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In Arabidopsis thaliana, the acyl acid amido synthetase Gretchen Hagen 3.5 (AtGH3.5) conjugates both indole-3-acetic acid (IAA) and salicylic acid (SA) to modulate auxin and pathogen response pathways. To understand the molecular basis for the activity of AtGH3.5, we determined the X-ray crystal structure of the enzyme in complex with IAA and AMP. Biochemical analysis demonstrates that the substrate preference of AtGH3.5 is wider than originally described and includes the natural auxin phenylacetic acid (PAA) and the potential SA precursor benzoic acid (BA). Residues that determine IAA versus BA substrate preference were identified. The dual functionality of AtGH3.5 is unique to this enzyme although multiple IAA-conjugating GH3 proteins share nearly identical acyl acid binding sites. In planta analysis of IAA, PAA, SA, and BA and their respective aspartyl conjugates were determined in wild-type and overexpressing lines of A thaliana This study suggests that AtGH3.5 conjugates auxins (i.e., IAA and PAA) and benzoates (i.e., SA and BA) to mediate crosstalk between different metabolic pathways, broadening the potential roles for GH3 acyl acid amido synthetases in plants.

Advances in sequencing technology and the development of metagenomic and bioinformatics methods have opened up new ways to investigate the 1014 microorganisms inhabiting the human gut. The gene composition of human gut microbiome in a large and deeply sequenced cohort highlighted an overall non-redundant genome size 150 times larger than the human genome. The in silico predictions based on metagenomic sequencing are now actively followed, compared and challenged using additional ‘omics’ technologies. Interactions between the microbiota and its host are of key interest in several pathologies and applying meta-omics to describe the human gut microbiome will give a better understanding of this crucial crosstalk at mucosal interfaces. Adding to the growing appreciation of the importance of the microbiome is the discovery that numerous phages, that is, viruses of prokaryotes infecting bacteria (bacteriophages) or archaea with a high host specificity, inhabit the human gut and impact microbial activity. In addition, gene exchanges within the gut microbiota have proved to be more frequent than anticipated. Taken together, these innovative exploratory technologies are expected to unravel new information networks critical for gut homeostasis and human health. Among the challenges faced, the in vivo validation of these networks, together with their integration into the prediction and prognosis of disease, may require further working hypothesis and collaborative efforts.