Chemical signaling in plants Flashcards
Describe the movement of plants
Called directional growth or tropism, in response to stimuli – there is positive phototropism and negative geotropism of the plant and of the root – positive phototropism is directional growth in response to lateral light; apical meristem cells (located at the root and shoot tip) in the shade divide or elongate to increase the amount of light absorbed by a shoot’s leaves for photosynthesis
What determines the growth response in plants
Type and direction of stimulus – plants control the direction of growth of their roots and shoots, the plant won’t grow differently due to stimulus if its tip of the shoot is covered (where apical meristem is situated) – a pigment detects light intensities and auxin is redistributed to cause changes in growth
Phytohormones and their effect
They are signaling molecules that:
1. Promote or inhibit growth by affecting rates of cell division and enlargement – gibberellin
2. Promote or inhibit aspects of development – ethylene promotes fruit ripening by promoting the conversion of acids and starch into disaccharides
3. Respond to stimuli – curling of tendrils, flytrap capturing an insect, auxin directional growth
Auxin
phytohormone that promotes differential stem growth in phototropic responses, produced in the shoot (apical) meristem and axillary buds and it can pass into neighboring cells – exists in the neutral and charged form
Auxin movement through cells
1) enters cells by passive diffusion when its carboxyl group is uncharged
2) cytoplasm is alkaline, so auxin loses a proton and its carboxyl group becomes negatively charged and it is trapped
3) auxin efflux carriers can pump negatively charged auxins across the plasma membrane into the cell wall
4) surrounding cell wall is acidic so auxin reverts to its neutral state and then passively diffuses into the next cell
How exactly does a plant grow (in directional growth)?
Cellulose molecules are inelastic, so a microfibril cannot stretch. The extension of walls during cell growth involves microfibrils moving further apart or sliding past each other. Microfibrils are crosslinked by carbohydrates and the strength of these links is influenced by pH (decreases in pH weaken them, the wall extends)
Auxin promotes the synthesis of proton pumps by a cell and their insertion in the plasma membrane. These pumps transport H+ ions from the cytoplasm to the cell wall, acidify it and thus weaken the links so the cells enlarge (when they absorb more water – become turgid and enlarge)
Auxin and cytokine relationship and mechanism
Phytohormones used for communication between the upper and lower plants of a plant as a means of regulating shoot and root growth
Auxin is transported via the phloem to the roots from the shoot and cytokines transported via xylem from roots to the shoot.
1) Synergism: both promote cell division and cell growth in the root and shoot tip meristems.
2) Antagonism: auxin promotes and cytokinin inhibits development of new roots and branching of roots (when the upper part is satisfied, it produces a lot of auxins which signal to the roots that things above are in order so they can produce more roots).
3) Antagonism: while auxin inhibits and cytokinin promotes branching of stems by growth of axillary buds (cytokines signal to the branches that they have the support of the roots to produce more branches)
What happens when the main shoot is eaten?
As that is the origin of most auxin produced within the plant, it can no longer inhibit the growth of axillary buds, so they grow and produce more auxin – also when there is less auxin, roots slow down their growth which allows for a stronger growth of the shoot to replace the missing part
Role of ethylene in chemical signaling in plans
Fruits are stimulated to ripen by ethylene and produce more of it as they ripen (positive feedback mechanism). Ethylene is volatile so it can diffuse to other fruits and promote their ripening too. This helps synchronize the ripening of fruits, increasing attractiveness of a plant with fruit to animals and encouraging dispersal of the seeds
