Growth In Tubes Research Articles

Gamma Irradiation Effects on Durum Wheat (Triticum durum Desf.) under Various Conditions

The effects on morphological and physiological characters of durum wheat (Triticum durum Desf.) plants issued from seeds upon irradiation with low doses of cobalt gamma rays (i.e., 0, 10, 20 and 30 Gy), were studied. The study is carried out in the Experimental Research Station of Ecole Supérieure d'Agriculture du Kef (North West Tunisia) in 2008/2009. In Petri dishes, the 20 Gy dose caused an increase of the speed and Germination Capacity (GC) of the seeds as compared to non irradiated ones. Plants from these treated seeds maintained on Knops' culture medium (culture medium used to study plant growth in test tubes), improved root system in terms of length, volume and weight when compared to the plants issued from the non treated seeds. This irradiation dose (20 Gy) also improved in a significant way the above ground system growth of the plants. Under glass house conditions with a water stress, the plants issued from seeds treated with 20 Gy, had higher water content and membrane stability as compared of those from the non irradiated ones. Furthermore, seed irradiation with this dose had a positive effect on the chlorophyll content and maximum quantum yield of the irradiated plants. These results suggest that ionizing irradiation may be considered as an alternative in improving root growth of the plant and therefore controlling drought.

Polar growth in pollen tubes is associated with spatially confined dynamic changes in cell mechanical properties

Cellular morphogenesis involves changes to cellular size and shape which in the case of walled cells implies the mechanical deformation of the extracellular matrix. So far, technical challenges have made quantitative mechanical measurements of this process at subcellular scale impossible. We used micro-indentation to investigate the dynamic changes in the cellular mechanical properties during the onset of spatially confined growth activities in plant cells. Pollen tubes are cellular protuberances that have a strictly unidirectional growth pattern. Micro-indentation of these cells revealed that the initial formation of a cylindrical protuberance is preceded by a local reduction in cellular stiffness. Similar cellular softening was observed before the onset of a rapid growth phase in cells with oscillating growth pattern. These findings provide the first quantitative cytomechanical data that confirm the important role of the mechanical properties of the cell wall for local cellular growth processes. They are consistent with a conceptual model that explains pollen tube oscillatory growth based on the relationship between turgor pressure and tensile resistance in the apical cell wall. To further confirm the significance of cell mechanics, we artificially manipulated the mechanical cell wall properties as well as the turgor pressure. We observed that these changes affected the oscillation profile and were able to induce oscillatory behavior in steadily growing tubes.

Calcium‐dependent protein kinases regulate polarized tip growth in pollen tubes

Calcium signals are critical for the regulation of polarized growth in many eukaryotic cells, including pollen tubes and neurons. In plants, the regulatory pathways that code and decode Ca(2+) signals are poorly understood. In Arabidopsis thaliana, genetic evidence presented here indicates that pollen tube tip growth involves the redundant activity of two Ca(2+)-dependent protein kinases (CPKs), isoforms CPK17 and -34. Both isoforms appear to target to the plasma membrane, as shown by imaging of CPK17-yellow fluorescent protein (YFP) and CPK34-YFP in growing pollen tubes. Segregation analyses from two independent sets of T-DNA insertion mutants indicate that a double disruption of CPK17 and -34 results in an approximately 350-fold reduction in pollen transmission efficiency. The near sterile phenotype of homozygous double mutants could be rescued through pollen expression of a CPK34-YFP fusion. In contrast, a transgene rescue was blocked by mutations engineered to disrupt the Ca(2+)-activation mechanism of CPK34 (CPK34-YFP-E465A,E500A), providing in vivo evidence linking Ca(2+) activation to a biological function of a CPK. While double mutant pollen tubes displayed normal morphology, relative growth rates for the most rapidly growing tubes were reduced by more than three-fold compared with wild type. In addition, while most mutant tubes appeared to grow far enough to reach ovules, the vast majority (>90%) still failed to locate and fertilize ovules. Together, these results provide genetic evidence that CPKs are essential to pollen fitness, and support a mechanistic model in which CPK17 and -34 transduce Ca(2+) signals to increase the rate of pollen tube tip growth and facilitate a response to tropism cues.

Uncovering hidden treasures in pollen tube growth mechanics

The long-standing model of tip growth in pollen tubes considers that exocytosis and growth occur at the apex and that the pool of very small vesicles in the apical dome contains secretory (exocytic) vesicles. However, recent work on vesicle trafficking dynamics in tobacco pollen tubes shows that exocytosis occurs in the subapical region. Taking these and other new results into account, we set out to resolve specific problems that are endemic in current models and present a two-part ACE (apical cap extension)-H (hydrodynamics) growth model. The ACE model involves delivery and recycling of materials required for new cell synthesis and the H model involves mechanisms that integrate and regulate key cellular pathways and drive cell elongation during growth.

Prediction of oxide scale growth in superheater and reheater tubes

In this paper a procedure on how to estimate the oxide scale growth in superheater and reheater tube utilizing the empirical formulae and the finite element modeling is proposed. An iterative procedure consisting of empirical formulae and numerical simulation is used to determine scale thickness as both temperature and time increase. Results of the scale thickness over period of time for two different design temperatures of the steam and different heat transfer parameters are presented. The procedures may provide better estimation on the oxide scale growth, provided that all the heat transfer parameters are well specified.

Laser damage to marine plankton and its application to checking biofouling and invasion by aquatic species: a laboratory study

In this laboratory study, the ability of low-power pulsed laser irradiation to kill planktonic organisms in a flowing water system was examined, thus, to test the possibility of using this technique as a water treatment strategy to reduce biofouling growth in condenser tubes of power plants and to reduce bioinvasion via the ballast water of ships. Two flow rates (4.6 and 9.0 l h−1) were tested on three planktonic organisms: two marine centric diatoms viz. Skeletonema costatum and Chaetoceros gracilis and a dinoflagellate, Heterocapsa circularisquama. A low-power pulsed laser irradiation at 532 nm with a fluence of 0.1 J cm−2 from a frequency-doubled Nd:YAG laser was used as the irradiation source. The laser irradiation resulted in a heavy mortality of the test cells. The mortality observed was >90% for S. costatum and H. circularisqama and >70% for C. gracilis. The results suggest that laser irradiation has the potential to act as a water treatment strategy to reduce biofouling of condenser tubes in power plants as well as to reduce species invasion via the ballast water of ships.

Reactive oxygen species (ROS) as early signals in root hair cells responding to rhizobial nodulation factors

Reactive oxygen species (ROS) are involved in supporting polar growth in pollen tubes, fucoid cells and root hair cells. However, there is limited evidence showing ROS changes during the earliest stages of the interaction between legume roots and rhizobia. We recently reported using Phaseolus vulgaris as a model system, the occurrence of a transient increase of ROS, within seconds, at the tip of actively growing root hair cells after treatment with Nod factors (NFs).1 This transient response is NFs-specific, and clearly distinct from the ROS changes induced by a fungal elicitor, with which sustained increases in ROS signal, is observed. Since ROS levels are transiently elevated after NFs perception, we propose that this ROS response is specific of the symbiotic interaction. Furthermore, the observed ROS changes correlate spatially and temporarily with the reported transient increases in calcium levels suggesting key roles for calcium and ROS during the early NF perception.Addendum to: Cárdenas L, Martinez A, Sanchez F, Quinto C. Fast transient and specific intracellular ROS Changes in root hair cells responding to Nod factors (NFs). Plant J 2008; DOI: 10.1111/j.1365-313X.2008.03644.x.

A Tip-Localized RhoGAP Controls Cell Polarity by Globally Inhibiting Rho GTPase at the Cell Apex

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 Rho-like GTPase from Plant 1 (ROP1) are critical for tip growth in pollen tubes. However, the underlying mechanism for the generation and maintenance of this dynamic apical cap is poorly understood. 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 disruption of ROP1-dependent F-actin and vesicular trafficking, which indicates that REN1 targeting and function is regulated by ROP1 downstream signaling. 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.

Cell Polarity Signaling in Arabidopsis

Cell polarization is intimately linked to plant development, growth, and responses to the environment. Major advances have been made in our understanding of the signaling pathways and networks that regulate cell polarity in plants owing to recent studies on several model systems, e.g., tip growth in pollen tubes, cell morphogenesis in the leaf epidermis, and polar localization of PINs. From these studies we have learned that plant cells use conserved mechanisms such as Rho family GTPases to integrate both plant-specific and conserved polarity cues and to coordinate the cytoskeketon dynamics/reorganization and vesicular trafficking required for polarity establishment and maintenance. This review focuses upon signaling mechanisms for cell polarity formation in Arabidopsis, with an emphasis on Rho GTPase signaling in polarized cell growth and how these mechanisms compare with those for cell polarity signaling in yeast and animal systems.

Tip growth: signaling in the apical dome.

Signaling molecules, such as ROP/RAC GTPases and their regulators, reactive oxygen species (ROS) and phospholipids, play pivotal roles in the control of tip growth in pollen tubes and root hairs. They are often localized to the apical growing region of these cells, where their functions are tightly interconnected with cytoskeletal rearrangement and polar vesicle trafficking, which participate in tip growth as well as affect the generation and maintenance of the apical growing region. Recent advances in our understanding of the interface between these cellular activities and signaling in tip growth will be discussed.

F-Actin Organization and Pollen Tube Tip Growth inArabidopsisAre Dependent on the Gametophyte-Specific Armadillo Repeat Protein ARO1

The signal-mediated and spatially controlled assembly and dynamics of actin are crucial for maintaining shape, motility, and tip growth of eukaryotic cells. We report that a novel Armadillo repeat protein in Arabidopsis thaliana, ARMADILLO REPEAT ONLY1 (ARO1), is of fundamental importance for polar growth and F-actin organization in tip-growing pollen tubes. ARO1 is specifically expressed in the vegetative cell of pollen as well as in the egg cell. ARO1-GFP (for green fluorescent protein) fusion proteins accumulate most notably in pollen tube tips and partially colocalize with F-actin in the shank of pollen tubes. ARO1 knockout results in a highly disorganized actin cytoskeleton, growth depolarization, and ultimately tube growth arrest. Tip-localized ARO1-GFP is spatially shifted toward the future site of tip growth, indicating a role of ARO1 in the signaling network controlling tip growth and regulating actin organization. After the pollen tube discharges its contents into the receptive synergid, ARO1-GFP colocalizes with emerging F-actin structures near the site of sperm cell fusion, suggesting additional participation in the mechanism of sperm cell tracking toward the female gametes. The variable localization of ARO1 in the cytoplasm, the nucleus, and at the plasma membrane, however, indicates a multifunctional role like that of beta-catenin/Armadillo and the p120 catenins.

Exclusion of a proton ATPase from the apical membrane is associated with cell polarity and tip growth in Nicotiana tabacum pollen tubes.

Polarized growth in pollen tubes results from exocytosis at the tip and is associated with conspicuous polarization of Ca(2+), H(+), K(+), and Cl(-) -fluxes. Here, we show that cell polarity in Nicotiana tabacum pollen is associated with the exclusion of a novel pollen-specific H(+)-ATPase, Nt AHA, from the growing apex. Nt AHA colocalizes with extracellular H(+) effluxes, which revert to influxes where Nt AHA is absent. Fluorescence recovery after photobleaching analysis showed that Nt AHA moves toward the apex of growing pollen tubes, suggesting that the major mechanism of insertion is not through apical exocytosis. Nt AHA mRNA is also excluded from the tip, suggesting a mechanism of polarization acting at the level of translation. Localized applications of the cation ionophore gramicidin A had no effect where Nt AHA was present but acidified the cytosol and induced reorientation of the pollen tube where Nt AHA was absent. Transgenic pollen overexpressing Nt AHA-GFP developed abnormal callose plugs accompanied by abnormal H(+) flux profiles. Furthermore, there is no net flux of H(+) in defined patches of membrane where callose plugs are to be formed. Taken together, our results suggest that proton dynamics may underlie basic mechanisms of polarity and spatial regulation in growing pollen tubes.

Model for calcium dependent oscillatory growth in pollen tubes

Experiments have shown that pollen tubes grow in an oscillatory mode, the mechanism of which is poorly understood. We propose a theoretical growth model of pollen tubes exhibiting such oscillatory behaviour. The pollen tube and the surrounding medium are represented by two immiscible fluids separated by an interface. The physical variables are pressure, surface tension, density and viscosity, which depend on relevant biological quantities, namely calcium concentration and thickness of the cell wall. The essential features generally believed to control oscillating growth are included in the model, namely a turgor pressure, a viscous cell wall which yields under pressure, stretch-activated calcium channels which transport calcium ions into the cytoplasm and an exocytosis rate dependent on the cytosolic calcium concentration in the apex of the cell. We find that a calcium dependent vesicle recycling mechanism is necessary to obtain an oscillating growth rate in our model. We study the variation in the frequency of the growth rate by changing the extracellular calcium concentration and the density of ion channels in the membrane. We compare the predictions of our model with experimental data on the frequency of oscillation versus growth speed, calcium concentration and density of calcium channels.

On Solar Intermediate Drift Radio Bursts at Decimeter and Meter Wavelength

Fiber – or intermediate drift – bursts are a continuum fine structure in some complex solar radio events. We present the analysis of such bursts in the X17 flare on 28 Oct. 2003. Based on the whistler wave model of fiber bursts we derive the 3D magnetic field structures that carry the radio sources in different stages of the event and obtain insight into the energy release evolution in the main flare phase, the related paths of nonthermal particle propagation in the corona, and the involved magnetic field structures. Additionally, we test the whistler wave model of fiber bursts for the meter and the decimeter wave range. Radio spectral data (Astrophysikalisches Institut Potsdam, Astronomical Observatory Ondřejov) show a continuum with fibers for ≈ 6 min during the main flare phase. Radio imaging data (Nancay Radio Heliograph) yield source centroid positions of the fibers at three frequencies in the spectrometer band. We compare the radio positions with the potential coronal magnetic field extrapolated from SOHO/MDI data. Given the detected source site configuration and evolution, and the change of the fiber burst frequency range with time, we can also extract those coronal flux tubes where the high-frequency fiber bursts are situated even without decimeter imaging data. To this aim we use a kinetic simulation of whistler wave growth in sample flux tubes modeled by selected potential field lines and a barometric density model. The whistler wave model of fiber bursts accurately explains the observations on 28 Oct. 2003. A laterally extended system of low coronal loops is found to guide the whistler waves. It connects several neighboring active regions including the flaring AR 10486. For varying source sites the fiber bursts are emitted at the fundamental mode of the plasma frequency over the whole range (1200 – 300 MHz). The present event can be understood without assuming two different generation mechanisms for meter and decimeter wave fiber bursts. It gives new insight into particle acceleration and propagation in the low flare and post-CME corona.

Differential organelle movement on the actin cytoskeleton in lily pollen tubes

We have examined the arrangement and movement of three major compartments, the endoplasmic reticulum (ER), mitochondria, and the vacuole during oscillatory, polarized growth in lily pollen tubes. These movements are dependent on the actin cytoskeleton, because they are strongly perturbed by the anti-microfilament drug, latrunculin-B, and unaffected by the anti-microtubule agent, oryzalin. The ER, which has been labeled with mGFP5-HDEL or cytochalasin D tetramethylrhodamine, displays an oscillatory motion in the pollen tube apex. First it moves apically in the cortical region, presumably along the cortical actin fringe, and then periodically folds inward creating a platform that transects the apical domain in a plate-like structure. Finally, the ER reverses its direction and moves basipetally through the central core of the pollen tube. When subjected to cross-correlation analysis, the formation of the platform precedes maximal growth rates by an average of 3 s (35-40 degrees ). Mitochondria, labeled with Mitotracker Green, are enriched in the subapical region, and their movement closely resembles that of the ER. The vacuole, labeled with carboxy-dichlorofluorescein diacetate, consists of thin tubules arranged longitudinally in a reticulate network, which undergoes active motion. In contrast to the mitochondria and ER, the vacuole is located back from the apex, and never extends into the apical clear zone. We have not been able to decipher an oscillatory pattern in vacuole motion. Because this motion is dependent on actin and not tubulin, we think this is due to a different myosin from that which drives the ER and mitochondria.

NAD(P)H oscillates in pollen tubes and is correlated with tip growth.

The location and changes in NAD(P)H have been monitored during oscillatory growth in pollen tubes of lily (Lilium formosanum) using the endogenous fluorescence of the reduced coenzyme (excitation, 360 nm; emission, >400 nm). The strongest signal resides 20 to 40 microm behind the apex where mitochondria (stained with Mitotracker Green) accumulate. Measurements at 3-s intervals reveal that NAD(P)H-dependent fluorescence oscillates during oscillatory growth. Cross-correlation analysis indicates that the peaks follow growth maxima by 7 to 11 s or 77 degrees to 116 degrees, whereas the troughs anticipate growth maxima by 5 to 10 s or 54 degrees to 107 degrees. We have focused on the troughs because they anticipate growth and are as strongly correlated with growth as the peaks. Analysis of the signal in 10-microm increments along the length of the tube indicates that the troughs are most advanced in the extreme apex. However, this signal moves basipetally as a wave, being in phase with growth rate oscillations at 50 to 60 microm from the apex. We suggest that the changes in fluorescence are due to an oscillation between the reduced (peaks) and oxidized (troughs) states of the coenzyme and that an increase in the oxidized state [NAD(P)(+)] may be coupled to the synthesis of ATP. We also show that diphenyleneiodonium, an inhibitor of NAD(P)H dehydrogenases, causes an increase in fluorescence and a decrease in tube growth. Finally, staining with 5-(and-6)-chloromethyl-2',7'-dichlorohydrofluorescein acetate indicates that reactive oxygen species are most abundant in the region where mitochondria accumulate and where NAD(P)H fluorescence is maximal.

Membrane trafficking and polar growth in root hairs and pollen tubes

Root hairs and pollen tubes extend by rapid elongation that occurs exclusively at the tip. Fundamental for such local, tip-focused growth (so-called 'tip growth') is the polarization of the cytoplasm that directs secretory events to the tip, and the presence of internal gradients and transmembrane flux of ions, notably Ca2+, H+, K+, and Cl-. Electrophysiological and imaging studies using fluorescent markers have sought to link ion gradients with growth and membrane trafficking. Current models recognize membrane trafficking as fundamental to tip growth, notably its role in supplying lipid and protein to the new plasma membrane and cell wall that extend the apex of the cell, and a complementary role for endocytosis in retrieving excess membrane and in recycling various protein fractions. The current state of knowledge is reviewed here in order to highlight the major gaps in the present understanding of trafficking as it contributes to polar growth in these cells and recent results, that suggest a role for membrane trafficking in the active regulation of ion channel turnover and activity during polar tip growth, are discussed.

Nt‐RhoGDI2 regulates Rac/Rop signaling and polar cell growth in tobacco pollen tubes

Rac/Rop-type Rho-family small GTPases accumulate at the plasma membrane in the tip of pollen tubes and control the polar growth of these cells. Nt-RhoGDI2, a homolog of guanine nucleotide dissociation inhibitors (GDIs) regulating Rho signaling in animals and yeast, is co-expressed with the Rac/Rop GTPase Nt-Rac5 specifically in tobacco (Nicotiana tabacum) pollen tubes. The two proteins interact with each other in yeast two-hybrid assays, preferentially when Nt-Rac5 is prenylated. Transient over-expression of Nt-Rac5 and Nt-RhoGDI2 depolarized or inhibited tobacco pollen tube growth, respectively. Interestingly, pollen tubes over-expressing both proteins grew normally, demonstrating that the two proteins functionally interact in vivo. Nt-RhoGDI2 was localized to the pollen tube cytoplasm and effectively transferred co-over-expressed YFP-Nt-Rac5 fusion proteins from the plasma membrane to this compartment. A single amino acid exchange (R69A), which abolished binding to Nt-RhoGDI2, caused Nt-Rac5 to be mis-localized to the flanks of pollen tubes and strongly compromised its ability to depolarize pollen tube growth upon over-expression. Based on these observations, we propose that Nt-RhoGDI2-mediated recycling of Nt-Rac5 from the flanks of the tip to the apex has an essential function in the maintenance of polarized Rac/Rop signaling and cell expansion in pollen tubes. Similar mechanisms may generally play a role in the polarized accumulation of Rho GTPases in specific membrane domains, an important process whose regulation has not been well characterized in any cell type to date.

The Maize aberrant pollen transmission 1 Gene Is a SABRE/KIP Homolog Required for Pollen Tube Growth

Maize (Zea mays) pollen tubes grow in the styles at a rate of >1 microm/sec. We describe here a gene required to attain that striking rate. The aberrant pollen transmission 1 (apt1) gene of maize was identified by an Ac-tagged mutation that displayed a severe pollen transmission deficit in heterozygotes. Rare apt1 homozygotes can be recovered, aided by phenotypic selection for Ac homozygotes. Half of the pollen in heterozygotes and most of the pollen in homozygotes germinate short and twisted pollen tubes. The apt1 gene is 26 kb long, makes an 8.6-kb pollen-specific transcript spliced from 22 exons, and encodes a protein of 2607 amino acids. The APT1 protein is homologous to SABRE and KIP, Arabidopsis proteins of unknown function involved in the elongation of root cortex cells and pollen tubes, respectively. Subcellular localization analysis demonstrates that APT1 colocalizes with a Golgi protein marker in growing tobacco pollen tubes. We hypothesize that the APT1 protein is involved in membrane trafficking and is required for the high secretory demands of tip growth in pollen tubes. The apt1-m1(Ac) mutable allele is an excellent tool for selecting Ac transpositions because of the strong negative selection pressure operating against the parental Ac site.

Members of a Novel Class of Arabidopsis Rho Guanine Nucleotide Exchange Factors Control Rho GTPase-Dependent Polar Growth

Rho family small GTPases are signaling switches controlling many eukaryotic cellular processes. Conversion from the GDP- to GTP-bound form is catalyzed by guanine nucleotide exchange factors (GEFs). Rho GEFs in animals fall into two structurally distinct classes containing DH and DOCKER catalytic domains. Using a plant Rho GTPase (ROP1) as bait in yeast two-hybrid screens, we identified a family of Rho GEFs, named RopGEFs. The Arabidopsis thaliana RopGEF family of 14 members contains a conserved central domain, the domain of unknown function 315 (DUF315), and variable N- and C-terminal regions. In vitro GEF assays show that DUF315 but not the full-length version of RopGEF1 has high GEF activity toward ROP1. Our data suggest that the variable regions of RopGEF1 are involved in regulation of RopGEF through an autoinhibitory mechanism. RopGEF1 overexpression in pollen tubes produced growth depolarization, as does a constitutively active ROP1 mutant. The RopGEF1 overexpression phenotype was suppressed by expression of a dominant-negative mutant of ROP1, probably by trapping RopGEF1. Deletion mutant analysis suggested a requirement of RopGEF activity for the function of RopGEF1 in polar growth. Green fluorescent protein-tagged RopGEF1 was localized to the tip of pollen tubes where ROP1 is activated. These results provide strong evidence that RopGEF1 activates ROP1 in control of polar growth in pollen tubes.