INTRODUCTIONMicrotubules are arranged in different arrays, which perform a variety of essential functions within the cell (Lloyd, 1991). During interphase, microtubules are organized predominately into a cortical array, where they are involved in directing cellulose deposition, which consequently plays a fundamental role in cellular morphogenesis (Giddings and Staehelin, 1991; Cyr, 1994). Individual cortical microtubules are up to 10 m long and are arranged parallel to one another with overlapping ends (Williamson, 1991;Vesk et al., 1994). They may be cross-bridged with other microtubules and/or linked to the plasma membrane or to other cytoskeletal components, and they may assemble into strands that are continuous from one cell face to another (Flanders et al., 1989;Yuan et al., 1995). Within the cells of elongating tissues, microtubules generally form cylindrical arrays in which their strands are oriented transversely or at slightly oblique angles to the direction of cell elongation (reviewed in Green, 1980; Gunning and Hardham, 1982; Cyr and Palevitz, 1995;Wymer and Lloyd, 1996; Fischer and Schopfer, 1997). At the completion of the elongation phase, microtubules usually reorient into a more oblique or even longitudinal direction. Unique arrays operate during the differentiation of specialized cells, such as the banded patterns in tracheary elements (Fukuda, 1997) or radial arrays in stomatal guard cells (Marc et al., 1989). The cortical array's significance in morphogenesis has been documented by studies with antimicrotubule drugs, which depolymerize microtubules and cause cells to grow isodiametrically (Morejohn, 1991), and by the mutants ton and fass of Arabidopsis, which have aberrant cortical microtubules and possess abnormally shaped cells (Traas et al., 1995;McClinton and Sung, 1997).The spatial orientation of microtubules in the cortex is complex and likely involves interactions with a variety of auxiliary molecules and complexes. For example, microtubule-organizing centers nucleate microtubules and thereby affect their appearance in the cortex (reviewed in Marc, 1997;Vaughn and Harper, 1998), whereas microtubule-associated proteins (MAPs) may serve to link microtubules to each other and to other organelles (reviewed in Cyr, 1991; Hirokawa, 1994;Mandelkow and Mandelkow, 1995) and also to modify microtubule stability, thereby affecting their organization and dynamics (Hirokawa, 1994; Desai and Mitchison, 1997). Phosphorylation of MAPs may play a role in organization because this post-translational modification alters their affinity for microtubules, thereby causing rearrangements of the microtubular network (Preuss et al., 1995;Hush et al., 1996;Shelden and Wadsworth, 1996); moreover, treatments with inhibitors of protein phosphatases and kinases disorganize cortical microtubules (Mizuno, 1994; Baskin and Wilson, 1 To whom correspondence should be addressed. E-mail rjc8@ psu.edu; fax 814-865-9131. 1928The Plant Cell 1997). Mechanochemical motor proteins could affect microtubule organization by fa...

INTRODUCTIONMicrotubules are arranged in different arrays, which perform a variety of essential functions within the cell (Lloyd, 1991). During interphase, microtubules are organized predominately into a cortical array, where they are involved in directing cellulose deposition, which consequently plays a fundamental role in cellular morphogenesis (Giddings and Staehelin, 1991; Cyr, 1994). Individual cortical microtubules are up to 10 m long and are arranged parallel to one another with overlapping ends (Williamson, 1991;Vesk et al., 1994). They may be cross-bridged with other microtubules and/or linked to the plasma membrane or to other cytoskeletal components, and they may assemble into strands that are continuous from one cell face to another (Flanders et al., 1989;Yuan et al., 1995). Within the cells of elongating tissues, microtubules generally form cylindrical arrays in which their strands are oriented transversely or at slightly oblique angles to the direction of cell elongation (reviewed in Green, 1980; Gunning and Hardham, 1982; Cyr and Palevitz, 1995;Wymer and Lloyd, 1996; Fischer and Schopfer, 1997). At the completion of the elongation phase, microtubules usually reorient into a more oblique or even longitudinal direction. Unique arrays operate during the differentiation of specialized cells, such as the banded patterns in tracheary elements (Fukuda, 1997) or radial arrays in stomatal guard cells (Marc et al., 1989). The cortical array's significance in morphogenesis has been documented by studies with antimicrotubule drugs, which depolymerize microtubules and cause cells to grow isodiametrically (Morejohn, 1991), and by the mutants ton and fass of Arabidopsis, which have aberrant cortical microtubules and possess abnormally shaped cells (Traas et al., 1995;McClinton and Sung, 1997).The spatial orientation of microtubules in the cortex is complex and likely involves interactions with a variety of auxiliary molecules and complexes. For example, microtubule-organizing centers nucleate microtubules and thereby affect their appearance in the cortex (reviewed in Marc, 1997;Vaughn and Harper, 1998), whereas microtubule-associated proteins (MAPs) may serve to link microtubules to each other and to other organelles (reviewed in Cyr, 1991; Hirokawa, 1994;Mandelkow and Mandelkow, 1995) and also to modify microtubule stability, thereby affecting their organization and dynamics (Hirokawa, 1994; Desai and Mitchison, 1997). Phosphorylation of MAPs may play a role in organization because this post-translational modification alters their affinity for microtubules, thereby causing rearrangements of the microtubular network (Preuss et al., 1995;Hush et al., 1996;Shelden and Wadsworth, 1996); moreover, treatments with inhibitors of protein phosphatases and kinases disorganize cortical microtubules (Mizuno, 1994; Baskin and Wilson, 1 To whom correspondence should be addressed. E-mail rjc8@ psu.edu; fax 814-865-9131. 1928The Plant Cell 1997). Mechanochemical motor proteins could affect microtubule organization by fa...

Full-length cloned cDNAs of lettuce infectious yellows closterovirus (LIYV) RNAs 1 and 2 were constructed and fused to the bacteriophage T3 RNA polymerase promoter. To assess RNA replication, Nicotiana benthamiana protoplasts were inoculated with LIYV virion RNAs and LIYV cDNA-derived in vitro transcripts. Analysis of protoplasts inoculated with LIYV virion RNAs or capped (m7GpppG) in vitro transcripts from LIYV RNA 1 and 2 cDNAs showed accumulation of LIYV genomic and putative subgenomic RNAs (sgRNAs), synthesis of LIYV coat protein, and formation of LIYV virions. Furthermore, protoplasts inoculated with only capped in vitro transcripts from LIYV RNA 1 cDNA showed accumulation of LIYV RNA 1 and its putative sgRNA, indicating that LIYV RNA 1 can replicate in the absence of LIYV RNA 2. Conversely, accumulation of LIYV RNA 2 was not detectable in protoplasts inoculated with only LIYV RNA 2 cDNA-derived capped in vitro transcripts. These data demonstrate that LIYV genomic RNAs are competent for replication in mesophyll protoplasts and that infectious in vitro transcripts can be derived from the cloned cDNAs of a closterovirus genome.

The halophytic genus Suaeda (Chenopodiaceae) includes species with the C3 and C4 photosynthetic pathways. North American species of this genus were investigated to determine whether C3 and C4 leaf anatomy are consistent within the two sections of Suaeda, Chenopodina and Limbogermen, present on this continent. All species from section Chenopodina were found to possess C3 anatomy, whereas all species from section Limbogermen were found to be C4 species. Characteristics of leaf anatomy and chloroplast ultrastructure are similar to those reported from C3 and C4 species, respectively, from the Eastern Hemisphere. All species from section Limbogermen have the suaedoid type of leaf anatomy, characterized by differentiation of the mesophyll into palisade parenchyma and a chlorenchymatous sheath surrounding central water-storage tissue, as well as leaf carbon isotope ratios (_13C) of above -20. All species from section Chenopodina have austrobassioid leaf anatomy without a chlorenchymatous sheath and _13C values of below -20. According to our literature review, the photosynthetic pathway has now been reported for about half (44) of the Suaeda species worldwide. The C3 and C4 photosynthetic syndromes are with few exceptions distributed along sectional or subsectional lines. These findings throw new light on the infrageneric taxonomy of this genus.

The cortical microtubule array provides spatial information to the cellulose-synthesizing machinery within the plasma membrane of elongating cells. Until now data indicated that information is transferred from organized cortical microtubules to the cellulosesynthesizing complex, which results in the deposition of ordered cellulosic walls. How cortical microtubules become aligned is unclear. The literature indicates that biophysical forces, transmitted by the organized cellulose component of the cell wall, provide a spatial cue to orient cortical microtubules. This hypothesis was tested on tobacco (Nicotiana tabacum L.) protoplasts and suspension-cultured cells treated with the cellulose synthesis inhibitor isoxaben. Isoxaben (0.25-2.5 M) inhibited the synthesis of cellulose microfibrils (detected by staining with 1 g mL ؊1 fluorescent dye and polarized birefringence), the cells failed to elongate, and the cortical microtubules failed to become organized. The affects of isoxaben were reversible, and after its removal microtubules reorganized and cells elongated. Isoxaben did not depolymerize microtubules in vivo or inhibit the polymerization of tubulin in vitro. These data are consistent with the hypothesis that cellulose microfibrils, and hence cell elongation, are involved in providing spatial cues for cortical microtubule organization. These results compel us to extend the microtubule/microfibril paradigm to include the bidirectional flow of information.