Mix of metabolites tunes root microbiota: Uncharacterized biosynthetic genes in plant genomes suggest that plants make a plethora of specialized metabolites. Huang et al. reconstructed three biosynthetic networks from the small mustard plant Arabidopsis thaliana . Promiscuous acyltransferases and dehydrogenases contributed to metabolite diversification. The plant may use these specialized metabolites to modulate the microbiota surrounding its roots. Disruption of the pathways and intervention with purified compounds caused changes in the root microbiota. Science , this issue p. eaau6389

Summary: Plant natural products are of great value for agriculture, medicine and a wide range of other industrial applications. The discovery of new plant natural product pathways is currently being revolutionized by two key developments. First, breakthroughs in sequencing technology and reduced cost of sequencing are accelerating the ability to find enzymes and pathways for the biosynthesis of new natural products by identifying the underlying genes. Second, there are now multiple examples in which the genes encoding certain natural product pathways have been found to be grouped together in biosynthetic gene clusters within plant genomes. These advances are now making it possible to develop strategies for systematically mining multiple plant genomes for the discovery of new enzymes, pathways and chemistries. Increased knowledge of the features of plant metabolic gene clusters – architecture, regulation and assembly – will be instrumental in expediting natural product discovery. This review summarizes progress in this area. Contents Summary 771 I. Introduction 771 II. Plant metabolic gene clusters – some rules and anomalies 772 III. Cluster regulation 780 IV. Why are the genes clustered? 781 V. Conclusions and outlook 786 Acknowledgements 786 References 786

Sesterterpenoids are a rare terpene class harboring untapped chemodiversity and bioactivities. Their structural diversity originates primarily from the scaffold-generating sesterterpene synthases (STSs). In fungi, all six known STSs are bifunctional, containing C-terminal trans-prenyltransferase (PT) and N-terminal terpene synthase (TPS) domains. In plants, two colocalized PT and TPS gene pairs from Arabidopsis thaliana were recently reported to synthesize sesterterpenes. However, the landscape of PT and TPS genes in plant genomes is unclear. Here, using a customized algorithm for systematically searching plant genomes, we reveal a suite of physically colocalized pairs of PT and TPS genes for the biosynthesis of a large sesterterpene repertoire in the wider Brassicaceae. Transient expression of seven TPSs from A. thaliana, Capsella rubella, and Brassica oleracea in Nicotiana benthamiana yielded fungal-type sesterterpenes with tri-, tetra-, and pentacyclic scaffolds, and notably (−)-ent-quiannulatene, an enantiomer of the fungal metabolite (+)-quiannulatene. Protein and structural modeling analysis identified an amino acid site implicated in structural diversification. Mutation of this site in one STS (AtTPS19) resulted in premature termination of carbocation intermediates and accumulation of bi-, tri-, and tetracyclic sesterterpenes, revealing the cyclization path for the pentacyclic sesterterpene (−)-retigeranin B. These structural and mechanistic insights, together with phylogenetic analysis, suggest convergent evolution of plant and fungal STSs, and also indicate that the colocalized PT–TPS gene pairs in the Brassicaceae may have originated from a common ancestral gene pair present before speciation. Our findings further provide opportunities for rapid discovery and production of sesterterpenes through metabolic and protein engineering. Significance Sesterterpenoids are a rare terpene class with largely unexplored chemical diversity and bioactivities, representing a potential new drug source. Sesterterpene synthases catalyze the first committed biosynthesis step and shape sesterterpene structural diversity. Here, using a customized algorithm for systematically surveying plant genomes, we unearth a sesterterpene synthase gene repertoire in the Brassicaceae that synthesizes fungal-type sesterterpenes with diverse scaffolds, thus fueling the drug-discovery pipeline. Mechanistic studies shed light on the origin of structural diversification and offer insights for expanding nature’s diversity by protein engineering. Our work also supports the concept of convergent evolution in natural product biosynthesis.

Plants are sessile organisms that have evolved various mechanisms to adapt to complex and changing environments. One important feature of plant adaption is the production of specialised metabolites. Terpenes are the largest class of specialised metabolites, with over 80 000 structures reported so far, and they have important ecological functions in plant adaptation. Here, we review the current knowledge on plant terpenes that mediate below‐ground interactions between plants and other organisms, including microbes, herbivores and other plants. The discovery, functions and biosynthesis of these terpenes are discussed, and prospects for bioengineering terpenoids for plant protection are considered. © 2019 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Summary: Plants and microbes coinhabit the earth and have coevolved during environmental changes over time. Root metabolites are the key to mediating the dynamic association between plants and microbes, yet the underlying functions and mechanisms behind this remain largely illusive. Knowledge of metabolite‐mediated alteration of the root microbiota in response to environmental stress will open avenues for engineering root microbiotas for improved plant stress resistance and health. Here, we synthesize recent advances connecting environmental stresses, the root metabolome and microbiota, and propose integrated synthetic biology‐based strategies for tuning the plant root metabolome in situ for microbe‐assisted stress resistance, offering potential solutions to combat climate change. The current limitations, challenges and perspectives for engineering the plant root metabolome for modulating microbiota are collectively discussed.