Fig. 8. One model that explains how RLD1 normally functions in the abaxial epidermis to promote adaxial identity. In this model, dorsoventrality is initiated by a peripheral signal and RLD1 is involved in its maintenance. At P0, the leaf founder initials lack polarity. As these initials develop into the early leaf founder cells, they develop polarity in response to a peripheral/adaxialization signal emanating from the meristem. The surface closest to the meristem takes on adaxial identity and the surface further from the meristem might have abaxial identity as a default or abaxial identity might be specified on tissue with undefined cell fate (see inset) as a result of a concentration differential or via a signal cascade started by the peripheral signal. By the late leaf primordium, as the developing primordium is moving further away from the meristem, the peripheral signal is no longer a strong enough influence on the primordia and another mechanism for polarity maintenance is necessary. One possible model for polarity maintenance is a polarity maintenance signal (PMS) emanating from tissue with adaxial identity (light blue arrow). This reinforces its adaxial identity via adaxial factors (AdFs) such as LEAFBLADELESS1 (LBL1), and signals to the opposite half of the leaf to maintain abaxial identity. Abaxial factors (AbFs) such as ROLLED LEAF1 (RLD1) are responsible for maintenance of abaxial polarity. As the blade primordium develops, this ‘I’m AD, you be AB maintenance signal remains important to maintain correct polarity. In Rld1 mutants, the mutant RLD1 interferes with the function of wild-type RLD1. The mutant AbF is unable to maintain abaxial identity in the abaxial tissue and results in a blade phenotype of abaxial tissue being switched to adaxial identity as a mutant response to either the weaker peripheral/adaxialization signal or to the PMS from the adaxial tissue. The lower tissue having taken on adaxial identity, now sends out the PMS to the upper surface, and sometimes in Rld1 mutants, some part of this upper surface (adaxial tissue) responds to the signal, becoming abaxialized. This results in the dorsoventral axis being flipped over in some portion of a Rld1 mutant leaf. However, an alternative model where the abaxial epidermis generates an ‘I’m ab so you are ad’ trans-tissue signal is equally likely (not shown).