Lec 10:Recap from previous lec:Secondary xylem: dead cells when functionalSecondary phloem: consists of living cells, sieve element without nucleus

Notion of “organ-forming” substances:Sachs: believed in the idea of organ/tissue forming substances

Root-forming substance Shoot-forming substance

Charles and Francis Darwin: Used coleoptiles of grass seedlings (monocot) to investigate tropism

Phototropism = movement in response to light. Performed a series of experiments, discovered that the tip perceives the light. Later discovered to be the effect of some plant hormone

Referred to as plant growth regulators

IAA = indole-3-acetic acid: AuxinEffects:

1. Apical dominance: IAA in SAM. Axillary buds inhibited from growing out2. Activity of vascular cambium. Thickening of stems3. Causes branch roots to differentiate: lateral root formation4. Fruit development: esp form and shape5. Regulates leaf drop: abscission 6. Orientation of plant: negative geotropism of shoots due to change of

concentration of IAA

Gibberellin(GA): 2nd plant growth regulatornaturally occurring in healthy plants. Effects:

1. Stimulate cell elongation: increase stem length2. Affect flowering: causes some plants to flower3. State of maturity of plants: causes juvenility in some plants (non-flowering)4. Commercial uses: overcome dormancy of some seeds

Mech by which gibberellins overcome dormancy:1. Signal to germinate received 2. Embryo makes GA3. GA moves to aleurone 4. Enzyme breaks down endosperm: alpha amylase

Lec 11:Cytokinin: third class of plant growth regulatorCharacteristics:

1. Stimulate cell division2. Delay senescence (death)3. In combination with auxin, can affect plant development: ratio of auxin to

cytokinina. High auxin = roots. High cytokinin: shoots

4. Release axillary buds from apical dominance: promote outgrowth of lateral buds

5. Tumors, witches broom disease

Ethylene: 4th group of growth substances:Gas Characteristics

1. Petals discolored, fell off (abscised)2. Leaves exhibited a type of development called epinasty (curl up)3. Leaves abscised 4. Produced in fruits that are ripening5. Promoting flowering

a. Mangos, pineapples6. Harvest certain fruits involving human labor

a. cherries, walnuts

autocatalytic: apples oranges, bananas, applesalso produced by damaged fruits

change in concentration: commercial usage: carbon dioxide, ethylene compete for same site that regulates ripening: high CO2 can delay ripening

Abscisic Acid (ABA):Discovery of abscisic acid:UK & UC Davis

UK: identified dormin Davis: cotton “June drop”

Later discovered dormin = abscisic acid

Characteristics —associated with plant stress:1. Seed germination:

a. When seed is made: high level of ABA, suppresses seed germinationb. ABA degraded by the cold (cold-labile): causes seed germination

2. Plant roots sense drought3. Makes ABA4. ABA moves to leaves5. Causes stomata to close

In desert: seed coat contains ABA, rain washes out inhibitor germination In Saline environment: synthesized in roots, leads to the production of

proteins that deal with salt stress

Summary:Balance between different regulatorsDifferent sensitivity/ change in concentration