Symbiosis

Salinity is one of the major constraints for agricultural productivity and the problem of salinity is likely to increase further. The aquatic fern Azolla is employed as an important bio-inoculant for the cultivation of rice paddy. In addition to this it is also used as feed supplement for cattle and poultry. However, increasing soil salinity is a serious threat for its wider exploitation. Stressors such as salinity leads to drastic changes in the metabolic activities especially the photosynthesis and nitrogen fixation. The potential of plants to tolerate salinity is determined by efficient mechanisms operating at cellular and sub-cellular levels. Several adaptation strategies are required to cope with salinity stress. Complicated interactions between the host plant Azolla and its symbiont should be considered if this organism is employed as bio-inoculant. Therefore, in the present review we discuss on the effect of salinity on Azolla and its cyanobiont with an emphasis on the physiological, biochemical and molecular mechanisms of salinity tolerance.

Arbuscular mycorrhiza (AM) development in five non-nodulating pea (Pisum sativum L.) mutants (sym7, sym11, sym14, sym34, sym38) grown in nurse pots was analyzed at the early stages of colonization by Rhizophagus irregularis. Abundance of external mycelium attached to the root surface and parameters of internal fungal colonization, i.e. intensity of colonization, and arbuscule and vesicle/spore abundance, were evaluated. Mutations in SYM7 (putative ortholog of NSP2, ‘nodulation signaling pathway 2’), SYM11, and SYM14 genes resulted in considerable increase in root surface colonization and a substantial decrease in internal colonization as compared to corresponding wild-type pea lines (wt). In addition, the mutants sym7 and sym11 displayed a highly reduced amount of arbuscules. In contrast, plants mutated for the SYM34 gene displayed strongly reduced root surface colonization. The mutant sym34 also had strongly reduced internal colonization after 10 days of growth, but did not differ from wt 10 days later. The sym38 mutant did not differ from wt. These data indicate that all pea genes analyzed, except SYM38, are essential for both nodule and AM development. The phenotype of sym34 suggested that the pea SYM34 gene is an ortholog of the Medicago truncatula NSP1 (‘nodulation signaling pathway 1’), and early stop codons were in fact detected in an NSP1 homologous sequence of two sym34 mutants. In addition, full co-segregation of the allelic states of a molecular marker representing a polymorphic fragment of the hypothetical pea NSP1 gene, together with the nodulation phenotype of 50 F2 plants, support our hypothesis. A possible explanation for the different AM phenotypes of sym7 and sym34 is given.

We quantified relative growth rates (RGR) in shade-adapted and melanin-deficient thalli of Cetraria islandica and Lobaria pulmonaria cultivated in short-term growth chamber experiments with and without UV-B radiation. In the first experiment done under optimal PAR (125 μmol m−2 s−1), but high UV-B radiation (1 W m−2), UV-B radiation significantly reduced RGR (P < 0.001). The second experiment with higher PAR, but more natural ratios between wavelength ranges (PAR: 500 μmol m−2 s−1; UV-A: 7 W m−2; UV-B: 0.4 W m−2), caused a reduction in mean RGR in L. pulmonaria to just 45% of rates in experiment 1. Lobaria pulmonaria screened from UV-B radiation had 1.9 and 1.6 times higher RGR than non-screened thalli in experiment 1 and 2, respectively. UV-B radiation significantly induced melanin synthesis in the second experiment only, causing significantly less photoinhibition than in thalli receiving just PAR. This is consistent with PAR-protective roles of melanins. Chlorophylls were not affected by UV-B radiation in any experiment. Because UV-B radiation affected RGR more than pure photobiont responses, the mycobiont is likely the more UV-B-susceptible partner. Apart from reduced RGR, we found little evidence for adverse UV-B effects.

Chlorophyll synthesis is an ongoing requirement for photosynthesis and a ubiquitous, diagnostic characteristic of plants and algae amongst eukaryotes. However, we have discovered that chlorophyll a (Chla) is synthesized in the symbiotic chloroplasts of the sea slug, Elysia chlorotica, for at least 6 months after the slugs have been deprived of the algal source of the plastids, Vaucheria litorea. In addition, using transcriptome analysis and PCR with genomic DNA, we found 4 expressed genes for nuclear-encoded enzymes of the Chla synthesis pathway that have been horizontally transferred from the alga to the genomic DNA of the sea slug. These findings demonstrate the first discovery of Chla production in an animal using transferred nuclear genes from its algal food.

Endophytes are a large and diverse group of fungi that colonize healthy plant tissues without causing any symptoms. The majority of studies have focused on angiosperm and conifer hosts and few have examined the endophytes of lycophytes. In the present study, we characterized culturable endophytic fungi in two closely related Lycopodium species (L. annotinum and L. clavatum) from pine, beech, oak and spruce forests across Poland. More than 400 strains were isolated but only 18 Ascomycete species were identified. Members of the Dothideomycetes dominated the fungal endophyte communities in Lycopodium. The most abundant taxa cultured were Phoma brasiliensis (from L. clavatum) and Paraconiothyrium lycopodinum (from L. annotinum). Five taxa were isolated exclusively from L. annotinum, but only two of them (Paraconiothyrium lycopodinum and Mycosphaerella sp.) were relatively abundant. Two taxa were only found in L. clavatum, namely: Stagonospora pseudovitensis and an unidentified Dothideomycete. The taxon assigned as Ascomycota 2 (SH219457.06FU) was isolated only from strobili of both host species. Direct PCR and cloning from L. annotinum shoots revealed a substantially greater endophyte richness compared with the results from culturing.

The algal photobiont of the lichen Porpidia crustulata from the Guancen Mountains was successfully cultivated under axenic conditions and tentatively designated as Chlorella sp. GC. The phylogenetic analysis of ribulose bisphosphate carboxylase (rbcL) and internal transcribed spacer (ITS) nucleotide sequence data revealed that this alga is closely related to C. sorokiniana. There are clear morphological differences between Chlorella sp. GC associated with Porpidia crustulata and the free-living C. sorokiniana, the former having a larger central pyrenoid. Based on our genetic and morphological characterization, the Chlorella photobiont of Porpidia crustulata is distinct from closely related Chlorella species.

Twenty four rhizobial strains were isolated from root nodules of Melilotus, Medicago and Trigonella plants growing wild in soils throughout Egypt. The nearly complete 16S rRNA gene sequence from each strain showed that 12 strains (50 %) were closely related to the Ensifer meliloti LMG6133T type strain with identity values higher than 99.0 %, that 9 (37.5 %) strains were more than 99 % identical to the E. medicae WSM419T type strain, and that 3 (12.5 %) strains showed 100 % identity with the type strain of N. huautlense S02T. Accordingly, the diversity of rhizobial strains nodulating wild Melilotus, Medicago and Trigonella species in Egypt is marked by predominance of two genetic types, E. meliloti and E. medicae, although the frequency of isolation was slightly higher in E. meliloti. Sequencing of the symbiotic nodC gene from selected Medicago and Melilotus strains revealed that they were all similar to those of the E. meliloti LMG6133T and E. medicae WSM419T type strains, respectively. Similarly, nodC sequences of strains identified as members of the genus Neorhizobium were more than 99 % identical to that of N. galegae symbiovar officinalis HAMBI 114.

Reindeer lichens (such as Cladonia rangiferina) are important winter forage for caribou and reindeer and are widely distributed in northern ecosystems. Widespread lichen communities may be explained by dispersal from thallus fragments or by fungal ascospores interacting with algal cells. Since three early stages of interactions between symbionts of the lichen C. rangiferina have already been established, this study investigated the effect of three temperatures (5, 20, and 35 °C) and pH levels (4.5, 6.5, and 8.5) on the early interaction of C. rangiferina by quantifying morphological differences for three fungal (internode length, number of lateral branches, number of appressoria) and one algal (cell diameter) characters using Scanning Electron Microscopy. The results showed that the fungal characters were significantly altered by the extreme temperatures (5 and 35 °C) and the pH level produced differences in the fungal characters at pH 8.5. The alga was more tolerant of the wide temperature range than the fungus while the fungus was more tolerant of pH changes than the alga. An interaction effect by temperature and pH on the symbiont characters was also observed. The study raises questions regarding the range of conditions tolerated by other species of lichens and their symbionts.