Phylometagenomics of cycad coralloid roots reveals shared symbiotic signals.

Cycads are known to host symbiotic cyanobacteria, including Nostocales species, as well as other sympatric bacterial taxa within their specialized coralloid roots. Yet, it is unknown if these bacteria share a phylogenetic origin and/or common genomic functions that allow them to engage in facultative symbiosis with cycad roots. To address this, we obtained metagenomic sequences from 39 coralloid roots sampled from diverse cycad species and origins in Australia and Mexico. Culture-independent shotgun metagenomic sequencing was used to validate sub-community co-cultures as an efficient approach for functional and taxonomic analysis. Our metanalysis shows a host-independent microbiome core consisting of seven bacterial orders with high species diversity within the identified taxa. Moreover, we recovered 43 cyanobacterial metagenome-assembled genomes, and in addition to Nostoc spp., symbiotic cyanobacteria of the genus Aulosira were identified for the first time. Using this robust dataset, we used phylometagenomic analysis to reveal three monophyletic cyanobiont clades, two host-generalist and one cycad-specific that includes Aulosira spp. Although the symbiotic clades have independently arisen, they are enriched in certain functional genes, such as those related to secondary metabolism. Furthermore, the taxonomic composition of associated sympatric bacterial taxa remained constant. Our research quadruples the number of cycad cyanobiont genomes and provides a robust framework to decipher cyanobacterial symbioses, with the potential of improving our understanding of symbiotic communities. This study lays a solid foundation to harness cyanobionts for agriculture and bioprospection, and assist in conservation of critically endangered cycads.

DISTINCT MECHANISMS OF INHIBITION OF VSV REPLICATION IN NEURONS MEDIATED BY TYPE I AND TYPE II IFN.

Acute viral infection of neurons presents a difficult problem to the host, since neurons are essential and not replaced, therefore cell-autonomous pathway(s) of suppressing viral replication are critical. We have examined the mechanisms by which neurons respond to exogenous interferons (IFNs) and observed that novel pathways inhibit acute vesicular stomatitis virus (VSV) replication. For both type I (IFN-beta) and Type II (IFN-gamma) interferons, post-translational modification of viral proteins contributed to the replication blockade, diminishing the efficiency of viral assembly and budding from the host neuron. IFN-gamma treatment induces the accumulation of NOS-1 in the absence of an increase of mRNA encoding this enzyme; a NOS-1-inhibiting protein, PIN, is rapidly ubiquitinated and eliminated in the presence of IFN-gamma. NOS-1 produces NO which combines with superoxide to form peroxynitrite (ONOO-), this binds tyrosines, cysteines, and serines; antagonism of NOS-1 with either non-specific or selective inhibitors block the antiviral effect of IFN-gamma. VSV proteins are decorated with -NO(2) in IFN-gamma-treated neurons, probably resulting in their diminished ability to interact properly and mature into budding virus. For IFN-beta, protein phosphorylation of the Matrix protein (M) and Phosphoprotein (P) were altered in infected neurons, with hyperphosphorylation of M (but not hypophosphorylated P) found in released virions. Hyperphosphorylated M protein does not immunoprecipitate with the viral ribonucleoprotein complex in IFN-beta-treated neurons. Thus both types of IFN interfere with viral assembly and release of infectious particles, but by distinct pathways.