Type IV pili are involved in plant–microbe and fungus–microbe interactions

Adherence of bacteria to eukaryotic cells is essential for the initiation of infection in many animal and human pathogens, e.g. Neisseria gonorrhoeae and Pseudomonas aeruginosa . Adhesion-mediating type IV pili, filamentous surface appendages formed by pilin subunits, are crucial virulence factors. Here, we report that type IV pilus-dependent adhesion is also involved in plant–bacteria and fungus–bacteria interactions. Nitrogen-fixing, endophytic bacteria, Azoarcus sp., can infect the roots of rice and spread systemically into the shoot without causing symptoms of plant disease. Formation of pili on solid media was dependent on the pilAB locus. PilA encodes an unusually short (6.4 kDa) putative pilin precursor showing 100% homology to the conserved N-terminus of the Pseudomonas aeruginosa type IV pilin. PilB encodes for a 14.2 kDa polypeptide showing similarity to FimF, a component of type I fimbriae of Escherichia coli . It was found to be extruded beyond the cell surface by immunofluorescence studies, and it may, therefore, be part of a pilus assembly complex or the pilus itself. Both genes are involved in the establishment of bacteria on the root surface of rice seedlings, as detected by fluorescence microscopy. Moreover, both genes are necessary for bacterial adhesion to the mycelium of an ascomycete, which was isolated from the same rhizosphere as the bacteria. In co-culture with the fungus, Azoarcus sp. forms complex intracytoplasmic membranes, diazosomes, which are related to efficient nitrogen fixation. Adhesion to the mycelium appears to be crucial for this process, as diazosomes were absent and nitrogen fixation rates were decreased in pilAB mutants in co-culture.     

The functional consequences of diversity in plant–pollinator interactions

The role of biological diversity in maintaining ecosystem functioning is a central issue in ecology. Most studies on diversity–functioning relationships have focused on ecosystem and community levels, leaving the extension of those relationships to other organization levels, such as populations, as a challenging and unsolved issue. Empirical studies have shown links between pollinator diversity and plant fecundity, suggesting that a diversity–functioning relationship at the population level may occur in pollination systems. We theoretically explored the effect of pollinator diversity on plant reproduction. We found that low pollinator diversity is beneficial when the most abundant pollinators are the most effective. In contrast, when the most effective pollinators are not the most abundant, we found an optimal value of pollinator diversity at which plant fecundity is maximized. When we parametrized our model with real data, we obtained that an increase in pollinator diversity was beneficial for the reproduction of some plants whereas it was harmful for other plants, the outcome depending exclusively on the differences in effectiveness among pollinators. Consequently, our theoretical approach suggests that in pollination systems the diversity–function relationship may be explained as the consequence of the interaction between among-pollinator differences in effectiveness and frequency of interaction, without the need to invoke additional ecological mechanisms.     

Conditional outcomes in plant–herbivore interactions: neighbours matter

Spatial distribution of palatable and unpalatable plants can influence the foraging behaviour of herbivores, thereby changing plant‐damage probabilities. Moreover, the immediate proximity to certain plants can benefit other plants that grow below them, where toxicity or spines act as a physical barrier or concealment against herbivores. This paper presents the results of a multi‐scale experiment performed to test the effect of shrubs as protectors of tree saplings against herbivores and the mechanism involved in Mediterranean ecosystems. We performed a factorial design in two mountain ranges, similar in physiognomy and vegetation, planting saplings of a palatable tree, the maple (Acer opalus subsp. granatense), and an unpalatable tree, the black pine (Pinus nigra), under three different types of shrubs. We considered four experimental microhabitats: highly palatable shrub (Amelanchier ovalis), palatable but spiny shrub (Crataegus monogyna or Prunus ramburii), unpalatable spiny shrub (Berberis vulgaris subsp. australis) and control (gaps of bare soil without shrubs). Three main factors were found to determine the probability of sapling attack: sapling palatability, experimental microhabitat and plot. Palatable saplings (maples) were browsed much more than unpalatable ones (pines). The degree of protection provided by the shrub proved greater as its palatability decreased with respect to sapling palatability, the unpalatable spiny shrub being the safest microhabitat for palatable saplings and bare soil for unpalatable ones. The differences found in number of attacked saplings between plots may be attributable to differences in herbivore pressure. The community context in which interaction takes place, namely the characteristics of the neighbours and the intensity of herbivore pressure, are determining factors for understanding and predicting the damage undergone by a target plant species. The mechanism that best explains these results is associational avoidance of saplings that grow near to unpalatable shrubs. It is necessary to introduce this neighbour effect in theoretical models and food‐web approaches that analyse the plant–herbivore relationships, since it can strongly determine not only the intensity of the interaction, but also the spatial distribution and diversity of the plant community.     

Latitudinal variation in plant–herbivore interactions in European salt marshes

Ecological interactions often vary geographically. Work in salt marshes on the Atlantic Coast of the United States has documented community-wide latitudinal gradients in plant palatability and plant traits that may be driven in part by greater herbivore pressure at low latitudes. To determine if similar patterns exist elsewhere, we studied six taxa of saltmarsh plants ( Atriplex , Juncus , Limonium , Salicornia , Spartina and Suaeda ) at European sites at high (Germany and the Netherlands) and low (Portugal and Spain) latitudes. We conducted feeding assays using both native and non-native consumers, and documented patterns of herbivore damage in the field. As in the United States, high-latitude plants tended to be more palatable than low-latitude plants when offered to consumers in paired feeding assays in the laboratory, although assays with grasshopper consumers were less consistent than those with crab consumers, and plants in the field at low-latitude sites tended to experience greater levels of herbivore pressure than plants at high-latitude sites. Similarly, high-latitude leaf litter was more palatable than litter from low-latitude plants when offered to consumers in paired feeding assays in the laboratory. Latitudinal gradients in plant palatability and herbivore pressure may be a general phenomenon, and may contribute to latitudinal gradients in decomposition processes.     

First insights into the genes that control plant–bacterial interactions

The events leading up to the cloning of the first bacterial avirulence gene, avrA , from Pseudomonas syringae pv. glycinea are described. The cloning of this gene marked the beginning of the molecular analyses of bacterial effectors and has paved the way for determining of the role of bacterial effectors in pathogen virulence and the triggering of plant innate immunity.     

Expression of fluorescent proteins in Lactobacillus rhamnosus to study host–microbe and microbe–microbe interactions

Probiotic Lactobacillus strains are widely used to benefit human and animal health, although the exact mechanisms behind their interactions with the host and the microbiota are largely unknown. Fluorescent tagging of live probiotic cells is an important tool to unravel their modes of action. In this study, the implementation of different heterologously expressed fluorescent proteins for the labelling of the model probiotic strains Lactobacillus rhamnosusGG (gastrointestinal) and Lactobacillus rhamnosusGR‐1 (vaginal) was explored. Heterologous expression of mTagBFP2 and mCherry resulted in long‐lasting fluorescence of L. rhamnosusGG and GR‐1 cells, using the nisin‐controlled expression (NICE) system. These novel fluorescent strains were then used to study in vitro aspects of their microbe–microbe and microbe–host interactions. Lactobacillus rhamnosusGG and L. rhamnosusGR‐1 expressing mTagBFP2 and mCherry could be visualized in mixed‐species biofilms, where they inhibited biofilm formation by Salmonella Typhimurium–gfpmut3 expressing the green fluorescent protein. Likewise, fluorescent L. rhamnosusGG and L. rhamnosusGR‐1 were implemented for the visualization of their adhesion patterns to intestinal epithelial cell cultures. The fluorescent L. rhamnosus strains developed in this study can therefore serve as novel tools for the study of probiotic interactions with their environment.     

The ubiquitin/26S proteasome system in plant–pathogen interactions: a never-ending hide-and-seek game

The ubiquitin/26S proteasome system (UPS) plays a central role in plant protein degradation. Over the past few years, the importance of this pathway in plant–pathogen interactions has been increasingly highlighted. UPS is involved in almost every step of the defence mechanisms in plants, regardless of the type of pathogen. In addition to its proteolytic activities, UPS, through its 20S RNase activity, may be part of a still unknown antiviral defence pathway. Strikingly, UPS is not only a weapon used by plants to defend themselves, but also a target for some pathogens that have evolved mechanisms to inhibit and/or use this system for their own purposes. This article attempts to summarize the current knowledge on UPS involvement in plant–microbe interactions, a complex scheme that illustrates the never-ending arms race between hosts and microbes.     

Symbiotic soil microorganisms as players in aboveground plant–herbivore interactions – the role of rhizobia

Rhizobia play a key role for performance of leguminous plants and ecosystem productivity. However, no studies to date have investigated the importance of the rhizobial symbiosis for legume–herbivore interactions. The additional nitrogen provided by the rhizobia improves the nutritional quality of plants, but may also be used for the synthesis of defence compounds. We performed greenhouse experiments with nodulating and non-nodulating, as well as cyanogenic and acyanogenic strains of Trifolium repen s to study the effects of rhizobia Rhizobium leguminosarum on plant growth and the performance of the chewing herbivore Spodoptera littoralis and the phloem-sucking aphid Myzus persicae . We demonstrate that for nodulating strains of T. repens rhizobia increased plant growth and the performance of Spodoptera littoralis . However, this positive effect of rhizobia on the caterpillars did not occur in a cyanogenic clover strain. Reproduction of the phloem-sucking aphid Myzus persicae was inconsistently affected by rhizobia. Our study provides evidence that the additional nitrogen provided by the rhizobia may be used for the production of nitrogen-based defence compounds, thereby counteracting positive effects on the performance of chewing herbivores. The symbiosis with rhizobia is therefore an important driver of legume–herbivore interactions.     

Efficient generation of transgenic barley: The way forward to modulate plant–microbe interactions

Stable genetic transformation represents the gold standard approach to the detailed elucidation of plant gene functions. This is particularly relevant in barley, an important experimental model widely employed in applied molecular, genetic and cell biological research, and biotechnology. Presented are details of the establishment of a protocol for Agrobacterium-mediated gene transfer to immature embryos, which enables the highly efficient generation of transgenic barley. Advancements were achieved through comparative experiments on the influence of various explant treatments and co-cultivation conditions. The analysis of representative numbers of transgenic lines revealed that the obtained T-DNA copy numbers are typically low, the generative transmission of the recombinant DNA is in accordance with the Mendelian rules and the vast majority of the primary transgenics produce progeny that expresses the respective transgene product. Moreover, the newly established protocol turned out to be useful to transform not only the highly amenable cultivar (cv.) ‘Golden Promise’ but also other spring and winter barley genotypes, albeit with substantially lower efficiency. As a major result of this study, a very useful tool is now available for future functional gene analyses as well as genetic engineering approaches. With the aim to modify the expression of barley genes putatively involved in plant–fungus interactions, numerous transgenic plants have been generated using diverse expression cassettes. These plants represent an example of how transformation technology may contribute to further our understanding of important biological processes.