Question

(Essay format) Define phytohormones, name them and explain their main general roles in the plant life cycle. As an example of phytohormone, explain the specific role of auxin in the plant and how the auxin signaling pathway works.

Answer 1

1. Phytohormones: Phytohormones are Plant hormones which are characterized by the property of serving as signal molecules or chemical messengers by which the activity of certain organs is coordinated with that of others within plants. These hormones occur in extremely low concentrations.

2. These are namely Abscisic acid, Cytokinins, Auxins, Ethylene, Gibberellins, Salicylic acid, Brassinosteroids, Jasmonates, Strigolactones etc.

Major roles: The major roles they performed within plant are the regulation of growth and development, cell division, elongation, cellular differentiation, organogenesis, pathogen defense, reproduction, pattern formation, sex determination, and abiotic and biotic stress tolerance.

3. Role of Auxin: The main function of Auxin are to influence positively in cell enlargement, root initiation and bud formation. These hormones also acts as inducer and promote the production of other hormones. Moreover, they can conjugate with other phytohormone. As for example, they conjugate with cytokinins which control the growth of stems, fruits, and roots, as well as convert stems into flowers.

4. Auxin signalling: Auxin facilitates rapid switching between transcriptional repression and gene activation through the auxin-dependent degradation of transcriptional repressors. The nuclear auxin can trigger broad and specific transcriptional responses. The major components of the auxin signaling mechanisms belong to three (3) protein families: the Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) transcriptional repressors, the F-box TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX PROTEIN (TIR1/AFB) auxin co-receptors and the AUXIN RESPONSE FACTOR (ARF) transcription factors. Auxin promotes an interaction between TIR1/AFB and Aux/IAA proteins. As a result the degradation of the Aux/IAAs occurs and the release of ARF repression.