Plant Hormones and Physiological Processes
Physiological Controls and Plant Hormones
Plant hormones, also known as phytohormones, control many different aspects regarding the physiology of plants, functioning similarly to human hormones.
These hormones control growth by either promoting or inhibiting cell division (the addition of new cells) or cell elongation.
Gibberellin is a specific hormone responsible for causing stem elongation, which means the cells grow larger, ultimately causing the plant to grow taller.
Hormones control plant development by promoting or inhibiting the differentiation of plant tissues.
Differentiation is a process where undifferentiated stem cells begin to express certain genes; once these genes are expressed, the cells differentiate into specific cell types, such as a flower, a leaf, or phloem.
Ethylene is an example of a plant hormone that is a gas and influences the process of differentiation.
Hormones help plants respond to various stimuli. While plants do not possess skeletal muscle to facilitate movement in the way animals do, their response to stimuli often manifests as growth.
Specific examples of responses to stimuli using hormones include phototropisms (response to light) and gravitational tropisms (response to gravity).
Auxin and Tropisms
Auxin is a primary phytohormone that controls tropisms and various other physiological processes.
For auxin to be active in a specific area, a high concentration of the hormone is required, which necessitates coordination and communication between cells.
In the presence of lateral sunlight, the plant must coordinate to move auxin molecules to the part of the plant that needs to grow and elongate, specifically the side away from the sun.
The coordination of auxin movement involves transmembrane proteins called auxin efflux carriers.
Auxin efflux carriers are proteins embedded within the cell membranes that help pump auxin into cells. Because these proteins are embedded in membranes, they have the ability to move around.
Nearby cells communicate and coordinate to relocate these auxin efflux carriers to the specific part of the cell where auxin is needed. This allows auxin to be pumped into those cells faster to facilitate elongation.
This cellular coordination allows the plant to exhibit positive phototropism, which is the bending of the plant toward the light source.
Molecular Mechanism of Cell Elongation
Auxin acts as a hormone that regulates gene expression within plant cells.
In the context of phototropism, auxin promotes the synthesis of a proton pump, which is a protein embedded in the cell wall, also known as the apoplast.
These proton pumps are concentrated in the cell walls of the cells farthest away from the sun.
The pumps move protons () into the cell wall, which lowers the of the apoplast.
The acidification of the cell wall (lowering the ) causes the cell wall to elongate.
Elongation occurring specifically on the side of the shoot farthest from the light source creates an imbalance that results in the physical bending of the plant toward the light, facilitating positive phototropism.
Hormonal Coordination and Transport
Multiple phytohormones influence growth and often work in tandem or in opposition.
Auxin is generally produced in the shoots at the top of the plant and is transported downward toward the roots.
Cytokinin is a different phytohormone produced in the roots and is transported upward toward the shoots.
Hormones can work together to accomplish growth simultaneously, or they can work antagonistically, meaning one hormone inhibits the effects of the other.
The coordination between different hormones ensures that shoot growth is appropriate for the specific stimuli the plant receives.
Examples of stimuli requiring hormonal coordination include:
The direction and presence of light.
Physical damage, such as someone chopping off the shoots of the plant.
Underground obstacles, such as roots running into hard ground.
Fruit Ripening and Ethylene
Coordination is required for processes beyond growth, such as fruit ripening in flowering plants.
Fruit ripening is necessary so that the seeds inside can be dispersed. Plants aim to have their seeds dispersed so that offspring do not have to compete with the parent plant for resources like space, water, and sunlight.
To encourage animals to eat the fruit and disperse the seeds, the fruit must become attractive. Ripening signals include:
A change in color (e.g., green bananas turning yellow).
Softening of the tissue.
Improved scent.
Increased sweetness.
Ethylene is the phytohormone responsible for coordinating the ripening of fruit.
Ethylene works through a positive feedback loop. The production of ethylene causes cellular changes that lead to the ripening of the fruit; these changes then trigger the production of even more ethylene.
This positive feedback loop is expressed as:
This process ensures that ripening progresses through stages to full maturity rather than reverting to an unripened state.
Ethylene is unique because it is a gas. Unlike water-soluble hormones like auxin that are restricted to traveling within a single plant, ethylene can travel through the air to affect nearby plants.
The gaseous nature of ethylene allows nearby plants to synchronize their ripening, ensuring they are all ripe at the same time when a seed-dispersing animal arrives.
This mechanism represents a significant example of biological interdependence and interaction mediated by phytohormones.