The accelerated evolution and spread of pathogens are threats to host species. Agrobacteria require an oncogenic Ti or Ri plasmid to transfer genes into plants and cause disease. We developed a strategy to characterize virulence plasmids and applied it to analyze hundreds of strains collected between 1927 and 2017, on six continents and from more than 50 host species. In consideration of prior evidence for prolific recombination, it was surprising that oncogenic plasmids are descended from a few conserved lineages. Characterization of a hierarchy of features that promote or constrain plasticity allowed inference of the evolutionary history across the plasmid lineages. We uncovered epidemiological patterns that highlight the importance of plasmid transmission in pathogen diversification as well as in long-term persistence and the global spread of disease.

Understanding how bacteria affect plant health is crucial for developing sustainable crop production systems. We coupled ecological sampling and genome sequencing to characterize the population genetic history of Rhodococcus and the distribution patterns of virulence plasmids in isolates from nurseries. Analysis of chromosome sequences shows that plants host multiple lineages of Rhodococcus, and suggested that these bacteria are transmitted due to independent introductions, reservoir populations, and point source outbreaks. We demonstrate that isolates lacking virulence genes promote beneficial plant growth, and that the acquisition of a virulence plasmid is sufficient to transition beneficial symbionts to phytopathogens. This evolutionary transition, along with the distribution patterns of plasmids, reveals the impact of horizontal gene transfer in rapidly generating new pathogenic lineages and provides an alternative explanation for pathogen transmission patterns. Results also uncovered a misdiagnosed epidemic that implicated beneficial Rhodococcus bacteria as pathogens of pistachio. The misdiagnosis perpetuated the unnecessary removal of trees and exacerbated economic losses.

SignificanceInternational trade has resulted in the introduction of plant diseases into natural ecosystems around the world. These introductions have potentially catastrophic impacts on ecosystem structure and function. Leveraging genomic tools, natural variation within a tree species, and a high-throughput phenotyping platform, we present a framework that can be broadly applied to rapidly identify candidate genes associated with resistance and susceptibility to introduced plant diseases. The unprecedented speed and accuracy with which the candidate genes can be identified in woody trees demonstrates the potential of genomics to mitigate the impacts of invasive diseases on forest health.

NFAT-133 is a Streptomyces-derived aromatic polyketide compound with immunosuppressive, antidiabetic, and antitrypanosomal activities. It inhibits transcription mediated by nuclear factor of activated T cells (NFAT), leading to the suppression of interleukin-2 expression and T cell proliferation. It also activates the AMPK pathway in L6 myotubes and increases glucose uptake. In addition to NFAT-133, a number of its congeners, e.g., panowamycins and benwamycins, have been identified. However, little is known about their modes of formation in the producing organisms. Through genome sequencing of Streptomyces pactum ATCC 27456, gene inactivation, and genetic complementation experiments, the biosynthetic gene cluster of NFAT-133 and its congeners has been identified. The cluster contains a highly disordered genetic organization of type I modular polyketide synthase genes with several genes that are necessary for the formation of the aromatic core unit and tailoring processes. In addition, a number of new analogs of NFAT-133 were isolated and their chemical structures elucidated. It is suggested that the heptaketide NFAT-133 is derived from an octaketide intermediate, TM-123. The current study shows yet another unusual biosynthetic pathway involving a noncanonical polyketide synthase assembly line to produce a group of small molecules with valuable bioactivities.

Genetic variation is fundamental to evolution yet is paradoxical in symbiosis. Symbionts exhibit extensive variation in the magnitude of services they provide despite hosts having mechanisms to select and increase the abundance of beneficial genotypes.