Role of an indole alkaloid in the resistance of barley seedlings to aphids

The content of the simple indole alkaloid gramine in barley leaves decreased with age. Conversely, susceptibility to aphids increased in older plants. Population growth rate of the greenbug Schizaphis graminum correlated with gramine content of leaves of several barley cultivars. Gramine decreased rate of feeding, survival and reproductive index of aphids feeding on artificial diets at concentrations similar to those found in plant leaves. Thus, it is suggested that gramine plays a role in the resistance of barley seedlings to S. graminum. Benzyl alcohol, a previously reported insect resistance factor from barley, was absent from all barley cultivars analysed.     

Effect of cysteine on stability and toxicity to aphids of a cyclic hydroxamic acid from gramineae

Toxicity of DIMBOA, the major cyclic hydroxamic acid in maize extracts, to the aphid Schizaphis graminum, was decreased by addition of cysteine to the insect diet. The ld₅₀ for DIMBOA on aphids was, after 24 hr, 2.1 and 0.9 mM in diets with and without added cysteine, respectively. DIMBOA decomposed 1.5 times faster in diets or buffer with added cysteine. Decomposition products of DIMBOA (4 mM) in insect diets with or without added cysteine were not toxic. It is suggested that the observed variations in toxicity of DIMBOA are a consequence of differences in its rate of disappearance from the diet.     

Effect of gramine on the feeding behavior of the aphids Schizaphis graminum and Rhopalosiphum padi

The effects of the indole alkaloid gramine on the behavior of the aphids Schizaphis graminum and Rhopalosiphum padi feeding on barley seedlings and on artificial diets were studied. On barley cv. F. Union, which lacks gramine, S. graminum ingested from phloem tissue for 30–80 min and non-phloem for 20–40 min, over a period of 3 h. In cultivar Datil S, gramine was found only in the epidermis and parenchyma mesophyll cells. On this cultivar, the non-phloem wave form of S. graminum was not observed. On F. Union, R. padi ingested non-phloem with short periods of ingestion from phloem. On cv. Datil S, this aphid did not show the non-phloem wave form. In experiments performed with cv. F. Union seedlings that contained exogenous gramine S. graminum did not ingest from phloem. Exogenous gramine was found only in the vascular bundles. It is suggested that gramine content and location may affect the feeding behavior of aphids in barley.

---

Résumé Les effects de la gramine sur le comportement alimentaire des pucerons Schizaphis graminum et Rhopalosiphum padi on été étudiés en plantules d'orge (Hordeum distichum) et aussi avec des diètes artificielles à l'aide d'un moniteur électronique. S.graminum se nourri pendant 30–80 minutes à partir du phloème et il a pris 20–40 minutes en se nourrisant d'autres tissues divers, d'un total de 3 heures d'exposition à la plante. Dans la culture Datil S la gramine a été trouvée seulement dans les cellules de l'epidermis et du parenchyme du mesophyle. Dans ce cas, l'onde non-phloematique de S. graminum, n'a été pas observé; le temps pour le première onde X a été plus prolongé par rapport à la culture F. Union. Dans la culture F. Union, R. padi se nourri principalement de tissue non-phloematique (35–100 minutes) et du phloème par des brèves périodes (15–25 minutes). Dans le cas de la culture Datil S ce puceron n'a pas montré l'onde X. D'autre part, quand les plantules de F. Union ont été poussées dans des solution avec gramine, S. graminum ne se nourri pas du phloeme, tandis que R. padi montre un comportement similaire a celui-ce observé avec plantules sans gramine. La gramine absorbé par les plantules a été retrouvée seulement dans les faisceaux vasculaires. D'autre part, la gramine dans des diètes artificielles agit comme repulsif alimentaire à partir de concentrations de 0,5 mM. Avec des diètes contenant 3 mM de gramine l'alimentation des pucerons a été inhibée. Pourtant, ces resultats sugèrent que le contenuet et la localisation de la gramine peuvent modifier le comportement alimentaire des pucerons en plantules d'orge.

     

Glycine-betaine accumulation influences susceptibility of water-stressed barley to the aphid Schizaphis graminum

Water deficit increased susceptibility of barley to the aphid Schizaphis graminum. Proline and glycine-betaine accumulated in the stressed plants. These compounds were incorporated into artificial diets to test their effects on aphids. Survival of S. graminum was not affected by proline and glycine-betaine. In addition, glycine-betaine increased reproduction of the greenbug at concentrations similar to those found in stressed barley plants. When glycine-betaine was added to detached shoots of barley, population growth rate of S. graminum increased in that plant material kept in the betaine solutions. It is suggested that glycine-betaine accumulation may be responsible for the increased susceptibility of water-stressed barley to the greenbug.     

Feeding deterrency of flavonoids and related phenolics towards Schizaphis graminum and Myzus persicae: Aphid feeding deterrents in wheat

A number of naturally occurring flavonoids have been tested for their feeding deterrent activity against two aphid species, Schizaphis graminum and Myzus persicae. Most flavonoids, including a number of dihydrochalcones related to phloretin, showed strong deterrency at concentrations well within the range often found in plants. Flavanone and flavone glycosides showed weak feeding deterrency relative to their corresponding aglycones. S. graminum and M. persicae responded similarly towards the compounds tested. The feeding deterrency of wheat extracts towards S. graminum was confined to the phenolic fraction, which included the flavone tricin. The more polar phenolic fraction showed the strongest feeding deterrency towards S. graminum.     

Sensitivity of Meloidogyne incognita and Rotylenchulus reniformis to Abamectin

Avermectins are macrocyclic lactones produced by Streptomyces avermitilis. Abamectin is a blend of B_1a and B_1b avermectins that is being used as a seed treatment to control plant-parasitic nematodes on cotton and some vegetable crops. No LD₅₀ values, data on nematode recovery following brief exposure, or effects of sublethal concentrations on infectivity of the plant-parasitic nematodes Meloidogyne incognita or Rotylenchulus reniformis are available. Using an assay of nematode mobility, LD₅₀ values of 1.56 μg/ml and 32.9 μg/ml were calculated based on 2 hr exposure for M. incognita and R. reniformis, respectively. There was no recovery of either nematode after exposure for 1 hr. Mortality of M. incognita continued to increase following a 1 hr exposure, whereas R. reniformis mortality remained unchanged at 24 hr after the nematodes were removed from the abamectin solution. Sublethal concentrations of 1.56 to 0.39 μg/ml for M. incognita and 32.9 to 8.2 μg/ml for R. reniformis reduced infectivity of each nematode on tomato roots. The toxicity of abamectin to these nematodes was comparable to that of aldicarb.     

Effect of gramine in the resistance of barley seedlings to the aphid Rhopalosiphum padi

Gramine (N,N-Dimethyl-3-aminomethylindole) content in various barley cultivars varied from 0 to 2.6 mmoles/kg fresh weight. Those cultivars which were lacking gramine were the most susceptible to the aphid Rhopalosiphum padi (L.). The population growth rate of R. padi negatively correlated with gramine content in leaves of barley seedlings. In addition, gramine incorporated in artificial diets decreased survival, amount of diet ingested and reproduction of aphids at concentrations similar to those found in plant leaves. Thus, it is suggested that gramine may be one of the factors responsible for the resistance of barley seedlings to R. padi.

---

Résumé La teneur de gramine (N,N-dimethyl-3-aminomethyl-indole) dans différentes cultures de seigle présente des variations comprises entre 0 et 2,8 mmoles/Kg (poids frois). Les varietés dépourvues de gramine sont plus sensibles à l'attaque des pucerons. Le taux de croissance de la population des Rhopalosiphum padi a une correlation négative avec la teneur en gramine des feuilles de plantules de seigle. D'ailleurs, la gramine diminue les taux de nourrissement, de survie et de réproduction des pucerons alimentés avec des diètes artificielles contenant des concentrations du produit testé, similaire à celles trouvées pour les feuilles des plantes. Donc, on suggèrent que la gramine peut être un des facteurs responsables de la résistance des plantules de seigle contre l'attaque de Rhopalosiphum padi.

     

Susceptibility of Asian citrus psyllid, <Emphasis Type="Italic">Diaphorina citri</Emphasis> (Hemiptera: Liviidae), to the insecticide afidopyropen: a new and potent modulator of insect transient receptor potential channels

A relatively new insecticide chemistry for controlling sucking insects, afidopyropen, was investigated for toxicity against Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae). We evaluated the mortality of D. citri eggs, nymphs, and adults treated with afidopyropen using both laboratory-reared and field populations. We also quantified the effects of sublethal doses of afidopyropen on D. citri feeding, host choice selection, and fecundity. For laboratory susceptible adults, the contact LC₅₀, topical application LD₅₀, and leaf dip LC₅₀ were 2.13, 2.00, and 3.08 ng/µL, respectively. For adults collected from a commercially managed citrus grove in Florida, the contact LC₅₀, topical application LD₅₀, and leaf dip LC₅₀ were 1.37, 1.92, and 4.89 ng/µL, respectively. Egg hatch was significantly reduced following exposure to afidopyropen at 100 ng/µL. Furthermore, afidopyropen reduced D. citri nymph survival and adult emergence at concentrations ranging between 0.01 and 100 ng/µL. Diaphorina citri adult feeding decreased on citrus leaves treated with afidopyropen in a concentration-dependent manner as measured indirectly by honeydew excretion, and appeared almost completely inhibited after treatment with 10 and 100 ng/µL solutions of afidopyropen. In choice tests, significantly fewer D. citri adults settled on afidopyropen-treated plants than on control plants at 24, 48, and 72 h after release, with no differences in settling between males and females. Afidopyropen reduced the fecundity of D. citri in a concentration-dependent manner. Collectively, the results suggest that afidopyropen could contribute to the integrated management of D. citri and may therefore be useful in rotational programs to improve resistance management.     

The susceptibility of the pea aphid to intrahemocoelic infection by Serratia marcescens

The bacterium Serratia marcescens is pathogenic for the pea aphid, Acyrthosiphon pisum. When S. marcescens cells are inoculated into the hemocoel of the aphid, there is a variable lag period of 2–3 hr duration. This phase is followed by a rapid increase in pathogen numbers causing a general septicemia and the death of the aphid. The LD₅₀ for fourth-instar pea aphid nymphs was estimated as 190 viable S. marcescens cells (95% confidence limits, 123–288). It is suggested that being sucking insects aphids are protected against infection by bacterial pathogens in the field.     

How aphids decide what is good for them: experiments to test aphid feeding behaviour on Tanacetum vulgare (L.) using different nitrogen regimes

Leaf-chewing herbivores select food with a protein/carbohydrate ratio of 0.8–1.5, whereas phloem sap, which aphids feed on, has a ratio of ~0.1. Enhanced N fertilization increases the amino acid concentration in phloem sap and elevates the N/C ratio. The study examines: (1) whether aphids select between plants of different N nutrition, (2) whether feeding time correlates with the amino acid composition of phloem sap, and (3) at which stage of probing aphids identify the quality of the plant. Uroleucon tanaceti (Mordvilko) and Macrosiphoniella tanacetaria (Kaltenbach), specialist aphids feeding on tansy (Tanacetum vulgare L.), were reared on this host plant grown essentially hydroponically (in Vermiculite) in the greenhouse on 1, 3, 6, or 12 mM NH₄NO₃. One and 3 mM NH₄NO₃ corresponds to the situation found in natural tansy stands. Aphid stylet penetration was monitored by electrical penetration graphs whilst phloem sap was sampled by stylectomy. Both aphid species settled 2–3 times more frequently on plants fertilized with 6 or 12 mM NH₄NO₃. The phloem sap of these plants contained up to threefold higher amino acid concentrations, without a change in the proportion of essential amino acids. No time differences were observed before stylet penetration of plant tissue. After the first symplast contact, most aphids penetrated further, except M. tanacetaria on low-N plants, where 50% withdrew the stylet after the first probing. The duration of phloem feeding was 2–3 times longer in N-rich plants and the time spent in individual sieve tubes was up to tenfold longer. Aphids identified the nutritional quality of the host plant mainly by the amino acid concentration of phloem sap, not by leaf surface cues nor the proportion of essential amino acids. However, U. tanaceti infestation increased the percentage of methionine plus tryptophan in phloem tenfold, thus manipulating the plants nutritional quality, and causing premature leaf senescence.     

Carbohydrate-binding activity of the type-2 ribosome-inactivating protein SNA-I from elderberry (Sambucus nigra) is a determining factor for its insecticidal activity

In recent years, different classes of proteins have been reported to promote toxic effects when ingested. Type-2 ribosome-inactivating proteins (RIPs) are a group of chimeric proteins built up of an A-chain with RNA N-glycosidase activity and a B-chain with lectin activity. These proteins are thought to play a role in plant protection. Sambucus nigra agglutinin I (SNA-I) is a type-2 RIP, isolated from the bark of elderberry (S. nigra L.). This study demonstrated the insecticidal potency of SNA-I on two Hemipteran insect species using two different methods. An artificial diet supplemented with different concentrations of the purified RIP reduced survival and fecundity of pea aphids Acyrthosiphon pisum. In addition, feeding of tobacco aphids, Myzus nicotianae, on leaves from transfected plants constitutively expressing SNA-I, resulted in a delayed development and reduced adult survival and also the fertility parameters of the surviving aphids were reduced, suggesting that a population of aphids would build up significantly slower on plants expressing SNA-I. Finally, a series of experiments with transgenic lines in which a mutant RIP was expressed, revealed that the carbohydrate-binding activity of SNA-I is necessary for its insecticidal activity. In a first set of mutants, the B-chain was mutated at one position (Asp231ΔGlu), and in the second set both carbohydrate-binding sites were mutated (Asn48ΔSer and Asp231ΔGlu). Mutation of one carbohydrate-binding site strongly reduced the insecticidal activity of SNA-I, whereas mutation of both lectin sites (almost) completely abolished the SNA-I effect on tobacco aphids.     

The effect of fertilizer‐N on the inter‐specific competition among three wheat aphids under elevated CO2

Increasing atmospheric carbon dioxide (ab. CO₂) and fertilizer‐nitrogen (ab. N) applications may have marked direct effects on the plant growth of agricultural crops, and in turn affect the higher trophic level of insect herbivores. In this study, the effects of elevated CO₂ (i.e., 650 µl/L vs. ambient 400 µl/L) and fertilizer‐N (0, 50, 100, 200 kg/ha) on the population abundances and the inter‐specific competition among three co‐occurring species of wheat aphids, Sitobion avenae, Rhopalosiphum padi and Schizaphis graminum, were studied. The grain weight per ear and the 1,000‐grain weight were generally increased when grown under elevated CO₂ and showed a significant effect at the 100 kg/ha (grain weight per ear) and 0, 50 and 100 kg/ha (1,000‐grain weight) N. These two yield indexes increased with increasing fertilizer‐N levels within reasonable limits and reached a maximum at 100 kg/ha. Elevated CO₂ combined with fertilizer‐N levels formed complex indirect effects on the three wheat aphids through the wheat crops they fed on. Elevated CO₂ significantly decreased the niche overlap index (ab. NOI) between S. avenae and R. padi under 0 and 100 kg/ha and that between R. padi and S. graminum under 0 kg/ha, while significantly increased the three NOIs under 50 kg/ha and that between R. padi and S. graminum under 100 and 200 kg/ha. S. avenae and R. padi had the larger population and stronger competition in low‐N condition (0 and 50 kg/ha), which was harmful to wheat yield and quality when combined with its own poor nutrition. Overall, the 100 kg/ha N level was the best option based on the aphid population, competition and wheat yields. Therefore, the balanced relationship formed among fertilizers, plants and insects under 100 kg/ha N was vital for the interactive ecosystem.     

Purification of toxic protease from Brevibacterium otitidis KB76 with both metal and hydrosulfuryl at the active site

An extracellular toxic protease, KB76 from Brevibacterium otitidis was successfully purified to 31.3-fold by anion-exchange and gel filtration chromatography. The molecular mass was determined to be 47 kDa using SDS-PAGE. The optimum temperature and pH of the protease were 7.4 and 40°C, respectively. Ethylenediaminetetraacetic acid and phenylmethylsulfonyl fluoride inhibited the activity of the enzyme but soybean trypsin inhibitor and aprotinin had no obvious inhibition, which suggested the presence of both metal and hydrosulfuryl at or near the active site. Additionally, the isoelectric point of this protein was 5.5 ± 0.2. Its apparent K _m and V _max for the synthetic substrate N-succinyl-L-phenylalanine p-nitroanilide were 2.41 mM and 21.74 μM/min, respectively. Further, studying the lethality of the protease on mice by intraperitoneal injection, it exhibited 48-h LD₅₀ value of 9.6 mg/kg body weight. Gross and electron microscopic study in mice revealed that purified protease was capable of eliciting a variety of tissue responses resulted in liver necrosis. In conclusion, this protease produced by B. otitidis represents a potential toxic agent.     

Resistance to multiple cereal aphids in wheat–alien substitution and translocation lines

Rhopalosiphum padi, Schizaphis graminum, and Sitobion avenae are three of the most destructive aphid species of wheat (Triticum aestivum L.). They can significantly reduce wheat yields directly by feeding and indirectly by transmitting viruses. This study aimed to search for resistance to these aphid species among lines derived from different rye (Secale cereale) origins and from Aegilops speltoides, all in the genetic background of the wheat cultivar Pavon F76. Resistance was quantified as aphid weight (R. padi, S. avenae, and S. graminum) and the number of aphids and percentage of infested leaf area exhibiting chlorosis (S. graminum). The most resistant genotypes reduced R. padi and S. avenae weight by 24.2 and 34.3 %, respectively, at the seedling stage, compared with Pavon F76 control plants. Strong S. graminum resistance was found only in A. speltoides-derived lines, the most resistant of which (7A.7S-L5) sustained just 3 % chlorosis and reduced S. graminum colony weight by 67.7 %. One line carrying the 1AL.1RS_am wheat–rye translocation from Amigo wheat (originally from Insave rye) reduced S. avenae weight by 23.2 and 21.8 % in seedling and adult plants, respectively. Single genotypes carrying the complete 1R chromosome or the 1RS chromosome arm derived from E12165 wheat and Presto triticale proved to be resistant to both R. padi and S. avenae at the seedling stage. Further research should be conducted to unravel the genetic basis of resistance to these aphids in 1RS genotypes. The sources of resistance identified here may be useful for incorporating multiple aphid species resistance in wheat breeding programs, particularly for R. padi and S. avenae, to which no resistant wheats have been bred.     

Salt resistance in two cashew species is associated with accumulation of organic and inorganic solutes

This study establishes relationships between salt resistance and solute accumulation in roots and leaves of two contrasting cashew species. The sensitive (Anacardium microcarpum) and resistant (A. occidentale) species showed maximum root LD₅₀ values (the external NaCl concentration required for a 50% reduction in dry weight) of 63 and 128?mM NaCl, whereas the shoot LD₅₀ values were 90 and 132?mM, respectively. The salt sensitivity was directly associated with Na⁺ accumulation and especially with the Cl^? content in leaves and to a minor extent in roots. The accumulation of saline ions was associated with higher net uptake rates by roots and transport rates from root to shoot in the sensitive cashew species. The K⁺/Na⁺ ratios were not associated with salt resistance either in roots or leaves. Proline and free amino acid concentrations were strongly increased by salinity, especially in the leaves of the resistant species. The soluble sugar concentrations were not influenced by NaCl treatments in leaves of both species. In contrast, the root soluble sugar content was significantly decreased by salinity in the sensitive species only. In conclusion, the higher salt sensitivity of A. microcarpum is associated to an inefficient salt exclusion system of the leaves, especially for Cl^?. On the other hand, the resistant species displays higher concentrations of organic solutes especially a salt-induced accumulation of proline and free amino acids in leaves.     

Intracranial Administration of P Gene siRNA Protects Mice from Lethal Chandipura Virus Encephalitis

Background

In parts of India, Chandipura Virus (CHPV) has emerged as an encephalitis causing pathogen in both epidemic and sporadic forms. This pediatric disease follows rapid course leading to 55–75% mortality. In the absence of specific treatment, effectiveness of RNA interference (RNAi) was evaluated.

Methods and Findings

Efficacy of synthetic short interfering RNA (siRNA) or short hairpin RNA (shRNA) in protecting mice from CHPV infection was assessed. The target genes were P and M genes primarily because important role of the former in viral replication and lethal nature of the latter. Real time one step RT-PCR and plaque assay were used for the assessment of gene silencing. Using pAcGFP1N1-CHPV-P, we showed that P-2 siRNA was most efficient in reducing the expression of P gene in-vitro. Both quantitative assays documented 2logs reduction in the virus titer when P-2, M-5 or M-6 siRNAs were transfected 2hr post infection (PI). Use of these siRNAs in combination did not result in enhanced efficiency. P-2 siRNA was found to tolerate four mismatches in the center. As compared to five different shRNAs, P-2 siRNA was most effective in inhibiting CHPV replication. An extended survival was noted when mice infected intracranially with 100 LD₅₀ CHPV were treated with cationic lipid complexed 5 µg P-2 siRNA simultaneously. Infection with 10LD₅₀ and treatment with two doses of siRNA first, simultaneously and second 24 hr PI, resulted in 70% survival. Surviving mice showed 4logs less CHPV titers in brain without histopathological changes or antibody response. Gene expression profiles of P-2 siRNA treated mice showed no interferon response. First dose of siRNA at 2hr or 4hr PI with second dose at 24hr resulted in 40% and 20% survival respectively suggesting potential application in therapy.

Conclusions

The results highlight therapeutic potential of siRNA in treating rapid and fatal Chandipura encephalitis.     

Comparative toxicity,growth inhibitory and biochemical effects of terpenes and phenylpropenes on Spodoptera littoralis (Boisd.)

Eleven monoterpenes, phenylpropenes and sesquiterpenes were evaluated for their insecticidal and growth inhibitory activities against the second and fourth larval instars of Spodoptera littoralis. Among the tested compounds, 1,8-cineole revealed the highest fumigant toxicity against the 2nd and 4th larval instars with LC₅₀ values of 2.32 and 3.13 mg/L air, respectively. The monoterpenes, p-cymene, α-terpinene, (−)α-pinene and (−)-carvone were highly toxic to both larval stages as their LC₅₀ values ranged between 7.35 and 13.79 mg/L air against 2nd larval instar and between 14.66 and 32.02 mg/L air against 4th larval instar. In topical application assay against the 4th larval instar, (−)-carvone (LD₅₀ = 0.15 mg/larva) and cuminaldehyde (LD₅₀ = 0.27 mg/larva) were the most potent contact toxicants. In residual film assay, trans-cinnamaldehyde, (−)-citronellal and p-cymene showed the highest insecticidal activity against the 2nd larval instar, while α-terpinene and (−)-carvone were most effective compounds against the 4th larval instar. Moreover, the tested compounds caused strong growth reduction of both larval stages with growth inhibition higher than 80% in the 2nd larval instar and higher than 70% in the 4th larval instar. On the other hand, (−)-carvone, cuminaldehyde and (Z,E)-nerolidol showed pronounced inhibitory effects on acetylcholinesterase (AChE) and adenosine triphosphatases (ATPases) activity of S. littoralis larvae. Cuminaldehyde (IC₅₀ = 1.04 mM) and (Z,E)-nerolidol (IC₅₀ = 0.02 mM) caused the highest inhibition of AChE and ATPases, respectively. Taken together, the results indicate that monoterpenes, phenylpropenes and phenylpropenes could be used to develop new botanical insecticides for S. littoralis management.     

Response of Lymantria dispar L. (Lepidoptera: Lymantriidae) to Bacillus thuringiensis subsp. kurstaki at different ingested doses and temperatures

We examined mortality and feeding inhibition response of Lymantria dispar L. (Lepidoptera: Lymantriidae) larvae to ingested doses of Bacillus thuringiensis subsp. kurstaki as a function of dose, instar and temperature. We developed generalized (logistic) linear mixed models and a mixture survival model, commonly used in medical statistics, to analyze the complex data set. We conducted bioassays of Foray 48B with larvae from the NJSS laboratory stock, using droplet imbibing or force-feeding to ensure dose ingestion. The dose causing mortality in 50% of the test population (LD₅₀) under standard test conditions (22 °C) ranged from 0.019 International Units (IU)/larva for first instar larvae (L₁) to 1.6 IU/larva for L₄. Temperature affected larval mortality in two ways. Mortality occurred sooner and progressed more rapidly with increasing temperature (13-25 °C) at each dose level and instar, while the maximum level of mortality attained by each instar decreased with increasing rearing temperature. The mechanisms underlying this effect are being investigated. Larvae that survived exposure to B. thuringiensis resumed feeding after a period that was dependent on instar, dose, and temperature. The equations describing observed mortality and feeding recovery responses were used to construct a simulation model, which was able to predict both processes, and which forms the basis for a process-oriented model that can be used as a decision support tool in aerial sprays.     

Gramine Accumulation in Leaves of Barley Grown under High-Temperature Stress

The indole alkaloid gramine is toxic to animals and may play a defensive role in plants. Under certain conditions, shoots of barley cultivars such as `Arimar' and CI 12020 accumulate gramine (N,N-dimethyl-3-aminomethylindole) and lesser amounts of its precursors 3-aminomethylindole (AMI) and N-methyl-3-aminomethylindole (MAMI); other cultivars such as `Proctor' do not. When grown at optimal temperatures (21°C/16°C, day/night), Arimar contained a high level of gramine in the first leaf (approximately 6 milligrams per gram dry weight), but progressively less accumulated in successive leaves so that the gramine level in the shoot as a whole fell sharply with age. In Arimar and CI 12020 plants transferred at the two- to three-leaf stage from 21°C/16°C to supra-optimal temperatures (≥30°C/25°C), there was massive gramine accumulation in leaves which developed at high temperature, so that gramine level in the whole shoot remained high (about 3-8 milligrams per gram dry weight).

Proctor lacked both constitutive gramine accumulation in the first leaf and heat-induced gramine accumulation in later leaves. The following evidence indicates that this results from a lesion in the pathway of synthesis (tryptophan →→ AMI → MAMI → gramine) between tryptophan and AMI. (a) Proctor and Arimar leaves readily absorbed [¹⁴C]gramine, but neither cultivar degraded it extensively. (b) Arimar leaf tissue incorporated [¹⁴C]formate label into the N-methyl groups of gramine and MAMI, and converted [methylene-¹⁴C]tryptophan to AMI, MAMI, and gramine; Proctor leaf tissue did not, even when a trapping pool of unlabeled gramine was supplied. (c) Proctor converted [¹⁴C]MAMI to gramine as actively as Arimar. (d) Proctor incorporated [¹⁴C]formate label into gramine and MAMI when supplied with AMI; the ratio [¹⁴C]gramine/[¹⁴C]MAMI fell with leaf age, suggesting that the two N-methylations involve different enzymes. Inasmuch as Proctor leaf tissue did not methylate added tryptamine or tyramine, the N-methyltransferase(s) of gramine synthesis may be substrate specific.

In sterile culture at optimal temperatures, 10 millimolar gramine did not affect autotrophic growth of Arimar or Proctor plantlets or heterotrophic growth of callus. At supra-optimal temperature, plantlet growth was reduced by gramine although callus growth was not. We speculate that gramine-accumulating cultivars may suffer autotoxic effects at high leaf temperatures.

     

Detection of fungal metabolites of various Trichoderma species by the aphid Schizaphis graminum

The feeding preferences of alate and apterous morphs of the aphid Schizaphis graminum (Rondani) (Homoptera: Aphididae) were evaluated using leaves treated with powdered rice cultures of four fungal isolates belonging to different species of the genus Trichoderma (Deuteromycotina: Hyphomycetes). All of the fungal isolates restrained alate morphs of S. graminum from visiting treated leaves, but only Trichoderma citrinoviride Bisset also influenced the preference of apterous morphs. Trials carried out with supernatants obtained by centrifuging aqueous suspensions of the fungal cultures showed that the feeding preference of aphids was maintained in the absence of fungal spores and mycelia, supporting the hypothesis that at least part of the fungal metabolites responsible for this effect were water‐soluble compounds. Electrophysiological studies showed that the structures involved in the perception of the fungal metabolites are located on the aphid tarsomeres.