BIOL 266 MT2: Ethylene (Plant Growth)

What is Ethylene?

A gaseous plant hormone. It is associated with the stress (stress responses of a plant), senescence (the period of growth from young to old) of a plant, and fruit ripening of plants. It can be produced in almost all parts of plants (due to it being important in stress response) and detected in almost all parts of a plant.

Acts as a defoliating (trees losing leaves) agent. Found out when lights powered by goal gas (which has ethylene) caused trees near it to lose their leaves.

Effects of Ethylene

Decreased stem elongation, increased stem radial growth (the stem’s radius grew/its circumfrence), prevents the “hook” of fresh seedlings from opening/flowering. It also produces degrative enzymes that speed up the ripening of fruits, causing it to spoil faster. Spoiled fruits produce ethylene, which causes it to spread to nearby fruits.

Climactic Fruit

Fruit that continues to ripen after harvesting and need Ethylene (C2H4). Ethylene fruits follow a sequence of events:

1. Produce ethylene to ripen.

2. Increased respiration rate (exchange gases to make energy)

3. Increased synthase activity.

4. Increased ethylene production but also the synthesis of degradative enzymes.

Auto-Catalytic

Ethylene is auto-catalytic, meaning ethylene can signal and produce more of itself.

Ripening

A complex biochemical process.

It softens cell walls due to increased activity of enzymes which decrades pectin (component of cell walls). Ripening also makes fruit sweeter by converting starch to sugar. It breaks down chlorophyll. Fruits with less ethylene have longer lasting shelf life. Fruits that are artificially engineered have less polygalacturonase, which degrades pectin.

Wounding (Stressors)

Stressing plants or slicing plant dissues increase ethylene production. It serves as a warning for pathogen invasion.

Waterlogging

Also a stressor for plants. When the soil is saturated with water, this leads to oxygen deprivation. This triggers a stress response for plants and leads to increased ethylene production. This is because soil normally has air space. Waterlogging saturates the soil and removes the air spaces. Leads to hypoxia (inadequate O2) and produces aerenchyma (spongy tissue with air spaces.)

The air spaces in the tissue allows for some O2 diffusion from the shoot and maintains root aerobic cellular respiration in case of waterlogging.

Ethylene and stem elongation

Inhibits it, but otherwise increases stem radial growth (stem thickening). The Apical hook (the hook helps seedlings push past the soil) of budding seedlings are affected by ethylene by making it stronger and wider, allowing it to push past the soil.

Ethylene and flowering

Does not promote flowering but rather inhibits it. For a certain few plants like mangoes and pineapples, ethylene actually promotes flowering. Its effect on flowering sequence is similar to ripening, where it causes it to age faster (senescence). It is also auto-catalytic (meaning ethylene stimulates more ethylene.)

Ethylene and Tomatoes

Some tomatoes are transformed with Antisense ACC synthase or ACC oxidase genes (these are genes that reduce ethylene production in plants). Its effects is that it responds to ethylene but doesn’t make more of it.

The vines on a tomato reduce ethylene linkage, allowing it to stay firmer and fresher for longer.

Ethylene and Christmas Tree/Transport

1-MCP greatly reduces needle loss from ethylene and promotes needle retention.

Abscisic Acid (ABA)

A stress hormone in response to water (or high salt conc.) and low temperature stress. Abscission causes plants to shed parts. ABA is synthesized from carotenoids and close stomatas and harden plant material. This helps it respectively conserve water (when closing stomata) and hardening the plant material helps it be more resistant to lower temperatures.