Abstract

In flowering plants, the shoot apical meristem (SAM) consists of a few hundred stem cells that ultimately give rise to all shoot structures. This stem cell population continues to proliferate throughout vegetative development, forming leaves and branches in an indeterminate fashion. In contrast, thorns arise from vegetative SAMs that, instead of maintaining a stem cell fate, switch from indeterminate to determinate growth, resulting in terminal differentiation. Thorns appear to have arisen multiple times during angiosperm evolution, and are thought to have evolved as a protection against herbivory. Using Citrus as a model system, we are exploring the molecular mechanisms controlling stem cell proliferation and arrest in thorns. The evolution of Citrus thorn identity appears to have resulted from shifts in the timing and function of several gene regulatory networks, resulting in stem cell quiescence and concomitant terminal differentiation to produce the thorn. We hope to determine whether Citrus thorn development represents a novel mode of stem cell arrest, or reflects the redeployment of an ancestral mechanism to restrict stem cell proliferation.

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