The expression domain of PHANTASTICA determines leaflet placement in compound leaves.

Diverse leaf forms in nature can be categorized as simple or compound. Simple leaves, such as those of petunia, have a single unit of blade, whereas compound leaves, such as those of tomato, have several units of blades called leaflets. Compound leaves can be pinnate, with leaflets arranged in succession on a rachis, or palmate, with leaflets clustered together at the leaf tip. The mechanisms that generate these various leaf forms are largely unknown. The upper (adaxial) surface is usually different from the bottom (abaxial) surface in both simple and compound leaves. In species with simple leaves, the specification of adaxial and abaxial cells is important for formation of the leaf blade, and the MYB transcription factor gene PHANTASTICA (PHAN) is involved in maintaining the leaf adaxial (upper) domain. Here we show that downregulation of PHAN is sufficient to reduce the adaxial domain of leaf primordia and to change pinnate compound leaves into palmate compound leaves. Furthermore, this mechanism seems to be shared among compound leaves that arose independently.