Is the lodicule a petal: molecular evidence?

Lodicules are grass-specific floral organs with scale-like shapes that play an important role in flower opening. Because of their position just outside the stamens, lodicules have been suggested as analogous to eudicot petals. Previous molecular genetic studies in maize and rice revealed that identities of lodicule and stamen are specified by members of the AP3-lineage of B-class MADS-box genes, which specify petal and stamen identities in eudicots. This supported the hypothesis that lodicules may be equivalent to eudicot petals. Recent studies in rice, maize, and barley further demonstrated that the molecular genetic mechanism of lodicule development includes the PI-lineage of B-class, C-class, SEP-like, AGL6-like, and AP2-like genes. These findings consistently suggest that the genetic mechanisms behind lodicule and petal development are similar. Nevertheless, remarkable divergence in the appearances of lodicules and petals suggests that their developmental processes are very different. Critical mutations in cis-elements and coding sequences of the key regulatory genes may be major driving forces of the divergence between lodicules and petals.