Pengmin Li

Quercetin glycosides are abundant flavonoids in apple (Malus domestica) leaves; however, how their structural variation shapes plant–insect interactions remain unclear. Here, we show that quercetin glycosylation and hydrolysis underlie a biochemical arms race between apple and multiple herbivorous insects. Herbivory by Helicoverpa armigera and Spodoptera litura activated a jasmonic acid (JA)-regulated transcriptional response that selectively induced the quercetin-glycoside pathway, resulting in elevated accumulation of 5 major quercetin glycosides. Methyl jasmonate treatment and JA content-boosting transgenic plants increased quercetin glycosides, whereas altering quercetin glycoside content did not feedback to JA signaling. Functional analyses revealed strong structure-dependent variation in anti-insect activity. Quercetin 3-O-glucoside (Q3Glc), but not its isomer quercetin 3-O-galactoside (Q3Gal), deterred larval feeding. During herbivory, apple leaves hydrolyzed Q3Glc to release toxic quercetin, while insects displayed species-specific detoxification strategies. H. armigera and S. exigua re-glycosylated quercetin to regenerate Q3Glc, neutralizing toxicity, whereas S. litura and S. frugiperda lacked this ability and suffered reduced growth. These findings reveal a reciprocal metabolic interplay in which apple activates glycoside hydrolysis to deploy toxic aglycones, while insects counter-evolve re-glycosylation to attenuate toxicity. Quercetin glycosylation patterns, therefore, serve as dynamic determinants of apple defense and herbivore susceptibility, offering opportunities for breeding insect-resistant cultivars.

Flavonols contribute significantly to both plant defense and human health. In apple (Malus domestica), flavonols accumulate at high levels in leaves and fruit skin but are present at low levels in the fruit flesh, the primary tissue consumed by humans. Enhancing flavonol content in the flesh requires a deeper understanding of the underlying molecular mechanisms. In this study, we investigated the expression patterns and functional roles of four apple flavonol synthase genes (MdFLS1, 2, 3, 4). MdFLS1, 2, 3 exhibited higher expression in fruit skin than in flesh, while MdFLS4 was not expressed in either tissue. Transient overexpression of each gene promoted flavonol accumulation in apple fruit flesh, indicating that all four genes encode functional flavonol synthases. This enzyme activity was further confirmed through stable overexpression in transgenic Arabidopsis and tomato plants. Notably, light exposure of apple fruit flesh enhanced the expression of MdFLS1 and MdFLS2 and increased flavonol accumulation. These findings advance our understanding of flavonol biosynthesis in apple.

Summary: ABA signaling is a central regulator of plant responses to abiotic stress. Similarly, LysM receptor‐like kinases (LYKs) are well‐established for their role in biotic stress responses. However, a potential function for LYKs in abiotic stress adaptation and their crosstalk with the ABA pathway remains largely unexplored. Here, we demonstrate that the apple LysM receptor‐like kinase MdLYK3 negatively regulates drought resistance by targeting ABA receptors. MdLYK3 directly interacts with 11 of 13 ABA receptors and phosphorylates MdPYR1 (T113, S131) and MdPYL4 (S112, S130). This phosphorylation promotes their ubiquitination and subsequent proteasomal degradation, thereby suppressing ABA signaling. Intriguingly, ABA itself attenuates the MdLYK3‐ABA receptor interaction, thereby stabilizing the receptors to sustain ABA signaling during stress. This dual regulatory mechanism uncovers a previously unrecognized link between microbial perception pathways and ABA‐mediated abiotic stress adaptation. Our findings reveal how plants co‐opt immune signaling components for environmental stress resilience, expanding the functional repertoire of LYKs beyond biotic interactions.

Summary: Polyphenols play key roles in plants' resistance to biotic and abiotic stresses. However, in fruits, these compounds are often antinutritive and unattractive to seed dispersers. How plants balance these competing requirements is critical to their survival. Here, we report that phlorizin (P2G), the predominant polyphenol in apples ( Malus spp.), and phloretin‐2'‐ O ‐xyloglucoside (P2XG), the xylosylated product of P2G, show different accumulation patterns in apple fruits. The addition of P2G to the diet of quails, zebra finches, and budgerigars reduced feeding, while the addition of P2XG did not. P2G treatment also lowered the blood sugar concentrations and significantly reduced egg weight in quail, consistent with its ability to inhibit glucose transport mediated by sodium‐glucose transporters. Two glycoside‐specific glycosyltransferase genes, PGGT1.1 and 1.2 , which catalyze the conversion of P2G to P2XG, were identified and shown to be specifically expressed in apple fruits. Transgenic apple plants overexpressing PGGT1 in leaves produced more P2XG, but less P2G. Consequently, these plants became more susceptible to lepidopteran pests and to spider mites. Our results show that the differential expression of glycosyltransferases in apple provides a mechanism to regulate polyphenol metabolism in different tissues to balance the requirements for plant‐seed dispersal and plant‐pest interactions.

Low temperature is a critical abiotic stress that imposes major constraints on the sustainable development of the fruit tree industry. Although exogenous dopamine has been shown to enhance cold tolerance in plants, its molecular mechanisms in apple ( Malus domestica ) remain poorly understood. In this study, we systematically investigated the role of dopamine in cold stress using exogenous dopamine application, overexpression (OE), and RNA interference (RNAi) of the MdTYDC (a key enzyme in dopamine biosynthesis). Our findings demonstrate that dopamine enhances cold resistance in apple through multiple mechanisms, including reducing reactive oxygen species accumulation, improving photosynthesis and stomatal function, promoting anthocyanin biosynthesis, and upregulating CBF genes. Molecular genetic analyses further revealed that MdICE1, a central transcriptional regulator, directly binds to cis‐regulatory elements in the MdTYDC promoter, thereby activating its transcription. Notably, we identified another bHLH transcription factor, MdFAMA, which interacts with MdICE1 and facilitates its binding to the MdTYDC promoter. This interaction amplifies dopamine biosynthesis and strengthens cold resistance. Moreover, exogenous dopamine treatment synergistically induced MdICE1 and MdFAMA expression, forming a positive feedback loop. This feedback mechanism establishes a hierarchical amplification of signalling, further reinforcing tolerance to low temperatures. Collectively, this study elucidates, for the first time, the molecular framework through which the MdICE1/MdFAMA ‐MdTYDC regulatory module orchestrates dopamine‐mediated cold tolerance in apple, providing novel insights into stress adaptation in perennial fruit crops.

Acetophenones, which show scattered distribution across phylogenetically distant plants and fungi, play diverse roles in plant–plant, plant–insect, plant–microbiome and even animal–insect interactions. However, the enzymatic basis of acetophenone biosynthesis in plants remains unknown. Here we elucidate the complete biosynthetic pathway of picein (4-hydroxyacetophenone glucoside) from 4-coumaroyl-CoA using pear (Pyrus) as a study system. We demonstrate that in certain pear cultivars, the acetophenone moiety originates from an impaired side-chain shortening reaction of an aromatic 3-ketoacyl-CoA intermediate, a key step in the β-oxidative biosynthesis of benzoic acid. This impairment results from a loss-of-function mutation in a peroxisomal 3-ketoacyl-CoA thiolase. The accumulated aromatic 3-ketoacyl-CoA is subsequently hydrolysed by a thioesterase and undergoes spontaneous decarboxylation to yield the acetophenone moiety. This rare metabolic phenomenon highlights that not only neofunctionalization but also loss-of-function mutations can drive diversification in plant secondary metabolism. Forward genetic approaches are powerful to shed light on such ‘hidden’ or recessive pathways in plants.

Summary: The clustered distribution of genes involved in metabolic pathways within the plant genome has garnered significant attention from researchers. By comparing and analyzing changes in the flanking regions of metabolic genes across a diverse array of species, we can enhance our understanding of the formation and distribution of biosynthetic gene clusters (BGCs). In this study, we have designed a workflow that uncovers and assesses conserved positional relationships between genes in various species by using synteny neighborhood networks (SNN). This workflow is then applied to the analysis of flanking genes associated with oxidosqualene cyclases (OSCs). The method allows for the recognition and comparison of homologous blocks with unique flanking genes accompanying different subfamilies of OSCs. The examination of the flanking genes of OSCs in 122 plant species revealed multiple genes with conserved positional relationships with OSCs in angiosperms. Specifically, the earliest adjacency of OSC genes and CYP716 genes first appeared in basal eudicots, and the nonrandom occurrence of CYP716 genes in the flanking region of OSC persists across different lineages of eudicots. Our study showed the substitution of genes in the flanking region of the OSC varies across different plant lineages, and our approach facilitates the investigation of flanking gene rearrangements in the formation of OSC‐related BGCs.