Oscillation regulates a wide variety of processes ranging from chemotaxis in Dictyostelium through segmentation in vertebrate development to circadian rhythms. Most studies on the molecular mechanisms underlying oscillation have focused on processes requiring a rhythmic change in gene expression, which usually exhibit a periodicity of >10 min. Mechanisms that control oscillation with shorter periods (<10 min), presumably independent of gene expression changes, are poorly understood. Oscillatory pollen tube tip growth provides an excellent model to investigate such mechanisms. It is well established that ROP1, a Rho-like GTPase from plants, plays an essential role in polarized tip growth in pollen tubes. In this article, we demonstrate that tip-localized ROP1 GTPase activity oscillates in the same frequency with growth oscillation, and leads growth both spatially and temporally. Tip growth requires the coordinate action of two ROP1 downstream pathways that promote the accumulation of tip-localized Ca2+ and actin microfilaments (F-actin), respectively. We show that the ROP1 activity oscillates in a similar phase with the apical F-actin but apparently ahead of tip-localized Ca2+. Furthermore, our observations support the hypothesis that the oscillation of tip-localized ROP activity and ROP-dependent tip growth in pollen tubes is modulated by the two temporally coordinated downstream pathways, an early F-actin assembly pathway and a delayed Ca2+ gradient-forming pathway. To our knowledge, our report is the first to demonstrate the oscillation of Rho GTPase signaling, which may be a common mechanism underlying the oscillation of actin-dependent processes such as polar growth, cell movement, and chemotaxis.

Summary Background Highly elongated eukaryotic cells (e.g., neuronal axons, fungal hyphae, and pollen tubes) are generated through continuous apically restricted growth (tip growth), which universally requires tip-localized Rho GTPases. We used the oscillating pollen tube as a model system to determine the function and regulation of Rho GTPases in tip growth. Our previous work showed that the spatiotemporal dynamics of the apical cap of the activated ROP1 Rho GTPase is critical for tip growth in pollen tubes. However, the underlying mechanism for the generation and maintenance of this dynamic apical cap is poorly understood. Results A screen for mutations that enhance ROP1 overexpression-induced depolarization of pollen tube growth identified REN1 (ROP1 enhancer 1) in Arabidopsis, whose null mutations turn elongated pollen tubes into bulbous cells. REN1 encodes a novel Rho GTPase-activating protein (RhoGAP) required for restricting the ROP1 activity to the pollen tube tip. REN1 was localized to exocytic vesicles accumulated in the pollen tube apex as well as to the apical plasma membrane at the site of ROP1 activation. The apical localization of REN1 and its function in controlling growth polarity was compromised by disrupting ROP1-dependent F-actin and vesicular trafficking, which indicates that REN1 targeting and function is regulated by ROP1 downstream signaling. Conclusions Our findings suggest that the REN1 RhoGAP controls a negative feedback-based global inhibition of ROP1. This function provides a critical self-organizing mechanism, by which ROP signaling is spatially limited to the growth site and temporally oscillates during continuous tip growth. Similar spatiotemporal control of Rho GTPase signaling may also play an important role in cell polarity control in other systems, including tip growth in fungi and cell movement in animals.

Actin antagonists have previously been shown to alter responses of Commelina communis stomata to physiological stimuli, implicating actin filaments i n the control of guard cell volume changes (M. Kim, P.K. Hepler, 5-0. Eun, K.S. Ha, Y. Lee [1995] Plant Physiol 109: 1077-1084). Since K+ channels i n the guard cell play an important role i n stomatal movements, we examined the possible regulation of K+-channel activities by the state of actin polymerization. Agents affecting actin polymerization altered light-induced stomatal opening and inward K+-channel activities measured by patch clamping in Vicia faba. Cytochalasin D, which induces depolymerization of actin filaments, promoted light-induced stomatal opening and potentiated the inward K+ current in guard cell protoplasts. Phalloidin, a stabilizer of filamentous actin, inhibited both light-induced stomatal opening and inward K+ current. lnward K+-channel activities in outside-out membrane patches showed responses to these agents that support results at the whole-cell current levei, suggesting that cytochalasin D facilitates and phalloidin inhibits K+ influx in intact guard cells, thus resulting in enhancement and inhibition of stomatal opening, respectively. To our knowledge, this is the first report that provides evidence that actin filaments may regulate an important physiological process by modulating the activities of ion channels i n plant cells.The regulation of stomatal aperture is critica1 to a plant's ability to balance the need for a C source while avoiding the deleterious effects of water loss. The size of the stomatal opening is established through volume changes of stomatal guard cells under the concerted influence of light, temperature, CO,, and phytohormones (Assmann, 1993). Recently, Kim et al. (1995) showed that cortical actin filaments are distributed radially, fanning out from the stomatal pore site in mature guard cells of Commelinu communis. Moreover, fungal toxins that interfere with the polymerization or depolymerization of actin filaments brought about altered stomatal responses to physiological stimuli. Thus, dynamic changes in the actin cytoskeleton

The antimicrobial susceptibilities of Staphylococcus isolated from clinical isolates and raw meats were tested for six different antimicrobial agents that are in widespread clinical use in Korea and four new antimicrobials, linezolid, quinupristin/dalfopristin, daptomycin, and tigecycline. And this study analyzed the mecA genes and genetic patterns of MRSA by performing epidemiological studies using the PCR method. 46%, 51%, and 79% of clinical isolates were identified as MRSA in 1998, 1999, and 2005, respectively, and the mecA gene was detected in 82% of these isolates. Of the 133 staphylococci isolated from raw meats, 18% of the isolates were found to be resistant to methicillin, but none of these isolates showed the presence of the mecA gene. New antimicrobials, which have rarely or not yet been used in Korean hospitals, showed high activity against all staphylococcal isolates including methicillin-resistant isolates. The randomly amplified polymorphic DNA (RAPD) patterns of MRSA isolates differed significantly between clinical isolates and raw meat isolates.