Using plant EST collections, we obtained 1392 potential gene duplicates across 8 plant species: Zea mays, Oryza sativa, Sorghum bicolor, Hordeum vulgare, Solanum tuberosum, Lycopersicon esculentum, Medicago truncatula, and Glycine max. We estimated the synonymous and nonsynonymous distances between each gene pair and identified two to three mixtures of normal distributions corresponding to one to three rounds of genome duplication in each species. Within the Poaceae, we found a conserved duplication event among all four species that occurred approximately 50-60 million years ago (Mya); an event that probably occurred before the major radiation of the grasses. In the Solanaceae, we found evidence for a conserved duplication event approximately 50-52 Mya. A duplication in soybean occurred approximately 44 Mya and a duplication in Medicago about 58 Mya. Comparing synonymous and nonsynonymous distances allowed us to determine that most duplicate gene pairs are under purifying, negative selection. We calculated Pearson's correlation coefficients to provide us with a measure of how gene expression patterns have changed between duplicate pairs, and compared this across evolutionary distances. This analysis showed that some duplicates seemed to retain expression patterns between pairs, whereas others showed uncorrelated expression.

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...

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Arabidopsis thaliana plants were transformed with GFP-MBD (J. Marc et al., Plant Cell 10: 1927-1939, 1998) under the control of a constitutive (35S) or copper-inducible promoter. GFP-specific fluorescence distributions, levels, and persistence were determined and found to vary with age, tissue type, transgenic line, and individual plant. With the exception of an increased frequency of abnormal roots of 35S GFP-MBD plants grown on kanamycin-containing media, expression of GFP-MBD does not appear to affect plant phenotype. The number of leaves, branches, bolts, and siliques as well as overall height, leaf size, and seed set are similar between wild-type and transgenic plants as is the rate of root growth. Thus, we conclude that the transgenic plants can serve as a living model system in which the dynamic behavior of microtubules can be visualized. Confocal microscopy was used to simultaneously monitor growth and microtubule behavior within individual cells as they passed through the elongation zone of the Arabidopsis root. Generally, microtubules reoriented from transverse to oblique or longitudinal orientations as growth declined. Microtubule reorientation initiated at the ends of the cell did not necessarily occur simultaneously in adjacent neighboring cells and did not involve complete disintegration and repolymerization of microtubule arrays. Although growth rates correlated with microtubule reorientation, the two processes were not tightly coupled in terms of their temporal relationships, suggesting that other factor(s) may be involved in regulating both events. Additionally, microtubule orientation was more defined in cells whose growth was accelerating and less stringent in cells whose growth was decelerating, indicating that microtubule-orienting factor(s) may be sensitive to growth acceleration, rather than growth per se.