Lecture16 - DB321 Plant Physiology DB321 r. 15, 2011 cture...

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Unformatted text preview: DB321 Plant Physiology DB321 r. 15, 2011 cture 16 nt Flowering/Photoperiodism Floral induction Day-length and flowering How do plants measure How the day-length the (circadian clock/photoperiodism/ the coincidence model) Florigen Floral transition (phase change) (Aukerman and Amasino, 1998) Transition in developmental programs: from vegetative growth to reproduction Autonomous regulation Floral Induction The floral evocation requires the apical bud to pass through 2 developmental stages: 1. acquisition of competence A. developmental age B. vernalization B. vernalization 2. Determination photoperiodism Developmental age Vernalization Developmental age is a determining factor in many plant species Developmental Many plants also requires a cold temperature before they can become competent Many Vernalization: a process required to remove flowering inhibitors process For some plants, flowering won’t occur unless they receive an appropriate For environmental signal (photoperiod). environmental The initial discovery of the importance The of day-length in regulating flowering of day-length (by Garner & Allard, 1920) A mutant tobacco plant, Maryland mammoth, failed to flower in summer under natural condition but was induced to flowering under an artificial light condition with short day-length The classification of plants according to their photoperiodic responses with regard to flowering with The terms “short-days” & “long-days” are relative The relative Long-day plants Long-day flower when the dayflower length exceeds (or the night length is less than) a certain critical duration in a 24 hour cycle Short-day plants flower when the day length is less than (or the night length exceeds) a certain critical duration in a 24-hour cycle 3. Day-neutral plants (DNPs) flower under any photoperiodic condition Their flowering is regulated by internal developmental processes Their Day-length alone is an ambiguous signal because it can not distinguish between spring and fall because Two common mechanisms for avoiding such an ambiguity of day-length 1. The coupling of day-length with temperature (cold requirement) 2. Measuring day-length shortening or lengthening 2. shortening A. long-short-day plants long-short-day flower only after a sequence of long days followed by short-days flower (late summer or fall) (late B. short-long-day plants short-long-day flower only after a sequence of short days followed by long days flower (they flower in the early spring) Plants monitor day-length by actually measuring the length of the night night “night-break” experiments Short-day plants Short-day Require long-night Require Long-day plants actually Need short night for flowering Phytochrome is the primary photoreceptor in photoperiodism Phytochrome How to measure the duration of the night (timing mechanism) How 1. An hourglass (or cumulative) timer: Plants measure the Pfr/P ratio 1. Plants 2. Oscillating or rhythmic timer 2. These measure time intervals between regular oscillations, such as the sweeps of a pendulum The time when a night break is given determine the flowering process The The SDP soybean received cycles of an 8-hour light period followed by a 64-hour dark period The A 4-hour night break was given at various times during the long inductive dark period If this SDP were simply measure the length of night by the accumulation of a particular substance in the dark, any dark period greater than the critical night-length should cause flowering Long dark periods are not inductive for flowering if the light break is given at a time (i.e.40h, 64 h) that does Long not properly coincide with a certain phase of the endogenous circadian oscillator not Flowering in SDPs requires both a dark period of sufficient duration and Flowering dark a dawn signal at an appropriate time in the circadian cycle In 1936 Erwin Bunning proposed that the control of flowering by photoperiodism Erwin is achieved by interaction of light/dark with an endogenous circadian oscillator The leaf is the site of perception of the photoperiodic signal The . Julius Sachs (1865): darkened parts of plants to conclude that leaves in the light produces flowering-forming substances . Knott (1934): exposing only leaves but not the shoot apex . Chailakhyan(1936) formalized the FLORIGEN hypothesis a substance generated in leaves under the inductive photoperiods and translocated from leaves to the shoot apex. . Naylor (1941): a single leaf is sufficient to induce flowering . King (1968): phloem is the conduit for florigen King phloem . Grafting experiments: induced leaves onto non-induced plants A. Melchers (1937) showed that leaves of both SD and LD strains of A. Hyoscyamus produce a signal that can cause a biennial Hyoscyamus strain produce to flower in the first year. to B. Zeevaart’s (1958) experiments B. Florigen? A substance made in leaves which induces the shoot to flower, and it is “graft-transmissible” Only problem is “no one could ever isolate or purify such a substance (and many tried!)”. “Florigen” remained a hypothesis for decades until…… Genetics came to the rescue Arabidopsis: Long Day Plant Flowering is induced by Long Day (LD) Certain late flowering mutants are blind to photoperiod i.e) constans (co), flowering locus t (ft) CO: transcriptional co-activator FT: transcriptional co-activator (?) Inductive Photoperiod CO FT (& other floral integrators) Floral Transition The rice homolog of CO, Hd1 (Heading date 1), functions as a suppressor functions for the rice homolog of for FT, Hd3a. Export of FT Protein from Phloem Companion Cells Is Sufficient for Floral Induction in Arabidopsis SAM-specific prom CC-specific prom FT:TEV:3xYFP:NLS 1. When driven by an SAM-specific promoter, the FT:TEV:3xYFP:NLS can promote flowering When driven by a companion-cell specific promoter, this fusion protein is incapable of promoting flowering. 2. Strong YFP signal was detected in companion cells in transgenic plants expression CCpro:FT:TEV:3xYFP:NLS but no YFP signal was detected in SAM indicating that neither the mRNA nor protein of the FT:TEV: 3xYFP:NLS is mobile. The CCpro:FT:TEV:3YFP:NLS was crossed into an Arabidopsis plant making TEV protease in the companion cells, the released FT protein can promote flowering. ...
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