Is there a subplot that contributes to the main story? how ? 

The death of Holden's brother Allie has a rather large impact on the main story. Holden's childhood is the reason he is the way he is. As Holden was growing up he had to deal with the death of his brother, which took a rather large toll on him. Holden had a break down ( smashing the garage windows...) which led to Holden going to boarding schools, many boarding schools. Holden brings Allies baseball glove with poetry on it to boarding school  because it is important to him. This is the reason Holden winds up walking the streets of New York and also contributes to the state of Holden's mental health.