Much of plant shape is defined by the number and location of branches. A combination of genetics and environment controls plant branching. Plants growing beneath the canopy of a dense forest will not produce many side branches. As a consequence, the available energy can be channeled into the main shoot so it can reach upward into sunlight. On the other hand, species growing on an open plain will be highly branched to extend outward and optimize sun exposure. Breeding desired branching traits has been very important in the course of human agriculture. For example, modern maize (Zea mays, corn) has fewer branches (tillers) than the wild teosinte progenitor. This reduction in tillers seems to be due to enhanced expression of a single gene, TEOSINTE BRANCHED1 (TB1), which inhibits bud outgrowth in plants.