Bioherbicidal potential of rhizosphere microorganisms for ecofriendly weed management

Abstract

Weeds pose a serious constraint to agricultural production and usually result in average ~37% losses of the world's agricultural output. Thus, weed control is indispensable in every crop production system. For weed management, chemical herbicides are usually applied due to uncertainty of effects caused by mechanical methods, and it also involves more labour. Although these herbicides are quite effective in controlling the weeds, their indiscriminate use causes environmental problems and human health hazards; moreover continuous use of herbicides may lead to evolution of resistant weed biotypes and shift in the weed flora. These prob- lems necessitated the search for an alternate ecofriendly method of weed manage- ment through the biological approach, in which microorganisms or their products could be used to suppress the growth of weed species. Many naturally occurring microorganisms in the rhizosphere have the potential to suppress the growth of weeds through the manipulation of rhizosphere ecosystem. These rhizosphere microorganisms colonize the root surfaces of weed seedlings and suppress the growth of weed plant by reducing weed density, biomass and its seed production. Many rhizosphere bacteria such as Pseudomonas aeruginosa, P. fluorescens, Erwinia herbicola, Alcaligenes sp.; strains of Xanthomonas campestris pv. poan-nua, Pseudomonas syringae pv. tagetis and P syringae pv. phaseolicola, Serratia plymuthica and S. marcescens; and the fungi including Colletotrichum gloeospori-oides, Aeschynomene virginica, Phomo chenopodicola and Exserohilum monoceras have been characterized as bioherbicides. The mode of action of each biocontrol agent is variable, and it may range from simple compounds like cyanide, organic acids, secondary metabolites (antibiotic 2,4-diacetylphloroglucinol) and plant growth regulators, such as auxins (indole acetic acid and 8-aminolevulinic acid). Bacterial and fungal microbes also produce a wide array of phytotoxins that inter- fere with metabolism of weed plants and cause plant mortality. Thus, there are immense possibilities for developing microbial bioherbicides that could reduce the application of chemical herbicides for weed control and may increase the produc- tion of cereal, oil seeds and legume crops.