During embryonic pattern formation, the main body axes are established and cells of different developmental fates are specified from a single-cell zygote. Despite the fundamental importance of this process, in plants, the underlying mechanisms are largely unknown. We show that expression dynamics of novel WOX (WUSCHEL related homeobox) gene family members reveal early embryonic patterning events in Arabidopsis. WOX2 and WOX8 are co-expressed in the egg cell and zygote and become confined to the apical and basal daughter cells of the zygote, respectively, by its asymmetric division. WOX2 not only marks apical descendants of the zygote, but is also functionally required for their correct development,suggesting that the asymmetric division of the plant zygote separates determinants of apical and basal cell fates. WOX9 expression is initiated in the basal daughter cell of the zygote and subsequently shifts into the descendants of the apical daughter apparently in response to signaling from the embryo proper. Expression of WOX5 shows that identity of the quiescent center is initiated very early in the hypophyseal cell, and highlights molecular and developmental similarities between the stem cell niches of root and shoot meristems. Together, our data suggest that during plant embryogenesis region-specific transcription programs are initiated very early in single precursor cells and that WOX genes play an important role in this process.

Plant microtubules are organized into specific cell cycle-dependent arrays that have been implicated in diverse cellular processes, including cell division and organized cell expansion. Mutations in four Arabidopsis genes collectively called the PILZ group result in lethal embryos that consist of one or a few grossly enlarged cells. The mutant embryos lack microtubules but not actin filaments. Whereas the cytokinesis-specific syntaxin KNOLLE is not localized properly, trafficking of the putative auxin efflux carrier PIN1 to the plasma membrane is normal. The four PILZ group genes were isolated by map-based cloning and are shown to encode orthologs of mammalian tubulin-folding cofactors (TFCs) C, D, and E, and associated small G-protein Arl2 that mediate the formation of ␣/␤-tubulin heterodimers in vitro. The TFC C ortholog, PORCINO, was detected in cytosolic protein complexes and did not colocalize with microtubules. Another gene with a related, although weaker, embryo-lethal phenotype, KIESEL, was shown to encode a TFC A ortholog. Our genetic ablation of microtubules shows their requirement in cell division and vesicle trafficking during cytokinesis, whereas cell growth is mediated by microtubule-independent vesicle trafficking to the plasma membrane during interphase. The microtubule cytoskeleton plays important roles in both nondividing and dividing cells of eukaryotes, assisting in vesicle trafficking and mediating proper segregation of daughter chromosomes to opposite poles during mitosis (for review, see Cole and Lippincott-Schwartz 1995;Straight and Field 2000). In fission yeast, microtubules have also been implicated in maintaining cell shape and polarity (Sawin and Nurse 1998). Higher-plant cells form their own specific microtubule arrays, such as cortical hoops of interphase microtubules that organize cell elongation (Whittington et al. 2001), and two arrays related to the plant-specific mode of cell division (for review, see Lloyd and Hussey 2001). The preprophase band forms transiently at the onset of mitosis and marks the cortical division site (for review, see Smith 2001). The phragmoplast forms at the center of the division plane in late anaphase and guides the delivery of Golgiderived vesicles. Their fusion results in the formation and lateral expansion of the cell plate that matures into a cell wall and flanking plasma membranes (for review, see Staehelin and Hepler 1996). Both preprophase band and phragmoplast also contain actin filaments; however, their specific role in plant cell division is not understood at present (Smith 2001).Microtubules are polymerized from ␣/␤-tubulin heterodimers (for review, see Nogales 2000). Newly synthesized ␣-and ␤-tubulin polypeptides undergo a sequence of folding steps catalyzed by chaperones. Initially, the tubulins are associated with the hexameric prefoldin complex that passes them on to the cytosolic chaperonin complex (Geissler et al. 1998;Vainberg et al. 1998;Hansen et al. 1999; for review, see Leroux and Hartl 2000;Llorca et al. 2000). Whereas fully func...

The gene coding for phytoene desaturase of the baderium Erwinia uredovora (crtl) was inserted into the chromosome of the cyanobacterium Synechococcus PCC7942 strain R2-PIM8. For expression of crtl in the heterologous host, two constructs with different promoters were introduced into Synechococcus. In the first, crtl was fused to the 5' region of the psbA gene of the xanthophycean microalga Bumilleriopsis filiformis. The second construct carried crtl inserted downstream of the neomycin phosphotransferase II gene (nptll) from the transposon Tn5. Expression of crtl under the control of the respective promoter was shown by immunodetection of the gene product. The functionality of the heterologously expressed phytoene desaturase CRTI in the transformants was demonstrated by enzymic assays. The transformants acquired very strong resistance toward the bleaching herbicide norflurazon.

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