Free proline content is positively correlated with copper tolerance of the lichen photobiont Trebouxia erici (Chlorophyta)

Abstract

Free proline accumulation has been observed in response to heavy metal stress in plants including algae, however nothing is known about its role in copper sensitivity or copper tolerance in lichen photobionts. Copper uptake, potassium efflux and free proline accumulation were determined in copper-enriched liquid cultures of wild-type Trebouxia erici as well as in copper-tolerant strain. By using inductively coupled plasma atomic emission spectrometry we found that the highest intracellular copper uptake from 2 mM copper media occurred within 4 h in both strains, but significantly less was accumulated by the tolerant strain. The copper-tolerant strain exhibited significantly more intracellular proline and significantly less potassium efflux than the wild strain. By 24 h differences between strains in intracellular copper diminished, as concentrations in both strains reached their highest levels. At the same time proline accumulations decreased significantly. Growth, pigment content, chlorophyll a degradation and chlorophyll a fluorescence were decreased by high copper concentration in the agar media after 2 weeks of cultivation, more pronounced in wild-type of T. erici. Adding proline alleviated the toxic effects of copper in both strains, but markedly so in the case of the tolerant strain.