Ricerc@Sapienza

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.