Enhanced Desiccation Tolerance in Mature Cultures of the Streptophytic Green Alga Zygnema circumcarinatum Revealed by Transcriptomics.

Desiccation tolerance is commonly regarded as one of the key features for the colonization of terrestrial habitats by green algae and the evolution of land plants. Extensive studies, focused mostly on physiology, have been carried out assessing the desiccation tolerance and resilience of the streptophytic genera Klebsormidium and Zygnema. Here we present transcriptomic analyses of Zygnema circumcarinatum exposed to desiccation stress. Cultures of Z. circumcarinatum grown in liquid medium or on agar plates were desiccated at ∼86% relative air humidity until the effective quantum yield of PSII [Y(II)] ceased. In general, the response to dehydration was much more pronounced in Z. circumcarinatum cultured in liquid medium for 1 month compared with filaments grown on agar plates for 7 and 12 months. Culture on solid medium enables the alga to acclimate to dehydration much better and an increase in desiccation tolerance was clearly correlated to increased culture age. Moreover, gene expression analysis revealed that photosynthesis was strongly repressed upon desiccation treatment in the liquid culture while only minor effects were detected in filaments cultured on agar plates for 7 months. Otherwise, both samples showed induction of stress protection mechanisms such as reactive oxygen species scavenging (early light-induced proteins, glutathione metabolism) and DNA repair as well as the expression of chaperones and aquaporins. Additionally, Z. circumcarinatum cultured in liquid medium upregulated sucrose-synthesizing enzymes and strongly induced membrane modifications in response to desiccation stress. These results corroborate the previously described hardening and associated desiccation tolerance in Zygnema in response to seasonal fluctuations in water availability.

Pectin metabolism and assembly in the cell wall of the charophyte green alga Penium margaritaceum.

The pectin polymer homogalacturonan (HG) is a major component of land plant cell walls and is especially abundant in the middle lamella. Current models suggest that HG is deposited into the wall as a highly methylesterified polymer, demethylesterified by pectin methylesterase enzymes and cross-linked by calcium ions to form a gel. However, this idea is based largely on indirect evidence and in vitro studies. We took advantage of the wall architecture of the unicellular alga Penium margaritaceum, which forms an elaborate calcium cross-linked HG-rich lattice on its cell surface, to test this model and other aspects of pectin dynamics. Studies of live cells and microscopic imaging of wall domains confirmed that the degree of methylesterification and sufficient levels of calcium are critical for lattice formation in vivo. Pectinase treatments of live cells and immunological studies suggested the presence of another class of pectin polymer, rhamnogalacturonan I, and indicated its colocalization and structural association with HG. Carbohydrate microarray analysis of the walls of P. margaritaceum, Physcomitrella patens, and Arabidopsis (Arabidopsis thaliana) further suggested the conservation of pectin organization and interpolymer associations in the walls of green plants. The individual constituent HG polymers also have a similar size and branched structure to those of embryophytes. The HG-rich lattice of P. margaritaceum, a member of the charophyte green algae, the immediate ancestors of land plants, was shown to be important for cell adhesion. Therefore, the calcium-HG gel at the cell surface may represent an early evolutionary innovation that paved the way for an adhesive middle lamella in multicellular land plants.

Phytoremediation potential of charophytes: bioaccumulation and toxicity studies of cadmium, lead and zinc.

The ability for usage of common freshwater charophytes, Chara aculeolata and Nitella opaca in removal of cadmium (Cd), lead (Pb) and zinc (Zn) from wastewater was examined. C. aculeolata and N. opaca were exposed to various concentrations of Cd (0.25 and 0.5 mg/L), Pb (5 and 10 mg/L) and Zn (5 and 10 mg/L) solutions under hydroponic conditions for 6 days. C. aculeolata was more tolerant of Cd and Pb than N. opaca. The relative growth rate of N. opaca was drastically reduced at high concentrations of Cd and Pb although both were tolerant of Zn. Both macroalgae showed a reduction in chloroplast, chlorophyll and carotenoid content after Cd and Pb exposure, while Zn exposure had little effects. The bioaccumulation of both Cd and Pb was higher in N. opaca (1544.3 microg/g at 0.5 mg/L Cd, 21657.0 microg/g at 10 mg/L Pb) whereas higher Zn accumulation was observed in C. aculeolata (6703.5 microg/g at 10 mg/L Zn). In addition, high bioconcentration factor values (> 1000) for Cd and Pb were observed in both species. C. aculeolata showed higher percentage of Cd and Pb removal (> 95%) than N. opaca and seemed to be a better choice for Cd and Pb removal from wastewater due to its tolerance to these metals.

Oleosin of subcellular lipid droplets evolved in green algae.

In primitive and higher plants, intracellular storage lipid droplets (LDs) of triacylglycerols are stabilized with a surface layer of phospholipids and oleosin. In chlorophytes (green algae), a protein termed major lipid-droplet protein (MLDP) rather than oleosin on LDs was recently reported. We explored whether MLDP was present directly on algal LDs and whether algae had oleosin genes and oleosins. Immunofluorescence microscopy revealed that MLDP in the chlorophyte Chlamydomonas reinhardtii was associated with endoplasmic reticulum subdomains adjacent to but not directly on LDs. In C. reinhardtii, low levels of a transcript encoding an oleosin-like protein (oleolike) in zygotes-tetrads and a transcript encoding oleosin in vegetative cells transferred to an acetate-enriched medium were found in transcriptomes and by reverse transcription-polymerase chain reaction. The C. reinhardtii LD fraction contained minimal proteins with no detectable oleolike or oleosin. Several charophytes (advanced green algae) possessed low levels of transcripts encoding oleosin but not oleolike. In the charophyte Spirogyra grevilleana, levels of oleosin transcripts increased greatly in cells undergoing conjugation for zygote formation, and the LD fraction from these cells contained minimal proteins, two of which were oleosins identified via proteomics. Because the minimal oleolike and oleosins in algae were difficult to detect, we tested their subcellular locations in Physcomitrella patens transformed with the respective algal genes tagged with a Green Fluorescent Protein gene and localized the algal proteins on P. patens LDs. Overall, oleosin genes having weak and cell/development-specific expression were present in green algae. We present a hypothesis for the evolution of oleosins from algae to plants.