BIOL 371 - Plant growth and development

When is the shoot-root axis established?

the embryo already establishes which end will become the root and which end will become the shoot

What is modular growth in plants?

Plants grow by producing repeating units of the same types of tissues/organs. Plants continue developing new structures throughout their lives and are not just larger versions of their embryo.

What are nodes and internodes?

• Nodes: regions where organs are formed

• Internodes: stem tissues in between the nodes

What is indeterminate growth and why is it beneficial?

Plants continue growing throughout their life, and the location/pattern of growth is not predetermined.

It allows plants to adjust their growth and development in response to their environment. Without one-size fits all plants can modify where and how they grow depending on their surroundings.

What is primary growth, and what maintains it?

Primary growth lengthens the shoot–root axis upwards/downwards. It is maintained by meristematic tissues, which produce new plant organs.

What are meristematic tissues?

Undifferentiated plant cells that continuously divide to produce new cells and enable plant growth giving rise to plant organs.

ex. organs include: stems, branches, leaves, reproductive organs, lateral roots

What are the two types of meristems?

• Apical meristems: shoot and root tips/apex

• Lateral meristems: axillary buds (tiny embryonic shoot that forms in the angle between a leaf and a plant stem) and pericycle cells (form a specialized cylinder of plant tissue located just inside the endodermis and surrounding the vascular core (xylem and phloem) in roots and stems)

Where are the youngest and oldest parts of a plant?

Youngest: at the tips

Oldest: closer to the soil surface

What is secondary growth, and what maintains it?

Thickening/widening of the shoot–root axis, mainly in older regions of woody plants, to provide support for continued primary growth. This growth is maintained by proliferation (rapid increase of cell) of the internal meristems.

What do lateral meristems do?

make stems and roots grow thicker and wider

What do apical meristems do?

drive primary growth, making plants longer or taller

What are meristems?

special areas in plants made of tiny, active cells that keep dividing to create new growth. You can think of them like plant stem cells.

Was are the types of lateral meristems?

• Vascular Cambium

• Cork Cambium

What do the vascular cambium and cork cambium produce?

Vascular cambium: secondary xylem inside + secondary phloem outside

• this thickens the plant while maintaining sufficient connections to the vascular network

Cork cambium: new dermal layers outside

• this accounts for further thickening of the vasculature, maintaining the outer surface

What are wood and bark made of? What makes it?

• Bark: outer layers composed of periderm and secondary phloem

• Wood: inner structure composed of secondary phloem

the vascular cambium creates wood by growing cells inward, and it forms the inner part of bark by growing cells outward

How do plants respond to their environment if they can't move?

Plants are sessile, so they adjust their development and defenses in response to environmental signals, mainly using chemical signals.

Why do plants produce many different metabolites?

Many metabolites help with defense and interactions with the environment. Some are also useful to humans—about 40% of medicines are directly derived from plants.

What is auxin and where is it produced?

Auxin is a plant hormone; its main active form is IAA. It is produced mainly in shoot apical meristems, young stems, and leaves.

What are tropisms?

Directional growth responses of plants to environmental stimuli such as light and gravity, regulated partly by auxin.

What is the Acid Growth Hypothesis?

1. Auxin signals the cell

2. H⁺ ions are released into the cell wall

3. Cell wall becomes acidic

4. Expansin proteins are activated

5. Expansins loosen hemicellulose cross-bridges between cellulose microfibrils

6. Cellulose microfibrils separate

7. Cell wall loosens → cell expands/elongates

What is the process of phototropism in shoots?

1. Directional light is detected by blue-light receptors

2. Auxin moves to the shaded side

3. Auxin promotes cell elongation on the shaded side

4. Unequal elongation causes the shoot to bend toward the light

5. This helps the plant access light for photosynthesis

What are the steps of gravitropism?

1. Gravity is detected by pressure sensors

2. Auxin moves to the lower side

3. Shoot: lots of auxin on the lower side → stimulates elongation there → lower side grows longer → shoot bends upward.

4. Root: lots of auxin on the lower side → inhibits elongation there → upper side grows longer → root bends downward.

5. This keeps shoots growing toward light and roots growing into the soil nutrients.

What is apical dominance and what causes it?

Apical dominance is when the main shoot grows while axillary buds are inhibited from developing into branches. It is caused by auxin produced at the shoot apex, which moves down the stem and suppresses axillary bud growth.

What happens to axillary buds farther from the shoot apex?

Generally, the farther a bud is from the shoot tip, the lower accumulation of auxin it will experience so branch development can occur causing the triangle shape observed in many Conifers

Where and how does auxin travel through tissues? What does it do?

Auxin is synthesized at the shoot apex and surrounding tissues and is then trafficked downwards via the cell-to-cell pathway

As it transits down the shoot, auxin can accumulate the axillarybuds, preventing their development into new branches

What are axillary buds?

dormant meristematic tissues located at the nodes of the shoot

How can branching be increased? What does this cause a loss of?

In other plants, either by reducing auxin production or having decreased perception at the axillary buds results in greater amounts of branching and a loss of apical dominance

What happens if the shoot apex is removed?

Removal of the shoot apex causes an immediate reduction in auxin production allowing for greater branch outgrowth

• New branches will eventually take the place of the main shoot

Remove shoot apex → ↓ auxin → side branches grow → one branch becomes dominant → new shoot apex forms → ↑ auxin → other side buds are inhibited again.

What do gibberellins (GA₃) do?

Promote flowering stem initiation and elongation (bolting) and can promote fruit enlargement.

What are examples of Green Revolution semi-dwarf varieties?

often GA mutants (reduced synthesis or sensitivity) that prevented excessive stem elongation but still allowed for flowering

How do plants defend themselves?

Plants defend against viruses, bacteria, fungi, herbivores, and parasitic plants using physical defenses (spines, thorns, hairs) and chemical defenses.

How can plants respond to damage or infection?

Damaged/infected plants can release volatile compounds that trigger defensive responses in themselves and nearby plants, involving salicylic acid and jasmonate signaling.

How do plants defend against microbial infection?

Strengthen cell walls, close stomata, selectively plug xylem/phloem to prevent spread and limit resource loss, and release antimicrobial proteins and toxic defensive compounds.

What is the hypersensitive response? When can it occur?

Uninfected cells surrounding the infection undergo programmed cell death to contain the pathogen and prevent its spread. Only when the plant can recognize the type of infection, so it occurs in resistant plants.

How can an infection trigger defense responses in distant parts of a plant?

Chemical signals such as salicylic acid travel through the vascular system and methyl salicylate can travel through the air, triggering defenses at distant sites. Some viral defenses also use traveling RNA signals to target viral RNA.

How do plants warn nearby plants about herbivore damage?

Damaged plants release volatile compounds that trigger jasmonic acid signaling in undamaged regions and nearby plants, causing them to produce defensive compounds that make them less palatable or toxic to herbivores.

How can volatile compounds help plants fight herbivores indirectly?

Some plants release volatiles that attract predatory insects, which feed on the herbivorous pests attacking the plant.