Pilar Alonso

The genus Micromonospora is widespread in legume root nodules: the example of Lupinus angustifolius

Our current knowledge of plant-microbe interactions indicate that populations inhabiting a host plant are not restricted to a single microbial species but comprise several genera and species. No one knows if communities inside plants interact, and it has been speculated that beneficial effects are the result of their combined activities. During an ecological study of nitrogen-fixing bacterial communities from Lupinus angustifolius collected in Spain, significant numbers of orange-pigmented actinomycete colonies were isolated from surface-sterilized root nodules. The isolates were analysed by BOX-PCR fingerprinting revealing an unexpectedly high genetic variation. Selected strains were chosen for 16S rRNA gene sequencing and phylogenetic analyses confirmed that all strains isolated belonged to the genus Micromonospora and that some of them may represent new species. To determine the possibility that the isolates fixed atmospheric nitrogen, chosen strains were grown in nitrogen-free media, obtaining in some cases, significant growth when compared with the controls. These strains were further screened for the presence of the nifH gene encoding dinitrogenase reductase, a key enzyme in nitrogen fixation. The partial nifH-like gene sequences obtained showed a 99% similarity with the sequence of the nifH gene from Frankia alni ACN14a, an actinobacterium that induces nodulation and fixes nitrogen in symbiosis with Alnus. In addition, in situ hybridization was performed to determine if these microorganisms inhabit the inside of the nodules. This study strongly suggests that Micromonospora populations are natural inhabitants of nitrogen-fixing root nodules.

Diversity of Micromonospora strains isolated from nitrogen fixing nodules and rhizosphere of Pisum sativum analyzed by multilocus sequence analysis

It was recently reported that Micromonospora inhabits the intracellular tissues of nitrogen fixing nodules of the wild legume Lupinus angustifolius. To determine if Micromonospora populations are also present in nitrogen fixing nodules of cultivated legumes such as Pisum sativum, we carried out the isolation of this actinobacterium from P. sativum plants collected in two man-managed fields in the region of Castilla and León (Spain). In this work, we describe the isolation of 93 Micromonospora strains recovered from nitrogen fixing nodules and the rhizosphere of P. sativum. The genomic diversity of the strains was analyzed by amplified ribosomal DNA restriction analysis (ARDRA). Forty-six isolates and 34 reference strains were further analyzed using a multilocus sequence analysis scheme developed to address the phylogeny of the genus Micromonospora and to evaluate the species distribution in the two studied habitats. The MLSA results were evaluated by DNA-DNA hybridization to determine their usefulness for the delineation of Micromonospora at the species level. In most cases, DDH values below 70% were obtained with strains that shared a sequence similarity of 98.5% or less. Thus, MLSA studies clearly supported the established taxonomy of the genus Micromonospora and indicated that genomic species could be delineated as groups of strains that share > 98.5% sequence similarity based on the 5 genes selected. The species diversity of the strains isolated from both the rhizosphere and nodules was very high and in many cases the new strains could not be related to any of the currently described species.

THE LEXICO-STRUCTURAL/SEMANTIC PROGRESSION IN DISCOURSE TOPIC CONSTRUCTION: EVIDENCE FROM COMMENT ARTICLES

In this article, Seuren’s notion of mod-proposition is taken as point of departure to address the question of discourse topic. Seuren distinguishes between the classic notion of proposition (flat-propositions) and modulated propositions (mod-propositions), and assigns to the latter a topic-comment modulated structure which becomes essential to decide on the information structure of discourse, especially in terms of incrementation. Drawing from this propositional modulated status, different theoretical approaches to the notion of topic at sentence level are reviewed and special attention is paid to the syntactic functions identified by these theoreticians as predominantly topical. Then the empirical analysis of a small corpus of comment articles in English is used to classify the linguistic strategies employed in the construction of a global discourse topic; this procedure makes it possible to focus on those topics which gradually acquire global projection by re-use, in opposition to those pieces of information which are topical only at the local level. Based on this evidence, our research shows the linguistic mechanisms which allow for re-used topical information to gradually acquire subject status as discourse progresses and proposes thinking in terms of intermediate information structures as subsumers of local information and builders of global topics.