Model Answer
0 min readIntroduction
Mycoplasmas are a group of bacteria lacking a cell wall, placing them among the smallest self-replicating organisms. Historically, these organisms were identified as the causative agents of several plant diseases where traditional bacterial cultures failed. Initially termed ‘pleuropneumonia-like organisms’ (PPLO), they are now recognized as significant plant pathogens causing a wide range of diseases, often characterized by yellowing, stunting, and phyllody (flower-like growth of leaves). Their unique cellular structure and limited metabolic capabilities contribute to their obligate parasitic lifestyle and complex interactions with host plants.
What is Mycoplasma?
Mycoplasmas are prokaryotic microorganisms belonging to the class Mollicutes. They are characterized by the absence of a cell wall, resulting in a pleomorphic (variable shape) appearance. They possess a triple-layered cell membrane and are typically 0.3-1.0 μm in size. Due to the lack of a cell wall, they are resistant to many antibiotics that target cell wall synthesis. They are obligate parasites, meaning they require a living host to survive and reproduce.
Types of Mycoplasma
Mycoplasmas can be classified based on various criteria, including host range, serological characteristics, and genetic makeup. Here's a breakdown:
- Phytoplasma: These are mycoplasmas that infect plants and are transmitted primarily by insect vectors, particularly leafhoppers and planthoppers. They reside in the phloem tissue of plants.
- Spiroplasma: These mycoplasmas are also plant pathogens, but they are typically motile due to the presence of helical filaments. They can inhabit both phloem and xylem tissues.
- Acholeplasma: Primarily found in animals, some species can infect plants, though less commonly.
A more detailed classification based on 16S rRNA gene sequence analysis has identified numerous phytoplasma groups, often designated by Roman numerals (e.g., Phytoplasma australis, Phytoplasma mali). These groups often correlate with specific host ranges and disease symptoms.
Importance in Plant Pathogenesis
Mycoplasmas are significant plant pathogens due to their ability to cause a wide range of economically important diseases. Their pathogenesis involves several key mechanisms:
- Phloem Limitation: Phytoplasmas colonize the phloem, obstructing nutrient transport and leading to symptoms like yellowing, stunting, and reduced yield.
- Hormonal Imbalance: They can disrupt plant hormone balance, causing phyllody (leafy growth in floral structures), virescence (green coloration of flowers), and other abnormal growth patterns.
- Vector Transmission: Insect vectors play a crucial role in spreading mycoplasmas, making disease control challenging.
- Induced Systemic Resistance (ISR): While primarily pathogens, some mycoplasma infections can induce ISR in plants, enhancing their resistance to other pathogens.
Examples of Plant Diseases caused by Mycoplasmas:
- Aster Yellows: Affects various plants like aster, carrot, and celery, causing yellowing, distortion, and reduced yield.
- Witch’s Broom: Common in plants like coffee and cocoa, characterized by excessive shoot proliferation, resembling a broom.
- Sandle Spike: Affects sugarcane, leading to reduced sugar content and stunted growth.
- Grassy Shoot of Rice: Causes significant yield losses in rice crops, characterized by elongated shoots and reduced grain formation.
| Disease | Host Plant | Symptoms | Mycoplasma Type |
|---|---|---|---|
| Aster Yellows | Aster, Carrot, Celery | Yellowing, Distortion, Stunting | Phytoplasma |
| Witch’s Broom | Coffee, Cocoa | Excessive Shoot Proliferation | Phytoplasma |
| Grassy Shoot | Rice | Elongated Shoots, Reduced Grain | Phytoplasma |
Conclusion
Mycoplasmas, particularly phytoplasmas, represent a significant threat to agricultural productivity worldwide. Their unique characteristics – lack of a cell wall, obligate parasitism, and insect vector transmission – make them difficult to control. Understanding their diverse types, pathogenic mechanisms, and the diseases they cause is crucial for developing effective management strategies, including vector control, disease-resistant cultivars, and diagnostic tools. Further research into their interactions with host plants and the environment is essential for mitigating their impact on global food security.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.