Switching from vegetative (indeterminate) to reproductive (determinate) growth is a major transition in the development of the shoot in higher plants. This transition occurs at the shoot apical meristem, the region that initiates primordia that differentiate as leaves separated by stem tissue during vegetative development or as floral organs during reproductive development. The timing of this transition has profound effects on the architecture of the shoot, as well as influencing yield in crop plants. The mechanism regulating the transition to flowering is beginning to be understood in plants in which the transition occurs in response to environmental cues. However, many other plants flower in a variety of environments. In these plants it is believed that the primary determinant of the timing of the transition from vegetative growth to flowering is a developmental program, of which little is known. The objective of the experiments proposed here is to examine various aspects of the developmental program underlying the switch to flowering in a plant of the latter group, maize. The meristem receives information from elsewhere in the plant and responds by either continuing leaf initiation (indeterminate) or becoming determined for determinate growth and forming an inflorescence, the tassel. This investigator will examine potential sources for that information by organ culture techniques. These experiments will lay the groundwork for the eventual identification of the signals that inform meristems of their progress through ontogeny, which results in the precise regulation of the morphology of maize shoots. The maize vegetative shoot apical meristem has been deduced to be organized in tiers, in which typically the lowermost cells (at the periphery) of the meristem will give rise to the first-formed nodes of the plant, midmeristem cells to the later nodes, and upper cells to the top of the plant. Cells most likely to participate in the formation of reproduct ive structures are located in the apex of the meristem. Resetting of the meristem by excision and culture results in more nodes being formed by the meristem than would normally occur. An inevitable result of this is a change in the contribution of at least some cells in the meristem to the shoot. This investigator will use clonal analysis to determine the origin within the meristem of the extra nodes of reset plants. This information will be useful for understanding the flexibility of cells within a meristem with respect to the portions of the plant to which they will contribute and the nature of the reorganization that takes place as a meristem is reset by excision and culture. This may also have practical implications for transformation of maize via the shoot meristem. Experiments are also planned to examine the molecular nature of the resetting of the shoot's developmental program by culturing plants carrying a photosynthetic mutation that can become suppressed through the course of normal development of a shoot. These experiments are expected to provide insight into normal changes at the level of gene expression in differentiation of successive nodes as part of the developmental program of the plant.
