Reduced expression of alpha-tubulin genes in Arabidopsis thaliana specifically affects root growth and morphology, root hair development and root gravitropism

Pelargonic acid's interaction with the auxin transporter PIN1: A potential mechanism behind its phytotoxic effects on plant metabolism

Pelargonic acid (PA) is a saturated fatty acid commonly found in several organisms, that is known for its phytotoxic effect and its use as bioherbicide for sustainable weed management. Although PA is already commercialised as bioherbicide, its molecular targets and mode of action is unknown according to the Herbicide Resistance Action Committee. Therefore, the aim of this work was focusing on the way this natural active substance impacts the plant metabolism of the model species Arabidopsis thaliana. PA caused increase of secondary and adventitious roots, as well as torsion, loss of gravitropism and phytotoxic effects. Moreover, PA altered the cellular arrangement and the PIN proteins activity. Computational simulations revealed that the intermolecular interactions between PA and the polar auxin transporter protein PIN1 are very similar to those established between the natural auxin IAA and PIN1. However, under intracellular conditions, the PA-PIN1 binding is more energetically stable than the IAA-PIN1. These results suggest that PA could act as an auxin-mimics bioherbicide. The exogenous application of PA would be responsible for the alterations observed both at structural and ultrastructural levels, which would be caused by the alteration on the transport of auxins into the plant, inducing root inhibition and ultimately total stop of root growth.

Genome-wide identification and evolution of the tubulin gene family in Camelina sativa

BACKGROUND

Tubulins play crucial roles in numerous fundamental processes of plant development. In flowering plants, tubulins are grouped into α-, β- and γ-subfamilies, while α- and β-tubulins possess a large isotype diversity and gene number variations among different species. This circumstance leads to insufficient recognition of orthologous isotypes and significantly complicates extrapolation of obtained experimental results, and brings difficulties for the identification of particular tubulin isotype function. The aim of this research is to identify and characterize tubulins of an emerging biofuel crop Camelina sativa.

RESULTS

We report comprehensive identification and characterization of tubulin gene family in C. sativa, including analyses of exon-intron organization, duplicated genes comparison, proper isotype designation, phylogenetic analysis, and expression patterns in different tissues. 17 α-, 34 β- and 6 γ-tubulin genes were identified and assigned to a particular isotype. Recognition of orthologous tubulin isotypes was cross-referred, involving data of phylogeny, synteny analyses and genes allocation on reconstructed genomic blocks of Ancestral Crucifer Karyotype. An investigation of expression patterns of tubulin homeologs revealed the predominant role of N6 (A) and N7 (B) subgenomes in tubulin expression at various developmental stages, contrarily to general the dominance of transcripts of H7 (C) subgenome.

CONCLUSIONS

For the first time a complete set of tubulin gene family members was identified and characterized for allohexaploid C. sativa species. The study demonstrates the comprehensive approach of precise inferring gene orthology. The applied technique allowed not only identifying C. sativa tubulin orthologs in model Arabidopsis species and tracking tubulin gene evolution, but also uncovered that A. thaliana is missing orthologs for several particular isotypes of α- and β-tubulins.

Chalkiness and premature controlled by energy homeostasis in OsNAC02 Ko-mutant during vegetative endosperm development

BACKGROUND

Chalkiness is a common phenotype induced by various reasons, such as abiotic stress or the imbalance of starch synthesis and metabolism during the development period. However, the reason mainly for one gene losing its function such as NAC (TFs has a large family in rice) which may cause premature is rarely known to us.

RESULTS

The Ko-Osnac02 mutant demonstrated an obviously early maturation stage compared to the wild type (WT) with 15 days earlier. The result showed that the mature endosperm of Ko-Osnac02 mutant exhibited chalkiness, characterized by white-core and white-belly in mature endosperm. As grain filling rate is a crucial factor in determining the yield and quality of rice (Oryza sativa, ssp. japonica), it's significant that mutant has a lower amylose content (AC) and higher soluble sugar content in the mature endosperm. Interestingly among the top DEGs in the RNA sequencing of N2 (3DAP) and WT seeds revealed that the OsBAM2 (LOC_Os10g32810) expressed significantly high in N2 mutant, which involved in Maltose up-regulated by the starch degradation. As Prediction of Protein interaction showed in the chalky endosperm formation in N2 seeds (3 DAP), seven genes were expressed at a lower-level which should be verified by a heatmap diagrams based on DEGs of N2 versus WT. The Tubulin genes controlling cell cycle are downregulated together with the MCM family genes MCM4 ( ↓), MCM7 ( ↑), which may cause white-core in the early endosperm development. In conclusion, the developing period drastically decreased in the Ko-Osnac02 mutants, which might cause the chalkiness in seeds during the early endosperm development.

CONCLUSIONS

The gene OsNAC02 which controls a great genetic co-network for cell cycle regulation in early development, and KO-Osnac02 mutant shows prematurity and white-core in endosperm.

Short-term exposition to acute Cadmium toxicity induces the loss of root gravitropic stimuli perception through PIN2-mediated auxin redistribution in Arabidopsis thaliana (L.) Heynh

Cadmium (Cd), one of the most widespread and water-soluble polluting heavy metals, has been widely studied on plants, even if the mechanisms underlying its phytotoxicity remain elusive. Indeed, most experiments are performed using extensive exposure time to the toxicants, not observing the primary targets affected. The present work studied Cd effects on Arabidopsis thaliana (L.) Heynh's root apical meristem (RAM) exposed for short periods (24h and 48h) to acute phytotoxic concentrations (100 and 150µM). The effects were studied through integrated morpho-histological, molecular, pharmacological and metabolomic analyses, highlighting that Cd inhibited primary root elongation by affecting the meristem zone via altering cell expansion. Moreover, Cd altered Auxin accumulation in RAM and affected PINs polar transporters particularly PIN2. In addition, we observed that high Cd concentration induced accumulation of reactive oxygen species (ROS) in roots, which resulted in an altered organization of cortical microtubules and the starch and sucrose metabolism, altering the statolith formation and, consequently, the gravitropic root response. Our results demonstrated that short Cd exposition (24h) affected cell expansion preferentially, altering auxin distribution and inducing ROS accumulation, which resulted in an alteration of gravitropic response and microtubules orientation pattern.

The plant cytoskeleton takes center stage in abiotic stress responses and resilience

Stress resilience behaviours in plants are defensive mechanisms that develop under adverse environmental conditions to promote growth, development and yield. Over the past decades, improving stress resilience, especially in crop species, has been a focus of intense research for global food security and economic growth. Plants have evolved specific mechanisms to sense external stress and transmit information to the cell interior and generate appropriate responses. Plant cytoskeleton, comprising microtubules and actin filaments, takes a center stage in stress-induced signalling pathways, either as a direct target or as a signal transducer. In the past few years, it has become apparent that the function of the plant cytoskeleton and other associated proteins are not merely limited to elementary processes of cell growth and proliferation, but they also function in stress response and resilience. This review summarizes recent advances in the role of plant cytoskeleton and associated proteins in abiotic stress management. We provide a thorough overview of the mechanisms that plant cells employ to withstand different abiotic stimuli such as hypersalinity, dehydration, high temperature and cold, among others. We also discuss the crucial role of the plant cytoskeleton in organellar positioning under the influence of high light intensity.

Physiological and protein profiling analysis provides insight into the underlying molecular mechanism of potato tuber development regulated by jasmonic acid in vitro

BACKGROUND

Jasmonates (JAs) are one of important phytohormones regulating potato tuber development. It is a complex process and the underlying molecular mechanism regulating tuber development by JAs is still limited. This study attempted to illuminate it through the potential proteomic dynamics information about tuber development in vitro regulated by exogenous JA.

RESULTS

A combined analysis of physiological and iTRAQ (isobaric tags for relative and absolute quantification)-based proteomic approach was performed in tuber development in vitro under exogenous JA treatments (0, 0.5, 5 and 50 μΜ). Physiological results indicated that low JA concentration (especially 5 μM) promoted tuber development, whereas higher JA concentration (50 μM) showed inhibition effect. A total of 257 differentially expressed proteins (DEPs) were identified by iTRAQ, which provided a comprehensive overview on the functional protein profile changes of tuber development regulated by JA. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis indicated that low JA concentration (especially 5 μM) exhibited the promotion effects on tuber development in various cellular processes. Some cell wall polysaccharide synthesis and cytoskeleton formation-related proteins were up-regulated by JA to promote tuber cell expansion. Some primary carbon metabolism-related enzymes were up-regulated by JA to provide sufficient metabolism intermediates and energy for tuber development. And, a large number of protein biosynthesis, degradation and assembly-related were up-regulated by JA to promote tuber protein biosynthesis and maintain strict protein quality control during tuber development.

CONCLUSIONS

This study is the first to integrate physiological and proteomic data to provide useful information about the JA-signaling response mechanism of potato tuber development in vitro. The results revealed that the levels of a number of proteins involved in various cellular processes were regulated by JA during tuber development. The proposed hypothetical model would explain the interaction of these DEPs that associated with tuber development in vitro regulated by JA.

Comparative de novo transcriptome analysis of flower and root of Oliveria decumbens Vent. to identify putative genes in terpenes biosynthesis pathway

The Oliveria decumbens Vent. is a wild, rare, annual medicinal plant and endemic plant of Iran that has metabolites (mostly terpenes) which make it a precious plant in Persian Traditional Medicine and also a potential chemotherapeutic agent. The lack of genetic resources has slowed the discovery of genes involved in the terpenes biosynthesis pathway. It is a wild relative of Daucus carota. In this research, we performed the transcriptomic differences between two samples, flower and root of Oliveria decumbens, and also analyze the expression value of the genes involved in terpenoid biosynthesis by RNA-seq and its essential oil’s phytochemicals analyzed by GC/MS. In total, 136,031,188 reads from two samples of flower and root have been produced. The result shows that the MEP pathway is mostly active in the flower and the MVA in the root. Three genes of GPP, FPPS, and GGPP that are the precursors in the synthesis of mono, di, and triterpenes are upregulated in root and 23 key genes were identified that are involved in the biosynthesis of terpenes. Three genes had the highest upregulation in the root including, and on the other hand, another three genes had the expression only in the flower. Meanwhile, 191 and 185 upregulated genes in the flower and root of the plant, respectively, were selected for the gene ontology analysis and reconstruction of co-expression networks. The current research is the first of its kind on Oliveria decumbens transcriptome and discussed 67 genes that have been deposited into the NCBI database. Collectively, the information obtained in this study unveils the new insights into characterizing the genetic blueprint of Oliveria decumbens Vent. which paved the way for medical/plant biotechnology and the pharmaceutical industry in the future.

In-depth assembly of organ and development dissected Picrorhiza kurroa proteome map using mass spectrometry

BACKGROUND

Picrorhiza kurroa Royle ex Benth. being a rich source of phytochemicals, is a promising high altitude medicinal herb of Himalaya. The medicinal potential is attributed to picrosides i.e. iridoid glycosides, which synthesized in organ-specific manner through highly complex pathways. Here, we present a large-scale proteome reference map of P. kurroa, consisting of four morphologically differentiated organs and two developmental stages.

RESULTS

We were able to identify 5186 protein accessions (FDR < 1%) providing a deep coverage of protein abundance array, spanning around six orders of magnitude. Most of the identified proteins are associated with metabolic processes, response to abiotic stimuli and cellular processes. Organ specific sub-proteomes highlights organ specialized functions that would offer insights to explore tissue profile for specific protein classes. With reference to P. kurroa development, vegetative phase is enriched with growth related processes, however generative phase harvests more energy in secondary metabolic pathways. Furthermore, stress-responsive proteins, RNA binding proteins (RBPs) and post-translational modifications (PTMs), particularly phosphorylation and ADP-ribosylation play an important role in P. kurroa adaptation to alpine environment. The proteins involved in the synthesis of secondary metabolites are well represented in P. kurroa proteome. The phytochemical analysis revealed that marker compounds were highly accumulated in rhizome and overall, during the late stage of development.

CONCLUSIONS

This report represents first extensive proteomic description of organ and developmental dissected P. kurroa, providing a platform for future studies related to stress tolerance and medical applications.

Genome-Wide Analysis of Tubulin Gene Family in Cassava and Expression of Family Member FtsZ2-1 during Various Stress

Filamentous temperature-sensitive protein Z (Tubulin/FtsZ) family is a group of conserved GTP-binding (guanine nucleotide-binding) proteins, which are closely related to plant tissue development and organ formation as the major component of the cytoskeleton. According to the published genome sequence information of cassava (Manihot esculenta Crantz), 23 tubulin genes (MeTubulins) were identified, which were divided into four main groups based on their type and phylogenetic characteristics. The same grouping generally has the same or similar motif composition and exon–intron structure. Collinear analysis showed that fragment repetition event is the main factor in amplification of cassava tubulin superfamily gene. The expression profiles of MeTubulin genes in various tissue were analyzed, and it was found that MeTubulins were mainly expressed in leaf, petiole, and stem, while FtsZ2-1 was highly expressed in storage root. The qRT-PCR results of the FtsZ2-1 gene under hormone and abiotic stresses showed that indole-3-acetic acid (IAA) and gibberellin A3 (GA3) stresses could significantly increase the expression of the FtsZ2-1 gene, thereby revealing the potential role of FtsZ2-1 in IAA and GA3 stress-induced responses.

Phytohormone cytokinin guides microtubule dynamics during cell progression from proliferative to differentiated stage

Cell production and differentiation for the acquisition of specific functions are key features of living systems. The dynamic network of cellular microtubules provides the necessary platform to accommodate processes associated with the transition of cells through the individual phases of cytogenesis. Here, we show that the plant hormone cytokinin fine‐tunes the activity of the microtubular cytoskeleton during cell differentiation and counteracts microtubular rearrangements driven by the hormone auxin. The endogenous upward gradient of cytokinin activity along the longitudinal growth axis in Arabidopsis thaliana roots correlates with robust rearrangements of the microtubule cytoskeleton in epidermal cells progressing from the proliferative to the differentiation stage. Controlled increases in cytokinin activity result in premature re‐organization of the microtubule network from transversal to an oblique disposition in cells prior to their differentiation, whereas attenuated hormone perception delays cytoskeleton conversion into a configuration typical for differentiated cells. Intriguingly, cytokinin can interfere with microtubules also in animal cells, such as leukocytes, suggesting that a cytokinin‐sensitive control pathway for the microtubular cytoskeleton may be at least partially conserved between plant and animal cells.

A natural indole alkaloid, norharmane, affects PIN expression patterns and compromises root growth in Arabidopsis thaliana

Norharmane is an indole alkaloid that can be found in several terrestrial plants, as well as in some dinoflagellates and cyanobacteria. The aim of this study was to focus on the way this metabolite impacts the plant metabolism of the model species Arabidopsis thaliana. This metabolite caused increase of secondary and adventitious roots, as well as torsion, toxic effects, and a decrease in root length. Moreover, norharmane altered the cellular arrangement, resulting in unfinished cell walls, decreased auxin content and inhibited PIN proteins activity. All the alterations suggest that norharmane alters polar auxin transport by inhibiting PIN2, PIN3 and PIN7 transport proteins, thus causing a significant inhibitory effect on the growth of A. thaliana seedlings.

Natural Root Cellular Variation in Responses to Osmotic Stress in Arabidopsis thaliana Accessions

Arabidopsis naturally occurring populations have allowed for the identification of considerable genetic variation remodeled by adaptation to different environments and stress conditions. Water is a key resource that limits plant growth, and its availability is initially sensed by root tissues. The root’s ability to adjust its physiology and morphology under water deficit makes this organ a useful model to understand how plants respond to water stress. Here, we used hyperosmotic shock stress treatments in different Arabidopsis accessions to analyze the root cell morphological responses. We found that osmotic stress conditions reduced root growth and root apical meristem (RAM) size, promoting premature cell differentiation without affecting the stem cell niche morphology. This phenotype was accompanied by a cluster of small epidermal and cortex cells with radial expansion and root hairs at the transition to the elongation zone. We also found this radial expansion with root hairs when plants are grown under hypoosmotic conditions. Finally, root growth was less affected by osmotic stress in the Sg-2 accession followed by Ws, Cvi-0, and Col-0; however, after a strong osmotic stress, Sg-2 and Cvi-0 were the most resilience accessions. The sensitivity differences among these accessions were not explained by stress-related gene expression. This work provides new cellular insights on the Arabidopsis root phenotypic variability and plasticity to osmotic stress.

Proteomic Investigation of S-Nitrosylated Proteins During NO-Induced Adventitious Rooting of Cucumber

Nitric oxide (NO) acts an essential signaling molecule that is involved in regulating various physiological and biochemical processes in plants. However, whether S-nitrosylation is a crucial molecular mechanism of NO is still largely unknown. In this study, 50 μM S-nitrosoglutathione (GSNO) treatment was found to have a maximum biological effect on promoting adventitious rooting in cucumber. Meanwhile, removal of endogenous NO significantly inhibited the development of adventitious roots implying that NO is responsible for promoting the process of adventitious rooting. Moreover, application of GSNO resulted in an increase of intracellular S-nitrosothiol (SNO) levels and endogenous NO production, while decreasing the S-nitrosoglutathione reductase (GSNOR) activity during adventitious rooting, implicating that S-nitrosylation might be involved in NO-induced adventitious rooting in cucumber. Furthermore, the identification of S-nitrosylated proteins was performed utilizing the liquid chromatography/mass spectrometry/mass spectrometry (LC-MS/MS) and biotin-switch technique during the development of adventitious rooting. Among these proteins, the activities and S-nitrosylated level of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), tubulin alpha chain (TUA), and glutathione reductase (GR) were further analyzed as NO direct targets. Our results indicated that NO might enhance the S-nitrosylation level of GAPDH and GR, and was found to subsequently reduce these activities and transcriptional levels. Conversely, S-nitrosylation of TUA increased the expression level of TUA. The results implied that S-nitrosylation of key proteins seems to regulate various pathways through differential S-nitrosylation during adventitious rooting. Collectively, these results suggest that S-nitrosylation could be involved in NO-induced adventitious rooting, and they also provide fundamental evidence for the molecular mechanism of NO signaling during adventitious rooting in cucumber explants.

Proteome Profiling of the Mutagen-Induced Morphological and Yield Macro-Mutant Lines of Nigella sativa L

In the present investigation, the leaf proteome profile of the macro-mutant lines of Nigella sativa L. was analyzed to identify the key proteins involved in the expression of traits associated with the morphology, seed yield, and content of thymoquinone. In our earlier study, the macro-mutants were generated with contrasting morphological traits and seed yields through induced mutagenesis, using ethyl methyl sulfonate, gamma rays, and combinations of both. Analysis of the leaf proteome of the control and macro-mutant lines of N. sativa showed that twenty-three proteins were differentially expressed. These differentially expressed proteins were sequenced through mass spectrometry and identified using the MASCOT software. On the basis of their function, these proteins were categorized into several groups. Most proteins were found in the categories of signal transduction (18%) and carbon metabolism (18%). A total of 13% of proteins belonged to the categories of energy and metabolism. Proteins in the categories of secondary plant metabolism, stress defense, cytoskeleton, and protein synthesis were also found. The polycomb group protein (FIE1), transcription factor (PRE1), and geranyl diphosphate synthase were notable proteins, in addition to some proteins of signal transduction and carbon metabolism. Expression patterns of the differentially expressed proteins were also studied at the transcript level by using qRT-PCR. Transcriptomics analysis was consistent with the proteomics data. This study shows the changes that take place at the proteomic level through induced mutagenesis, as well as the involvement of some proteins in the expression traits associated with plant height, seed yield, and the thymoquinone content of N. sativa. The identified proteins might help elucidate the metabolic pathways involved in the expression of traits, including seed yield, and the active compounds of medicinal plants.

Identification of TPX2 Gene Family in Upland Cotton and Its Functional Analysis in Cotton Fiber Development

Microtubules (MTs) are of importance to fiber development. The Xklp2 (TPX2) proteins as a class of microtubule-associated proteins (MAPs) play a key role in plant growth and development by regulating the dynamic changes of microtubules (MTs). However, the mechanism underlying this is unknown. The interactions between TPX2 proteins and tubulin protein, which are the main structural components, have not been studied in fiber development of upland cotton. Therefore, a genome-wide analysis of the TPX2 family was firstly performed in Gossypiumhirsutum L. This study identified 41 GhTPX2 sequences in the assembled G. hirsutum genome by a series of bioinformatic methods. Generally, this gene family is phylogenetically grouped into six subfamilies, and 41 G. hirsutum TPX2 genes (GhTPX2s) are distributed across 21 chromosomes. A heatmap of the TPX2 gene family showed that homologous GhTPX2 genes, GhWDLA2/7 and GhWDLA4/9, have large differences in expression levels between two upland cotton recombinant inbred lines (69307 and 69362) that are different in fiber quality at 15 and 20 days post anthesis. The relative data indicate that these four genes are down-regulated under oryzalin, which causes microtubule depolymerization, as determined via qRT-PCR. A subcellular localization experiment suggested that GhWDLA2 and GhWDLA7 are localized to the microtubule cytoskeleton, and GhWDLA4 and GhWDLA9 are only localized to the nucleus. However, only GhWDLA7 between GhWDLA2 and GhWDLA7 interacted with GhTUA2 in the yeast two-hybrid assay. These results lay the foundation for further function study of the TPX2 gene family.

Flax tubulin and CesA superfamilies represent attractive and challenging targets for a variety of genome- and base-editing applications

Flax is both a valuable resource and an interesting model crop. Despite a long history of flax genetic transformation only one transgenic linseed cultivar has been so far registered in Canada. Implementation and use of the genome-editing technologies that allow site-directed modification of endogenous genes without the introduction of foreign genes might improve this situation. Besides its potential for boosting crop yields, genome editing is now one of the best tools for carrying out reverse genetics and it is emerging as an especially versatile tool for studying basic biology. A complex interplay between the flax tubulin family (6 α-, 14 β-, and 2 γ-tubulin genes), the building block of microtubules, and the CesA (15-16 genes), the subunit of the multimeric cellulose-synthesizing complex devoted to the oriented deposition of the cellulose microfibrils is fundamental for the biosynthesis of the cell wall. The role of the different members of each family in providing specificities to the assembled complexes in terms of structure, dynamics, activity, and interaction remains substantially obscure. Genome-editing strategies, recently shown to be successful in flax, can therefore be useful to unravel the issue of functional redundancy and provide evidence for specific interactions between different members of the tubulin and CesA gene families, in relation to different phase and mode of cell wall biosynthesis.

Effects of glucose and ethylene on root hair initiation and elongation in lettuce (Lactuca sativa L.) seedlings

Root hair formation occurs in lettuce seedlings after transfer to an acidic medium (pH 4.0). This process requires cortical microtubule (CMT) randomization in root epidermal cells and the plant hormone ethylene. We investigated the interaction between ethylene and glucose, a new signaling molecule in plants, in lettuce root development, with an emphasis on root hair formation. Dark-grown seedlings were used to exclude the effect of photosynthetically produced glucose. In the dark, neither root hair formation nor the CMT randomization preceding it occurred, even after transfer to the acidic medium (pH 4.0). Adding 1-aminocyclopropane-1-carboxylic-acid (ACC) to the medium rescued the induction, while adding glucose did not. Although CMT randomization occurred when glucose was applied together with ACC, it was somewhat suppressed compared to that in ACC-treated seedlings. This was not due to a decrease in the speed of randomization, but due to lowering of the maximum degree of randomization. Despite the negative effect of glucose on ACC-induced CMT randomization, the density and length of ACC-induced root hairs increased when glucose was also added. The hair-cell length of the ACC-treated seedlings was comparable to that in the combined-treatment seedlings, indicating that the increase in hair density caused by glucose results from an increase in the root hair number. Furthermore, quantitative RT-PCR revealed that glucose suppressed ethylene signaling. These results suggest that glucose has a negative and positive effect on the earlier and later stages of root hair formation, respectively, and that the promotion of the initiation and elongation of root hairs by glucose may be mediated in an ethylene-independent manner.

Auxin-like effects of the natural coumarin scopoletin on Arabidopsis cell structure and morphology

The mode of action and phytotoxic potential of scopoletin, a natural compound belonging to the group of coumarins, has been evaluated in detail. Analysis conducted by light and electron transmission microscopy showed strong cell and tissue abnormalities on treated roots, such as cell wall malformations, multi-nucleated cells, abnormal nuclei and tissue disorganization. Scopoletin compromised root development by inducing wrong microtubule assembling, mitochondrial membrane depolarization and ultimate cell death, in a way similar to auxin herbicides. The structural similarities of the natural compound scopoletin and the auxin herbicide 2,4-D, as well as the ability of scopoletin to fit into the auxin-binding site TIR1, were analyzed, suggesting that the phytotoxic activity of scopoletin matches with that exhibited by auxinic herbicides.

Root morphogenic pathways in Eucalyptus grandis are modified by the activity of protein arginine methyltransferases

BACKGROUND

Methylation of proteins at arginine residues, catalysed by members of the protein arginine methyltransferase (PRMT) family, is crucial for the regulation of gene transcription and for protein function in eukaryotic organisms. Inhibition of the activity of PRMTs in annual model plants has demonstrated wide-ranging involvement of PRMTs in key plant developmental processes, however, PRMTs have not been characterised or studied in long-lived tree species.

RESULTS

Taking advantage of the recently available genome for Eucalyptus grandis, we demonstrate that most of the major plant PRMTs are conserved in E. grandis as compared to annual plants and that they are expressed in all major plant tissues. Proteomic and transcriptomic analysis in roots suggest that the PRMTs of E. grandis control a number of regulatory proteins and genes related to signalling during cellular/root growth and morphogenesis. We demonstrate here, using chemical inhibition of methylation and transgenic approaches, that plant type I PRMTs are necessary for normal root growth and branching in E. grandis. We further show that EgPRMT1 has a key role in root hair initiation and elongation and is involved in the methylation of β-tubulin, a key protein in cytoskeleton formation.

CONCLUSIONS

Together, our data demonstrate that PRMTs encoded by E. grandis methylate a number of key proteins and alter the transcription of a variety of genes involved in developmental processes. Appropriate levels of expression of type I PRMTs are necessary for the proper growth and development of E. grandis roots.

A Model Analysis of Mechanisms for Radial Microtubular Patterns at Root Hair Initiation Sites

Plant cells have two main modes of growth generating anisotropic structures. Diffuse growth where whole cell walls extend in specific directions, guided by anisotropically positioned cellulose fibers, and tip growth, with inhomogeneous addition of new cell wall material at the tip of the structure. Cells are known to regulate these processes via molecular signals and the cytoskeleton. Mechanical stress has been proposed to provide an input to the positioning of the cellulose fibers via cortical microtubules in diffuse growth. In particular, a stress feedback model predicts a circumferential pattern of fibers surrounding apical tissues and growing primordia, guided by the anisotropic curvature in such tissues. In contrast, during the initiation of tip growing root hairs, a star-like radial pattern has recently been observed. Here, we use detailed finite element models to analyze how a change in mechanical properties at the root hair initiation site can lead to star-like stress patterns in order to understand whether a stress-based feedback model can also explain the microtubule patterns seen during root hair initiation. We show that two independent mechanisms, individually or combined, can be sufficient to generate radial patterns. In the first, new material is added locally at the position of the root hair. In the second, increased tension in the initiation area provides a mechanism. Finally, we describe how a molecular model of Rho-of-plant (ROP) GTPases activation driven by auxin can position a patch of activated ROP protein basally along a 2D root epidermal cell plasma membrane, paving the way for models where mechanical and molecular mechanisms cooperate in the initial placement and outgrowth of root hairs.

Arabidopsis SABRE and CLASP interact to stabilize cell division plane orientation and planar polarity

The orientation of cell division and the coordination of cell polarity within the plane of the tissue layer (planar polarity) contribute to shape diverse multicellular organisms. The root of Arabidopsis thaliana displays regularly oriented cell divisions, cell elongation and planar polarity providing a plant model system to study these processes. Here we report that the SABRE protein, which shares similarity with proteins of unknown function throughout eukaryotes, has important roles in orienting cell division and planar polarity. SABRE localizes at the plasma membrane, endomembranes, mitotic spindle and cell plate. SABRE stabilizes the orientation of CLASP-labelled preprophase band microtubules predicting the cell division plane, and of cortical microtubules driving cell elongation. During planar polarity establishment, sabre is epistatic to clasp at directing polar membrane domains of Rho-of-plant GTPases. Our findings mechanistically link SABRE to CLASP-dependent microtubule organization, shedding new light on the function of SABRE-related proteins in eukaryotes.

MAPs: cellular navigators for microtubule array orientations in Arabidopsis

Microtubules are subcellular nanotubes composed of α- and β-tubulin that arise from microtubule nucleation sites, mainly composed of γ-tubulin complexes [corrected]. Cell wall encased plant cells have evolved four distinct microtubule arrays that regulate cell division and expansion. Microtubule-associated proteins, the so called MAPs, construct, destruct and reorganize microtubule arrays thus regulating their spatiotemporal transitions during the cell cycle. By physically binding to microtubules and/or modulating their functions, MAPs control microtubule dynamic instability and/or interfilament cross talk. We survey the recent analyses of Arabidopsis MAPs such as MAP65, MOR1, CLASP, katanin, TON1, FASS, TRM, TAN1 and kinesins in terms of their effects on microtubule array organizations and plant development.

Multiple tubulins: evolutionary aspects and biological implications

Plant tubulin is a dimeric protein that contributes to formation of microtubules, major intracellular structures that are involved in the control of fundamental processes such as cell division, polarity of growth, cell-wall deposition, intracellular trafficking and communications. Because it is a structural protein whose function is confined to the role of microtubule formation, tubulin may be perceived as an uninteresting gene product, but such a perception is incorrect. In fact, tubulin represents a key molecule for studying fundamental biological issues such as (i) microtubule evolution (also with reference to prokaryotic precursors and the formation of cytomotive filaments), (ii) protein structure with reference to the various biochemical features of members of the FstZ/tubulin superfamily, (iii) isoform variations contributed by the existence of multi-gene families and various kinds of post-translational modifications, (iv) anti-mitotic drug interactions and mode of action, (v) plant and cell symmetry, as determined using a series of tubulin mutants, (vi) multiple and sophisticated mechanisms of gene regulation, and (vii) intron molecular evolution. In this review, we present and discuss many of these issues, and offer an updated interpretation of the multi-tubulin hypothesis.

The Arabidopsis embryo as a miniature morphogenesis model

Four basic ingredients of morphogenesis, oriented cell division and expansion, cell-cell communication and cell fate specification allow plant cells to develop into a wide variety of organismal architectures. A central question in plant biology is how these cellular processes are regulated and orchestrated. Here, we present the advantages of the early Arabidopsis embryo as a model for studying the control of morphogenesis. All ingredients of morphogenesis converge during embryogenesis, and the highly predictable nature of embryo development offers unprecedented opportunities for understanding their regulation in time and space. In this review we describe the morphogenetic principles underlying embryo patterning and discuss recent advances in their regulation. Morphogenesis is under tight transcriptional control and most genes that were identified as important regulators of embryo patterning encode transcription factors or components of signaling pathways. There exists, therefore, a large gap between the transcriptional control of embryo morphogenesis and the cellular execution. We describe the first such connections, and propose future directions that should help bridge this gap and generate comprehensive understanding of the control of morphogenesis.

MicroFilament Analyzer, an image analysis tool for quantifying fibrillar orientation, reveals changes in microtubule organization during gravitropism

Image acquisition is an important step in the study of cytoskeleton organization. As visual interpretations and manual measurements of digital images are prone to errors and require a great amount of time, a freely available software package named MicroFilament Analyzer (MFA) was developed. The goal was to provide a tool that facilitates high-throughput analysis to determine the orientation of filamentous structures on digital images in a more standardized, objective and repeatable way. Here, the rationale and applicability of the program is demonstrated by analyzing the microtubule patterns in epidermal cells of control and gravi-stimulated Arabidopsis thaliana roots. Differential expansion of cells on either side of the root results in downward bending of the root tip. As cell expansion depends on the properties of the cell wall, this may imply a differential orientation of cellulose microfibrils. As cellulose deposition is orchestrated by cortical microtubules, the microtubule patterns were analyzed. The MFA program detects the filamentous structures on the image and identifies the main orientation(s) within individual cells. This revealed four distinguishable microtubule patterns in root epidermal cells. The analysis indicated that gravitropic stimulation and developmental age are both significant factors that determine microtubule orientation. Moreover, the data show that an altered microtubule pattern does not precede differential expansion. Other possible applications are also illustrated, including field emission scanning electron micrographs of cellulose microfibrils in plant cell walls and images of fluorescent actin.

Invasive cells in animals and plants: searching for LECA machineries in later eukaryotic life

Invasive cell growth and migration is usually considered a specifically metazoan phenomenon. However, common features and mechanisms of cytoskeletal rearrangements, membrane trafficking and signalling processes contribute to cellular invasiveness in organisms as diverse as metazoans and plants – two eukaryotic realms genealogically connected only through the last common eukaryotic ancestor (LECA). By comparing current understanding of cell invasiveness in model cell types of both metazoan and plant origin (invadopodia of transformed metazoan cells, neurites, pollen tubes and root hairs), we document that invasive cell behavior in both lineages depends on similar mechanisms. While some superficially analogous processes may have arisen independently by convergent evolution (e.g. secretion of substrate- or tissue-macerating enzymes by both animal and plant cells), at the heart of cell invasion is an evolutionarily conserved machinery of cellular polarization and oriented cell mobilization, involving the actin cytoskeleton and the secretory pathway. Its central components - small GTPases (in particular RHO, but also ARF and Rab), their specialized effectors, actin and associated proteins, the exocyst complex essential for polarized secretion, or components of the phospholipid- and redox- based signalling circuits (inositol-phospholipid kinases/PIP2, NADPH oxidases) are aparently homologous among plants and metazoans, indicating that they were present already in LECA.

Reviewer: This article was reviewed by Arcady Mushegian, Valerian Dolja and Purificacion Lopez-Garcia.

Dissecting the cellular functions of plant microtubules using mutant tubulins

α- and β-tubulins, the building blocks of the microtubule (MT) polymer, are encoded by multiple genes that are largely functionally redundant in plants. Null tubulin mutants are thus phenotypically indistinguishable from the wild type, but miss-sense or deletion mutations of critical amino acid residues that are important for the assembly, stability, or dynamics of the polymer disrupt the proper organization and function of the resultant MT arrays. Mutant tubulins co-assemble with wild-type tubulins into mutant MTs with compromised functions, and thus mechanistically act as dominant-negative MT poisons. Cortical MT arrays in interphase plant cells are most sensitive to tubulin mutations, and are transformed into helical structures or random orientation, which produce twisted or radially swollen cells. Mutant plants resistant to MT-targeted herbicides may possess tubulin mutations at the binding sites of the herbicides. Tubulin mutants are valuable tools for investigating how individual MTs are organized into particular patterns in cortical arrays, and for defining the functional contribution of MTs to various MT-dependent or -assisted cellular processes in plant cells.

Microtubule and male sterility in a gene-cytoplasmic male sterile line of non-heading Chinese cabbage

BACKGROUND

Microtubules are the basic components of the cytoskeleton in eukaryotic cells and are made up of 13 parallel protofilaments, each composed of α- and β-tubulin unit molecules aligned along the longitudinal axis of the microtubule.

RESULTS

α-Tubulin gene TUBA2 from non-heading Chinese cabbage (Brassica campestris ssp. chinensis Makino) was expressed at the highest level in stamens and at lower levels in other organs. In addition, it was expressed at a much lower level in the cytoplasmic male sterile (CMS) line than in the maintainer line. Furthermore, at the microsporocyte stage of development in the CMS line the microtubule bundles were knitted together in random organisation, which differed significantly from the radiate microtubule bundles running circumferentially around the nucleus in the maintainer line. Also, large vacuoles appeared within the cytoplasm in the CMS line with no dyed microtubules.

CONCLUSION

TUBA2 was very important to pollen development, which might be closely related to male sterility. Large vacuoles might replace the nuclei close to the cell walls and lead to a lack of microtubules when the cells abort. Abnormalities and defects in the organisation and composition of microtubules in the male sterile line highlighted the complex interaction between microtubules and cytoplasmic male sterility.

Identification of soybean proteins from a single cell type: the root hair

Root hairs (RH) are a terminally differentiated single cell type, mainly involved in water and nutrient uptake from the soil. The soybean RH cell represents an excellent model for the study of single cell systems biology. In this study, we identified 5702 proteins, with at least two peptides, from soybean RH using an accurate mass and time tag approach, establishing a comprehensive proteome reference map of this single cell type. We also showed that trypsin is the most appropriate enzyme for soybean proteomic studies by performing an in silico digestion of the soybean proteome using different proteases. Although the majority of proteins identified in this study are involved in basal metabolism, the function of others are more related to RH formation/function and include proteins involved in nutrient uptake (transporters) or vesicular trafficking (cytoskeleton and ras-associated binding proteins). Interestingly, some of these proteins appear to be specifically detected in RH and constitute promising candidates for further studies to elucidate unique features of this single-cell model.

Formins: emerging players in the dynamic plant cell cortex

Formins (FH2 proteins) are an evolutionarily conserved family of eukaryotic proteins, sharing the common FH2 domain. While they have been, until recently, understood mainly as actin nucleators, formins are also engaged in various additional aspects of cytoskeletal organization and signaling, including, but not limited to, the crosstalk between the actin and microtubule networks. A surprising diversity of domain organizations has been discovered among the FH2 proteins, and specific domain setups have been found in plants. Seed plants have two clades of formins, one of them (Class I) containing mostly transmembrane proteins, while members of the other one (Class II) may be anchored to membranes via a putative membrane-binding domain related to the PTEN antioncogene. Thus, plant formins present good candidates for possible mediators of coordination of the cortical actin and microtubule cytoskeletons, as well as their attachment to the plasma membrane, that is, aspects of cell cortex organization likely to be important for cell and tissue morphogenesis. Although experimental studies of plant formin function are hampered by the large number of formin genes and their functional redundancy, recent experimental work has already resulted in some remarkable insights into the function of FH2 proteins in plants.

Plant microtubule cytoskeleton complexity: microtubule arrays as fractals

Biological systems are by nature complex and this complexity has been shown to be important in maintaining homeostasis. The plant microtubule cytoskeleton is a highly complex system, with contributing factors through interactions with microtubule-associated proteins (MAPs), expression of multiple tubulin isoforms, and post-translational modification of tubulin and MAPs. Some of this complexity is specific to microtubules, such as a redundancy in factors that regulate microtubule depolymerization. Plant microtubules form partial helical fractals that play a key role in development. It is suggested that, under certain cellular conditions, other categories of microtubule fractals may form including isotropic fractals, triangular fractals, and branched fractals. Helical fractal proteins including coiled-coil and armadillo/beta-catenin repeat proteins and the actin cytoskeleton are important here too. Either alone, or in combination, these fractals may drive much of plant development.

Emerging aspects of ER organization in root hair tip growth: lessons from RHD3 and Atlastin

Cell polarity is a fundamental aspect of eukaryotic cells. A central question for cell biologists is how the polarity of a cell is established and maintained. Root hairs are exceptionally polarized structures formed from specific root epidermal cells. The morphogenesis of root hairs is characterized by the localized cell growth in a small dome at the tip of the hair, a process called tip growth. Root hairs are thus an attractive model system to study the establishment and maintenance of cell polarity in eukaryotes. Research on Arabidopsis root hairs has identified a plethora of molecular and cellular components that are important for root hair tip growth. Recently, studies on RHD3 and Atlastin have revealed a surprising similarity with respect to the role of the tubular ER network in tip growth of root hairs in plants and the axonal outgrowth of corticospinal neurons in neurological disorders known as hereditary spastic paraplegia (HSP). In this mini-review, we highlight recent progress in understanding of the function and regulation of RHD3 in the generation of the tubular ER network and discussed ways in which RHD3 could be involved in the establishment and maintenance of root hair tip growth.

New insights into the mechanism of development of Arabidopsis root hairs and trichomes

Epidermis cell differentiation in Arabidopsis thaliana is a model system for understanding the mechanisms leading to the developmental end state of plant cells. Both root hairs and trichomes differentiate from epidermal cells and molecular genetic analyses using Arabidopsis mutants have demonstrated that the differentiation of root hairs and trichomes is regulated by similar molecular mechanisms. Molecular-genetic approaches have led to the identification of many genes that are involved in epidermal cell differentiation, most of which encode transcription factors that induce the expression of genes active in both root hair and trichome development. Control of cell growth after fate determination has also been studied using Arabidopsis mutants.

Root hair systems biology

Plant functional genomic studies have largely measured the response of whole plants, organs and tissues, resulting in the dilution of the signal from individual cells. Methods are needed where the full repertoire of functional genomic tools can be applied to a single plant cell. Root hair cells are an attractive model to study the biology of a single, differentiated cell type because of their ease of isolation, polar growth, and role in water and nutrient uptake, as well as being the site of infection by nitrogen-fixing bacteria. This review highlights the recent advances in our understanding of plant root hair biology and examines whether the root hair has potential as a model for plant cell systems biology.

Arabidopsis homologs of nucleus- and phragmoplast-localized kinase 2 and 3 and mitogen-activated protein kinase 4 are essential for microtubule organization

A double homozygous recessive mutant in the Arabidopsis thaliana homologs of nucleus- and phragmoplast-localized kinase 2 (ANP2) and 3 (ANP3) genes and a homozygous recessive mutant in the mitogen-activated protein kinase 4 (MPK4) gene of Arabidopsis exhibit deficiencies in the overall microtubule (MT) organization, which result in abnormal cell growth patterns, such as branching of root hairs and swelling of diffusely growing epidermal cells. Genetic, pharmacological, molecular, cytological, and biochemical analyses show that the major underlying mechanism for these phenotypes is excessive MT stabilization manifested in both mutants as heavy MT bundling, disorientation, and drug stability. The above defects in MAPK signaling result in the adverse regulation of members of the microtubule-associated protein (MAP65) protein family, including strongly diminished phosphorylation of MAP65-1. These data suggest that ANP2/ANP3, MPK4, and the microtubule-associated protein MAP65-1, a putative target of MPK4 signaling, are all essential for the proper organization of cortical microtubules in Arabidopsis epidermal cells.

Changes in the accumulation of alpha- and beta-tubulin during bud development in Vitis vinifera L

Microtubules play important roles during growth and morphogenesis of plant cells. Multiple isoforms of alpha- and beta-tubulin accumulate in higher plant cells and originate either by transcription of different genes or by post-translational modifications. The use of different tubulin isoforms involves the binding of microtubules to different associated proteins and therefore generates microtubules with different organizations and functions. Tubulin isoforms are differentially expressed in vegetative and reproductive structures according to the developmental program of plants. In grapevine (Vitis vinifera L.), vegetative and reproductive structures appear on the same stem, making this plant species an excellent model to study the accumulation of tubulin isoforms. Proteins were extracted from grapevine samples (buds, leaves, flowers and tendrils) using an optimized extraction protocol, separated by two-dimensional electrophoresis and analyzed by immunoblot with anti-tubulin antibodies. We identified eight alpha-tubulin and seven beta-tubulin isoforms with pI around 4.8-5 that group into separate clusters. More acidic alpha-tubulin isoforms were detected in buds, while more basic alpha-isoforms were prevalently found in tendrils and flowers. Similarly, more acidic beta-tubulin isoforms were used in the bud stage while a basic beta-tubulin isoform was essentially used in leaves and two central beta-tubulin isoforms were characteristically used in tendrils and flowers. Acetylated alpha-tubulin was not detected in any sample while tyrosinated alpha-tubulin was essentially found in large latent buds and in bursting buds in association with a distinct subset of tubulin isoforms. The implication of these data on the use of different tubulin isoforms during grapevine development is discussed.

Exocytosis and cell polarity in plants - exocyst and recycling domains

In plants, exocytosis is a central mechanism of cell morphogenesis. We still know surprisingly little about some aspects of this process, starting with exocytotic vesicle formation, which may take place at the trans-Golgi network even without coat assistance, facilitated by the local regulation of membrane lipid organization. The RabA4b guanosine triphosphatase (GTPase), recruiting phosphatidylinositol-4-kinase to the trans-Golgi network, is a candidate vesicle formation organizer. However, in plant cells, there are obviously additional endosomal source compartments for secretory vesicles. The Rho/Rop GTPase regulatory module is central for the initiation of exocytotically active domains in plant cell cortex (activated cortical domains). Most plant cells exhibit several distinct plasma membrane domains, established and maintained by endocytosis-driven membrane recycling. We propose the concept of a 'recycling domain', uniting the activated cortical domain and the connected endosomal compartments, as a dynamic spatiotemporal entity. We have recently described the exocyst tethering complex in plant cells. As a result of the multiplicity of its putative Exo70 subunits, this complex may belong to core regulators of recycling domain organization, including the generation of multiple recycling domains within a single cell. The conventional textbook concept that the plant secretory pathway is largely constitutive is misleading.

Genetic evidence that cellulose synthase activity influences microtubule cortical array organization

To identify factors that influence cytoskeletal organization we screened for Arabidopsis (Arabidopsis thaliana) mutants that show hypersensitivity to the microtubule destabilizing drug oryzalin. We cloned the genes corresponding to two of the 131 mutant lines obtained. The genes encoded mutant alleles of PROCUSTE1 and KORRIGAN, which both encode proteins that have previously been implicated in cellulose synthesis. Analysis of microtubules in the mutants revealed that both mutants have altered orientation of root cortical microtubules. Similarly, isoxaben, an inhibitor of cellulose synthesis, also altered the orientation of cortical microtubules while exogenous cellulose degradation did not. Thus, our results substantiate that proteins involved in cell wall biosynthesis influence cytoskeletal organization and indicate that this influence on cortical microtubule stability and orientation is correlated with cellulose synthesis rather than the integrity of the cell wall.

A class I ADP-ribosylation factor GTPase-activating protein is critical for maintaining directional root hair growth in Arabidopsis

Membrane trafficking and cytoskeletal dynamics are important cellular processes that drive tip growth in root hairs. These processes interact with a multitude of signaling pathways that allow for the efficient transfer of information to specify the direction in which tip growth occurs. Here, we show that AGD1, a class I ADP ribosylation factor GTPase-activating protein, is important for maintaining straight growth in Arabidopsis (Arabidopsis thaliana) root hairs, since mutations in the AGD1 gene resulted in wavy root hair growth. Live cell imaging of growing agd1 root hairs revealed bundles of endoplasmic microtubules and actin filaments extending into the extreme tip. The wavy phenotype and pattern of cytoskeletal distribution in root hairs of agd1 partially resembled that of mutants in an armadillo repeat-containing kinesin (ARK1). Root hairs of double agd1 ark1 mutants were more severely deformed compared with single mutants. Organelle trafficking as revealed by a fluorescent Golgi marker was slightly inhibited, and Golgi stacks frequently protruded into the extreme root hair apex of agd1 mutants. Transient expression of green fluorescent protein-AGD1 in tobacco (Nicotiana tabacum) epidermal cells labeled punctate bodies that partially colocalized with the endocytic marker FM4-64, while ARK1-yellow fluorescent protein associated with microtubules. Brefeldin A rescued the phenotype of agd1, indicating that the altered activity of an AGD1-dependent ADP ribosylation factor contributes to the defective growth, organelle trafficking, and cytoskeletal organization of agd1 root hairs. We propose that AGD1, a regulator of membrane trafficking, and ARK1, a microtubule motor, are components of converging signaling pathways that affect cytoskeletal organization to specify growth orientation in Arabidopsis root hairs.

Plant virus infection-induced persistent host gene downregulation in systemically infected leaves

Understanding of virus infection-induced alterations in host plant gene expression and metabolism leading to the development of virus disease symptoms is both scientifically and economically important. Here, we show that viruses belonging to various RNA virus families are able to induce efficient host gene mRNA downregulation (shut-off) in systemically infected leaves. We demonstrate that the host gene mRNA shut-off overlaps spatially with virus-occupied sectors, indicating the direct role of virus accumulation in this phenomenon. The establishment of shut-off was not directly connected to active viral replication or the RNA-silencing machinery. Importantly, the induced shut-off phenomenon persisted for several weeks, resulting in severe deficiency of mRNA for important housekeeping genes in the infected plants. Interestingly, we found that some other RNA viruses do not induce or only slightly induce the shut-off phenomenon for the same set of genes, implicating genetic determination in this process. Nuclear run-on experiments suggest that plant viruses, similarly to animal viruses, mediate suppression of host mRNA synthesis in the nucleus. By investigating various host-virus interactions, we revealed a correlation between the intensity of the shut-off phenomenon and the severity of disease symptoms. Our data suggest that efficient and persistent downregulation of host genes may be an important component of symptom development in certain host-virus interactions.

Molecular cloning, expression profiling, and yeast complementation of 19 beta-tubulin cDNAs from developing cotton ovules

Microtubules are a major structural component of the cytoskeleton and participate in cell division, intracellular transport, and cell morphogenesis. In the present study, 795 cotton tubulin expressed sequence tags were analysed and 19 beta-tubulin genes (TUB) cloned from a cotton cDNA library. Among the group, 12 cotton TUBs (GhTUBs) are reported for the first time here. Transcription profiling revealed that nine GhTUBs were highly expressed in elongating fibre cells as compared with fuzzless-lintless mutant ovules. Treating cultured wild-type cotton ovules with exogenous phytohormones showed that individual genes can be induced by different agents. Gibberellin induced expression of GhTUB1 and GhTUB3, ethylene induced expression of GhTUB5, GhTUB9, and GhTUB12, brassinosteroids induced expression of GhTUB1, GhTUB3, GhTUB9, and GhTUB12, and lignoceric acid induced expression of GhTUB1, GhTUB3, and GhTUB12. When GhTUBs were transformed into the Saccharomyces cerevisiae inviable mutant, tub2, which is deficient in beta-tubulin, one ovule-specific and eight of nine fibre-preferential GhTUBs rescued this lethality. This study suggests that the proteins encoded by cotton GhTUBs are involved during cotton fibre development.

Armadillo repeat-containing kinesins and a NIMA-related kinase are required for epidermal-cell morphogenesis in Arabidopsis

The involvement of kinesin motor proteins in both cell-tip growth and cell-shape determination has been well characterized in various organisms. However, the functions of kinesins during cell morphogenesis in higher plants remain largely unknown. In the current study, we demonstrate that an armadillo repeat-containing kinesin-related protein, ARMADILLO REPEAT KINESIN1 (ARK1), is involved in root-hair morphogenesis. Microtubule polymers are more abundant in ark1 null allele root hairs, but analysis shows that these extra microtubules are concentrated in the endoplasm, and not in the cortical array, suggesting that ARK1 regulates tip growth by limiting the assembly and distribution of endoplasmic microtubules. The ARK1 gene has two homologues in the Arabidopsis genome, ARK2 and ARK3, and our results show that ARK2 is involved in root-cell morphogenesis. We further reveal that a NIMA-related protein kinase, NEK6, binds to the ARK family proteins and has pleiotropic effects on epidermal-cell morphogenesis, suggesting that NEK6 is involved in cell morphogenesis in Arabidopsis via microtubule functions associated with these armadillo repeat-containing kinesins. We discuss the function of NIMA-related protein kinases and armadillo repeat-containing kinesins in the cell morphogenesis of eukaryotes.

A genetic regulatory network in the development of trichomes and root hairs

Trichomes and root hairs differentiate from epidermal cells in the aerial tissues and roots, respectively. Because trichomes and root hairs are easily accessible, particularly in the model plant Arabidopsis, their development has become a well-studied model of cell differentiation and growth. Molecular genetic analyses using Arabidopsis mutants have demonstrated that the differentiation of trichomes and root hair/hairless cells is regulated by similar molecular mechanisms. Transcriptional complexes regulate differentiation into trichome cells and root hairless cells, and formation of the transcriptional complexes is inhibited in neighboring cells. Control of cell growth after fate determination has also been analyzed using Arabidopsis mutants. The progression of endoreduplication cycles, reorientation of microtubules, and organization of the actin cytoskeleton play important roles in trichome growth. Various cellular components such as ion channels, the actin cytoskeleton, microtubules and cell wall materials, and intracellular signal transduction act to establish and maintain root hair tip growth.

Microtubule reorientation in shoots precedes bending during the gravitropic response of cut snapdragon spikes

The microtubule reorientation during the gravitropic bending of cut snapdragon (Antirrhinum majus L.) spikes was investigated. Using indirect immunofluorescence methods, we examined changes in microtubule orientation in the cortex, endodermis and pith tissues of the shoot bending zone, in response to gravistimulation. Our results show that dense microtubule arrays were visible throughout the cortical, endodermal and pith shoot tissues, and that the transverse orientation of the microtubules (perpendicular to the growth axis) was specifically associated with the shoot growing bending zone. Microtubules showed gravity-induced kinetics of changes in their orientation, which occurred only in the upper stem flank and preceded shoot bending. While this observation, that the gravity-induced microtubule orientation precedes bending, was previously reported only in special above-ground organs such as coleoptiles and hypocotyls, our present study is the first to show that such patterns of change occur in mature flowering shoots. These changes were exhibited first in the upper flank of the cortex and then in the upper flank of the endodermis. No changes in microtubule orientation were observed in the cortex or endodermis tissues of the lower flanks or in the pith, suggesting that these tissues continue to grow during shoot gravistimulation. Our results imply that microtubules may be involved in growth cessation of the upper shoot flank occurring during the gravitropic bending of snapdragon cut spikes.

A mutation in MRH2 kinesin enhances the root hair tip growth defect caused by constitutively activated ROP2 small GTPase in Arabidopsis

Root hair tip growth provides a unique model system for the study of plant cell polarity. Transgenic plants expressing constitutively active (CA) forms of ROP (Rho-of-plants) GTPases have been shown to cause the disruption of root hair polarity likely as a result of the alteration of actin filaments (AF) and microtubules (MT) organization. Towards understanding the mechanism by which ROP controls the cytoskeletal organization during root hair tip growth, we have screened for CA-rop2 suppressors or enhancers using CA1-1, a transgenic line that expresses CA-rop2 and shows only mild disruption of tip growth. Here, we report the characterization of a CA-rop2 enhancer (cae1-1 CA1-1) that exhibits bulbous root hairs. The cae1-1 mutation on its own caused a waving and branching root hair phenotype. CAE1 encodes the root hair growth-related, ARM domain-containing kinesin-like protein MRH2 (and thus cae1-1 was renamed to mrh2-3). Cortical MT displayed fragmentation and random orientation in mrh2 root hairs. Consistently, the MT-stabilizing drug taxol could partially rescue the wavy root hair phenotype of mrh2-3, and the MT-depolymerizing drug Oryzalin slightly enhanced the root hair tip growth defect in CA1-1. Interestingly, the addition of the actin-depolymerizing drug Latrunculin B further enhanced the Oryzalin effect. This indicates that the cross-talk of MT and AF organization is important for the mrh2-3 CA1-1 phenotype. Although we did not observe an apparent effect of the MRH2 mutation in AF organization, we found that mrh2-3 root hair growth was more sensitive to Latrunculin B. Moreover, an ARM domain-containing MRH2 fragment could bind to the polymerized actin in vitro. Therefore, our genetic analyses, together with cell biological and pharmacological evidence, suggest that the plant-specific kinesin-related protein MRH2 is an important component that controls MT organization and is likely involved in the ROP2 GTPase-controlled coordination of AF and MT during polarized growth of root hairs.