Plant Growth & Development

Chapter 27

Plant Growth and Development

How do cells differentiate and form organs of the primary body of a plant?• Internal and external factors

Growth• cell enlargement

Development• genetically programmed succession from simple to

more advanced formDifferentiation

• progressive, developmental change to more specialized form

Totipotency• can undifferentiate and redifferentiate into other cell

types

Plant Hormones• Chemical signal that regulates and

coordinates metabolism, growth, and morphogenesis

• Plant hormones = plant growth regulating substances

• Organic compound molecule made in one part of a plant • where it elicits a response • and/or transported to another part where it elicits a

response• made in several locations in the plant body

• Five classic groupsAuxinGibberellinsCytokininsEthyleneAbscisic acid

Important Points

1. Single hormone elicits multiple responses2. Effect of hormone depends on many

factors• presence of other hormones• amount• sensitivity of tissue

3. Difficult to predict specific (single) response

4. Small quantities are required for response• 6µg IAA/kg plant• weight of needle/44,000lbs

Initial hormone experiments (Charles & Francis Darwin, 1881)

1. Auxin

• “when seedlings are freely exposed to a lateral light some influence is transmitted from the upper to the lower part, causing the latter to bend.”

1. Auxin

• Synthesized primarily in shoot apical meristems, young leaves, developing fruits/seeds

• Cellular elongation• Stem elongation• Root growth • Phototropism & Gravitropism

• Apical dominance• Differentiation of vascular tissue• Fruit development• Formation of adventitious roots

tryptophan

• IAA (indole-3-acetic acid) is most abundant auxin in nature

• Synthetic auxins are economically important• NAA (1-Naphthaleneacetic acid)

• induces adventitious roots in cuttings• reduces fruit drop in commercial crops

• 2,4-D (2,4-dichlorophenoxyacetic acid)• herbicide• kills broadleaf dicots but not monocots

• Agent orange• 1:1 auxin and 2,4,5-T (2,4,5-trichlrophenoxyaceticd

acid)• sprayed to kill vegetation during Vietnamese war• may have long-term health effects on soldiers and

Vietnamese citizens

1. Auxin

Auxins

Differentiation of Vascular Tissue

• Physiological response to many hormones is influenced by non-hormonal factors• auxin with 2% sucrose • differentiation of xylem

• auxin with 3% sucrose• xylem and phloem

• auxin with 4% sucrose• differentiation of phloem

1. Auxin

Apical Dominance• Axillary buds near shoot apex grow

more slowly than those further from shoot apex• remember axillary buds at base of petioles

• High IAA high ethylene inhibition of bud growth cone-shaped plant• IAA is released from apical bud, thus highest

levels close to that bud

• Single aspect of growth and development influenced by several hormones

1. Auxin

2. Gibberellins

a. Extensive growth: internode elongation• Growth retardants are manufactured

to inhibit action of gibberellins• leads to stunted plants

• chrysanthemums to produce flowers with thicker, sturdier stalks

• turf-grass for slowing stem elongation Promotes seed/bud germination

b. Promotes seed and bud germination c. Promotes flowering

• biennials are rosettes first year• bolt to produce flowers 2nd year• GA can cause bolting in 1st year

3.Cytokinins

a. Stimulates cellular division• hastens G2 to Mitosis transition• depends on presence of auxin

also

b. Organogenesis (formation of organs)• high auxin:cytokinin ratio roots• low auxin:cytokinin ratio shoots

c. Senescence (leaf or other organ death)• delays breakdown of chlorophyll

and thus initiation of senescence

4. Ethylenea. Fruit ripening

• what happens if you put an orange and an unripe banana in a paper bag?

• “one bad apple spoils the barrel”

b. Opposes or reduces some effects of auxin

c. Promotes or inhibits growth and development of roots, leaves, flowers, depending on species

H2C=CH2

ethylene

5. Abscisic Acid

a. Inhibits growthb. Closure of stomatac. Counteracts

breaking dormancy (buds, seeds)