Specification of adaxial cell fate during maize leaf development

Dorsoventral (adaxial/abaxial) polarity of the maize leaf is established in the meristem and is maintained throughout organ development to coordinate proper outgrowth and patterning of the leaf. rolled leaf1 (rld1) and leafbladeless1 (lbl1) are required for the specification of the adaxial/upper leaf surface. rld1 encodes a class III homeodomain-leucine zipper (HD-ZIPIII) protein whose adaxial expression is spatially defined by miRNA166-directed transcript cleavage on the abaxial side. The semi-dominant Rld1-Original (Rld1-O) mutation, which results from a single nucleotide substitution in the miRNA166 complementary site, leads to persistent expression of mutant transcripts on the abaxial site. This causes the adaxialization or partial reversal of leaf polarity. By contrast, recessive mutations in lbl1 cause the formation of abaxialized leaves. The lbl1 and Rld1-O mutations mutually suppress each other, indicating that these two genes act in the same genetic pathway. Adaxial and meristematic expression of rld1 is reduced in lbl1 mutants, indicating that lbl1 acts upstream of rld1 to specify adaxial fate during primordium development. However, rld1 expression in the vasculature of lbl1 is normal, suggesting that the specification of adaxial/abaxial polarity during vascular and primordia development is governed by separate but overlapping pathways. We also show that members of the maize yabby gene family are expressed on the adaxial side of incipient and developing leaf primordia. This expression pattern is unlike that observed in Arabidopsis, where YABBY expression is correlated with abaxial cell fate. The yabby expression patterns in lbl1 and Rld1-O mutants suggest that the yabby genes act downstream in the same pathway as lbl1 and rld1. Moreover, our observations suggest that maize yabby genes may direct lateral organ outgrowth rather than determine cell fate. We propose that a single genetic pathway involving lbl1, rld1 and the yabby genes integrates positional information within the SAM, and leads to adaxial/abaxial patterning and mediolateral outgrowth of the leaf.