Control of Flowering PDF

CONTROL OF FLOWERING Chapter 20 + Chapter 16 Introduction At every stage in the life of a plant, sensitivity to the environment and coordination of responses are evident • All organisms have the ability to receive specific environmental and internal signals and respond in ways that enhance surviv...

CONTROL OF FLOWERING Chapter 20 + Chapter 16 Introduction At every stage in the life of a plant, sensitivity to the environment and coordination of responses are evident • All organisms have the ability to receive specific environmental and internal signals and respond in ways that enhance survival and reproductive success – Like animals, plants have cellular receptors that they use to detect changes in their environment: increase in concentration of a growth hormone, an injury from a caterpillar, or decreases in day length as winter approaches • Plant hormones help coordinate growth, development and responses to environmental stimuli – Internal and external stimuli, and each of these initiates a specific signal transduction pathway SUNLIGHT • Regulates multiple developmental processes and cues for plant growth: • • • • • • • Chloroplast movement Solar tracking Photoblasty? Photomorphogenesis Phototropism Photonasty – nycinasty Photoperiodism • Plants have to protect from UV radiation (by photoreceptors) • Perception of light quality, quality, intensity and duration A light perception system is critical for the meaningful regulation of plant metabolism • Plants are sessile: must deal with environmental limitations to survive • Plants use photoreceptors to detect environmental light changes • They absorb a photon and initiate a photoresponse • All photoreceptors consist of proteins bound to light absorbing pigments i.e. Chromophores (except for UVR8) • Main types: 1. Phytochromes (most strongly red and far-red light, but also blue and UV-A radiation): many aspects of vegetative and reproductive development 2. Cryptochrome (blue and UV-A): seedling development and flowering 3. Phototropin (blue and UV-A): differential growth in a light gradient, phototropism, chloroplast movement, stomatal opening 4. Zeitlupe (blue and UV-A): day length perception and circadian rhythms 5. UV-B receptors**: discovered in Arabidopsis (UVR8) PHYTOCHROMES Phytochromes are photochromic – Absorb red (665nm) and far-red (730nm) light. – Two forms, red-absorbing form (Pr) and far red-absorbing form (Pfr)* Present in all land plants – Predates the appearance of eukaryotes! – Bacteria have something very similar too! Responses: seed germination, control of flowering, etc. • Never 100% converted • Saturating irradiation red light: 88% Pfr • Far-red: 98% Pr, 2% Pfr (not much far red) • Conversion to Pfr is faster than to Pr Photostationary equilibrium Reversability: helps plants adjust to their environment Phytochrome is down regulated after activation Enzymatic destruction Slow conversion in darkness (some plants) Light under canopy is far-red enriched • Leaves in the canopy absorb red light • Shaded plants receive more far-red than red light • In the “shade avoidance” response: phytochrome ratio shifts in favor of Pr • Let’s plants sense shading and strategize how to deal with it Signals (internal or external) à detected by receptors à proteins change shape in response to stimulus – Sensitive to very weak environmental and chemical signals, which are amplified by second messengers (Ca2+) – E.g., just a few seconds of moonlight slow stem elongation in dark-grown oak seedlings BLUE LIGHT RESPONSES • Plants, algae, fungi, prokaryotes • Unicellular org.: mediates phototaxis and infection process • Plants/algae: chloroplast movement, solar tracking, stomatal opening, photoblasty, phototropism, photomorphogenesis • Significant lag times & persistence of the response after the light signal has been switched off • Cryptochromes, Phototropin, Zeitlupe, UV-8 CRYPTOCHROMES • Blue/UV-A photoreceptors • Functions: – – – – – – – Mediates seedling development/flowering responses in plants Suppression hypocotyl elongation (long term**) Cotyledon expansion Membrane depolarization Inhibition petiole elongation Anthocyanin production Circadian clock entrainment • In Arabidopsis, there are two cryptochromes, cry1 and cry2 PHOTOTROPIN • Functions: – Chloroplast movements, stomatal opening, leaf movements, leaf expansion dim light bright light The Effect of Light on the Biological Clock & Circadian rhythms Many plant processes oscillate during the day: many legumes lower their leaves in the evening and raise them in the morning, even when kept under constant light or dark conditions These rhythms are ubiquitous features of eukaryotic life Noon Midnight Circadian rhythms are cycles that are ~24h long and are governed by an internal “clock” • Can be entrained to exactly 24 hours by the day/night cycle • May depend on synthesis of a protein regulated through feedback control and may be common to all eukaryotes • Phytochrome and cryptochrome entrain the clock Period: time between comparable points. Phase: any point recognizable by its relationship to the rest of the cycle. Amplitude: distance between peak and trough CR entrained to 24-h, light-dark cycle. In constant conditions: free-running periods ranging from 21 to 27 h CRs are endogenous but need environmental signal Suspension of CR in continuous bright light and release or restarting after transfer to darkness CIRCADIAN RHYTMS (CR) • When an organism is entrained to a 12 h light and 12 h dark period: the CR responds to the light period of the previous cycle • Light pulses – First hours of night: plant interprets as light from the end of the previous day. Rhythm is delayed – End of night: plant interprets as light from the next day. Advances rhythm Phase-shifting response to light pulse given shortly after transfer to darkness. The rhythm is rephased without changes in period Phytochrome and cryptochromes entrain clock! • In darkness, the phytochrome ratio shifts gradually in favor of the Pr form, in part from synthesis of new Pr molecules and, in some species, by slow biochemical conversion of Pfr to Pr • When the sun rises, the Pfr level suddenly increases by rapid photoconversion of Pr • This sudden increase in Pfr each day at dawn resets the biological clock • Plants sense light through cryptochromes Photoperiodism & Responses to Seasons Photoperiodism à ability to detect day length • Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year • Seasonal events are of critical importance in the life cycles of most plants – Examples: seed germination, flowering, and the onset and breaking of bud dormancy – One of the earliest clues came from a mutant variety of tobacco studied in 1920; didn’t flower in summer but winter WHEN TO FLOWER?? • Floral evocation • Trade-off: delaying vs flowering – Reproductive strategies: annuals vs perennials • Responds to developmental factors (autonomous) and/or environmental cues. Facultative: depends on environmental cues but can occur without them • Photoperiodism and vernalization: 2 of the most important mechanisms – Plants have to find the optimal time to reproduce – Climate change?! Phase change from vegetative to reproductive. Plants have 3 phases: juvenile, adult vegetative, adult reproductive WHAT CAUSES PHASE CHANGES? • Light, carbohydrates, gibberellins, environment • Gibberellin-induced stem elongation: rapid formation of the floral stalk – As the plant switches to reproductive growth, a surge of gibberellins induces internodes to elongate rapidly, which elevates the floral buds that develop at the tips of the stems Main photoperiodic responses Short-day plants (SDP): flower when days are short & nights long. Spring or autumn Long-day plants (LDP): flower when days are long & nights short. Summer Day-neutral plants: photoperiod not a factor in flowering (tobacco). Autonomous regulation The photoperiodic control of flowering ensures that flowers are produced in a favorable season and allows floral synchronization in local populations leading to more efficient cross-pollination Distinguishing spring/autumn? • Some plants need additional information to avoid day-length ambiguity • Couple temperature into the response or detecting if days are shortening (autumn) or lengthening (spring) – – Long-short day plants (LSDP) Short-long day plants (SLDP) Leaves detect photoperiodic signals • Plants can measure the length of periods of darkness to an accuracy of a few minutes • Leaves sense the signals using phytochrome/cryptochrome and send the signals to the rest of the plant • Plants lacking leaves will not flower, even if exposed to the appropriate photoperiod Critical Night Length In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length •SDP have a critical long night. Nighttime has to exceed a particular length before flowering •LDP have a critical short night. Nighttime must be shorter than a critical length before flowering • Night breaks have a profound effect on the onset of flowering for SDP and LDP: – Plants distinguish if the critical night lengths set a maximum (longday plants) or minimum (short-day plants) number of hours of darkness required for flowering • At night: no red light and thus more infra-red (far) light. So phytochrome exists in an inactive PR form during the NIGHT Phytochrome Far-red (Pfr) affects plants differently • Pfr activates flowering in long day plants • Pfr inhibits/ prevents flowering in short day plants • Red light is the most effective color in interrupting the nighttime portion of the photoperiod. • Blue-light (cry): promote flowering too LDP Humans exploit the photoperiodic control of flowering to produce flowers “out of season”. By punctuating each long night with a flash of light, the floriculture industry can induce chrysanthemums, normally a short-day plant that blooms in fall, to delay their blooming until Mother’s Day in May. The plants interpret this as not one long night, but two short nights. 24 hours R R FR R FR R R FR R FR Critical dark period Short-day Long-day (long-night)(short-night) plant plant Vernalization: promoting flowering with cold! • Vernalization: repression of flowering is alleviated by cold treatment – E.g., winter wheat will not flower unless it has been exposed to several weeks of temperatures below 10 ºC • Effective Temp range: from just below 0 ºC to 10 ºC Vernalization: promoting flowering with cold! • Takes place primarily in the shoot apical meristem – Acquisition of the competence of the meristem to undergo the floral transition – Common for plants in high latitudes: lets them “know” sufficient wintertime has gone by – Major groups of angiosperms evolved in warm climates • Plants developed a way to measure winter (bud dormancy also). Likely à independent evolution A Flowering Hormone? • Photoperiod is detected by leaves, which cue buds to develop as flowers • The flowering signal is called florigen • Translocated in the phloem to the apical meristem – Source-sink relations. Leaves close the apex are most likely to cause the induction Flowering has even been induced by grafts between different genera 24 hours 24 hours 24 hours Graft Short-day plant Long-day plant grafted to short-day plant Long-day plant Photoreceptors and circadian clock in the regulation of CO expression