Unit 3: Introduction to Bacteria, Mollicutes, Viruses, Phanerogamic Plant Parasites and other Plant pathogens

1. Plant Pathogenic Bacteria

A. General Morphological Characters

Plant pathogenic bacteria are unicellular, prokaryotic microorganisms (lacking a true nucleus and membrane-bound organelles).

1. Shape:
   • Rod-shaped (Bacilli): The vast majority of plant pathogenic bacteria (e.g., Pseudomonas, Xanthomonas, Erwinia).
   • Filamentous (Thread-like): Example: Streptomyces (causes Potato Scab).
   • Spherical (Cocci): Rare in plant pathology.
   • Comma/Spiral: Rare, though Spiroplasma (a mollicute) is helical.
2. Size: generally 0.6 to 3.5 µm in length and 0.5 to 1.0 µm in diameter.
3. Arrangement: Usually exist as single cells, occasionally in pairs (diplobacilli) or chains (streptobacilli).
4. Cell Structure:
   • Cell Wall: Rigid, determines shape.
     • Gram-Negative: Most plant pathogens (thin peptidoglycan layer, outer lipopolysaccharide membrane). Stains pink/red.
     • Gram-Positive: Clavibacter, Streptomyces, Curtobacterium (thick peptidoglycan layer). Stains purple.
   • Capsule/Slime Layer: Extracellular polysaccharide (EPS) layer. Acts as a virulence factor (causes wilting by blocking xylem) and protects against desiccation.
   • Flagella: Organs of locomotion.
     • Monotrichous: Single polar flagellum (Xanthomonas).
     • Lophotrichous: Tuft of flagella at one end (Pseudomonas).
     • Peritrichous: Flagella all over the surface (Erwinia).
     • Atrichous: No flagella (Xylella).

B. Reproduction of Bacteria

Bacteria reproduce primarily through Binary Fission (asexual).

1. DNA Replication: The circular chromosome replicates.
2. Cell Elongation: The cell grows in size; chromosomes move to opposite ends.
3. Septum Formation: The plasma membrane creates a transverse septum (cross-wall) at the center.
4. Separation: The cell splits into two genetically identical daughter cells.
   • Rate: Under optimal conditions, division occurs every 20 minutes (exponential growth).

C. Basic Methods of Classification

Taxonomy is based on Bergey’s Manual of Systematic Bacteriology. Classification relies on a hierarchy: Domain Phylum Class Order Family Genus Species Pathovar.

1. Phenotypic Classification:
   • Morphology: Shape, flagella, Gram staining.
   • Physiological/Biochemical: Carbon source utilization, enzyme production (oxidase, catalase), fluorescence on King's B medium (Pseudomonas fluoresces).
2. Pathovar (pv.): A sub-species classification distinct to plant pathology. It distinguishes strains based on the specific host plant they infect (e.g., Xanthomonas campestris pv. campestris infects cabbage; X. campestris pv. vesicatoria infects tomato).
3. Molecular Classification (Genotypic):
   • 16S rRNA gene sequencing (the gold standard for identification).
   • Fatty Acid Methyl Ester (FAME) analysis.

2. Mollicutes (Phytoplasma and Spiroplasma)

A. Nature and Structure

Mollicutes are prokaryotes belonging to the class Mollicutes. They differ from true bacteria mainly by the absence of a cell wall.

1. Phytoplasma:

   • Morphology: Pleomorphic (variable shape—rounded, filamentous, or irregular) because they lack a rigid cell wall.
   • Size: 0.2 to 0.8 µm.
   • Habitat: Restricted to the Phloem sieve tubes of plants.
   • Nature: Obligate parasites (mostly unculturable on artificial media).
   • Sensitivity: Resistant to Penicillin (target cell wall) but sensitive to Tetracycline (targets ribosomes).

2. Spiroplasma:

   • Morphology: Helical or spiral filaments. Motile (flexing/corkscrew motion).
   • Culturable: unlike phytoplasmas, spiroplasmas can often be cultured on nutrient media.
   • Example: Spiroplasma citri (Citrus stubborn disease).

B. Reproduction

• Reproduce by binary fission, segmentation, or budding.
• Since they lack a cell wall, the division involves constriction of the cell membrane rather than septum formation via peptidoglycan synthesis.

C. Transmission

• Insect Vectors: Primarily leafhoppers, planthoppers, and psyllids. The relationship is often persistent and propagative (pathogen multiplies inside the vector).
• Grafting: Vegetative propagation.
• Dodder: Parasitic plant (Cuscuta) bridges.

3. Fastidious Vascular Bacteria (FVB)

These are bacteria that are difficult to culture (fastidious) and are restricted to vascular tissues. They possess a cell wall (unlike Mollicutes).

A. Xylem-Limited Bacteria (RLO - Rickettsia-Like Organisms)

• Habitat: Restricted to Xylem vessels.
• Characteristics: Gram-negative, rod-shaped, non-motile, usually transmitted by xylem-feeding insects (sharpshooters, spittlebugs).
• Key Example: Xylella fastidiosa.
  • Disease: Pierce’s disease of grapes, Citrus Variegated Chlorosis.
  • Mechanism: Bacteria aggregate in xylem, form biofilms, and block water transport, causing scorch symptoms.

B. Phloem-Limited Bacteria (BLO - Bacteria-Like Organisms)

• Habitat: Restricted to Phloem sieve tubes.
• Characteristics: Gram-negative, bacterial rods.
• Key Example: Candidatus Liberibacter spp.
  • Disease: Citrus Greening (Huanglongbing).
  • Vector: Citrus Psyllid.

4. Plant Viruses

A. Nature and Structure

Plant viruses are sub-microscopic, obligate intracellular parasites consisting of nucleic acid (genome) surrounded by a protein coat.

1. The Genome (Nucleic Acid):
   • ssRNA (Single-stranded RNA): The majority of plant viruses (e.g., Tobacco mosaic virus - TMV).
   • dsRNA (Double-stranded RNA): e.g., Wound tumor virus.
   • ssDNA: e.g., Gemini viruses.
   • dsDNA: e.g., Cauliflower mosaic virus.
2. The Capsid (Protein Coat):
   • Composed of protein subunits called capsomeres.
   • Protects the genome and aids in transmission.
3. Morphology/Shape:
   • Rod-shaped (Rigid): Tobacco mosaic virus (300 x 18 nm).
   • Filamentous (Flexuous): Potato virus Y.
   • Isometric (Polyhedral/Spherical): Cucumber mosaic virus.
   • Bacilliform: Alfalfa mosaic virus.
   • Geminate (Twin particles): Maize streak virus.

B. Replication of Plant Viruses

Viruses do not divide; they replicate by hijacking the host cell's machinery.

1. Entry/Penetration: Virus enters via wounds (mechanical) or vectors. It cannot penetrate intact cuticle/cell wall actively.
2. Uncoating: The protein coat is removed, releasing the viral genome into the host cytoplasm.
3. Gene Expression and Replication:
   • Positive-sense ssRNA viruses: The RNA acts directly as mRNA. Host ribosomes translate it into Replicase (enzyme).
   • Replicase synthesizes complementary RNA strands to create new viral genomes.
4. Synthesis of Coat Protein: Late in the cycle, the viral genome directs the synthesis of capsid proteins.
5. Assembly (Maturation): Viral genomes and capsid proteins self-assemble into complete virions.
6. Movement:
   • Cell-to-Cell: Through plasmodesmata via "Movement Proteins."
   • Systemic: Through the phloem.

C. Transmission of Plant Viruses

1. Mechanical Transmission: Through sap via contact (tools, hands, rubbing leaves). Example: TMV.
2. Vegetative Propagation: Infected tubers, bulbs, cuttings, grafting.
3. Seed and Pollen: Example: Bean common mosaic virus.
4. Vectors (Most Common):
   • Insects:
     • Aphids: Most important vectors (Stylet-borne/non-persistent or Circulative/persistent). Example: Potato virus Y.
     • Whiteflies: Transmit Geminiviruses (e.g., Tomato yellow leaf curl virus).
     • Thrips: Transmit Tospoviruses (e.g., Tomato spotted wilt virus).
     • Leafhoppers/Beetles.
   • Nematodes: E.g., Xiphinema transmits Grapevine fanleaf virus.
   • Fungi/Protists: Olpidium transmits Lettuce big vein virus.
   • Dodder (Cuscuta): Forms a bridge between plants.

5. Phanerogamic Plant Parasites

These are flowering plants (angiosperms) that parasitize other plants for water and nutrients. They produce specialized absorbing organs called Haustoria which penetrate the host tissue (xylem/phloem).

Classification of Parasitic Plants

Detailed Study of Parasites

1. Cuscuta spp. (Dodder)

• Type: Total Stem Parasite.
• Morphology: Yellow/orange, thread-like, leafless vines lacking chlorophyll.
• Hosts: Alfalfa, clover, flax, onions, ornamentals.
• Symptoms: Chlorosis, stunted growth, plants bound together by wire-like stems.
• Dissemination: Seeds and vegetative fragmentation.

2. Loranthus spp. / Dendrophthoe (Giant Mistletoe)

• Type: Partial Stem Parasite.
• Morphology: Green leaves, woody stem, produces berries. Grows on branches of trees.
• Hosts: Mango, Citrus, Teak, Guava.
• Symptoms: Formation of gall at infection site, dieback of the host branch distal to infection.
• Dissemination: Birds eat the berries and deposit sticky seeds on tree branches.

3. Orobanche spp. (Broomrape)

• Type: Total Root Parasite.
• Morphology: Whitish/brownish/purple scaly leaves, lacks chlorophyll. Shoots emerge from the soil at the base of the host.
• Hosts: Solanaceous crops (Tobacco, Tomato, Potato), Mustard, Sunflower.
• Symptoms: Severe stunting, chlorosis, wilting of host.
• Germination: Seeds require chemical stimulants from host root exudates to germinate.

4. Striga spp. (Witchweed)

• Type: Partial (Semi) Root Parasite.
• Morphology: Green leafy shoots, pink/purple flowers. Photosynthetic but relies on host for water/nutrients.
• Hosts: Sorghum, Maize, Sugarcane, Millets (Cereal crops).
• Symptoms: "Bewitched" appearance—host looks drought-stricken even with water available. Yellowing and stunting.
• Significance: Major constraint in African and Asian agriculture.

6. Green Algae and Other Pathogens

A. Parasitic Green Algae

Algae are rare as plant pathogens, but the genus Cephaleuros is significant.

• Organism: Cephaleuros virescens / Cephaleuros parasiticus.
• Classification: Green Algae (Chlorophyta).
• Disease: Red Rust (mostly on fruit trees and plantation crops).
  • Examples: Red rust of Tea, Mango, Guava, Litchi.
• Symptoms:
  • Rusty, red-orange, velvet-like circular spots on leaves or stems.
  • The red color is due to the pigment haematochrome masking the chlorophyll.
  • Severe infection leads to defoliation and twig dieback.
• Structure: Thallus grows beneath the cuticle; reproductive structures (sporangia) break through to the surface.

B. Flagellate Protozoa

• Organism: Phytomonas spp.
• Classification: Protozoa (Trypanosomatids).
• Habitat: Laticifers (latex-bearing cells) or Phloem.
• Diseases:
  • Hartrot of Coconut.
  • Phloem necrosis of Coffee.
• Transmission: Usually by heteropteran insects (bugs).