Adaxial/abaxial specification of the maize leaf

Michelle T. Juarez^1,2 and Marja Timmermans¹

¹Cold Spring Harbor Laboratory Cold Spring Harbor, NY; ²State University of New York, Stony Brook, NY

Specification of adaxial(upper)/abaxial(lower) polarity is responsible for normal outgrowth and patterning of the maize leaf. Establishment of adaxial/abaxial polarity requires signals from the shoot apical meristem as well as from the leaf itself. Through molecular and genetic analyses we have identified several genes involved in setting up adaxial identity. Recessive mutations in leafbladeless1 (lbl1) lead to development of radial symmetric abaxialized outgrowths suggesting lbl1 is required for proper adaxial specification. Semi-dominant mutations in Rolledleaf1 (Rld1) develop adaxialized leaves or cause partial inversions of adaxial and abaxial domains in the leaf blade. lbl1Rld1 double mutants display a mutual suppression of both the single mutant phenotypes suggesting the two mutations act in the same adaxial/abaxial pathway.

Several maize genes were cloned with high homology to two Arabidopsis gene families; YABBY (YAB) and Homeodomain Leucine Zipper class III (HD-ZIP III), which are known to play a role in abaxial and adaxial cell fate, respectively. In contrast to Arabidopsis, the maize yab genes are expressed in the adaxial domain of young leaf primordia. In both the lbl1 and Rld1 mutants yab expression is altered. This data places the maize yab genes downstream of lbl1 and rld1 in the adaxial/abaxial specification pathway.

We recently cloned rld1, which encodes the homolog of Arabidopsis revoluta, a member of the hd-zip III family. Members of this gene family are thought to be regulated by microRNAs (miRNA). rld1 is expressed in the SAM and the adaxial domain of young leaf primordia. Dominant mutant alleles of Rld1 result from mutations in the miRNA166 complementary site and misexpress rld1 on the abaxial side. miRNA166 is expressed in the abaxial domain of leaf primordia, consistent with the misexpression of rld1 in the dominant Rld1 mutants. miRNA166, thus, has a role in adaxial/abaxial specification by spatial restriction of hd-zip III. rld1 expression is reduced in lbl1 mutant tissue. This together with the mutual suppressive interaction between lbl1 and Rld1 suggests that lbl1 is upstream of rld1. The combination of the genetic and molecular analyses outlines an adaxial/abaxial specification pathway leading to proper development of the maize leaf.