Dual turgor regulation response to hypotonic stress in Lamprothamnium papulosum

MJ Beilby, CA Cherry and VA Shepherd

Plant,Cell and Environment, 1999, 22:347 - 359

Abstract

    Cells of the salt-tolerant charophyte Lamprothamnium respond differently to hypotonic challenge according to their position on the plant (ie. cell age). Differences in electrophysiological response are coupled with differences in cell fine structure, and presence or absence of extracellular mucilage.
(1). Young, apical (fast-regulating) cells respond with sudden cessation of cyclosis, depolarisation 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 hour post hypotonic challenge. Patch-clamping cytoplasmic droplets reveals a large conductance K+ channel. Extracellular mucilage is thin (<5 microns ) 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) cells do not cease streaming on hypotonic challenge and depolarise only slightly (by ~20mV) with small or no change in membrane conductance. After 1 hour intracellular [K+]v, [Na+]v and  [Cl-]v scarcely change. Patch-clamping cytoplasmic droplets reveals predominance of a small conductance K+ channel. The cells are coated with extracellular mucilage up to 50 microns thick. Similarly to freshwater Chara, these cells compartment
6 carboxyfluorescein in a large central vacuole. In 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.

Key words: Lamprothamnium, hypotonic effect, vacuolar concentrations of  K+, Na+ and Cl -, vacuolar structure, patch clamp