Question

Choose the correct order of the following microbes based on their requirement of host - from dead to living
(A). Saprotrophs
(B). Biotrophs
(C). Hemibiotrophs
(D). Nectrotrophs

• A. (A), (B), (C), (D)
• B. (A), (D), (C), (B)
• C. (B), (A), (D), (C)
• D. (C), (B), (D), (A)

Hint:

To remember this scale: Saprotrophs feed on dead material; Necrotrophs kill first then feed; Hemibiotrophs do both sequentially; Biotrophs must keep the host alive to feed.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Plant-associated microbes utilize different nutritional strategies to obtain carbon and energy from host plants, ranging from living cells to dead organic matter.

Step 2: Detailed Explanation:
Let us order the micro-organisms based on their ecological requirement of host tissue state, progressing from purely dead tissue to strictly living host cells:
1. Saprotrophs (A): These organisms are non-parasitic decomposers.
They grow exclusively on dead, decaying organic matter and have no requirement for a living host.
2. Necrotrophs (D): (Note: "Nectrotrophs" is a typographical error in the paper for "Necrotrophs").
These are pathogens that actively kill host plant cells using toxins and cell-wall-degrading enzymes.
Once the host tissue is dead, they colonize and feed on the dead plant matter.
Their requirement is therefore focused on dead/killed cells.
3. Hemibiotrophs (C): These are transitional pathogens.
They spend the initial phase of their life cycle feeding on living host tissues without causing visible cell death (biotrophic phase).
Eventually, they transition to a necrotrophic phase, killing host cells to feed on the dead tissue.
Hence, they represent an intermediate requirement between living and dead.
4. Biotrophs (B): These are obligate parasites that depend entirely on the host's viable cytoplasm for survival and reproduction throughout their life cycle.
They require active, living cells and establish specialized structures like haustoria to absorb nutrients without killing the host cell.
Thus, the correct sequence from dead to living host requirement is: Saprotrophs (A) \(\rightarrow\) Necrotrophs (D) \(\rightarrow\) Hemibiotrophs (C) \(\rightarrow\) Biotrophs (B).

Step 3: Final Answer:
The correct order is (A), (D), (C), (B).