In response to pathogen invasion, plants synthesize and deposit the polysaccharide callose at plasmodesmata, between the cell wall and plasma membrane of infected cells, and within sieve tubes.
Which of the following describes the primary mechanism by which callose maintains plant health?
It acts as a physical barrier to prevent the symplastic and systemic transport of the pathogen.
It functions as a chemical defence by enzymatically degrading the pathogen's cell wall.
It acts as an antigen-presenting molecule to recruit plant immune cells to the site of infection.
It is released into the vascular system to act as an opsonin, marking pathogens for phagocytosis.