Final answer:
(B) PTI, or Pathogen-Triggered Immunity, is a plant defense mechanism responding to pathogen invasion by initiating immune reactions.
Step-by-step explanation:
PTI stands for Pathogen-Triggered Immunity, a fundamental part of a plant's defense mechanisms against pathogens. When plants detect the presence of pathogens like bacteria or fungi, they activate PTI as an initial response, triggering immune reactions to fend off the invasion.
However, PTI is distinct from effector-triggered immunity (ETI), a more robust defense mechanism activated upon the recognition of specific pathogen effectors. PTI acts as the plant's first line of defense, initiating responses such as the production of defense molecules, reinforcement of cell walls, and activation of signaling pathways to combat pathogen attacks. It's crucial for protecting plants against various diseases and stresses caused by pathogens.