Relationships between Sphaerulina musiva Infection and the Populus Microbiome and Metabolome

Unraveling the shifts in the belowground microbiota and metabolome of Pinus pinaster trees affected by forest decline

Pinus pinaster Aiton (maritime pine) stands are suffering a generalized deterioration due to different decline episodes throughout all its distribution area. It is well known that external disturbances can alter the plant associated microbiota and metabolome, which ultimately can entail the disruption of the normal growth of the hosts. Notwithstanding, very little is known about the shifts in the microbiota and the metabolome in pine trees affected by decline. The aim of our work was to unravel whether bacterial and fungal communities inhabiting the rhizosphere and root endosphere of P. pinaster trees with symptoms of decline and affected by Matsucoccus feytaudi in the National Park of Sierra Nevada (Granada, Spain) showed alterations in the structure, taxonomical profiles and associative patterns. We also aimed at deciphering potential changes in the rhizosphere and root metabolome. Trees infected by M. feytaudi and healthy individual harbored distinct microbial communities at both compositional and associative patterns. Unhealthy trees were enriched selectively in certain plant growth promoting microorganisms such as several ectomycorrhizal fungi (Clavulina) and Streptomyces, while other beneficial microorganisms (Micromonospora) were more abundant in unaffected pines. The rhizosphere of unhealthy trees was richer in secondary metabolites involved in plant defense than healthy pines, while the opposite trend was detected in root samples. The abundance of certain microorganisms was significantly correlated with several antimicrobial metabolites, thus, being all of them worthy of further isolation and study of their role in forest decline.

Microbiome selection and evolution within wild and domesticated plants

Microbes are ubiquitously found across plant surfaces and even within their cells, forming the plant microbiome. Many of these microbes contribute to the functioning of the host and consequently affect its fitness. Therefore, in many contexts, including microbiome effects enables a better understanding of the phenotype of the plant rather than considering the genome alone. Changes in the microbiome composition are also associated with changes in the functioning of the host, and there has been considerable focus on how environmental variables regulate plant microbiomes. More recently, studies suggest that the host genome also preconditions the microbiome to the environment of the plant, and the microbiome is therefore subject to evolutionary forces. Here, we outline how plant microbiomes are governed by both environmental variables and evolutionary processes and how they can regulate plant health together.

Seasonality and longer-term development generate temporal dynamics in the Populus microbiome

Temporal variation in community composition is central to our understanding of the assembly and functioning of microbial communities, yet the controls over temporal dynamics for microbiomes of long-lived plants, such as trees, remain unclear. Temporal variation in tree microbiomes could arise primarily from seasonal (i.e., intra-annual) fluctuations in community composition or from longer-term changes across years as host plants age. To test these alternatives, we experimentally isolated temporal variation in plant microbiome composition using a common garden and clonally propagated plants, and we used amplicon sequencing to characterize bacterial/archaeal and fungal communities in the leaf endosphere, root endosphere, and rhizosphere of two Populus spp. over four seasons across two consecutive years. Microbial community composition differed among seasons and years (which accounted for up to 21% of the variation in microbial community composition) and was correlated with seasonal dissimilarity in climatic conditions. However, microbial community dissimilarity was also positively correlated with time, reflecting longer-term compositional shifts as host trees aged. Together, our findings demonstrate that temporal patterns in tree microbiomes arise from both seasonal fluctuations and longer-term changes, which interact to generate unique seasonal patterns each year. In addition to shedding light on two important controls over the assembly of plant microbiomes, our results also suggest future studies of tree microbiomes should account for background temporal dynamics when testing the drivers of spatial patterns in microbial community composition and temporal responses of plant microbiomes to environmental change.IMPORTANCEMicrobiomes are integral to the health of host plants, but we have a limited understanding of the factors that control how the composition of plant microbiomes changes over time. Especially little is known about the microbiome of long-lived trees, relative to annual and non-woody plants. We tested how tree microbiomes changed between seasons and years in poplar (genus Populus), which are widespread and ecologically important tree species that also serve as important biofuel feedstocks. We found the composition of bacterial, archaeal, and fungal communities differed among seasons, but these seasonal differences depended on year. This dependence was driven by longer-term changes in microbial composition as host trees developed across consecutive years. Our findings suggest that temporal variation in tree microbiomes is driven by both seasonal fluctuations and longer-term (i.e., multiyear) development.

Streptomyces and their specialised metabolites for phytopathogen control - comparative in vitro and in planta metabolic approaches

In the search for new crop protection microbial biocontrol agents, isolates from the genus Streptomyces are commonly found with promising attributes. Streptomyces are natural soil dwellers and have evolved as plant symbionts producing specialised metabolites with antibiotic and antifungal activities. Streptomyces biocontrol strains can effectively suppress plant pathogens via direct antimicrobial activity, but also induce plant resistance through indirect biosynthetic pathways. The investigation of factors stimulating the production and release of Streptomyces bioactive compounds is commonly conducted in vitro, between Streptomyces sp. and a plant pathogen. However, recent research is starting to shed light on the behaviour of these biocontrol agents in planta, where the biotic and abiotic conditions share little similarity to those of controlled laboratory conditions. With a focus on specialised metabolites, this review details (i) the various methods by which Streptomyces biocontrol agents employ specialised metabolites as an additional line of defence against plant pathogens, (ii) the signals shared in the tripartite system of plant, pathogen and biocontrol agent, and (iii) an outlook on new approaches to expedite the identification and ecological understanding of these metabolites under a crop protection lens.