Regulatory Systems in Plants (Ch 39)

• Plants show complex responses to environmentalstimuli

• Problem: no nervous system (detection) & nomuscular system (response)

• Various mechanisms for detecting stimuli• Plants respond to stimulus by:

– cell division– cell elongation– cell differentiation

Regulatory Systems: RESPONSE TO LIGHT

• Light is a major environmental stimulus– seed germination– shade/sun response– photosynthesis– flowering

• Plants respond:– presence of light (on/off response)– light quantity– light quality

The Reversible phytochrome system

Pr

Pfr

Darkness=slow conversionback to Pr

Far-red light =rapid conversionback to Pr

Red lightrapid conversionto Pfr

Photoperiodic Control

• Monitoring daylength– ratio Pr:Pfr

• Many plants respond to length of day– Long-night plants (short day- flower fall/winter)– Short-night plants (long day- flower spring/summer)– Night-neutral plants (response is daylength independent)

Short-night plant Long-night plant

Flashes of light can fool a plant!Short-night plant Long-night plant

Disco plants...

Plant Hormones:

• Most plant responses controlled by HORMONES

• What are hormones?

• 5 major classes of plant hormones

Five Plant Hormones:

• 1) Auxins– Stem apical meristems are major site of auxin production

• promote cell elongation• responsible for phototropic response

Control- turns to light

(READ pp 750-751!)

Auxin produced at growing tip.

Auxin migrates away from light

Auxin stimulates cells to elongation

Five Plant Hormones:

• 1) Auxins– Other effects

• produced in seeds --> stimulates fruit production• promotes growth of adventitious roots

Five Plant Hormones:

• 2) Cytokinins– Root apical meristems are major

site of cytokinin production• stimulates cytokinesis (cell

division)• “anti-aging” hormone- delays

scenescence

Five Plant Hormones:

• Auxin:CytokininInteractions– Cell differentiation in

tissue culture• auxin>cytokinin --> roots

develop!

• auxin<cytokinin --> shootbud develops

• Auxin:CytokininInteractions– Shoot and root apical

dominance• auxins inhibit lateral bud

growth in stems, cytokininsstimulate it!

• auxins stimulate lateral rootgrowth, cytokinins inhibitthem

Five Plant Hormones:

• 3) Abscisic Acid (growth inhibitor)– despite name… isn’t really involved in leaf abscission!– Prepares plant for winter dormancy– maintains seed dormancy– shuts down stomates under serious water stress

Five Plant Hormones:

• 4) Gibberellin (growth promoter)– foolish rice disease– promotes stem (but not root) cell division and

elongation (may work w/ auxins)– promotes seed germination (antagonistic effect w/

abscissic acid)– promotes spring bud break (antagonistic effect w/

abscissic acid)

Spray developing grape clusterw/ gibberellin = stem elongatesand grapes are farther apart= larger grapes!

Five Plant Hormones:• 5) Ethylene

– fruit ripening– leaf abscission in the fall Ripe apple releases ethylene-

results in leaf drop!

Plant Control Systems: Tropism

• Tropism- growth responseof plant to environmentalstimulus

• Phototropism- Growth inresponse to light

Plant Control Systems: Tropism• Gravitropism - growth in

response to gravity• Positive gravitropism-

roots grow DOWN!– Amyloplasts

• Negative gravitropism-stems grow UP!– auxin response…

Plant Control Systems: Tropism• Thigmotropism - directed

growth in response totouch

• examples– vine tendrils– wind-induced shortness

Plant Control Systems:Biological Clocks

• Plants have bio-rhythms• “Circadian rhythms” = 24 hour internal clock

– usually… reset daily– plant “jet lag”– under constant conditions may drift-

• Examples of rhythmic movements-– Stomatal opening– sleep movement of leaves

Noon position Midnight position

Tugor Movement• Movements based not on growth, but reversible

changes in cell turgor pressure• Examples- flowers which open/close @ day/night• Rapid leaf movement in Mimosa plant