Plant pathogenic Bacteria

A large group of bacteria cause disease in plants. they have specific characteristics and structure. There are different mechanism by which bacteria affect the plant and cause disease symptom. It is generally survive in soil and dead and decay organic matters and spread by water, agricultural implements, propagating materials, insects and humans. Hence, management practices are designed accordingly. Crop rotation, field sanitation, disinfestation of agricultural implements, use of disease free or resistant varieties and use of antibiotics are few of them.

1. Plant pathogenic bacteria Dr Bhagyashree Khamari Assistant Professor (Plant Pathology) Department of Plant Pathology Institute of Agricultural Sciences Siksha O Anusandhan (Deemed to be University) Bhubaneswar, Odisha .

2. Characteristics of bacteria Prokaryotes Single celled or unicellular Single cells usually group in masses or in chains Absence of well defined nucleus Microscopic Ubiquitous Parasitic, saprophytic and photosynthetic but most are obligatory saprophyte ( necrotrophs ).

3. Can be gram + ve or gram – ve . Can be cultured in artificial media. Motile or Non-motile Flagellate Bacteria grows in the intercellular spaces and do not invade them . High degree of resistance to high temperature (75 ℃) , low temperature(-19 ℃) and severe drought . All the plant pathogenic bacteria are mesophilic (they can grow at a temperature of 20-35ºC)

4. Reproduction (asexual method) Binary fission Budding Conidia

5. Binary fission

6. Bacteria enters through wound or natural openings such as stomata, lenticels or hydathodes . They can survive in soil (as soil inhabitant or soil invaders), plant debris, organic matters. Spread by water (irrigation water or rain water), agricultural implemen ts, seed, insects, human and animals.

7. 80 species consisting many sub species ( Pathovars or pv ) cause plant disease. Important plant pathogenic genera:  Pseudomonas, Xanthomonas , Erwinia , Pectobacterium , Pantoea , Agrobacterium, Ralstonia , Burkholderia , Acidovorax , Clavibacter , Streptomyces, Xylella .

8. Bacteria produces toxins or enzymes or special proteins that break down key structural components of plant cells and their walls.  Important symptoms: Galls and overgrowths, wilts, leaf spots, specks, blights, soft rots, scabs and cankers .

9. Gall Soft rot Canker Scab

10. Ooze exudation Ooze test Wilt

11. Shape of Bacteria Rod Shaped (Bacilli), Spherical ( Cocci ), Spiral ( Spirilli ), Coma Shaped ( Vibrios ) Or Thread Like (Filamentous).

12. Colour of Bacteria Single bacterium mostly appears as hyaline or yellowish white under the compound microscope, when grown on a medium The colonies of most of bacteria have a whitish or greyish appearance but some of them develop yellow, red or other colours.

13. Structure of Bacteria

14. Bacterial Cell wall A bacterium has a thin, relatively tough, rigid cell wall. It composed of peptidoglycan consisting of chain of alternating N-acetyl muramic acid and N-acetyl glucosamine units cross linked by tetrapeptide and pentaglycine units Cell wall provide immuological distinction and variation among strains of bacteria. It allows the inward passage of nutrients and the outward passage of waste matter and digestive enzymes .

15. Difference between Gram+ve and Gram - ve Gram + ve Cell wall is thicker and homogemous . Contains lower content of lipids (5-10%) Peptidoglycan comprises up to 90% of the cell wall and hence maximum lipid. Techoic acid present. Cell wall has higher amino sugar content (10-20%) Cell wall is simple in shape and is single layered . Mesosomes more prominent. Retains violet dye Examples : Bacillus, Clavibacter , Streptomyces Gram – ve Cell wall is thinner and usually thin layered . Contains higher content of lipids (up to 40 %). Peptidoglycan comprises only 10%. Techoic acid absent . Low content of amino sugars Varying cell wall shape and is tripartite (3-layered ). Mesosomes less prominent. Retains red dye Examples : Erwinia , Pseudomonas, Xanthomonas , Agrobacterium , Xylell

16. Cell membrane: It is composed phospholipid (bilayer) and protein. Permeable to nutrients, ions and others. Function: Regulate the passage of substance due to permeability. Synthesis of membrane lipids Synthesis of cell wall peptidoglycan Assembly and secreation of extra cytoplasmic proteins Co-ordination of DNA replication and segregation with septum formation during cell division.

17. Mesosome Cytoplasmic membrane invagination in the form of tubular or vesicle shaped is called as mesosomes . Mesosome in the centre (central mesosome ) is used in cell division . Peripheral mesosome is used in transport of extracellulaar enzymes from cytoplasm to exterior regions.

18. Capsule If many cells embedded in a common matrix then it is slime layer . It is made up of viscous gummy material, mostly composed of polysaccharides but may rarely contain amino sugars, sugar acids, etc. When the layer is thick and firm, it is called capsule. Generally plant pathogenic bacteria lack capsule but some of them like Pseudomonas and Xanthomonas produce slime . Capsule or slime or micro capsule often called glycocalyx . Most capsule are polyssacharide but some are polypeptide.

19. Function Protection from temporary dryness Block attachment of bacteriophage Prevent phagocytosis. Provide virulence to the bacteria Promote attachment of bacteria Prevent cell aggregation in suspension by electric charge. Important role in biofilm application

20. Flagella These are thread like structure, which are usually longer than the cell Used for locomotion. They are very delicate and fragile, 10-12 nm in width . Thinner than eukaryotic flagella Made up of flagellin protein All spiral, few sperical and half of the rod shaped bacteria are motile by flagella

21. Parts of a Flagellum Filament: It is helical, outermost region of flagellum, composed of flagellin (protein), synthesized in the cell, which moves to the hollow core of the flagellum to the tip. Hook : Filament is attached to hook which is wider than the flagellum. This is 45 nm wide and made up of different types of protein. The hook of gram positive bacterium is longer than that of gram negative bacteria. Basal body: It consists of small central rod which is inserted into a system of rings.

22. c lassification of rod shaped plant pathogenic bacteria on the basis of flagellation Atrichous : Without flagella. Monotrichous : Single flagellum at one end. Cephalotrichous / Lophotrichous : Several flagella at one end. Amphitrichous : Atleast one flagellum at each end Peritrichous : Flagella present on whole periphery/surface.

23. Atrichous Monotrichous Amphitrichous Peritrichous Lophotrichous Cephalotrichous

24. Pili or Fimbriae These are hollow, non-helical, filamentous, thinner, shorter, numerous hair like structures. Generally shorter than the flagella and are thicker (3-15 nm in diameter). They are made up of protein sub-units known as pilin . Common in gram – ve but also present in archae and gram + ve Both flagella and pili originate from cell membrane and extend outward through the cell wall. Function: Adherence to the substrate, determine the virulence, provide resistance from phagocytosis, involve in conjugation (sex pilli ).

25. Genome: It includes both chrmosomal DNA and plasmid. Chromosomal DNA is a long coiled, double stranded, single circular DNA. It appears as a spherical, ellipsoidal, dumb-bell or Y-shaped body in the cytoplasm, but without any membrane. The nuclear material consists of large circular chromosome, composed of DNA. It is also known as nucleoid or bacterial chromosome or chromatin body. Some species also have additionally single or multiple copies of smaller circular genetic material called plasmids

26. Plasmid These are small, circular, covalently closed, self replicating, extrachromosomal DNA. They control specific characters like pathogenicity, nodulation etc. Plasmids can move from bacterium to bacterium or bacterium to plants. This special property is used in genetic engineering for transformation of desired genes from one plant to another by using it as vector.

27. Plasmid Chromosomal DNA

28. Ribosomes: It is used in protein synthesis. Never bound to any organelles in cytoplasm . Endospore It is a spore within the cells Resistant to unfavourable or stress condition . On arrival of adverse condition protoplast concentrates into spherical form and a thick wall develops around it. On return of favorable condition endospore comes out of bacterial cell and protoplast germinates to give rise to a new bacteria.

29. Classification of Bacteria Gram – ve Do not form endospores Thin cell walls consisting of outer membrane E .g. Psudomonas Xanthomonas , Erwinia Gram+ve Some of them produce endospore. Thick (firm) cell wall and unit membrane but without any outer membrane. E.g. Streptomyces Lack of cell wall and cells are enclosed by a unit membrane only. E.g. Phytoplasma

30. Detailed Classification of Phytopathogenic Bacteria Kingdom : Prokaryotae Division I: Gracilicutes Class: Proteobacteria (mostly single-celled, non-photosynthetic ) Order : Eubacteriales Family1 : Enterobacteriaceae (They are peritrichous bacteria) Genus: Erwinia Family2: Rhizobiaceae Genus: Agrobacterium, Rhizobium (Symbiotic, Nitrogen fixing bacteria) Family3: Azotobacteriaceae Genus: Azotobacter Order: Pseudomonadales Family: Pseudomonadaceae Genus: Pseudomonas Genus : Xanthomonas

31. Division 2: Firmicutes (Gram + ve ) Class 1: Firmibacteria (Simple gram positivebacteria ) Genus : Bacillus Bacillus subtilis – biocontrol agent Genus: Clavibacter ( Corynebacterium ) Class 2: Thallobacteria (Gram positive, branching bacteria) Order : Actinomycetales Family 1: Actinomycetaceae Genus: Actinomyces Family 2: Streptomycetaceae Genus : Streptomyces Family 3: Mycobacteriaceae Genus: Mycobacterium

32. Division 3: Tenericutes Class: Mollicutes (wall less prokaryotes) Family: Spiroplasmataceae Spiroplasma citri causing citrus stubborn Spiroplasma kunkelii causing corn stunt Phytoplasma

33. Use Resistant varieties Use Bacteria-free seed or propagation materials. Maintain proper sanitation Disinfestation of pruning tools . Crop rotation to reduce over-wintering. Preventing surface wounds that permit the entrance of bacteria into the inner tissues. Prolonged exposure to dry air, heat, and sunlight will sometimes kill bacteria in plant material. Applications of copper-containing compounds or Bordeaux mixture. Apply antibiotic s such as streptomycin and/or oxytetracycline prior to infection and reduce spread of the disease, but they will not cure plants that are already diseased. Insect control will help to eliminate vectors or reduce feeding wounds that can provide points of entry. The use of antagonistic for managing bacterial diseases of plants. Implementation of Government Regulatory Measures to restrict the introduction or movement of pathogens or infected plant material. Management of Bacterial disease in Plants

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