Question

(a) What are plant hormones?
(b) Write two functions of Auxin.

Answer 1

Plant hormones are not nutrients, but chemicals that in small amounts promote and influence the growth, development and differentiation of cells and tissues. The biosynthesis of plant hormones within plant tissues is often diffuse and not always localized.

Complete step by step answer: Plant hormones, also known as phytohormones are signalling molecules produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, i.e. from embryogenesis to the regulation of organ size, pathogen defence, stress tolerance and reproductive development. Hormones are transported within the plant by utilizing four types of movements. For localized movement, cytoplasmic streaming within cells and slow diffusion of ions and molecules between cells are utilized. Vascular tissues are used to move hormones from one part of the plant to another; these include sieve tubes or phloem that move sugars from the leaves to the roots and flowers and xylem that transport water and mineral solutes from the roots to the foliage. Initial research in plant hormones identified five major types: abscisic acid, auxins, cytokinins, ethylene and gibberellins. Auxins have some morphogen-like characteristics. Auxins play a cardinal role in the coordination of many growths and behavioural processes in the plant life cycle and are essential for plant body development.
Functions of Auxin include:
1. Auxins promote root initiation. It induces both pre-existing roots and adventitious root formation. As more native, auxin is transported down the stem to the root where the overall development of roots is stimulated.
2. Auxin induces shoot apical dominance; the axillary buds are inhibited by auxin. The high concentration of auxin directly stimulates ethylene synthesis in axillary buds, causing inhibition of their growth and potentiation of apical dominance.

Note: Plants use different pathways to regulate internal hormone quantities and moderate their effects; they can regulate the number of chemicals used to biosynthesize hormones. They can store them in cells, inactivate them or cannibalize already-formed hormones by conjugating them with carbohydrates, amino acids or peptides. Plants can also break down hormones chemically, effectively destroying them. Plant hormones frequently regulate the concentrations of other plant hormones.