The past, present, and future of vegetative phase change.

Many things change during shoot development. Although the most obvious of these is the production of reproductive structures, the morphology and physiology of leaves and vegetative buds changes as well. Some traits change continuously (either increasing or decreasing), while others are expressed in bimodal (present or absent) or even more complex patterns. Some traits vary in the same way in every species, while others vary in different ways in different species. For example, cell size declines gradually in successively higher leaves, but trichomes may be produced uniformly on all leaves, early in shoot development, late in shoot development, in different patterns on vegetative and reproductive parts of the shoot, or not at all. This phenomenon is termed heteroblasty (Goebel, 1900). Despite the complexity of the changes that occur during vegetative development, it is usually possible to recognize several more-or-less discrete phases based on correlated changes in the expression of multiple traits. These phases are particularly evident in certain woody plants, including Hedera helix and some species of Acacia and Eucalyptus, but are obvious in other species as well (Godley, 1985; Boland et al., 2006). The transition between these phases is referred to as vegetative phase change (Poethig, 1990). Recent studies in Arabidopsis (Arabidopsis thaliana) and maize (Zea mays) are beginning to provide a clearer picture of the molecular mechanism of vegetative phase change, and may eventually make it possible to manipulate this process in woody species, where it is of major economic and social significance (Brunner and Nilsson, 2004).