Plant responses to internal and external signals (continued)

What are cytokinin’s?

A plant hormone that stimulates more cell division, cell differentiation, apical dominance and acts as growth promoting hormones

where are cytokinins produced?

In actively growing tissues

• such as in roots, embryos, fruits

what do the many effects of the cytokinins also rely on?

On auxin

what is abscisic acid (ABA)?

A plant hormone with the primary role to slow growth

• once was thought to promote leaf abscission

where can abscisic acid be produced?

In most plant tissues

what are the two main effects of abscisic acid?

Seed dormancy and drought tolerance

what is the effect of seed dormancy?

As the seed is maturing, the levels of ABA increases, ensuring the seed germinates only when optimal

• the high levels of ABA inhibits seed germination

what happens if there are low levels of abscisic acid for seed dormancy?

It can lead to early seed germination

• Ex: seen in red mangroves that are common in costal swamps, their seeds may still germinate while still on the plant

what is the effect of drought tolerance?

when a turgid cell has their stoma open, ABA accumulates and water is released along with potassium, causing the cell to become flaccid and close their stoma shut to prevent anymore water lose

where is abscisic acid produced during drought tolerance? (where is it transported?)

in water-stressed roots and leaves, then they are transported through the xylem

what is the process of a plants response to drought stress?

1. ABA accumulates in the leaves

2. The guard cell potassium channels open

3. the stomate closes to prevent water lose

4. Transpiration decreases

What is ethylene?

A plant hormone that is produced in response to stress from the plant

• such as from drought, flooding, mechanical pressure, injury, or infection

what are the effects produced from ethylene?

• Response to mechanical stress

• senescence

• leaf abscission

• Fruit ripening

what is mechanical stress?

A response that occurs when ethylene is produced from the seedling tip of a plant thats being pushed against an obstacle.

what is the response to mechanical stress? (What occurs once ethylene wears off?)

The triple response, causing the stem to grow thicker and elongate in a horizontal direction, allowing the shoot to grow around obstacles. Once ethylene wears off, the plant will grow normally again

what is the senescence?

the programmed death of certain cells, organs, or entire plants

• and usually there might be a burst of ethylene that causes an on sent of senescence

what is leaf abscission?

It is the loss of leaves caused by more ethylene being present than auxin as a leaf ages

• Ex: During the fall, the chlorophyll in leaves begin to break down and the nutrients are reabsorbed and relocated into other parts of the plant

what is fruit ripening?

It is an effect thats triggered when more ethylene is produced overtime in a chan reaction, causing the plant to ripen, making the fruit more softer and sweeter

Since ethylene is a gas, what occurs if one fruit is ripening?

It will release a signal that spreads from fruit to fruit, causing the other fruits to ripen

what do external signals received by plants produce?

internal chemical signals, aka hormones

• salinity and nutrient availability are a few other external cues that plants respond to

what does light cue for plants?

It cues many key events in plant growth and development, and the passage of days and seasons

What is the cue from light that causes the passage of days and seasons in plants?

the cue is change in day-length and change in temperature

What are examples of light cuing the passage of days and seasons?

• In winter, there are short periods of sunlight and long periods of night time

• In summer, there are long periods of sunlight and short periods of night time

what does light help plants do?

Helps plants time daily and seasonal activities to optimize their function and survival

• Ex: on a daily basis, plants will open their stomata in the daytime when sunlight is present and will close their stomata in the nighttime when sunlight isn’t present, preventing water loss

• Some seasonal activities are preparing for the winter, leaf senescence + abscission, and timing for pollinator availability

what do plants detect?

They detect presence, direction, intensity, and wavelength

what do plants tend to be more photosensitive to?

to red and blue lights

what does the detection of light in plants involve?

It involves photoreceptors, which are sensitive to specific wavelength

what are blue-light photoreceptors?

They are photoreceptors that detect blue light and tend to be important for phototropism and light-induced opening of the stomata, with just the presence of the light being able to cause the stomata to open

what are red-light photoreceptors (phytochrome)?

They are photoreceptors that detect red light that tend to be important for specific seed germination, shade avoidance, and de-etiolation

• de-etiolation being the greening of shoots

what do both photoreceptor types help with?

With the timing of daily and seasonal activities

why is light sensitive germination more pronounced in small seeds compared to germination in large seeds?

Small seeds require light for germination to ensure germination occurs only when they are close to the soil surface since they have relatively smaller internal resources

• they have to be close to the soil surface to ensure the shoot reaches the surface to begin photosynthesis

what is gravitropism?

it is the response to gravity

how do plants detect gravity?

Through the settling of statoliths (dense cytoplasmic components)

• Ex: if the root changes orientation, the statoliths moves down

what accumulates near the statoliths where the new bottom of the shoot/root is?

Auxin

Why does the root of a plant grow downward while the shoot grows upward after the statoliths and auxin redistributes?

The auxin triggers cell elongation in the shoots but not in the roots, causing the shoots to bend up and the roots to bend down, with the root cells elongating the most when there is less auxin

• as auxin inhibits branching

what are a few mechanical stimuli that plants respond to?

• Wind exposure alters growth form in some plant

• Vines and other climbing plants grow in response to touch, enabling them to cling to structural supports

• Some plants undergo rapid leaf movement in response to touch

How does water harm plants?

If there’s too little water, such as from drought, or too much water, such as from flooding, it can harm the plant

what causes drought stress?

Occurs when evapotranspiration exceeds water uptake

what happens in a plant that is in drought stress?

Abscisic acid produced in the mesophyll of leaf and root cells triggers stomatal closure to prevent water loss through the pores

what happens during drought stress?

Auxin stimulates root growth, helping plants take up what little water is available

What causes flooding stress?

it occurs when the soil pores are filled with water and not with air, causing too little oxygen to be present in the roots

what does flooding stress cause in some plants?

In some plants, ethylene triggers the death of root cortex cells and creates “air tubes”, allowing for movement and air flow of oxygen into the roots