The glutamate level in the mushroom increases as the mushroom matures from a button to a flat mushroom. Natural glutamate is also responsible for much of the flavour in Parmesan cheese, soy sauce, anchovies, tomato juice, Vegemite and Marmite.
When extra glutamate is added to food, the salt content can be reduced by 30-40% without affecting the palatability (Mouritsen 2012). We suggest that when you add mushrooms to a meal, you can add far less salt. In fact, you may be able to get away with no salt at all and let the flavour of mushrooms do the talking.