Raven Biology of Plants PDF

11/21/24 Ray F. Evert Susan E. Eichhorn Raven Biology of Plants Eighth Edition CHAPTER 28 External Factors and Plant Growth © 2013 W. H. Freeman and Company 1 Tropisms: Movement of plant part in r...

11/21/24 Ray F. Evert Susan E. Eichhorn Raven Biology of Plants Eighth Edition CHAPTER 28 External Factors and Plant Growth © 2013 W. H. Freeman and Company 1 Tropisms: Movement of plant part in response to external stimulus – e.g gravitropism, phototropism, hydrotropism, thigmotropism Positive – towards the stimulus Negative – away from the stimulus https://plantsinmotion.bio.indiana.ed u/plantmotion/movements/tropism/tr opisms.html Cool corn! 2 1  11/21/24 Gravitropism – the response of plants to gravity shoots – display negative gravitropism roots – display positive gravitropism 3 In roots, change in relative direction of gravity results in increased auxin response on the side towards gravity Results in decreased cell elongation on the lower side of the root and curving of the root towards gravity Yan et al., 2013. PLOS Mahonen et al., 2014. Nature 515, 125–129 4 2  11/21/24 Friml, 2003 PIN1 is apically positioned in stele cells, PIN4 basally positioned in quiescent centre, PIN3 laterally positioned in columella, and PIN2 basally positioned in lateral root cap, cortex and epidermis 5 PIN3 is important in root and shoot gravitropism pin3 mutants show reduced response to gravity in both root and hypocotyl Friml et al., 2002. Nature 415: 806-809 6 3  11/21/24 PIN3 is expressed in the starch sheath of stem and the root columella – gravity sensing cells PIN3 protein by immunolocalization: a) hypocotyl starch sheath b) Stem starch sheath c) Stem starch sheath d) Root pericycle inner e) Columella in wt inner and pin3 mutant (inset) Friml et al., 2002. Nature 415: 806-809 7 Does the PIN3 protein relocalize in response to gravity? h-j) relocalization of PIN3 after change in gravity ( ), 2 min, 10 min, 1 hr a-b) PIN3 in different cell compartments (a) PM (b)vesicles c-d) pericycle e-g) following inhibition of e) vesicles (BFA) f) protein synthesis (cycloheximide) g) actin polymerization (latrunculin) Friml et al., 2002. Nature 415: 806-809 8 4  11/21/24 Model: response to gravity causes targeting of vesicles containing PIN3 to new basal membrane of the columella cells results in increased auxin efflux on side towards gravity and inhibition of cell elongation (root); promotion of cell elongation in shoot PIN3 9 How is gravity sensed? Statocytes in the columella of the Arabidopsis root cap contain amyloplasts (=statoliths) 10 5  11/21/24 Statoliths sediment to the bottom of the statocyte Change in gravity relative to cell causes statoliths to reposition Innermost layer of shoot cortex = starch sheath (or shoot endodermis) – contains amyloplasts that sediment in response to gravity 11 Starch biosynthetic mutants (pgm-1) show reduced response to gravity Fail to shift auxin maximum (DR5:GFP) toward lateral root cap Wolverton et al., 2011. Physiologia Plantarum 141: 373–382. 12 6  11/21/24 Distribution of auxin depends on actin-dependent localization of vesicles carrying auxin efflux carriers gravity gravity Sedimentation of amyloplasts causes shift in actin filaments targeting vesicles towards basal side Estelle, 2001. Nature 413, 374-375 13 Model: sedimentation of statoliths in response to gravity causes shift in actin cytoskeleton alters vesicle trafficking directs PIN3 to basal membrane more auxin on basal side causes inhibition of cell elongation (root); promotion of cell elongation in shoot 14 7  11/21/24 Evolution of “fast” gravitropic response occurred in seed plants Zhang et al., 2019 s://doi.org/10.1038/s41467-019- 11471 15 Statoliths first appear in the root cap of seed plants Moss (Physcomitrella ) has no statoliths Fern (Ceratopteris) has statoliths Lycophyte in root cap and (Selaginella) above the has meristem statoliths above the meristem Zhang et al., 2019 s://doi.org/10.1038/s41467-019- 11471 16 8  11/21/24 PIN amplification, statoliths in root cap all specific to seed plants associated with evolution of fast gravitropic response in seed plants; gravitropism is more ancient – exists in Charophytes (rhizoid and protonema) Statoliths in root cap Original PIN proteins PINs (PIN2,PIN3) specialized for gravitropism Zhang et al., 2019 s://doi.org/10.1038/s41467-019- 11471 17 Thigmotropism Differential growth of plants to touch - asymmetric growth in shoots involves changes to Ca2+, but molecular mechanism unclear Roots grow around obstacle (e.g. compacted soil, rock), then resume positive gravitropism 18 9  11/21/24 Roots rapidly respond to an obstacle (scalpel blade) encountered by the root tip Lee et al., 2019 New Phytologist, Volume: 225, Issue: 3, Pages: 1285-1296, First published: 23 July 2019, DOI: (10.1111/nph.16076) 19 Root bending is correlated with asymmetric auxin response Asymmetric auxin response and root bending is eliminated in eir1 (pin2) mutant Lee et al., 2019 New Phytologist, Volume: 225, Issue: 3, Pages: 1285-1296, First published: 23 July 2019, DOI: (10.1111/nph.16076) 20 10  11/21/24 Root gravitropism evolved early; thigmotropism unknown Zhang, Y., Friml, J., 2020. Auxin guides roots to avoid obstacles during gravitropic growth. New Phytologist 225, 1049–1052. https://doi.org/10.1111/nph.16203 21 Plants show multiple responses to light of different wavelengths via phytochromes (PHY – red/far-red), cryptochromes (CRY-blue) and phototropins (PHOT - blue) Red/far-red/blue blue 22 11  11/21/24 Photoreceptors: proteins which have a photosensory domain (through interaction with chromophores - light absorbing pigments) and an “output” domain whose activity depends on light absorption Chromophore binding sites Phytochromes Red, far-red light 600-750 nm Phototropins blue, UVA light 320-500 nm Output domains Cryptochromes blue, UVA light 320-500 nm Sullivan and Deng, 2003 23 Phototropism Darwin’s The Power of Shoots display positive phototropism Movement of Plants (movement towards light) (1880) Charles Darwin and his son Francis performed experiments on canary grass and oat coleoptiles Coleoptile bent towards light unless the tip was covered Suggested that a “mysterious substance Tip Elongation moves” from the tip to covered zone – no covered – the lower part, causing bending bending the coleoptile to bend 24 12  11/21/24 What “mysterious substance” induces elongation? Frits Went 1926: 1) removed tips from oat coleoptiles and placed them on agar for 1 hour 2) Put the agar onto each side of the decapitated coleoptile 3) Seedlings bent away from the side with the agar block 4) Concluded that a chemical accumulated on side away from light, causing coleoptile to bend towards light 5) Called the substance he isolated “auxin”: from Greek auxein "to increase" Increased elongation 25 Briggs: a) +b) same amount of auxin from both illuminated and un-illuminated maize seedlings c) +d) if seedling is split in half with a piece of glass, same amount on both sides e) + f) if seedling is split with glass but not to tip, dark side accumulates more IAA Suggests that auxin moves from illuminated to un- illuminated side 26 13  11/21/24 Cholodny-Went hypothesis (Went and Thimann, 1937): Phototropism: auxin is re- distributed from the lit flank to the shaded flank leading to auxin-stimulated cell elongation in the shaded side and thus curvature toward the light source Plants with DR5:GFP reporter gene show higher GFP on un- illuminated side = higher auxin response on shaded side Haga and Sakai, 2012. Plant and Cell Physiology 27 How is light perceived? Blue light is most effective in promoting phototropism Blue light uniquely induces coleoptile bending © Pearson 28 14  11/21/24 How is light perceived? Wild type seedlings (Col) move towards unilateral blue Blue light light source Non-phototropic hypocotyl (nph1) mutants allowed identification of photoreceptor responsible for phototropism Liscum and Briggs, Plant Cell 1995 29 Blue light source similar gene = NPH1-LIKE1 Double mutants lack all blue-light induced phototropic responses Renamed PHOTOTROPIN1 and PHOTOTROPIN2 (double mutant – p1p2) 30 15  11/21/24 phototropins = N-terminal photosensory input region C-terminal output region; classic serine/threonine kinase motif N-terminal region = two LOV domains (sensing Light, Oxygen, or Voltage; also in bacteria, algae and fungi), each bind to the vitamin-B derived cofactor flavin mononucleotide (FMN) = blue light-absorbing http://www.photobiology.info/Christie.html chromophore 31 In dark, LOV domain is FMN non-covalently associated with the FMN active chromophore; LOV inactive domains associate with C-terminal kinase domain inhibiting its activity blue light causes a covalent bond to form between C of FMN and sulfur of cysteine in LOV displaces α-helix and activates kinase Autophosphorylation results in active protein http://www.photobiology.info/Christie.html 32 16  11/21/24 Blue light, perceived by PHOT photoreceptors, induces gradient of auxin which drives differential cell elongation Emmanuel Liscum et al. Plant Cell 2014;26:38-55 ©2014 by American Society of Plant Biologists 33 How does blue light activation of PHOT1/PHOT2 result in increased auxin on shaded side? pin3 mutants do not respond to unilateral light as well as wild type Friml et al., 2002. Nature 415: 806-809 34 17  11/21/24 DR5::GFP is asymmetrically expressed on shaded side of wild type (a and d), but is symmetric in phot1 (b and d) and is less asymmetric in pin3 (c and d) light Ding et al., 2011. Nature Cell Biology. 13: 447-453. 35 PIN3:GFP is relocalized asymmetrically in wild type but not phot1 mutants Unillateral illumination of hypocotyls (arrow indicates direction from left) is followed by progressive PIN3:GFP relocalization to inner side of wild type cortex cells (starch sheath, endodermis), but not phot1 – suggests PHOT1 required for PIN3 relocalization Ding et al., 2011. Nature Cell Biology. 13: 447-453. 36 18  11/21/24 Blue light induces relocation of PIN auxin transporters In absence Blue light PIN3 of PIN3 activates Auxin directional PIN3 kinase activity efflux light PIN3 of PHOT vesicles are PHOT Phosphorylated trafficked Auxin PIN3 is equally to efflux trafficked to all sides shaded side Auxin efflux Auxin efflux equivalent, results in elongation increased equal elongation of cells on shaded side Ding et al., 2011. Nature Cell Biology. 13: 447-453. 37 Control of tropisms by PIN-FORMED mediated auxin distribution present in nonvascular terrestrial plants Marchantia polymorpha gametangiophore stalk grows towards a block of auxin media Marchantia polymorpha has a single PIN-FORMED gene (MpPIN1) Localized to basal membrane of gametangiophore stalks Generated mutants mppin1 Complemented by proMpPIN1:MpPIN1- Citrine fusion protein Archegonial and antheridia stalks of mppin1 lack gravi- and Fisher et al., 2023. New Phytologist, DOI: (10.1111/nph.18854) phototropism 38 19  11/21/24 Circadian rhythms – phenomena that occur in an approximately daily rhythm – control many plant responses to light e.g. leaf folding and unfolding, photosynthetic activity, seed germination, stomatal opening, flower opening… First recorded in 4th century BCE by Androsthenes – Greek soldier, observing the tamarind tree http://science.sciencemag.org/content/suppl/201 6/08/03/353.6299.587.DC1?_ga=2.48569381.20 84906094.1511903659- 1871957354.1414425433 39 Studied by de Mairan in Mimosa pudica in 1729 – Leaves are perpendicular to stem during day and drawn upward at night leaf movements persisted in cellar Persist even in absence of light cues Persist in absence of all environmental cues Endogenous timing mechanism = circadian clock 40 20  11/21/24 Circadian clock: 3 components 1.Oscillator – generates rhythmic behaviour Under constant conditions, circadian rhythm is free-running with an intrinsic period (22-29 hours) that does not have to be reset at each cycle 2.Input pathways – carry synchronizing environmental information synchronizing agent = Zeitgeber (time giver) – keeps circadian rhythm in time with the 24 hour day/night cycle (entrainment) Light/dark cycles, temperature cycles 3.Output pathways – regulate physiological, developmental and biochemical pathways – 30-40% of Arabidopsis genes oscillate (25% estimated in Drosophila) 41 Chlorophyll a/b binding protein 2 (CAB2) part of PSII - LHCIIb light harvesting complex rhythmically expressed – comes on at dawn Ligated the CAB2 promoter to luciferase, used these plants as a way to screen for clock mutants – found toc mutant (timing of CAB::LUC) https://www.youtube.com/watch?v=hOlO1C u6E9I 42 21  11/21/24 Central oscillator in Arabidopsis: Three major genes: TOC1 (TIMING OF CAB EXPRESSION 1) LHY (LATE ELONGATED HYPOCOTYL) CCA1 (CIRCADIAN CLOCK LHY/CCA Decay ASSOCIATED 1 all encode transcription factors TOC1 +ve regulates LHY and CCA, LHY and CCA negatively regulate TOC Stability of LHY/CCA negative regulation determines timing of oscillation 43 A similar (non-homologous) mechanism acts in Drosophila and Mouse activation of negative regulator defines oscillatory behaviour Most organisms have multiple interconnected loops Doherty and Kay, Annu Rev Genet. 2010; 44: 419–444. 44 22  11/21/24 Circadian clock: Controls daily events Allows organism to detect seasonal change by measuring changing day length Allows synchronization of flowering, dormancy, tuber formation etc. with seasons Oravek et al. 2022 Plant Physiol, https://doi.org/10.1093/plphys/kiac337 45 Effect of daylength on flowering time 1920s- Varieties of soybean and tobacco that would not flower unless the day length was shorter than a particular number of hours - photoperiodism Threshold for flowering < 16hr < 14 hr < 12 hr 46 23  11/21/24 Short-day plants – flower when light period is shorter than a critical length (flower in spring or fall – e.g. Chrysanthemums, poinsettias, strawberries) Long-day plants – flower when light period is longer than a critical length (flower in summer – e.g. some potatoes, some wheat, spinach, lettuce) Day-neutral – flower without respect to daylength (cucumber, sunflower, rice, pea, maize) 47 Chrysanthemum – a short day plant Short day Long day 48 24  11/21/24 Spinach – a long day plant Short day Long day 49 Plants show natural variation in photoperiodic response along a latitudinal cline Collected different soybean “landraces”(wild or domesticated) from different regions of China Grew in “common garden” experiment – same LD conditions, near Beijing Under LD conditions, flowering time was inversely correlated with latitude Related to level of cryptochrome latitude Zhang et al., PNAS Dec2008 105: 21028-21033; https://doi.org/10.1073/pnas.0810585105 50 25  11/21/24 Hammer and Bonner 1938 If expose cocklebur to a short day cycle (8 hours light, 16 hour night) will flower, whereas henbane (LD) flowers under 16 hours light, 8 hours dark If interrupt night with short light flash, cocklebur will not flower, henbane will Interruption of day with dark period has no effect Suggests that plant detects night length 51 Previously shown that lettuce seeds require light to germinate, that red light is most effective and that it is reversed by far-red light Red light Red light Red light Red light Far-red light Far-red light Far-red light Red light Red light Far-red light 52 26  11/21/24 Similar effect in flowering: Interrupting night with light of different wavelengths indicated that red light was most effective. The red light flash could be “undone” by a flash of far-red light. =LD =SD =LD =SD =LD =SD http://plantphys.info/plant_physiology/photoperiodism.shtml 53 Phytochrome Photoreceptor required for effect in lettuce seeds and flowering was called phytochrome Pr form – absorbs red light Pfr form – absorbs far red light Absorbance spectrum for phytochrome: (UVB light = 280 nm) (red light = 660 nm) (far red light = 730 nm) 54 27  11/21/24 Isomeric forms of phytochrome interconvert: when Pr (inactive, newly synthesized form) absorbs light (red 660 nm), it is converted into Pfr (active form) When Pfr absorbs light (far-red 730 nm), it is converted into Pr Pfr also reverts to Pr independent of light = dark reversion – provides a “night measurement” function (activates) (deactivates) 55 http://plantphys.info/pl ant_physiology/photop eriodism.shtml Like phototropins, plant pigment phytochrome has a protein component and a chromophore component (tetrapyrrole (bilin) derived from heme) Light absorption causes chromophore to convert between Pr form (cis isomer – inactive, absorbs red light) and Pfr form (trans isomer, active, absorbs far-red light) Protein encoded by a family of genes (PHYA to PHYE in Arabidopsis) that have different properties (transcription, stability) 56 28  11/21/24 active form Inactive form kinase Nuclear localization Trans-isomer Cis-isomer Bae and Choi. 2008. Annu. Rev. Plant Biol. 59:281–311 Photoisomerization changes structure so that domains for nuclear localization and kinase activity are accessible autophosphorylation enters nucleus and activates transcription through interaction with PIF3 (Phytochrome Interacting Factor), a MYB transcription factor 57 phytochromes control photomorphogenesis (de-etioloation), shade avoidance, flowering, as well as providing “Zeitgeber” function Red/far-red/blue blue 58 29  11/21/24 Photomorphogenesis etiolated Phytochome is also important Light grown for seedling photomorphogenesis: unbending of apical hook reduced stem elongation leaf opening and expansion Chloroplast formation - greening 59 Phytochrome is important for shade avoidance low red/far- low red/far- Light transmitted red red through plants is depleted in red (chlorophyll) and blue (carotenoid) wavelengths and enriched in far-red light low red/far- low red/far- Detection of low red red red/far-red initiates shade avoidance – stem elongation, vertical leaves, accelerated flowering 60 30  11/21/24 Phytochrome (and cryptochrome) act as Zeitgeber – increase CCA1 and LHY activity at dawn 61 Importance of phytochrome to controlling flowering time How do plants know if days (nights) are long or short? External coincidence model – detection of external signal (photoperiod) relative to internal rhythm Sensitivity to night breaks varies on a 24 hour period, suggesting that the internal rhythm may be the circadian clock 24 hours – internal Maximum sensitivity to night breaks Lagercrantz, J. Exp Bot 2009 62 31  11/21/24 What are the cellular events that are coinciding? Mutants unresponsive to photoperiod – flower at a constant time: Under LD, late flowering mutants (e.g. co, ft) flower late compared to wild type (as if they were in short days) Under SD, early flowering mutants flower as if under LD Mutant e.g. co Wild type Early Wild type constans flowering mutant Long days (LD) short days (SD) 63 CO transcription is regulated in a circadian fashion – light input (via phytochrome = zeitgeber) causes CO mRNA to peak earlier in LD than in SD Peaks Peaks Fits in light in dark “coincidence” model Under LD, transcription of CO mRNA is high at dawn, decreases at midday, rises again to peak at end of light period Under SD, transcription of CO mRNA is low throughout the light period, peaks during dark period high CO mRNA is present during light period under LD but not SD Hypothesis: LD flowering occurs through coincidence of circadian clock regulated CO transcription with light-mediated CO post-transcriptional regulation (e.g. translation) 64 32  11/21/24 light (external signal) affects CO protein stability Wt, peak at 16 hr cry1cry2, phyA mutant peak at 16 hr is reduced CO protein level Histone 3A protein level dawn phyB mutant, no reduction Express CO from 35S promoter (why?) Look at protein amounts by antibody to CO on western blot In wild type, see that CO is high in morning (0.5hr), has another peak at 16hr In a phyB mutant, CO is not degraded in the morning - PHYB required for degradation In cry1cry2 or phyA mutants, peak of CO in evening is not as high - required for stabilization Valverde et al., 2004 65 CO induces FT Circadian input via phytochrome Stabilization of protein via CRY and PHY light affects protein levels of CONSTANS in two ways: 1) transcription: the circadian output (synchronized by PHY) increases CO transcription – peaks at end of day under LD 2) Protein stabiltiy: CRY and PHYA stabilizes the CO protein – results in a high level of CO in late afternoon Under LD, increased transcription and protein stabilization coincide sufficiently to induce flowering; under SD, they do not 66 33  11/21/24 The flowering stimulus is produced in leaves and is graft transmissible, between species 1) Leaves are required for induction of flowering (Chailakhyan, 1920s) 2) grafting a single leaf from a SD treated Perilla plant (environmentally induced) to a non-SD treated plant (uninduced) can induce flowering Graft transmissible, phloem transported substance called florigen 67 Both CO and FT are expressed mainly in the leaf Expression of CO induces FT in leaf FT moves to SAM FT might be florigen 68 34  11/21/24 FT in phloem or in SAM sufficient to rescue ft FT driven by 2 different promoters: SUC2 = phloem companion cells ft = late flowering, KNAT1 - SAM many leaves FT driven by either of these is able to wild type = normal rescue late flowering, few leaves flowering phenotype ft transformed by FT in phloem or SAM = normal flowering (increased leaf number) of ft-7 mutant Corbesier et al., 2007 69 FT from one species can induce flowering in another: E) Expression of 35S:SFT (Tomato FT homologue) in tobacco can induce flowering in non-inductive conditions F) Tomato expressing 35S:SFT when grafted onto tobacco induces flowering in tobacco Lifschitz et al. 2006, PNAS 70 35  11/21/24 FT is expressed in leaves, transported to SAM via phloem FT acts with FD to activate AP1, which with LFY, induces flowering 71 FT important in inducing flowers in numerous plants, including Populus tremuloides (Pt) A and B) 35S::PtFT callus culture initiates flowers C-E) 6 month old plants, produce flowers (normally 8-20 years) Bohlenius et al., 2006 Science 72 36  11/21/24 Formation of dormant buds is also controlled by photoperiod Apical meristem +nodes + internodes enclosed in modified leaves (bud scales) Slowing/cessation of vegetative growth precedes dormancy often induced by changes in photoperiod 73 Correlation between daylength and plant development 74 37  11/21/24 FT controls bud dormancy: LD 32 SD 63 SD LD Wild type = stop Wild type = growth under continue short days growth Wild under long type days RNAi 35SptFT = RNAi = stop continue growth growth 35S:: under short under long days days PtFT Bohlenius et al., 2006 Science SD induced growth cessation (i.e. bud dormancy in preparation for winter) does not occur in 35S::PtFT Inhibit FT with RNAi (H), see growth cessation in LD low FT is required for bud cessation, high FT inhibits bud cessation 75 Bud growth dormancy in different accessions of P. tremuloides has different photoperiodic requirements at different latitudes: Occurs at 21 hours in N. Sweden (63oN), 15 hours in Germany (51oN) If grow in a “common garden experiment” with 19 hour day: Daylength is below threshold for bud growth in northern accessions (red and green) – buds become dormant Daylength is above threshold for bud growth in southern accessions (blue and purple) – buds continue growing * Bud cessation in N. Sweden (63oN) ecotypes occurs when days *are shorter than 21 hours * * * * Bud cessation in Germany (51oN) ecotypes occurs when days are shorter than 15 hours * * Bohlenius et al., 2006 Science 76 38  11/21/24 Bud growth cessation in P. tremuloides is dependent on CO and FT (recall that high FT maintains bud growth, low FT required for bud dormancy) In same LD condition, timing of PtCO peak (*) is different in different accessions – occurs in day in southern (blue and purple) accessions, but in night in northern (red and green) accessions Coincidence of high CO with light in southern accessions turns on FT(*) in southern trees in northern accessions– reduced FT induced bud growth cessation in southern accessions– high FT causes buds to continue growing Climate change will disrupt the power of photoperiod to predict temperature, precipitation, etc. * Bud dormancy * Bud dormancy All accessions grown at 19 hour day Bud growth * * * Bohlenius et al., 2006 Science Bud growth ** * 77 Tuber formation in Potato is promoted by SD Under long days, induction is repressed Rodríguez-Falcón, et al. 2006. https://doi.org/10.1146/annurev.arplant.57.032905.105224 78 39  11/21/24 Phytochrome (PHYB) represses tuber formation – signal is graft transmissible (transported from shoot to root) PhyB mutants produce tubers under LD PhyB mutant shoot on wild type root produces tubers under LD Grafting wild type shoot onto phyB mutant root prevents tuber formation Rodríguez-Falcón, et al. 2006. https://doi.org/10.1146/annurev.arplant.57.032905.105224 79 High levels of FT causes tuber formation Expression of Rice FT gene (Hd3a) in potato causes tuber formation under LD conditions Navarro et al., 2011. Nature volume 478, pages119–122 80 40  11/21/24 StCO3 and PhyB regulate FT expression StCO3 undergoes diurnal rhythm – peaks at dawn SD In LD, peak correlates with phyB activation - together repress FT – no tubers LD In SD, peak does not correlate - FT is activated - tubers LD SD Low FT = No tubers High FT = tubers 81 Seasonal clock variation influences numerous developmental/physiological processes seasonally regulated developmental events: Flowering Bud set Tubers Vascular cambium activity… Oravek et al. 2022 Plant Physiol, https://doi.org/10.1093/plphys/kiac337 82 41  11/21/24 Vernalization – flowering in response to cold Some plants require a cold treatment to flower = vernalization Beet plant (biennial, flowers in the second year following a winter) has been grown in a greenhouse for many years No winter, continues to grow vegetatively 83 Thlapsi arvense - obligate vernalization Chill different parts of the plant Shoot apex needs to be vernalized for flowering – memory persists from seed – reproductive phase of plants Chilling leaves does not induce vernalization But, if chill leaves (or roots or cells in tissue culture), and then induce meristem formation from leaves, will flower The same cells that are chilled must form shoot apex for activation Cells have a memory that persists through mitosis 84 42  11/21/24 Vernalization in Arabidopsis - Different ecotypes (accessions) of Arabidopsis may be a) summer annuals (no vernalization requirement), or b and c) winter annuals (facultative vernalization requirement) Sung and Amasino, 2004 85 FLC is a MADS box gene – encodes transcription factor, key repressor of floral activation in Arabidopsis Vernalization represses expression of FLC The autonomous floral- promotion pathway also Photoperio represses FLC (means that (SOC1) d (CO) requirement for repression by vernalization is not absolute) FLC expression is high in plants that require vernalization, and expression diminishes with increasing length of cold period Figure 8-11 86 43  11/21/24 Variation in vernalization requirement amongst ecotypes depends on allelic variation at FLC and FRI Summer annual = e.g. Winter annual = Functional FLC loss-of-function alleles at FLC 87 Vernalization response: 1) cells producing SAM must be cold treated 2) stable through mitoses - cellular memory 3) Achieved gradually over a lengthy cold period Suggests epigenetic modification - chromatin remodeling Cold treatment can be mimicked by methylating DNA (inhibits transcription) - target unknown Cold treatment causes decreased H3 and H4 acetylation in FLC chromatin (represses transcription) Cold treatment causes trimethylation of FLC Histone H3 at lysine27 (H3K27me3) and H3K9me2 (heterochromatin represses transcription) Cold treatment removes H3K4me3 and H3K36me (activate transcription) 88 44