Meristems Are Niches for Stem Cells in Plants

In plants, as in their multicellular animal counterparts, the production of all tissues and organs relies on small populations of stem cells. Like animal stem cells, these stem cells are defined by their ability to undergo self-renewal and to generate daughter cells that produce differentiated tissues. And like animal stem cells, plant stem cells reside in specialized microenvironments—stem-cell niches—where extracellular signals are produced that maintain the stem cells in a multipotent state. Because the last common ancestor of plants and animals was a unicellular eukaryote, it would appear that, despite common organizing principles, stem cells and their niches evolved independently and by different pathways in plants and animals—an example of convergent evolution.

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The niches in which plant stem cells are located, called meristems, can persist for thousands of years in long-lived species such as bristlecone pines. The body axis of the plant is defined by two primary meristems that are established during embryogenesis, the shoot apical meristem and the root apical meristem. In contrast to animal development, very few tissues or organs are specified during plant embryogenesis. Instead, organs such as leaves, flowers, and even germ cells are continuously generated as the plant grows and develops. The aboveground part of the plant is derived from the shoot apical meristem and the belowground part from the root apical meristem. Classic clonal analysis experiments have demonstrated that plant cell fate depends on the cell’s position, not its lineage. A cell’s identity is reinforced by intercellular signals such as hormones, mobile signaling peptides, and miRNAs.

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Unlike somatic stem cells in metazoan animals, somatic plant stem cells give rise to entire organs, not just specific tissues or lineages. Slowly dividing pluripotent stem cells are located at the apex of the shoot apical meristem, with more rapidly dividing multipotent transient amplifying daughter cells on the periphery. Descendants of the shoot stem cells are displaced to the periphery of the meristem and are recruited to form primordia of new organs, including leaves and stems. Division ceases as these cells acquire the characteristics of specific cell types, and most organ growth occurs by cell expansion and elongation (Figure 21-20a). New shoot stem-cell niches can form in the axils of leaf primordia, which then grow to form lateral branches. Floral meristems give rise to the four floral organs—sepals, stamens, carpels, and petals—that form flowers. Unlike shoot apical meristems, floral meristems gradually become depleted as they give rise to the floral organs.

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FIGURE 21-20 Structures of the Arabidopsis thaliana shoot and root meristems. (a) Transverse section through the apex of the shoot apical meristem. The organizing center cells signal to maintain the overlying stem cells. The stem cells produce daughters by division in the direction of the black arrows, generating rapidly dividing transient amplifying cells that will eventually differentiate and give rise to entire organs, such as a leaf. (b) Transverse section through the root meristem. Stem cells surround the mitotically less active quiescent center, four cells that send signals to prevent stem-cell differentiation. Each stem cell divides asymmetrically: one daughter remains adjacent to the quiescent center and becomes a stem cell (self-renewal); the other daughter becomes a transient amplifying cell that divides a number of times before exiting the cell cycle, elongating, and assuming a specific differentiated state. See R. Heidstra and S. Sabatini, 2014, Nat. Rev. Mol. Cell. Biol. 15:301.