Plant Growth And Development Flashcards
What are tropisms in plants?
Tropisms are directional growth responses to external stimuli like light, gravity, and touch, involving perception, transduction, and auxin redistribution.
What is gravitropism, and how do roots and shoots respond?
Gravitropism is a plant’s growth response to gravity:
- Roots grow toward gravity.
- Shoots grow away from gravity.
Where is gravity perceived in roots and shoots?
Roots: In the root cap.
Shoots: In the endodermis near the stem tip.
What are statocytes and statoliths, and how do they function?
Statocytes: gravity-sensing cells in the root cap and shoot endodermis.
Statoliths: dense, starch-filled plastids (amyloplasts) inside statocytes.
How They Work:
When the plant’s orientation changes, statoliths settle to the lowest part of statocytes due to gravity.
This mechanical signal triggers auxin redistribution, initiating a growth response.
How does auxin affect gravitropic responses in roots and shoots?
Roots: High auxin concentration on the lower side inhibits cell elongation, causing bending.
Shoots: High auxin concentration on the lower side promotes cell elongation, causing upward bending.
What is the role of PIN proteins in auxin transport?
PIN proteins are auxin efflux carriers that transport auxin directionally.
For example: PIN3 relocalizes to the basal side of statocytes after reorientation to accumulate auxin.
What is the role of the LAZY protein in gravitropism?
LAZY proteins connect statolith sedimentation to auxin redistribution, ensuring proper gravity perception and bending responses.
How is auxin transported in plants?
Long-distance: Via phloem sap.
Cell-to-cell: Through active, polar transport mediated by PIN proteins.
How do ‘lazy mutants’ affect gravitropism?
Lazy mutants have defective auxin relocation, leading to agravitropic behavior despite normal statolith sedimentation.
What experimental evidence shows auxin redistribution in gravitropism?
DR5::GFP reporter gene visualizes auxin accumulation.
Experiments show higher auxin levels in the lower part of reoriented roots or shoots.
Summarize the key steps of gravitropism.
Perception: Gravity sensed in statocytes.
Transduction: Auxin redistribution via PIN proteins.
Response: Differential cell elongation causes bending.
What did the experiment involving the removal and inversion of the hypocotyl of a seedling prove?
That despite gravity, auxin movement is basipetal not acropetal but even when the hypocotyl was inverted this polar transport remained true, the base end didn’t transport the auxin to the apical end.
Summarise gravotropism mechanism
What is the role of photoperiod in plant development?
Photoperiod regulates key processes in plant development such as flowering, tuber formation, leaf fall, and bud dormancy. Plants use photoperiodism to synchronize their growth and reproduction with the seasons.
How was photoperiodism discovered?
Garner and Allard (1920) studied the mutant tobacco plant Maryland Mammoth. They found it only flowered when daylength was shorter than a critical period (~14 hours). This demonstrated that daylength, not other factors like temperature, influences flowering.
What are short-day and long-day plants?
Short-day plants flower when daylength is shorter than a critical period (e.g., chrysanthemum). Long-day plants flower when daylength is longer than a critical period (e.g., spinach).
How is photoperiodism ecologically relevant?
Photoperiodism aligns plant growth and reproduction with the seasons, ensuring flowering, seed production, or dormancy occurs under optimal conditions. It is also critical in animals, influencing behaviors like migration and hibernation.
What is phytochrome, and how does it function?
Phytochrome is a light-sensitive pigment that exists in two interconvertible forms: Pr (inactive) absorbs red light (660 nm) and converts to Pfr; Pfr (active) absorbs far-red light (730 nm) and converts to Pr. In sunlight, red light dominates, so phytochrome is mostly in the Pfr form. At night, Pfr slowly decays to Pr, helping plants measure night length.
How did photoperiodism studies implicate red/far-red light in flowering?
Studies showed that interrupting the dark period of short-day plants with red light inhibited flowering, while subsequent exposure to far-red light reversed this effect. This demonstrated the role of phytochrome in regulating flowering based on light signals.
What are circadian rhythms in plants?
Circadian rhythms are internal, ~24-hour biological cycles in plants that regulate processes like leaf movement, gene expression, and photosynthesis. They allow plants to anticipate daily changes, like sunrise.
What are the key properties of circadian rhythms?
- ~24-hour periodicity. 2. Self-sustained in constant light or dark conditions. 3. Resettable by external signals like light. 4. Temperature compensated, maintaining a consistent period across temperature variations.
Why are circadian rhythms important for photoperiodism?
Circadian rhythms allow plants to distinguish between long and short days by providing an internal timekeeper. For example, plants ‘know’ whether dusk coincides with high or low levels of specific proteins, enabling flowering in response to the correct daylength.
What is the external coincidence model?
This model suggests that flowering occurs when: 1. A light-sensitive phase of an internal rhythm (e.g., protein accumulation) coincides with daylight. 2. For long-day plants, light exposure during this phase promotes flowering, while for short-day plants, it inhibits flowering.
What is florigen, and how does it function?
Florigen is the universal mobile signal that triggers flowering. It has been identified as the FT protein, which is expressed in leaves under specific photoperiods and transported through the phloem to the shoot apex to induce flowering.
What evidence supports that florigen is FT?
- FT protein moves across grafts between plants, promoting flowering in non-transgenic plants. 2. Overexpression of FT induces flowering even in day-neutral plants. 3. Mutations in FT-related genes (e.g., in Maryland Mammoth) disrupt photoperiodic flowering.
How is CONSTANS (CO) involved in flowering?
The CO gene promotes flowering in long-day plants. Its mRNA levels peak at dusk in long days, but the CO protein is stabilized only in light, making flowering light-dependent.
How does light effect CO protein?
CO protein is unstable in darkness but stabilized by blue and far-red light. This ensures that flowering is promoted only when CO protein coincides with light at the end of long days.
What happens in clock mutants like LHY overexpressors?
LHY mutants alter circadian rhythms, affecting the plant’s ability to measure daylength accurately. Result: Abnormal flowering times, with late flowering in long days and disrupted circadian processes.
How does manipulating FT expression benefit agriculture?
Manipulating FT expression can shorten flowering times, enabling speed breeding in crops like apple trees, which normally take years to flower. This reduces generation time and accelerates breeding programs.
How do plants perceive photoperiod?
Photoperiod is detected in leaves via light receptors like phytochromes (red/far-red light) and cryptochromes (blue light). Signals like FT protein are then transmitted to the shoot apex to induce flowering.
