The leaf is the most visible part of the repeating shoot segment in plants. Monocotyledonous plants have leaves in two parts: the sheath at the base and the blade that extends out from the plant. Our data supports the concept that the leaf is actually a patchwork of gene domains. At the sheath-blade boundary is the ligular region. In maize, we have many mutant alleles in 20 different genes that alter all or a portion of the blade-sheath boundary. Five of these are cloned and several more are characterized to the level that we can say something about their function. Some encode homeodomain proteins of the type known to be particularly important transcriptional regulators of animal development. In general, our aim is to integrate the function of these genes into a system of fields, domains, inductions, transductions and responses where the related concepts of """"""""positional information"""""""" and """"""""developmental time schedules"""""""" might be distinguished. Perhaps the most unique aspect of our work is the way we are able to genetically perturb various developmental time-keeping functions. We can obtain molecular mechanisms for heterochronic shifts that would be very difficult to do in animal systems. We propose a suppressor screen to obtain mutants that specifically accelerate development or transform juvenile parts of the plant to be more adult. We have begun to reduce heterochronic theory, the prevailing concepts that relate evolution to development, to molecular mechanisms.
