Semi-constitutive expression of an Arabidopsis thaliana alpha-tubulin gene

Accumulation and post-translational modifications of plant tubulins

The microtubular cytoskeleton of plant cells provides support for several functions (including the anchoring of proteins, assembly of the mitotic spindle, cytoplasmic streaming and construction of cell walls). Both α- and β-tubulins are encoded through multigene families that are differentially expressed in different organs and tissues. To increase the variability of expression, both protein subunits are subjected to post-translational modifications, which could contribute to the assembly of specific microtubule structures. This review aims to highlight the role of specific post-translational modifications of tubulin in plant cells. We initially describe the expression and accumulation of α- and β-tubulin isoforms in different plants and at different stages of plant development. Second, we discuss the different types of post-translational modifications that, by adding or removing specific functional groups, increase the isoform heterogeneity and functional variability of tubulin. Modifications are proposed to form a 'code' that can be read by proteins interacting with microtubules. Therefore, the subpopulations of microtubules may bind to different associated proteins (motor and non-motor), thus creating the physical support for various microtubule functions.

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.

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.

Differentially expression of Tua1, a tubulin-encoding gene, during flowering of tea plant Camellia sinensis (L.) O. Kuntze using cDNA amplified fragment length polymorphism technique

The complementary DNA (cDNA) amplified fragment length polymorphism technique was used to isolate transcript-derived fragments corresponding to genes involved in the flowering of tea plant. Comparative sequence analysis of an approximately 300 bp differential fragment amplified by primer combination E11M11 revealed 80%-84% similarity to the corresponding part of an a-tubulin gene of other species. The complete cDNA sequence of this a-tubulin was cloned by the rapid amplification of cDNA ends technique; its full length is 1537 bp and contains an open reading frame of 450 amino acid residues with two N-glycosylation sites and four protein kinase C phosphorylation sites. The deduced amino acid sequences did show significant homology to the a-tubulin from other plants that has been reported to be a pollen-specific protein and could be correlated with plant cytoplasm-nucleus-interacted male sterility. We named this complete cDNA Tua1. The nucleotide and amino acid sequence data of Tua1 have been recorded in the GenBank sequence database. This Tua1 gene was cloned into the pET-32a expression system and expressed in Escherichia coli BL21trxB(DE3). The molecular weight of expressed protein was deduced to be approximately 49 kDa. Western blot analysis was used to identify the temporal expression of Tua1 in tea plant. Further studies of the effect of Tua1 protein on pollen tube growth indicated the Tua1 solution obviously promoted the growth of tea pollen tube.

Pathogen-responsive expression of glycosyltransferase genes UGT73B3 and UGT73B5 is necessary for resistance to Pseudomonas syringae pv tomato in Arabidopsis

The genome sequencing of Arabidopsis (Arabidopsis thaliana) has revealed that secondary metabolism plant glycosyltransferases (UGTs) are encoded by an unexpectedly large multigenic family of 120 members. Very little is known about their actual function in planta, in particular during plant pathogen interactions. Among them, members of the group D are of particular interest since they are related to UGTs involved in stress-inducible responses in other plant species. We provide here a detailed analysis of the expression profiles of this group of Arabidopsis UGTs following infection with Pseudomonas syringae pv tomato or after treatment with salicylic acid, methyljasmonate, and hydrogen peroxide. Members of the group D displayed distinct induction profiles, indicating potential roles in stress or defense responses notably for UGT73B3 and UGT73B5. Analysis of UGT expression in Arabidopsis defense-signaling mutants further revealed that their induction is methyljasmonate independent, but partially salicylic acid dependent. T-DNA tagged mutants (ugt73b3 and ugt73b5) exhibited decreased resistance to P. syringae pv tomato-AvrRpm1, indicating that expression of the corresponding UGT genes is necessary during the hypersensitive response. These results emphasize the importance of plant secondary metabolite UGTs in plant-pathogen interactions and provide foundation for future understanding of the exact role of UGTs during the hypersensitive response.

Microtubule Defects and Cell Morphogenesis in the lefty1lefty2 Tubulin Mutant of Arabidopsis thaliana

lefty1 and lefty2 are semi-dominant left-handed helical growth mutants of Arabidopsis thaliana, which result from identical dominant-negative amino acid substitutions in alpha-tubulin 6 and alpha-tubulin 4, respectively. Here we characterized the expression patterns of the affected tubulin genes and the phenotypes of the lefty double mutant to address increasing effects of microtubule defects on cell morphogenesis. Both tubulin genes were expressed ubiquitously in examined tissue and cell types, and the alpha-tubulin 2/4/6 subfamily transcripts predominated over other alpha-tubulin transcripts in Arabidopsis seedlings. The lefty double mutant seedlings showed helical growth in hypocotyls and radial cell expansion in the root elongation zone where mutant cortical microtubule arrays were more fragmented and less well aligned than wild-type arrays. Branching of leaf trichomes was highly reduced. In adult mutant plants, anisotropic growth of anther filament cells was severely impaired. These results suggest that left-handed twisted elongation is an intermediate state that leads to full isotropic expansion as the cortical microtubules are increasingly destabilized.

A long leader intron of the Ostub16 rice beta-tubulin gene is required for high-level gene expression and can autonomously promote transcription both in vivo and in vitro

A 2 kb DNA fragment, upstream of the rice beta-tubulin isotype 16 (Ostub16) coding sequence, was isolated using inverse PCR and screening of a tubulin-enriched lambda library. An intron (863 bp) present in the 5' untranslated region (5' UTR) is spliced out to produce the most abundant mRNA species which corresponds to the previously cloned Ostub16 cDNA. Transient expression assays performed on rice embryogenic calluses with chimeric Ostub16::GUS constructs demonstrated that the entire 2 kb upstream sequence has a strong promoter activity, and that the 863 bp intron is required for high-level GUS expression. In addition, the intron sequence is capable per se of sustaining a weak but consistent GUS expression. Two rare Ostub16 transcripts, with a start site mapping within this intron sequence, were detected in rice coleoptile cells. The transcription start site mapped at position -290 with respect to the ATG codon, and the shorter molecule originated from splicing of the same precursor mRNA. Therefore transcriptional expression of rice beta-tubulin isotype 16 results in the synthesis of two premRNA molecules (I and II) encoding for three different mRNA species. We discuss these findings in terms of function and molecular evolution of the mechanisms that control plant beta-tubulin gene expression.

Molecular motors and their functions in plants

Molecular motors that hydrolyze ATP and use the derived energy to generate force are involved in a variety of diverse cellular functions. Genetic, biochemical, and cellular localization data have implicated motors in a variety of functions such as vesicle and organelle transport, cytoskeleton dynamics, morphogenesis, polarized growth, cell movements, spindle formation, chromosome movement, nuclear fusion, and signal transduction. In non-plant systems three families of molecular motors (kinesins, dyneins, and myosins) have been well characterized. These motors use microtubules (in the case of kinesines and dyneins) or actin filaments (in the case of myosins) as tracks to transport cargo materials intracellularly. During the last decade tremendous progress has been made in understanding the structure and function of various motors in animals. These studies are yielding interesting insights into the functions of molecular motors and the origin of different families of motors. Furthermore, the paradigm that motors bind cargo and move along cytoskeletal tracks does not explain the functions of some of the motors. Relatively little is known about the molecular motors and their roles in plants. In recent years, by using biochemical, cell biological, molecular, and genetic approaches a few molecular motors have been isolated and characterized from plants. These studies indicate that some of the motors in plants have novel features and regulatory mechanisms. The role of molecular motors in plant cell division, cell expansion, cytoplasmic streaming, cell-to-cell communication, membrane trafficking, and morphogenesis is beginning to be understood. Analyses of the Arabidopsis genome sequence database (51% of genome) with conserved motor domains of kinesin and myosin families indicates the presence of a large number (about 40) of molecular motors and the functions of many of these motors remain to be discovered. It is likely that many more motors with novel regulatory mechanisms that perform plant-specific functions are yet to be discovered. Although the identification of motors in plants, especially in Arabidopsis, is progressing at a rapid pace because of the ongoing plant genome sequencing projects, only a few plant motors have been characterized in any detail. Elucidation of function and regulation of this multitude of motors in a given species is going to be a challenging and exciting area of research in plant cell biology. Structural features of some plant motors suggest calcium, through calmodulin, is likely to play a key role in regulating the function of both microtubule- and actin-based motors in plants.

Tissue-preferential expression of a rice alpha-tubulin gene, OsTubA1, mediated by the first intron

The genomic clone encoding an alpha-tubulin, OsTubA1, has been isolated from rice (Oryza sativa L.). The gene consists of four exons and three introns. RNA-blot analysis showed that the gene is strongly expressed in actively dividing tissues, including root tips, young leaves, and young flowers. Analysis of chimeric fusions between OsTubA1 and beta-glucuronidase (GUS) revealed that the intron 1 was required for high-level GUS expression in actively dividing tissues, corresponding with normal expression pattern of OsTubA1. Fusion constructs lacking the intron 1 showed more GUS staining in mature tissues rather than young tissues. When the intron 1 was placed at the distal region from 5'-upstream region or at the 3'-untranslated region, no enhancement of GUS expression was observed. Sequential deletions of the OsTubA1 intron 1 brought about a gradual reduction of GUS activity in calli. These results suggest that tissue-preferential expression of the OsTubA1 gene is mediated by the intron 1 and that it may be involved in a mechanism for an efficient RNA splicing that is position dependent.

The evolution of new structures: clues from plant cytoskeletal genes

How large numbers of genes were recruited simultaneously to build new organ structures is one of the greatest puzzles in evolutionary biology. Here, we present data suggesting that the vegetative and reproductive classes of actins and other cytoskeletal proteins arose concurrently with the macroevolutionary divergence of leaves and reproductive structures in the earliest land plants. That the cytoskeleton is essential for physically programming the development of organs and tissues is well established. Thus, we propose that this regulatory dichotomy represents an ancient landmark event in the global regulation of hundreds of higher-plant genes, an event that is linked to the macroevolution of plant vegetative and reproductive organs. The recent availability of sequence and expression data for large numbers of plant genes should make it possible to dissect this and other major macroevolutionary events.

Codon usage and gene function are related in sequences of Arabidopsis thaliana

In this paper, the relationship between codon usage and the physiological pattern of expression of a gene is investigated while considering a dataset of 815 nuclear genes of Arabidopsis thaliana. Factorial Correspondence Analysis, a commonly used multivariate statistical approach in codon usage analysis, was used in order to analyse codon usage bias gene by gene. The analysis reveals a single major trend in codon usage among genes in Arabidopsis. At one end of the trend lie genes with a highly G/C biased codon usage. This group contains mainly photosynthetic and housekeeping genes which are known to encode the most abundant proteins of the vegetal cell. At the other extreme lie genes with a weaker A/T-biased codon usage. This group contain genes with various functions which exhibits most of the time a strong tissue-specific pattern of expression in relation, for example, to stress conditions. These observations were confirmed by the detailed analysis of codon usage in the multigene family of tubulins and appear to be general in plant species, even as distant from Arabidopsis thaliana as a monocotyledonous plant such as maize.

Isolation and characterization of two beta-tubulin cDNA clones from rice

Two cDNA clones encoding two different beta-tubulins, RTUB-1 and RTUB-2, were isolated from a rice cDNA library and their nucleotide sequences were analyzed. The deduced amino acid sequences showed amino acid sequence identity between 92% and 97% with other plant beta-tubulins. Southern blot analysis using gene-specific and coding-region probes suggested that beta-tubulins in rice are encoded by multigene families. The two cDNA clones represent two subfamilies of rice tubulins, RTUB-1 and RTUB-2, consisting of 3 to 4 genes and a single gene, respectively. The transcript levels of RTUB-1 and RTUB-2 genes were higher in actively elongating tissues such as etiolated shoot tissues and light-grown root tissues of four-day old seedlings.

The plant cytoskeleton

Particles that can nucleate microtubules in vitro have been isolated from higher plant cells. Observations of living cells injected with fluorescent probes have improved our understanding of plant cytoskeleton dynamics. Despite growing recognition of the need for biochemical studies on cytoskeleton-associated proteins, little progress has been made in this field in the past year, although plant lamins have been isolated and partially characterized.