34.10: Morphogenesis

Plant morphogenesis—the development of a plant’s form and structure—involves several overlapping developmental processes, including growth and cell differentiation. Precursor cells differentiate into specific cell types, which are organized into the tissues and organ systems that make up the functional plant.

Plant growth and cell differentiation are under complex hormonal control. Plant hormones regulate gene expression, often in response to environmental stimuli. For example, many plants form flowers. Unlike stems and roots, flowers do not grow throughout a plant’s life. Flowering involves a change in the identity of meristems—regions of the plant containing actively-dividing cells that form new tissues.

In addition to internal signals, environmental cues—such as temperature and day length—trigger the expression of meristem identity genes. Meristem identity genes enable the conversion of the shoot apical meristem into the inflorescence meristem, allowing the meristem to produce floral rather than vegetative structures.

The inflorescence meristem produces the floral meristem. Cells in the floral meristem differentiate into one of the flower organs—sepals, petals, stamens, or carpels—according to their radial position, which dictates the expression of organ identity genes.

The ABC hypothesis posits that the four flower organs form under the direction of three classes of organ identity genes: A, B, and C. If only A genes are expressed, sepals form. If only C genes are expressed, carpels are produced. Co-expression of B and C genes gives rise to stamens, whereas that of A and B genes produces petals.

In summary, flowering—and other aspects of plant morphogenesis—are contingent on multiple, overlapping developmental processes.

Plant morphogenesis is the development of a plant’s form and structure. Many overlapping processes and environmental factors contribute to plant morphogenesis. 

Among these processes are growth, cell differentiation, and cell-to-cell communication.

Growth includes cell division and elongation. During cell division, the symmetry, rate, and plane—or orientation—of division greatly influence a cell’s fate.

For example, guard cells, which control gas exchange in plants, typically form through asymmetrical division and by a change in the plane of cell division. 

However, most plant growth is caused by elongation, which is the permanent enlargement of differentiated cells. 

Plant cells expand primarily by taking in water. Most of the water is stored in a large central vacuole.  

Cell division and cell enlargement determine a plant’s shape and direction of growth. However, these processes vary among different types of plant cells. 

The specification of immature plant cells into distinct cell types is called cell differentiation and is also a critical component of plant morphogenesis. 

Cell differentiation is guided by gene expression changes, which inactivate or activate protein-coding genes. Cell-to-cell communication likely regulates the expression of genes that influence cell differentiation.  

For example, the root epidermis of Arabidopsis thaliana produces hair cells and hairless epidermal cells. Immature epidermal cells contacting one cortical cell differentiate into hairless cells, while those contacting two cortical cells develop into root hair cells. This pattern is associated with differential gene expression. 

Environmental factors, such as light, temperature, and the availability of water and nutrients, also greatly influence plant morphology.