76 Growth and Development in Arabidopis thallana. 35出微2a地88bae。 HAP d ceser the d two s ete.Ho the ca d in the rotcrationo ell was a ntry,bu of m 2337 37. “二声空 ization 3a眼2含8pR湍88是72s63品。 39.umGadFertiocalclum a key sper 40. he c rather than the e 7 vas iden R19 h “ M 5oganond ssP】 46. 8 ns Panta 2011. of male g with the P ago zeylanica IPT ger xpresed ini的& 。 ion of 50. 61. ant Chen TannoMrrChromin Current Opinion in Plant Biology 2012.15:70-7 www.scie edirect.com species, and a complementation assay was performed. The N-terminus was functional when exchanged with that of closely related species, such as Sisymbrium irio, whereas it was nonfunctional when exchanged with that of distantly related species such as Oryza sativa. The positive charge of the histidines in the C-terminus contributed to the full function of HAP2. 35. Liu Y, Tewari R, Ning J, Blagborough AM, Garbom S, Pei J, Grishin NV, Steele RE, Sinden RE, Snell WJ et al.: The conserved plant sterility gene HAP2 functions after attachment of fusogenic membranes in Chlamydomonas and Plasmodium gametes. Genes Dev 2008, 22:1051-1068. 36. Yu F, Shi J, Zhou J, Gu J, Chen Q, Li J, Cheng W, Mao D, Tian L, Buchanan BB et al.: ANK6, a mitochondrial ankyrin repeat protein, is required for male-female gamete recognition in Arabidopsis thaliana. Proc Natl Acad Sci USA 2010, 107:22332- 22337. The mitochondrial ankyrin repeat protein ANK6 was abundantly expressed in the male and female gametophyte, both of which were unexpectedly involved in gamete recognition for fusion. 37. Tsukamoto T, Qin Y, Huang Y, Dunatunga D, Palanivelu R: A role for LORELEI, a putative glycosylphosphatidylinositol￾anchored protein, in Arabidopsis thaliana double fertilization and early seed development. Plant J 2010, 62:571-588. 38. 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The Arabidopsis thaliana mutant of RETINOBLASTOMA RELATED1 (rbr1) contains an additional egg cell and a sterile central cell. The two egg cells in the rbr1 ovule fused with sperm cells to intiate normal embryogenesis, indicating that both sperm cells in a single pollen tube have equal ability to fertilize an egg cell. 44. Kranz E, Vonwiegen P, Lorz H: Early cytological events after induction of cell-division in dgg cells and zygote development following in-vitro fertilization with angiosperm gametes. Plant J 1995, 8:9-23. 45. Mogensen HL: The male germ unit: concept, composition and significance. Int Rev Cytol: Surv Cell Biol 1992, 140:129-147. 46. Russell SD, Cass DD: Ultrastructure of the sperms of Plumbago zeylanica. 1. Cytology and association with the vegetative nucleus. Protoplasma 1981, 107:85-107. 47. Sodmergen, Chen GH, Hu ZM, Guo FL, Guan XL: Male gametophyte development in plumbago-zeylanica – cytoplasm localization and cell determination in the early generative cell. Protoplasma 1995, 186:79-86. 48. Saito C, Nagata N, Sakai A, Mori K, Kuroiwa H, Kuroiwa T: Angiosperm species that produce sperm cell pairs or generative cells with polarized distribution of DNA-containing organelles. Sex Plant Reprod 2002, 15:167-178. 49. Russell SD: Preferential fertilization in Plumbago: Ultrastructural evidence for gamete-level recognition in an angiosperm. Proc Natl Acad Sci USA 1985, 82:6129-6132. 50. Pagnussat GC, Yu HJ, Sundaresan V: Cell-fate switch of synergid to egg cell in Arabidopsis eostre mutant embryo sacs arises from misexpression of the BEL1-like homeodomain gene BLH1. Plant Cell 2007, 19:3578-3592. 51. Chen Z, Tan JL, Ingouff M, Sundaresan V, Berger F: Chromatin assembly factor 1 regulates the cell cycle but not cell fate during male gametogenesis in Arabidopsis thaliana. Development 2008, 135:65-73. 52. Aw SJ, Hamamura Y, Chen Z, Schnittger A, Berger F: Sperm entry is sufficient to trigger division of the central cell but the paternal genome is required for endosperm development in Arabidopsis. Development 2010, 137:2683-2690. Loss-of-function mutations in Arabidopsis CYCLIN-DEPENDENT KINASE A1 (CDKA;1) cause the production of a single sperm cell. In this study, the authors showed that cdka;1 single spermcells fertilizedeitherthe eggcell or the central cell in Arabidopsis. They also revealed that a significant propor￾tion of cdka;1 pollen tubes contained two sperm cells, which appeared to fuse with each female gamete. However, the karyogamy of the cdka;1 sperm nucleus failed in the fertilized central cell. The following autonomous proliferation of the central cell was activated by cdka;1 sperm entry, but incorporation of the paternal genome (i.e. karyogamy of male and female nuclei in the fertilized central cell) was required for early development of the endosperm. 53. Kim HJ, Oh SA, Brownfield L, Hong SH, Ryu H, Hwang I, Twell D, Nam HG: Control of plant germline proliferation by SCF(FBL17) degradation of cell cycle inhibitors. Nature 2008, 455: 1134-U1114. 54. Frank AC, Johnson MA: Expressing the diphtheria toxin A subunit from the HAP2(GCS1) promoter blocks sperm maturation and produces single sperm-like cells capable of fertilization. Plant Physiol 2009, 151:1390-1400. Expression of the translation inhibitor diphtheria toxin A subunit (DTA) under the control of the male germ line-specific HAP2 (GCS1) promoter led to the production of a single sperm-like cell rather than two sperm cells. The single sperm-like cell preferentially fused with the central cell rather than the egg cell, suggesting the possibility of a differential gene expression program between the two sperm cells in directing preference to the targeted female gametes. 55. Ron M, Saez MA, Williams LE, Fletcher JC, McCormick S: Proper regulation of a sperm-specific cis-nat-siRNA is essential for double fertilization in Arabidopsis. Genes Dev 2010, 24:1010- 1021. A male gametophytic mutant of Arabidopsis, kokopelli (kpl), was identi- fied, which showed single-fertilization events and resulted in the reduc￾tion of seed set. The KPL and inversely transcribed ARIADNE14 (ARI14; putatively encoding a ubiquitin E3 ligase) genes generated cis-nat-siRNA that targeted ARI14 specifically in sperm cells. ARI14 overexpression in sperm cells also induced the reduced seed set phenotype, indicating that cis-nat-siRNA played a key role in successful double fertilization of plants by regulating the ARI14 transcript level. 56. Bayer M, Nawy T, Giglione C, Galli M, Meinnel T, Lukowitz W: Paternal control of embryonic patterning in Arabidopsis thaliana. Science 2009, 323:1485-1488. Mutations in the IRAK/Pelle-like kinase SHORT SUSPENSOR (SSP) gene resulted in the suppression of suspensor formation such as yoda (yda) mitogen-activated protein kinase pathway mutants. SSP transcripts existed in sperm cells, but the protein was undetectable. Alternatively, SSP protein accumulated in the zygote and the endosperm transiently after fertilization, suggesting the presence of paternal effects over zygote growth. Because the ectopically expressed SSP in seedlings solely activated YDA-dependent signaling, the authors concluded that SSP plays an important role upon fertilization via the YDA-dependent signaling cascade. 57. Ge L, Gou X, Yuan T, Strout GW, Nakashima J, Blancaflor EB, Tian HQ, Russell SD: Migration of sperm cells during pollen tube elongation in Arabidopsis thaliana: behavior during transport, maturation and upon dissociation of male germ unit associations. Planta 2011, 233:325-332. A GFP reporter gene fused with the Plumbago zeylanica IPT gene promoter of the sperm associated with the vegetative nucleus was expressed in both sperm cells at an equal level in Arabidopsis. The authors used Arabidopsis plants transformed with this pPzIPT:GFP con￾struct to examine movement of sperm cells during pollen tube elongation and to show that the male germ unit is an essential assemblage for transmission of sperm cells. 58. Ueda M, Zhang Z, Laux T: Transcriptional activation of Arabidopsis axis patterning genes WOX8/9 links zygote polarity to embryo development. Dev Cell 2011, 20:264-270. The plant specific zinc-finger transcription factor WRKY2 directly activates the transcription of WUSCHEL-RELATED HOMEOBOX (WOX) 8 and its redundant gene WOX9 in the Arabidopsis zygote. In wrky2-1 mutant plants, the zygote failed to reestablish polar distribution of organelles, and the zygote underwent abnormal cell division. The WRKY2-WOX8/9 76 Growth and Development Current Opinion in Plant Biology 2012, 15:70–77 www.sciencedirect.com