Victor H. Argandoña

Role of hydroxamic acids in the resistance of cereals to aphids.

Abstract Hydroxamic acid concentration in Gramineae, both natural and incorporated, correlates with resistance to the aphid Metopolophium dirhodum . 2,4-Dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one, a hydroxamic acid isolated from corn extracts, is deleterious to aphids fed on artificial diets. It is proposed that hydroxamic acids act as naturally-occurring protective factors against M. dirhodum .

Effect of content and distribution of hydroxamic acids in wheat on infestation by the aphid Schizaphis graminum

Abstract The content of hydroxamic acids in wheat plants shows substantial variations between different species and cultivars (1.0–6.3 mmol/kg fr. wt). It also varies with the age of the plant and the organ assayed. The maximum concentration is reached by the fourth day in epicotyls and roots. The amount in leaves at different plant ages is always higher in the younger leaves. Based on feeding and infestation experiments, it is proposed that the distribution of aphids on leaves of different ages is regulated by the hydroxamic acid content.

TOXICITY AND FEEDING DETERRENCY OF HYDROXAMIC ACIDS FROM GRAMINEAE IN SYNTHETIC DIETS AGAINST THE GREENBUG, SCHIZAPHIS GRAMINUM

2,4‐Dihydroxy‐7‐methoxy‐1,4‐benzoxazin‐3‐one (DIMBOA), the main hydroxamic acid isolated from maize extracts, increased the mortality of Schizaphis graminum when fed in artificial diets. Electrically‐monitored feeding assays showed that DIMBOA acted as a feeding deterrent at concentrations as low as 1 mM. On 12 mM DIMBOA diets, feeding by aphids was completely inhibited. Additional feeding experiments showed that when DIMBOA was ingested there was an increase in aphid mortality relative to that of aphids which did not ingest the compound. Thus, the deleterious effects of DIMBOA on aphids are due to feeding deterrency and toxicity. The 2‐ß‐0 glucoside of DIMBOA (DIMBOA‐Glc), the form in which DIMBOA naturally occurs in Gramineae, had a slight effect on lowering aphid survival and an appreciable feeding‐deterrent effect on diet‐fed aphids. The relevance of the effects of DIMBOA and DIMBOA‐Glc on aphids to resistance of certain graminaceous crops against aphids is discussed.

Hydroxamic acid content in wild and cultivated gramineae

Abstract The content of two hydroxamic acids, 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), in cultivated and wild species of Gramineae was determined. Zea mays and Triticum durum contained both DIBOA and DIMBOA, the latter being in greater concentrations. Secale cereale and Arundo donax contained only DIBOA, while Elymus gayanus and Chusquea cumingii contained only DIMBOA. Poa annua, Bromus unioloides, Dactylis glomerate, Phalaris canariense, Lolium perenne, Hordeum species, Setaria verticilata, Cynodon dactylon and a Sorghum hybrid lacked these hydroxamic acids. The maximum concentration of hydroxamic acid in A. donax was found at the end of summer, and the minimum at the beginning of winter. In annual plants, such as wheat, while neither acid was found in the fruits, their concentrations in coleoptiles and leaves increased rapidly reaching a maximum 4 days after germination and decreasing gradually afterwards. DIBOA and DIMBOA had toxic and feeding deterrent effects on the greenbug Schizaphis graminum at concentrations similar to those found in both cultivated and wild Gramineae.

Ethylene production and peroxidase activity in aphid-infested barley.

The purpose of this work was to investigate whether ethylene is involved in the oxidative and defensive responses of barley to the aphids Schizaphis graminum (biotype C) and Rhopalophum padi. The effect of aphid infestation on ethylene production was measured in two barley cultivars (Frontera and Aramir) that differ in their susceptibility to aphids. Ethylene evolution was higher in plants infested for 16 hr than in plants infested for 4 hr in both cultivars. Under aphid infestation, the production of ethylene was higher in cv. Frontera than in Aramir, the more aphid susceptible cultivar. Ethylene production also increases with the degree of infestation. Maximum ethylene evolution was detected after 16 hr when plants were infested with 10 or more aphids. Comparing the two species of aphids, Schizaphis graminum induced more ethylene evolution than Rhopalosiphum padi. Infestation with S. graminum increased hydrogen peroxide content and total soluble peroxidase activity in cv. Frontera, with a maximum level of H2O2 observed after 20 min of infestation and the maximum in soluble peroxidase activity after 30 min of infestation. When noninfested barley seedlings from cv. Frontera were exposed to ethylene, an increase in hydrogen peroxide and in total peroxidase activity was detected at levels similar to those of infested plants from cv. Frontera. When noninfested plants were treated with 40 ppm of ethylene, the maximum levels of H2O2 and soluble peroxidase activity were at 10 and 40 min, respectively. Ethylene also increased the activity of both cell-wall-bound peroxidases types (ionically and covalently bound), comparable with infestation. These results suggest that ethylene is involved in the oxidative responses of barley plants induced by infestation.

Distribution of gramine and hydroxamic acids in barley and wheat leaves

Abstract The first leaf of 12-day-old barley seedlings, contained 1.3 μmol/g fr. wt gramine while the concentrations were 2.2, 2.0 and 3.1 μmol/g fr. wt in lower and upper epidermis and mesophyll parenchyma, respectively. Gramine was not detected in the vascular bundles, nor in xylem exudates or guttation drops. Thus, about 70% of total gramine in the leaf was found in the mesophyll parenchyma and 30% in epidermal tissue. The content of hydroxamic acids was 3.0 μmol/g fr. wt in the first leaf of 12-day-old wheat plants, while the concentrations were 6.8 and 4.1 μmol/g fresh wt in the vascular bundle and mesophyll parenchyma, respectively. These compounds were not detected in epidermis, xylem exudates and guttation drops. About 50% of total hydroxamic acids were found in the vascular bundles. The significance of these results in plant protection against aphids is discussed.

Effect of infestation by aphids on the water status of barley and insect development

Abstract To compare the effects of aphid infestation with some effects of wounding and drought-stress, several physiological parameters and metabolite concentrations were measured in infested, mechanically wounded or water-stressed young barley plants ( Hordeum vulgare cv Aramir). Barley plants infested with the greenbug ( Schizaphis graminum ) had lower water potentials and CO 2 assimilation than non-infested plants. Abscisic acid content increased by 55% in leaves after 72 hr of infestation. Water potentials and stomatal resistance of barley plants changed only as a consequence of infestation by the greenbug or by drought-stress. Proline concentration increased in leaves subjected to infestation or drought by 11- and 14-fold, respectively. Leaves with artificial damage showed the same reduction in chlorophyll contents as leaves of drought-stressed plants. Greenbug infestation caused higher chlorosis than other treatments. Contents of soluble carbohydrates and proteins decreased 52 and 38% by infestation, 38 and 28% by drought, and 14 and 8% by artificial leaf damage, respectively. To study the influence of these treatments on the quality of barley plants as a food source for the aphids, developmental rates ( 1 D ) and the mean relative growth rates (MRGR) of nymphs reared on these seedlings were compared. New Aphids reared on previously infested seedlings had the lowest MRGR and 1 D ( ca 82 and 68%, respectively) compared to aphids on control plants without previous infestation. Aphids reared on plants subjected to drought also had lower MRGR and 1 D ( ca 89% and 77%). Greenbugs on wounded leaves had similar MRGR and 1 D rates to nymphs reared on control plants. These results show that greenbug infestation of barley produced changes similar to those observed in plants subjected to drought-stress and that aphids feeding on both groups of seedlings had lower developmental and mean relative growth rates. Water-stress caused in barley by aphid infestation or drought would probably affect greenbug development due to the effects of stress on the chemical composition of the plant.

Distribution of hydroxamic acids in zea mays tissues

Abstract The hydroxamic acid content of leaves of cereals correlates well with resistance to aphids. In maize these compounds were absent from xylem exudates and guttation drops. Lateral veins of leaves of 7-day-old maize plants contained 8 mmol/kg fr. wt. while the entire leaf contained only 4.2 mmol/kg fr. wt. In leaves of 20-day-old plants, these amounts decreased by ca one-third. In mesocotyls, the cortex and central vascular cylinder contained 1.3 and 2.2 mmol/kg fr. wt, respectively. In 12-day-old wheat plants, the complete leaves and their veins contained 2.4 and 6.4 mmol/kg fr. wt respectively. Thus, the concentration of hydroxamic acid was always higher in the vascular bundles.

Metabolic changes in barley seedlings at different aphid infestation levels

Abstract Different infestation levels with the greenbug ( Schizaphis graminum Rondani) were correlated with physiological parameters and several metabolite concentrations in young barley plants ( Hordeum vulgare cv Aramir). Leaf water potential, chlorophyll content, carbon dioxide assimilation and total soluble carbohydrates decreased significantly with more than 18 aphids per plant. Conversely, proline concentration increased with more than 18 aphids. With 100 individuals per plant, sugar concentrations decreased five times with respect to the non-infested control. Similar changes were observed in plants subjected to drought stress. These results suggest that several metabolic changes that occur in barley infested by a large number of aphids may be due in part to the lower leaf water potential caused by the insect.

Changes in ferulic acid and lipid content in aphid-infested barley

Abstract Aphid infestation reduced the susceptibility of barley cultivars to new infestations. The intrinsic growth rate of aphids was lower in the populations reared on previously infested plants, than in populations reared on barley without previous infestation. Changes in the content of ferulic acid, and other aromatic compounds and lipids, were detected in barley seedlings infested by the aphid, Schizaphis graminum . Ferulic acid concentration increased from 4 to 14 μ mol kg − 1 fresh wt, as a function of infestation level by aphids. Survival of aphids reared with diets containing this compound was lower than the controls. Aphid survival at 24 hours was under 58% and 40% in diets with 10 and 20 μM ferulic acid, respectively. In diets without the compound, survival was 84%. Analysis of alicyclic and aliphatic compounds in extracts of infested and non-infested barley plants showed differences in the relative concentrations of compounds. Our results suggest that the increase of ferulic acid induced by initial infestation may protect the plant against new infestations. Changes in lipid concentrations could be an acclimatization response of the plant to water-stress caused by the aphids.