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Plant Physiology 138:571-572 (2005) © 2005 American Society of Plant Biologists On The InsideCytoskeleton Regulating ProteinsCytoskeletal elements, including microfilaments and microtubules, are highly dynamic structures whose assembly, disassembly, and organization are under the control of many regulatory proteins. For example, microtubule-associated proteins (MAPs) regulate the organization and dynamics of microtubules. In Arabidopsis (Arabidopsis thaliana), nine genes encode proteins of the evolutionarily conserved MAP65 family. Mao et al. (pp. 654–662) have examined two AtMAP65 proteins, AtMAP65-1 and AtMAP65-6, to determine whether they have distinct roles in their respective interactions with microtubules. They report that AtMAP65-1 promoted tubulin polymerization, induced the formation of large microtubule bundles by forming cross-bridges between microtubules, and increased the cold resistance of microtubules. In contrast, AtMAP65-6 did not promote tubulin polymerization and induced microtubules to form a mesh-like network with individual microtubules. AtMAP65-6 was also found to be associated with mitochondria in Arabidopsis cells and to render microtubules more resistant to high NaCl concentrations. Thus, these two MAP65 proteins were targeted to distinct sites and apparently perform distinct functions in Arabidopsis cells. Another example of cytoskeleton regulating proteins is formins, which are required for many actin-related processes, including cytokinesis and the maintenance of cell polarity. Based on the study of many organisms, formins appear to act by nucleating actin filaments. All formins contain a unique, highly conserved formin homology domain (FH2) that interacts with actin, as well as a proline-rich domain (FH1) that binds to the actin monomer-binding protein profilin. The Arabidopsis genome contains at least 21 formin genes that have been divided into two classes: group I and group II, defined by the presence or absence of an N-terminal transmembrane domain that does not appear in formins from other organisms. In this issue, Yi et al. (pp. 1071–1082) provide the first biochemical study of the function of conserved domains of a forming-like protein (AtFH8) from Arabidopsis. Fluorescent microscopy showed that profilin, in the presence of AtFH8(FH1FH2), facilitates nucleation of actin filaments. The overexpression of AtFH8 in Arabidopsis causes a prominent change in root hair cell development and actin organization, indicating the involvement of AtFH8 in polarized cell growth. Molecular Biology of Pollination The stigma and transmitting tract of the style play critical roles in triggering, promoting, and guiding the growth of pollen tubes toward their ovule targets. In crucifers, the stigma epidermis is also the major site for intraspecific and interspecific pollen recognition. The high degree of specificity in pollen-pistil interactions and the precision of directional pollen tube growth suggest that signals are continually being exchanged between pollen/pollen tubes and cells of the pistil that line their path. The specialized functions of stigma epidermal cells and transmitting tract cells are likely to depend on the activity of genes expressed specifically in these cells. Tung et al. (pp. 977–989) have used the Arabidopsis ATH1 microarray to compare the whole-genome transcriptional profiles of stigmas and ovaries isolated from wild-type Arabidopsis and from transgenic plants in which cells of the stigma epidermis and transmitting tract were specifically ablated by expression of a cellular toxin. Among the 23,000 genes represented on the array, they identified 115 and 34 genes predicted to be expressed specifically in stigma epidermis and transmitting tract, respectively. Although the biological roles of most of these genes have yet to be determined, it is likely that at least some of the papillar cell-specific genes have functions related to the development of the stigma epidermis in pollen recognition or in the promotion of adhesion, hydration, and germination of pollen grain. The transmitting tract-specific genes might function in the development of transmitting tract cells or in the promotion and guidance of the pollen tube. Dong et al. (pp. 778–787) examine the role of the small extracellular matrix (ECM) protein plantacyanin in the pollination process of Arabidopsis. Plantacyanins are ECM proteins of unknown function that belong to the ancient, plant-specific phytocyanins, a subfamily of blue copper proteins. The Arabidopsis genome contains a single plantacyanin gene. The authors present data that support the hypothesis that Arabidopsis plantacyanin may function in pollination since it is a component of the transmitting tract ECM and its overexpression in the pistil disrupts pollen tube guidance from the stigma to the style. It also appears to affect anther development since overexpression results in an indehiscent anther with a degenerated endothecium. Plantacyanin is not only expressed in the vegetative and reproductive sporophytic tissues but also in the female gametophyte. These findings suggest a role for plantacyanin both in anther development and in pollination in Arabidopsis. Auxin Transport and Leaf Venation Patterns The basipetal transport of auxin from the leaf margin directs leaf venation patterns. When this transport is inhibited either chemically or genetically, vein proliferation occurs near the leaf margin, suggesting that the leaf margin is a major source of auxin. In this issue, Clay and Nelson (pp. 767–777) describe a recessive mutant called thickvein (tkv) that develops thicker veins in leaves and in inflorescence stems. The increased vein thickness is attributable to an increased recruitment of cells into veins. Although floral development is normal, auxin transport in the inflorescence stem is significantly reduced, suggesting that a defect in auxin transport is responsible for the vascular phenotypes. The tkv mutation was found to reside in the ACL5 gene, which encodes a spermine synthase and whose expression is specific to provascular cells. Thus, it appears that ACL5/TKV is involved in vein definition (defining the boundaries between veins and nonvein regions) and in polar auxin transport and that polyamines are involved in this process. Polar auxin flow is believed to be essential for continuous vascular formation, and mutants that demonstrate discontinuities in vascular formation usually have defects in auxin transport. For example, the GNOM gene encodes a brefeldin A-sensitive ADP-ribosylation factor guanine-nucleotide exchange factor (ARF-GEF) that is responsible for recycling of the auxin transport protein PIN1. Mutation of the GNOM gene causes the formation of tracheary elements arranged in clusters or as single cells, instead of a formation lined up to form continuous strands. These results support the role of the vesicle trafficking system, which includes PIN1 and GNOM as being responsible for vascular formation. Recently, the VAN3 gene, whose mutation induces fragmented venation, has been shown to encode a unique type of ARF-GTPase-activating protein (GAP), in which the VAN3 protein is shown to locate in the trans-Golgi network (TGN). A phenotypic analysis of the van3 mutant suggests that the VAN3 ARF-GAP may play an important role in the vesicle transport responsible for the auxin signaling that is required for vascular differentiation. Sawa et al. (pp. 819–826) have isolated and characterized van3, an Arabidopsis mutant that exhibits a discontinuous vascular pattern. To elucidate the molecular nature controlling the vein patterning process through membrane trafficking, they examined VAN3-interacting proteins using a yeast two hybrid system and successfully identified the plant dynamin DRP1A as a VAN3 interacting protein. The spatial and temporal expression patterns of DRP1A::GUS and VAN3::GUS are very similar even at the subcellular level. drp1a showed a disconnected vascular network, and the drp1a mutation enhanced the phenotype of vascular discontinuity of the van3 mutant in the drp1a van3 double mutant. Furthermore, the drp1 mutation enhanced the discontinuous vascular pattern of the gnom mutant. These results indicate that DRP1 modulates the VAN3 function in vesicle budding from the TGN, which regulates vascular formation in Arabidopsis. GA Controls Cortex Formation The Arabidopsis root is usually described as having four concentric single-celled layers surrounding the central vascular tissue. From the outside of the root, these are the epidermis, cortex, endodermis, and pericycle. This radial pattern is established in the apical meristem, and SHR and SCR, which are members of the GRAS family of putative transcription factors, play a central role in this patterning process. However, as the Arabidopsis root ages, an additional layer of ground tissue, termed the middle cortex, forms by periclinal cell divisions and rapidly takes on a cortex-like character. Paquette and Benfey (pp. 636–640) demonstrate that that GA and SCR additively regulate the timing of formation of the middle cortex and that SHR is required independently of SCR for this developmental transition to occur. They propose a model in which the periclinal cell divisions that give rise to the middle cortex depend upon the activation of a hypothetical, GA-repressible "gene X" that is induced in the endodermis by SHR. A Constitutively Stressed Phosphoinositide Mutant There is accumulating evidence that phosphoinositide-derived signals are involved in plant stress responses. Salt, cold, and osmotically stressed plants accumulate the phosphoinositide PtdIns(4,5)P2, which can serve as a direct precursor to other signaling molecules, including Ins(1,4,5)P3. PtdIns(4,5)P2 signaling is terminated through the action of two types of phosphatases. The type II inositol polyphosphate 5-phosphatases (5PTases) act on both phosphoinositides and soluble inositol phosphates, while the SAC domain phosphatases are thought to act only on phosphoinositides. Arabidopsis encodes nine SAC domain phosphoinositide phosphatase-like proteins, falling into three different classes. Williams et al. (pp. 686–700) have identified and characterized mutants in the Arabidopsis SAC9 gene. The sac9 mutants constitutively express a systemic stressed phenotype, which may result from altered cellular signaling. As expected, both PtdIns(4,5)P2 and Ins(1,4,5)P3 levels were elevated in the sac9 mutants. Specific phenotypes displayed by the sac9 mutant plants include overexpression of stress-induced genes, elevated accumulation of reactive oxygen species, constitutively closed guard cells, and a slow-growing dwarf phenotype. All of these phenotypic features are consistent with the sac9 mutant being in a constitutively stressed condition.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900162. Related articles in Plant Physiol.:
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