The cytosolic aminotransferase VAS1 coordinates aromatic amino acid biosynthesis and metabolism.

The aromatic amino acids (AAAs) phenylalanine, tyrosine, and tryptophan are basic protein units and precursors of diverse specialized metabolites that are essential for plant growth. Despite their significance, the mechanisms that regulate AAA homeostasis remain elusive. Here, we identified a cytosolic aromatic aminotransferase, REVERSAL OF SAV3 PHENOTYPE 1 (VAS1), as a suppressor of arogenate dehydrogenase 2 (adh2) in Arabidopsis (Arabidopsis thaliana). Genetic and biochemical analyses determined that VAS1 uses AAAs as amino donors, leading to the formation of 3-carboxyphenylalanine and 3-carboxytyrosine. These pathways represent distinct routes for AAA metabolism that are unique to specific plant species. Furthermore, we show that VAS1 is responsible for cytosolic AAA biosynthesis, and its enzymatic activity can be inhibited by 3-carboxyphenylalanine. These findings provide valuable insights into the crucial role of VAS1 in producing 3-carboxy AAAs, notably via recycling of AAAs in the cytosol, which maintains AAA homeostasis and allows plants to effectively coordinate the complex metabolic and biosynthetic pathways of AAAs.

Insight into neofunctionalization of 2,3-oxidosqualene cyclases in B,C-ring-opened triterpene biosynthesis in quinoa.

Bioactive triterpenes feature complex fused-ring structures, primarily shaped by the first-committed enzyme, 2,3-oxidosqualene cyclases (OSCs) in plant triterpene biosynthesis. Triterpenes with B,C-ring-opened skeletons are extremely rare with unknown formation mechanisms, harbouring unchartered chemistry and biology. Here, through mining the genome of Chenopodium quinoa followed by functional characterization, we identified a stress-responsive and neofunctionalized OSC capable of generating B,C-ring-opened triterpenes, including camelliol A and B and the novel (-)-quinoxide A as wax components of the specialized epidermal bladder cells, namely the quinoxide synthase (CqQS). Protein structure analysis followed by site-directed mutagenesis identified key variable amino acid sites underlying functional interconversion between pentacyclic ß-amyrin synthase (CqbAS1) and B,C-ring-opened triterpene synthase CqQS. Mutation of one key residue (N612K) in even evolutionarily distant Arabidopsis ß-amyrin synthase could generate quinoxides, indicating a conserved mechanism for B,C-ring-opened triterpene formation in plants. Quantum computation combined with docking experiments further suggests that conformations of conserved W613 and F413 of CqQS might be key to selectively stabilizing intermediate carbocations towards B,C-ring-opened triterpene formation. Our findings shed light on quinoa triterpene skeletal diversity and mechanisms underlying B,C-ring-opened triterpene biosynthesis, opening avenues towards accessing their chemistry and biology and paving the way for quinoa trait engineering and quality improvement.

Making small molecules in plants: A chassis for synthetic biology-based production of plant natural products.

Plant natural products have been extensively exploited in food, medicine, flavor, cosmetic, renewable fuel, and other industrial sectors. Synthetic biology has recently emerged as a promising means for the cost-effective and sustainable production of natural products. Compared with engineering microbes for the production of plant natural products, the potential of plants as chassis for producing these compounds is underestimated, largely due to challenges encountered in engineering plants. Knowledge in plant engineering is instrumental for enabling the effective and efficient production of valuable phytochemicals in plants, and also paves the way for a more sustainable future agriculture. In this manuscript, we briefly recap the biosynthesis of plant natural products, focusing primarily on industrially important terpenoids, alkaloids, and phenylpropanoids. We further summarize the plant hosts and strategies that have been used to engineer the production of natural products. The challenges and opportunities of using plant synthetic biology to achieve rapid and scalable production of high-value plant natural products are also discussed.

Chemical genetic screening identifies nalacin as an inhibitor of GH3 amido synthetase for auxin conjugation.

Auxin inactivation is critical for plant growth and development. To develop plant growth regulators functioning in auxin inactivation pathway, we performed a phenotype-based chemical screen in Arabidopsis and identified a chemical, nalacin, that partially mimicked the effects of auxin. Genetic, pharmacological, and biochemical approaches demonstrated that nalacin exerts its auxin-like activities by inhibiting indole-3-acetic acid (IAA) conjugation that is mediated by Gretchen Hagen 3 (GH3) acyl acid amido synthetases. The crystal structure of Arabidopsis GH3.6 in complex with D4 (a derivative of nalacin) together with docking simulation analysis revealed the molecular basis of the inhibition of group II GH3 by nalacin. Sequence alignment analysis indicated broad bioactivities of nalacin and D4 as inhibitors of GH3s in vascular plants, which were confirmed, at least, in tomato and rice. In summary, our work identifies nalacin as a potent inhibitor of IAA conjugation mediated by group II GH3 that plays versatile roles in hormone-regulated plant development and has potential applications in both basic research and agriculture.

Functions and biosynthesis of plant signaling metabolites mediating plant-microbe interactions.

Covering: 2015-2022Plants and microbes have coevolved since their appearance, and their interactions, to some extent, define plant health. A reasonable fraction of small molecules plants produced are involved in mediating plant-microbe interactions, yet their functions and biosynthesis remain fragmented. The identification of these compounds and their biosynthetic genes will open up avenues for plant fitness improvement by manipulating metabolite-mediated plant-microbe interactions. Herein, we integrate the current knowledge on their chemical structures, bioactivities, and biosynthesis with the view of providing a high-level overview on their biosynthetic origins and evolutionary trajectory, and pinpointing the yet unknown and key enzymatic steps in diverse biosynthetic pathways. We further discuss the theoretical basis and prospects for directing plant signaling metabolite biosynthesis for microbe-aided plant health improvement in the future.

Biosynthesis of saponin defensive compounds in sea cucumbers.

Soft-bodied slow-moving sea creatures such as sea stars and sea cucumbers lack an adaptive immune system and have instead evolved the ability to make specialized protective chemicals (glycosylated steroids and triterpenes) as part of their innate immune system. This raises the intriguing question of how these biosynthetic pathways have evolved. Sea star saponins are steroidal, while those of the sea cucumber are triterpenoid. Sterol biosynthesis in animals involves cyclization of 2,3-oxidosqualene to lanosterol by the oxidosqualene cyclase (OSC) enzyme lanosterol synthase (LSS). Here we show that sea cucumbers lack LSS and instead have two divergent OSCs that produce triterpene saponins and that are likely to have evolved from an ancestral LSS by gene duplication and neofunctionalization. We further show that sea cucumbers make alternate sterols that confer protection against self-poisoning by their own saponins. Collectively, these events have enabled sea cucumbers to evolve the ability to produce saponins and saponin-resistant sterols concomitantly.

Crafting the plant root metabolome for improved microbe-assisted stress resilience.

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.

Linderaggrenolides A-N, Oxygen-Conjugated Sesquiterpenoid Dimers from the Roots of Lindera aggregata.

Linderaggrenolides A-N (1-14), 14 new lindenane sesquiterpenoid dimers with oxygen bridges were isolated from the roots of Lindera aggregata. Their structures were elucidated on the basis of comprehensive spectroscopic data analysis, with the absolute configurations established by empirical approaches, electronic circular dichroism calculations, and X-ray crystallography. Compounds 8 and 9 were found to exhibit significant transforming growth factor-ß inhibitory activity, with IC50 values of 25.91 and 21.52 µM, respectively.

Modulation of Arabidopsis root growth by specialized triterpenes.

Plant roots are specialized belowground organs that spatiotemporally shape their development in function of varying soil conditions. This root plasticity relies on intricate molecular networks driven by phytohormones, such as auxin and jasmonate (JA). Loss-of-function of the NOVEL INTERACTOR OF JAZ (NINJA), a core component of the JA signaling pathway, leads to enhanced triterpene biosynthesis, in particular of the thalianol gene cluster, in Arabidopsis thaliana roots. We have investigated the biological role of thalianol and its derivatives by focusing on Thalianol Synthase (THAS) and Thalianol Acyltransferase 2 (THAA2), two thalianol cluster genes that are upregulated in the roots of ninja mutant plants. THAS and THAA2 activity was investigated in yeast, and metabolite and phenotype profiling of thas and thaa2 loss-of-function plants was carried out. THAA2 was shown to be responsible for the acetylation of thalianol and its derivatives, both in yeast and in planta. In addition, THAS and THAA2 activity was shown to modulate root development. Our results indicate that the thalianol pathway is not only controlled by phytohormonal cues, but also may modulate phytohormonal action itself, thereby affecting root development and interaction with the environment.

Chromatin Analysis of Metabolic Gene Clusters in Plants.

Plant metabolic gene clusters consist of neighboring genes that are involved in the biosynthesis of secondary or specialized metabolites. The genes within clusters are typically co-regulated, share a common set of chromatin marks, and code for the biosynthesis enzymes of a single metabolic pathway. Here, we describe three essential protocols for the basic analysis of metabolic gene clusters at transcription, histone modification, and metabolite level. The protocols are specified to clusters in the Arabidopsis thaliana genome and are transferable to other plant species.

A specialized metabolic network selectively modulates Arabidopsis root microbiota.

Plant specialized metabolites have ecological functions, yet the presence of numerous uncharacterized biosynthetic genes in plant genomes suggests that many molecules remain unknown. We discovered a triterpene biosynthetic network in the roots of the small mustard plant Arabidopsis thaliana. Collectively, we have elucidated and reconstituted three divergent pathways for the biosynthesis of root triterpenes, namely thalianin (seven steps), thalianyl medium-chain fatty acid esters (three steps), and arabidin (five steps). A. thaliana mutants disrupted in the biosynthesis of these compounds have altered root microbiota. In vitro bioassays with purified compounds reveal selective growth modulation activities of pathway metabolites toward root microbiota members and their biochemical transformation and utilization by bacteria, supporting a role for this biosynthetic network in shaping an Arabidopsis-specific root microbial community.

Plant terpenes that mediate below-ground interactions: prospects for bioengineering terpenoids for plant protection.

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.

Diverged Plant Terpene Synthases Reroute the Carbocation Cyclization Path towards the Formation of Unprecedented 6/11/5 and 6/6/7/5 Sesterterpene Scaffolds.

Sesterterpenoids are a relatively rare class of plant terpenes. Sesterterpene synthase (STS)-mediated cyclization of the linear C25 isoprenoid precursor geranylfarnesyl diphosphate (GFPP) defines sesterterpene scaffolds. So far only a very limited number of STSs have been characterized. The discovery of three new plant STSs is reported that produce a suite of sesterterpenes with unprecedented 6/11/5 and 6/6/7/5 fused ring systems when transiently co-expressed with a GFPP synthase in Nicotiana benthamiana. Structural elucidation, feeding experiments, and quantum chemical calculations suggest that these STSs catalyze an unusual cyclization path involving reprotonation, intramolecular 1,6 proton transfer, and concerted but asynchronous bicyclization events. The cyclization is diverted from those catalyzed by the characterized plant STSs by forming unified 15/5 bicyclic sesterterpene intermediates. Mutagenesis further revealed a conserved amino acid residue implicated in reprotonation.

Unearthing a sesterterpene biosynthetic repertoire in the Brassicaceae through genome mining reveals convergent evolution.

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.