74. Which one of the following is NOT a typical feature of the hypersensitive response observed in
plants during the attack of invading microbes?
A. Onset of programmed cell death in the region surrounding the infection site
B. Rapid accumulation of reactive oxygen species
C. Rapid spike in photosynthetic rate
D. Rapid spike in nitric oxide production accompanying the oxidative burst

Introduction:

The hypersensitive response (HR) is a crucial defense mechanism in plants that occurs when they are attacked by pathogens. The HR involves a series of rapid cellular changes, including localized programmed cell death (PCD), production of reactive oxygen species (ROS), and other molecular responses that limit pathogen spread. While the HR is a protective measure, it comes with various physiological changes that enhance the plant’s ability to fight off the infection.

Features of the Hypersensitive Response:

Let’s explore the typical features of the hypersensitive response and identify which option is NOT characteristic of this defense process:

1. A. Onset of programmed cell death in the region surrounding the infection site

   • Correct. Programmed cell death (PCD) is a hallmark of the hypersensitive response. The plant triggers PCD in cells surrounding the infection site to limit the spread of the pathogen and isolate the infection. This is a controlled cell death that helps prevent further damage to the plant.

2. B. Rapid accumulation of reactive oxygen species

   • Correct. During the HR, the plant undergoes a rapid oxidative burst, leading to the accumulation of reactive oxygen species (ROS) such as hydrogen peroxide. ROS are toxic to pathogens and act as signaling molecules to activate other defense mechanisms in the plant.

3. C. Rapid spike in photosynthetic rate

   • Incorrect. A rapid spike in photosynthetic rate is NOT a typical feature of the hypersensitive response. In fact, during the HR, energy is diverted from processes like photosynthesis to defense mechanisms such as the oxidative burst and the activation of protective proteins. The plant’s metabolic activity is often temporarily slowed down to redirect energy toward combating the infection.

4. D. Rapid spike in nitric oxide production accompanying the oxidative burst

   • Correct. Nitric oxide (NO) is another important signaling molecule in the hypersensitive response. The production of NO is often observed alongside the accumulation of ROS, and it plays a role in activating downstream defense responses, including the regulation of programmed cell death.

Conclusion:

The hypersensitive response in plants is a complex defense mechanism involving multiple molecular changes aimed at limiting pathogen spread. While programmed cell death, reactive oxygen species accumulation, and nitric oxide production are typical features, a rapid spike in photosynthetic rate is NOT associated with the HR. The correct answer is:

C. Rapid spike in photosynthetic rate