Final answer:
Bacteria can enter plants via endocytosis or through natural openings and wounds. Effector proteins often facilitate entry, and plant viruses typically need cellular damage to penetrate due to plant cell walls. The correct option is A.
Step-by-step explanation:
The question relates to how bacteria can enter plant cells. There are various mechanisms by which this can occur. One common method is endocytosis, a process where the plant cell membrane engulfs the bacteria to take it inside the cell.
Some pathogens use effector proteins to trigger this process, causing membrane ruffling that envelops the bacteria. Another notable technique bacteria use for entry is through wounds or natural openings in the plant, including those caused by insect damage or growth processes, like where new branches form or where leaves have fallen.
Specific bacteria use different methods to invade plant cells. For instance, Erwinia amylovora, the causative agent of fire blight, can live harmlessly on the plant surface before becoming necrogenic and producing effector molecules that kill host cells, aiding its entry and spread.
By contrast, plant viruses, which are different from bacteria, generally require physical damage to plant cells to gain entry since they do not utilize receptor-mediated endocytosis because of the rigid plant cell walls.