Entry Eight: Signal Transduction Pathway in Plants and Plant Hormones

Carolus Linnaeus, the father of taxonomy, was keen naturalist, noting that each plant species opened and closed its flowers at a characteristic time of the day. Why does the time vary? Well, the time at which flowers open presumably reflects the time when their insect pollinators are most active. This is just an example of the numerous environmental factors that a plant must sense to compete successfully. In this entry let’s talk a look at the mechanisms by which plants sense and respond to external and internal cues. Because, you see, unlike animals, plants are stationary and generally respond to environmental cues by adjusting their individual patterns of growth and development.

Like animals, plants receive specific signals and respond to them in ways that enhance survival and reproductive success. Here, we will explore the de-etiolation, the changes a plant shoot undergoes in response to sunlight, or plant as an example of how a plant cell’s reception of a signal – in his case, light – is transduced into a response.

Reception:

Signals are first detected by receptors, proteins that undergo changes in shape in response to a specific stimulus. The receptor involved in de-etiolation is a type of phytochrome, a member of a class of photo receptors. Unlike most receptors, which are built in the plasma membrane, they type of phytochrome that functions in de-etiolation is located in the cytoplasm.

Transduction: 

Receptors can be sensitive to very weak environmental or chemical signals. Some de-etiolation responses are triggered by extremely low levels of light. The transduction of these extremely weak signals involves second messengers – small molecules and ions in the cell that amplify the signal and transfer it from the receptor to other proteins that carry out the response. In de-etiolation, two second messengers, calcium ions and cyclic GMP must be produced for a complete de-etiolation response.

Response: 

Ultimately, second messengers regulate one or more cellular activities. In most cases, these responses involve the increased activity of particular enzymes. There are two main mechanisms by which a signaling pathway can enhance an enzymatic step in a biochemical pathway: post-translational modification and transcriptional regulation. Post-translational modification activates preexisting enzymes. Transcriptional regulation increases or decreases the synthesis of mRNA encoding a specific enzyme.

Post-translation Modifications:

In most signal transduction pathways, preexisting proteins are modified by the phosphorylation of specific amino acids, which alters the protein’s hydrophobicity and activity. Many second messengers, including calcium ions and cyclic GMP, activate kinases directly. Often, one protein kinase will phosphorylate another protein kinase, which then phosphorylate another, causing a phosphorylation cascade. Such kinase cascades may link initial stimuli to responses at the level of gene expression.

Transcriptional Regulation:

The proteins we call transcription factors bind to specific regions of DNA and control the transcription of specific genes. In the case of the phytochrome-induced de-etiolation, several such transcription factors are activated by phosphorylation in response to the appropriate light conditions.

Hormones:

A hormone is a signaling molecule that is produced in tiny amounts by one part of an organism’s body and transported to other parts, where it binds to a specific receptor and triggers responses in target cells and tissues. We will explore four separate plant hormones and their effects on plants.

Auxin (IAA) is a plant hormone produced in the apical meristems and young leaves. Developing seeds and fruits contain high levels of auxin, but it is unclear whether it is newly synthesized or transported from maternal tissues. Auxin primarily stimulates stem elongation, promotes the formation of lateral and adventitious roots, regulates development of fruit, enhances apical dominance; functions in phototropism and gravitropism, promotes vascular differentiation, and retards leaf abscission.

Cytokinin is a plant hormone that are synthesized primarily in roots and transported to other organs. It helps regulate cell division in shoots and roots, modify apical dominance and promote lateral bud growth, promote movement of nutrients into sink tissues, stimulate seed germination, and delay leaf senescence.

Gibberellins are primarily produced in meristems of apical buds and roots, young leaves, and developing seeds. They play a role in stem elongation, pollen development, pollen tube growth, fruit growth, seed development and germination, regulation of sex determination and transition from juvenile to adult phases.

Abscisic acid (ABA) is a plant hormone found and synthesized in almost all plant cells and in every major organ and living tissue. It may be transported in the phloem or xylem. The acid helps inhibit growth, promote stomatal closure during drought stress, promotes seed dormancy and inhibits early germination, promote leaf senescence, and promote desiccation tolerance.