Trading acyls and swapping sugars: metabolic innovations inSolanumtrichomes

Abstract

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 from sugars and acyl-Coenzyme A esters by acylsugar acyltransferases in glandular trichomes. We characterized trichome acylsugars of the Clade II species Solanum melongena (brinjal eggplant) using liquid chromatography-mass spectrometry (LC-MS), gas chromatography-MS and nuclear magnetic resonance (NMR) spectroscopy. This led to the identification of eight unusual structures with inositol cores, inositol glycoside cores, and hydroxyacyl chains. LC-MS analysis of 31 species in the megadiverse Solanum genus revealed striking acylsugar diversity with some traits restricted to specific clades and species. The Acylinositols were found throughout each clade while acylglucoses were restricted to DulMo and VANAns species. Medium length hydroxyacyl chains were found in many species. Analysis of tissue-specific transcriptomes and interspecific acylsugar acetylation differences led to characterization of the S. melongena Acylsugar AcylTransferase 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 is a functionally divergent ASAT3. This study provides a foundation for investigating the evolution of diverse Solanum acylsugar structures and harnessing this diversity in breeding and synthetic biology.

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

“…Further, inositol-based acyldisaccharides in S. quitoense and S. melongena trichomes and GI5:26 (2,5,6,6,7) in cultivated tomato roots ( Fig. 3B ) are all acylated on the inositol ring at positions 1, 2, and 3 ( 54 , 62 , 79 ). These results support the hypothesis that root and trichome inositol-based acylsugars are synthesized through similar biosynthetic routes anchored by ASAT1-L .…”

“…S10). Thereafter, we examined gene expression within the cluster using publicly available RNA sequencing (RNAseq) data from all three species ( 39 , 54 , 55 ). S. pennellii has one ASAT1-L ortholog within the cluster, SpASAT1-L ( Sopen07g023230 ), which is highly expressed in roots, consistent with the observed acylsugar phenotype ( Fig.…”

“…In addition to cultivated tomato roots, inositol-based acylsugars accumulate in trichomes of several dozen species in different clades of the large and diverse Solanum genus, including S. melongena (brinjal eggplant), S. quitoense , and S. nigrum ( 51 , 54 , 62 , 78 , 79 ), and their spatial distribution across species is correlated with ASAT1-L expression ( Fig. 6 ).…”

“…The GI5:26 structures were determined with a series of one-dimensional (1D) and 2D NMR experiments (data S1), an approach used successfully in previous studies ( 54 , 70 , 79 ). We used total correlation spectroscopy (TOCSY) to identify which proton signals correspond to each ring in the sugar core and then determined correlations between individual 1 H signals with correlation spectroscopy ( 1 H- 1 H COSY), heteronuclear single quantum coherence (HSQC), heteronuclear multiple bond correlation (HMBC), and heteronuclear two-bond correlation (H2BC) experiments.…”