Direct evidence for pressure‐generated closure of plasmodesmata

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Abstract
Pressure differentials were generated between adjacent leaf trichome cells of Nicotiana clevelandii using a modified micropressure probe/injection system. The location of the pressure differentials was altered using two main treatments: (a) puncturing one of the cells in the trichome to induce a large differential at its junction with the next basal cell, or (b) raising the pressure of the impaled cell and ‘clamping’ it at a higher value, thus creating a differential at its upper, as well as lower, wall. In both treatments basipetal intercellular transport of Lucifer yellow was impeded exactly at the sites predicted by the generated pressure differentials; at the base of punctured cells and at the top of clamped cells. In flaccid trichomes puncturing was without effect because the turgor of all the cells in the trichome was too low (<50 kPa) to generate a sufficiently large pressure differential between the punctured cell and its neighbours. Elevations of cell turgor (ΔP) in excess of 200 kPa or more were required to impede intercellular transport When cells were clamped at a high pressure and then subsequently allowed to lose their turgor by withdrawal of the probe the effects of the original pressure clamp were partially retained, indicating an inability to resume intercellular transport.

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