Specialized metabolism by trichome-enriched Rubisco and fatty acid synthase components

Role of LEAFLESS, an AP2/ERF family transcription factor, in the regulation of trichome specialized metabolism

Acylsugars, specialized metabolites produced by trichomes of many solanaceous species, provide protection against biotic and abiotic stresses. Many acylsugar metabolic enzymes have been identified; however, regulatory factors remain unknown. Our multidisciplinary approaches identified LEAFLESS (APETALA 2/ ETHYLENE RESPONSE FACTOR (AP2/ERF) family member) as a positive regulator of acylsugar biosynthesis. Virus-induced gene silencing (VIGS) of LEAFLESS in Solanum pennellii (SpLFS/Sopen05g008450) revealed its distinct roles in two related but separate processes: acylsugar biosynthesis and trichome development. Most acylsugar (and several flavonoid) metabolic genes were downregulated in SpLFS-silenced plants and showed strong co-expression with SpLFS. Phylogenetic and additional data analyses indicated trichome-enriched expression of SpLFS orthologs in other acylsugar-producing solanaceous species, and VIGS of SpLFS orthologs in Nicotiana benthamiana reduced acylsugar production. Transcriptional reporter showed expression of SpLFS in type I/IV trichome tip cells, the site of acylsugar biosynthesis. Electrophoretic mobility shift assays indicated that SpLFS directly binds to promoters of several acylsugar (and flavonoid) metabolic genes. Additionally, data mining suggested remarkable spatiotemporal functional diversity: from coordinating leaf initiation at incipient primordia (previously reported for the S. lycopersicum ortholog SlLFS/Solyc05g013540) to regulating trichome specialized metabolism (acylsugar and flavonoid). Our work highlights a critical role of LEAFLESS in trichome specialized metabolism, paving the way to disentangle the acylsugar regulatory network.

A Metabolic Complex Involved in Tomato Specialized Metabolism

Specialized metabolites mediate diverse plant-environment interactions. Although, recent work has begun to enzymatically characterize entire plant specialized metabolic pathways, little is known about how different pathway components organize and interact within the cell. Here we use acylsugars - a class of specialized metabolites found across the Solanaceae family - as a model to explore cellular localization and metabolic complex formation of pathway enzymes. These compounds consist of a sugar core decorated with acyl groups, which are connected through ester linkages. In Solanum lycopersicum (tomato) four acylsugar acyltransferases (SlASAT1-4) sequentially add acyl chains to specific hydroxyl positions on a sucrose core leading to accumulation of tri and tetraacylated sucroses in the trichomes. To elucidate the spatial organization and interactions of tomato ASATs, we expressed SlASAT1-4 proteins fused with YFP in N. benthamiana and Arabidopsis protoplasts. Our findings revealed a distributed ASAT pathway with SlASAT1 and SlASAT3 localized to the mitochondria, SlASAT2 localized to the cytoplasm and nucleus, and SlASAT4 localized to the endoplasmic reticulum. To explore potential pairwise protein-protein interactions in acylsugar biosynthesis, we used various techniques, including co-immunoprecipitation, split luciferase assays, and bimolecular fluorescence complementation. These complementary approaches based on different interaction principles all demonstrated interactions among the different SlASAT pairs. Following transient expression of SlASAT1-4 in N. benthamiana , we were able to pull down a complex consisting of SlASAT1-4, which was confirmed through proteomics. Size exclusion chromatography of the SlASAT pulldown suggests a heteromultimeric complex consisting of SlASATs and perhaps other proteins involved in this interaction network. This study sheds light on the metabolic coordination for acylsugar biosynthesis through formation of an interaction network of four sequential steps coordinating efficient production of plant chemical defenses.

Trading acyls and swapping sugars: metabolic innovations in Solanum trichomes

Solanaceae (nightshade family) species synthesize a remarkable array of clade- and tissue-specific specialized metabolites. Protective acylsugars, one such class of structurally diverse metabolites, are produced by ACYLSUGAR ACYLTRANSFERASE (ASAT) enzymes from sugars and acyl-coenzyme A esters. Published research has revealed trichome acylsugars composed of glucose and sucrose cores in species across the family. In addition, acylsugars have been analyzed across a small fraction of the >1,200 species in the phenotypically megadiverse Solanum genus, with a handful containing inositol and glycosylated inositol cores. The current study sampled several dozen species across subclades of Solanum to get a more detailed view of acylsugar chemodiversity. In depth characterization of acylsugars from the clade II species brinjal eggplant (Solanum melongena) led to the identification of eight unusual structures with inositol or inositol glycoside cores and hydroxyacyl chains. Liquid chromatography-mass spectrometry analysis of 31 additional species in the Solanum genus revealed striking acylsugar diversity, with some traits restricted to specific clades and species. Acylinositols and inositol-based acyldisaccharides were detected throughout much of the genus. In contrast, acylglucoses and acylsucroses were more restricted in distribution. Analysis of tissue-specific transcriptomes and interspecific acylsugar acetylation differences led to the identification of the brinjal eggplant ASAT 3-LIKE 1 (SmASAT3-L1; SMEL4.1_12g015780) enzyme. This enzyme is distinct from previously characterized acylsugar acetyltransferases, which are in the ASAT4 clade, and appears to be a functionally divergent ASAT3. This study provides a foundation for investigating the evolution and function of diverse Solanum acylsugar structures and harnessing this diversity in breeding and synthetic biology.

Cutting-edge plant natural product pathway elucidation

Plant natural products (PNPs) play important roles in plant physiology and have been applied across diverse fields of human society. Understanding their biosynthetic pathways informs plant evolution and meanwhile enables sustainable production through metabolic engineering. However, the discovery of PNP biosynthetic pathways remains challenging due to the diversity of enzymes involved and limitations in traditional gene mining approaches. In this review, we will summarize state-of-the-art strategies and recent examples for predicting and characterizing PNP biosynthetic pathways, respectively, with multiomics-guided tools and heterologous host systems and share our perspectives on the systematic pipelines integrating these various bioinformatic and biochemical approaches.

Woolly mutation with the Get02 locus overcomes the polygenic nature of trichome-based pest resistance in tomato

Type-IV glandular trichomes, which only occur in the juvenile developmental phase of the cultivated tomato (Solanum lycopersicum), produce acylsugars that broadly protect against arthropod herbivory. Previously, we introgressed the capacity to retain type-IV trichomes in the adult phase from the wild tomato, Solanum galapagense, into the cultivated species cv. Micro-Tom (MT). The resulting MT-Galapagos enhanced trichome (MT-Get) introgression line contained 5 loci associated with enhancing the density of type-IV trichomes in adult plants. We genetically dissected MT-Get and obtained a subline containing only the locus on Chromosome 2 (MT-Get02). This genotype displayed about half the density of type-IV trichomes compared to the wild progenitor. However, when we stacked the gain-of-function allele of WOOLLY, which encodes a homeodomain leucine zipper IV transcription factor, Get02/Wo exhibited double the number of type-IV trichomes compared to S. galapagense. This discovery corroborates previous reports positioning WOOLLY as a master regulator of trichome development. Acylsugar levels in Get02/Wo were comparable to the wild progenitor, although the composition of acylsugar types differed, especially regarding fewer types with medium-length acyl chains. Agronomical parameters of Get02/Wo, including yield, were comparable to MT. Pest resistance assays showed enhanced protection against silverleaf whitefly (Bemisia tabaci), tobacco hornworm (Manduca sexta), and the fungus Septoria lycopersici. However, resistance levels did not reach those of the wild progenitor, suggesting the specificity of acylsugar types in the pest resistance mechanism. Our findings in trichome-mediated resistance advance the development of robust, naturally resistant tomato varieties, harnessing the potential of natural genetic variation. Moreover, by manipulating only 2 loci, we achieved exceptional results for a highly complex, polygenic trait, such as herbivory resistance in tomato.

Tomato root specialized metabolites evolved through gene duplication and regulatory divergence within a biosynthetic gene cluster

Tremendous plant metabolic diversity arises from phylogenetically restricted specialized metabolic pathways. Specialized metabolites are synthesized in dedicated cells or tissues, with pathway genes sometimes colocalizing in biosynthetic gene clusters (BGCs). However, the mechanisms by which spatial expression patterns arise and the role of BGCs in pathway evolution remain underappreciated. In this study, we investigated the mechanisms driving acylsugar evolution in the Solanaceae. Previously thought to be restricted to glandular trichomes, acylsugars were recently found in cultivated tomato roots. We demonstrated that acylsugars in cultivated tomato roots and trichomes have different sugar cores, identified root-enriched paralogs of trichome acylsugar pathway genes, and characterized a key paralog required for root acylsugar biosynthesis, SlASAT1-LIKE (SlASAT1-L), which is nested within a previously reported trichome acylsugar BGC. Last, we provided evidence that ASAT1-L arose through duplication of its paralog, ASAT1, and was trichome-expressed before acquiring root-specific expression in the Solanum genus. Our results illuminate the genomic context and molecular mechanisms underpinning metabolic diversity in plants.

Genetic and physiological requirements for high-level sesquiterpene-production in tomato glandular trichomes

Type-VI glandular trichomes of wild tomato Solanum habrochaites PI127826 produce high levels of the sesquiterpene 7-epizingiberene and its derivatives, making the plant repellent and toxic to several pest insects and pathogens. How wild tomato trichomes achieve such high terpene production is still largely unknown. Here we show that a cross (F1) with a cultivated tomato produced only minute levels of 7-epizingiberene. In the F2-progeny, selected for the presence of the 7-epizingiberene biosynthesis genes, only three percent produced comparable amounts the wild parent, indicating this trait is recessive and multigenic. Moreover, trichome density alone did not explain the total levels of terpene levels found on the leaves. We selected F2 plants with the "high-production active-trichome phenotype" of PI127826, having trichomes producing about 150 times higher levels of terpenes than F2 individuals that displayed a "low-production lazy-trichome phenotype". Terpene quantities in trichomes of these F2 plants correlated with the volume of the storage cavity and shape of the gland. We found that trichome morphology is not a predetermined characteristic, but cavity volume rather depended on gland-cell metabolic activity. Inhibitor assays showed that the plastidial-precursor pathway (MEP) is fundamental for high-level production of both cytosolic as well as plastid-derived terpenes in tomato trichomes. Additionally, gene expression profiles of isolated secretory cells showed that key enzymes in the MEP pathway were higher expressed in active trichomes. We conclude that the MEP pathway is the primary precursor-supply route in wild tomato type-VI trichomes and that the high-production phenotype of the wild tomato trichome is indeed a multigenic trait.