Dual turgor regulation response to hypotonic stress in Lamprothamnium papulosum

Cells of the salt-tolerant charophyte Lamprothamnium respond differently to hypotonic challenge according to their position on the plant (i.e. cell age). Differences in electrophysiological response are coupled with differences in cell fine structure, and the presence or absence of extracellular mucilage. (1) Young, apical (fast-regulating, FR) cells respond with sudden cessation of cyclosis, depolarization to -50 mV (in some cells by more than 100 mV) and increase in membrane conductance by up to an order of magnitude. Intracellular [K+](v), [Na+](v) and [Cl-](v) decrease 1 h after hypotonic challenge. Patch-clamping cytoplasmic droplets reveals two types of K+ channel, 150 pS and 35 pS, and a small conductance Cl- channel, 35 pS (conductances at estimated tonoplast resting potential between zero and 20 mV). Extracellular mucilage is thin (<5 mu m thick) or lacking, similar to freshwater Chara. Unlike freshwater charophytes these cells have a canalicular vacuolar system of large surface area and compartment the fluorochrome 6 carboxyfluorescein in the cytoplasm rather than the vacuolar system. (2) Older basal (slow-regulating, SR) cells do not cease streaming on hypotonic challenge and depolarize only slightly (by approximately 20 mV) with small or no change in membrane conductance. After 1 h the intracellular [K+](v), [Na+](v) and [Cl-](v) scarcely change. Patch-clamping cytoplasmic droplets reveals two types of KC channel, medium conductance 90 pS and low conductance (as in FR cells). The large conductance K+ channel was not observed. The CT channel was more active in SR cells. The cells were coated with extracellular mucilage more than 10 mu m thick. In a similar manner to freshwater Chara, these cells compartment 6 carboxyfluorescein in a large central vacuole. In the older cells, making up the bulk of any given plant, the simultaneous development of extracellular mucilage and a large central vacuole which compartments 6 carboxyfluorescein is associated with a minimal electrophysiological response to hypotonic challenge. The significance of these findings for salt-tolerance is discussed.