The mechanisms underlying the growth of fungal hyphae are rooted in the physical property of cell pressure. Internal hydrostatic pressure (turgor) is one of the major forces driving the localized expansion at the hyphal tip which causes the characteristic filamentous shape of the hypha. Calcium gradients regulate tip growth, and secretory vesicles that contribute to this process are actively transported to the growing tip by molecular motors that move along cytoskeletal structures. Turgor is controlled by an osmotic mitogen-activated protein kinase cascade that causes de novo synthesis of osmolytes and uptake of ions from the external medium. However, as discussed in this Review, turgor and pressure have additional roles in hyphal growth, such as causing the mass flow of cytoplasm from the basal mycelial network towards the expanding hyphal tips at the colony edge.

Hyperosmotic stress is known to significantly enhance net uptake of inorganic ions into plant cells. Direct evidence for cell turgor recovery via such a mechanism, however, is still lacking. In the present study, we performed concurrent measurements of net ion fluxes (with the noninvasive microelectrode ion flux estimation technique) and cell turgor changes (with the pressure-probe technique) to provide direct evidence that inorganic ion uptake regulates turgor in osmotically stressed Arabidopsis epidermal root cells. Immediately after onset of hyperosmotic stress (100/100 mm mannitol/sorbitol treatment), the cell turgor dropped from 0.65 to about 0.25 MPa. Turgor recovery started within 2 to 10 min after the treatment and was accompanied by a significant (30-80 nmol m Ϫ2 s Ϫ1 ) increase in uptake of K ϩ , Cl Ϫ , and Na ϩ by root cells. In most cells, almost complete (Ͼ90% of initial values) recovery of the cell turgor was observed within 40 to 50 min after stress onset. In another set of experiments, we combined the voltage-clamp and the microelectrode ion flux estimation techniques to show that this process is, in part, mediated by voltage-gated K ϩ transporters at the cell plasma membrane. The possible physiological significance of these findings is discussed.Improving crop resistance to drought stresses is a long-standing challenge for generations of plant physiologists and agricultural biotechnologists. In the last 15 years, major efforts have been focused on molecular engineering of transgenic species that overexpress genes responsible for biosynthesis of various compatible solutes (Bohnert et al., 1995; Bray, 1997). This approach has been extensively reviewed (Smirnoff, 1998; Bajaj et al., 1999; Bohnert and Shen, 1999; Nuccio et al., 1999;Serrano et al., 1999b; Cushman and Bohnert, 2000). Among the genes targeted were those responsible for biosynthesis of amino acids (Pro, ectoine, and Gly betaine), sugars (Suc, trehalose, and fructan), polyols (mannitol and sorbitol) and quaternary amines (Winicov, 1998; Bajaj et al., 1999; Cushman and Bohnert, 2000; and refs. therein).The practical outcomes of these extensive studies surprisingly are only marginal (Bajaj et al., 1999; Bohnert and Shen, 1999). To our knowledge, there are no reports of any significant improvements in drought tolerance in transgenic crop species in field trials. This is probably due to the complexity of whole-plant responses to water stress. But at the cellular level, are we on the right track in our attempts to improve the plant's ability to withstand water stress?It was traditionally believed that the major function of compatible solutes is osmoregulation (Wyn Jones and Pritchard, 1989; Delauney and Verma, 1993; Bajaj et al., 1999). However, it recently became evident that the functions of compatible solutes are not likely to be as straightforward as initially believed. More and more papers question whether compatible solutes are directly involved in regulation of cell turgor, suggesting instead that their possible regulatory role i...

Mass flow of cytoplasm in Neurospora crassa trunk hyphae was directly confirmed by injecting oil droplets into the hyphae. The droplets move in a manner similar to cytoplasmic particles and vacuoles within the hyphae. The direction of mass flow is towards the growing hyphal tips at the colony edge. Based on flow velocities (about 5 μm s−1), hyphal radius and estimates of cytoplasm viscosity, the Reynolds number is about 10−4, indicating that mass flow is laminar. Therefore, the Poiseulle equation can be used to calculate the pressure gradient required for mass flow: 0·0005–0·1 bar cm−1 (depending on the values used for septal pore radius and cytoplasmic viscosity). These values are very small compared to the normal hydrostatic pressure of the hyphae (4–5 bar). Mass flow stops after respiratory inhibition with cyanide, or creation of an extracellular osmotic gradient. The flow is probably caused by internal osmotic gradients created by differential ion transport along the hyphae. Apical cytoplasm migrates at the same rate as tip extension, as do oil droplets injected near the tip. Thus, in addition to organelle positioning mediated by molecular motors, pressure-driven mass flow may be an integral part of hyphal extension.

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