PREPARATION:

1. Preheat oven to 350°F. Coat 9-inch square pan with nonstick cooking spray. For topping, combine 1/4 cup flour, brown sugar, cinnamon and margarine in small bowl with fork until crumbs form; set aside.
2. Combine remaining 2 cups flour, baking powder, baking soda and salt in large bowl. Beat yogurt, granulated sugar, egg whites, egg and lemon peel in medium bowl with electric mixer until well blended. Stir yogurt mixture into flour mixture just until ingredients are combined.
3. Pour batter into prepared pan. Arrange plums over batter; sprinkle evenly with reserved topping. Bake 30 to 35 minutes or until toothpick inserted in center of cake comes out clean. Cool in pan on wire rack. Serve warm or at room temperature.