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Difficulty: HardPlant Science and Agricultural Technology

With reference to transgenic Bt (*Bacillus thuringiensis*) technology used in agricultural crop protection, consider the following statements:

1. The crystalline insecticidal proteins produced by *Bacillus thuringiensis* are inactive protoxins that require the alkaline pH environment of the insect midgut for solubilization and activation.
2. Transgenic crops expressing *Cry1Ac* genes provide broad-spectrum protection primarily against sap-sucking insect pests such as aphids and whiteflies.
3. The mandatory planting of non-Bt 'refuge' crops surrounding Bt crop fields is an agronomic management strategy intended to delay the development of insect resistance to Bt toxins.

Which of the statements given above is/are correct?

  1. A
    1 and 2 only
  2. 1 and 3 onlyAnswer
  3. C
    2 and 3 only
  4. D
    1, 2 and 3

Answer

1 and 3 only
The correct response identifies statements 1 and 3 as true. The Bt crystal protoxin requires an alkaline midgut pH for conversion into an active toxin that disrupts gut cell membrane integrity. Furthermore, non-Bt refuge planting is a established IPM strategy designed to preserve susceptible alleles within target pest populations, slowing down natural selection for resistance.

Step-by-Step Solution

1
Analyze Statement 1 regarding the mechanism of action of Bt endotoxins.
Bt bacteria produce inactive crystal proteins (protoxins). Upon ingestion by susceptible insects, these protoxins are solubilized in the alkaline environment of the insect midgut and cleaved by proteases into active toxins, which bind to midgut epithelial cells and create pores. Thus, statement 1 is correct.
Understanding the biochemical trigger for Bt toxin activation is essential for evaluating plant biotechnology applications.
2
Analyze Statement 2 regarding the target pest spectrum of Cry1Ac gene products.
The Cry1Ac gene specifically encodes toxins active against lepidopteran pests (such as the American bollworm, Pink bollworm, and tobacco budworm). Sucking pests (order Hemiptera, such as aphids, jassids, and whiteflies) are not susceptible to Cry1Ac toxins. Thus, statement 2 is incorrect.
Distinguishing between pest orders and their specific susceptibility to Cry protein sub-classes is key to agronomic crop protection analysis.
3
Analyze Statement 3 regarding the agronomic purpose of refuge crop strategies.
Planting non-Bt refuge crops ensures a localized population of susceptible insects that mate with any rare homozygous resistant insects emerging from Bt fields. This dilutes resistance genes in the pest population and significantly delays resistance development. Thus, statement 3 is correct.
Insect resistance management (IRM) strategies are vital components of sustainable agricultural biotechnology policy.

Key Concept

Bt Transgenic Technology and Insect Resistance Management
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Topics

Plant Science and Agricultural Technology

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