Author: Sonam Singh Chandel
It is very evident that plant pathogens also have the foe to maintain their population in natural ecosystem as well as in agroecosystem. Interestingly, potential to contain plant pathogens makes theses biological entities very special to agriculturists or scientific fraternity and well known as bio-agents/ biological control agents. Bio-control of plant pathogens involves the use of an organism to inhibit the pathogen and reduce disease incidence or severity by direct and indirect manipulation of microorganisms. The term “Biological control” has been used in different fields of biology, most notably entomology and plant pathology. In both fields, the organism that suppresses the pest or pathogen is referred to as the Biological Control Agent (BCA). In entomology, it has been used to describe the use of live predatory insects, entomopathogenic nematodes, or microbial pathogens to suppress populations of different pest insects. In plant pathology, the term applies to the use of microbial antagonists to suppress diseases as well as the use of host-specific pathogen to control weed populations.
It is quite clear, to develop an effective disease management program, use of potential bio-agents is essential. Bio-agents of plant diseases are most often referred to as antagonists and a viable alternative strategy for the control of plant diseases. Fungi belonging to the genusTrichoderma spp. and bacteria such as Pseudomonas spp., Bacillus subtilis are the most promising bio- agents against a wide range of plant pathogens under a variety of environmental conditions. Integration of bio-agents in a compatible manner may enhance the effectiveness of disease control and provide better management of soil-borne diseases with no environmental hazards. Compatible bio-agents in an IDM strategy protects the seeds and seedlings from soil-borne and seed-borne inoculum. Interestingly, the use of antagonists eliminates the chance of resistance development and reduces the fungicide and insecticide application. A variety of biological control agents is available for use. However, inconsistent performance of most bio-agents under field conditions is the major challenge in the adoption at a vast level. Therefore, understanding of underlying mechanisms of action and to identify the compatibility of the potential bio-agents for the eco-friendly management of the plant diseases is a way forward for sustainable disease management in an agroecosystem. Moreover, identification and cloning of genes that offer great promise as transgenes to produce crop resistance against plant pathogens. So, application of biotechnological tools in a disease management system can full fill our dreams to facilitate a healthier and cleaner agriculture. Therefore, this article is presented as the key mechanisms employed by the bio-agents and a survey of the use of biological control as it is applied to the suppression the plant diseases.
The mechanisms employed by Bio-agents
Mainly the plant pathogens antagonized by the presence and activities of other organisms that they counter and it is directionality related to the amount of interspecies contact and specificity of the host-pathogen interaction. There're various mechanisms employed by the bio-agents to counteract various plant diseases broadly classified into 3 categories:
- Direct antagonism
- Mixed-path antagonism
- Indirect antagonism
Direct antagonism
It results from the physical contact and/or high degree of selectivity for the pathogens by bio-agents. It includes hyper-parasitism/predation, hyper-parasitism would be considered the most direct form of antagonism due to its tropic growth of bio-agent towards the target organism, attack and dissolution of the cell wall or membrane by the activity of enzymes. The pathogen is directly attacked by a bio-agent that kills it or its propagules. There are several fungal parasites of plant pathogens, including those that attack Sclerotia (e.g.Coniothyrium minitans) while others on living hyphae (e.g. Pythium oligandrum) and, a few of the fungi that have the capacity to parasitize powdery mildew pathogens shows multiple hyper-parasitism (e.g. Cladosporium oxysporum, and Acremonium alternatum, Gliocladium virens etc.)
Some bio-agents exhibit predatory behaviour under nutrient-limited conditions. For example, some species of Trichoderma spp. produce certain enzymes like chitinase and β-1,3 glucanase has excellent mycoparasitic activity against Rhizoctonia solani hyphae.
Mixed-path antagonism
Antibiotic-mediated suppression
Antibiotics are microbial toxins that act at low concentrations, poisonous or kill other micro-organisms which are secreted by some microbes having antibiotic activity. In some instances, antibiotics have been shown to be particularly effective at suppressing the growth of the target pathogen in vitro and/or in situ. Several bio-agent strains are known to produce multiple antibiotics which can suppress one or more pathogens. For example, Bacillus cereus strain UW85 is known to produce both zwittermycin and kanosamine which are effective against the suppression of damping-off diseases. More recently, Pseudomonas putida WCS358r strains genetically engineered to produce phenazine and DAPG displayed improved capacities to suppress plant diseases in field-grown wheat.
Secretion of Lytic Enzymes
Many microorganisms produce and release lytic enzymes that can interfere with pathogen growth and/or activities directly. It can hydrolyze a wide variety of polymeric compounds including chitin, cellulose, hemicellulose, and DNA. For example, control of Sclerotium rolfsii by Serratia marcesens appeared to be mediated by chitinase expression and β-1,3 glucanase contributes significantly to biocontrol activities of Lysobacter enzymogenes strain C3. Similarly, the production of HCN by certain Fluorescent pseudomonads involved in the suppression of root pathogens by blocking the cytochrome oxidase pathway.
Indirect antagonism
Competition
The competition between pathogens and non-pathogens for space and nutrient resources is very important for restricting disease incidence and severity. Both the bio control agents and the pathogens compete with one another for the nutrients and space to get established in the environment. To survive in such an environment, microorganisms secrete iron-binding ligands called siderophores as in the case of Erwinia caratovora. Siderophores chealate Fe (II) ions and the membrane binding protein receptors specifically recognize and take up the Siderophores-Fe-complex. Thus the iron becomes unavailable to the pathogen resulting fewer chances of
Table:1 List of plant diseases control by Bio-control agents in different crops
| Crop | Disease | Target pathogen | Species/strains of Bio-control agents | References |
| Beans | Halo blight Graymold | Pseudomonas syringae pv. phaseolicola Botrytis cinerea | Pseudomonas putida & P. fluorescens Pseudomonas aeruginosa | Alstrom, 1991 Meyer et. al., 1997 |
| Cucumber | Bacterial wilt Fusarium wilt Angular leaf spot Anthracnose Viral disease | Erwinia tracheiphila Fusarium oxysporum f.sp.cucumerinum P. syringae pv. lachrymans Colletotrichum orbiculare Cucumber mosaic virus (CMV) | Pseudomonas putida, Serratia marcescens 90-166 & Flavimonas oryzihabitans ENR-5 Bacillus pumilus INR-7 & Pseudomonas putida 89B-27 P. fluorescens 89B-27 & S. marcescens | Kloepper et al.,1993, Liu et al., 1995a&b Wei et al., 1996 Raupach et al.; 1996 |
| Rice | Sheath blight Bacterial blight Blast | Rhizoctonia solani Xanthomonas oryzae pv. oryzae Pyricularia oryzae | Bacillus subtilis MBI600 Delftia tsuruhatensis Burkholderia cepacia | Krishna Kumar et al., 2012 Jigang Han et al., 2005 Homma et al., 1989 |
| Sugarcane | Red rot | Colletotrichum falcatum | P. fluorescens & P. putida | Viswanathan & Samiyappan, 2002 |
| Carnation | Fusarium wilt | Fusarium oxysporum f. sp. dianthi | P. putida | Van Peer et al., 1991 |
| Wheat | Damping-off & root rot Take-all | Pythium ultimum Gaeumannomyces graminis var. tritici | P. fluorescens, Streptomyces griseoviridis, Candida oleophila & Trichoderma virens P. fluorescens 2- 79 & 30-84 | Howell & Stipanovic, 1980 Thomashow et al., 1990 |
| Tomato | Damping-off Root-rot | Rhizoctonia solani Pythium aphanidermatum | Bacillus subtilis QST713 B. subtilis BBG100, Enterobactercloaceae, T. hamatum | Paulitz & Belanger, 2001 Kloepper et al., 2004 Leclere et al., 2005 |
| Citrus, Pineapple, Black pepper | Root rots | Fusarium solani Phytophthora spp. Rhizoctonia solani | Pseudomonas stutzeri Trichoderma harzianum, T. virens | Ho-Seong Lim et al., 1991 Wilhite et al., 2001 |
| Apple & Pear | Graymold diseases Fire blight | Botrytis cinerea Erwinia amylovora | Candida membranifaciens & Rhodotorula mucilaginosa Pantoea agglomerans C9-1 | Sui et al., 2012 Sandra et al., 2001 |
| Oilseed crops | Aflatoxin contamination | Aspergillus flavus | Bacillus subtilis AU195 | Moyne et al., 2001 |
| Cereals | Seed-borne diseases | Microdochium nivale, Cochliobolus sativus, Drechslera graminea, Ustilago avenae, Tilletia caries | Pseudomonas chlororaphis | Johnsson, 1998 |
Induction of host resistance
Induction of host defenses can be localized and/or systemic in nature. Plants actively respond to a variety of environmental stimuli and chemical stimuli such as high temperature, water, physical stress and nutrient availability. Chemical stimuli can either induce host plant defenses through biochemical changes that enhance resistance against subsequent infection by a variety of pathogens. The first of these pathways, called systemic acquired resistance (SAR), mediated by salicylic acid (SA) which typically leads to the expression of pathogenesis-related (PR) proteins including a variety of enzymes. A second phenotype, referred to as induced systemic resistance (ISR), mediated by jasmonic acid (JA) and/or ethylene, which are produced following applications of some non-pathogenic rhizobacteria. Some biocontrol strains of Pseudomonas spp. and Trichoderma spp. are known to strongly induce plant host defenses. Likewise, Bacillus mycoides strains able to produce peroxidase, chitinase and β-1,3-glucanase in sugar beet andPseudomonas putida strains producing a lipopolysaccharide in Arabidopsis are the most striking example of bacterial determinants and types of diseases resistance.
References
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About Author / Additional Info:
I am currently pursuing MSc in Plant Pathology from SVBP Univ. Agri. & Tech. Meerut (Uttar Pradesh)-250110.