Leaf shape varies dramatically among plant species, ranging from the needle leaves of pine trees and the strap-like leaves of grasses to the broad lamina of most eudicots. Also the arrangement and types of cells within the leaf can vary between species, leading to adaptations to, for instance, protect against predators or optimize the utilization of sunlight for photosynthesis. The specification of adaxial/abaxial leaf polarity, i.e. the formation of distinct upper and lower leaf surfaces, directs both the flattened outgrowth and the patterning of leaves. Early surgical experiments suggested that a signal from the undetermined growing tip of the plant, the shoot apical meristem, is required to set up adaxial/abaxial polarity in the leaf. Recent genetic studies have identified several genes required for the specification of adaxial or abaxial cell identity in Arabidopsis leaves, and have led to the following model. Adaxial and abaxial promoting factors are uniformly expressed throughout newly initiated leaves. Polarity is established when transcripts of the HD-ZIPIII family members are degraded on the abaxial side of the leaf via a microRNA-directed cleavage reaction. In addition, these HD-ZIPIII proteins, which contain a lipid-sterol-binding domain, become activated on the adaxial side of the leaf in response to the meristem-born signal. This activation results in the down-regulation of abaxial determinants such as the KANADI and YABBY genes and in the specification of adaxial cell fate. In contrast, abaxial cell identity results from the continued expression of the KANADI and YABBY genes.
Members of the yabby and hd-zipIII gene families are also required for adaxial/abaxial patterning in maize. However, the yabby genes are expressed on the adaxial side of the maize leaf suggesting that these genes have distinct functions in Arabidopsis and maize. Also, several recessive mutations that abaxialize the leaf have been identified in maize, whereas the recessive mutations in Arabidopsis adaxialize the leaf. This suggests either that additional genes controlling adaxial/abaxial polarity remain to be described in each species, or that aspects of adaxial/abaxial patterning are different in monocots and dicots. Given the differences in monocot and dicot leaf initiation, morphology and anatomy, it may be expected that the mechanisms underlying adaxial/abaxial pattern formation have diverged between these two branches of angiosperms. The goal of this proposal is to further investigate the mechanism by which adaxial/abaxial polarity is established in the maize leaf. The specific aims are: 1) To determine the genetic pathway(s) that underlies adaxial/abaxial pattern formation in maize, 2) To characterize the rolled leaf1 (rld1) gene and the microRNAs that may regulate rld1 expression(rld1 is a member of the hd-zipIII gene family and like the Arabidopsis homologs, rld1 is required to specify the adaxial domain of the maize leaf), and 3) To clone the leafbladeless1 gene, which functions downstream of rld1 in the specification of adaxial cell fate. This project will broaden our understanding of the molecular mechanisms that underlie the initiation and patterning of the maize leaf. The comparative analysis of adaxial/abaxial axis specification in Arabidopsis and maize may provide an understanding of how variations in the expression patterns of just a few genes can contribute to the diversity of leaf shapes observed among plant species. Such knowledge may ultimately facilitate the manipulation of plant architecture, which has become increasingly more important for aspects of crop yield such as shade tolerance in maize and insect resistance in cotton. Moreover, this project will also make important educational contributions by providing training for two graduate students, including one underrepresented minority student. Also, during the course of this award, several high school and undergraduate students will have to opportunity to participate in some of the proposed experiments, exposing them to a variety of topics, including maize genetics, plant genomics, and PCR genotyping of developmental mutants in plants.