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Cell-cell communication during double fertilization BL 5400 Generalized preparatory events in double fertilization Generation of embryo sac/ megagametophyte from haploid functional megaspore Eight nucleate immature female gametophyte (FG) generation Migration of these cells according to their cell fate Contd… Formation of egg apparatus consists of egg nucleus and flanked by two synergids Antipodal cells formation and degeneration by Programmed Cell Death (PCD) Polar nuclei migrate towards EA and fuse before/after fertilization QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Generalized model of key cell-cell communication events before and after fertilization in angiosperms a. b. c. Embryo sac maturation and guidance molecules for male gametophyte Events towards fertilization by the two male gametophytes Karyogamy, cell-cell signaling and cell division Genetic studies Ds transposon insertion lines identified 130 mutants having defects in FG development Thirty-three lines were defective in embryo sac maturation Defective fertilization in 18 lines. Of these lines defective pollen tube attraction (6), unfertilized mature ovum (12) Development (Pagnussat et al, 2005) Problems in identification of genes in haploid cells (FG/MG) Mutations in housekeeping genes could arrest development and nuclear division Gene redundancy in plant genomes One member of gene family can fully or partially complement another member in the same gene family by which important genes can be overlooked Important mutants identifies key factors during fertilization ZmEA1 gene product encodes a small protein that contains predicted TM domain secreted by egg apparatus guides the pollen tube at micropylar end Science (Marton et al, 2005) Female control of male gamete delivery during fertilization First evidence of female control over male gametophyte In the pistils of sirene (srn) mutant, half of the embryo sacs remained unfertilized Srn embryo sacs could have been structurally or physiologically incapable of correctly receiving pollen tube Embryo sac could not have emitted signals which can arrest pollen tube growth or discharge A. Unfertilized embryo sac in a srn/SRN pistil B. sirene embryo sac penetrated by a pollen tube QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. C. Abnormal growth of pollen tube in srn mutant D. Wild type embryo sac penetrated by pollen tube, with a degenerated synergid cell (dsy) Current Biology (Rotman et al, 2003) A. Wild type embryo sac penetrated by a pollen tube reaches B. Tangled structure in embryo sac (pollen tube outgrowth) QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. C. Two pollen tubes penetration in the micropyle of an ovule D. Interpretative scheme for the serene phenotype sirene affects the pollen tube morphology within the embryo sac Current Biology (Rotman et al, 2003) Arabidopsis mutant feronia disrupts the female gametophytic control pollen tube reception Mutant identified from the Ds tagged population and screened for reduced seed set, obtained feronia as a result of loss-of-function mutation affecting female gametophyte Embryo sac does not arrest the pollen tube growth in feronia mutants and sperm cells are not released Supernumerary pollen tubes were attracted by feronia mutant embryo sac Synergid specification and degeneration in feronia normal? Development (Huck etal, 2003) QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. A. WT FG mature B. Fertilized WT showing elongated zygote and free endosperm C. Unfertilized feronia after 24 HAP D. WT ovule after fertilization E. Feronia PT invadingmicropylar area F. GUS activity in WT G. Feronia mutant GUS activity H. & I. invading PT enters central cell J. Formation of free nuclear endosperm after PT has invaded the FG Development (Huck et al, 2003) magatama mutant delays the FG development Wandering and polyspermy phenotype of pollen tubes of maa1 and maa3 mutants Delay in the fusion of polar cell nuclei thereby maintaining seven-celled eight nuclear state QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Schematic drawing of the monogamy model explain how one pollen tube (PT) but not more, grows and fertilizes a female gametophyte (FG). Pol, pollen grain; Sp, stigmatic papilla; tt, transmitting tissue; MMR, male-male repulsion; FS, funiculus guidance signal; MS, micropylar signal Development (Shimizu & Okada, 2000) Cell-cell communication highlights The maize gene, ZmES1-4 is specifically expressed in FG and downregulated immediately after fertilization Several genes expressed immediately after fertilization shows homology to anti-microbial proteins, indicating that they are signaling molecules Several proteases have been reported to be expressed after fertilization to degrade the signals by those extra-cellular proteases thereby preventing polyspermy References Dresselhaus T. (2006). Current Opinion in Plant Biology 9:41-47 Huck N., Moore J. M., Federer M., Grossniklaus U. (2003) Development 130: 2149-2159 Rotman N., Rozier F., Boavida L., Dumas C., Berger F., Faure, J-E. (2003) Current Biology 13: 432-436 Shimizu K.K. and Okada K. (2000). Development 127:4511-4518