Chapter 16

2  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? 3

 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? 4

 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 5

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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 7

 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 8

 Transition from vegetative to reproductive  Flowering (ch. 25)  Fruit development 9

 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 10

 Arabidopsis – model organism  Monocots -- weird.  Zygote (a)  Globular (b-d)  Heart (e-f)  Torpedo (g)  Mature (h) 11

12

 Polarity  Apical-basal axis  Radial axis  Apical-basal begins with zygote  Apical cell -> nearly entire embryo  Basal cell -> transient suspensor 13

 Apical cell  Apical region  cotyledons + apical meristem  Middle region  hypocotyl, root, and meristem  Hypophysis  quiescent center and root cap 14

 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 15

 Auxin important  tissue culture  Embryogenic patterns in total absence of plant  Auxin deficient mutants morphologically similar to normal plants with altered auxins 16

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18

 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 19

 Radial patterning  Mechanism unknown  Work with Gibberellin mutants 20

21

 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.) 22

 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 23

 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 24

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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 26

 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 27

 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! 28

 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 29

 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 30

 Similar mechanisms maintain initials in SAM and RAM 31

 Phyllotaxy  Position dependant mechanisms  Auxins (remember Vi Hart?)  Leaf initiation depends on auxin accumulation 32

 Planar form of the leaf 33

 Distinct mechanisms for formation of lateral organs  Root  series of periclinal divisions in pericycle  growth in plane perpendicular to root 34

 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) 35

 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 36

 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.) 37

 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 38

 Chloroplast first to degrade  Significant in terms on nutrient reallocation  N  Releases potentially phototoxic chlorophyll 39

 Programmed cell death  Pathogens  necrotic lesions  DNA replication errors  Xylem trachery elements 40