Agriculture is facing tremendous challenges. To keep pace with population growth it is estimated that food production must double over the next 40 years. This must be achieved in the face of climate change and an ever-increasing demand for biofuels. Improvement of plant yield and biomass is therefore critical. Leaf shape is in this regard an extremely important trait. For instance, the continuing gains in US maize yield have not come from more grain per plant, but rather from adaptation of hybrids to continually higher plant densities. Particularly, changes in leaf angle and size have altered plant architecture, allowing more efficient light capture as planting density increased. This project investigates how newly initiated leaves establish top-bottom polarity, a process that drives the flattened outgrowth of the leaf and optimizes its capacity for light capture and photosynthesis. The specific mechanism under investigation will also provide insight into gene regulation by so-called small RNAs. Despite their importance for many fundamental processes in plants and animals, exactly how small RNAs act to coordinate development remains poorly understood.
Adaxial-abaxial patterning of the leaf is an excellent model to study the role of small RNAs in development; it relies on two opposing mobile small RNA gradients. The recognition that small RNAs can move from cell to cell has broadened our thinking on how they are utilized during development, serving as positional, possibly morphogen-like, signals. Yet, major questions regarding the properties of mobile small RNAs remain; how do small RNAs move, how is mobility regulated, and what are their distinguishing patterning properties? This project will address these questions. Artificial miRNAs directed against visual reporters will be used to characterize miRNA mobility between key domains in the shoot apex and establish whether this is dependent solely on small RNA abundance or regulated developmentally. Similar technologies will also be used to test mathematical predictions that small RNA gradients are uniquely suited to generate stable gene expression domains. This question will be addressed using GFP reporters that act independent of the leaf developmental programs, as well as using reporters for ARF3 and HD-ZIPIII that monitor the behavior of small RNAs within the context of leaf polarity. The latter experiments will resolve the mechanisms via which the miR166 and tasiR-ARF gradients pattern their targets to yield sharply delineated domains of expression and will reveal how their opposing gradients interact to create a robust adaxial-abaxial boundary. Creating sharp expression patterns and robust developmental boundaries present novel roles for small RNAs. Thus, the findings will have far-reaching implications for patterning processes in plants, as well as animals. In addition, the project will make important educational contributions by providing scientific and professional training for a postdoctoral fellow and research opportunities for undergraduate and high school students, as well as through lectures by PI Timmermans in the NSF funded CSHL "Plant Course" and in a summer training program for high school teachers at the CSHL DNA learning center on small RNA biology.
