NOTES: CH 39 - Plant Responses to Internal &

External Signals

Factors Affecting Plant Development:

1) The plant senses and responds to ENVIRONMENTAL CUES.

2) The plant’s GENOME encodes for enzymes that take part in development.

3) The plant uses RECEPTORS, such as photoreceptors, that absorb light.

4) HORMONES (chemical messengers) regulate the effects of environmental cues on receptors.

PLANT HORMONES:

● HORMONES = regulatory compounds that act at very low concentrations at sites distant from where they are produced.

● they mediate / regulate developmental phenomena, such as:

– Stem growth

– Autumn leaf fall

Plant hormones & responses:

Some important plant hormones:

● Abscisic acid: inhibits growth; maintains seed dormancy & winter dormancy; closes stomata during drought stress

● Auxin: promotes stem elongation & fruit growth; inhibits lateral bud outgrowth; gravitropism

● Gibberellins: promote seed dev. & germination, stem growth, flowering, & fruit development

Some important plant hormones:

● Cytokinins: root growth and differentiation; cell division and growth; seed germination; delay leaf senescence (aging / death)

● Ethylene: (gaseous hormone) fruit ripening; promotes leaf abscission (autumn) & enhances rate of senescence

Daily and Seasonal Responses● Circadian rhythm (24 hour periodicity) Transpiration & synthesis of certain

enzymes are plant processes that oscillate during the course of the day

Responses to environmental changes in: light levels, temperature, relative humidity, however…

these cyclic processes often continue even when the environmental cues are removed (“biological clock”)

Daily and Seasonal Responses● Photoperiodism (a response to a change in

the relative lengths of night and day)

Seasonal events, such as: flowering, seed germination, the onset and breaking of bud dormancy – all occur at specific times of the year

Daily and Seasonal Responses

FROM SEED TO SEEDLING:● recall: seeds may remain

dormant for weeks,

months, years,

or centuries!

● the mechanisms of dormancy include:

exclusion of water by an impermeable seed coat

mechanical restraint of embryo by tough seed coat

chemical / hormonal inhibition of embryo dev.

Seed dormancy can be broken by…

● seed coat is weakened from

tumbling across the ground,

or passing through an

animal’s digestive tract;

● soil microorganisms soften

the seed coat;

● fire (can release a mechanical restraint or remove the waterproofing of the seed coat)

● leaching (prolonged exposure to water)

FROM SEED TO SEEDLING:● Germination begins with:

1) IMBIBITION (absorption of water)

-hydration causes the seed

to swell and rupture the

seed coat

-triggers metabolic changes in embryo resume growth

-storage nutrients are digested by enzymes and nutrients are transferred to growing regions of embryo.

Imbibition of water in peas!

FROM SEED TO SEEDLING:

Germination continues as the:

2) radicle emerges from the seed;

3) shoot tip breaks through soil surface

How does hypocotyl/shoot emerge from the soil?

*in many dicots:

-the hypocotyl is in shape of a

hook (pushed above ground)

-light stimulates the hypocotyl to straighten

-hypocotyl raises the cotyledons and epicotyl above ground

-epicotyl then spreads the first leaves which become green and begin photosynthesis

**Many seeds will remain quiescent (dormant) until suitable environmental conditions are available; other seeds await a specific environmental cue (e.g. heavy rainfall; brush fire; exposure to cold or sunlight; passage through an animal’s digestive system) before they will break dormancy.

Flowering…

● FLOWERING = the formation of reproductive organs

● May be initiated by: Plant reaching an appropriate age / size

Particular time of year (sensed by the length of night)…photoreceptors involved!

It is likely that a “flowering hormone” is sent from the leaves to where the flowers form

Fruit formation…

● As already studied, fruits form following FERTILIZATION

● Fruit ripening is under hormonal control

Plant death…● Some plants are PERENNIALS: continue to

grow year after year

their buds typically enter a state of winter dormancy during the cold season

the hormone ABSCISIC ACID helps maintain this dormancy

Plant death…● many plans undergo SENESCENCE of certain

cells / organs / entire plant…during LEAF ABSCISSION, leaves die & fall off at the end of the growing season

● both of these processes involve turning on specific genes leading to apoptosis (programmed cell death);

newly formed enzymes break down many chemical components (DNA, RNA, proteins, membrane lipids) that the plant will salvage for a later date…

Plant Responses to STRESS:ABIOTIC STRESSES:

● DROUGHT: close stomata; wilt / roll up; promote root growth deeper down

● FLOODING: ethylene production stimulates apoptosis of root cortex cells, producing air tubes (“snorkels”) to provide O2 to roots

● SALT STRESS

● HEAT STRESS / COLD STRESS

Plant Responses to STRESS:BIOTIC STRESSES:

● HERBIVORY: animals eating plants

physical defenses: thorns, trichomes

chemical defenses: distasteful / toxic compounds

“recruit” predatory animals to kill the insect parasites

● PATHOGENS: infection (bacteria, virus)

YUMMY!Thank you RUBISCO for making this all possible!