Leaf primordia are born around meristem-containing stem cells at shoot apices, grow along three axes (proximal-distal, adaxial-abaxial, medial-lateral), and develop into flat symmetric leaves with adaxial-abaxial polarity. Axis development and polarity specification of Arabidopsis leaves require a network of genes for transcription factor-like proteins and small RNAs. Here, we summarize present understandings of adaxial-specific genes, ASYMMETRIC LEAVES1 (AS1) and AS2. Their complex (AS1-AS2) functions in the regulation of the proximal-distal leaf length by directly repressing class 1 KNOX homeobox genes (BP, KNAT2) that are expressed in the meristem periphery below leaf primordia. Adaxial-abaxial polarity specification involves antagonistic interaction of adaxial and abaxial genes including AS1 and AS2 for the development of two respective domains. AS1-AS2 directly represses the abaxial gene ETTIN/AUXIN RESPONSE FACTOR3 (ETT/ARF3) and indirectly represses ETT/ARF3 and ARF4 through tasiR-ARF. Modifier mutations have been identified that abolish adaxialization and enhance the defect in the proximal-distal patterning in as1 and as2. AS1-AS2 and its modifiers synergistically repress both ARFs and class 1 KNOXs. Repression of ARFs is critical for establishing adaxial-abaxial polarity. On the other hand, abaxial factors KANADI1 (KAN1) and KAN2 directly repress AS2 expression. These data delineate a molecular framework for antagonistic gene interactions among adaxial factors, AS1, AS2, and their modifiers, and the abaxial factors ARFs as key regulators in the establishment of adaxial-abaxial polarity. Possible AS1-AS2 epigenetic repression and activities downstream of ARFs are discussed.
© 2015 The Authors. WIREs Developmental Biology published by Wiley Periodicals, Inc.