Understanding Plant Hormones and Responses: A Comprehensive Guide

1. Plant Responses to Internal & External Signals

2. Plant Hormones • Hormones are chemical signals that coordinate the various parts of an organism • A hormone is a compound produced in one part of the body which is then transported to other parts of the body, where it triggers responses in target cells and tissues • Examples of human hormones: • Adrenaline, testosterone, estrogen, epinephrine…

3. Plant Hormones • There are 5 major classes of plant hormones, each with specific functions: • Auxin • Cytokinins • Gibberellins • Abscisic acid • Ethylene

4. Auxin • Stimulates stem elongation • Stimulates development of fruit • Involved in phototropism and gravitropism

5. Cytokinins • Stimulate cell division and growth • Stimulate cytokinesis • Stimulate germination and flowering

6. Gibberelins • Trigger seed and bud germination • Promote stem elongation and leaf growth • Important in the growth of fruit

7. Ethylene • Promotes fruit ripening • Senescence (aging) is a progression of irreversible change that eventually leads to death • Caused, at least in part, by ethylene • “One bad apple spoils the whole bunch”

8. Abscisic Acid • Induces seed dormancy • Anti-gibberellin • Inhibits cell growth • Anti-cytokinin • Inhibits fruit ripening • Anti-ethylene • Closes stomata during water stress, allowing many plants to survive droughts

9. Tropisms • Tropisms are growth responses that result in curvatures of whole plant organs toward or away from a stimuli • There are three major stimuli that induce tropisms • Light (Phototropism) • Gravity (Gravitropism) • Touch (Thigmotropism)

10. Phototropism • Phototropism is the growth of a shoot towards light • This is primarily due to the action of auxin • Auxin elongates the cells on the non-light side

11. Biological Clocks/Circadian Rhythms • A physiological cycle with a frequency of about 24 hours is called a circadian rhythm • Even without external, environmental cues, circadian rhythms persist in humans and in all eukaryotes • Example: jet lag in humans

12. Photoperiodism • A physiological response to day length (differs in winter, summer, spring, and fall) is known as photoperiodism • Short-day plants • Require a shorter light period • Flower in later summer/fall/winter • Example: poinsettias • Long-day plants • Require a longer light period • Flower in late spring/early summer • Example: spinach • Day-neutral plants • Are unaffected by photoperiod • Example: tomatoes • But it’s actually the night that matters!!

13. Plant Defenses • Plants defend themselves against herbivores in several ways • Physical defenses, such as thorns • Chemical defenses, such as producing distasteful/toxic compounds