Nancy A. Fujishige

Rhizobium common nod genes are required for biofilm formation.

In legume nitrogen‐fixing symbioses, rhizobial nod genes are obligatory for initiating infection thread formation and root nodule development. Here we show that the common nod genes, nodD1ABC, whose products synthesize core Nod factor, a chitin‐like oligomer, are also required for the establishment of the three‐dimensional architecture of the biofilm of Sinorhizobium meliloti. Common nod gene mutants form a biofilm that is a monolayer. Moreover, adding Nod Factor antibody to S. meliloti cells inhibits biofilm formation, while chitinase treatment disrupts pre‐formed biofilms. These results attest to the involvement of core Nod factor in rhizobial biofilm establishment. However, luteolin, the plant‐derived inducer of S. melilotis nod genes, is not required for mature biofilm formation, although biofilm establishment is enhanced in the presence of this flavonoid inducer. Because biofilm formation is plant‐inducer‐independent and because all nodulating rhizobia, both alpha‐ and beta‐proteobacteria have common nod genes, the role of core Nod factor in biofilm formation is likely to be an ancestral and evolutionarily conserved function of these genes.

Complete Genome Sequence of Micromonospora Strain L5, a Potential Plant-Growth-Regulating Actinomycete, Originally Isolated from Casuarina equisetifolia Root Nodules

ABSTRACT Micromonospora species live in diverse environments and exhibit a broad range of functions, including antibiotic production, biocontrol, and degradation of complex polysaccharides. To learn more about these versatile actinomycetes, we sequenced the genome of strain L5, originally isolated from root nodules of an actinorhizal plant growing in Mexico.

Molecular Signals and Receptors: Communication Between Nitrogen-Fixing Bacteria and Their Plant Hosts

Our understanding of the extent of communication taking place between the plant and its underground microbiome (rhizosphere microbes) as well as with other soil organisms has grown exponentially in the last decade. Much of this information has been obtained from studies of nitrogen-fixing organisms, particularly members of the family Rhizobiaceae(Alphaproteobacteria) that establish nodules on legume roots in which atmospheric nitrogen is converted to plant-utilizable forms. Signals exchanged among organisms in the rhizosphere via quorum sensing (QS) and the responses to these signals have been identified, but it is unclear how they influence the downstream stages of nodulation and nitrogen fixation. An exchange of signal molecules ensures that a high level of specificity takes place to optimize the nitrogen-fixing interaction between host legume and symbiont. Chitin-related molecules appear to be the microbial currency for communication between the symbiotic partners in both mutualistic and pathogenic interactions. Exceptions to the paradigms based on the legume-Rhizobiuminteraction, including the discovery of Betaproteobacteria (now called beta-rhizobia) that nodulate and fix nitrogen with legumes and the lack of nodulation (nod) genes in certain alpha-rhizobia, particularly those that nodulate Aeschynomeneand Arachis, bring into question the universality of some of the previous models. Moreover, new frontiers have opened that examine the coordination of information exchange that is needed for the induction and maintenance of nitrogen fixation and for bacteroid differentiation. Nevertheless, nitrogen-fixing organisms are just one small part of a highly interactive rhizosphere community. The challenge of the next decade will be to understand in greater depth the community dynamics that occur in soil, one of our planet’s most precious yet limited resources, in the hopes of maintaining the key signal webs that are critical not only for the promotion of agriculture but also for the preservation of the environment overall.

Expression of MsLEC1 Transgenes in Alfalfa Plants Causes Symbiotic Abnormalities

Legume lectins have been proposed to have important symbiotic roles during Rhizobium-legume symbioses. To test this hypothesis, the symbiotic responses of transgenic alfalfa plants that express a portion of the putative alfalfa lectin gene MsLEC1 or MsLEC2 in either the antisense or sense orientation were analyzed following inoculation with wild-type Sinorhizobium meliloti 1021. MsLEC1-antisense (LEC1AS) plants were stunted, exhibited hypernodulation, and developed not only abnormally large nodules but also numerous small nodules, both of which senesced prematurely. MsLEC2-antisense plants were intermediate in growth and nodule number compared with LEC1AS and vector control plants. The symbiotic abnormalities of MsLEC1-sense transgene plants were similar to but milder than the responses shown by the LEC1AS plants, whereas MsLEC2-sense transgene plants exhibited symbiotic responses that were identical to those of vector and nontransgenic control plants. MsLEC1 mRNA accumulation was not detected in nodule RNA by Northern blot analysis but was localized to alfalfa nodule meristems and the adjacent cells of the invasion zone by in situ hybridization; transcripts were also detected in root meristems. A similar spatial pattern of MsLEC2 expression was found by using a whole-mount in situ hybridization procedure. Moreover, mRNAs for an orthologous lectin gene (MaLEC) were detected in white sweetclover (Melilotus alba) nodules and root tips.

Mining the phytomicrobiome to understand how bacterial coinoculations enhance plant growth.

In previous work, we showed that coinoculating Rhizobium leguminosarum bv. viciae 128C53 and Bacillus simplex 30N-5 onto Pisum sativum L. roots resulted in better nodulation and increased plant growth. We now expand this research to include another alpha-rhizobial species as well as a beta-rhizobium, Burkholderia tuberum STM678. We first determined whether the rhizobia were compatible with B. simplex 30N-5 by cross-streaking experiments, and then Medicago truncatula and Melilotus alba were coinoculated with B. simplex 30N-5 and Sinorhizobium (Ensifer) meliloti to determine the effects on plant growth. Similarly, B. simplex 30N-5 and Bu. tuberum STM678 were coinoculated onto Macroptilium atropurpureum. The exact mechanisms whereby coinoculation results in increased plant growth are incompletely understood, but the synthesis of phytohormones and siderophores, the improved solubilization of inorganic nutrients, and the production of antimicrobial compounds are likely possibilities. Because B. simplex 30N-5 is not widely recognized as a Plant Growth Promoting Bacterial (PGPB) species, after sequencing its genome, we searched for genes proposed to promote plant growth, and then compared these sequences with those from several well studied PGPB species. In addition to genes involved in phytohormone synthesis, we detected genes important for the production of volatiles, polyamines, and antimicrobial peptides as well as genes for such plant growth-promoting traits as phosphate solubilization and siderophore production. Experimental evidence is presented to show that some of these traits, such as polyamine synthesis, are functional in B. simplex 30N-5, whereas others, e.g., auxin production, are not.