Plant Responses to Internal & External Stimuli

I. Etiolation vs. De-etiolation

• A. Etiolation – are the adaptations/responses plants have towards growing in complete darkness– Short stubby roots, unexpanded leaves, pale stems

• B. De-etiolation – when a plant(shoot) reaches light also known as “greening”– Stem elongation slows, leaves expand, roots elongate, shoot

produces chlorophyll

Through Etiolation & De-etiolation we will discuss how plants response to a signal and then transduce a response

II. Signal Transduction pathway• A. It occurs in 3 steps:– 1. Reception – (receptors)proteins along the cell membrane

or cytoplasm change shape due to a specific stimulus– 2. Transduction – if the signal is weak secondary messengers

inside the cell amplify the signal and transfer it from the receptors to other proteins that carry out the response Ex. Ca2+ and cyclic GMP

– 3. Response – 2 ways either post-translational modification (modifying preexisting enzymes) or transcriptional regulation (increase/decrease synthesis of mRNA encoding specific enzymes by binding to specific DNA sequences (transcriptional factors)

II. Plant Hormones

• A. Hormones – signaling molecule that is produced in tiny amounts in one part of the body and transported to other parts where it binds to a receptor and triggers a response. – 1. In plants some work together to influence one

response, others can regulate several processes– 2. The first hormone discovered was when

watching a plant’s growth toward/away light (phototropism) by auxin (IAA) indoleacetic acid

II. Plant Hormones

• B. Survey of Plant Hormones– 1. Auxin – 2. Cytokinins– 3. Gibberellins– 4. Brassinosteriods– 5. Abcisic Acid (ABA)– 6. Strigolactones– 7. Ethylene

• 1. Auxin – always moves from shoot tip downward (polar transport) and allows cells to elongate, promotes formation of lateral roots, regulates development of fruit, enhances apical dominance• a. By way of expansins that break cellulose microfibrils due to a

low pH by uptake of H+ ions in the cell walls

• 2. Cytokinins – stimulate cell division, apical dominance, differentiation of cells and aging• a. produced in growing tissues (roots, embryos & fruits)

• 3. Gibberellin – stem elongation, fruit growth & seed germination (break dormancy)• a. mostly found in young roots an leaves, work with auxin to help

fruit develop and can increase the size of fruit

• 4. Brassinosteroids – promote cell expansion and cell division in shoots, root growth in low conc., promote xylem growth

• 5. Abscisic Acid (ABA) – slows growth, inhibit germination in young seeds and allows seeds to withstand dehydration, causes stomata to close

• 6. Strigolactones – stimulate seed germination, help establish mycorrhizal associations and control apical dominance

• 7. Ethylene – promotes ripening of fruit, and the triple response in seedlings (inhibit stem elongation, promotion of lateral expansion and horizontal growth), leaf abscission