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Study Questions

Compare and contrast the fundamental differences between the use of light as a developmental signal versus light-harvesting during photosynthesis.
What early observations about lettuce seed germination led to the classical model of phytochrome action?
Diagram the two absorption spectra of the two forms of phytochrome. Where are the two peaks?
Is the photoconversion of phytochrome from one form to another ever 100 percent? Explain your answer.
Which form of phytochrome is the physiologically active form?
Define VLFR, LFR, and HIR phytochrome responses. How do they differ with respect to photoreversibility, reciprocity, and time-dependence?
Despite their lack of photoreversibility, the property that led to phytochrome’s discovery, both VLFR and HIR responses are known to involve phytochrome. What is the evidence?
Describe the overall structure and substructure of phytochrome. Where in the cell is phytochrome synthesized and what is the molecular basis of its photoreversibility?
Discuss the important functional domains of phytochrome. How do these domains regulate phytochrome responses?
Compare and contrast plant and bacterial phytochrome, and discuss the implications for phytochrome evolution.
What are the characteristics of “Type I” and “Type II” phytochromes? Identify which members of the Arabidopsis multigene family fall into each of these categories.
Compare and contrast Phytochrome A versus Phytochrome B with respect to their responses.
Give an example of a rapid phytochrome response. How does it differ from slower, long-term responses?
Differentiate between the concepts of primary versus secondary response genes during phytochrome signaling.
Pr, the physiologically active form of phytochrome, can exist in either a stable or labile form. Describe the roles of protein kinases, protein phosphatases, and the 26S proteasome in regulating this transition.
Discuss the roles of COP1 and the 26S proteasome during phytochrome-induced gene expression.
What is meant by a “circadian rhythm”? Define the following parameters of circadian rhythms: “period,” “temperature compensation,” “entrainment,” and “gating.”
What is a “circadian oscillator” and how is it regulated by phytochrome?
How do “sun plants” differ from “shade plants”?
What is the “shade avoidance response,” and how is it regulated by phytochrome?
Phytochrome responses are typically integrated with hormone responses. How is this integration achieved at the cellular level?
Give an example of how alterations in the phytochrome response can improve crop yields.