What is the Difference Between Biotrophs and Necrotrophs

The main difference between biotrophs and heterotrophs is that biotrophs derive nutrients from living cells, whereas necrotrophs derive nutrients from dead or dying cells.

Biotrophs and necrotrophs are two plant pathogens infecting plants. They are different types of fungi.

Key Areas Covered

1. What are Biotrophs
     – Definition, Facts, Examples
2. What are Necrotrophs
     – Definition, Facts, Examples
3. Similarities Between Biotrophs and Necrotrophs
     – Outline of Common Features
4. Difference Between Biotrophs and Necrotrophs
     – Comparison of Key Differences

Key Terms 

Biotrophs, Necrotrophs

What are Biotrophs

Biotrophs are a successful group of plant pathogens that live on living plants. Examples of biotrophs include rust fungi, powdery mildews, and genera, including Ustilago, Cladosporium, and Magnaporthe. However, they are destructive plant pathogens, causing substantial economic losses and threatening global food security. During the infection, biotrophs secrete effector proteins that translocate into the plant cell’s cytosol, altering the host’s response to the infected pathogen. They also suppress the immune system of plants. Additionally, they reprogram the host to produce nutrients required by the pathogen.

Figure 1: Rust Fungi

However, biotrophs are obligate plant pathogens that do not kill the host cells. But they penetrate the plant cell wall and colonize the intercellular space of the plant tissue. They also produce feeding structures such as haustoria to absorb plant nutrients. But, they do not penetrate the plasma membrane of plant cells. In addition, biotrophs maintain a strike balance between virulence and evading host defense. 

What are Necrotrophs

Necrotrophs are plant pathogens that complete their life cycle on dead plant tissues. They cause a significant economic loss in numerous agronomic hosts by causing pre- and post-harvest diseases. In contrast to obligate biotrophs, necrotrophs induce the destruction of host cells to fulfill the nutrient requirement of the necrotrophs. Therefore, necrotrophs kill host cells and thrive on the content of the dead or dying plant tissue.

Figure 2: Cochliobolus miyabeanus

In contrast to biotrophs, necrotrophs produce fewer infectious structures. But necrotrophs are more potent than biotrophs in pathogenicity as they secrete virulence factors throughout the infection. Therefore, necrotrophs have more destructive pathogenicity. Cochliobolus spp., Pyrenophora spp., Alternaria spp., Parastagonospora nodorum, Phytophthora capsici, Macrophomina phaseolina, and Moniliophthora river are examples of necrotrophs.

Similarities Between Biotrophs and Necrotrophs

• Biotrophs and necrotrophs are two types of plant pathogens.
• They are fungi.
• They cause a significant economic loss on plant crops.  

Difference Between Biotrophs and Necrotrophs

Definition

Biotrophs are organisms that can live and multiply only on another living organism. In contrast, necrotrophs refer to a parasitic organism that kills the living plant cells of its host and then feeds on the dead.

Type of Pathogen

Biotrophs are obligate pathogens, while necrotrophs are opportunistic non-obligate pathogens.  

The Killing of Host Cells

While biotrophs do not kill host cells, necrotrophs rapidly kill host cells. 

Entry

Biotrophs have a direct entry or through natural openings, while necrotrophs enter via wounds or natural openings.

Lytic Enzymes and Toxins

Biotrophs produce little lytic enzymes and toxins, while necrotrophs secrete lytic enzymes and toxins.

Appressoria/haustoria 

Furthermore, biotrophs produce appressoria/haustoria, while necrotrophs do not produce appressoria/haustoria.

Systemic

Biotrophs are often systemic, while necrotrophs are seldom systemic.  

Type of Attack

Biotrophs attack all ages of plants, while necrotrophs attack young, weak, and damaged plants.

Host Range

In addition, biotrophs have a narrow host range, while necrotrophs have a broad one.

Growth

Biotrophs grow intercellularly, while necrotrophs grow both intercellularly and intracellularly. 

Culturing

Biotrophs are difficult to culture axenically, while necrotrophs are easy to culture axenically.

Conclusion

In brief, biotrophs and necrotrophs are fungi and plant pathogens. Biotrophs are obligate pathogens that do not kill host cells. They have a direct entry and do not produce lytic enzymes or toxins. They attack all ages of plants, and their infection is often systemic. Moreover, they have a narrow host range, and they grow intercellularly. They are difficult to culture. In comparison, necrotrophs are non-obligate pathogens that rapidly kill host cells. They enter through wounds and natural openings of the plant. However, they produce lytic enzymes and toxins. Meanwhile, they attack plants that are weak, young, and damaged. They have a broad host range. In addition, they grow intercellularly and intracellularly and are easy to culture. Therefore, the main difference between biotrophs and necrotrophs is the type of pathogens. 

References:

1. Koeck M, Hardham AR, Dodds PN. The role of effectors of biotrophic and hemibiotrophic fungi in infection. Cell Microbiol. 2011 Dec;13(12):1849-57. doi: 10.1111/j.1462-5822.2011.01665.x. Epub 2011 Sep 14. PMID: 21848815; PMCID: PMC3218205.
2. Laluk K, Mengiste T. Necrotroph attacks on plants: wanton destruction or covert extortion? Arabidopsis Book. 2010;8:e0136. doi: 10.1199/tab.0136. Epub 2010 Aug 10. PMID: 22303261; PMCID: PMC3244965.

Image Courtesy:

1. “Rust fungus on Sorbus sp. (44405239292)” By Björn S.. – Own work (CC-BY SA 2.0) via Commons Wikimedia
2. “Cochliobolus miyabeanus” By Donald Groth, Louisiana State University AgCenter – Own Work (CC-BY 3.0) via Commons Wikimedia