Leaves of higher plants usually develop distinct upper (adaxial) and lower (abaxial) sides. The resulting leaf asymmetry contributes to the metabolic efficiency of land plants and is an important adaptive feature. The goal of this research is to elucidate the mechanism that sets up adaxial/abaxial leaf polarity. In addition to addressing a key developmental process, the study of adaxial/abaxial patterning provides a unique opportunity to unravel the role of small regulatory RNAs in development. Studies reveal that the adaxial domain of the leaf is defined by a 21-nucleotide small RNA, miR166, whereas delineation of the abaxial side involves a second small RNA, ta-siR2142. Small regulatory RNAs control many important processes in both plants and animals, however, their roles as developmental signals are less well understood. Genetic and histological approaches will be used to identify factors that act upstream or downstream of ta-siR2142 and miR166 to distinguish the adaxial and abaxial leaf surfaces. Mobile factors will be identified that provide positional information, i.e. that signal to the new leaf which side is up and which side is down. These experiments will help elucidate the relative contribution of two candidate signals, the above-mentioned small RNAs and the classical plant hormone auxin, as positional cues in adaxial/abaxial patterning. Moreover, this research will reveal whether small RNAs, like hormones and certain proteins, can function as mobile developmental signals. These experiments will help answer several pivotal questions in development biology; namely, how adaxial/abaxial polarity is established in plants, how small regulatory RNAs and their targets fit within gene networks, and whether small RNAs can provide or transmit the positional information essential to achieving normal development. Significant educational contributions will be made through the training of a postdoctoral fellow, as well as graduate, undergraduate and high school students.
