Plant Response

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