Lecture 7 Plant Hormones PDF

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Plant Hormones...

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Biology 01/31/2018 Lecture 7: Plant Hormones

Auxin: Apical Dominance    

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First molecule to be identified as a plant hormone Involved in many different plant processes Charles and Francis Darwin (father son) how plants grow towards light Apical dominance: presence of apical bud inhibits lateral bud growth o Apical dominance favors upwards growth over outwards growth…and up is (usually) towards the sun Observations: Presence of apical bud inhibits lateral bud growth o Removing tips of stems makes plants bushier (more lateral bud growth) Hypothesis: tips of growing plant stems produce a growth substance that inhibits lateral bud growth Preliminary experiments: o Determined amount (activity) of growth substance in tips Current Understanding (based on many analyses): o Growth substance isolated and identified as auxin – promotes ggowth by causing sell elongation  Auxin makes plant cell walls weaken in specific areas, which allows cells to swell with water and get longer o Specific role of auxin in apical dominance still being unraveled  Does NOT inhibit lateral bud growth in some plant species  Data suggests does NOT enter or interact with cells of lateral buds  Hypothesis (one of several): a second messenger (e.g. a different hormone) is involved in inhibition of lateral bud growth by auxin

Wrap-up 

Key Points (reminders): o Most plant processes are controlled by hormone signals o These hormones play roles in may other plant processes o Like most (all) processes, these examples are controlled by many other hormones

Plant Responses to External Stimuli 

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Plants must respond to many external stimuli o Primarily respond by altering growth and developmental processes o Fundamental external stimuli for ALL plants include light and gravity Two main categories of responses o Photoperiodism: developmental responses due to changes in relative lengths of day and night

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Tropisms: growth responses that promote growth towards or away from a specific stimulus  Phototropism (light)  Gravitropism (gravity)  Thigmotropism (touch)...