examined the ability of jasmonic acid (JA) to enhance drought tolerance in different Brassica species in terms of physiological parameters, antioxidants defense, and glyoxalase system. Ten-day-old seedlings were exposed to drought (15 % polyethylene glycol, PEG-6000) either alone or in combination with 0.5 mM JA. Drought significantly increased lipoxygenase activity and oxidative stress, levels of malondialdehyde and H₂O₂. Drought reduced seedling biomass, chlorophyll (chl) content, and leaf relative water content (RWC). Drought increased proline, oxidized ascorbate (DHA) and glutathione disulfide (GSSG) levels. Drought affected different species differently: in B. napus, catalase (CAT) and glyoxalase II (Gly II) activities were decreased, while glutathione-S-transferase (GST) and glutathione peroxidase (GPX) activities were increased in drought-stressed compared to unstressed plants; in B. campestris, activities of glutathione reductase (GR), glyoxalase I (Gly I), GST, and GPX were increased, monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), CAT and other enzymes were decreased; in B. juncea, activities of ascorbate peroxidase, GR, GPX, Gly I were increased; Gly II activity was decreased and other enzymes did not change. Spraying drought-stressed seedlings with JA increased GR and Gly I activities in B. napus; increased MDHAR activity in B. campestris; and increased DHAR, GR, GPX, Gly I and Gly II activities in B. juncea. JA improved fresh weight, chl, RWC in all species, dry weight increased only in B. juncea. Brassica juncea had the lowest oxidative stress under drought, indicating its natural drought tolerance capacity. The JA improved drought tolerance of B. juncea to the highest level among studied species.     

Effect of drought and ABA on growth, photosynthesis and antioxidant system of Cotinus coggygria seedlings under two different light conditions

We exposed seedlings of Cotinus coggygria var. cinerea to drought and exogenous abscisic acid (ABA) under two different light conditions. Two watering regimes (well-watered and drought), two exogenous ABA applications (no ABA and with ABA) and two light regimes (full sunlight and shade) were employed. Compared with well-watered treatment, drought treatment significantly reduced the relative growth rate, relative water content (RWC), net photosynthesis rate (A) and transpiration (E), but increased chlorophyll a (chla), carbon isotope (δ¹³C), endogenous ABA, malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) contents, and guaiacol peroxidase (POD) and catalase (CAT) activities. There was an apparent alleviation of drought effects by shade, as indicated by the lower relative growth rate, and chlorophyll, MDA and H₂O₂ contents, and increases in indoleacetic acid (IAA) and reduced glutathione (GSH) contents. On the other hand, the exogenous ABA application under shade induced protective effects on drought-stressed seedlings, as visible in RWC, MDA, A, stomatal conductance (g_s), E, δ¹³C, ABA and IAA values. In all, our results suggest that seedlings of C. coggygria are more sensitive to drought under full-light than under shade.     

Temperature-related effects of treatments with jasmonic and salicylic acids on Arabidopsis infected with cucumber mosaic virus

Plant-virus interactions are affected by environmental factors, including temperature. Plant defenses are often inhibited by high or low temperature. In this study, oxidative damage and gene expression were detected in Arabidopsis thaliana infected with cucumber mosaic virus (CMV) at different temperatures. Before virus inoculation, plants were treated with jasmonic acid (JA) and salicylic acid (SA), both of which are important signaling molecules in plant defense responses. The levels of MDA and hydrogen peroxide (H₂O₂), and electrolyte leakage were significantly higher in CMV-infected leaves at 15 and 37°C. The accumulation of H₂O₂ and superoxide radical (O ₂ ^·? ) was obviously suppressed by spraying with JA followed by SA (JA → SA) at different temperatures. The CMV-CP expression analysis showed that virus replication was inhibited efficiently in the (JA → SA) treatment. Therefore, many JA- and SA-responsible resistance genes were quantified; MPK4 was expressed highly and steadily in the (JA → SA) treatment. To further confirm the role of MPK4, the CMV-CP gene expression was evaluated in wild-type Arabidopsis and its mpk4 mutant infected with CMV. The results suggested that MPK4 might play an important role in the antagonism between JA and SA at temperature fluctuation.     

Relationship between H2O2 and jasmonic acid in pea leaf wounding response

The relationship of H₂O₂ and jasmonic acid (JA) in wound-induced defense response was investigated in the leaves of pea (Pisum sativum L.) plants. The results showed that both wounding and JA treatment led to a significant increase in activities of plasma membrane NADPH oxidase and phenylalanine ammonialyase. However, such an increase was blocked by the pretreatment with plasma membrane NADPH oxidase inhibitors, O ₂ ^? scavengers, or H₂O₂ scavenger, implying that H₂O₂ functions downstream of JA. Furthermore, wounding treatment activated two key enzymes of JA biosynthesis, lipoxygenase and allene oxide synthase, while JA biosynthetic inhibitors impaired the wounding-induced H₂O₂ burst. Thus, it is suggested that H₂O₂ burst depends on JA production in plant wounding response.     

Identification of genes differentially expressed in husk tomato (<Emphasis Type="Italic">Physalis philadelphica</Emphasis>) in response to whitefly (<Emphasis Type="Italic">Trialeurodes vaporariorum</Emphasis>) infestation

Plants respond to phloem-feeding whiteflies by extensive changes in gene expression. To identify differentially expressed genes in husk tomato plants (Physalis philadelphica) infested with Trialeurodes vaporariorum, young plants were challenged with adult whiteflies, and forward and reverse subtractive libraries were constructed from infested leaves at 5 and 15 days after infestation. Several genes were identified as up-regulated; these included a diversity of genes involved in plant defense responses, protein synthesis or degradation, and cell wall fortification or modification. Genes required for amino acid biosynthesis, lipid metabolism and synthesis, including cell surface components such as suberin, responses to stress, photosynthesis and other functions, were similarly induced. Down-regulated genes were also identified, most prominently kinases and aquaporin genes. Similarities in defense responses between tomato and P. philadelphica were noted regarding the expression of certain genes in response to nematode, aphid, or whitefly. A role for abscisic acid, brassinosteroids, and cytokinins in the regulated response to whitefly infestation in P. philadelphica was also implied by the expression pattern of phytohormone-associated genes, including genes coding for proteins containing F-box motifs. Differential expression of selected genes was validated by quantitative real-time PCR. The possible role played by some of these genes during whitefly infestation is discussed.     

Overexpression of Arabidopsis Molybdenum Cofactor Sulfurase Gene Confers Drought Tolerance in Maize (Zea mays L.)

Abscisic acid (ABA) is a key component of the signaling system that integrates plant adaptive responses to abiotic stress. Overexpression of Arabidopsis molybdenum cofactor sulfurase gene (LOS5) in maize markedly enhanced the expression of ZmAO and aldehyde oxidase (AO) activity, leading to ABA accumulation and increased drought tolerance. Transgenic maize (Zea mays L.) exhibited the expected reductions in stomatal aperture, which led to decreased water loss and maintenance of higher relative water content (RWC) and leaf water potential. Also, transgenic maize subjected to drought treatment exhibited lower leaf wilting, electrolyte leakage, malondialdehyde (MDA) and H₂O₂ content, and higher activities of antioxidative enzymes and proline content compared to wild-type (WT) maize. Moreover, overexpression of LOS5 enhanced the expression of stress-regulated genes such as Rad 17, NCED1, CAT1, and ZmP5CS1 under drought stress conditions, and increased root system development and biomass yield after re-watering. The increased drought tolerance in transgenic plants was associated with ABA accumulation via activated AO and expression of stress-related gene via ABA induction, which sequentially induced a set of favorable stress-related physiological and biochemical responses.     

Heat shock protein 70 regulates the abscisic acid-induced antioxidant response of maize to combined drought and heat stress

We investigated the interaction between heat shock protein 70 (HSP70) and abscisic acid (ABA)-induced antioxidant response of maize to the combination of drought and heat stress. First, the increased activities of enzymes, including superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR) and catalase (CAT), induced by drought were less than those by heat or combined drought and heat stress, except some individual cases (e.g. CAT in leaves, GR in roots). Second, both HSP70 synthesis and H₂O₂ production increased prominently under drought, heat or their combination stress; the increase in leaves induced by drought and heat combination was the highest, followed by heat and by drought, while the increase in roots had not visible difference. Third, either in leaves or roots, pretreatment with ABA inhibitor, HSP70 inhibitor and H₂O₂ scavenger, significantly arrested the stress-induced increase of antioxidant enzyme activities, and ABA inhibitor and H₂O₂ scavenger obviously suppressed HSP70 synthesis, while HSP70 inhibitor slightly heightened H₂O₂ accumulation. Finally, 100 μM ABA significantly enhanced the activities of antioxidant enzymes, HSP70 expression and H₂O₂ production under stresses in comparison with ABA-deficient mutant vp5 maize plants without pretreatment. Thus, ABA-induced H₂O₂ production enhances the HSP70 synthesis and up-regulates the activities of antioxidant enzymes, resulting in the suppression of cellular reactive oxygen species (ROS) levels. Our results suggest that HSP70 may play a crucial role in ABA-induced antioxidant defense of maize to drought and heat combination.     

Elevated O3 reduces the fitness of Bemisia tabaci via enhancement of the SA-dependent defense of the tomato plant

The effect of elevated O₃ on tomato plants of three different genotypes (wild-type, a jasmonic acid (JA) defense-enhanced genotype (35S) and a JA-deficient genotype (spr2)) grown in association with the whitefly Bemisia tabaci Gennadius biotype B was examined in the field in open-top chambers. We experimentally tested the hypothesis that elevated O₃ tends to reduce the nutrition of tomato plants, and to increase the SA-dependent pathway defenses and the secondary metabolites, and therefore decrease the population fitness of the whitefly. The results show that for all three tomato genotypes, elevated O₃ reduced the soluble sugars and free amino acids, increased the phenylalanine ammonia-lyase enzyme activity and the accumulated salicylic acid (SA), and up-regulated the pathogenesis-related protein (PR1), which is commonly considered to be the whitefly-resistance gene product involved in SA-dependent defense. Elevated O₃ did not affect the JA level in any of the three plant genotypes, but it increased the levels of some secondary metabolites, including total phenolics and condensed tannins. Elevated O₃ prolonged the developmental time of whiteflies fed on the three plant genotypes, and it also reduced the fecundity and the intrinsic rate of increase of whiteflies fed on either the 35S or the wild-type plants. These results suggest that elevated O₃ reduces the nutrition of tomato plants and enhances their SA content, relative PR mRNA expression and secondary metabolism, resulting in decreased fitness of whiteflies on these tomato plants.     

Overexpression of Rat Neurons Nitric Oxide Synthase in Rice Enhances Drought and Salt Tolerance

Nitric oxide (NO) has been shown to play an important role in the plant response to biotic and abiotic stresses in Arabidopsis mutants with lower or higher levels of endogenous NO. The exogenous application of NO donors or scavengers has also suggested an important role for NO in plant defense against environmental stress. In this study, rice plants under drought and high salinity conditions showed increased nitric oxide synthase (NOS) activity and NO levels. Overexpression of rat neuronal NO synthase (nNOS) in rice increased both NOS activity and NO accumulation, resulting in improved tolerance of the transgenic plants to both drought and salt stresses. nNOS-overexpressing plants exhibited stronger water-holding capability, higher proline accumulation, less lipid peroxidation and reduced electrolyte leakage under drought and salt conditions than wild rice. Moreover, nNOS-overexpressing plants accumulated less H₂O₂, due to the observed up-regulation of OsCATA, OsCATB and OsPOX1. In agreement, the activities of CAT and POX were higher in transgenic rice than wild type. Additionally, the expression of six tested stress-responsive genes including OsDREB2A, OsDREB2B, OsSNAC1, OsSNAC2, OsLEA3 and OsRD29A, in nNOS-overexpressing plants was higher than that in the wild type under drought and high salinity conditions. Taken together, our results suggest that nNOS overexpression suppresses the stress-enhanced electrolyte leakage, lipid peroxidation and H₂O₂ accumulation, and promotes proline accumulation and the expression of stress-responsive genes under stress conditions, thereby promoting increased tolerance to drought and salt stresses.     

Root and shoot jasmonic acid induced plants differently affect the performance of Bemisia tabaci and its parasitoid Encarsia formosa

Plants employ both direct and indirect defenses to protect themselves from attacks by herbivores and pathogens. To date most aboveground and belowground interaction studies have focused on interactions between plants and leaf-chewing herbivores, while the plant defence on the performance of phloem-feeding insects, induced by above- and belowground interaction, has been less explored. Here, jasmonic acid (JA) was used to mimic herbivore-induced responses in Chinese broccoli (Brassica oleracea var. alboglabra) roots (RJA) and shoots (SJA). The effects of JA-induced plant defenses on the performance of the phloem-feeding whitefly, Bemisia tabaci, and its aphelinid parasitoid Encarsia formosa were investigated. The results indicated that SJA induction has a much larger negative effect on B. tabaci than RJA: nymphs develop slower and have a lower survivorship. Also, females live shorter and have a lower fecundity on SJA plants compared to those on RJA and untreated control (CON) plants. The intrinsic rate of increase (r_m) of B. tabaci on SJA plants was 0.089, which was significantly lower than those on CON and RJA plants (0.115 and 0.104, respectively). The parasitoid E. formosa, on the other hand, shows a significantly faster development when parasitizing whitefly hosts feeding on SJA plants, whereas parasitism rate, longevity and fecundity were similar to those on RJA and CON plants. The current study reveals that plants induced with exogenous JA vary in both their resistance to whitefly and suitability for parasitoids, depending on the organ to which the JA was applied. Root and shoot JA applications also have contrasting effects on the phloem-feeding insect B. tabaci and its parasitoid, that is, SJA induction leads to more negative effects on whitefly performance than RJA induction, but its parasitoid performs better on hosts reared on SJA plants. These results show that top-down and bottom-up processes governing herbivore populations on Chinese broccoli are working in concert to increase plant resistance when plants are induced by SJA application.