Lesson 4: Plant Growth and Development

The growing parts of angiosperms are called meristems. In eudicot plants, there are two types of meristem. Apical Meristem – found at the tip of the root and the shoot Lateral Meristem – found in the vascular bundles of the stem.

Growth in the main stem occurs as cells divide and by mitosis. These new cells provide the tissues required to elongate the stem and form new leaves. This allows plants to grow upwards from the soil towards the light. Growth in the root occurs as cells divide by mitosis. This allows the root to extend further into the soil

Secondary plant growth occurs at lateral meristems that are found in the vascular cambium. Secondary growth makes stems and roots thicker. Side growth may develop from the main stem as shoots or branches If the apical meristem is damaged, the first lateral meristem will take over its role. If a flower on the apical meristem is cut off, the lateral meristems will switch to producing flowers.

Plants can alter their growth in response to stimuli in the environment. This is known as tropism. Plant responses that are not growth related are called nastic responses. The most easily observed tropism is the bending of plants to light (phototropism). The opening and closing of flowers is a nastic response to light. (photonasty)

Plants will direct the their growth towards the sunlight in order to maximize photosynthesis. Cells on the dark side of the plant will elongate more rapidly than those on the bright side causing the plant to curve rather than grow straight upwards.

There are five major types of plant hormones Auxins Cytokinins Gibberelins Abscisic Acid Ethylene

Observations of phototropism led to the discovery of plant hormones, Auxins being the first. Auxins regulate stem elongation, cell differentiation, branching, development of fruit, apical dominance, phototropism and gravitropism. Auxins are found in embryos of seeds and in apical meristems. Auxins influence and coordinate the development of plants by influencing the expression of genes. Auxins may activate or inhibit genes. Auxin affects both cell division and cell enlargement. In a shoot auxin promotes the elongation of cells at the shoot apex Auxin is not produced in all cells and has to be moved to where it is needed. Auxin generally moves form roots to shoot but can move from one side of the shoot to the other using polar auxin transport. Movement only occurs in one direction and is controlled by auxin efflux pumps (protein channels unevenly distributed in the plasma membrane). This uneven distribution causes auxin build up in certain areas of the plant.

Auxin moves to the side of the plant that is not exposed to light. This uneven auxin distribution causes the cells on the shaded side to elongate faster, thereby bending the shoot towards the light.

Growth of the shoot apex (terminal shoot) usually inhibits the development of the lateral buds on the stem beneath. This phenomenon is called apical dominance. Apical dominance seems to result from the downward transport of auxin produced in the apical meristem. If the terminal shoot of a plant is removed, the inhibition is lifted, and lateral buds begin growth.

A group of hormones that promotes cell division, stimulate germination and delay aging. Made in the roots and transported throughout the plant. Cytokinins interact with auxins to stimulate cell division and differentiation. In the absence of cytokinins, a piece of parenchyma tissue grows large in culture, but the cells do not divide If the ratio of cytokinins and auxins is balanced, then the mass of growing cells, called a callus, divides but remains undifferentiated If cytokinin levels are raised, shoot buds form from the callus; if auxin levels are raised, roots form

Gibberellins promote seed germination, stem elongation and leaf growth and stimulate flowering and fruit development. Found in embryos and meristems of apical buds and roots.

Abscisic acid (ABA) inhibits growth, closes stomata during low water conditions and maintains dormancy. Found in leaves, stems, roots and green fruits. ABA counteracts the action of growth hormones, the ratio of ABA to growth hormones determines the growth outcome of the plant. ABA controls seed dormancy to ensure that germination occurs during favourable conditions.

Ethylene promotes fruit ripening. Ethylene is produced in the fruit and works by positive feedback. Ripening involves the breakdown of cell walls and the conversion of starches to sugars. Ethylene is a gas so the ripening effect can spread from fruit to fruit.