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...

Polarized movement of auxin generates concentration gradients within plant tissues to control cell division patterns and growth direction by modulating microtubule organization. In this study, we identify a reverse mechanism, wherein microtubules influence polar auxin transport. We show that the microtubule-associated protein CLASP interacts with the retromer component sorting nexin 1 (SNX1) to mediate an association between endosomes and microtubules. clasp-1 null mutants display aberrant SNX1 endosomes, as do wild-type plants treated with microtubule-depolymerizing drugs. Consistent with SNX1's role in trafficking of the auxin efflux carrier PIN-FORMED2 (PIN2), clasp-1 mutant plants have enhanced PIN2 degradation, and PIN2 movement to lytic vacuoles is rapidly induced by depolymerization of microtubules. clasp-1 mutants display aberrant auxin distribution and exhibit numerous auxin-related phenotypes. In addition to mechanistically linking auxin transport and microtubules, our data identify a ubiquitous endosome-microtubule association in plants.

;The organisation of plant microtubules into distinct arrays during the cell cycle requires interactions with partner proteins. Having recently identified a 90-kDa phospholipase D (PLD) that associates with microtubules and the plasma membrane [Gardiner et al. (2001) Plant Cell 13: 2143], we exposed seeds and young seedlings of Arabidopsis to 1-butanol, a specific inhibitor of PLD-dependent production of the signalling molecule phosphatidic acid (PA). When added to agar growth media, 0.2% 1-butanol strongly inhibited the emergence of the radicle and cotyledons, while 0.4% 1-butanol effectively blocked germination. When normal seedlings were transferred onto media containing 0.2% and 0.4% 1-butanol, the inhibitor retarded root growth by about 40% and 90%, respectively, by reducing cell elongation. Inhibited plants showed significant swelling in the root elongation zone, bulbous or branched root hairs, and modified cotyledon morphology. Confocal immunofluorescence microscopy of root tips revealed that 1-butanol disrupted the organisation of interphase cortical microtubules. Butanol isomers that do not inhibit PLD-dependent PA production, 2-and 3-butanol, had no effect on seed germination, seedling growth, or microtubule organisation. We propose that production of PA by PLD may be required for normal microtubule organisation and hence normal growth in Arabidopsis.

The organization of microtubule arrays in the plant cell cortex involves interactions with the plasma membrane, presumably through protein bridges. We have used immunochemistry and monoclonal antibody 6G5 against a candidate bridge protein, a 90-kD tubulin binding protein (p90) from tobacco BY-2 membranes, to characterize the protein and isolate the corresponding gene. Screening an Arabidopsis cDNA expression library with the antibody 6G5 produced a partial clone encoding phospholipase D (PLD), and a full-length gene was obtained by sequencing a corresponding expressed sequence tag clone. The predicted protein of 857 amino acids contains the active sites of a phospholipidmetabolizing enzyme and a Ca 2 ؉-dependent lipid binding domain and is identical to Arabidopsis PLD ␦. Two amino acid sequences obtained by Edman degradation of the tobacco p90 are identical to corresponding segments of a PLD sequence from tobacco. Moreover, immunoprecipitation using the antibody 6G5 and tobacco BY-2 protein extracts gave significant PLD activity, and PLD activity of tobacco BY-2 membrane proteins was enriched 6.7-fold by tubulin-affinity chromatography. In a cosedimentation assay, p90 bound and decorated microtubules. In immunofluorescence microscopy of intact tobacco BY-2 cells or lysed protoplasts, p90 colocalized with cortical microtubules, and taxol-induced microtubule bundling was accompanied by corresponding reorganization of p90. Labeling of p90 remained along the plasma membrane when microtubules were depolymerized, although detergent extraction abolished the labeling. Therefore, p90 is a specialized PLD that associates with membranes and microtubules, possibly conveying hormonal and environmental signals to the microtubule cytoskeleton.

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...