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 form Differentiation 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 groups Auxin Gibberellins Cytokinins Ethylene Abscisic acid

Important Points 1. Single hormone elicits multiple responses 2. 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. Ethylene a. 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 H 2 C=CH 2 ethylene

5. Abscisic Acid a. Inhibits growth b. Closure of stomata c. Counteracts breaking dormancy (buds, seeds)