Anthropogenic deposition of heavy metals and phosphorus may reduce biological N2 fixation in boreal forest mosses.

A study was undertaken to test the effects of molybdenum (Mo) and phosphorus (P) amendments on biological nitrogen (N) fixation (BNF) by boreal forest moss-associated cyanobacteria. Feather moss (Pleurozium schreberi) samples were collected on five sites, on two dates and at different roadside distances (0-100m) corresponding to an assumed gradient of reactive N deposition. Potential BNF of Mo and P amended moss samples was measured using the acetylene reduction assay. Total N, P and heavy metal concentrations of mosses collected at 0 and 100m from roadsides were also measured. Likewise, the needles from Norway spruce trees (Picea abies) at different roadside distances were collected in late summer and analyzed for total N, P and heavy metals. There was a significant increase in BNF with roadside distance on 7-of-10 individual Site×Date combinations. We found no clear evidence of an N gradient across roadside distances. Elemental analyses of feather moss and Norway spruce needle tissues suggested decreasing deposition of heavy metals (Mo-Co-Cr-Ni-V-Pb-Ag-Cu) as well as P with increasing distance from the roadside. The effects of Mo and P amendments on BNF were infrequent and inconsistent across roadside distances and across sites. One particular site, however, displayed greater concentrations of heavy metals near the roadside, as well as a steeper P fertility gradient with roadside distance, than the other sites. Here, BNF increased with roadside distance only when moss samples were amended with P. Also at this site, BNF across all roadside distances was higher when mosses were amended with both Mo and P, suggesting a co-limitation of these two nutrients in controlling BNF. In summary, our study showed a potential for car emissions to increase heavy metals and P along roadsides and underscored the putative roles of these anthropogenic pollutants on BNF in northern latitudes.

An event of alternative splicing affects the expression of the NTRC gene, encoding NADPH-thioredoxin reductase C, in seed plants.

The NTRC gene encodes a NADPH-dependent thioredoxin reductase with a joint thioredoxin domain, exclusive of photosynthetic organisms. An updated search shows that although most species harbor a single copy of the NTRC gene, two copies were identified in different species of the genus Solanum, Glycine max and the moss Physcomitrella patens. The phylogenetic analysis of NTRCs from different sources produced a tree with the major groups of photosynthetic organisms: cyanobacteria, algae and land plants, indicating the evolutionary success of the NTRC gene among photosynthetic eukaryotes. An event of alternative splicing affecting the expression of the NTRC gene was identified, which is conserved in seed plants but not in algae, bryophytes and lycophytes. The alternative splicing event results in a transcript with premature stop codon, which would produce a truncated form of the enzyme. The standard splicing/alternative splicing (SS/AS) transcripts ratio was higher in photosynthetic tissues from Arabidopsis, Brachypodium and tomato, in line with the higher content of the NTRC polypeptide in these tissues. Moreover, environmental stresses such as cold or high salt affected the SS/AS ratio of the NTRC gene transcripts in Brachypodium seedlings. These results suggest that the alternative splicing of the NTRC gene might be an additional mechanism for modulating the content of NTRC in photosynthetic and non-photosynthetic tissues of seed plants.

Evolutionary relationships among barley and Arabidopsis core circadian clock and clock-associated genes.

The circadian clock regulates a multitude of plant developmental and metabolic processes. In crop species, it contributes significantly to plant performance and productivity and to the adaptation and geographical range over which crops can be grown. To understand the clock in barley and how it relates to the components in the Arabidopsis thaliana clock, we have performed a systematic analysis of core circadian clock and clock-associated genes in barley, Arabidopsis and another eight species including tomato, potato, a range of monocotyledonous species and the moss, Physcomitrella patens. We have identified orthologues and paralogues of Arabidopsis genes which are conserved in all species, monocot/dicot differences, species-specific differences and variation in gene copy number (e.g. gene duplications among the various species). We propose that the common ancestor of barley and Arabidopsis had two-thirds of the key clock components identified in Arabidopsis prior to the separation of the monocot/dicot groups. After this separation, multiple independent gene duplication events took place in both monocot and dicot ancestors.

Responses of photosynthetic properties and chloroplast ultrastructure of Bryum argenteum from a desert biological soil crust to elevated ultraviolet-B radiation.

Our understanding of plant responses to enhanced ultraviolet-B (UV-B) radiation has improved over recent decades. However, research on cryptogams is scarce and it remains controversial whether UV-B radiation causes changes in physiology related to photosynthesis. To investigate the effects of supplementary UV-B radiation on photosynthesis and chloroplast ultrastructure in Bryum argenteum Hedw., specimens were cultured for 10 days under four UV-B treatments (2.75, 3.08, 3.25 and 3.41 W m(-2) ), simulating depletion of 0% (control), 6%, 9% and 12% of stratospheric ozone at the latitude of Shapotou, a temperate desert area of northwest China. Analyses showed malondialdehyde content significantly increased, whereas chlorophyll (Chl) fluorescence parameters and Chl contents decreased with increased UV-B intensity. These results corresponded with changes in thylakoid protein complexes and chloroplast ultrastructure. Overall, enhanced UV-B radiation leads to significant decreases in photosynthetic function and serious destruction of the chloroplast ultrastructure of B. argenteum. The degree of negative influences increased with the intensity of UV-B radiation. These results may not only provide a potential mechanism for supplemental UV-B effects on photosynthesis of moss crust, but also establish a theoretical basis for further studies of adaptation and response mechanisms of desert ecosystems under future ozone depletion.

The moss Physcomitrella patens contains cyclopentenones but no jasmonates: mutations in allene oxide cyclase lead to reduced fertility and altered sporophyte morphology.

• Two cDNAs encoding allene oxide cyclases (PpAOC1, PpAOC2), key enzymes in the formation of jasmonic acid (JA) and its precursor (9S,13S)-12-oxo-phytodienoic acid (cis-(+)-OPDA), were isolated from the moss Physcomitrella patens. • Recombinant PpAOC1 and PpAOC2 show substrate specificity against the allene oxide derived from 13-hydroperoxy linolenic acid (13-HPOTE); PpAOC2 also shows substrate specificity against the allene oxide derived from 12-hydroperoxy arachidonic acid (12-HPETE). • In protonema and gametophores the occurrence of cis-(+)-OPDA, but neither JA nor the isoleucine conjugate of JA nor that of cis-(+)-OPDA was detected. • Targeted knockout mutants for PpAOC1 and for PpAOC2 were generated, while double mutants could not be obtained. The ΔPpAOC1 and ΔPpAOC2 mutants showed reduced fertility, aberrant sporophyte morphology and interrupted sporogenesis.

Enhanced expression of a calcium-dependent protein kinase from the moss Funaria hygrometrica under nutritional starvation.

Among the downstream targets of calcium in plants, calcium-dependent protein kinases (CDPKs) form an interesting class of kinases which are activated by calcium binding. They have been implicated in a diverse array of responses to hormonal and environmental stimuli. In order to dissect the role of CDPKs in the moss Funaria hygrometrica, a polymerase chain reaction (PCR)-based approach was adopted to clone the gene. Using degenerate PCR primers against conserved regions of CDPKs, a 900 bp amplicon was obtained from the genomic DNA of Funaria. Southern hybridization under low stringency conditions indicated the presence of several CDPK related sequences in the Funaria genome. This observation is consistent with reports of multigene families of CDPKs in other plants. The 900 bp fragment was subsequently used to isolate a 2.2 kb partial genomic clone of the CDPK gene from Funaria. The genomic clone encodes an open reading frame (ORF) of 518 amino acids. Interestingly, unlike other CDPK genes from plants, the entire 1.5 kb ORF is not interrupted by introns. The deduced amino acid sequence of the Funaria gene shows extensive homology with CDPKs from higher plants, 73% identity with the Fragaria CDPK and 71% identity with CDPK isoform 7 of Arabidopsis. Phylogenetic analysis revealed that the Funaria CDPK is closer to the CDPKs from higher plants like strawberry and Arabidopsis as compared to those from lower plants such as the liverwort Marchantia, the green alga Chlamydomonas or another moss Tortula. Northern analysis shows enhanced expression of the CDPK transcript within 24-48 h of starvation for nitrogen, phosphorus or sulphur. So far the only other kinase which is known to be induced by nutrient starvation in plants is the wpk 4 which is a snf-1 related kinase (SnRKs). To our knowledge this is the first report that implicates a CDPK in the starvation response.