Chapter 11 Plant responses to hormones & environmental stimuliResponses includeDevelopmental transitionsDormancyGerminationFloweringGrowth

Hormones & environmental signals involve signal transduction pathways

Internal and external signals

Hormones influence gene expressionGene expression regulated bymicroRNAstranscription factors

Plant hormones & growth(abscisic acid)

Hormones interact to promote/inhibit development

Auxinstryptophan

Responses involving auxinPhototropismGravitropismCellular elongationInitiation of leaf primordiaApical dominanceRoot developmentFruit development

TropismsPermanent, directional growth in response to an external stimulus

Positive tropismsNegative tropisms

PhototropismStems are positively phototropic

How can plants grow towards light?

Auxin and cell elongationAcidified cell walls have increased elasticity

Phototropism researchPhototropin (NPH1) and phototropism-initiates a signal transduction pathway-nph1 mutants non-phototropic

Gravitropism

GravitropismHow can it be demonstrated that auxin is involved in gravitropism?

Gravitropism and root cap amyloplastsGravity regulated auxin transport, Ottenschlaumlger, Iris et al. (2003) Proc. Natl. Acad. Sci. USA 100, 2987-2991

Auxin and initiation of leaf primordiaPin mutant link

Responses involving auxinApical dominance

Responses involving auxinFormation of adventitious roots

Auxin produced by seeds promotes ovary tissue growth

Plant hormonesAre proteins encoded for by genesAct individually to bring about changes in plant developmentFunction as receptors for environmental signalsBoth 1 and 3None of these

AuxinPrevents apical dominanceIs produced in shoot apical meristemsPromotes seed development inside fruitAll of theseNone of these

Phototropin Is a type of auxinPromotes apical dominanceIs involved in stem growth towards lightIs produced by seedsAll of these

Cytokinins cell division and differentiation

Cytokinin & tissue culture

From callus to somatic embryos

GibberellinsPromotes: Germination Stem elongation Flowering Fruit development

GibberellinsBreaking dormancySeed germination

GibberellinsPromotes cell division & elongation

GibberellinsPromotes bolting in biennials

GibberellinsPromotes: Germination Stem elongation Flowering Fruit development

Gibberellin is part of a complex signal transduction pathway(see supplemental reading, for related information)

Della proteins restrain growthGA and GID2 degrade Della proteins

Gibberellins and germination

Gibberellin promotes vegetative growth

Abscisic acidInhibits growthPromotes dormancyCloses stomata

Abscisic acid inhibits germination

Promotes dormancyLeached by imbibition

ABA and stomatal closure

ABA delays floweringFCA an RNA binding proteinFY an mRNA processing factorFlowering Locus C a flowering repressor

Ethylene (CH2=CH2)Fruit ripening (promotes)Flowering (inhibits)Abscission (promotes)Sex expression in monoecious species (ratio of to )Thigmomorphogenesis (reduced stem elongation in some environments)

Thigmomorphogenesis

Brassinosteroids (BRs)60 types, brassinolide most commonStimulates cell elongation, leaf expansionBR mutants extreme dwarfs, small crinkled leavesDark grown BR mutants de-etiolated

Plant Genes on Steroids Science, Vol 307, Issue 5715, 1569-1570 , 11 March 2005BIN2 catalyzes breakdown of BES1 & BZR2 proteins (phosphorylation)BR regulates activity of key growth transcription factors-BES1(activator)-BZR1(repressor)

Figure 13.12 (p.290)

Figure 13.12 (p.290)

Plant Genes on Steroids Science, Vol 307, Issue 5715, 1569-1570 , 11 March 2005BIN2 catalyzes breakdown of BES1 & BZR2 proteins (phosphorylation)BR regulates activity of key growth transcription factors-BES1(activator)-BZR1(repressor)

Responses to environmental stimuli: lightPhototropismStomata openingStem elongationPhotodormancy (photoblastism)Photoperiodism

PhytochromePhytochromes are proteins with a light absorbing pigment attached (chromophore) Mediates stem elongation, seed germination, timing of flowering

Phytochrome structure

Two forms of phytochrome

Phytochrome & stem growthEtiolation occurs in low light or dark why?

Does Pfr inhibit or promote stem elongation?Phytochrome and hormonal control of stem elongation

Phytochrome and seed germinationPhotodormancy & photoblastic seedsGermination activated by lightSome plants, by red lightSome plants, by far-red lightNegative photoblastism (tomato), Pfr inhibits germinationPositive photoblastism (lettuce), Pfr promotes germination

Lettuce is positively photoblastic30-60% lettuce seed germinate in dark85-95% lettuce seed germinate in light

Phytochrome, photoperiodism & flowering

Manipulation of photoperiodPoinsettia industryChrysanthemums

Why wont my Christmas cactus bloom?

BrassinosteroidsPromote seed germination in response to lightPromotes flowering in response to day lengthAre proteins with an attached light absorbing chromophoreRegulate transcription factors involved in growthAll of these

Which of the following is true of phytochrome?Pfr absorbs red light and Pr absorbs far red lightPr is the active form of phytochrome and Pfr is the inactive form of phytochromePfr promotes germination in seeds requiring lightAll of these None of these

PhotoperiodismDetermines seed dormancy/germination in response to light/darkDetermines flowering in response to day lengthIs a protein with an attached light absorbing chromophoreControls stem growth in response to light/darkAll of these

Circadian rhythms sleep movements (nyctinasty)

Nyctinasty

Solar tracking (heliotropism)

Response to mechanical stimuli (seismonasty)

Seismonasty Mimosa pudica

Seismonasty

SeismonastyVenus flytrap

Response to environmental stimuli:Induced resistanceHerbivore attack, systemin (18aa polypeptide hormone) & jasmonic acid (1-alpha, 2-beta-3-oxo-2-(cis-2-pentenyl)-cyclopentane acetic acid)

Figure 1 Model for the activation of defense genes in tomato in response to wounding and insect attack. After wounding, systemin is released from its precursor prosystemin by proteolytic processing. Systemin subsequently binds a membrane-bound receptor to initiate an intracellular signaling cascade, including the activities of a MAP kinase, a phospholipase, a calcium dependent protein kinase, an extracellular alkalinization, and the release of linlenic acid from membranes. Linolenic acid is converted to jasmonic acid, a messenger for early defense gene activation. Catalytic activity of polygalaturonase, an early gene, leads to generation of hydrogen peroxide acting as a second messenger for late gene activation. R, receptor; MAPK, MAP kinase; Ca2+PK; calcium dependent protein kinase; PLA2, phospholipase A2; LA, linolenic acid; JA, jasmonic acid; pm, plasma membrane.H2O2 prevents cell wall digestion by fungal pectinases

Response to environmental stimuli:Induced resistancePathogens & the hypersensitive response (HR)

HR response & systemic acquired resistance (SAR)

SAR responsesLignification of cell wallsAntimicrobial moleculesPR-proteins (pathogen related proteins)ChitinasesPhytoalexins (inhibit protein synthesis

SAR model

Acidification of the cell wall increases elasticity