Regulation of Shoot Development by the Maize Meristem

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Irish 9728518 The shoot apical meristem of higher plants is ultimately responsible for the post-embryonic addition of new cells, tissues, and organs to the shoot through-out the life of the plant. During vegetative development, the meristem initiates leaves and stem tissue in a repetitive manner. As vegetative development progresses, the manner in which newly formed organs differentiate may vary, in a process termed phase change. Once in the reproductive phase, shoot meristems form or become floral meristems, which initiate a predictable and determinate sequence of floral organs. Much remains to be learned about the functioning of the vegetative shoot apical meristem, such as how it initiates lateral organs, to what extent it determines how those organs differentiate, and how the vegetative meristem is convened to an inflorescence or floral meristem. The focus of the research proposed here is to ask to what controls various developmental processes that occur at the shoot apex. The experiments will be conducted using the maize plant, and will take advantage of a technique, shoot meristem culture, that we have developed for maize. Our previous experiments using meristem culture indicated that both the change from vegetative to reproductive development and the process of phase change are under the control of global signaling that occurs throughout the plant. We propose a set of experiments that are aimed at understanding how global signaling in maize is coordinated with meristem-autonomous activities. The following questions will be addressed: 1. How is the expression of indeterminate, a gene required for the transition to flowering in maize, affected by the culture-induced resetting of a shoot apex? 2. How is the meristem reorganized when it is forced to undergo a prolonged period of vegetative development? 3. Are all phase-specific phenotypes reset when an adult meristem is isolated and forms a complete new shoot? If resetting occurs in a specific gene (PI-Blotched), how is the methylation state of that locus affected? 4. In mutant backgrounds in which the juvenile phase is prolonged and reproduction delayed, is this slower ontogeny the result of altered (weaker) signaling from leaf primordia? Is the expression of heterochronic mutations dependent on the phase of the plant, or on a free-running measurement o1' time after germination? The answers to these questions will increase our understanding of the global changes that occur during shoot ontogeny and the role of the shoot meristem, the site of organogenesis, in directing these changes.