Sentence:  The <WSD>arbuscle</WSD> formation increases the metabolic activity of the host cell which is mainly due to the bidirectional transfer of metabolites and nutrients to and from the fungus. <WSD>Arbuscles</WSD> live for only 4–15 days. They degenerate and are digested by the host cell .... 
Meaning of the word marked with <WSD> tags:  The site at which a symbiotic fungus attaches to the roots of a plant and exchanges nutrients, etc., with it.