Chapter 16

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Growth form through postembryonic processes

Animal form during embryogenesis

Are there increases in complexity? To what extent is growth coupled to

division, expansion & differentiation How does the environment affect or

influence growth processes?

How are characteristic patterns genetically controlled?

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Second set details nature of the underlying mechanisms How are characteristic growth patterns genetically

determined? How is development tied to external influences?

Nutrients, energy, stress What mechanisms deal with external influences? What physical components involved & how do they work?

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Plants rigid anatomy compared to animals Animal development characterized by cellular

migration Plant cells in inflexible/woody matrix

Sporophyte development Embryogenesis Germination/Vegetative development Reproductive development

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Process transforming the zygote into a multicellular entity having a characteristic organization Within the ovule Predictable sequence Basic patterning – establishes polarity Cells differentiate positionally Concluding with changes allowing the embryo to

survive dormancy

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The breaking of the dormant state and the beginning of vegetative growth Many factors can trigger germination Early – stored reserves in the seed Meristematic activity Photomorphogenesis (ch. 17) – seedling

photosynthesizes Indeterminate growth

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Transition from vegetative to reproductive Flowering (ch. 25) Fruit development

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Developmental processes by which basic plant architecture established Morphogenesis – elaboration of form Organogenesis – formation of functionally organized

structures Histogenesis – differentiation producing tissues

Apical meristems – sustain indeterminate growth Development enables dormancy and germination

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Arabidopsis – model organism Monocots -- weird.

Zygote (a) Globular (b-d) Heart (e-f) Torpedo (g) Mature (h)

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Polarity Apical-basal axis Radial axis

Apical-basal begins with zygote Apical cell -> nearly entire

embryo Basal cell -> transient suspensor

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Apical cell Apical region cotyledons + apical meristem Middle region hypocotyl, root, and meristem Hypophysis quiescent center and root cap

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Lineage-dependent signaling Cell fate is fixed fixed programs of development

Position-dependent signaling Cell fate depends on position

BUT Cells have to have cues to signify position Cells have to assess their location Cells have to respond to that information

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Auxin important tissue culture Embryogenic patterns in total absence of plant

Auxin deficient mutants morphologically similar to normal plants with altered auxins

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TRIVIA! GURKE – encodes acetyl-CoA carboxylase –

required for synthesis of very-long-chain fatty acids and sphingolipids

FACKEL – encodes a sterol C-14 reductase GNOM – guanine nucleotide exchange factor which

enables polar distribution of auxin MONOPTEROS – encodes an auxin response factor

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Radial patterning Mechanism unknown Work with Gibberellin mutants

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Meristems ≈ Stem Cells Mitotic potential persists

RAM/SAM – most important Intercalary meristems – meristems flanked by

differentiated tissues Marginal meristems – edges of developing organs Meristemoids – superficial clusters of cells

(trichomes, stomata, etc.)

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Similarities Initials – slow dividing & undetermined fate Are the underlying mechanisms the same?

Differences Lateral root formation back from root tip Leaves form at meristem – specialized terminology

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4 zones with distinct behaviors Root Cap

Covers meristem; secreted mucigel Perceives gravity

Meristematic Zone Initials that produce the root tissues

Elongation Zone Rapid and extensive cell elongation Rate decreases with distance

Maturation Zone Cells acquire differentiated characteristics Elongation/differentiation have ceased Lateral organs form

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Quiescent Center – low rate of cell division Close functional relationship between QC and

other initials – apparently SPECIES DEPENDANT! Removal of QC results in

abnormal division and

precocious differentiation QC -- auxin concentration maximum Derived from apical cell of hypophysis

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Auxin vs Cytokinin Auxin largely synthesized in shoot

transported to root Promotes root growth

Cytokinin synthesized in root transported to shoot

Promotes shoot growth; suppresses roots Signaling begins in hypophysis

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Maintain sets of undetermined cells that enable indeterminate growth SAM – initials and undifferentiated derivatives Shoot apex – SAM plus developmentally

committed cells (e.g., most recently formed leaf primordia)

Species specific!

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Zones and layers Central Zone – cluster of infrequently dividing

cells (c.f., QC) Peripheral Zone – dense;

incorporated into lateral organs

(e.g., leaves) Rib Zone – gives rise to internal

tissues

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Zones and Layers Tunica

L1 epidermis Anticlinal divisions

Corpus L2 & L3 internal tissues L2 – anticlinal L3 – randomly oriented

Identities are position dependant L2 cell divides periclinally and in L1 becomes epidermis

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Similar mechanisms maintain initials in SAM and RAM

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Phyllotaxy Position dependant mechanisms Auxins (remember Vi Hart?)

Leaf initiation depends

on auxin accumulation

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Planar form of the leaf

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Distinct mechanisms for formation of lateral organs Root series of periclinal divisions in pericycle

growth in plane perpendicular to root

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Distinct mechanisms for formation of lateral organs Shoot cells from several distinct layers Axillary meristems

Pattern of branch formation directly related to phyllotaxy

Apical dominance ( Auxins)

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Senescence ≠ Necrosis Senescence – energy-dependant developmental

process Necrosis – death brought about by physical

damage, poisons, or external injury Senescence – ordered degradation of cellular

contents; remobilization of nutrients Associated with abscission Early senescence – nutrients mobilization reversible

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Occurs variety of organs; in response to different cues Monocarpic senescence – senescence of entire plant after a

single reproductive cycle Senescence of aerial shoots in herbaceous perennials Seasonal leaf senescence Sequential leaf senescence (leaves of an age die) Senescence of fruits Senescence of storage cotyledons Senescence of floral organs Senescence of specialized cell types (e.g., trichomes,

tracheids, vessel elements, etc.)

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Triggers Reproductive processes Environmental cues Day length; temperature Pathogens Hormonal control ethylene; cytokinins Oxidative stress Metabolic status sugar sensor hexokinase Macromolecule degredation Intrinsic developmental factors age-related Programmed cell death apoptosis

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Chloroplast first to degrade Significant in terms on nutrient reallocation N Releases potentially phototoxic chlorophyll

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Programmed cell death Pathogens necrotic lesions DNA replication errors

Xylem trachery elements

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