For fertilization to occur in plants, the pollen tube must be guided to enter the ovule via the micropyle. Previous reports have implicated actin filaments, actin binding proteins, and the tip-focused calcium gradient as key contributors to polar growth of pollen tubes; however, the regulation of directional pollen tube growth is largely unknown. We reported previously that Arabidopsis thaliana MICROTUBULE-ASSOCIATED PROTEIN18 (MAP18) contributes to directional cell growth and cortical microtubule organization. The preferential expression of MAP18 in pollen and in pollen tubes suggests that MAP18 also may function in pollen tube growth. In this study, we demonstrate that MAP18 functions in pollen tubes by influencing actin organization, rather than microtubule assembly. In vitro biochemical results indicate that MAP18 exhibits Ca2+-dependent filamentous (F)-actin-severing activity. Abnormal expression of MAP18 in map18 and MAP18 OX plants was associated with disorganization of the actin cytoskeleton in the tube apex, resulting in aberrant pollen tube growth patterns and morphologies, inaccurate micropyle targeting, and fewer fertilization events. Experiments with MAP18 mutants created by site-directed mutagenesis suggest that F-actin-severing activity is essential to the effects of MAP18 on pollen tube growth direction. Our study demonstrates that in Arabidopsis, MAP18 guides the direction of pollen tube growth by modulating actin filaments.

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[1] An ensemble-based four-dimensional variational data assimilation (4DVar) method is proposed to fit the model field to 4-D observations in an increment form in the analysis step of data assimilation. The fitting is similar to that in the 4DVar but the analysis increment is expressed by a linear combination of the leading singular vectors extracted from an ensemble of 4-D perturbation solutions, so the fitting is computationally very efficient and does not require any adjoint integration. In the cost function used for the fitting, the background error covariance matrix is constructed implicitly by the perturbation solutions (through their representative singular vectors) similarly to that in the ensemble Kalman filter, but the perturbation solutions are not updated by the analysis into the next assimilation cycle, so the analysis is simpler and more efficient than that in the ensemble Kalman filter. The potential merits of the method are demonstrated by three sets of observing system simulation experiments performed with a shallow-water equation model. The method is shown to be robust even when the model is imperfect and the observations are incomplete.Citation: Qiu, C., A. Shao, Q. Xu, and L. Wei (2007), Fitting model fields to observations by using singular value decomposition: An ensemble-based 4DVar approach,

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The bending movement of the pulvinus of Mimosa pudica is caused by a rapid change in volume of the abaxial motor cells, in response to various environmental stimuli. We investigated the relationship between the actin cytoskeleton and changes in the level of calcium during rapid contractile movement of the motor cells that was induced by electrical stimulation. The bending of the pulvinus was retarded by treatments with actin-affecting reagents and calcium channel inhibitors. The actin filaments in the motor cells were fragmented in response to electrical stimulation. Further investigations were performed using protoplasts from the motor cells of M. pudica pulvini. Calcium-channel inhibitors and EGTA had an inhibitory effect on contractile movement of the protoplasts. The level of calcium increased and became concentrated in the tannin vacuole after electrical stimulation. Ruthenium Red inhibited the increase in the level of calcium in the tannin vacuole and the contractile movement of the protoplasts. However, treatment with latrunculin A abolished the inhibitory effect of Ruthenium Red. Phalloidin inhibited the contractile movement and the increase in the level of calcium in the protoplasts. Our study demonstrates that depolymerization of the actin cytoskeleton in pulvinus motor cells in response to electrical signals results in increased levels of calcium.

The previous idealized two-fluid model of a density current in constant shear is extended to the case where the inflow shear is confined to the low levels. The analytical solution is determined by the conservation of mass, momentum, vorticity, and energy. It is found that a low-level shear acts in a similar manner to a uniform vertical shear in controlling the depth of a steady-state density current. When the shear enhances the low-level flow against the density current propagation, the current is deeper than half of the domain depth. Time-dependent numerical experiments are conducted for a variety of parameter settings, including various depths and strengths of the shear layer. The numerical results agree closely with the theoretical analyses.Numerical experiments are also performed for a case where the initial depth of the density current is set to be comparable to the low-level shear, which is much shallower than that given by the steady-state solution. The circulation at the density current head remains shallow and is nonsteady in this case, whereas the time-averaged flow still exhibits a deep jump updraft pattern that is close to the theoretical solution, suggesting the applicability of the theoretical results to even more transient flows.The simulated flow features are discussed in terms of balanced and unbalanced dynamics, and in the context of forcing and uplifting at the frontal zone in long-lived convective systems. Here the term balance refers to a flow configuration that satisfies the steady-state solution of the idealized theoretical model.