Morphogenèse

Plant morphogenesis involves three overlapping processes: growth, cell differentiation, and cell-to-cell communication. Growth includes cell division and elongation, where cells expand by taking in water stored in the central vacuole. Cell differentiation specifies immature cells into distinct types through gene expression changes. Cell-to-cell communication regulates genes influencing differentiation, as seen in root epidermis patterning.

The symmetry, rate, and plane of cell division greatly influence a cell's fate. Asymmetrical division and changes in division orientation produce specialized cells like guard cells that control gas exchange. These divisions determine a plant's overall shape and direction of growth by controlling how cells are organized and positioned within developing tissues.

Most plant growth results from cell elongation, the permanent enlargement of differentiated cells. Plant cells expand primarily by taking in water, which is stored in a large central vacuole. Cell enlargement, combined with cell division, determines a plant's shape and growth direction, though these processes vary among different cell types.

Environmental factors such as light, temperature, and the availability of water and nutrients greatly influence plant morphology. Additionally, environmental cues like temperature and day length trigger the expression of meristem identity genes, enabling meristems to convert from vegetative to floral development and produce different organ structures.

The ABC hypothesis explains flower organ formation through three classes of organ identity genes. Cells in the floral meristem differentiate into sepals, petals, stamens, or carpels based on their radial position and gene expression patterns. A genes alone produce sepals; C genes produce carpels; B and C together produce stamens; A and B together produce petals.

Cell-to-cell communication regulates gene expression that influences cell differentiation. In Arabidopsis root epidermis, immature epidermal cells contacting one cortical cell differentiate into hairless cells, while those contacting two cortical cells develop into root hair cells. This differential contact pattern triggers distinct gene expression patterns, determining cell fate.

Flowering involves a change in meristem identity, converting the shoot apical meristem into the inflorescence meristem. This conversion allows the meristem to produce floral rather than vegetative structures. The inflorescence meristem then produces the floral meristem, whose cells differentiate into the four flower organs based on organ identity gene expression.