Héctor R. Bravo

Changes in hydroxamic acid levels of wheat plants induced by aphid feeding

Abstract Seedlings of four wheat cultivars were infested with Metopolophium dirhodum nymphs. After aphids had fed for 40 hr on the plants, the levels of the defense metabolite 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one had changed. The changes depended on the cultivar and the portion of the leaf examined. The increase was greater in cultivars Naofen (45.0% at the tip of the leaf, 96.2% at the base where aphids were feeding) and Quilafen (14.7 and 35.8% respectively). The increase was not significant in cultivars Huenufen and Sonka. A simple and sensitive high performance liquid chromatographic method is described for the quantitation of the above benzoxazinone and its demethoxylated analogue using small amounts of plant tissue.

Decomposition in aprotic solvents of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one, a hydroxamic acid from cereals

Abe&ad-The decomposition of the title compound (DIMBOA, 1) in aprotic solvents was analysed in terms of linear salvation energy relationships using donor numbers. The results indicate rate-limiting cyclic hemiacetal opening in low donor number solvents and rate-limiting isccyanate formation in high donor numbcrsolvents.TheadditionofH,O toDIMBOAdecomposingin highdonornumbersolventshadnoeffect upon the reaction rate, allowing one of the two proposed mechanisms lo be rejected. maize contain hydroxamic acids ‘ which inhibit growth and development of plant pathogens2*’ and are involved in cereal resistance to various insects.4*5 Knowledge of the reactivity of these compounds in solution is essential for the molecular interpretation of their widespread toxicity.6*7 Two mechanisms for the decomposition of these acids in aqueous solutions have been proposed. Both coincide in proposing the fast opening of the hemiacetal as the first step. In one mechanism,” an isocyanate intermediate (3) would be formed in the rate-limiting step of the reaction by attack of the hydroxamic oxygen atom on the aldehyde function ofthe a-ketoaldehyde(2) (Scheme 1, path A). In the other,’ cyclisationof2 would form the Smembered hemiketal5, water would then add in the rate-limiting step to the aldehyde function of 5 converting it to a better electrophilic leaving group, and finally formic acid and water would be formed from this electrophilic residue and the hydroxamic hydroxyl group, leaving a compound which tautomerizes to the corresponding benzoxazolinone (4). The participation of water may be assessed by studying the effect of added water on the decomposition reaction in aprotic solvents. In this paper, we describe the decomposition of 2,4dihydroxy - 7 - methoxy - 1,4 - benzoxazin - 3 - one (DIMBOA, l), the main hydroxamic acid in maize extracts,“inaproticsolvents. Apreliminaryaccount of this work has been published.”

Antimicrobial activity of cereal hydroxamic acids and related compounds

Abstract Hydroxamic acids derived from 2,4-dihydroxy-1,4-benzoxazin-3-one or 4-hydroxy-1,4-benzoxazin-3-one inhibited the growth of Staphylococcus aureus , Escherichia coli and Candida albicans . On the basis of structure-activity relationships, the hydroxamic acid function is suggested to be responsible for the antimicrobial effects.

Hydroxamic acids in Secale cereale L. and the relationship with their antifeedant and allelopathic properties.

Abstract Contents of the hydroxamic acids 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA), and 2,4- dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) in leaves and roots of 14 cultivars of rye, Secale cereale L., were determined. Dynamics of accumulation in three cultivars were evaluated. DIBOA was the main cyclic hydroxamic acid in leaves but the contents differed significantly between the cultivars. Both DIBOA and DIMBOA were present in the roots. Maximum concentration of DIBOA in leaves and DIMBOA in roots was reached between 48-54 h and 54-72 h after germination, respectively. Antifeedant activity of DIBOA towards the aphid Rhopalosiphum padi and the feeding behavior were studied by electronic recording in barley leaves treated with different contents of DIBOA. The deleterious activity of DIBOA could arise by starvation and/or a toxic effect. Additionally, allelopathic potential of pure DIBOA and aqueous extracts of leaves and roots of rye (Tetra-Baer) on the germination of lettuce (Lactuca sativa) and rye (Tetra-Baer) seeds was evaluated. A high percentage of germination inhibition of pure DIBOA and the extracts of leaves and roots was observed. The activity is in agreement with the contents of hydroxamic acids in the plants. The substrates had no allelopathic effect on rye seeds.

1,4-benzoxazin-3-one, 2-benzoxazolinone and gallic acid from Calceolaria thyrsiflora graham and their antibacterial activity

Secondary metabolites, DIBOA, HBOA, 7-OH-HBOA, BOA and gallic acid, were isolated and quantified from Calceolaria thyrsiflora Graham, a native medicinal plant of Chile belonging to the Scrophulariaceae family. The highest DIBOA contents were determined in leaves (145 mmol kg-1 dry wt) and flowers (161 mmol kg-1 dry wt). Antibacterial activities of DIBOA, HBOA, BOA, gallic acid and infusions of flowers and leaves were determined. The phytomedicinal properties attributed to C. thyrsiflora Graham could be understood on the basis of its antibacterial activity

Hydroxamic acids accumulation by wheat callus.

Abstract Hydroxamic acids were quantified by HPLC in leaves of one-week-old seedlings and in 30 day-old callus tissues of four wheat cultivars. Leaves accumulated preferentially 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA). 2,4-Dihydroxy-1,4 (DIBOA) was detected only in cv SNA-3. By contrast, callus tissues accumulated both compounds and in cv Millaleu the only compound detected was DIBOA. The amount of hydroxamic acids in 30-day-old callus tissues was higher than in seven-day-old leaves in all cultivars. In addition, as cv SNA-3 callus aged the concentration of DIBOA decreased, and DIMBOA concentration increased. The results suggest that DIMBOA originates from DIBOA.

Relationship between Grain Yield, Osmotic Adjustment and Benzoxazinone Content in Triticum aestivum L. Cultivars

Abstract Fifteen wheat genotypes were grown under water deficit to ascertain the role of osmotic adjustment (OA) and the concentration of benzoxazinones in sustaining grain yield. A positive correlation between osmotic adjustment capacity and yield was observed in wheat genotypes cultivated under field conditions. The weight gain of plants exposed to drought was in agreement with the OA values (R2 = 0.93). However, when wheat plants were infested by cereal aphids, this correlation was not found. The benzoxazinones 2,4-dihydroxy-1,4-benzoxazin- 3-one (DIBOA) and 2,4-dihydroxy-7-methoxy-1,4 benzoxazin-3-one (DIMBOA) are defensive secondary metabolites present in wheat and others cereals. The content of these compounds varied in wheat genotypes and increased with drought and aphid infestation. A positive correlation between weight gain of irrigated-infested plants and drought-infested plants and the contents of benzoxazinones was observed. These results suggest that plants with better OA capacity and high benzoxazinone content should have better field yields.

ADSORPTION OF FUNGICIDES IN CHILEAN SOILS INCUBATED WITH BIOSOLIDS

ABSTRACT The efficiency of the adsorption process of the fungicides Captan and Thiram was studied in four Chilean soils from the VI [O´Higgins, (HGS)], VII [Talcarehue, (THL)], VIII [Diguillin, (DIG)] and Metropolitan [Maipo, (MAO)] regions of Chile. Changes in the efficiency of adsorption when the natural soils were incubated with biosolids were analyzed.The values of parameters Kf and n from the Freundlich equation indicated an increase in the adsorption of fungicides incubated with biosolids (1% and 10%) with respect to natural soil. A high Kf value (12.1) for DIG-Thiram and a lower Kf value (4.3) for MAO-Captan interaction were observed for natural soils, while in soils incubated with biosolids (10%) the greatest value was (20.3) for DIG-Thiram and the lowest Kf value (10.2) for MAO-Captan. In general the high Kf values for Captan and Thiram were correlated with organic carbon content in the natural and incubated soils, except for the interaction THL-Thiram, in which inorganic compounds of soils (clay) were dominant.The distribution constant between solid-liquid phases (Kd) increased in soils incubated with biosolids; the magnitude of this constant was more significant with Thiram. The same behavior was observed for the constant related to organic carbon constants (Koc).The results of this work confirm that amending soils with biosolids is beneficial for immobilizing fungicides and helps prevent the percolation of Captan and Thiram through the soil profile and into groundwater

Contents of 1,4-Benzoxazin-3-ones and 2-Benzoxazolinone from Stenandrium dulce (Nees)

Secondary metabolites, DIBOA, HBOA, 7-OH-HBOA and BOA, were isolated and quantified from S. dulce (Nees), a native species in Chile belonging to the Acanthaceae family. The highest DIBOA and HBOA contents were determined in leaves (9.25 mmol kg-1 fr. wt) and root (6.81 mmol kg-1 fr. wt), respectively. Aglycones, 7-OH-HBOA and HBOA, were isolated together from root extracts of Acanthaceae species. Both, HBOA and 7-OH-HBOA should be direct precursors in the biosynthesis of DIBOA and DIMBOA, respectively.

Chemical Basis for the Phytotoxicity of N-Aryl Hydroxamic Acids and Acetanilide Analogues

Germination inhibition activity of N-aryl hydroxamic acids and acetanilide analogues was measured on lettuce seeds (Lactuca sativa). Lipophilicity of the compounds was determined by HPLC. A correlation between lipophilicity values and percentage of germination inhibition was established. A model mechanism of action for auxin was used for analyzing the effect of the substituent at the alpha carbon atom (Cα) on the polarization of hydroxamic and amide functions in relation to the germination inhibition activity observed. Results suggest that the lipophilic and acidic properties play an important role in the phytotoxicity of the compounds. A test with the microalga Chlorella vulgaris was used to evaluate the potential herbicide activity of the hydroxamic acids and acetanilides.