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Completely sequenced plastomes provide a valuable source of information about the duplication, loss, and transfer events of chloroplast genes and phylogenetic data for resolving relationships among major groups of plants. Moreover, they can also be useful for exploiting chloroplast genetic engineering technology. Ericales account for approximately six per cent of eudicot diversity with 11,545 species from which only three complete plastome sequences are currently available. With the aim of increasing the number of ericalean complete plastome sequences, and to open new perspectives in understanding Mediterranean plant adaptations, a genomic study on the basis of the complete chloroplast genome sequencing of Arbutus unedo and an updated phylogenomic analysis of Asteridae was implemented. The chloroplast genome of A. unedo shows extensive rearrangements but a medium size (150,897 nt) in comparison to most of angiosperms. A number of remarkable distinct features characterize the plastome of A. unedo: five-fold dismissing of the SSC region in relation to most angiosperms; complete loss or pseudogenization of a number of essential genes; duplication of the ndhH-D operon and its location within the two IRs; presence of large tandem repeats located near highly re-arranged regions and pseudogenes. All these features outline the primary evolutionary split between Ericaceae and other ericalean families. The newly sequenced plastome of A. unedo with the available asterid sequences allowed the resolution of some uncertainties in previous phylogenies of Asteridae. [ABSTRACT FROM AUTHOR]

The chloroplast-encoded large subunit ribosomal RNA gene of several free-living green algae contains group I introns at Escherichia coli genic positions 1917, 1931, 1951, and 2449. Herein we report the presence of group I introns at these -positions within the chloroplast-encoded large subunit ribosomal RNA gene of several lichen-forming green algae belonging to the Trebouxia genus. In contrast to the introns inserted at position 2449, all introns inserted at positions 1917, 1931, and 1951 contained LAGLIDADG homing endonuclease genes. Phylogenetic analyses show that: (i) introns inserted at positions 1917, 1931, and 1951 are closely related to introns located at homologous insertion sites in bacterial rDNA genes; and (ii) introns inserted at position 2449 are closely related to fungal introns located at homologous insertion sites in mitochondrial rDNA genes. The symbiogenetic thalli of some lichens are proposed as the likely setting of horizontal transfer of genetic material among distantly related organisms such as bacteria, fungi, and green algae. [ABSTRACT FROM AUTHOR]

An Ndh-deficient mutant of tobacco (Nicotiana tabacum cv. Petit Havana) was prepared by disrupting the ndhF gene in a transplastomic approach. The mutant (ΔndhF) showed 10% of the Ndh complex activity (EC 1.6.5.3) and 8% of the NDH-F polypeptide of that of non-transformed plants. However, in ΔndhF, NDH-A, another Ndh polypeptide, was still present at 50% of the level in non-transformed plants. ΔndhF tobacco showed higher sensitivity than non-transformed plants to photo-oxidative stress (as judged by chlorophyll bleaching) caused by increased light intensity and paraquat applications. These photo-oxidative treatments increased the amount and activity of the Ndh complex, thylakoid peroxidase, post-illumination chlorophyll fluorescence and non-photochemical quenching (NPQ) of chlorophyll fluorescence in non-transformed but not in ΔndhF tobacco. Highly stressed non-transformed plants showed a rapid post-rise decline of chlorophyll fluorescence, probably indicating a re-oxidation of reduced plastoquinone. The results indicate that, in normal plants, the Ndh complex and thylakoid peroxidase (EC 1.11.1.7) provide and remove electrons, respectively, to balance the redox level of the intermediates of cyclic electron transport. In this way, they optimize the generation of the transmembrane H+ gradient of thylakoids and, as a consequence, increase the NPQ and the protection against photo-oxidative stress. [ABSTRACT FROM AUTHOR]

About 350 nucleotide sequences around the start codon of the plastid ndhD gene were determined in six species to investigate the requirement of C to U editing of the cryptic start codons. The comparison with other sequences in databanks showed that, in contrast to grasses and similarly to dicots, Liliaceae and Aloaceae require editing of the primary transcript to generate the AUG start codon. Primary transcripts of Liliaceae and Aloaceae also show a new editing site at the 59th nucleotide, which has not been described in other plants. In some cases, transcripts showed partial editing which suggests that the synthesis of the protein may be controlled at the level of transcript processing. [ABSTRACT FROM AUTHOR]

Trebouxia sp. TR9 and Coccomyxa simplex are desiccation-tolerant microalgae with flexible cell walls, which undergo species-specific remodelling during dehydration-rehydration (D/R) due to their distinct ultrastructure and biochemical composition. Here, we tested the hypothesis that extracellular polysaccharides excreted by each microalga could be quantitatively and/or qualitatively modified by D/R. Extracellular polysaccharides were analysed by size exclusion and anion exchange chromatography, specific stains after gel electrophoresis and gas chromatography/mass spectrometry of trimethylsilyl derivatives (to determine their monosaccharide composition). The structure of a TR9-sulfated polymer was deduced from nuclear magnetic resonance (NMR) analyses. In addition, sugar-sulfotransferase encoding genes were identified in both microalgae, and their expression was measured by RT-qPCR. D/R did not alter the polydispersed profile of extracellular polysaccharides in either microalga but did induce quantitative changes in several peaks. Furthermore, medium-low-sized uronic acid-containing polysaccharides were almost completely substituted by higher molecular mass carbohydrates after D/R. Sulfated polysaccharide(s) were detected, for the first time, in the extracellular polymeric substances of both microalgae, but only increased significantly in TR9 after cyclic D/R, which induced a sugar-sulfotransferase gene and accumulated sulfated ß-D-galactofuranan(s). Biochemical remodelling of extracellular polysaccharides in aeroterrestrial desiccation-tolerant microalgae is species-specific and seems to play a role in the response to changes in environmental water availability., (© 2020 Society for Applied Microbiology and John Wiley & Sons Ltd.)

Background and Aims: One of the most distinctive features of desiccation-tolerant plants is their high cell wall (CW) flexibility. Most lichen microalgae can tolerate drastic dehydration-rehydration (D/R) conditions; however, their mechanisms of D/R tolerance are scarcely understood. We tested the hypothesis that D/R-tolerant microalgae would have flexible CWs due to species-specific CW ultrastructure and biochemical composition, which could be remodelled by exposure to cyclic D/R., Methods: Two lichen microalgae, Trebouxia sp. TR9 (TR9, adapted to rapid D/R cycles) and Coccomyxa simplex (Csol, adapted to seasonal dry periods) were exposed to no or four cycles of desiccation [25-30 % RH (TR9) or 55-60 % RH (Csol)] and 16 h of rehydration (100 % RH). Low-temperature SEM, environmental SEM and freeze-substitution TEM were employed to visualize structural alterations induced by D/R. In addition, CWs were extracted and sequentially fractionated with hot water and KOH, and the gel permeation profile of polysaccharides was analysed in each fraction. The glycosyl composition and linkage of the main polysaccharides of each CW fraction were analysed by GC-MS., Key Results: All ultrastructural analyses consistently showed that desiccation caused progressive cell shrinkage and deformation in both microalgae, which could be rapidly reversed when water availability increased. Notably, the plasma membrane of TR9 and Csol remained in close contact with the deformed CW. Exposure to D/R strongly altered the size distribution of TR9 hot-water-soluble polysaccharides, composed mainly of a β-3-linked rhamnogalactofuranan and Csol KOH-soluble β-glucans., Conclusions: Cyclic D/R induces biochemical remodelling of the CW that could increase CW flexibility, allowing regulated shrinkage and expansion of D/R-tolerant microalgae., (© The Author(s) 2019. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

The Trebouxiophyceae is the class of Chlorophyta algae from which the highest number of chloroplast genome (cpDNA) sequences has been obtained. Several species in this class participate in symbioses with fungi to form lichens. However, no cpDNA has been obtained from any Trebouxia lichen-symbiont microalgae, which are present in approximately half of all lichens. Here, we report the sequence of the completely assembled cpDNA from Trebouxia sp. TR9 and a comparative study with other Trebouxio-phyceae. The organization of the chloroplast genome of Trebouxia sp. TR9 has certain features that are unusual in the Trebouxiophyceae and other green algae. The most remarkable characteristics are the presence of long intergenic spacers, a quadripartite structure with short inverted repeated sequences (IRs), and the loss of the rps4 gene. The presence of long intergenic spacers accounts for a larger cpDNA size in comparison to other closely related Trebouxiophyceae. The IRs, which were thought to be lost in the Trebouxiales, are distinct from most of cpDNAs since they lack the rRNA operon and uniquely includes the rbcL gene. The functional transfer of the rps4 gene to the nuclear genome has been confirmed by sequencing and examination of the gene architecture, which includes three spliceosomal introns as well as the verification of the presence of the corresponding transcript. This is the first documented transfer of the rps4 gene from the chloroplast to the nucleus among Viridiplantae. Additionally, a fairly well-resolved phylogenetic reconstruction, including Trebouxia sp. TR9 along with other Trebouxiophyceae, was obtained based on a set of conserved chloroplast genes., (© 2019 Phycological Society of America.)

The epiphytic lichen Ramalina farinacea is distributed throughout the northern hemisphere in which the same two algal Trebouxia species (provisionally named TR1 and TR9) coexist in every thallus. Ramalina farinacea symbionts were characterized based on the two fungal nuclear loci (nrITS and rpb2) along with the primary and secondary structures of nrITS from each Trebouxia species in the Iberian Peninsula and Canary Islands. The results indicated a noticeable genetic differentiation between mycobionts from these two geographic areas and also suggested concerted changes in the three partners of a lichen symbiosis toward two clearly distinguishable 'holobiont' lineages. Modeling of ITS2 RNA secondary structures suggested their temperature sensitivity in TR1 but not in TR9, which was consistent with the observed superior physiological performance of TR9 phycobionts under relatively high temperatures. Both TR1 and TR9 phycobionts have been also found in a variety of taxonomically distinct lichens with a preferably Mediterranean distribution, being TR1 much more widespread than TR9. Our observations support a model in which ecological diversification and speciation of lichen symbionts in different habitats could include a transient phase consisting of associations with more than one photobiont in individual thalli. Such diversification is likely to be promoted by different physiological backgrounds., (© 2012 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd. All rights reserved.)