Place bread in a food processor; pulse 10 times or until coarse crumbs measure 1 cup. Set aside.
Halve peppers lengthwise; discard seeds. Place pepper halves in a microwave-safe dish; microwave at HIGH 6 minutes or until peppers begin to soften.
Preheat oven to 375°.
Heat oil in a large skillet over medium heat; add garlic, and sauté 30 seconds. Add zucchini and mushrooms; cook 5 minutes or until vegetables begin to soften. Remove from heat; stir in breadcrumbs, corn, and next 7 ingredients.
Coat a shallow baking dish with cooking spray. Place pepper halves in dish; divide vegetable mixture evenly among peppers. Bake at 375° for 25 to 30 minutes or until thoroughly heated.