Microbial Allelochemicals and Pathogens as Bioherbicidal Agents¹

Abstract

The initiative to use plant pathogens and allelochemicals from pathogens and other microorganisms as biological weed control agents (bioherbicides) began about 30 yr ago. Since then, numerous plant pathogens (bacteria and fungi) and microbial allelochemicals have been isolated, identified, and tested for their bioherbicidal potential. Pathogens (and in some cases microbial phytotoxins) may be used directly on target weed species, or such allelochemicals may provide unique chemical templates for the synthesis of new herbicide classes with novel molecular modes of action. To date, the most successful microbial products that have led to the development of commercial herbicides are bialaphos (commercially available in Japan) and glufosinate (marketed worldwide). Glufosinate is the ammonium salt of phosphinothricin, which is the active ingredient of bialaphos derived from a nonphytopathogenic Streptomyces species. This overview will examine selected advances in the isolation and identification of novel plant pathogens that have weed hosts, and some microbial allelochemicals with phytotoxic properties. Perspectives on the use of these bioherbicides in weed control, relative to their allelopathic interactions with plants will be discussed.

Phytotoxic effects of metabolites from Alternaria euphorbiicola against its host plant Euphorbia heterophylla

“…Fungal phytotoxins are regarded as a promising source for the discovery of new natural herbicide chemotypes, 19,20 or at the very least, suggest molecules that could serve as templates in the search for novel synthetic analogs with useful herbicidal action. 21,22 Because many of these metabolites are non-selective toxins, we decided to evaluate the phytotoxicity of the isolated metabolites against other important weeds by the leaf spray assay (Figure 3).…”

“…Particularly for necrotrophic fungi, secretion of low-molecular-weight phytotoxins constitutes a strategy to kill plant cells in the initial stages of the interaction and gain access to host nutrients. 4 The alleged involvement of the metabolites produced by A. euphorbiicola in the early process of invasion of host plant tissues is reinforced by the fast development of lesions (which became evident in <24 h after application) produced by these compounds on the host plant leaves and by the similarity between the damage caused by these metabolites and the first symptoms observed after the application of spore suspensions of A. euphorbiicola on E. heterophylla.Fungal phytotoxins are regarded as a promising source for the discovery of new natural herbicide chemotypes, 19,20 or at the very least, suggest molecules that could serve as templates in the search for novel synthetic analogs with useful herbicidal action. 21,22 Because many of these metabolites are non-selective toxins, we decided to evaluate the phytotoxicity of the isolated metabolites against other important weeds by the leaf spray assay (Figure 3).…”

Persistent and Transient Inhibition of Seed Germination by Fractions of Methanol-Water Extract of Wheat Straw

“…Such chemicals may have a high potential to provide alternative and more natural ways of controlling and managing weed populations [3] and in the framework of allelopathic investigation the search for chemicals with herbicide activity has grown over the years. Microbes [4], fungi [5,6] and a variety of higher plants [7][8][9] have been investigated with that purpose.…”

Bioherbicides: Research and Risks

“…Since then, numerous microbes have been screened for phytotoxic potential, and several dozens evaluated as bioherbicides as reported by various researchers and summarized (e.g., Hoagland, 1990Hoagland, , 2001TeBeest, 1991). Due to the interest in this area, many other weed pathogens and phytotoxins (from pathogenic and nonpathogenic microorganisms) will be discovered that possess bioherbicidal activity.…”