https://abulafia.ciencias.uchile.cl/publicaciones/pdf/49-Phytochemistry-27-3349-1988.pdf

Hydroxamic acids (4-hydroxy-1,4-benzoxazin-3-ones), defence chemicals in the Gramineae

Weeds are known to cause enormous losses due to their interference in agroecosystems. Because of environmental and human health concerns, worldwide efforts are being made to reduce the heavy reliance on synthetic herbicides that are used to control weeds. In this regard the phenomenon of allelopathy, which is expressed through the release of chemicals by a plant, has been suggested to be one of the possible alternatives for achieving sustainable weed management. The use of allelopathy for controlling weeds could be either through directly utilizing natural allelopathic interactions, particularly of crop plants, or by using allelochemicals as natural herbicides. In the former case, a number of crop plants with allelopathic potential can be used as cover, smother, and green manure crops for managing weeds by making desired manipulations in the cultural practices and cropping patterns. These can be suitably rotated or intercropped with main crops to manage the target weeds (including parasitic ones) selectively. Even the crop mulch/residues can also give desirable benefits. Not only the terrestrial weeds, even allelopathy can be suitably manipulated for the management of aquatic weeds. The allelochemicals present in the higher plants as well as in the microbes can be directly used for weed management on the pattern of herbicides. Their bioefficacy can be enhanced by structural changes or the synthesis of chemical analogues based on them. Further, in order to enhance the potential of allelopathic crops, several improvements can be made with the use of biotechnology or genomics and proteomics. In this context either the production of allelochemicals can be enhanced or the transgenics with foreign genes encoding for a particular weed-suppressing allelochemical could be produced. In the former, both conventional breeding and molecular genetical techniques are useful. However, with conventional breeding being slow and difficult, more emphasis is laid on the use of modern techniques such as molecular markers and the selection aided by them. Although the progress in this regard is slow, nevertheless some promising results are coming and more are expected in future. This review attempts to discuss all these aspects of allelopathy for the sustainable management of weeds.

Allelopathic Interactions and Allelochemicals: New Possibilities for Sustainable Weed Management

Allelopathy studies the interactions among plants, fungi, algae and bacteria with the organisms living in a certain ecosystem, interactions that are mediated by the secondary metabolites produced and exuded into the environment. Consequently, allelopathy is a multidisciplinary science where ecologists, chemists, soil scientists, agronomists, biologists, plant physiologists and molecular biologists offer their skills to give an overall view of the complex interactions occurring in a certain ecosystem. As a result of these studies, applications in weed and pest management are expected in such different fields as development of new agrochemicals, cultural methods, developing of allelopathic crops with increased weed resistance, etc. The present paper will focus on the chemical aspects of allelopathy, pointing out the most recent advances in the chemicals disclosed, their mode of action and their fate in the ecosystem. Also, attention will be paid to achievements in genomics and proteomics, two emerging fields in allelopathy. Rather than being exhaustive, this paper is intended to reflect a critical vision of the current state of allelopathy and to point to future lines of research where in the authors' opinion the main advances and applications could and should be expected.

Allelopathy--a natural alternative for weed control.

Many cereals accumulate hydroxamic acids derived from 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one. These benzoxazinoid hydroxamic acids are involved in defense of maize against various lepidopteran pests, most notably the European corn borer, in defense of cereals against various aphid species, and in allelopathy affecting the growth of weeds associated with rye and wheat crops. The role of benzoxazinoid hydroxamic acids in defense against fungal infection is less clear and seems to depend on the nature of the interactions at the plant-fungus interface. Efficient use of benzoxazinoid hydroxamic acids as resistance factors has been limited by the inability to selectively increase their levels at the plant growth stage and the plant tissues where they are mostly needed for a given pest. Although the biosynthesis of benzoxazinoid hydroxamic acids has been elucidated, the genes and mechanisms controlling their differential expression in different plant tissues and along plant ontogeny remain to be unraveled.

Hydroxamic acids derived from 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one: key defense chemicals of cereals.

Benzoxazinoid biosynthesis, a model for evolution of secondary metabolic pathways in plants.

https://abulafia.ciencias.uchile.cl/publicaciones/pdf/51-Phytochemistry-28-447-1989.pdf

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

Analogues of the aglucones of naturally occurring cyclic hydroxamic acids (2,4-dihydroxy-1,4-benzoxazin-3-ones) from Gramineae (Poaceae) have been synthesized by the reductive cyclization of the ring-substituted methyl α-(o-nitrophenoxy)-α-methoxyacetates, followed by demethylation of the C-2 methoxy group with BBr 3 or BCl 3 to reveal the 2-hydroxy group. A structure-activity series was produced by varying the substituent at C-7 on the aromatic ring [R=MeO, t-BU, Me, H, Cl, F, CO 2 Me]. The remarkable rate enhancement provided by an oxa functionality suggests that reduction occurs by direct attack of thiolate on the hydroxamic nitrogen of a resonance-stabilized ion pair

Analogs of the cyclic hydroxamic acid 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3-one (DIMBOA): decomposition to benzoxazolinones and reaction with .beta.-mercaptoethanol

Natural hydroxamic acids and related compounds derived from 1,4-benzoxazin-3-one structures inhibited the growth of the alga Chlorella xanthella and the fungus Candida albicans. On the basis of structure−activity relationships the lipophilic character of the substituent in the aromatic ring and the electrophilic character of the hydroxamic function are suggested to be responsible for antialgal and antifungal activity, respectively. Keywords: Hydroxamic acids; Chlorella xanthella; Candida albicans; antialgal/antifungal activity

Antialgal and Antifungal Activity of Natural Hydroxamic Acids and Related Compounds

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.

/pdf/hydroxamic-acids-in-secale-cereale-l-and-the-relationship-1yzdhaddbw.pdf

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

Analogues of the Cyclic Hydroxamic Acid 2,4‐Dihydroxy‐7‐methoxy‐2H‐1,4‐ benzoxazin‐3‐one: Decomposition to Benzoxazolinones and Reaction with . beta.‐Mercaptoethanol.

Héctor R. Bravo

Papers

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

Analogs of the cyclic hydroxamic acid 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3-one (DIMBOA): decomposition to benzoxazolinones and reaction with .beta.-mercaptoethanol

Antialgal and Antifungal Activity of Natural Hydroxamic Acids and Related Compounds

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

Analogues of the Cyclic Hydroxamic Acid 2,4‐Dihydroxy‐7‐methoxy‐2H‐1,4‐ benzoxazin‐3‐one: Decomposition to Benzoxazolinones and Reaction with . beta.‐Mercaptoethanol.