Problem Sets Flashcards
What type of curve (function) best describes plant growth in theory? Do plants fit this model in practice? Why or why not?
Plant growth is theorized to operate like a population following an exponential curve. Plants in real life do not fit this model because relative growth rates (the exponent) slows with plant age and size. Plants also have parts that are not metabolically active and shift their investment in those organs over time
Plant growth is sometimes considered in absolute terms and sometime in relative terms. What is an advantage and a disadvantage of each? Provide examples to help support your argument.
Absolute growth rate has the advantage of predicting absolute yield in forest or agricultural systems and can predict terminal size of a plant. Relative growth rates allow us to extract the causes of rapid changes in growth rate (otherwise these factors are hidden by major effect of size), can be broken down into structural components, and allows for standardized comparisons of plants across species or experimental treatments.
Explain how plants regulate directional cell-expansion. What experiments revealed this?
The direction of cell expansion is controlled by the relative extensibility of cell walls. This is most commonly regulated by auxins via acid growth. Auxin stimulates expression of an H+-ATPase, which increases the cell wall extensibility by stimulating expansins, proteins in the cell walls that loosen cell walls under acidic pH by weakening the hydrogen bonds between polysaccharides in the wall. At the cellular scale, H+-ATPases can preferentially act on radial or axial cell walls causing cells to expand in the opposite plane. At the whole plant level auxins can be differentially distributed to one side of a stem (e.g., phototropism), or roots (e.g., gravitropism) stimulating or inhibiting cell expansion on that side resulting in bending. These phenomenon were first described by Charles and Francis Darwin, in 1880. Then Boysen and Jensen determined this was an asymmetric response and the substance was water soluble in 1913 and Went discovered the substance could be extracted.
Auxin regulates numerous physiological events. Is it the absolute amount of auxin or the amount of auxin relative to other hormones that determines whether a physiological response is induced? Site an example from your text or notes.
Both the absolute and relative amounts of auxin regulate responses. Most processes are sensitive to auxin concentration, where auxin in low concentrations will stimulate cell growth overall, but will inhibit growth at high concentrations in coleoptiles that cannot synthesize their own auxin. Relative concentrations of auxin drive tropism responses, where auxin is preferentially transduced to one side of the plant causing bending towards light or gravity. Auxin is also dependent on the relative concentrations of other hormones, e.g., cytokinins have an antagonistic relationship with auxin and their relative concentrations control the activation of axillary buds.
How is cell division regulated? What plant hormones induce cell division?
Cell division is induced by cytokinins, gibberellins, and Brassinosteroids, but in the majority of cell divisions occurs within meristems where it is regulated by a the relative concentrations of cytokinins and auxins.
Explain how gravity is detected in roots.
Plants detect gravity by the activity of dense amyloplasts (called statoliths in this role), which detect gravity by falling through the cytoplasm when roots are place horizontally. This triggers a stimulus to redistribute auxin either by interacting with the cytoskeleton or be exerting pressure on the endoplasmic reticulum at the bottom of the cell. The gravity sensing cells cause redistribution of auxin,
What substance initiates root gravitropism in response to the signal generated in question 2. How does this physiological mechanism differ from the mechanism driving plant phototropism?
Gravitropism involves lateral redistribution of auxin. Placing a plant on its side causes a rapid differentiation in the expression of auxin-stimulated mRNAs (small auxin up-regulated RNAs = SAURs) driving the redistribution of auxin to the lower side. Unlike in shoots, where increased IAA on the shady side of the plant, stimulates rapid cell expansion on that side, IAA is inhibitory in roots (decreases growth). The lower IAA concentration in the upper side of the horizontal root causes rapid growth and bending downward.
describe GA signaling
Gibberellins (aka GAs) include at least 136 naturally occurring compounds, with new ones still being discovered, most are precursors of active GAs. These are tetracyclic skeleton with 19 or 20 carbons that are synthesized throughout the life cycle. GAs operate in a signaling cascade either in response to an exogenic signal (dark drives etiolation, circadian rhythm drives flowering) or endogenous signal (e.g., transition from juvenile to adult)
describe GA physiological response
GAs drive etiolation and stem elongation in rosette and dwarf plants and internode elongation in grasses. They also control transition from juvenile to adult phase, pollen development, seed set, parthenocarpy, seed development and germination
describe GA discovery
Japanese rice farms know of the actions of Gibberellins for a long time. Fungal disease caused rice plants to grow too tall and not make seed. Pathologist isolated that function (Gibberella fujikuroi) and extracted a substance the promoted growth. In the 1950s scientists purified compounds from the fungal culture and named it gibberellic acid. Many other GAs were also discovered, but GA3 was the most prevalent and active. They caused elongation responses, especially in dwarf varieties, and in rosette plants (“bolting”). Bioassays of extracts of a variety of plant species showed the GA-like substances were present endogenously. Higher concentrations were found in immature seeds than mature plant parts
How is cell division regulated? What plant hormones induce cell division?
Cell division is induced by cytokinins, gibberellins, and Brassinosteroids, but in the majority of cell divisions occurs within meristems where it is regulated by the relative concentrations of cytokinins and auxins.
What is phytochrome? What are its ecological functions?
Phytochrome is a protein pigment that absorbs red and far-red light most strongly, but also blue and UVA light. Phytochrome is responsible for photoreversible photomorphogenesis, or broadly developmental changes in response to light. These responses include germination, de-etiolation, unrolling of leaves, formation of leaf primordia, inhibition of stem elongation, inhibition of flowering, enhancement of chlorophyll accumulation, promotion of growth, replication of plastids, organization of chloroplasts, and nyctinastic leaf movements.
What are ABC genes? Where is each located in a normally functioning plant? What structures do they control?
ABC genes are floral identity genes. Their expression controls the development of petals, sepals, carpels, and stamens in floral meristems. They are spatially limited and are expressed in different concentric whorls in the developing floral bud. Type A activity controls organ identity in the first and second whorls, B activity controls the second and third whorls, and C controls the third and fourth whorls. Each whorl is determined by a unique combination of activity by the three organ identity genes. Type A alone specifies sepals, A+B specifies petals, B+C specifies stamens, and type C alone specifies carpels.
How do plants know when to flower? What endogenous signal(s) are used to induce flowering?
Some specific hormones promote flowering in some plants, but the more general floral stimulus is florigen. Florigen is transduced in response to phytochrome detection of light patterns (circadian rhythms and photoperiod) or in response to temperature (vernalization and growing degree days).
