Turgor-driven Processes
Turgor pressure is a natural byproduct of the fact that the concentration of solutes within a cell is often higher than the concentration within the cell's environment. However, turgor is also tightly regulated by cells and may be selected for by evolution such that cells can exploit the force provided by turgor to perform physical processes.
Turgor-driven tip growth in plants and fungi
One way in which cells use turgor is to stretch their stiff cell wall during growth. The relationship between turgor and cell-wall expansion has long been recognized in a select few single-celled algae. I explored this concept more deeply by testing it in a diversity of plant, fungal, and protistan tip-growing cells. Tip growth is a strategy by which cells can explore their environment by extending from one pole. The mechanical stresses within the cell wall, which result from turgor pressure, naturally vary as a function of space due to the curved geometry of the cell pole. Thus, tip-growing cells are ideal systems in which to study the relationship between stress and cell-wall expansion. In a number of organisms, I found that expansion rate scales with stress. In pollen tubes, we also elucidated the biochemical feedback system that regulates mechanical expansion of the wall.

Rojas, Hotton, Dumais (2011)

Dual regulation of Bacillus subtilis growth by turgor pressure
While the pressure dependence of cell growth in plants is well established, this relationship in bacteria had not been measured. To achieve this, I altered the turgor pressure of bacterial cells by subjecting them to osmotic shock, and then measured the effect of this treatment on cell growth-rate. While reducing turgor pressure of the Gram-positive bacterium Bacillus subtilis causes proportional reduction in growth rate, suprisingly, hypoosmotic shock, which tends to increase turgor, causes growth oscillations. I showed that these oscillations are initiated when mechanical strain in the cell membrane actually inhibits cell wall synthesis, causing a reduction in cell growth and a build-up of cell-wall precursors in the cytoplasm. That is, this organism has actually co-opted turgor pressure to control cell growth in two opposing ways: turgor pressure up-regulates cell-wall expansion by driving mechanical deformation of the wall, but down-regulates expansion if the cell is growing faster than membrane synthesis can keep up with, by inhibiting cell wall synthesis.

Short video explaining this work
Webinar: For Whom the Cell Tolls

Stored growth in Escherichia coli
In sharp contrast to B. subtilis, the Gram-negative bacterium Escherichia coli is robust to osmotic shock. Not only was E. coli insensitive to hypoosmotic shock, but hyperosmotic shock, which reduces turgor pressure, only reduced cell-wall expansion when the shock was large enough to plasmolyze the cell, that is, to separate the plasma membrane from the cell wall. However, in this case, if turgor pressure was reestablished, the cells quickly elongated to the length that they would have attained had pressure never been depleted. The cells apparently "stored" growth during the period of reduced cell-wall expansion, which implied that cell-wall synthesis was unaffected by depletion of turgor. As confirmation of this, I found that the motion of the cell-wall synthesis enzyme MreB was unaffected by osmotic shock.

Rojas, Theriot, Huang (2014)

Short video explaining this work
Webinar: For Whom the Cell Tolls
Press: Stanford Gazette, Microbe Magazine Podcast

Turgor-driven cell division in Staphylococcus aureus
In collaboration with Xiaoxue "Snow" Zhou and David Halladin, I am working on project to characterize the mechanics of cell division of the pathogenic bacterium Staphylococcus aureus. David showed that separation of daughter cells in this organism occurs extremely rapidly, in less than 1 ms. To demonstrate that cell separation was mediated by turgor pressure, I subjected cells to oscillatory osmotic shock and showed that daughter cells are more likely to separate upon each hypoosmotic shock, when turgor is increasing. Snow is characterizing in-depth the enzymes responsible for initiating this mechanical cleavage.

Zhou*, Halladin*, Rojas* (2015)

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