Regulation of cell division and the cytoskeleton by mitogen-activated protein kinases in higher plants

AM fungal exudates activate MAP kinases in plant cells in dependence from cytosolic Ca(2+) increase

The molecular dialogue occurring prior to direct contact between the fungal and plant partners of arbuscular-mycorrhizal (AM) symbioses begins with the release of fungal elicitors, so far only partially identified chemically, which can activate specific signaling pathways in the host plant. We show here that the activation of MAPK is also induced by exudates of germinating spores of Gigaspora margarita in cultured cells of the non-leguminous species tobacco (Nicotiana tabacum), as well as in those of the model legume Lotus japonicus. MAPK activity peaked about 15 min after the exposure of the host cells to the fungal exudates (FE). FE were also responsible for a rapid and transient increase in free cytosolic Ca(2+) in Nicotiana plumbaginifolia and tobacco cells, and pre-treatment with a Ca(2+)-channel blocker (La(3+)) showed that in these cells, MAPK activation was dependent on the cytosolic Ca(2+) increase. A partial dependence of MAPK activity on the common Sym pathway could be demonstrated for a cell line of L. japonicus defective for LjSym4 and hence unable to establish an AM symbiosis. Our results show that MAPK activation is triggered by an FE-induced cytosolic Ca(2+) transient, and that a Sym genetic determinant acts to modulate the intensity and duration of this activity.

Mitogen-activated protein kinase 4 is involved in the regulation of mitotic and cytokinetic microtubule transitions in Arabidopsis thaliana

• A mitogen-activated protein kinase kinase kinase (MAPKKK) double mutant, Arabidopsis homologue of nucleus and phragmoplast associated kinase (anp) anp2anp3, and the mitogen-activated protein kinase (MAPK) 4 mutant mpk4 of Arabidopsis thaliana show prominent cytokinetic defects. This prompted the analysis of mitotic and cytokinetic progression as a function of MAPK signalling. Mutants were compared with wild types untreated or treated with the specific MAPKK inhibitor PD98059. • This study included phenotype analysis, expression analysis of the MPK4 promoter, immunofluorescent localization of MPK4, tubulin and MAP65-1, and time-lapse microscopic visualization of the mitotic microtubule (MT) transitions in control, mutant and inhibitor-treated cells. • Mutant and inhibitor-treated cells showed defects in mitosis and cytokinesis, including aberrant spindle and phragmoplast formation and drastically delayed or abortive mitosis and cytokinesis. As a result, bi- and multinucleate cells were formed, ultimately disturbing the vegetative tissue patterning. MPK4 was localized to all stages of the expanding phragmoplast, in a pattern similar to that of its putative substrate MAP65-1. • In this study, MPK4 is shown to be involved in the regulation of mitosis/cytokinesis through modulation of the cell division plane and cytokinetic progression.

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.

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.

Nanoscale intracellular organization and functional architecture mediating cellular behavior

Cells function based on a complex set of interactions that control pathways resulting in ultimate cell fates including proliferation, differentiation, and apoptosis. The inter-workings of this immensely dense network of intracellular molecules are influenced by more than random protein and nucleic acid distribution where their interactions culminate in distinct cellular function. By probing the design of these biological systems from an engineering perspective, researchers can gain great insight that will aid in building and utilizing systems that are on this size scale where traditional large-scale rules may fail to apply. The organized interaction and gradient distribution in intracellular space imply a structural architecture that modulates cellular processes by influencing biochemical interactions including transport and binding-reactions. One significant structure that plays a role in this modulation is the cell cytoskeleton. Here, we discuss the cytoskeleton as a central and integrating functional structure in influencing cell processes and we describe technology useful for probing this structure. We explain the nanometer scale science of cytoskeletal structure with respect to intracellular organization, mechanotransduction, cytoskeletal-associated proteins, and motor molecules, as well as nano- and microtechnologies that are applicable for experimental studies of the cytoskeleton. This biological architecture of the cytoskeleton influences molecular, cellular, and physiological processes through structured multimodular and hierarchical principles centered on these functional filaments. Through investigating these organic systems that have evolved over billions of years, understanding in biology, engineering, and nanometer-scaled science will be advanced.

Hyperosmotic stress induces formation of tubulin macrotubules in root-tip cells of Triticum turgidum: their probable involvement in protoplast volume control

Treatment of root-tip cells of Triticum turgidum with 1 M mannitol solution for 30 min induces microtubule (Mt) disintegration in the plasmolyzed protoplasts. Interphase plasmolyzed cells possess many cortical, perinuclear and endoplasmic macrotubules, 35 nm in mean diameter, forming prominent arrays. In dividing cells macrotubules assemble into aberrant mitotic and cytokinetic apparatuses resulting in the disturbance of cell division. Putative tubulin paracrystals were occasionally observed in plasmolyzed cells. The quantity of polymeric tubulin in plasmolyzed cells exceeds that in control cells. Root-tip cells exposed for 2-8 h to plasmolyticum recover partially, although the volume of the plasmolyzed protoplast does not change detectably. Among other events, the macrotubules are replaced by Mts, chromatin assumes its typical appearance and the cells undergo typical cell divisions. Additionally, polysaccharidic material is found in the periplasmic space. Oryzalin and colchicine treatment induced macrotubule disintegration and a significant reduction of protoplast volume in every plasmolyzed cell type examined, whereas cytochalasin B had only minor effects restricted to differentiated cells. These results suggest that Mt destruction by hyperosmotic stress, and their replacement by tubulin macrotubules and putative tubulin paracrystals is a common feature among angiosperms and that macrotubules are involved in the mechanism of protoplast volume regulation.

Altered patterns of tubulin polymerization in dividing leaf cells of Chlorophyton comosum after a hyperosmotic treatment

•  Microtubule organization and tubulin polymerization in meristematic leaf cells of Chlorophyton comosum treated with an aqueous solution of 1 M mannitol, inducing plasmolysis, were examined with immunofluorescence and transmission electron microscopy. •  Hyperosmotic treatment induced disintegration of the interphase microtubule systems. Free tubulin, either liberated from the depolymerized microtubules or pre-existing as a nonassembled pool, was incorporated into a network of paracrystals. In most of the dividing cells, mitotic and cytokinetic microtubule systems were replaced by atypical spindle-like structures displaying bipolarity and atypical phragmoplasts, respectively. These atypical mitotic and cytokinetic structures consisted of large densely packed bundles of macrotubules (32 nm diameter) or macrotubules and paracrystals. Tubulin paracrystals also occurred in ectopic positions in plasmolysed mitotic and cytokinetic cells. Dividing cells displaying paracrystals only did not form atypical mitotic and cytokinetic apparatuses. •  Short hyperosmotic stress causes disintegration of all microtubule arrays in dividing cells of C. comosum. Free tubulin is incorporated into macrotubules and tubulin paracrystals. The latter exhibit definite periodicity and characteristic fine structure.