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.

Photolysis of caged cytokinin in single cells of Arabidopsis thaliana.

BACKGROUND: Cytokinins are a class of phytohormone that play a crucial role in the development of plants. They are involved in the regulation of nearly every aspect of plant growth, from germination to senescence. The role of cytokinins in many developmental programs is complex and varies both spatially and temporally. Current techniques used to investigate the functions of cytokinins in plant development lack this spatial and temporal resolution required to observe cell-type specific effects. RESULTS: To this end, we present a method of activating a caged cytokinin in single cells. A caged benzyladenine was synthesized, along with caged adenine as a negative control. In vitro testing confirmed ultraviolet light-mediated uncaging, and subsequent root growth assays demonstrated that uncaging produced a cytokinin phenotype. This uncaging was confined to single cells using multiphoton confocal microscopy. Using an Arabidopsis thaliana cytokinin reporter line expressing TCSn::GFP, the resulting GFP expression was confined to the uncaging region, including in single cells. This study presents a novel cell-targeted method of cytokinin delivery, which has the potential to elucidate a broad range of processes in plant development. CONCLUSIONS: We combined multiphoton confocal microscopy and a caged cytokinin treatment, allowing cell type-specific uncaging of a cytokinin in Arabidopsis roots.

How Do Quorum-Sensing Signals Mediate Algae-Bacteria Interactions?

Quorum sensing (QS) describes a process by which bacteria can sense the local cell density of their own species, thus enabling them to coordinate gene expression and physiological processes on a community-wide scale. Small molecules called autoinducers or QS signals, which act as intraspecies signals, mediate quorum sensing. As our knowledge of QS has progressed, so too has our understanding of the structural diversity of QS signals, along with the diversity of bacteria conducting QS and the range of ecosystems in which QS takes place. It is now also clear that QS signals are more than just intraspecies signals. QS signals mediate interactions between species of prokaryotes, and between prokaryotes and eukaryotes. In recent years, our understanding of QS signals as mediators of algae-bacteria interactions has advanced such that we are beginning to develop a mechanistic understanding of their effects. This review will summarize the recent efforts to understand how different classes of QS signals contribute to the interactions between planktonic microalgae and bacteria in our oceans, primarily N-acyl-homoserine lactones, their degradation products of tetramic acids, and 2-alkyl-4-quinolones. In particular, this review will discuss the ways in which QS signals alter microalgae growth and metabolism, namely as direct effectors of photosynthesis, regulators of the cell cycle, and as modulators of other algicidal mechanisms. Furthermore, the contribution of QS signals to nutrient acquisition is discussed, and finally, how microalgae can modulate these small molecules to dampen their effects.

The Multifaceted Inhibitory Effects of an Alkylquinolone on the Diatom Phaeodactylum tricornutum.

The mechanisms underlying interactions between diatoms and bacteria are crucial to understand diatom behaviour and proliferation, and can result in far-reaching ecological consequences. Recently, 2-alkyl-4-quinolones have been isolated from marine bacteria, both of which (the bacterium and isolated chemical) inhibited growth of microalgae, suggesting these compounds could mediate diatom-bacteria interactions. The effects of several quinolones on three diatom species have been investigated. The growth of all three was inhibited, with half-maximal inhibitory concentrations reaching the sub-micromolar range. By using multiple techniques, dual inhibition mechanisms were uncovered for 2-heptyl-4-quinolone (HHQ) in Phaeodactylum tricornutum. Firstly, photosynthetic electron transport was obstructed, primarily through inhibition of the cytochrome b6 f complex. Secondly, respiration was inhibited, leading to repression of ATP supply to plastids from mitochondria through organelle energy coupling. These data clearly show how HHQ could modulate diatom proliferation in marine environments.

Complete genome sequence of Dyadobacter sp. 32, isolated from a culture of the freshwater diatom Cymbella microcephala.

Bacteria have been shown to be involved in different species-specific interactions with eukaryotic algae such as diatoms, impacting important ecosystem processes. Recently, a strain assigned to Dyadobacter, named 'species 32', has been shown to be involved in a number of ecologically relevant diatom processes, such as biofilm formation or growth enhancement, depending on the diatom species. This bacterium was originally isolated from a culture of freshwater benthic diatoms that originated from an epilithic biofilm, in which both bacteria and diatoms coexist. A single complete circular chromosome of Dyadobacter sp. 32 was assembled with a length of 7,101,228 bp, containing 6062 protein coding genes and 3 rRNA operons. A number of interesting genetic features were found, such as a putative zeaxanthin biosynthetic gene cluster. A large number of polysaccharide utilizing gene clusters were also detected, along with genes potentially acquired from other bacteria through horizontal gene transfer, and genes previously identified in other algae-bacteria interactions. These data serve to increase our understanding of specific interactions within freshwater biofilms, and identify a number of gene targets with which to study the molecular basis of diatom-bacteria interactions.

N-Acyl Homoserine Lactone Derived Tetramic Acids Impair Photosynthesis in Phaeodactylum tricornutum.

Marine bacteria contribute substantially to nutrient cycling in the oceans and can engage in close interactions with microalgae. Many microalgae harbor characteristic satellite bacteria, many of which participate in N-acyl homoserine lactone (AHL) mediated quorum sensing. In the diffusion-controlled phycosphere, AHLs can reach high local concentrations, with some of them transforming into tetramic acids, compounds with a broad bioactivity. We tested a representative AHL, N-(3-oxododecanoyl) homoserine lactone, and its tetramic acid rearrangement product on the diatom Phaeodactylum tricornutum. While cell growth and photosynthetic efficiency of photosystem II were barely affected by the AHL, exposure to its tetramic acid rearrangement product had a negative effect on photosynthetic efficiency and led to growth inhibition and cell death in the long term, with a minimum inhibitory concentration between 20 and 50 µΜ. These results strengthen the view that AHLs may play an important role in shaping the outcome of microalgae-bacteria interactions.