Plant Meristems and Growth

Plants grow from meristems — self-renewing pools of stem cells at their tips and flanks — that let a plant add new organs and extend itself throughout its life, an open-ended growth strategy unlike that of animals.

Definition

Meristems are regions of undifferentiated, dividing cells that generate new tissues and organs, and plant growth is the increase in size and number of cells produced through their continued activity.

Scope

This topic covers the shoot and root apical meristems and lateral meristems, the organization and maintenance of plant stem-cell niches, the patterning of organ initiation including phyllotaxis, and how cell division and expansion drive plant growth.

Core questions

• How are the shoot and root apical meristems organized?
• How is a stable pool of stem cells maintained while cells are also being used for growth?
• How do meristems pattern the regular arrangement of leaves and other organs?

Key theories

Stem-cell niche maintenance

A feedback loop between an organizing center and the overlying stem cells keeps the meristem in balance, replacing cells lost to differentiation so the meristem persists indefinitely.

Indeterminate, open growth

Because meristems are retained, plants grow indeterminately and modularly, adding repeated units and adjusting their architecture in response to the environment.

Mechanisms

The shoot apical meristem is organized into a central zone of slowly dividing stem cells, a peripheral zone where organs are initiated, and an underlying organizing center; signaling between the organizing center and stem cells stabilizes their number. Directional auxin transport and inhibitory fields position successive organ primordia, generating the regular spirals and whorls of phyllotaxis. Growth itself combines oriented cell division with anisotropic cell expansion driven by turgor against the controlled yielding of the cell wall. Mutant collections in model plants have been instrumental in identifying the regulators of meristem function.

Clinical relevance

Meristem biology underlies horticultural propagation and tissue culture: meristem-tip culture produces disease-free plants, and the control of branching and architecture through meristem activity is a target for improving crop yield and form.

History

Classical anatomy described the apical meristems and their zonation; later microsurgical experiments and, decisively, molecular genetics in model plants revealed the signaling that maintains the stem-cell niche and patterns organ initiation.

Key figures

• Katherine Esau
• Ian Sussex

Seminal works

• taiz2015
• raven2013

Frequently asked questions

What is a meristem?

A meristem is a region of undifferentiated, continuously dividing cells; apical meristems at shoot and root tips drive lengthening, while lateral meristems add girth, allowing plants to grow throughout their lives.

Why do leaves grow in regular patterns?

New leaf primordia form at positions set by inhibitory fields, largely involving the hormone auxin, around the shoot apex, producing the regular spirals and other arrangements known as phyllotaxis.