Chemical Interactions with Bioherbicides to Improve Efficacy

Abstract

Bioherbicides can be defined as plant pathogens, phytotoxins derived from pathogens or other microorganisms, augmentatively applied to control weeds. Although many pathogens with bioherbicidal potential have been discovered, most lack sufficient aggressiveness to overcome weed defenses to achieve adequate control. Plants use various physical and biochemical mechanisms to defend against pathogen infectivity, including callose deposition, hydroxyproline-rich glycoprotein accumulation, pathogenesis-related proteins (PR-proteins), phytoalexin production, lignin and phenolic formation, and free radical generation. Some herbicides, plant growth regulators, specific enzyme inhibitors, and other chemicals can alter these defenses. Various pathogens also produce chemical suppressors of plant defenses. Secondary plant metabolism is a major biochemical pathway related to several defense processes. Increased activity of a key enzyme of this pathway, phenylalanine ammonia-lyase (PAL), is often a response to pathogen attack, as demonstrated in two weeds and their associated bioherbicidal pathogens:Alternaria cassiaeon sicklepod andA. crassaon jimsonweed. Weakening of physical and biochemical defenses, and lowering of resistance to pathogen attack, may result from reduced production of phenolics, lignin, and phytoalexins caused by herbicides and other chemicals that affect cuticular component biosynthesis and/or key aspects of secondary plant metabolism. Potent PAL inhibitors [aminooxyacetic acid, α-aminooxy-β-phenylpropionic acid, and (l-amino-2-phenylethyl)phosphonic acid] have some regulatory action on secondary plant metabolism and pathogenicity. Various herbicides and other chemicals dramatically affect extractable PAL activity levels and/or substantially alter PAL product production. Some non-pathogenic organisms can alter herbicide efficacy, and some herbicides influence disease development in plants. Research has shown some synergistic interactions of microbes and chemicals with relevance to weed control. Further research on pathogen interactions with agrochemicals (or other chemicals/regulators) could result in increased efficacy of pathogen-herbicide combinations, reduction of herbicide and pathogen levels required for weed control, and expanded pathogen host range.

Selected Phenolic Compounds in Cultivated Plants: Ecologic Functions, Health Implications, and Modulation by Pesticides

“…Possibly as a protective reaction to the oxidative damages, plants increase the PAL synthesis and synthesize more flavonoids. For example, xanthotoxin content in celery is increased after the application of acifluorfen ( (52,53) amide, and carbamate classes) are able to reduce the PAL synthesis in soy seedlings (54). Some fungicides can also modulate concentrations of secondary compounds.…”

“…Direct effects involve the colonization of plants with pathogenic microorganisms or the change of the plant's attractivity to herbivores (54). Pesticides can also affect the secondary metabolism indirectly by eliminating nontarget plants, that compete for light and nutrients or serve as habitats for pathogens and herbivores (53).…”

Rhizobacteria able to produce phytotoxic metabolites

“…Although these substances are essential to obtain the necessary food and fiber, they also present some negative aspects as the increase of cost production and the contamination of human beings and the environment with toxic residues (13,23,24). Consequently, methodologies less expensive and toxic to control weed are greatly welcome.…”

Investigation of the fungi associated with dieback of prickly acacia (Vachellia nilotica subsp. indica) in Northern Australia

“…Although the use of fungi is considered safer and more economically viable compared to other weed control tools, its efficacy is influenced by several factors such as the extent of inherent or elicited host resistance to the potential bioherbicide fungi, desired level of weed control, variation in soil type, topography and other ecological factors such as temperature and moisture availability (Auld & Morin 1995;Hoagland 1996;Ghorbani et al 2005;Iffat et al 2011;Bailey 2014 Chalak et al 2011;Bailey 2014). …”