Phytoalexins produced in plants act as toxins to the attacking organism. They may puncture the cell wall, delay maturation, disrupt metabolism or prevent reproduction of the pathogen in question. Their importance in plant defense is indicated by an increase in susceptibility of plant tissue to infection when phytoalexin biosynthesis is inhibited. Mutants incapable of phytoalexin production exhibit more extensive pathogen colonization as compared to wild type. As such, host-specific pathogens capable of degrading phytoalexins are more virulent than those unable to do so.

When a plant cell recognizes particles from damaged cells or particles from the pathogen, the plant launches a two-pronged resistance: a general short-term response and a delayed long-term specific response.

As part of the induced resistance, the short-term response, the plant deploys reactive oxygen species such as superoxide and hydrogen peroxide to kill invading cells. In pathogen interactions, the common short-term response is the hypersensitive response, in which cells surrounding the site of infection are signaled to undergo apoptosis, or programmed cell death, in order to prevent the spread of the pathogen to the rest of the plant.

Long-term resistance, or systemic acquired resistance (SAR), involves communication of the damaged tissue with the rest of the plant using plant hormones such as jasmonic acid, ethylene, abscisic acid or salicylic acid. The reception of the signal leads to global changes within the plant, which induce genes that protect from further pathogen intrusion, including enzymes involved in the production of phytoalexins. Often, if jasmonates or ethylene (both gaseous hormones) is released from the wounded tissue, neighboring plants also manufacture phytoalexins in response. For herbivores, common vectors for disease, these and other wound response aromatics seem to act as a warning that the plant is no longer edible.[citation needed] Also, in accordance with the old adage, "an enemy of my enemy is my friend," the aromatics may alert natural enemies of the plant invaders to the presence thereof.

Explanation:
Phytoalexins are substances that are produced by plants in response to microbial infection. They play a crucial role in the defense mechanisms of plants against pathogens. Here's a detailed explanation of why phytoalexins are secreted in the cell of the host plant due to infection caused by fungi:
1. Infection caused by fungi:
- Fungi are a type of microorganism that can cause various diseases in plants, such as fungal infections.
- When a plant is infected by fungi, it recognizes the presence of the pathogen and triggers a defense response.
- One of the defense mechanisms is the production and secretion of phytoalexins.
2. Role of phytoalexins:
- Phytoalexins are toxic compounds that are produced by plants in response to microbial attack.
- They act as chemical barriers to inhibit the growth and spread of pathogens, including fungi.
- Phytoalexins can directly inhibit the growth of fungi or interfere with their metabolic processes, ultimately reducing the severity of the infection.
3. Mechanism of phytoalexin production:
- When a plant detects the presence of fungal pathogens, it activates specific genes involved in phytoalexin biosynthesis.
- These genes are responsible for the production of enzymes that catalyze the synthesis of phytoalexins.
- The synthesized phytoalexins are then transported to the site of infection and accumulate in the infected cells, where they exert their antifungal activity.
4. Examples of phytoalexins:
- Different plants produce different types of phytoalexins, depending on the specific pathogen they are defending against.
- For example, isoflavonoids are phytoalexins produced by leguminous plants like soybeans in response to fungal infection.
- Other examples of phytoalexins include resveratrol in grapes, pisatin in peas, and camalexin in Arabidopsis.
Therefore, in the given options, the correct answer is D: Infection caused by fungi.