1. Plant Response Signal Reception/Transduction Pathways… the plant response to external stimuli determines the internal patterns of development – Etiolation … adaptations for growing in darkness Elongation of roots to obtain substrates – Root shoots dominate over extensive root systems as transpiration is low Underdeveloped leaves – Small size & lacking in chlorophyll – De-etiolation… process of greening when light is available Leaves expand and production of chlorophyll increases Elongation of stems (previously root shoots)

2. Signal Reception-Transduction- Response Studies in de-etiolation demonstrate the model – Reception… the availability of light activates phytochrome in the cytoplasm of the leaf – Transduction… even the slightest amount of light causes signal amplification from the phytochrome cGMP opens Ca+ channels in the cell membrane – Activates various kinases that signal production of de-etiolation response proteins

3. – Response … Transcription Regulation – By activation of specific positive transcription factors by cGMP & Ca+ Post-translational Modification – Modification of existing proteins by phosphorylation (kinase activity) – Protein phosphatases turn off process through dephosphorylating if light is no longer available

4. Plant Hormones Hormones affect plant growth by affecting growth, elongation, and differentiation. – Due to positive or negative tropism Phototropism causes the dark side of the grass to grow faster due to a release in a growth hormone (auxin) resulting in bending toward light Each hormone has multiple effects causing a profound effect on plant development – Most through signal amplification or redirection of metabolism

5. Hormone types Auxins… stimulate elongation (growth) – Travel from tip to root … polar travel from apical to basal end of the cell Used in broadleaf herbicides as they are taken in by leaves causing a hormone overdose Cytokinins…activate growth through cell division (cytokinesis) – Produced in roots (travel up) – Works in concert with auxins The ratio of each determines whether roots or shoots develop Apical dominance is exhibited in plants as auxins dominate. – Apical inhibition is exhibited through decapitation as cytokinins to dominate causing later bud formation

6. Hormone types Gibberellins (GA)… active in growth in leaves and stems, fruit growth, and seed germination – More than 100 different forms identified – Facilitates the penetration of expansion proteins through the cell wall – Work in concert with auxins to promote fruit formation Sprayed on fruit to increase size for market – Activated in the seed after imbibition to signal embryogenisis

7. Hormone types Brassinosteroids… promote cell elongation and differentiation on stems and seedlings. – Similar to auxins – Retard abscission (loss of leaves) Abscisic Acid (ABA)… antagonizes the other growth hormones to retard growth – Ratio determines effect – Present in concentrated amounts in the seed to prevent maturation in the absence of water or light “washed out” during imbibition The ratio of ABA to GA determines germination – Helps in drought by causing stomata to stay closed decreasing transpiration

8. Hormone types Ethylene… produced by the plant in times of stress (drought, flooding, mechanical pressure, infection, …) Production causes both cell death and fruit to ripen – Triple response… produced in high concentrations when a seedling reaches an obstruction causing it to grow sideways – Apoptosis… a burst of ethylene production is associated with deciduous leave removal in autum and the death of an annual plant Process of abscission causing hydrolyzation of polysaccharides holding the leave stem Recycling of nutrients and auxins signal new leaves in spring – Fruit ripening… a burst of production signals ripening by softening the cell walls and sweetening of the fruit through acid and sugar production. Storing unripened fruit in a paper bag concentrates ethylene gas to speed ripening Commercial fruits are stored in CO2 ventilated containers (retards ethylene production) and exposed to ethylene gas just prior to market A rotten apple does spoil the bunch by over production of ethylene causing nearby fruit to spoil