The introduction of t-DNA into plants involves:

Options

A. Altering the pH of the soil, then heat shocking the plants
B. Exposing the plants to cold for brief period
C. Allowing the plant roots to stand in water
D. Infection of the plant by Agrobacterium infections

Answer

To answer the question regarding the introduction of T-DNA into plants, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding T-DNA**: T-DNA stands for Transfer DNA. It is a segment of DNA that is transferred from a bacterium to a plant cell, which is crucial for genetic engineering in plants.

**Hint**: Remember that T-DNA is specifically designed for transferring genetic material into plant cells.

2. **Role of Agrobacterium**: The primary bacterium used for introducing T-DNA into plants is Agrobacterium tumefaciens. This bacterium naturally transfers T-DNA into the plant genome through its tumor-inducing plasmid (Ti plasmid).

**Hint**: Focus on Agrobacterium as a key player in the process of T-DNA transfer.

3. **Vector Requirement**: To introduce T-DNA into plants, a vector is needed. The Ti plasmid from Agrobacterium serves as a vector to carry the desired DNA segment into the plant cells.

**Hint**: Think of the vector as a delivery system for the T-DNA.

4. **Infection Process**: The process begins with the infection of the plant by Agrobacterium. The bacterium attaches to the plant cell and transfers the T-DNA into the plant's genome.

**Hint**: The infection step is crucial as it initiates the transfer of T-DNA into the plant cells.

5. **Transformation**: Once the T-DNA is integrated into the plant's genome, the plant can express the new traits encoded by the introduced DNA.

**Hint**: Transformation is the final step where the plant exhibits new characteristics due to the T-DNA integration.

### Final Answer:
The introduction of T-DNA into plants involves the infection of the plant by Agrobacterium, which transfers the T-DNA into the plant cells.