(e) How does the infection of a pathogen affect the host plant physiology ?

Effects of Pathogen Infection on Host Plant Physiology

Pathogen infection significantly alters various physiological processes in host plants, leading to a range of symptoms and ultimately impacting plant health and yield. These disruptions are primarily due to the pathogen's direct actions (e.g., tissue destruction, toxin production, nutrient consumption) and the plant's defense responses.

1. Impact on Photosynthesis

• Chlorophyll Degradation: Many pathogens, especially foliar ones, cause chlorosis (yellowing of leaves) by destroying chlorophyll or inhibiting its synthesis. This directly reduces the plant's ability to capture light energy.
• Reduced Photosynthetic Area: Leaf spots, blights, and defoliation decrease the total green leaf area available for photosynthesis.
• Enzyme Inhibition: Pathogens can produce toxins or interfere with host enzymes crucial for the photosynthetic process, such as those involved in CO₂ fixation (e.g., Rubisco).
• Chloroplast Disruption: Viral and bacterial infections can lead to a decrease in chloroplast numbers, disorientation, or destruction of chloroplast integrity, further impairing photosynthesis.

2. Alterations in Respiration

• Increased Respiration Rate: Infected plants generally exhibit an increased respiration rate, particularly in the infected tissues. This is a common stress response, as the plant expends more energy for defense mechanisms and to repair damaged cells.
• Faster Carbohydrate Consumption: The increased respiration leads to a faster utilization of the plant's stored carbohydrates, diverting energy away from growth and development.
• Metabolic Shifts: Pathogens can induce shifts in host metabolism, favoring pathways that produce compounds beneficial for the pathogen's growth, often at the expense of host energy reserves.

3. Disruption of Water Relations

• Interference with Water Absorption: Root-infecting pathogens (e.g., damping-off fungi, root-rotting bacteria, nematodes causing root galls) damage root integrity, reduce root mass, and alter cell membrane permeability, thereby impairing water and nutrient absorption.
• Impaired Water Translocation: Vascular pathogens (e.g., those causing wilts) proliferate in the xylem vessels, physically blocking them with microbial cells, gums, or host defense compounds. This impedes the upward movement of water, leading to wilting, even when soil moisture is adequate.
• Altered Stomatal Behavior: Foliar pathogens can cause stomata to remain partially closed, reducing transpiration, or, in some cases, induce uncontrolled water loss.

4. Changes in Nutrient Uptake and Translocation

• Reduced Nutrient Absorption: Damage to roots directly impacts the uptake of essential mineral nutrients from the soil.
• Interference with Vascular Transport: Pathogens in xylem (water and minerals) and phloem (sugars and organic nutrients) can block the movement of these vital resources throughout the plant. For instance, obligate fungal parasites like rusts can cause accumulation of photosynthetic products and inorganic nutrients in infected areas, making them unavailable to other parts of the plant.
• Nutrient Diversion: Pathogens often act as "sinks," diverting host nutrients to the infection site for their own growth and reproduction.

5. Hormonal Imbalance and Growth Regulation

• Production of Plant Growth Regulators: Some pathogens produce plant growth regulators (e.g., auxins, gibberellins, cytokinins) or substances that mimic them, leading to abnormal growth patterns.
• Growth Promotion/Inhibition:
  • Hypertrophy (Overgrowth): Excess production of growth-promoting compounds can cause galls, tumors (e.g., Crown gall by Agrobacterium tumefaciens), or witches' broom.
  • Atrophy (Stunting/Retardation): Pathogens can also produce growth inhibitors or interfere with host hormone production, leading to stunted growth, reduced internode elongation, or smaller organs.
• Developmental Changes: Pathogen infection can alter the development of specific tissues or organs, such as the abnormal development of floral parts into leafy structures (phyllody).

6. Effects on Cell Membrane Permeability and Cellular Processes

• Membrane Damage: Pathogens can change the permeability of host cell membranes through mechanical injury, enzymatic degradation, or toxins. This often results in the leakage of electrolytes and other cellular contents.
• Disruption of Transcription and Translation: Viral infections, in particular, can reprogram the host cell's machinery to synthesize viral components, disrupting normal host protein synthesis and genetic expression.

The cumulative effect of these physiological disruptions weakens the plant, reduces its vigor, yield, and reproductive capacity, and often makes it more susceptible to secondary infections or environmental stresses.