Kerry Franklin Flashcards
dark-grown phenotype features
Plants undergo scotomorphogenesis and display an etiolated phenotype. Seedlings are elongated with, with closed unexpanded cotyledons in an apical hook. As chlorophyll synthesis requires light, these seedlings are also yellow in colour.
light-grown phenotype
Plants display photomorphogenesis. Hypocotyl elongation is inhibited, cotyledons open and expand and chlorophyll is synthesised.
how are light signals perceived and transduced?
By photoreceptors, which include the red/far-red light-absorbing phytochromes and the blue light absorbing cryptochromes and phototropins
From what are Phytochromes made of?
Phytochromes consist of 2 parts- a protein and a light-absorbing molecule called the chromophore. The chromophore is made in the chloroplast from the same pathway as chlorophyll. The protein component (termed apophytochrome) combines with the chromophore in the cytosol to make functional phytochrome. Apophytochrome+chromophore = holophytochrome.
What are cryptochromes?
hy4 was deficient in a blue light photoreceptor, cryptochrome 1. Unlike phytochromes, cryptochromes have 2 photoreceptors, a pterin (heterocyclic compound) and flavin adenine dinucleotide (FADH). The N-terminal domain of the cry1 protein resembles a DNA repair enzyme- photolyase
photomorphogenesis - definition
The regulation of plant growth and development by light
respond to the direction - term
heliotropism
phenotype in monocots
hypocotyl = mesocotyl, apical hook = coleoptile
types of photoreceptors
phutochromes = red & far red,
cryptochromes and phototropins = blue
UVR8 = UV
phytochrome syntesis
Phytochrome is synthesised in a biologically inactive Pr form. Following the absorption of red light, the chromophore and protein both change conformation. This changes phytochrome to a biologically active Pfr form. The Pr and Pfr forms of phytochrome are interconvertible. Pfr is converted back to the Pr form by far-red light. So…. If the amount of red and far-red light in the environment changes, the amount of phytochrome in the active Pfr form changes.
Note: All wavelengths of light can drive both reactions. It is just that Pr to Pfr is OPTIMAL in red light and Pfr to Pr is OPTIMAL in far-red light.
What do phytocromes do?
- promote germination
- seeding de-etiolation - inhibit stem elongation and promote leaf development
- plant architecture
- reproductive development
Classic photoresponses of Arabidopsis seedling
- dark - etiolated form, long hypocotyl with an epical hook, cotyledons haven’t opened or expanded.
- red or blue - much shorter hypocotyls, because photomorphogenesis has been triggered in red by phytochromes and in blue by cryptochromes.
- far red - also show photomorphogenesis (mediated by phytochrome A) - short hypocotyls, opena dn expanded cotyledons but they are bleached. Because photosynthesis requires light, plants cannot produce chlorophyll in dark.
phy A as a signal for photomorphognesis.
Seedlings grown in the dark are packed with phyA /because pfr has accumulated at slower rate so it is not degraded as fast/. When reaching light /i.e. out of the soil/ phyA acts as an antenna - sihgnaling (cycling b/n pr and pfr). phyB kicks in and together trigger photomorphogenesis. In FR rich envi, phyA inhibits elongation, so phy A is active.
shade tolerance phenotype
produce thinner leaves with higher chlorophyll content, increasing photosynthesis efficiency
How shade avoidance work?
Sunlight has a red to far-red light ratio greater than 1. This pushes phytochrome to the active Pfr form. This keeps stems short, enabling plants to direct resources towards leaf development for photosynthesis. Plants reflect wavebands which are photosynthetically useless (eg. Green and Far-red). If a plant is growing in close proximity to other plants, it perceives both reduced red light and an enrichment of far-red light, lowering the R:FR ratio. Low R:FR shifts phytochrome back to the inactive Pr form, relieving inhibition of elongation growth. Plants therefore elongate to overtop neighbours.
Auxin is the major plant growth hormone. Low R:FR triggers auxin (Indole-Acetic-Acid: IAA) synthesis in the leaves. This is then pumped down the plant to drive shade avoidance.
