Klebsormidium flaccidum, a charophycean green alga, exhibits cold acclimation that is closely associated with compatible solute accumulation and ultrastructural changes

Abstract

To elucidate the fundamental mechanisms and subsequent evolutionary aspects of plant cold acclimation, we examined the effect of cold acclimation on freezing tolerance in Klebsormidium flaccidum, a green alga belonging to Charophyceae, a sister group of land plants. Freezing tolerance of K. flaccidum was significantly enhanced by cold treatment: survival increased from 15% at -10°C when grown at 18°C to 55 and 85% after exposure at 2°C for 2 and 7 d, respectively. Accompanying the development of freezing tolerance, soluble sugars (glucose and sucrose), a putative glycoside and amino acids, including g-aminobutyric acid (GABA), accumulated to high levels in the alga, suggesting that these solutes play a crucial role in the cold acclimation of K. flaccidum. Interestingly, the application of abscisic acid (ABA) did not change the freezing tolerance of the alga. We also observed changes in cell structure, including increased numbers and sizes of starch grains in chloroplasts, chloroplast enlargement, vacuole size reduction and cytoplasmic volume increase. These results suggest that K. flaccidum responds well to cold treatment and develops freezing tolerance in a process comparable to that of land plants.

Ecophysiological performance of an urban strain of the aeroterrestrial green algaKlebsormidiumsp. (Klebsormidiales, Klebsormidiophyceae)

“…In K. flaccidum the accumulation of various low molecular weight carbohydrates (LMWCs) and amino acids together with ultrastructural changes such as chloroplast enlargement, vacuole size reduction, etc. contribute to an enhanced freezing tolerance (Nagao et al, 2008). However, to date no detailed data on the ecophysiological performances of Klebsormidium have been obtained.…”

“…However, several days without liquid water led to complete inhibition of CO 2 fixation, while rehydration was accompanied by fast and full recovery (De Winder et al, 1990), indicating efficient, but still unstudied protective mechanisms against desiccation. In contrast to desiccation, cold acclimation and freezing resistance in various Klebsormidium strains are better understood (Elster et al, 2008;Nagao et al, 2008). In K. flaccidum the accumulation of various low molecular weight carbohydrates (LMWCs) and amino acids together with ultrastructural changes such as chloroplast enlargement, vacuole size reduction, etc.…”

Sucrose phosphate phosphatase in the green alga Klebsormidium flaccidum (Streptophyta) lacks an extensive C-terminal domain and differs from that of land plants

“…Accumulation of sucrose plays a crucial role in cold acclimation of green plants, enhancing freezing tolerance (Anchordoguy et al 1987;Leslie et al 1995;. In K. flaccidum, sucrose accumulates significantly during cold acclimation (Nagao et al 2008). Thus, it is reasonable to expect that the expression of genes for sucrose synthesis would be associated with cold acclimation.…”

“…In our studies on the cold acclimation process, we have focused on Klebsormidium flaccidum (Streptophyta, Klebsormidiophyceae, Table 2) as a model green alga (Nagao et al 2008). This alga is a member of a closely related sister group of land plants (Bhattacharya and Medlin 1998;Graham et al 2000;Karol et al 2001).…”

“…In addition, K. flaccidum is interesting from an evolutionary viewpoint, because it may reveal the ancestral cold acclimation process of land plants. Previously, we showed that the freezing tolerance of K. flaccidum was significantly enhanced by cold acclimation and that the cells accumulated sucrose in response to cold (Nagao et al 2008). Both of these processes are quite similar to cold acclimation processes found in land plants (Kamata and Uemura 2004;Nagao et al 2006;Kaplan et al 2007).…”