Summary: Plant cell expansion is controlled by a fine‐tuned balance between intracellular turgor pressure, cell wall loosening and cell wall biosynthesis. To understand these processes, it is important to gain in‐depth knowledge of cell wall mechanics. Pollen tubes are tip‐growing cells that provide an ideal system to study mechanical properties at the single cell level. With the available approaches it was not easy to measure important mechanical parameters of pollen tubes, such as the elasticity of the cell wall. We used a cellular force microscope (CFM) to measure the apparent stiffness of lily pollen tubes. In combination with a mechanical model based on the finite element method (FEM), this allowed us to calculate turgor pressure and cell wall elasticity, which we found to be around 0.3 MPa and 20–90 MPa, respectively. Furthermore, and in contrast to previous reports, we showed that the difference in stiffness between the pollen tube tip and the shank can be explained solely by the geometry of the pollen tube. CFM, in combination with an FEM‐based model, provides a powerful method to evaluate important mechanical parameters of single, growing cells. Our findings indicate that the cell wall of growing pollen tubes has mechanical properties similar to rubber. This suggests that a fully turgid pollen tube is a relatively stiff, yet flexible cell that can react very quickly to obstacles or attractants by adjusting the direction of growth on its way through the female transmitting tissue.

Plant viruses act as triggers and targets of RNA silencing and have evolved proteins to suppress this plant defense response during infection. Although Tobacco mosaic tobamovirus (TMV) triggers the production of virus-specific small interfering RNAs (siRNAs), this does not lead to efficient silencing of TMV nor is a TMV-green fluorescent protein (GFP) hybrid able to induce silencing of a GFP-transgene in Nicotiana benthamiana , indicating that a TMV silencing suppressor is active and acts downstream of siRNA production. On the other hand, TMV-GFP is unable to spread into cells in which GFP silencing is established, suggesting that the viral silencing suppressor cannot revert silencing that is already established. Although previous evidence indicates that the tobamovirus silencing suppressing activity resides in the viral 126-kDa small replicase subunit, the mechanism of silencing suppression by this virus family is not known. Here, we connect the silencing suppressing activity of this protein with our previous finding that Oilseed rape mosaic tobamovirus infection leads to interference with HEN1-mediated methylation of siRNA and micro-RNA (miRNA). We demonstrate that TMV infection similarly leads to interference with HEN1-mediated methylation of small RNAs and that this interference and the formation of virus-induced disease symptoms are linked to the silencing suppressor activity of the 126-kDa protein. Moreover, we show that also Turnip crinkle virus interferes with the methylation of siRNA but, in contrast to tobamoviruses, not with the methylation of miRNA.

Leu-rich repeat extensins (LRXs) are chimeric proteins containing an N-terminal Leu-rich repeat (LRR) and a C-terminal extensin domain. LRXs are involved in cell wall formation in vegetative tissues and required for plant growth. However, the nature of their role in these cellular processes remains to be elucidated. Here, we used a combination of molecular techniques, light microscopy, and transmission electron microscopy to characterize mutants of pollen-expressed LRXs in Arabidopsis (Arabidopsisthaliana). Mutations in multiple pollen-expressed lrx genes cause severe defects in pollen germination and pollen tube growth, resulting in a reduced seed set. Physiological experiments demonstrate that manipulating Ca²⁺ availability partially suppresses the pollen tube growth defects, suggesting that LRX proteins influence Ca²⁺-related processes. Furthermore, we show that LRX protein localizes to the cell wall, and its LRR-domain (which likely mediates protein-protein interactions) is associated with the plasma membrane. Mechanical analyses by cellular force microscopy and finite element method-based modeling revealed significant changes in the material properties of the cell wall and the fine-tuning of cellular biophysical parameters in the mutants compared to the wild type. The results indicate that LRX proteins might play a role in cell wall-plasma membrane communication, influencing cell wall formation and cellular mechanics.

The aim of this work was to study the role of the cell wall protein expansin in elongation growth. Expansins increase cell wall extensibility in vitro and are thought to be involved in cell elongation. Here, we studied the regulation of two tomato (Lycopersicon esculentum cv Moneymaker) expansin genes,LeExp2 and LeExp18, in rapidly expanding tissues. LeExp2 was strongly expressed in the elongation zone of hypocotyls and in the faster growing stem part during gravitropic stimulation. LeExp18 expression did not correlate with elongation growth. Exogenous application of hormones showed a substantial auxin-stimulation of LeExp2 mRNA in etiolated hypocotyls and a weaker auxin-stimulation ofLeExp18 mRNA in stem tissue. Analysis of transcript accumulation revealed higher levels of LeExp2 andLeExp18 in light-treated, slow-growing tissue than in dark-treated, rapidly elongating tissue. Expansin protein levels and cell wall extension activities were similar in light- and dark-grown hypocotyl extracts. The results show a strong correlation between expansin gene expression and growth rate, but this correlation is not absolute. We conclude that elongation growth is likely to be controlled by expansin acting in concert with other factors that may limit growth under some physiological conditions.