Meristems, Stem Cells, and Stem Cell Niches in Vascular Land Plants-Chapter 6

All tissues and organs in vascular plants derive from a pool of undifferentiated, totipotent cells known as stem cells that divide repeatedly and asymmetrically to feed a growing organ with newly amplified cells. The evolution of stem cells and their organization within meristems of different types allowed the diversification of vascular land plant species, which have spread and diversified since the mid-Devonian period (about 400 Mya). Stem cells are set aside very early during de novo organogenesis to sustain the development of leaves, roots, and flowers throughout plant life cycles. The evolution of stem cells was essential for plant survival and integrating external/ exogenous stimuli with internal/endogenous mechanisms that allow coherent and plastic organ development and tissue replenishment. Stem cells are of pivotal importance for plant exploration of the surrounding space, both above and below the ground, for tissue repair and integration and to establish new generation during embryogenesis. This chapter highlights the basic principles of plant stem cell biology and their deployment in the evolution in vascular land plants. We discuss the advances made by studying model plants, particularly thale cress Arabidopsis thaliana, focusing on specification of plant meristems during early stages of embryogenesis and maintenance of meristem integrity during undetermined organ growth. Also, we examine the evolutionary appearance of stem cells and their organization in extinct and extant vascular land-plant phyla, the different types of meristematic structures in lycophytes, ferns, gymnosperms and angiosperms,1 and the importance of stem cells’ activity for root and shoot evolution and for strategies of branching morphogenesis.