The Gram-positive side of plant–microbe interactions

Plant growth and development are significantly influenced by the presence and activity of microorganisms. To date, the best-studied plant-interacting microbes are Gram-negative bacteria, but many representatives of both the high and low G+C Gram-positives have excellent biocontrol, plant growth-promoting and bioremediation activities. Moreover, actinorhizal symbioses largely contribute to the global biological nitrogen fixation and many Gram-positive bacteria promote other types of symbioses in tripartite interactions. Finally, several prominent and devastating phytopathogens are Gram-positive. We summarize the present knowledge of the beneficial and detrimental interactions of Gram-positive bacteria with plants to underline the importance of this particular group of bacteria.     

Congruence between visitation and pollen-transport networks in a California plant–pollinator community

Most recent studies describing pollination networks are based on observed flower visits, and few have explicitly tested if the floral visitors actually carry pollen. Since floral visitors can vary in their ability to remove and transfer pollen, it is important to show that visitation patterns reflect effective pollination. Given the difficulty of measuring per-visit pollen deposition at the community scale, a first step is to examine the amount of conspecific pollen carried by insect visitors. Here I compared the plant–animal visitation network with the pollen-transport network, estimated from insect pollen loads, for a montane meadow community from southern California, USA. Visitation and pollen-transport networks were positively associated with each other in both 2001 and 2002. However, the exclusion of visitors that do not carry any conspecific pollen reveals that pollen-transport networks are more specialized from the plants' perspective and that species are involved in fewer mutualistic interactions compared with estimates derived from visitation frequencies. Although conspecific pollen loads were smaller in 2002, bees tended to carry the largest conspecific loads in both years and were responsible for transporting the most pollen. These results suggest that, although visitation networks are suitable first-order approximations of pollination networks, information on which visitors carry conspecific pollen, and in what amounts, is crucial for distinguishing between antagonistic and mutualistic interactions.     

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.     

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.     

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.     

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.     

The diversity of p53 mutations among human brain tumors and their functional consequences

The p53 tumor suppressor is implicated in cell cycle control, DNA repair, replicative senescence and programmed cell death. Inactivation of the p53 contributes to the wide range of human tumors, including glial neoplasms. In this review, we describe the regulation and biochemical properties of p53 protein that may explain its ability to activate various genetic programs underlying cellular responses to stress conditions. The overall spectrum of p53 mutations is rather shared between tumor types indicating that these mutations are not tumor type-specific. However, there is one example of germ-line mutation of p53 gene (the deletion of the codon 236) that is associated with a familiar brain tumor syndrome. We compare the frequency and type of most common mutations among various brain tumours (focusing on glioblastomas) and their consequences on protein functions. Furthermore, we discuss the most promising approaches of potential brain tumor therapy, including an adenovirus-mediated p53 gene transfer. Human glioblastomas are highly sensitive to the effects of p53 activity when the wild-type p53 is introduced ectopically. It suggests that the genetic or pharmacological modulation of the p53 pathway is potentially important strategy in the treatment of human cancers.