Actin-Microtubule Interaction in Plants

Interactions between actins and microtubules play an important role in many fundamental cellular processes in eukaryotes. Although several studies have shown actins and microtubules to be involved in specific cellular activities, little is known about how actins and microtubules contribute together to a given process. Preprophase band formation, which plays an essential role in plant division site determination, is a cellular process that lends itself to studies of actin-microtubule interactions and how they contribute to important cellular functions. Recently, we have analyzed microtubule-associated microfilaments during preprophase band formation in onion cotyledon epidermal cells using a combination of high-pressure freezing/freeze substitution and electron tomography. Quantitative analysis of our electron tomography data showed that relatively short single microfilaments form bridges between two adjacent microtubules in the process of narrowing of the preprophase microtubule band. Two types of microtubule-microfilament-microtubule connections are observed, and these microfilament-microtubule interactions suggest a direct role of F-actins in microtubule bundling. Based on these observations, we discuss how different actin-microtubule linkers might contribute to preprophase band narrowing and to other changes in microtubule organization in plant cells

Ultrastructural study of plasmodesmata in the brown alga Dictyota dichotoma (Dictyotales, Phaeophyceae)

Plasmodesmata (PD) singular: plasmodesma, from the Greek plasma=cytoplasm and desmos=connection) are intercellular bridges of multicellular plants, directly connecting cytoplasms of neighboring cells. They play a crucial role in cell-to-cell communication and cell development. Although brown algae (Phaeophyceae, Heterokontophyta) are phylogenetically far from the lineage of land plants, they possess a complex multicellularity with PD like green plants. In this study, the ultrastructure and the formation of PD in the brown alga Dictyota dichotoma, were studied using transmission electron microscopy (TEM) and electron tomography with rapid freezing and freeze-substitution. D. dichotoma, possesses plasma membrane-lined simple PD without an internal endoplasmic reticulum (ER) (desmotubule), different from those of land plants. PD were clustered in thin cell wall regions forming pit fields. Fine proteinaceous internal bridges were observed in the cavity. Ultrastructural observations of cytokinesis of D. dichotoma revealed that the PD formation started from an early stage of cytokinesis with the formation of tubular pre-plasmodesmata (PPD) within the membranous sacs (MSs) of the cytokinetic diaphragm. Clustering of these PPD became into the future pit field. As cytokinesis proceeds, electron-dense material extended from the outer surface of the middle part of PPD and finally formed the nascent cell wall. From these results, it is suggested that PPD would be associated with the cell wall development during cytokinesis of D. dichotroma

Isolation of a Protein Interacting with Vfphot1a in Guard Cells of Vicia faba

A recent study has demonstrated that phototropins act as blue light receptors in stomatal guard cells. However, the downstream components responsible for phototropin signaling are largely unknown. In this study, using a yeast two-hybrid system, we isolated a Vicia faba protein that has a high similarity to dynein light chain in the C terminus, which interacts with Vicia faba phototropin 1a (Vfphot1a). Protein-blot and two-hybrid analyses revealed that Vfphot1a interacting protein (VfPIP) bound to the C-terminal region of Vfphot1a but did not bind to Vfphot1b. The interaction between VfPIP and Vfphot was indicated by a pull-down assay. Northern analysis revealed that the transcription level of VfPIP gene was more abundant in guard cells than in other tissues or cell types. The transiently expressed fusion protein of VfPIP-green fluorescent protein was localized on cortical microtubules in Vicia guard cells. Microtubule-depolymerizing herbicides partially inhibited both blue light-dependent H(+) pumping in Vicia guard cell protoplasts and stomatal opening in the Vicia epidermis. From these results, we conclude that VfPIP may act as a downstream component of phototropin (Vfphot1a) in blue light signaling in guard cells. The possible role of VfPIP in blue light signaling of guard cells is discussed

Surface IgM-Inducing Factor in the Culture Supernatant of Bursal Epithelial Cells Derived from Chick Embryos.

Bursal epithelial cells derived from 14-day-old chick embryos were cultured under a serum-free condition and the recovered culture supernatant was analyzed for surface immunoglobulin M (sIgM)-inducing activity on the bursacytes of 12-day-old chick embryos. Surface IgM positive rate of bursacytes incubated with the culture supernatant increased significantly, thus indicating the presence of sIgM-inducing factor(s) in the culture supernatant. The sIgM-inducing factor(s) in the supernatant was analyzed by gel filtration chromatography and HPLC. Results from these analyses indicated that the factor may be a polypeptide with a molecular weight of about 1300. This substance which is different from already known factors, such as Bursin, may be a newly discovered B cell differentiating factor in the bursa of Fabricius that promotes chicken B cell differentiation