Casano, L.; Del Campo, E.; García Breijo, FJ.; Reig Armiñana, J.; Del Hoyo, A.; Guéra, A.... (2011)

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.

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.

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.

Intergenic cleavages, intron splicing, and editing of primary transcripts of the plastid ndhH-D operon produce multiple overlapping RNAs, of which the most abundant by far is the monocistronic 400-nucleotide mRNA of psaC (encoding the PsaC protein of photosystem I), in contrast with the low level of transcripts of the six ndh genes. Like other plastid operons containing genes for functionally unrelated proteins, the contrasting accumulation of ndh and psaC transcripts provides a model to investigate the mechanisms of the post-transcriptional control of gene expression, a feature of chloroplast genetic machinery, with a minimum of interference by transcriptional control. In leek (Allium porrum L), the ndhD transcript (which follows the psaC gene and ends the ndhH-D operon) requires C 3 U editing to restore its start codon and may be used as a marker for the processing of psaC and ndhD transcripts. By determining the editing state and 5 end sequences of specific transcripts, we demonstrated that stable monocistronic psaC mRNA results from downstream cleavages in the ndhD sequence, which renders non-functional ndhD transcripts as by-products. Alternative psaC-ndhD intergenic cleavages produce complete mRNAs for both genes, but only take place in precursors containing editing-restored ndhD start codons. Hence, post-transcriptional control acts by promoting the ndhD cleavage alternative, which allows the accumulation of psaC mRNA at the expense of ndhD mRNA levels.Many chloroplast genes are expressed as polycistronic transcription units that are processed to complex sets of overlapping RNAs through steps controlled by nuclear encoded factors (1, 2). A lot remains to be learned about factors, intermediates, and the order in which transcript processing occurs as well as the mechanisms responsible for the accumulation of specific mRNAs (frequent in operons containing genes for functionally unrelated proteins). This last is a feature of chloroplast genetic machinery in which post-transcriptional processing controls the differential expression of specific genes within the same operon.The NdhH-D operon provides a model to investigate posttranscriptional control of gene expression, because it includes six ndh genes whose protein products are present at low levels in chloroplasts, together with the psaC gene, encoding the PsaC protein (9-kDa Fe-S subunit VII) of the abundant photosystem I. The polypeptides encoded by ndh genes are part of the thylakoid Ndh complex that has been purified from peas (3) and barley (4). The photosystem I/Ndh ratio in chloroplasts is estimated at 200. Accordingly, the level of monocistronic psaC mRNA is estimated to be around 2 orders of magnitude higher than that of ndh genes (5-8).The NdhH-D operon includes (in this order) ndhH, ndhA, ndhI, ndhG, ndhE, psaC, and ndhD genes (9) and produces a complex pattern of transcripts resulting from intergenic cleavages, intron splicing (within ndhA), and C 3 U editing at several specific sites (usually in ndhA and ndhD genes). The requirement of intron spl...