34.9 : מורפוגנזה

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.