BackgroundDespite considerable progress in our understanding of land plant phylogeny, several nodes in the green tree of life remain poorly resolved. Furthermore, the bulk of currently available data come from only a subset of major land plant clades. Here we examine early land plant evolution using complete plastome sequences including two previously unexamined and phylogenetically critical lineages. To better understand the evolution of land plants and their plastomes, we examined aligned nucleotide sequences, indels, gene and nucleotide composition, inversions, and gene order at the boundaries of the inverted repeats.ResultsWe present the plastome sequences of Equisetum arvense, a horsetail, and of Isoetes flaccida, a heterosporous lycophyte. Phylogenetic analysis of aligned nucleotides from 49 plastome genes from 43 taxa supported monophyly for the following clades: embryophytes (land plants), lycophytes, monilophytes (leptosporangiate ferns + Angiopteris evecta + Psilotum nudum + Equisetum arvense), and seed plants. Resolution among the four monilophyte lineages remained moderate, although nucleotide analyses suggested that P. nudum and E. arvense form a clade sister to A. evecta + leptosporangiate ferns. Results from phylogenetic analyses of nucleotides were consistent with the distribution of plastome gene rearrangements and with analysis of sequence gaps resulting from insertions and deletions (indels). We found one new indel and an inversion of a block of genes that unites the monilophytes.ConclusionsMonophyly of monilophytes has been disputed on the basis of morphological and fossil evidence. In the context of a broad sampling of land plant data we find several new pieces of evidence for monilophyte monophyly. Results from this study demonstrate resolution among the four monilophytes lineages, albeit with moderate support; we posit a clade consisting of Equisetaceae and Psilotaceae that is sister to the "true ferns," including Marattiaceae.
Light-induced coleoptile stimulation and mesocotyl suppression in etiolated Avena sativa (cv. Lodi) has been quantitated. Etiolated seedlings showed the greatest response to light when they were illuminated 48 to 56 hours after imbibition. Two low-irradiance photoresponses for each tissue have been described. Red steps found in oats (1). They did not find in corn the very sensitive response detected in oats. Generally, the characteristics of the two responses ofthe corn mesocotyl were similar in magnitude, relative sensitivity, R/FR reversibility, and time dependence to those described for oats (29, cf. 1). Corn coleoptiles, however, under the conditions used to study mesocotyl suppression, showed no response at all to R for doses from 100 to 105 nE cm-2 for any wavelengths from 600 to 700 nm (Vanderhoef, unpublished).Other workers have also failed to obtain reproducible red light responses with coleoptiles (e.g. 17).The vast literature on photomorphogenesis in Avena was confusing for two major reasons: first, growth ofthe etiolated seedlings was highly variable, and second, the responses to light were often not reproducible. This report described the elongation growth of etiolated oat coleoptiles and mesocotyls in a growth system in which these problems have been solved and which has facilitated analysis of large groups of seedlings. METHODSGrowth of Uniform Etiolated Seedlings. A 9 x 6 cm piece of absorbent paper (Kimpak, Perf'd, Kimberly-Clarke) was placed in the middle of a 17 x 3.5 cm glass plate so that one long side of the paper and glass were even. Groups of 25 dry oat seeds (Avena sativa L. cf. Lodi, Lot No. 0170-B, Dakota Seed and Grain Co., Inc., Watertown, SD) with husks present were placed side by side onto strips of masking tape 12.7 mm in width (Scotch Brand, Part No. 6201, 3M Co.) with the embryos against the tape. About 5 cm of bare tape extended on either side of each group of seeds. A group of seeds was then laid against absorbent paper with the bare ends of the tape affixed to the glass plate beyond the paper. The ends of the seeds farthest away from the embryo and the padding were even with the aligned edges of the glass plate and paper, and any projecting glumes, etc., were excised. These drymounted seeds could be stored for several weeks at 4 C without loss of viability. They were imbibed by inserting this asembly with the seeds up into distilled H20 just to the level of the middle of the seeds for 0.5 h in absolute darkness. Uniformity ofgermination and growth was improved if the seeds themselves were not entirely submerged during imbibition. The plates were then transferred to racks which held them at an angle 450 from the vertical with the seeds up and the other edge of the absorbent paper immersed about 5 mm deep in distilled H20 in plastic boxes. These boxes were then tightly closed and kept in darkness until seedling treatment. The absorbent paper under the seeds acted as a wick to provide water to the seedlings for the duration of the experiments. This method yielded fro...
Arsenic is an element that is ubiquitous in the environment and is known to form compounds with toxic, even carcinogenic properties. Arsenic toxicity is a function of its chemical form (species). Identification of arsenic species is necessary to accurately determine the transformation and fate of arsenicals as well as the actual risk posed by arsenic contamination. We report X-ray absorption near-edge structure (XANES) measurements of 16 biologically important arsenic compounds. Solid and aqueous standards were studied for differences in XANES spectral features, white line positions, stability during exposure to the beam, and stability between two beam exposures separated by 48 h. Samples containing As(III) (11870.0-11871.7+/-0.5 eV) and As(V) (11872.6-11875.3+/-0.5 eV) were easily distinguished by white line energies and could be further subdivided into a total of seven groups. Valuable examples include As(III)-sulfur compounds (11870.0+/-0.5 eV), arsenobetaine and arsenocholine (11872.6+/-0.5 eV), and a dimethyl arsinyl riboside (11873.3+/-0.5 eV). A growing number of environmental and biological studies use X-ray absorption spectroscopy (XAS) results to complement their more traditional analyses. Results provided here are intended to help make XAS more accessible to new users interested in the study of arsenic in the environment.
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