Electrophysiology of the Detached Cell Under Stress

Abstract

This chapter summarises the research on characean detached cells, subjected to calibrated and primarily primarily abiotic types of stress. Responses to touch, wounding, voltage clamp to both depolarised and hyperpolarised potential difference (PD) levels and to osmotic and saline stress are described. Both salt-tolerant and salt-sensitive Characeae have been investigated under osmotic and saline stress. Many of the cell responses involve an increase in cytoplasmic calcium concentration. However, the manner of calcium increase is specific to different stressors: mechanical stimulation opens stretch-activated (SA) channels on internal stores, depolarisation to excitation threshold opens IP6-activated channels on internal stores, hypo-osmotic stress opens SA channels on internal stores and possibly the vacuole, hyperpolarisation opens channels on the plasma membrane. The increase in the calcium concentration in the cytoplasm opens calcium-activated chloride channels on the plasma membrane (and possibly the tonoplast), leading to an outflow of chloride and depolarisation that can take a form of receptor potential (RPD), action potential (AP) or variation potential (VP). From all these responses, the AP fits the role of a signal: its form is relatively constant and it can propagate speedily from cell to cell. The responses to hyperosmotic and saline stress show interesting differences between salt-tolerant and salt-sensitive Characeae. In salt-tolerant Characeae, the proton pump is activated by both a decrease in turgor and an increase in salinity. The activation by turgor decrease is transient as turgor is regulated, while activation by salt is permanent and graded according to salt concentration. The turgor sensors may be located at the ends of nodal cells. In salt-sensitive Characeae, on the other hand, non-plasmolysing turgor decrease elicits no change in pump activity, while an increase in salinity shuts down the pump and opens H+/OH− channels, depolarising the membrane PD further. Spontaneous APs with long durations and opening of outward rectifying channels lead to a loss of potassium and death. Both salt-sensitive and salt-tolerant Characeae employ Na+/H+ antiporter to remove Na+ from the cytoplasmic compartment under saline stress and the water channels on the plasma membrane are closed by high osmolarity.