What can be used as a substitute for tomato sauce in typical italian dishes?
You could use Red Pepper Coulis:

Roast red peppers on grill, gas flame or cut off the sides and place skin side up under broiler until skins are blackened.  Cool and remove charred skin.

Sweat onions and garlic in a little olive oil with salt and pepper until softened.  Add the charred roasted red peppers and some chicken stock.  Simmer until vegetables are very soft and puree with an immersion blender, food processor or regular blender.  Taste and adjust seasoning as necessary.  Thin with additional stock according to consistency desired/needed.

Add basil, oregano, Italian seasoning blend to season as desired.