Myosins form a large superfamily of molecular motors that move along actin filaments. The functions of myosins in plant cells are thought to be related to various processes: cell division, movement of mitochondria and chloroplasts, cytoplasmic streaming, rearrangement of transvacuolar strands, and statolith positioning. Class VIII and XI myosins are represented in the Arabidopsis thaliana genome by 4 and 13 potential genes, respectively. The roles of individual class XI myosins and their cellular targets in A. thaliana are still unclear. In this work we implemented a reverse genetic approach to analyse the loss-of-function mutants of XIK, a representative of class XI myosins in A. thaliana. Three different T-DNA insertion mutants in the myosin XIK gene showed similar phenotypes: impaired growth of root hair cells, twisted shape of stem trichomes, and irregular size, branch positioning, and branch expansion of leaf trichomes. Morphometric analysis of mutant seedlings showed that the average length of root hairs was reduced up to 50% in comparison with wild-type root hairs, suggesting an involvement of the class XI myosin XIK in tip growth. On leaves, the proportion of trichomes with short branches was doubleed in mutant plants, and the mutant trichomes possessed a mildly twisted shape. Therefore, we concluded that myosin XIK is involved also in the elongation of stalks and branches of trichomes.

BackgroundThe positioning and dynamics of vesicles and organelles, and thus the growth of plant cells, is mediated by the acto-myosin system. In Arabidopsis there are 13 class XI myosins which mediate vesicle and organelle transport in different cell types. So far the involvement of five class XI myosins in cell expansion during the shoot and root development has been shown, three of which, XI-1, XI-2, and XI-K, are essential for organelle transport.ResultsSimultaneous depletion of Arabidopsis class XI myosins XI-K, XI-1, and XI-2 in double and triple mutant plants affected the growth of several types of epidermal cells. The size and shape of trichomes, leaf pavement cells and the elongation of the stigmatic papillae of double and triple mutant plants were affected to different extent. Reduced cell size led to significant size reduction of shoot organs in the case of triple mutant, affecting bolt formation, flowering time and fertility. Phenotype analysis revealed that the reduced fertility of triple mutant plants was caused by delayed or insufficient development of pistils.ConclusionsWe conclude that the class XI myosins XI-K, XI-1 and XI-2 have partially redundant roles in the growth of shoot epidermis. Myosin XI-K plays more important role whereas myosins XI-1 and XI-2 have minor roles in the determination of size and shape of epidermal cells, because the absence of these two myosins is compensated by XI-K. Co-operation between myosins XI-K and XI-2 appears to play an important role in these processes.

Movement protein binding 2C (MPB2C) is a plant endogenous microtubule-associated protein previously identified as an interaction partner of tobacco (Nicotiana tabacum) mosaic virus movement protein (TMV-MP). In this work, the role of MPB2C in cell-to-cell transport of TMV-MP, viral spread of TMV, and subcellular localization of TMV-MP was examined. To this end, plants with reduced MPB2C levels were generated by a gene-silencing strategy. Local and systemic spread of TMV and cell-tocell movement of TMV-MP were unimpaired in MPB2C-silenced plants as compared to nonsilenced plants, indicating that MPB2C is not required for intercellular transport of TMV-MP itself or spread of TMV. However, a clear change in subcellular distribution of TMV-MP characterized by a nearly complete loss of microtubular localization was observed in MPB2C-silenced plants. This result shows that the MPB2C is a central player in determining the complex subcellular localization of TMV-MP, in particular its microtubular accumulation, a phenomenon that has been frequently observed and whose role is still under discussion. Clearly, MPB2C mediated accumulation of TMV-MP at microtubules is not required for intercellular spread but may be a means to withdraw the TMV-MP from the cell-to-cell transport pathway.

The main sigma factor activating gene expression, necessary in stationary phase and under stress conditions, is S . In contrast to other minor sigma factors, RNA polymerase holoenzyme containing S (E S ) recognizes a number of promoters which are also recognized by that containing 70 (E 70 ). We have previously shown that transposon Tn4652 can activate silent genes in starving Pseudomonas putida cells by creating fusion promoters during transposition. The sequence of the fusion promoters is similar to the 70 -specific promoter consensus. The ؊10 hexameric sequence and the sequence downstream from the ؊10 element differ among these promoters. We found that transcription from the fusion promoters is stationary phase specific. Based on in vivo experiments carried out with wild-type and rpoS-deficient mutant P. putida, the effect of S on transcription from the fusion promoters was established only in some of these promoters. The importance of the sequence of the ؊10 hexamer has been pointed out in several published papers, but there is no information about whether the sequences downstream from the ؊10 element can affect In their natural environment, most bacteria are challenged by widely changing nutrient availability and by exposure to various forms of physical stress (temperature shock, oxidative stress, etc.). When starvation or various other stress factors cause a reduction or cessation of growth, many genes are shut down while others are induced to help the cells to survive. One way to modulate gene expression is replacement of the main sigma factor with an alternative sigma factor that recognizes a specific promoter of the stimulus response gene (reviewed in reference 19). In Escherichia coli, S regulates the expression of more than 100 genes involved in cell survival in the stationary phase and in response to different stresses (reviewed in references 14 and 15). The rpoS gene encoding S has been described also for nonenteric bacteria, e.g., fluorescent pseudomonads (21,36,37,40). Despite clearly different physiological roles, S is similar to the major sigma factor 70 in terms of structure and molecular function (7,26,42). No clear differences between 70 -and S -dependent promoters are apparent. A compilation of S -dependent promoters deduced a Ϫ10 consensus sequence, CTATACT, that is slightly different from the typical TATAAT sequence recognized by 70 (11). However, the binding patterns of E S and E 70 revealed by DNase I protection experiments are not completely identical (29,41). E S appears to be less dependent on contacts in the Ϫ35 region (7,17,41). The activity of E S and E 70 is differentially influenced by salt concentrations and by the degree of negative supercoiling of the DNA template (2, 10, 23). Additionally, a number of global regulators and histone-like proteins, such as H-NS, Lrp, CRP, IHF, and Fis, are involved in determination of sigma factor specificity (29; see also references 14 and 15 for a review and references cited therein).We have previously shown the generation of constitutively ex...

Expression of the phenol degradation pathway in Pseudomonas putida strain PaW85 requires coordinated transcription of the plasmid-borne pheBA operon encoding catechol 1,2-dioxygenase and phenol monooxygenase, respectively, and the chromosomally encoded catechol degradation catBCA operon. Transcriptional activation from the pheBA and catBCA promoters is regulated by CatR and the catechol degradation pathway intermediate cis,cis-muconate. Here it is shown that physiological control mechanisms are superimposed on this regulatory system. Transcriptional activation from the pheBA and catBCA promoters is growth-phase-regulated in P. putida cells grown on rich medium (LB medium). CatR-mediated transcription from these promoters is silenced on rich medium until the transition from exponential to stationary phase. A slight positive effect (threefold) of stationary-phase-specific sigma factor σ S on transcription from the pheBA promoter was observed. Expression of the catBCA promoter was not influenced by the activity of this sigma factor. In contrast to rich growth medium, transcription from the pheBA and catBCA promoters in minimal medium containing a mixture of glucose and sodium benzoate was rapidly induced in exponential culture. It was shown that the presence of amino acids in the culture medium causes exponential silencing of the pheBA and catBCA promoters. The possibility that a hypothetical repressor protein could be involved in physiological control of transcription from the pheBA and catBCA promoters is discussed.