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Transcript
Fucus life cycle F-actin patch defines position of future rhizoid •Originally at sperm entry site •Light/gravity/temp/current cause reassembly of actin patch (e.g. opposite light, towards gravity) Figure 4-1 Embryo develops apical/basal polarity Figure 2-1 •Deposition of unique membrane/cell wall material at rhizoid end •Ca 2+ channels asymmetrically positioned •Asymmetric localization of Ca2+ pumps results in a gradient of Ca2 + ions Belanger and Quatrano, 2000 Figure 2-1 1 •Both thallus and rhizoid walls have fate inducing components •Directed fusion of vesicles deposits compounds into cell wall - these fix cell fate •Following polarization, axis becomes fixed • cytoskelton is organized so that vesicles fuse at rhizoid end •If stop secretion using brefaldinA, no axis fixation Belanger and Quatrano, 2000 Figure 4.3 Berger et al., 1994 Marker AtLTP1 is restricted to the shoot epidermis - indicates apical region of embryo Arabidopsis zygotes also undergo an initial asymmetric cell division to generate the upper embryo and lower suspensor •Similar to Fucus, although different evolutionary origins •Angiosperm embryos are contained within maternal tissue, thus polarity may depend on maternal polarity In gnom mutants, first asymmetric division does not occur, subsequent divisions are not normal •Secretion defect improper cell wall targeting? Top cell of suspensor = hypophysis - QC and root cap columella Former micropyle Figure 4-4 But AtlTP1 expression may be normal, indicting proper Apical/basal polarity is independent of cell division 2 Figure 4-5 Polar auxin transport controls many aspects of post-embryonic development •Transported basipetally, in shoot vascular parencyma •Efflux carriers (PIN) at base of cells •Redirection of auxin at root tip results in “reverse fountain” Directional flow of auxin in globular and beyond is due to basipetal placement of PIN proteins (immunolocalization) early in embryonic development (mid-globular stage) Steinmann et al., 1999, Figure 4.6 a -c) DR5::GFP activity is highest in apical cell and derivatives d-e) activity is highest in hypophysis PIN1 in wild type DR5::DTA (diptheria toxin) made conditional by Gal4 transactivation •Early activation causes death of apical cell (g), later causes death of basal cells (h) PIN1 in gnom •In embryos mutant for gnom PIN1 is not localized to the basal membrane (but gnom defects are ealier) Steinmann et al., 1999 immunolocalization of IAA •I - pre-globular in apical •J - globular in basal Friml et al., 2003 3 a) Wild type a-d) PIN1 localization 1-16 cells - no polarity, 32 cells -basal Friml et al., 2003 f) - PIN3::GUS f inset) PIN3 in situ g) - PIN4 i-o) PIN7 localization i) PIN7::GUS j) in situ - basal cells 3k-o immunolocaliation To 32 cell stage - In basal cells on side facing apical At 32 cells - reverses b-e) pin7 mutant Many pin7 mutant embryos show defects: b, c) filamentous, d) apical/basal boundary not clearly defined, e) apical defects Friml et al., 2003 -similar to gnom gnom But, pin7 embryos seem to recover at 32 cell stage Friml et al., 2003 Friml et al., 2003 pin1 pin3 pin4 pin7 quadruple mutants show severe apical/basal defects, do not recover at 32-cell stage, and end up looking like gnom •Functional redundancy amongst PIN genes •gnom defects are explained by improper localization of ALL PIN proteins 4 Model of Experiments That Have Examined the Mechanisms of the Control of PIN1 Protein Localization CytochalasinInhibits actin untreated BFA inhibits vesicle cycling Friml et al., 2003 a) After asymmetric division, auxin accumulates in apical cell through apically localized PIN7-dependent transport b) Auxin is produced in apical region, and PIN1 localization to basal membranes causes basipetal transport, PIN 7 localization reverses (PIN7 function is now redundant) Muday, G. K., et al. Plant Cell 2002;14:293-299 PIN is internalized Therefore less on PM wt Copy right ©2002 American Society of Plant Biologists 35S::PID gnom Friml et al., 2003 •Functional redundancy amongst PIN genes •gnom defects are explained by improper localization of ALL PIN proteins •GNOM = ARF-GEF, functionally homologous to yeast protein, required for vesicle cycling wt RPS5A::PI D DR5::GUS in 35S::PID DR5::GUS RPS5A::PI D RPS5A::PI D Overexpression of PID results in •reversal in PIN1 polarity (basal (low) to apical (high)) •Lack of basal specific auxin maximum •Defects in basal structure development •Defects in basal structures, embryo symmetry 5 PID The Mammalian GLUT4 Asymmetric Vesicular Targeting Mechanism as a Model for the Localization of the Auxin Efflux Carrier GNOM - to plasma membrane allows vesicle trafficking High PID apical Low PID - basal exocytosis GNOM - allows vesicle cycling - if inhibit, PIN is internalized PID - directs vesicle cycling - if increase, PIN is apical instead of basal auxin - inhibits endocytosis, which results in an accumulation of PIN at the PM auxin causes inhibition of endocytosis, which results in an accumulation of PIN at the PM endocytosis Muday, G. K., et al. Plant Cell 2002;14:293-299 A Model for Auxin Response through the TIR1 Auxin Receptor Pathway But what causes the auxin response? BDL AXR6 MP Woodward, A. W., et al. Plant Cell 2005;17:2425-2429 Copy right ©2005 American Society of Plant Biologists 6 A Model for Auxin Response through the TIR1 Auxin Receptor Pathway •monopteros (mp) embryos show abnormal division patterns and never form a root •Plants can be made in culture - these have reduced levels of auxin transport •MP is expressed in subepidermis, then is restricted to procambium BDL AXR6 MP Woodward, A. W., et al. Plant Cell 2005;17:2425-2429 Figure 4.7 Copy right ©2005 American Society of Plant Biologists No degradation (no activator ARF) mp (Hardtke and Berleth, 1998) axr6 (Hobbie et al., 2000) (undegradable Repressor Aux/IAA) bdl (Hamann et al., 1999) Auxin response is necessary to pattern the basal parts of the plant 7 Auxin is directed to root by PIN4, where it induces BDL degradation and activation of auxin responsive genes via MP WOX2 expressio n WOX9 expression Weijers and Jurgens, 2005 Vogler and Kuhlemeier, 2003 WOX2 expressed only in apical cell, WOX9 only in basal cell WOX = WUSCHEL-related homeobox Later WOX9 expression depends on MP and BDL 8