Biological control agents in plant disease control 한혜수

Introduction

Competition for space and nutrients One of the classic demonstrations of competiton for space, infection sites or nutrients as a mode of action concerns the control of fireblight caused by the pathogenic bacterium, Erwinia amylovora by the non-pathogenic bacterium Pseudomonas fluorescens A506(Lindow&Leveau, 2002) There is also evidence that ectomycorrhizal fungi, by way of their physical sheathing of the root, may also occupy pathogen infection sites, thereby preventing infection(Whipps, 2004) competition for nutrients alone ranging from simple carbon-and nitrogen-containing compounds to complex plant residues, in a range of situations is also a commonly reported mode of action for both bacterial and fungal BCAs Consequently, most microorganisms produce iron-chelating compounds termed siderophores to competitively acquire ferric iron. Many siderophores produced by bacteria have a very high affinity for frerric iron, and their release sequesters the limited supply of iron making it unavailable to pathogenic fungi, thereby restricting their growth(Loper&Henkels, 1999)

Production of antibiotics Microorganisms commonly produce such metabolites during the course of their growth and only if production at the site of biocontrol is confirmed, or activity implied by use of either non-producing or over-producing mutants, or reporter strains, can a role in biocontrol be assured. The regulation of many of these bacterial antibiotics has been explored and involvement of regulatory genes, and sigma factors, and key signal molecules has been found. Gene clusters responsible for production of several antibiotics produced by bacteria have now been cloned and manipulated to examine the potential to enhance biocontrol activity in other strains(Hammer et al., 1997;Nowak-Thompson et al., 1999; Timms-Wilson et al., 2000; Chin-A-Woeng et al., 2001;Delany et al., 2001; Huang et al.,2004) The potential for detection of additional, and potentially novel, gene clusters responsible for antibiotic production and other modes of action is likely to ensue as complete genomic sequences such as that for the BCA strain Pseudomonas fluorescens Pf-5(Paulsen et al., 2005;Loper et al., 2007)become available. The situation is less clear cut with fungi where molecular manipulation methods are generally less advanced and more complex, especially for producing single insert gene mutants.

Parasitism and production of extracellular lytic enzymes This is a relatively simple phenomenon for bacteria where degradation of target cell walls is generally considered to reflect parasitism, and may range from simple attachment of bacterial cells to hyphae with minimal degradation, through biofilm formation to complete lysis and cell wall breakdown(Mitchell&Hurwitz,1965; Nelson et al., 1986; Bolwerk et al., 2003) With fungal BCAs the process of parasitism of fungal plant pathogens, or mycoparasitism, is more complex than that for bacteria and a series of interlinked phases of hyphal-hyphal interactions have been recorded especially for Trichoderma spp. Production of extracellular enzymes that degrade virulence factors such as N-acyl homoserine lactones have been mentioned previously as a mode of action involved with regulation of antibiotic production(Molina et al., 2003; Dong et al., 2004)but this general concept is growing in importance.

Induced resistance 생물또는 비생물적인 병원체에 의한 활성화 된 물리적 또는 화학적 방어벽 기주식물에로 인하여 저항을 나타내는 것을 유도 저항성이라고 한다. (Kloepper et al.,1992) 유도저항성(ISR)은 식물근권세균의 처리에 의하여 저항성이 유도되는 것으로 lasmonic acid(JA)나 ethylene이 신호물질의 역할을 한다. 획득저항성(SAR)은 화학물질이나 비병원성균의 처리에 의하여 저항성이 유도되는것으로 salicylic acd(SA)가 전달물질로의 역할을하고, 저항성 산물로 감염 특이적 단백질(PR-protein)이 생성되는경우를 말한다. 병 저항성이 가동되게 되면 식물체내의 항균성 Chitinase, β-1,3-glucanase, β-1,4-glucosidase 등 병저항성에 관련된 protein과 phytoalexine이 증가하게 되어 식물체는 차후의 병원균 침입으로부터 자신을 보호할 수 있게 된다. 저항성 유도 물질 - lipopolsaccharides(LPS) - siderophores - flagella or the protein subunit flagellin - bacterial volatiles - salicylate - the cyclic peptide syringilin - antibiotics

Pant-growth promotion 식물생장촉진근권세균(PGPR)은 식물근권에서 다양한 기작에 의해서 식물의 생장을 촉진하며 Bacillus 속과 Pseudomonas 속 등이 PGPR 미생물로 알려져 있다. → 질소고정, 인산의 가용화, mycorrhizal과 rhizobia function의 증진, 뿌리의 에틸렌 생성의 조절, phytohormanes의 release, 중금속 독성의 감소 식물생장촉진균(RGPs)은 Trichoderma spp.가 가장 잘 알려져 있다.

Hypovirulence 저병원성 균주 내에는 병원성 균주의 병원성을 약화시키는 겹가닥 RNA(dsRNA)가 존재하고 이들 dsRNA는 균사융합을 통해서 저병원성 균주로 이동됨으로써 병원성 균주가 저병원성으로 변환되고 병의 진전이 지연되거나 정지하게 된다. 유럽에서 Cryphonectria parasitica에 저병원성을 이용하여 밤나무 줄기마름병을 방제(Heiniger&Rigling, 1994)

Production, formulation and application