Summary
Dehydration leads to different physiological and biochemical responses in plants. We analysed the lipid composition and the expression of genes involved in lipid biosynthesis in the ...desiccation‐tolerant plant Craterostigma plantagineum. A comparative approach was carried out with Lindernia brevidens (desiccation tolerant) and two desiccation‐sensitive species, Lindernia subracemosa and Arabidopsis thaliana. In C. plantagineum the total lipid content remained constant while the lipid composition underwent major changes during desiccation. The most prominent change was the removal of monogalactosyldiacylglycerol (MGDG) from the thylakoids. Analysis of molecular species composition revealed that around 50% of 36:x (number of carbons in the acyl chains: number of double bonds) MGDG was hydrolysed and diacylglycerol (DAG) used for phospholipid synthesis, while another MGDG fraction was converted into digalactosyldiacylglycerol via the DGD1/DGD2 pathway and subsequently into oligogalactolipids by SFR2. 36:x‐DAG was also employed for the synthesis of triacylglycerol. Phosphatidic acid (PA) increased in C. plantagineum, L. brevidens, and L. subracemosa, in agreement with a role of PA as an intermediate of lipid turnover and of phospholipase D in signalling during desiccation. 34:x‐DAG, presumably derived from de novo assembly, was converted into phosphatidylinositol (PI) in C. plantagineum and L. brevidens, but not in desiccation‐sensitive plants, suggesting that PI is involved in acquisition of desiccation tolerance. The accumulation of oligogalactolipids and PI in the chloroplast and extraplastidial membranes, respectively, increases the concentration of hydroxyl groups and enhances the ratio of bilayer‐ to non‐bilayer‐forming lipids, thus contributing to protein and membrane stabilization.
Several studies on aeroterrestrial microalgae are unravelling their resistance mechanisms to different abiotic stressors, including hazardous metals, pointing to their future role as bioremediation ...microorganisms. In the present study, physiological and molecular alterations of four phycobionts of genus
Trebouxia (T.
TR1 and
T. TR9)
and
Coccomyxa
(
C. subellipsoidea
and
C. simplex
) exposed to Cd were studied. Cd accumulation and subcellular distribution, cell wall structure, production of biothiols (GSH and phytochelatins), reactive oxygen species (ROS) formation, expression of key antioxidant genes and ROS-related enzymes were evaluated to determine the physiological differences among the four microalgae, with the aim to identify the most suitable microorganism for further biotechnological applications. After 7 days of Cd exposure,
Coccomyxa
algae showed higher capacity of Cd intake than
Trebouxia
species, with
C. subellipsoidea
being the highest Cd accumulator at both intracellular and, especially, cell wall level. Cd induced ROS formation in the four microalgae, but to a greater extent in both
Coccomyxa
algae.
Trebouxia
TR9 showed the lowest Cd-dependent oxidative stress probably due to glutathione reductase induction. All microalgae synthetized phytochelatins in response to Cd but in a species-specific and a dose-dependent manner. Results from this study agree with the notion that each microalga has evolved a distinct strategy to detoxify hazardous metals like Cd and to cope with oxidative stress associated with them.
Coccomyxa subellipsoidea
and
Trebouxia
TR9 appear as the most interesting candidates for further applications.
Desiccation tolerance (DT) is relatively frequent in non-vascular plants and green algae. However, it is poorly understood how successive dehydration/rehydration (D/R) cycles shape their ...transcriptomes and proteomes. Here, we report a comprehensive analysis of adjustments on both transcript and protein profiles in response to successive D/R cycles in
Coccomyxa simplex
(Csol), isolated from the lichen
Solorina saccata
. A total of 1833 transcripts and 2332 proteins were differentially abundant as a consequence of D/R; however, only 315 of these transcripts/proteins showed similar trends. Variations in both transcriptomes and proteomes along D/R cycles together with functional analyses revealed an extensive decrease in transcript and protein levels during dehydration, most of them involved in gene expression, metabolism, substance transport, signalling and folding catalysis, among other cellular functions. At the same time, a series of protective transcripts/proteins, such as those related to antioxidant defence, polyol metabolism and autophagy, was upregulated during dehydration. Overall, our results show a transient decrease in most cellular functions as a result of drying and a gradual reactivation of specific cell processes to accommodate the hydration status along successive D/R cycles. This study provides new insights into key mechanisms involved in the DT of Csol and probably other dehydration-tolerant microalgae. In addition, functionally characterising the high number of genes/proteins of unknown functions found in this study may lead to the discovery of new DT mechanisms.
Trebouxiophyceae (Chlorophyta) is a species-rich class of green algae with a remarkable morphological and ecological diversity. Currently, there are a few completely sequenced mitochondrial genomes ...(mtDNA) from diverse Trebouxiophyceae but none from lichen symbionts. Here, we report the mitochondrial genome sequence of Trebouxia sp. TR9 as the first complete mtDNA sequence available for a lichen-symbiont microalga. A comparative study of the mitochondrial genome of Trebouxia sp. TR9 with other chlorophytes showed important organizational changes, even between closely related taxa. The most remarkable change is the enlargement of the genome in certain Trebouxiophyceae, which is principally due to larger intergenic spacers and seems to be related to a high number of large tandem repeats. Another noticeable change is the presence of a relatively large number of group II introns interrupting a variety of tRNA genes in a single group of Trebouxiophyceae, which includes Trebouxiales and Prasiolales. In addition, a fairly well-resolved phylogeny of Trebouxiophyceae, along with other Chlorophyta lineages, was obtained based on a set of seven well-conserved mitochondrial genes.
Speciation in oceanic islands has attracted the interest of scientists since the 19th century. One of the most striking evolutionary phenomena that can be studied in islands is adaptive radiation, ...that is, when a lineage gives rise to different species by means of ecological speciation. Some of the best-known examples of adaptive radiation are charismatic organisms like the Darwin finches of the Galapagos and the cichlid fishes of the great African lakes. In these and many other examples, a segregation of the trophic niche has been shown to be an important diversification driver. Radiations are known in other groups of organisms, such as lichen-forming fungi. However, very few studies have investigated their adaptive nature, and none have focused on the trophic niche. In this study, we explore the role of the trophic niche in a putative radiation of endemic species from the Macaronesian Region, the
group. The photobiont diversity was studied by Illumina MiSeq sequencing of the ITS2 region of 197 specimens spanning the phylogenetic breadth and geographic range of the group. A total of 66 amplicon sequence variants belonging to the four main clades of the algal genus
were found. Approximately half of the examined thalli showed algal coexistence, but in most of them, a single main photobiont amounted to more than 90% of the reads. However, there were no significant differences in photobiont identity and in the abundance of ITS2 reads across the species of the group. We conclude that a segregation of the trophic niche has not occurred in the
radiation.
Lichens are symbiotic associations (holobionts) established between fungi (mycobionts) and certain groups of cyanobacteria or unicellular green algae (photobionts). This symbiotic association has ...been essential in the colonization of terrestrial dry habitats. Lichens possess key mechanisms involved in desiccation tolerance (DT) that are constitutively present such as high amounts of polyols, LEA proteins, HSPs, a powerful antioxidant system, thylakoidal oligogalactolipids, etc. This strategy allows them to be always ready to survive drastic changes in their water content. However, several studies indicate that at least some protective mechanisms require a minimal time to be induced, such as the induction of the antioxidant system, the activation of non-photochemical quenching including the de-epoxidation of violaxanthin to zeaxanthin, lipid membrane remodeling, changes in the proportions of polyols, ultrastructural changes, marked polysaccharide remodeling of the cell wall, etc. Although DT in lichens is achieved mainly through constitutive mechanisms, the induction of protection mechanisms might allow them to face desiccation stress in a better condition. The proportion and relevance of constitutive and inducible DT mechanisms seem to be related to the ecology at which lichens are adapted to.
•Oligogalactolipids (OGLs) are lipids with a polar head of 3 or more galactoses.•OGLs are constitutive in desiccation tolerant algae, but inducible in plants.•DGD and SFR2 enzymes mediate two ...different pathways for OGLs synthesis.•DGD is ancestral, SFR2 evolved in Streptophytes likely as a response to desiccation.•SFR2 is stress-inducible and simultaneously protects against other stress factors.
Galactolipids (GLs) are the main lipids in chloroplast membranes and by default are also the most abundant polar lipids on earth. GLs with one or two galactose residues, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), are ubiquitous and essential for photosynthesis. GLs with a headgroup formed by three to five galactoses, the so-called oligogalactolipids (OGLs), are only detected in some taxa, organs and environmental conditions. OGLs can be synthesized by two metabolic pathways: successive galactosylation by DGDG synthase (DGD) or transgalactosylation from MGDG by the GL:GL galactosyltransferase (GGGT/SFR2). While the first route appeared early in the evolution (cyanobacteria), the second evolved associated to the process of terrestrialization in the streptophytes. Both routes also differ on the anomeric type of glycosidic linkages formed: α-type in DGD and β-type in GGGT/SFR2. Despite functional differences between both configurations, the anomeric analysis of OGLs allows tracking their biosynthetic origin. While α-OGLs are constitutive and present in some algae and non-vegetative organs of vascular plants, β-OGLs are typically stress-inducible in photosynthetic tissues. Land colonization by plants involved new challenges, such as the risk of dehydration, which required developing biochemical and physiological strategies to stabilize chloroplast membranes and safeguard their functioning. Based on the integrated assessment of data available we propose that the appearance of OGLs was one of those adaptations that simultaneously could have provided advantages against other environmental constraints such as freezing.
Abstract
An interesting biota of lichen-forming fungi occurs along rocky seashores of cold and warm-temperate regions in both hemispheres. Most of the species belong to the family Verrucariaceae and ...form symbioses with an extraordinarily diverse group of photobionts. We isolated the photobionts of three species:
Hydropunctaria maura
and
H. amphibia
from the supralittoral zone, and
Wahlenbergiella striatula
from the upper intertidal zone. We characterized the isolated strains structurally by means of transmission electron microscopy, and molecularly using the nrSSU and nrITS and chloroplast RPL10A regions. Additionally, we studied the response of the strains to different salt concentrations, analyzed the concentration of osmoregulatory solutes, and measured photosynthesis performance by chlorophyll fluorescence and CO
2
assimilation techniques. All strains belong to the recently described species
Halofilum ramosum
, although we found differences in the ITS and RPL10A regions among the strains shared by
H. maura
and
H. amphibia
and the strain isolated from
W. striatul
a. Differences were also found in the main osmoregulatory response of the strains growing under high salt concentrations:
W. striatula
accumulated glycerol, while
H. maura
and
H. amphibia
synthetized sucrose. Analyses of photosynthesis performance also indicated differences in physiological behavior between supralittoral-dwelling and intertidal-dwelling species,
W. striatula
showing lower photosynthetic activity under high irradiance. Our results highlight the role of photobionts in determining lichen zonation on rocky seashores.
The mechanisms involved in desiccation tolerance of lichens and their photobionts are still poorly understood. To better understand these mechanisms we have studied dehydration rate and desiccation ...time in Trebouxia, the most abundant chlorophytic photobiont in lichen. Our findings indicate that the drying rate has a profound effect on the recovery of photosynthetic activity of algae after rehydration, greater than the effects of desiccation duration. The basal fluorescence (F′o) values in desiccated algae were significantly higher after rapid dehydration, than after slow dehydration, suggesting higher levels of light energy dissipation in slow-dried algae. Higher values of PSII electron transport were recovered after rehydration of slow-dried Trebouxia erici compared to rapid-dried algae. The main component of non-photochemical quenching after slow dehydration was energy dependent (q E), whereas after fast dehydration it was photoinhibition (q I). Although q E seems to play a role during desiccation recovery, no significant variations were detected in the xanthophyll cycle components. Desiccation did not affect PSI functionality. Classical antioxidant activities like superoxide dismutase or peroxidase decreased during desiccation and early recovery. Dehydrins were detected in the lichen-forming algae T. erici and were constitutively expressed. There is probably a minimal period required to develop strategies which will facilitate transition to the desiccated state in this algae. In this process, the xanthophyll cycle and classical antioxidant mechanisms play a very limited role, if any. However, our results indicate that there is an alternative mechanism of light energy dissipation during desiccation, where activation is dependent on a sufficiently slow dehydration rate.
Reactive oxygen species (ROS) are normally produced in respiratory and photosynthetic electron chains and their production is enhanced during desiccation/rehydration. Nitric oxide (NO) is a ...ubiquitous and multifaceted molecule involved in cell signaling and abiotic stress. Lichens are poikilohydrous organisms that can survive continuous cycles of desiccation and rehydration. Although the production of ROS and NO was recently demonstrated during lichen rehydration, the functions of these compounds are unknown. The aim of this study was to analyze the role of NO during rehydration of the lichen Ramalina farinacea (L.) Ach., its isolated photobiont partner Trebouxia sp. and Asterochloris erici (Ahmadjian) Skaloud et Peksa (SAG 32.85 = UTEX 911).
Rehydration of R. farinacea caused the release of ROS and NO evidenced by the fluorescent probes DCFH₂-DA and DAN respectively. However, a minimum in lipid peroxidation (MDA) was observed 2 h post-rehydration. The inhibition of NO in lichen thalli with c-PTIO resulted in increases in both ROS production and lipid peroxidation, which now peaked at 3 h, together with decreases in chlorophyll autofluorescence and algal photobleaching upon confocal laser incidence. Trebouxia sp. photobionts generate peaks of NO-endproducts in suspension and show high rates of photobleaching and ROS production under NO inhibition which also caused a significant decrease in photosynthetic activity of A. erici axenic cultures, probably due to the higher levels of photo-oxidative stress.
Mycobiont derived NO has an important role in the regulation of oxidative stress and in the photo-oxidative protection of photobionts in lichen thalli. The results point to the importance of NO in the early stages of lichen rehydration.
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