Herbicide residues in soil decrease microbe-mediated plant protection

• The residues of glyphosate are found to remain in soils longer than previously reported, affecting rhizosphere microbes. This may adversely affect crop and other non-target plants because the plant's resilience and resistance largely rely on plant-associated microbes. Ubiquitous glyphosate residues in soil and how they impact mutualistic microbes inhabiting the aboveground plant parts are largely unexplored.
• We studied the effects of herbicide residues in soil on Epichloë sp., which are common endophytic symbionts inhabiting aerial parts of cool-season grasses. In this symbiosis, the obligate symbiont subsists entirely on its host plant, and in exchange, it provides alkaloids conferring resistance to herbivores for the host grass that invests little in its own chemical defence.
• We first show decreased growth of Epichloë endophytes in vitro when directly exposed to two concentrations of glyphosate or glyphosate-based herbicides. Second, we provide evidence for a reduction of Epichloë-derived, insect-toxic loline alkaloids in endophyte-symbiotic meadow fescue (F. pratensis) plants growing in soil with a glyphosate history. Plants were grown for 2 years in an open field site, and natural herbivore infestation was correlated with the glyphosate-mediated reduction of loline alkaloid concentrations.
• Our findings indicate that herbicides residing in soil not only affect rhizosphere microbiota but also aerial plant endophyte functionality, which emphasizes the destructive effects of glyphosate on plant symbiotic microbes, here with cascading effects on plant–pest insect interactions.

     

Examination of the success rate of secondary symbiont manipulation by microinjection methods in the pea aphid system

Insects harbor a wide range of microbial symbionts, but their influence on host phenotypes is described in a limited number of biological models. One experimental approach to gain knowledge on the effects of symbionts to their hosts is to create insect lines with and without symbionts and examine their phenotypes. However, the success rate of symbiont elimination and introduction methods is dependent on several parameters that are scarcely tested or described. The pea aphid, Acyrthosiphon pisum Harris (Hemiptera: Aphididae), is a model insect of symbiosis studies. It harbors a primary symbiont that supplies the host with essential amino acids, and an array of secondary symbionts whose effects have been assessed by manipulating their presence/absence in the insect. Here, we describe the influence of key parameters on the success rate of symbiont manipulation using the pea aphid–secondary symbiont system. We compared two elimination methods differing in antibiotic treatment using several aphid–symbiont combinations. We also created new aphid host–symbiont combinations by secondary symbiont introduction and examined the effects of larval stage of recipient aphids on introduction success. Our study revealed that the aphid–symbiont combination has strong influence on both symbiont introduction and elimination success rates, and that the type of antibiotics and the larval stage of recipient aphids influence the elimination and introduction success rate, respectively.     

Dynamics of the fungal symbionts in the gallery system and the mycangia of the ambrosia beetle,Xylosandrus mutilatus (Blandford) (Coleoptera: Scolytidae) in relation to its life history

The dynamics of the fungal symbionts in the gallery system and the mycangia of the ambrosia beetle,Xylosandrus mutilatus, were studied in relation to its life history using both isolation experiments and scanning electron microscopy (SEM). In the galleries,Ambrosiella sp. was predominant during the larval stages but its relative dominance gradually decreased during the development of the larvae. In contrast, yeasts (mainlyCandida sp.) andPaecilomyces sp. dominated continuously in the galleries after eclosion.Ambrosiella sp. was consistently stored in the mycangia in all adult stages, except in the teneral and overwintering adults when the other fungi were dominant. No fungal spores occurred in the mycangia of the adult beetles reared under aseptic conditions from the pupal stage, while onlyAmbrosiella sp. was stored in those reared from the teneral-adult stage. These results suggest that: (i) Xmutilatus is associated with at least three fungal species, among whichAmbrosiella sp. is the most essential food resource for development of the broods; (ii) immediately after eclosion, new female adults may take at least four associated fungal species, with no or incomplete selection, into their mycangia from the walls of the cradles; and (iii) conditions may well be produced in the mycangia of both matured and dispersing beetles whereby only the spores ofAmbrosiella sp. can proliferate.     

Cohabitation and roommate bias of symbiotic bacteria in insect hosts

Symbiotic interactions between insects and bacteria have long fascinated ecologists. Aphids have emerged as the model system on which to study the effect of endosymbiotic bacteria on their hosts. Aphid‐symbiont interactions are ecologically interesting as aphids host multiple secondary symbionts that can provide broad benefits, such as protection against heat stress or specialist natural enemies (parasitic wasps and entomopathogenic fungi). There are nine common aphid secondary symbionts and individual aphids host on average 1–2 symbionts. A cost‐benefit trade‐off for hosting symbionts is thought to explain why not all aphids host every possible symbiont in a population. Both positive and negative associations between various symbionts occur, and this could happen due to increased costs when cohosting certain combinations or as a consequence of competitive interactions between the symbionts within a host. In this issue of Molecular Ecology, Mathé‐Hubert, Kaech, Hertaeg, Jaenike, and Vorburger (2019) use data on the symbiont status of field‐collected aphids to inform a model on the evolution of symbiont co‐occurrence. They vary the effective female population size as well as the rate of horizontal and maternal transmission to infer the relative impact of symbiont‐symbiont interactions versus random drift. Additional data analysis revisits an association between two symbionts in a fruit fly species using a long‐term data set to highlight that such interactions are not limited to aphids.     

Induction and suppression of gene silencing in plants by nonviral microbes

Gene silencing is a conserved mechanism in eukaryotes that dynamically regulates gene expression. In plants, gene silencing is critical for development and for maintenance of genome integrity. Additionally, it is a critical component of antiviral defence in plants, nematodes, insects, and fungi. To overcome gene silencing, viruses encode effectors that suppress gene silencing. A growing body of evidence shows that gene silencing and suppression of silencing are also used by plants during their interaction with nonviral pathogens such as fungi, oomycetes, and bacteria. Plant–pathogen interactions involve trans-kingdom movement of small RNAs into the pathogens to alter the function of genes required for their development and virulence. In turn, plant-associated pathogenic and nonpathogenic microbes also produce small RNAs that move trans-kingdom into host plants to disrupt pathogen defence through silencing of plant genes. The mechanisms by which these small RNAs move from the microbe to the plant remain poorly understood. In this review, we examine the roles of trans-kingdom small RNAs and silencing suppressors produced by nonviral microbes in inducing and suppressing gene silencing in plants. The emerging model is that gene silencing and suppression of silencing play critical roles in the interactions between plants and their associated nonviral microbes.     

Endophyte research: going beyond isolation and metabolite documentation

Many fungi belonging to mostly Ascomycota inhabit living tissues of plants of all major lineages without causing any visible symptoms. Termed horizontally transmitted endophytes, they have been investigated mostly for their capacity to produce bioactive secondary metabolites. However, many questions regarding the interactions between endophytes and their plant hosts, phytophagous insects and other fungi remain unanswered. This review highlights some of these areas of endophyte biology about which very little or no knowledge exists. Information garnered' using modern methodologies' on these grey areas of ‘endophytism’ (endophytic mode of lifestyle) would help immensely in understanding the evolution of endophytes of aerial plant tissues and in exploiting endophytes in various fields of biotechnology.     

FINE STRUCTURE OF THE SNOW ALGA (CHLAMYDOMONAS NIVALIS) AND ASSOCIATED BACTERIA1

Chlamydomonas nivalis (Bau.) Wille is present in red snow as large spherical resting cells. Fine structural studies reveal an abundance of clear granules in the cytoplasm and occasional starch grains in the chloroplast. Individual cells display a thick cell wall with a smooth outer surface. Cells may be surrounded by a loose fibrous network in which encapsulated bacteria are seen. The bacteria have a characteristic Gram-negative cell wall and constrictive mode of division. The algal-bacterial association appears to be characteristic of red snow populations.