Unit3 - Subjective Questions

Plant pathogenic bacteria exhibit specific morphological characteristics, primarily defined by their shape and the arrangement of their flagella.

1. Shape:
Most plant pathogenic bacteria are rod-shaped (bacilli). They are typically cylindrical with rounded ends. However, the genus Streptomyces is filamentous.

• Dimensions: Usually range from $0.6$ to $3.5$ in length and $0.5$ to $1.0$ in diameter.

2. Flagellar Arrangement:
Bacteria may possess flagella, which are hair-like appendages used for locomotion. The arrangement is taxonomically significant:

• Monotrichous: A single flagellum at one end (polar). E.g., Xanthomonas.
• Lophotrichous: A tuft or cluster of flagella at one end. E.g., Pseudomonas (some species).
• Amphitrichous: One or more flagella at both ends.
• Peritrichous: Flagella distributed over the entire surface of the cell. E.g., Erwinia, Pectobacterium.
• Atrichous: No flagella (non-motile).

The distinction between Gram-positive and Gram-negative bacteria is based on the structure of their cell walls and how they react to the Gram staining procedure.

1. Gram-Positive Bacteria:

• Cell Wall: Contains a thick layer of peptidoglycan (murein) which comprises 50-90% of the cell wall. It also contains Teichoic acids.
• Staining Reaction: They retain the primary stain (Crystal Violet) because the thick peptidoglycan layer shrinks upon decolorization with alcohol, trapping the dye-iodine complex.
• Color: They appear purple/violet under a microscope.
• Examples: Clavibacter, Streptomyces, Curtobacterium.

2. Gram-Negative Bacteria:

• Cell Wall: Contains a thin layer of peptidoglycan (only 5-10%) and an outer membrane composed of lipopolysaccharides (LPS), lipoproteins, and phospholipids.
• Staining Reaction: They lose the primary stain during decolorization because the alcohol dissolves the outer lipid membrane and the thin peptidoglycan cannot retain the dye. They take up the counterstain (Safranin).
• Color: They appear pink/red.
• Examples: Pseudomonas, Xanthomonas, Erwinia, Ralstonia.

Binary fission is the primary method of asexual reproduction in bacteria, where a single parent cell divides into two genetically identical daughter cells.

Steps involved:

1. Replication of DNA: The bacterial chromosome (nucleoid) uncoils and replicates. The DNA attaches to a specialized part of the cell membrane (mesosome).
2. Cell Elongation: The cell grows in size, elongating to separate the two DNA molecules, pushing them to opposite poles of the cell.
3. Septum Formation: The cell membrane and cell wall begin to grow inward from the center (invagination) to form a transverse septum or cross-wall.
4. Separation: The septum fully divides the cytoplasm, creating two distinct chambers.
5. Cell Division: The two daughter cells separate. They are clones of the parent cell.

Depending on the species and environmental conditions, this process can occur rapidly (e.g., every 20 minutes), leading to exponential population growth.

Although bacteria reproduce asexually, they achieve genetic variation through horizontal gene transfer mechanisms:

1. Conjugation:

• This involves direct physical contact between two bacterial cells via a structure called a pilus (sex pilus).
• A plasmid (F-plasmid) or part of the chromosome is transferred from a donor cell () to a recipient cell ().
• It requires cell-to-cell contact.

2. Transformation:

• This is the uptake of naked DNA fragments from the surrounding environment by a competent bacterial cell.
• The DNA usually comes from dead or lysed bacteria. If there is homology, the new DNA is integrated into the recipient's genome.

3. Transduction:

• This process is mediated by a bacteriophage (virus infecting bacteria).
• Generalized Transduction: The phage accidentally packages a piece of bacterial DNA instead of viral DNA and injects it into a new host bacterium.
• Specialized Transduction: The phage integrates into the bacterial genome and later excises, taking adjacent bacterial genes with it to transfer to a new host.

Mollicutes are a class of bacteria distinguished by the absence of a cell wall. In plant pathology, the important members are Phytoplasmas and Spiroplasmas.

General Characteristics:

• Lack of Cell Wall: They are devoid of a rigid cell wall and are bounded only by a trilaminar unit membrane. Consequently, they are pleiomorphic (variable in shape).
• Size: They are very small, typically ranging from $100$ to $1000$ nm in diameter, passing through bacterial filters.
• Genome: They have a very small genome size compared to true bacteria.
• Habitat: They are obligate parasites found in the phloem sieve elements of infected plants.
• Transmission: They are transmitted primarily by phloem-feeding insects (vectors), such as leafhoppers and psyllids, and through vegetative propagation (grafting, tubers).
• Resistance: They are resistant to penicillin (which targets cell walls) but are sensitive to tetracycline antibiotics.
• Culture: Most plant pathogenic Mollicutes (Phytoplasmas) cannot be cultured on artificial media (fastidious), whereas Spiroplasmas can be cultured.

Phytoplasma vs. Spiroplasma:

Fastidious Vascular Bacteria (FVB) are plant pathogenic bacteria that are restricted to specific vascular tissues (xylem or phloem) of the host plant and are extremely difficult or impossible to culture on standard bacteriological media. They require complex, nutrient-rich media or can only be maintained in host tissue.

1. Xylem-Limited Bacteria (XLB):

• These bacteria inhabit the xylem vessels and cause symptoms like scorching, wilting, and stunting due to water blockage.
• Example: Xylella fastidiosa (Causes Pierce's disease of grapes, Citrus variegated chlorosis).
• Vector: Typically transmitted by xylem-feeding insects like sharpshooters.

2. Phloem-Limited Bacteria (PLB):

• These bacteria inhabit the phloem sieve tubes and disrupt nutrient transport, causing yellowing and stunting.
• Example: Candidatus Liberibacter species (Causes Citrus Greening or Huanglongbing).
• Vector: Transmitted by phloem-feeders like Psyllids.

Nature of Plant Viruses:
Plant viruses are sub-microscopic, obligate intracellular parasites. They are nucleoproteins, consisting of a nucleic acid core (genome) surrounded by a protein coat. They lack cellular machinery for energy production and protein synthesis, relying entirely on the host cell to replicate.

General Structure:

1. Capsid: The outer protective protein coat is called the capsid. It protects the genetic material.
2. Capsomeres: The capsid is made up of repeating protein subunits called capsomeres. The arrangement of capsomeres determines the virus shape (helical or icosahedral).
3. Nucleic Acid: Located centrally. Plant viruses primarily contain RNA (mostly ssRNA), but some contain DNA (e.g., Caulimoviruses have dsDNA, Geminiviruses have ssDNA).
4. Envelope: Some viruses possess an outer lipid membrane derived from the host cell, known as an envelope (e.g., Tospoviruses). Most plant viruses are non-enveloped.
5. Virion: The complete, infectious virus particle is called a virion.

Tobacco Mosaic Virus (TMV) is the most extensively studied plant virus. It is a rigid, rod-shaped virus.

• Morphology: It is a rigid rod measuring approximately $300$ nm in length and $18$ nm in diameter.
• Symmetry: It exhibits helical symmetry.
• Capsid: The protein coat is composed of approximately 2,130 identical protein subunits (capsomeres). These subunits are arranged in a helix around the RNA core.
• Nucleic Acid: It contains a single strand of positive-sense RNA (ssRNA).
• Assembly: The RNA strand is coiled within the hollow groove formed by the protein subunits. The RNA consists of about 6,400 nucleotides.
• Molecular Weight: The total molecular weight is approximately Daltons, with the protein constituting about 95% and RNA about 5%.

The replication of a positive-sense ssRNA plant virus involves several distinct stages within the host cell:

1. Entry and Uncoating: The virus enters the cell (usually through wounds or vectors). The protein coat (capsid) is removed, releasing the viral RNA into the cytoplasm.
2. Translation (Early): The viral genomic RNA acts as messenger RNA (mRNA). Host ribosomes translate it to produce Replicase (RNA-dependent RNA polymerase).
3. Genome Replication:
   • The Replicase synthesizes a complementary negative-sense RNA strand ( strand) using the genomic positive-sense RNA ( strand) as a template.
   • This forms a double-stranded intermediate (Replicative Form).
   • The strand then serves as a template to synthesize multiple copies of new viral ssRNA genomes.
4. Translation (Late): The newly synthesized RNA strands are translated to produce Coat Proteins (capsid proteins) and movement proteins.
5. Assembly (Maturation): The new viral RNA genomes interact with the coat proteins to self-assemble into complete virions.
6. Movement/Release: Virions move to adjacent cells via plasmodesmata (assisted by movement proteins) or are released when the cell dies/vector acquires them.

Plant viruses require a mechanism to breach the rigid plant cell wall to cause infection. Major modes of transmission include:

1. Mechanical Transmission: Occurs through the sap of infected plants coming into contact with wounds on healthy plants (e.g., via wind, tools, or human handling). Common for TMV.
2. Insect Vectors: The most common method. Insects feeding on sap carry the virus.
   • Aphids: Transmit Potyviruses, Cucumber mosaic virus.
   • Whiteflies: Transmit Geminiviruses (e.g., Leaf curl viruses).
   • Leafhoppers/Thrips: Transmit Tospoviruses.
3. Vegetative Propagation: Viruses are systemic; thus, using infected tubers, cuttings, bulbs, or graft scions spreads the virus (e.g., Potato virus X).
4. Seed and Pollen: Some viruses are seed-borne (transmitted via embryo) or carried in pollen (e.g., Bean common mosaic virus).
5. Nematode Vectors: Soil-inhabiting nematodes transmit viruses like Nepoviruses and Tobraviruses by feeding on roots.
6. Fungal/Protist Vectors: Some soil-borne fungi (e.g., Olpidium) and plasmodiophorids (e.g., Polymyxa) transmit viruses like Tobacco necrosis virus.

Phanerogamic (flowering) plant parasites are classified into four groups based on two criteria:

1. Degree of Dependence: Total (Holoparasite) vs. Partial (Hemiparasite).
2. Site of Attack: Stem vs. Root.

Classification:

1. Total Stem Parasite:

   • Depends entirely on the host for water and nutrients. Lacks chlorophyll.
   • Example: Cuscuta spp. (Dodder).

2. Partial (Semi) Stem Parasite:

   • Has chlorophyll and photosynthesizes but relies on the host for water and minerals.
   • Example: Loranthus spp. (Giant mistletoe), Viscum spp.

3. Total Root Parasite:

   • Attacks the roots and lacks chlorophyll. Usually subterranean until flowering.
   • Example: Orobanche spp. (Broomrape).

4. Partial (Semi) Root Parasite:

   • Attacks roots but possesses green leaves for photosynthesis.
   • Example: Striga spp. (Witchweed).

Characteristics:

• Cuscuta (Dodder) is a Total Stem Parasite.
• It appears as a yellow, orange, or pink thread-like vine intertwined around the host stems.
• It lacks leaves (reduced to scales) and chlorophyll.
• It produces haustoria (root-like structures) that penetrate the host stem to connect with both xylem and phloem to extract water and nutrients.

Disease Cycle:

1. Germination: Seeds germinate in the soil. The seedling creates a temporary root and a slender shoot that rotates (circumnutation) seeking a host.
2. Attachment: If the shoot contacts a susceptible host, it twines around it and develops haustoria to penetrate the vascular bundles.
3. Detachment: Once established, the connection to the soil withers away, making it fully dependent on the host.
4. Spread: The vine grows rapidly, spreading to adjacent plants, forming a dense mat.
5. Reproduction: It produces clusters of small white flowers and thousands of seeds, which can remain dormant in the soil for years.

Striga (Witchweed):

• Classification: It is a Semi-root parasite belonging to the family Orobanchaceae.
• Hosts: It is a serious pest of cereal crops like Sorghum, Maize, Sugarcane, and Pearl Millet.
• Characteristics: It has green leaves and can synthesize its own food (photosynthesis), but it lacks a developed root system. It relies on the host plant for water and minerals.
• Germination: Striga seeds require a chemical stimulant exuded by the host plant roots (strigolactones) to germinate.
• Damage: It attaches to the host roots via haustoria. The major damage occurs underground before the parasite emerges, causing severe stunting, yellowing, and wilting of the host, often resembling drought stress.
• Seed Production: A single plant can produce up to 500,000 microscopic seeds that remain viable in soil for decades.

Cephaleuros is a genus of parasitic green algae (Phylum: Chlorophyta).

Disease Caused: Red Rust

• Host: It commonly affects fruit trees like Mango, Guava, and plantation crops like Tea and Coffee.
• Symptoms:
  • The disease typically appears on the upper surface of the leaves (sometimes on twigs/stems).
  • It forms rusty, reddish-brown, velvet-like circular spots.
  • The red color is due to the haematochrome pigment in the algal thallus.
  • Severe infection can lead to defoliation, twig dieback, and reduced photosynthetic area, weakening the plant.
• Structure: The algal thallus grows beneath the cuticle of the leaf. During reproduction, sporangiophores emerge through the cuticle, releasing zoospores.

In the classification of bacteria (specifically according to the International Code of Nomenclature of Bacteria), many plant pathogenic bacteria cannot be distinguished based on standard physiological or biochemical tests.

• Definition: A Pathovar (pv.) is a sub-specific subdivision used to classify bacteria based on their pathogenicity to a specific host plant or group of plants.
• Usage: It is used when bacteria of the same species are morphologically and culturally identical but cause disease in different hosts.
• Example:
  • Xanthomonas campestris is a bacterial species.
  • Xanthomonas campestris pv. citri infects Citrus.
  • Xanthomonas campestris pv. malvacearum infects Cotton.
  • Xanthomonas campestris pv. oryzae infects Rice.
    These variants are distinguished solely by their host range.

Difference between Viruses and Viroids:

Plant pathogenic protozoa are microscopic, single-celled eukaryotes belonging to the family Trypanosomatidae. They are typically found in the latex or phloem of plants.

Characteristics:

1. Morphology: They are unicellular and possess a single flagellum used for locomotion. The body is typically spindle-shaped or twisted.
2. Genera: The two main genera associated with plant diseases are Phytomonas.
3. Habitat: They inhabit the laticiferous (latex-bearing) tubes or phloem sieve elements.
4. Transmission: They are transmitted by insect vectors, particularly phytophagous bugs (e.g., Lygaeid bugs, Pentatomid bugs).
5. Diseases:
   • Phloem Necrosis of Coffee: Caused by Phytomonas leptovasorum.
   • Hartrot of Coconut: Associated with Phytomonas species.
   • Symptoms include yellowing, wilting, and eventual death of the plant.

Comparison between Orobanche and Loranthus:

1. Orobanche (Broomrape):

• Type: Total Root Parasite.
• Chlorophyll: Absent (Achlorophyllous). It is yellowish or brownish.
• Host Dependency: Obligate. Relies entirely on the host for organic food, water, and minerals.
• Attachment: Attaches to the roots of the host (e.g., Tobacco, Mustard, Tomato, Brinjal).
• Visibility: Only the flowering spikes are seen above ground; the vegetative part is subterranean.

2. Loranthus (Dendrophthoe):

• Type: Partial (Semi) Stem Parasite.
• Chlorophyll: Present. It has green leaves and can photosynthesize.
• Host Dependency: Partial. Relies on the host primarily for water and minerals, which it extracts from the xylem.
• Attachment: Attaches to the branches/stem of the host (e.g., Mango, Teak, Citrus).
• Visibility: Grows as a bush on the branches of trees.

Plant viruses are of immense economic importance because they cause significant yield losses, reduce produce quality, and increase production costs due to vector management.

Economic Impact:

1. Yield Reduction: Viruses disrupt photosynthesis and metabolism, leading to stunting and reduced fruit/grain set.
2. Quality Deterioration: They cause deformations, discoloration (mosaics), and necrosis, making fruits, flowers, and vegetables unmarketable.
3. Longevity: Viral infections are systemic and incurable in the field; once a plant is infected, it remains so, often requiring the rogueing of entire fields.

Major Viral Diseases:

• Tobacco Mosaic Virus (TMV): Affects tobacco, tomato, and peppers. Causes mosaic patterns and leaf distortion.
• Tomato Spotted Wilt Virus (TSWV): Causes bronzing, ring spots, and necrosis in a wide range of crops.
• Rice Tungro Disease: Caused by a complex of two viruses (RTSV and RTBV), leading to severe stunting and yellow/orange discoloration in rice.
• Papaya Ring Spot Virus (PRSV): Causes ring spots on fruit and severe leaf distortion, devastating papaya plantations.
• Yellow Vein Mosaic of Bhindi (Okra): Causes vein clearing and chlorosis, significantly reducing fruit yield.