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Cytoskeletal regulation of epidermal pavement cell morphogenesis. ( a ) Schematic illustration representing a two-dimensional projection of the outer half of an expanding pavement cell. Actin filaments are shown in orange, with cortical actin around the cell periphery and cytoplasmic actin cables permeating the cytoplasm; cortical microtubules are shown in green. For the sake of clarity, cortical actin on the outer face of the cell is not shown. ( b ) Model schematically illustrating regulation of actin polymerization and microtubule organization in expanding pavement cells by ROPs and two ROP-interacting RIC proteins (based on data presented in Fu et al. 2005).
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The cytoskeleton plays important roles in plant cell shape determination by influencing the patterns in which cell wall materials are deposited. Cortical microtubules are thought to orient the direction of cell expansion primarily via their influence on the deposition of cellulose into the wall, although the precise nature of the microtubule-cellul...
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... trichome branch1 mutations disrupt one member of this family, SCAR2 , producing trichome morphology defects and associated alterations in the F-actin cytoskeleton similar to (although less severe than) those observed in Arp2/3 complex subunit mutants (Basu et al. 2005, Zhang et al. 2005b). Thus, SCAR2 is implicated to play an important role in activation of the Arp2/3 complex in expanding trichomes. However, since trichome morphology defects in scar2 mutants are less severe than those of Arp2/3 complex subunit mutants, other activators must also be involved, perhaps including other members of the Arabidopsis SCAR family. Homologs of Sra1, Nap1, Abi, and HSPC300 are also present in Arabidopsis ; recent analyses of these proteins and the cor- responding genes strongly suggest that, as in mammalian cells, these proteins form a complex that is essential for SCAR-mediated activation of the putative Arabidopsis Arp2/3 complex ( Figure 4 b ). Arabidopsis pirogi and gnarled mutations affecting SRA1 and NAP1, respectively, produce trichome morphology defects and alterations in the F-actin cytoskeleton of expanding trichomes similar to those observed in Arp2/3 complex subunit mutants (Brembu et al. 2004, Basu et al. 2004, El-Assal et al. 2004a, Li et al. 2004, Zimmerman et al. 2004). HSPC300 is the mammalian homolog of BRICK1 ( BRK1 ), originally discov- ered because of its essential role in formation of localized cortical F-actin enrichments and epidermal pavement cell lobe formation in maize (Frank & Smith 2002). Like mutations in SCAR2 , NAP1 , and SRA1 , mutations disrupting Arabidopsis BRK1 result in trichome morphology defects and alterations in the F- actin cytoskeleton similar to those of Arp2/3 complex subunit mutants (L.G. Smith, unpublished observation; Szymanski 2005). Analyses of double mutants lacking both an Arp2/3 complex subunit and NAP1, BRK1, or SCAR2 provide genetic evidence that all three of these putative SCAR complex components act in the same pathway with the putative Arp2/3 complex (Deeks et al. 2004; Basu et al. 2005; L.G. Smith, unpublished observation). Moreover, Arabidopsis SRA1 and NAP1 interact with each other in the yeast two-hybrid system (Basu et al. 2004, El Assal et al. 2004b), and Arabidopsis BRK1 binds directly to the N- terminal Scar homology domains of Arabidopsis SCAR1, SCAR2, and SCAR3 (Frank et al. 2004, Zhang et al. 2005b). Although no genetic evidence has yet been reported supporting the expectation that Arabidopsis homologs of Abi proteins also function to activate the putative Arp2/3 complex, one member of the family of four predicted Abi-related proteins in Arabidopsis has recently been shown to interact with the Scar homology domain of Arabidopsis SCAR2 in the yeast two-hybrid system (Basu et al. 2005). Given that the mammalian WAVE complex is activated by Rac (Eden et al. 2002, Innocenti et al. 2004, Steffen et al. 2004), an obvious question is whether ROPs function to activate the putative Arabidopsis SCAR complex. ROP2 is a member of the ROP family that is expressed in developing leaves and as discussed in detail below, it plays a critical role in the spatial regulation of epidermal pavement cell expansion owing in part to its ability to stimulate localized actin polymerization. The idea that ROP2 is similarly involved in trichome morphogenesis is supported by the finding that expression of a constitutively active version of ROP2 causes a mild distorted trichome phenotype (Fu et al. 2002). Support- ing the notion that ROP2 activates the putative Arabidopsis SCAR complex, ROP2 interacts with Arabidopsis SRA1 (homologous to the Rac-binding component of the mammalian WAVE complex) in the yeast two-hybrid system (Basu et al. 2004). Interestingly, the Arabidopsis SPIKE1 (SPK1) protein, which is required for the formation of trichome branches as well as for normal epidermal pavement cell morphogenesis, contains a domain found in a class of unconventional guanine nucleotide exchange factors that stimulate the GTPase activity of Rho family GTPases in animal cells (Qiu et al. 2002, Brugnera et al. 2002, Meller et al. 2002). Thus, SPK1 may function to activate ROP2 in developing trichomes and pavement cells. Further work will be needed to establish whether, as illustrated in Figure 4 b , ROP2 activates the putative Arabidopsis SCAR complex, as well as to elucidate the po- tential role of SPK1 in that activation. In any case, other proteins are likely to be involved in regulation of the putative Arabidopsis SCAR complex. For example, the mammalian WAVE complex is activated by Nck as well as by Rac, and this activation is thought to involve binding of Nck to Nap1 (Eden et al. 2002). Thus, an as yet unidentified Arabidopsis Nck ortholog may interact with NAP1 to activate the putative SCAR complex ( Figure 4 b ). Unspecialized leaf epidermal cells (so-called pavement cells) are an interesting case study in cytoskeletal regulation of cell growth pattern. As illustrated in Figure 5 a for Arabidopsis , epidermal pavement cells of most flowering plant species have lobed morphologies. The lobes of each pavement cell interdigitate with those of its nearest neighbors to form an in- terlocking cellular array. Thus, pavement cells not only acquire complex shapes, but they do so in a manner involving coordination of growth patterns between adjacent cells. Studies on the cytoskeletal basis of pavement cell morphogenesis have shown that microtubules are required for lobe formation and that they tend to be organized into parallel bundles in areas of the cell periphery where lobes are not emerging ( Figure 5 a ; reviewed in Smith 2003). Thus, microtubules have been thought to contribute to lobe formation by constrain- ing cell expansion between lobes. Recent observations indicate that actin also plays a critical role in the spatial regulation of pavement cell growth. In expanding leaf epidermal pavement cells, localized accumulations of dense, fine cortical F-actin are found at sites of lobe outgrowth in both maize and Arabidopsis ( Figure 5 a ). In maize, brk mutations eliminate the formation of these localized F-actin enrichments and also eliminate the formation of lobes (Frank & Smith 2002, Frank et al. 2003). As discussed above, BRK1 is the plant homolog of a mammalian WAVE complex component and is thereby implicated as a regulator of Arp2/3 complex–dependent actin polymerization, which therefore appears to be essential for pavement cell lobe formation in maize. The presence of dense, fine F-actin networks at sites of lobe outgrowth in pavement cells presents an intriguing parallel with tip- growing cells, where a related actin config- uration is observed near the growth site, as discussed above. In a further parallel with tip growth, two closely related members of the ROP GTPase family (ROP2 and ROP4) contribute to lobe outgrowth in part by stimulating localized F-actin assembly. Plasma membrane–localized ROP2-GFP is concentrated at sites of lobe outgrowth. Localized cortical F-actin accumulation and lobe outgrowth are both reduced in plants with im- paired ROP2 and ROP4 function, although cytoplasmic F-actin density and organization is normal (Fu et al. 2002, 2005). Conversely, expression of a constitutively active version of ROP2 results in delocalization of cortical F- actin accumulation and causes growth to be uniformly distributed as well. Interestingly, ROP2 and ROP4 have also been implicated in polarization of actin polymerization and growth in tip-growing root hairs (Molendijk et al. 2001, Jones et al. 2002). Moreover, like ROP1 in pollen tubes, ROP2-dependent localization of F-actin polymerization in expanding pavement cells involves an interaction with RIC4 (this interaction is discussed below in more detail). Thus, polarization of diffuse growth in pavement cells is mechanistically related to that in tip-growing cells. However, whether the contribution of ROP- dependent F-actin polymerization to growth polarization is the same in pavement cells and tip-growing cells is unclear. As discussed above, in pollen tubes exocytosis appears to be concentrated in the apical-most area of the tip where little or no F-actin is present. The subapical F-actin fringe, although important for vesicle delivery to and/or retention at the apex, has also been implicated in suppres- sion of exocytosis in the subapical area (e.g., Ketelaar et al. 2003) ( Figure 2 a ). In contrast, cortical F-actin is enriched at sites where exocytosis rates are presumably highest in expanding pavement cells, although patterns of exocytosis have not been directly examined. Although these findings may seem contradic- tory, there is evidence that cortical F-actin plays both inhibitory and stimulatory roles in exocytosis in neuroendocrine PC-12 cells (Lang et al. 2000). Thus, ROP-dependent actin polymerization may locally facilitate exocytosis in expanding pavement cells, and locally inhibit exocytosis in tip-growing cells. In any case, the effects of ROP2 and ROP4 on pavement cell morphogenesis are not lim- ited to their influence on F-actin polymerization. In plants with reduced ROP2 and ROP4 gene function, parallel bundles of transversely aligned microtubules are more broadly distributed throughout the cell cortex than they are in wild type (Fu et al. 2005). Conversely, expression of constitutively active ROP2 inhibits the formation of well-ordered arrays of cortical microtubules (Fu et al. 2002). Thus, ROP2 and ROP4 appear to have dual roles in promoting pavement cell lobe outgrowth: They locally activate F-actin polymerization at sites of lobe outgrowth and also suppress the formation of ordered arrays of transversely aligned cortical microtubule bundles in these areas. As discussed above, the observed interaction between ROP2 and Arabidopsis SRA1 suggests that ROP2 can activate the putative SCAR complex. However, pavement cell lobe outgrowth and localized F-actin accumulation are reduced considerably less ...
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... and ROP4 have also been implicated in polarization of actin polymerization and growth in tip-growing root hairs (Molendijk et al. 2001, Jones et al. 2002). Moreover, like ROP1 in pollen tubes, ROP2-dependent localization of F-actin polymerization in expanding pavement cells involves an interaction with RIC4 (this interaction is discussed below in more detail). Thus, polarization of diffuse growth in pavement cells is mechanistically related to that in tip-growing cells. However, whether the contribution of ROP- dependent F-actin polymerization to growth polarization is the same in pavement cells and tip-growing cells is unclear. As discussed above, in pollen tubes exocytosis appears to be concentrated in the apical-most area of the tip where little or no F-actin is present. The subapical F-actin fringe, although important for vesicle delivery to and/or retention at the apex, has also been implicated in suppres- sion of exocytosis in the subapical area (e.g., Ketelaar et al. 2003) ( Figure 2 a ). In contrast, cortical F-actin is enriched at sites where exocytosis rates are presumably highest in expanding pavement cells, although patterns of exocytosis have not been directly examined. Although these findings may seem contradic- tory, there is evidence that cortical F-actin plays both inhibitory and stimulatory roles in exocytosis in neuroendocrine PC-12 cells (Lang et al. 2000). Thus, ROP-dependent actin polymerization may locally facilitate exocytosis in expanding pavement cells, and locally inhibit exocytosis in tip-growing cells. In any case, the effects of ROP2 and ROP4 on pavement cell morphogenesis are not lim- ited to their influence on F-actin polymerization. In plants with reduced ROP2 and ROP4 gene function, parallel bundles of transversely aligned microtubules are more broadly distributed throughout the cell cortex than they are in wild type (Fu et al. 2005). Conversely, expression of constitutively active ROP2 inhibits the formation of well-ordered arrays of cortical microtubules (Fu et al. 2002). Thus, ROP2 and ROP4 appear to have dual roles in promoting pavement cell lobe outgrowth: They locally activate F-actin polymerization at sites of lobe outgrowth and also suppress the formation of ordered arrays of transversely aligned cortical microtubule bundles in these areas. As discussed above, the observed interaction between ROP2 and Arabidopsis SRA1 suggests that ROP2 can activate the putative SCAR complex. However, pavement cell lobe outgrowth and localized F-actin accumulation are reduced considerably less in Arp2/3 complex subunit mutants (Li et al. 2003) than they are in plants with reduced ROP2/4 function (Fu et al. 2002, 2005). Thus, in expanding pavement cells, activation of the putative SCAR complex apparently cannot be the only pathway through which ROP2 acts to stimulate F-actin assembly. Indeed, recent work analyzing interactions between ROP2 and CRIB-domain-containing RIC proteins has shown that, as for ROP1 in pollen tubes, ROP2/4 stimulates cortical F-actin assembly in expanding pavement cells via interaction with RIC4 (Fu et al. 2005). GFP-RIC4 is localized to sites of incipient lobe formation and lobe tips in young pavement cells, and this localization pattern is dependent upon ROP2/4 activity. Moreover, loss of RIC4 activity results in reduced accumulation of fine cortical F-actin and reduced outgrowth of lobes. Interestingly, ROP2/4-mediated suppres- sion of the formation of well-ordered cortical microtubule bundles also involves another CRIB-domain-containing protein, RIC1 (Fu et al. 2005). Similar to what was observed in plants with reduced ROP2/4 function, RIC1 overexpression causes transversely aligned microtubule bundles to form along the entire length of expanding pavement cells and reduces lobe outgrowth. Conversely, in ric1 loss- of-function mutants, cortical microtubules are fewer, less bundled, and less well-ordered than they are in the neck regions of expanding wild-type pavement cells, and excess expansion of neck regions occurs. RIC1-GFP colocalizes with cortical microtubules; this localization is inhibited by expression of constitutively active ROP2, but is increased in mutants with reduced ROP2/4 function. Thus, RIC1 appears to mediate the formation of ordered arrays of transversely aligned cortical microtubules via a direct association with microtubules, and ROP2/4 activity inhibits this function of RIC1. RIC1 activity, in turn, suppresses cortical F-actin accumulation by in- hibiting the interaction between ROP2 and RIC4. This effect of RIC1 is likely to be mediated by microtubules themselves because depolymerization of microtubules by oryzalin treatment or by shifting mor1-1 mutants to restrictive temperature enhances the RIC4- ROP2 interaction and increases cortical F- actin accumulation (Fu et al. 2005). Together, these observations support the following model to explain the patterning of pavement cell growth via ROP-dependent activities of RIC1 and RIC4 ( Figure 5 b ). Local enrichment of ROP2/4 activity at sites of lobe emergence promotes RIC4-dependent activation of cortical F-actin assembly and simul- taneously suppresses RIC1-dependent formation of well-ordered cortical microtubule arrays in these areas. These effects of ROP2/4 cooperatively promote lobe outgrowth. Between sites of lobe emergence where ROP2/4 and RIC4 are less abundant, RIC1-dependent formation of transversely aligned cortical microtubule bundles can take place. Promotion of cortical microtubule alignment by RIC1 is self-reinforcing because the resulting microtubule arrays inhibit the ROP2/RIC4 interaction, further reducing the inhibition of RIC1 activity in neck regions. RIC1-dependent cortical microtubule arrays restrict cell expansion between lobes, amplifying the difference in growth rates between areas of the cell surface where lobes are emerging and neck regions between these lobes. This model goes a long way toward explaining cytoskeletal regulation of pavement cell growth pattern, but the question remains open as to what initially deter- mines the sites where ROP2/4 will become enriched. Because growth patterns of adjacent cells must be coordinated, it seems likely that the initial localization of ROP2/4 enrichment sites depends on some form of cell-cell communication. Thus, important questions remain to be answered regarding the coordination of growth patterns among neighboring pavement cells. In recent years, dramatic advances have been made in our understanding of mechanisms regulating cytoskeletal dynamics and organization that are important for plant cell shape determination. These advances have come from studies combining tools of genetics, ge- nomics, molecular biology, cell biology, and biochemistry. However, much remains to be learned. For example, studies of the putative plant Arp2/3 complex have made it clear that the majority of F-actin in plant cells is nucleated in an Arp2/3-independent manner. Formins, which constitute a family of 21 predicted proteins in Arabidopsis (Deeks et al. 2002), are likely to serve as the primary F-actin nucleators in plant cells, but have only begun to be studied. A multitude of microtubule and actin-binding proteins are known to be important for cell growth and its spatial regulation in plants, but their precise roles remain to be elucidated. Another area still awaiting major breakthroughs is that of understanding how cytoskeletal filaments promote or ...
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... family. Homologs of Sra1, Nap1, Abi, and HSPC300 are also present in Arabidopsis ; recent analyses of these proteins and the cor- responding genes strongly suggest that, as in mammalian cells, these proteins form a complex that is essential for SCAR-mediated activation of the putative Arabidopsis Arp2/3 complex ( Figure 4 b ). Arabidopsis pirogi and gnarled mutations affecting SRA1 and NAP1, respectively, produce trichome morphology defects and alterations in the F-actin cytoskeleton of expanding trichomes similar to those observed in Arp2/3 complex subunit mutants (Brembu et al. 2004, Basu et al. 2004, El-Assal et al. 2004a, Li et al. 2004, Zimmerman et al. 2004). HSPC300 is the mammalian homolog of BRICK1 ( BRK1 ), originally discov- ered because of its essential role in formation of localized cortical F-actin enrichments and epidermal pavement cell lobe formation in maize (Frank & Smith 2002). Like mutations in SCAR2 , NAP1 , and SRA1 , mutations disrupting Arabidopsis BRK1 result in trichome morphology defects and alterations in the F- actin cytoskeleton similar to those of Arp2/3 complex subunit mutants (L.G. Smith, unpublished observation; Szymanski 2005). Analyses of double mutants lacking both an Arp2/3 complex subunit and NAP1, BRK1, or SCAR2 provide genetic evidence that all three of these putative SCAR complex components act in the same pathway with the putative Arp2/3 complex (Deeks et al. 2004; Basu et al. 2005; L.G. Smith, unpublished observation). Moreover, Arabidopsis SRA1 and NAP1 interact with each other in the yeast two-hybrid system (Basu et al. 2004, El Assal et al. 2004b), and Arabidopsis BRK1 binds directly to the N- terminal Scar homology domains of Arabidopsis SCAR1, SCAR2, and SCAR3 (Frank et al. 2004, Zhang et al. 2005b). Although no genetic evidence has yet been reported supporting the expectation that Arabidopsis homologs of Abi proteins also function to activate the putative Arp2/3 complex, one member of the family of four predicted Abi-related proteins in Arabidopsis has recently been shown to interact with the Scar homology domain of Arabidopsis SCAR2 in the yeast two-hybrid system (Basu et al. 2005). Given that the mammalian WAVE complex is activated by Rac (Eden et al. 2002, Innocenti et al. 2004, Steffen et al. 2004), an obvious question is whether ROPs function to activate the putative Arabidopsis SCAR complex. ROP2 is a member of the ROP family that is expressed in developing leaves and as discussed in detail below, it plays a critical role in the spatial regulation of epidermal pavement cell expansion owing in part to its ability to stimulate localized actin polymerization. The idea that ROP2 is similarly involved in trichome morphogenesis is supported by the finding that expression of a constitutively active version of ROP2 causes a mild distorted trichome phenotype (Fu et al. 2002). Support- ing the notion that ROP2 activates the putative Arabidopsis SCAR complex, ROP2 interacts with Arabidopsis SRA1 (homologous to the Rac-binding component of the mammalian WAVE complex) in the yeast two-hybrid system (Basu et al. 2004). Interestingly, the Arabidopsis SPIKE1 (SPK1) protein, which is required for the formation of trichome branches as well as for normal epidermal pavement cell morphogenesis, contains a domain found in a class of unconventional guanine nucleotide exchange factors that stimulate the GTPase activity of Rho family GTPases in animal cells (Qiu et al. 2002, Brugnera et al. 2002, Meller et al. 2002). Thus, SPK1 may function to activate ROP2 in developing trichomes and pavement cells. Further work will be needed to establish whether, as illustrated in Figure 4 b , ROP2 activates the putative Arabidopsis SCAR complex, as well as to elucidate the po- tential role of SPK1 in that activation. In any case, other proteins are likely to be involved in regulation of the putative Arabidopsis SCAR complex. For example, the mammalian WAVE complex is activated by Nck as well as by Rac, and this activation is thought to involve binding of Nck to Nap1 (Eden et al. 2002). Thus, an as yet unidentified Arabidopsis Nck ortholog may interact with NAP1 to activate the putative SCAR complex ( Figure 4 b ). Unspecialized leaf epidermal cells (so-called pavement cells) are an interesting case study in cytoskeletal regulation of cell growth pattern. As illustrated in Figure 5 a for Arabidopsis , epidermal pavement cells of most flowering plant species have lobed morphologies. The lobes of each pavement cell interdigitate with those of its nearest neighbors to form an in- terlocking cellular array. Thus, pavement cells not only acquire complex shapes, but they do so in a manner involving coordination of growth patterns between adjacent cells. Studies on the cytoskeletal basis of pavement cell morphogenesis have shown that microtubules are required for lobe formation and that they tend to be organized into parallel bundles in areas of the cell periphery where lobes are not emerging ( Figure 5 a ; reviewed in Smith 2003). Thus, microtubules have been thought to contribute to lobe formation by constrain- ing cell expansion between lobes. Recent observations indicate that actin also plays a critical role in the spatial regulation of pavement cell growth. In expanding leaf epidermal pavement cells, localized accumulations of dense, fine cortical F-actin are found at sites of lobe outgrowth in both maize and Arabidopsis ( Figure 5 a ). In maize, brk mutations eliminate the formation of these localized F-actin enrichments and also eliminate the formation of lobes (Frank & Smith 2002, Frank et al. 2003). As discussed above, BRK1 is the plant homolog of a mammalian WAVE complex component and is thereby implicated as a regulator of Arp2/3 complex–dependent actin polymerization, which therefore appears to be essential for pavement cell lobe formation in maize. The presence of dense, fine F-actin networks at sites of lobe outgrowth in pavement cells presents an intriguing parallel with tip- growing cells, where a related actin config- uration is observed near the growth site, as discussed above. In a further parallel with tip growth, two closely related members of the ROP GTPase family (ROP2 and ROP4) contribute to lobe outgrowth in part by stimulating localized F-actin assembly. Plasma membrane–localized ROP2-GFP is concentrated at sites of lobe outgrowth. Localized cortical F-actin accumulation and lobe outgrowth are both reduced in plants with im- paired ROP2 and ROP4 function, although cytoplasmic F-actin density and organization is normal (Fu et al. 2002, 2005). Conversely, expression of a constitutively active version of ROP2 results in delocalization of cortical F- actin accumulation and causes growth to be uniformly distributed as well. Interestingly, ROP2 and ROP4 have also been implicated in polarization of actin polymerization and growth in tip-growing root hairs (Molendijk et al. 2001, Jones et al. 2002). Moreover, like ROP1 in pollen tubes, ROP2-dependent localization of F-actin polymerization in expanding pavement cells involves an interaction with RIC4 (this interaction is discussed below in more detail). Thus, polarization of diffuse growth in pavement cells is mechanistically related to that in tip-growing cells. However, whether the contribution of ROP- dependent F-actin polymerization to growth polarization is the same in pavement cells and tip-growing cells is unclear. As discussed above, in pollen tubes exocytosis appears to be concentrated in the apical-most area of the tip where little or no F-actin is present. The subapical F-actin fringe, although important for vesicle delivery to and/or retention at the apex, has also been implicated in suppres- sion of exocytosis in the subapical area (e.g., Ketelaar et al. 2003) ( Figure 2 a ). In contrast, cortical F-actin is enriched at sites where exocytosis rates are presumably highest in expanding pavement cells, although patterns of exocytosis have not been directly examined. Although these findings may seem contradic- tory, there is evidence that cortical F-actin plays both inhibitory and stimulatory roles in exocytosis in neuroendocrine PC-12 cells (Lang et al. 2000). Thus, ROP-dependent actin polymerization may locally facilitate exocytosis in expanding pavement cells, and locally inhibit exocytosis in tip-growing cells. In any case, the effects of ROP2 and ROP4 on pavement cell morphogenesis are not lim- ited to their influence on F-actin polymerization. In plants with reduced ROP2 and ROP4 gene function, parallel bundles of transversely aligned microtubules are more broadly distributed throughout the cell cortex than they are in wild type (Fu et al. 2005). Conversely, expression of constitutively active ROP2 inhibits the formation of well-ordered arrays of cortical microtubules (Fu et al. 2002). Thus, ROP2 and ROP4 appear to have dual roles in promoting pavement cell lobe outgrowth: They locally activate F-actin polymerization at sites of lobe outgrowth and also suppress the formation of ordered arrays of transversely aligned cortical microtubule bundles in these areas. As discussed above, the observed interaction between ROP2 and Arabidopsis SRA1 suggests that ROP2 can activate the putative SCAR complex. However, pavement cell lobe outgrowth and localized F-actin accumulation are reduced considerably less in Arp2/3 complex subunit mutants (Li et al. 2003) than they are in plants with reduced ROP2/4 function (Fu et al. 2002, 2005). Thus, in expanding pavement cells, activation of the putative SCAR complex apparently cannot be the only pathway through which ROP2 acts to stimulate F-actin assembly. Indeed, recent work analyzing interactions between ROP2 and CRIB-domain-containing RIC proteins has shown that, as for ROP1 in pollen tubes, ROP2/4 stimulates cortical F-actin assembly in expanding pavement cells via interaction with RIC4 (Fu et al. 2005). GFP-RIC4 is localized to sites of incipient lobe formation and lobe tips ...
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... and alterations in the F-actin cytoskeleton of expanding trichomes similar to those observed in Arp2/3 complex subunit mutants (Brembu et al. 2004, Basu et al. 2004, El-Assal et al. 2004a, Li et al. 2004, Zimmerman et al. 2004). HSPC300 is the mammalian homolog of BRICK1 ( BRK1 ), originally discov- ered because of its essential role in formation of localized cortical F-actin enrichments and epidermal pavement cell lobe formation in maize (Frank & Smith 2002). Like mutations in SCAR2 , NAP1 , and SRA1 , mutations disrupting Arabidopsis BRK1 result in trichome morphology defects and alterations in the F- actin cytoskeleton similar to those of Arp2/3 complex subunit mutants (L.G. Smith, unpublished observation; Szymanski 2005). Analyses of double mutants lacking both an Arp2/3 complex subunit and NAP1, BRK1, or SCAR2 provide genetic evidence that all three of these putative SCAR complex components act in the same pathway with the putative Arp2/3 complex (Deeks et al. 2004; Basu et al. 2005; L.G. Smith, unpublished observation). Moreover, Arabidopsis SRA1 and NAP1 interact with each other in the yeast two-hybrid system (Basu et al. 2004, El Assal et al. 2004b), and Arabidopsis BRK1 binds directly to the N- terminal Scar homology domains of Arabidopsis SCAR1, SCAR2, and SCAR3 (Frank et al. 2004, Zhang et al. 2005b). Although no genetic evidence has yet been reported supporting the expectation that Arabidopsis homologs of Abi proteins also function to activate the putative Arp2/3 complex, one member of the family of four predicted Abi-related proteins in Arabidopsis has recently been shown to interact with the Scar homology domain of Arabidopsis SCAR2 in the yeast two-hybrid system (Basu et al. 2005). Given that the mammalian WAVE complex is activated by Rac (Eden et al. 2002, Innocenti et al. 2004, Steffen et al. 2004), an obvious question is whether ROPs function to activate the putative Arabidopsis SCAR complex. ROP2 is a member of the ROP family that is expressed in developing leaves and as discussed in detail below, it plays a critical role in the spatial regulation of epidermal pavement cell expansion owing in part to its ability to stimulate localized actin polymerization. The idea that ROP2 is similarly involved in trichome morphogenesis is supported by the finding that expression of a constitutively active version of ROP2 causes a mild distorted trichome phenotype (Fu et al. 2002). Support- ing the notion that ROP2 activates the putative Arabidopsis SCAR complex, ROP2 interacts with Arabidopsis SRA1 (homologous to the Rac-binding component of the mammalian WAVE complex) in the yeast two-hybrid system (Basu et al. 2004). Interestingly, the Arabidopsis SPIKE1 (SPK1) protein, which is required for the formation of trichome branches as well as for normal epidermal pavement cell morphogenesis, contains a domain found in a class of unconventional guanine nucleotide exchange factors that stimulate the GTPase activity of Rho family GTPases in animal cells (Qiu et al. 2002, Brugnera et al. 2002, Meller et al. 2002). Thus, SPK1 may function to activate ROP2 in developing trichomes and pavement cells. Further work will be needed to establish whether, as illustrated in Figure 4 b , ROP2 activates the putative Arabidopsis SCAR complex, as well as to elucidate the po- tential role of SPK1 in that activation. In any case, other proteins are likely to be involved in regulation of the putative Arabidopsis SCAR complex. For example, the mammalian WAVE complex is activated by Nck as well as by Rac, and this activation is thought to involve binding of Nck to Nap1 (Eden et al. 2002). Thus, an as yet unidentified Arabidopsis Nck ortholog may interact with NAP1 to activate the putative SCAR complex ( Figure 4 b ). Unspecialized leaf epidermal cells (so-called pavement cells) are an interesting case study in cytoskeletal regulation of cell growth pattern. As illustrated in Figure 5 a for Arabidopsis , epidermal pavement cells of most flowering plant species have lobed morphologies. The lobes of each pavement cell interdigitate with those of its nearest neighbors to form an in- terlocking cellular array. Thus, pavement cells not only acquire complex shapes, but they do so in a manner involving coordination of growth patterns between adjacent cells. Studies on the cytoskeletal basis of pavement cell morphogenesis have shown that microtubules are required for lobe formation and that they tend to be organized into parallel bundles in areas of the cell periphery where lobes are not emerging ( Figure 5 a ; reviewed in Smith 2003). Thus, microtubules have been thought to contribute to lobe formation by constrain- ing cell expansion between lobes. Recent observations indicate that actin also plays a critical role in the spatial regulation of pavement cell growth. In expanding leaf epidermal pavement cells, localized accumulations of dense, fine cortical F-actin are found at sites of lobe outgrowth in both maize and Arabidopsis ( Figure 5 a ). In maize, brk mutations eliminate the formation of these localized F-actin enrichments and also eliminate the formation of lobes (Frank & Smith 2002, Frank et al. 2003). As discussed above, BRK1 is the plant homolog of a mammalian WAVE complex component and is thereby implicated as a regulator of Arp2/3 complex–dependent actin polymerization, which therefore appears to be essential for pavement cell lobe formation in maize. The presence of dense, fine F-actin networks at sites of lobe outgrowth in pavement cells presents an intriguing parallel with tip- growing cells, where a related actin config- uration is observed near the growth site, as discussed above. In a further parallel with tip growth, two closely related members of the ROP GTPase family (ROP2 and ROP4) contribute to lobe outgrowth in part by stimulating localized F-actin assembly. Plasma membrane–localized ROP2-GFP is concentrated at sites of lobe outgrowth. Localized cortical F-actin accumulation and lobe outgrowth are both reduced in plants with im- paired ROP2 and ROP4 function, although cytoplasmic F-actin density and organization is normal (Fu et al. 2002, 2005). Conversely, expression of a constitutively active version of ROP2 results in delocalization of cortical F- actin accumulation and causes growth to be uniformly distributed as well. Interestingly, ROP2 and ROP4 have also been implicated in polarization of actin polymerization and growth in tip-growing root hairs (Molendijk et al. 2001, Jones et al. 2002). Moreover, like ROP1 in pollen tubes, ROP2-dependent localization of F-actin polymerization in expanding pavement cells involves an interaction with RIC4 (this interaction is discussed below in more detail). Thus, polarization of diffuse growth in pavement cells is mechanistically related to that in tip-growing cells. However, whether the contribution of ROP- dependent F-actin polymerization to growth polarization is the same in pavement cells and tip-growing cells is unclear. As discussed above, in pollen tubes exocytosis appears to be concentrated in the apical-most area of the tip where little or no F-actin is present. The subapical F-actin fringe, although important for vesicle delivery to and/or retention at the apex, has also been implicated in suppres- sion of exocytosis in the subapical area (e.g., Ketelaar et al. 2003) ( Figure 2 a ). In contrast, cortical F-actin is enriched at sites where exocytosis rates are presumably highest in expanding pavement cells, although patterns of exocytosis have not been directly examined. Although these findings may seem contradic- tory, there is evidence that cortical F-actin plays both inhibitory and stimulatory roles in exocytosis in neuroendocrine PC-12 cells (Lang et al. 2000). Thus, ROP-dependent actin polymerization may locally facilitate exocytosis in expanding pavement cells, and locally inhibit exocytosis in tip-growing cells. In any case, the effects of ROP2 and ROP4 on pavement cell morphogenesis are not lim- ited to their influence on F-actin polymerization. In plants with reduced ROP2 and ROP4 gene function, parallel bundles of transversely aligned microtubules are more broadly distributed throughout the cell cortex than they are in wild type (Fu et al. 2005). Conversely, expression of constitutively active ROP2 inhibits the formation of well-ordered arrays of cortical microtubules (Fu et al. 2002). Thus, ROP2 and ROP4 appear to have dual roles in promoting pavement cell lobe outgrowth: They locally activate F-actin polymerization at sites of lobe outgrowth and also suppress the formation of ordered arrays of transversely aligned cortical microtubule bundles in these areas. As discussed above, the observed interaction between ROP2 and Arabidopsis SRA1 suggests that ROP2 can activate the putative SCAR complex. However, pavement cell lobe outgrowth and localized F-actin accumulation are reduced considerably less in Arp2/3 complex subunit mutants (Li et al. 2003) than they are in plants with reduced ROP2/4 function (Fu et al. 2002, 2005). Thus, in expanding pavement cells, activation of the putative SCAR complex apparently cannot be the only pathway through which ROP2 acts to stimulate F-actin assembly. Indeed, recent work analyzing interactions between ROP2 and CRIB-domain-containing RIC proteins has shown that, as for ROP1 in pollen tubes, ROP2/4 stimulates cortical F-actin assembly in expanding pavement cells via interaction with RIC4 (Fu et al. 2005). GFP-RIC4 is localized to sites of incipient lobe formation and lobe tips in young pavement cells, and this localization pattern is dependent upon ROP2/4 activity. Moreover, loss of RIC4 activity results in reduced accumulation of fine cortical F-actin and reduced outgrowth of lobes. Interestingly, ROP2/4-mediated suppres- sion of the formation of well-ordered cortical microtubule bundles also involves another CRIB-domain-containing protein, RIC1 (Fu et al. 2005). Similar to what was observed in plants with ...
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Literature data and results of the studies carried out us concerning the involvement of plant cell cytoskeleton in cellular mechanisms of metal toxicity are summarized. Characteristics of cytotoxic effect of metals on plant cytoskeleton and, in particular, on microtubules and actin filaments are reviewed. Particular attention is paid to cellular an...
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... Proper cellular auxin gradient formation depends on the correct localization of the auxin transporter proteins to the membrane aided by the intracellular trafficking of these proteins (Geldner et al. 2001;Muday and DeLong 2001;Muday and Murphy 2002;Rakusová, Fendrych, and Friml 2015). Actin filament is one of the essential factors for regulating cellular protein trafficking and cycling (Staiger and Schliwa 1987;An et al. 1996;Shimmen and Yokota 2004;Smith and Oppenheimer 2005;Staiger and Blanchoin 2006;Kandasamy, McKinney, and Meagher 2009;Pollard and Cooper 2009;Wang and Hussey 2015). Not surprisingly, intracellular trafficking of both PIN1 and PIN2 had been shown to be actin-dependent (Geldner et al. 2001;Rahman et al. 2007), highlighting the role of actin in optimal auxin gradient formation. ...
Lateral root (LR) organogenesis is regulated by cellular flux of auxin within pericycle cells, which depends on the membrane distribution and polar localization of auxin carrier proteins. The correct distribution of auxin carrier proteins relies on the intracellular trafficking of these proteins aided by filamentous actin as a track. However, the precise role of actin in lateral root development is still elusive. Here, using vegetative class actin isovariant mutants, we revealed that loss of actin isovariant ACT8 led to increased lateral root formation. The distribution of auxin within pericycle cells was altered in act8 mutant, primarily due to the altered distribution of AUX1 and PIN7. Interestingly, incorporation of act2 mutant in act8 background ( act2act8 ) effectively nullified the LR phenotype observed in act8 mutant, indicating that ACT2 plays an important role in LR development. To explore further, we investigated the possibility that the act8 mutant's LR phenotype and cellular auxin distribution resulted from ACT2 overexpression. Consistent with the idea, enhanced lateral root formation, altered AUX1, PIN7 expression, and auxin distribution in pericycle cells were observed in ACT2 overexpression lines. Collectively, these results suggest that actin isovariant ACT2 but not ACT8 plays a pivotal role in regulating source‐to‐sink auxin distribution during lateral root organogenesis.
... The plant actin cytoskeleton is irreplaceable for its role in the connection of proteomic functions to cellular life activities [3][4][5]. A multitude of cellular processes in plant cells relies on the cytoplasmic actin cytoskeleton, which is key to cell fission and expansion regulation, propelling the flow of cytoplasm, nurturing growth at the tips, forming and sustaining cell contours and directional properties and orchestrating organelle transport and relocation [6][7][8]. Moreover, the cytoskeleton frequently acts as a mediator of stress response in plants [9,10]. ...
ACTINs are key structural proteins in plants, which form the actin cytoskeleton and are engaged in numerous routine cellular processes. Meanwhile, ACTIN, recognized as a housekeeping gene, has not yet been thoroughly investigated in Brassica napus. The current research has led to the detection of 69 actin genes in B. napus, which were organized into six distinct subfamilies on the basis of phylogenetic relationships. Functional enrichment analysis, along with the construction of protein interaction networks, suggested that BnACTINs play roles in Preserving cell morphology and facilitating cytoplasmic movement, plant development, and adaptive responses to environmental stress. Moreover, the BnACTIN genes presented a wide range of expression levels among different tissues, whereas the majority experienced a substantial increase in expression when subjected to various abiotic stresses, demonstrating a pronounced sensitivity to abiotic factors. Furthermore, association mapping analysis indicated that some BnACTINs potentially affected certain key agronomic traits. Overall, our research deepens the knowledge of BnACTIN genes, promotes the cultivation of improved B. napus strains, and lays the groundwork for subsequent functional research.
... Proper cellular auxin gradient formation depends on the correct localization of the auxin transporter proteins to the membrane aided by the intracellular trafficking of these proteins (Muday and DeLong, 2001;Muday and Murphy, 2002). Actin filament is one of the essential factors for regulating cellular protein trafficking and cycling (Staiger and Schliwa, 1987;An et al., 1996;Shimmen and Yokota, 2004;Smith and Oppenheimer, 2005;Staiger and Blanchoin, 2006;Kandasamy et al., 2009;Pollard and Cooper, 2009). Not surprisingly, intracellular trafficking of both PIN1 and PIN2 had been shown to be actin dependent (Geldner et al., 2001, Rahman et al., 2007, highlighting the role of actin in optimal auxin gradient formation. ...
Lateral root (LR) organogenesis is regulated by cellular flux of auxin within pericycle cells, which depends on the membrane distribution and polar localization of auxin carrier proteins. The correct distribution of auxin carrier proteins relies on the intracellular trafficking of these proteins aided by filamentous actin as a track. However, the precise role of actin in lateral root development is still elusive. Here, using vegetative class actin isovariant mutants, we revealed that loss of actin isovariant ACT8 led to an increase in lateral root formation. The distribution of auxin within pericycle cells was altered in act8 mutant, primarily due to the altered distribution of AUX1 and PIN7. Interestingly, incorporation of act2 mutant in act8 background ( act2act8 ) effectively nullified the LR phenotype observed in act8 mutant, indicating that ACT2 plays an important role in LR development. To explore further, we investigated the possibility that the act8 mutant’s LR phenotype and cellular auxin distribution resulted from ACT2 overexpression. Consistent with the idea, enhanced lateral root formation, altered AUX1, PIN7 expression and auxin distribution in pericycle cells were observed in ACT2 overexpression lines. Collectively, these results suggest that actin isovariant ACT2 but not ACT8 plays a pivotal role in regulating source to sink auxin distribution during lateral root organogenesis.
... A prominent feature of leaf epidermis, among various vascular plant species, is the wavy anticlinal contour of ordinary epidermal cells, which has been reported in ferns, gymnosperms, dicots and monocots [11][12][13][14], in samples as early as in the Paleozoic era [15]. Although this feature exhibits variation among species [14,16], the basic morphogenetic scenario is common, relying on cortical microtubule organization and local cell wall differentiation [17,18]. After the cessation of divisions, protodermal cells, either rectangular or polygonal, grow differentially to become wavy ordinary epidermal cells, making the epidermis look like a "jigsaw-puzzle" [17][18][19]. ...
... Although this feature exhibits variation among species [14,16], the basic morphogenetic scenario is common, relying on cortical microtubule organization and local cell wall differentiation [17,18]. After the cessation of divisions, protodermal cells, either rectangular or polygonal, grow differentially to become wavy ordinary epidermal cells, making the epidermis look like a "jigsaw-puzzle" [17][18][19]. During the above progression, cortical microtubules under the anticlinal walls are organized in bundles, while at the junctions of the external periclinal wall with the anticlinal walls they extend as radial arrays [17][18][19]. ...
... After the cessation of divisions, protodermal cells, either rectangular or polygonal, grow differentially to become wavy ordinary epidermal cells, making the epidermis look like a "jigsaw-puzzle" [17][18][19]. During the above progression, cortical microtubules under the anticlinal walls are organized in bundles, while at the junctions of the external periclinal wall with the anticlinal walls they extend as radial arrays [17][18][19]. These microtubule systems define the sites where cell walls are reinforced by local thickenings, the cellulose microfibrils of which follow the orientation of the underlying microtubules [12,14,17]. ...
The ordinary epidermal cells of various vascular plants are characterized by wavy anticlinal wall contours. This feature has not yet been reported in multicellular algal species. Here, we found that, in the leaf-like blades of the brown alga Sargassum vulgare, epidermal cells exhibit prominent waviness. Initially, the small meristodermal cells exhibit straight anticlinal contour, which during their growth becomes wavy, in a pattern highly reminiscent of that found in land plants. Waviness is restricted close to the external periclinal wall, while at inner levels the anticlinal walls become thick and even. The mechanism behind this shape relies on cortical F-actin organization. Bundles of actin filaments are organized, extending under the external periclinal wall and connecting its junctions with the anticlinal walls, constituting an interconnected network. These bundles define the sites of local thickening deposition at the anticlinal/periclinal wall junctions. These thickenings are interconnected by cellulose microfibril extensions under the external periclinal wall. Apart from the wavy anticlinal contour, outward protrusions also arise on the external periclinal wall, thus the whole epidermis exhibits a quilted appearance. Apart from highlighting a new role for F-actin in cell shaping, the comparison of this morphogenetic mechanism to that of vascular plants reveals a case of evolutionary convergence among photosynthetic organisms.
... The orientation and dynamics of cortical MTs directly affect the direction of cell expansion, which determines cell shape (Smith & Oppenheimer, 2005). We therefore first observed cortical MT arrays in the inner epidermal cells of immature lemmas from transgenic WT and Sng rice expressing an eGFP-b-tubulin fusion protein. ...
Grain notching is a common deformation that decreases rice (Oryza sativa) quality; however, the underlying molecular basis causing grain notching remains unclear.
We report mechanisms underlying grain notching in Small and notched grain (Sng) mutants, which contained an arginine to histidine substitution at amino acid position 422 (R422H) of the α‐tubulin protein OsTUBA3. The R422H mutation decreased cell length and increased cell width/height of glumes and caryopses, but led to elongated caryopses compressed within shortened glumes, thus giving rise to notched and small grains. Glume and caryopsis cells had different dimensional orientations relative to the directions of organ elongation. Thus, the abnormal cell expansion induced in glumes and caryopses by the R422H mutation had different effects on elongation of these organs.
The R422H mutation in OsTUBA3 compromised β‐tubulin binding and led to formation of defective heterodimers. This in turn affected tubulin incorporation and microtubule (MT) nucleation and regrowth, consequently leading to MT instability and reducing the transverse orientation. The defective MT dynamics affected cell expansion and shape, causing different alterations in glume and caryopsis dimensions and resulting in grain notching.
These data indicate that Arg422 in OsTUBA3 is crucial for MT dynamics and that substitution with His causes grain notching, reducing grain quality and yield. These findings offer valuable insights into the molecular regulation underlying grain development in rice.
... In plants signaling mechanisms modulating ABPs are emerging but remained poorly understood. Plantspecific ROP (Rho-like GTPase from plants) subfamily of the conserved Rho family of small GTPases, well known conserved molecular switch that control actin organization and dynamic in eukaryotes, were among the first signaling proteins known to regulate actin organization and dynamics in plants (Smith and Oppenheimer, 2005;Yang, 2008；Fu et al., 2005Fu et al., 2002;Fu et al., 2009;Humphries et al., 2011). In Arabidopsis, auxin promotes formation of the puzzle shape of leaf pavement cells by activating the antagonistic ROP2/ROP4 and ROP6 signaling pathways (Fu et al., 2005;Xu 2010). ...
Actin dynamic is critical for cell morphogenesis in plants, but the signaling mechanisms underlying its regulation are not well understood. Here we found PRL1 ( P leiotropic R egulatory L ocus1) modulates leaf pavement cell (PC) morphogenesis in Arabidopsis by maintaining the dynamic homeostasis of actin microfilaments (MF). Our previous studies indicated PC shape formation was mediated by the counteracting ROP2 and ROP6 signaling pathways that promote the organization of cortical MF and microtubules (MT), respectively. Our genetic screen for ROP6 enhancers identified prl1 alleles. Genetic analysis suggested that prl1 acted synergistically with ROP2 and ROP6 in regulation of PC morphogenesis. We further found that the activities of ROP2 and ROP6 were increased and decreased in prl1 mutants, respectively. Interestingly prl1 was found to prefer to depolymerize MF independent of ROP2 and ROP6. Stress (high salinity and low temperature) induced similar changes of ROP activities as do prl1 mutations. Together our findings provided evidence that PRL1 governed two signaling pathways that counteractively maintain actin dynamics and resultant cell morphogenesis.
... At the late stage, the extreme tip of the pollen tube is devoid of large organelles, including vacuoles, whereas fine, thread-shaped vacuolar strands are observed in the shank of the pollen tube ( Figure 4A). Moreover, the more basal part of the pollen tube is occupied by a large vacuole ( Figure 4A) [38][39][40]. ...
Large vacuoles are a predominant cell organelle throughout the plant body. They maximally account for over 90% of cell volume and generate turgor pressure that acts as a driving force of cell growth, which is essential for plant development. The plant vacuole also acts as a reservoir for sequestering waste products and apoptotic enzymes, thereby enabling plants to rapidly respond to fluctuating environments. Vacuoles undergo dynamic transformation through repeated enlargement, fusion, fragmentation, invagination, and constriction, eventually resulting in the typical 3-dimensional complex structure in each cell type. Previous studies have indicated that such dynamic transformations of plant vacuoles are governed by the plant cytoskeletons, which consist of F-actin and microtubules. However, the molecular mechanism of cytoskeleton-mediated vacuolar modifications remains largely unclear. Here we first review the behavior of cytoskeletons and vacuoles during plant development and in response to environmental stresses, and then introduce candidates that potentially play pivotal roles in the vacuole–cytoskeleton nexus. Finally, we discuss factors hampering the advances in this research field and their possible solutions using the currently available cutting-edge technologies.
... The main characteristic of apical growth is the expansion of a single region of the wall independently of the rest of cell. This extremely polarized type of growth is mediated and directed by the cytoskeleton (Wasteneys and Galway, 2003;Smith and Oppenheimer, 2005;Li et al., 2017). Cells with apical growth usually have microtubules in longitudinal or slightly helical organization in the cortical and endoplasmic cell regions. ...
Laticifers are secretory structures that produce latex, forming a specialized defense system against herbivory. Studies using anatomical approaches to investigate laticifer growth patterns have described their origin; however, their mode of growth, i.e., whether growth is intrusive or diffuse, remains unclear. Studies investigating how cytoskeleton filaments may influence laticifer shape establishment and growth patterns are lacking. In this study, we combined microtubule immunostaining and developmental anatomy to investigate the growth patterns in different types of laticifers. Standard anatomical methods were used to study laticifer development. Microtubules were labelled through immunolocalization of α-tubulin in three types of laticifers from three different plant species: nonanastomosing (Urvillea ulmacea), anastomosing unbranched with partial degradation of terminal cell walls (Ipomoea nil), and anastomosing branched laticifers with early and complete degradation of terminal cell walls (Asclepias curassavica). In both nonanastomosing and anastomosing laticifers, as well as in differentiating meristematic cells, parenchyma cells and idioblasts, microtubules were perpendicularly aligned to the cell growth axis. The analyses of laticifer microtubule orientation revealed an arrangement that corresponds to those cells that grow diffusely within the plant body. Nonanastomosing and anastomosing laticifers, branched or not, have a pattern which indicates diffuse growth. This innovative study on secretory structures represents a major advance in the knowledge of laticifers and their growth mode.
... Filamentous actin (F-actin) plays important roles in cell expansion by influencing the patterns in which cell wall materials are deposited (Smith & Oppenheimer, 2005). The application of actindepolymerizing drugs, latrunculins, or cytochalasins arrested cell growth (Hepler et al, 2001). ...
Plant cell expands via a tip growth or diffuse growth mode. In plants, RabA is the largest group of Rab GTPases that regulate vesicle trafficking. The functions of RabA protein in modulating polarized expansion in tip growth cells have been demonstrated. However, whether and how RabA protein functions in diffuse growth plant cells have never been explored. Here, we addressed this question by examining the role of GhRabA4c in cotton fibers. GhRabA4c was preferentially expressed in elongating fibers with its protein localized to endoplasmic reticulum and Golgi apparatus. Over- and down-expression of GhRabA4c in cotton lead to longer and shorter fibers, respectively. GhRabA4c interacted with GhACT4 to promote the assembly of actin filament to facilitate vesicle transport for cell wall synthesis. Consistently, GhRabA4c -overexpressed fibers exhibited increased content of wall components and the transcript levels of the genes responsible for the synthesis of cell wall materials. We further identified two MYB proteins that directly regulate the transcription of GhRabA4c . Collectively, our data showed that GhRabA4c promotes diffused cell expansion by supporting vesicle trafficking and cell wall synthesis.
... Cortical microtubules orient the deposition of cellulose microfibrils during cell wall biosynthesis and often correlate with plant cell expansion and a particular cell morphology (Paredez et al., 2006;Crowell et al., 2009). Well-ordered microtubule arrays arranged transversely relative to the cell axis are associated with an increase in cell elongation but a restriction in radial cell expansion (Wasteneys, 2004;Smith and Oppenheimer, 2005). Throughout the cell cycle, the precise spatial-temporal regulation of microtubule organization and dynamics plays important roles in the formation, proper functioning, and structural orientation of these cytoskeletal structures. ...
The organization of the microtubule cytoskeleton is critical for cell and organ morphogenesis. The evolutionarily conserved microtubule‐severing enzyme KATANIN plays critical roles in microtubule organization in the plant and animal kingdoms. We previously used conical cell of Arabidopsis thaliana petals as a model system to investigate cortical microtubule organization and cell morphogenesis and determined that KATANIN promotes the formation of circumferential cortical microtubule arrays in conical cells. Here, we demonstrate that the conserved protein phosphatase PP2A interacts with and dephosphorylates KATANIN to promote the formation of circumferential cortical microtubule arrays in conical cells. KATANIN undergoes cycles of phosphorylation and dephosphorylation. Using co‐immunoprecipitation coupled with mass spectrometry, we identified PP2A subunits as KATANIN‐interacting proteins. Further biochemical studies showed that PP2A interacts with and dephosphorylates KATANIN to stabilize its cellular abundance. Similar to the katanin mutant, mutants for genes encoding PP2A subunits showed disordered cortical microtubule arrays and defective conical cell shape. Taken together, these findings identify PP2A as a regulator of conical cell shape and suggest that PP2A mediates KATANIN phospho‐regulation during plant cell morphogenesis.
