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To evaluate the influence of soil type on the symbiosis between Mimosa spp. and rhizobia. A greenhouse experiment was carried out with trap plants using seeds of six species of Mimosa and soils from three different locations in central Brazil: Posse, Brasilia and Cavalcante. Plant dry biomass and number of nodules were measured after four months. Symbiotic bacteria were isolated from nodules and their molecular identification was performed. Three housekeeping genes (16S rRNA, recA and gyrB) plus the nodC and nifH symbiotic genes were used to determine the identity of the symbionts and to reconstruct the phylogenetic relationships among the isolated nitrogen-fixing bacteria. Rhizobia from the Betaproteobacterial genus Paraburkholderia (former Burkholderia) and the Alphaproteobacterial genus Rhizobium were isolated from different species of Mimosa. As in previous studies, the phylogenies of their symbiosis-essential genes, nodC and nifH, were broadly congruent with their core housekeeping genes (16S rRNA, recA and gyrB), which suggests limited or no horizontal gene transfer. Edaphic factors such as pH and fertility influenced the occurrence of these unrelated rhizobial types in the nodules on these Mimosa spp. Mimosa species have the ability to associate with different types of rhizobia (α- and β-proteobacteria), suggesting low specificity between host and bacterium in experimental conditions. Soil factors such as pH, nitrogen and fertility seem to favour the predominance of certain types of rhizobia, thus influencing the establishment of symbiotic relationships.

To describe the structure of nodules of Vachellia (Acacia) jacquemontii, and to characterise the rhizobia that occupy them. Light and electron microscopy were used to analyse nodules. Rhizobia were characterised using their 16S rRNA, housekeeping and symbiosis-related gene sequences. Nodules of V. jacquemontii were typical of all other described mimosoid legumes. All 73 of the isolates were strains of Ensifer, and concatenated phylogenetic analysis of their housekeeping genes (rrs, recA, atpD, glnII and dnaK) suggested that they are novel, forming separate lineages close to E. saheli. The phylogenies of the symbiosis-essential genes nodA and nifH were inconsistent with the housekeeping phylogenies. The nodA sequences of most isolates were close to that of E. arboris HAMBI 1552T, but the nifH gene was found to be related to that of E. kostiensis HAMBI 1489T. All the tested Ensifer strains, except for AJ24, were found to be capable of nodulating other species of Vachellia as well as native Indian Mimosa and Prosopis spp. Stressful conditions caused by the alkaline soil of the Thar Desert have resulted in V. jacquemontii being nodulated by diverse and promiscuous Ensifer species that are capable of nodulating other native members of the tribe Mimoseae.

Background Crops require adequate nutrition for the production of food, fibre and fuel, but soil conditions often limit the ability of crops to acquire mineral nutrients. To address this, mineral nutrients can be applied as inorganic or organic fertilisers to the soil or as liquid fertilisers to foliage. However, production and use of fertiliserscanhavenegativeenvironmentalimpacts.The articles in this Special Issue illustrate a number of ways to improve nutrient acquisition from the soil and their delivery through foliar application. Scope Articles highlighted here include those that discuss ways by which to assess a crop’s requirement for additional mineral elements, ways by which minerals can be supplied more effectively to crops both through roots and shoots, and ways by which the crop itself can be enhanced to acquire more mineral elements. Conclusions It is apparent from the information containedinthisSpecialIssue thattoimprovethe ability of crops to acquire mineral elements, a number of strategies are available. However, the success of any one intervention is dependent on how these strategies interact with the environment in which they are deployed and the suitability of the management system for the specific intervention.

Aims To survey the occurrence of nodulated legumes in the arid and semi-arid areas of Western Rajasthan and to characterize their associated symbiotic bacteria.

The 12th International Symposium on biological nitrogen fixation (BNF) with Non-Legumes took place in Buzios, Rio de Janeiro, Brazil from October 3 to 8, 2010. It was the latest in a series of meetings that, fittingly, first began in Brazil, in 1977 in the city of Piracicaba. This Special Issue of Plant and Soil comprises 21 papers from the Buzios Symposium, as well as a commentary and three obituaries. Given the volatility (and general upward trend) in oil prices and global attempts to alleviate greenhouse gas (GHG) emissions associated with the agricultural use of Ncontaining mineral fertilizers produced by the energyintensive Haber-Bosch process, it was inevitable that the main focus of the 2010 Symposium would be very much on the substitution of fertilizers in favour of the increased use of BNF in non-legume cropping systems. In the intervening years since the last time the symposium was held in Brazil (August 1987 in Rio de Janeiro), knowledge about non-legume BNF has increased enormously, encompassing everything from more accurate N-based quantification studies in the field through to the molecular identification of their associated diazotrophs (both culturable and unculturable) and the localization of these bacteria via high resolution microscopy. The completely sequenced genomes of many diazotrophic bacteria are also now available, and more are deposited in databases every year. Indeed, such technologies, which only a few years ago were considered remarkable, are now becoming standard tools in this field. In the case of sugarcane and other biofuel crops, the location of the 2010 meeting in Brazil was especially pertinent, as the highly advanced Brazilian bioethanol programme, which produces over 27 billion litres of ethanol per year, is based upon the cultivation of sugarcane, and Brazilian cane has long been known to be able to derive much of its N-requirements via BNF. The consequent low fertilizer use by Brazilian cane producers not only makes for enormous economic savings, but also mitigates against GHG emissions from the production of N-containing fertilizers, as well as their transport and application and nitrous oxide emissions. However, in spite of the strong likelihood that cane can benefit significantly from BNF, little is known about how much N it can really fix, and as field-based BNF quantification studies may take several years to complete, we are thus fortunate to include two such studies in this Special Issue (both utilizing the N natural abundance technique): one on sugarcane by Urquiaga et al., and one on the C-4 “energy crop” Plant Soil (2012) 356:1–3 DOI 10.1007/s11104-012-1317-1

Recent microscopic evidence acquired using strain-specific monoclonal antibodies and specific gene probes confirms earlier claims that some strains of Azospirillum lipoferum and A. brasilense, but not others, are capable of infecting the interior of wheat roots. The present study was performed to determine whether this strain specificity in the infection of the interior of wheat roots was apparent in the first 24 h of adsorption (`anchoring') of Azospirillum cells to the root surface. Strains of A. brasilense, originally isolated from surface-sterilised wheat roots (Sp 245, Sp 107) or with a proven ability to infect the interior of wheat roots (Sp 245), showed no greater ability to anchor to the roots than other Azospirillum strains isolated from the wheat rhizosphere (Sp 246) or from the rhizosphere or rhizosphere soil of other gramineae (Sp 7, Cd, S 82). The SEM images showed that at the root tip the Azospirillum cells were principally located in cracks between epidermal cells. In the root hair zone the bacteria were more numerous but again principally located in the depressions between epidermal cells. In all zones of the roots mucilage was present, and near the tip this appeared to have been partially digested, forming `halos' around the bacteria and revealing fibril-like strands attached to the bacteria. Subsequent studies were conducted using a technique originally developed for investigating competition of rhizobia for adsorption sites on legume roots. In the adaptation of this technique it was found that the presence of any significant concentration of Ca++ in the incubation medium reduced bacterial adsorption, as did concentrations of (PO4)3- above 50 mM. The influence of the pH of the incubation medium on the adsorption of ten different strains of Azospirillum showed, that with one exception, strains isolated from the roots or rhizosphere of wheat showed optimum adsorption at pH 6.0, and all other strains pH 7.0. Apart from this effect of pH no differences in adsorption were detected between strains with a proven capacity to infect wheat roots and those unable to do so. However, strains varied in their capability to compete for adsorption sites, there being a tendency for strains with a proven capacity to invade the internal tissues of wheat roots to be more competitive for adsorption sites.