Though numerous studies have focused on the cell wall disassembly of bananas during the ripening process, the modification of homogalacturonan (HG) during fruit development remains exclusive. To better understand the role of HGs in controlling banana fruit growth and ripening, RNA-Seq, qPCR, immunofluorescence labeling, and biochemical methods were employed to reveal their dynamic changes in banana peels during these processes. Most HG-modifying genes in banana peels showed a decline in expression during fruit development. Four polygalacturonase and three pectin acetylesterases showing higher expression levels at later developmental stages than earlier ones might be related to fruit expansion. Six out of the 10 top genes in the Core Enrichment Gene Set were HG degradation genes, and all were upregulated after softening, paralleled to the significant increase in HG degradation enzyme activities, decline in peel firmness, and the epitope levels of 2F4, CCRC-M38, JIM7, and LM18 antibodies. Most differentially expressed alpha-1,4-galacturonosyltransferases were upregulated by ethylene treatment, suggesting active HG biosynthesis during the fruit softening process. The epitope level of the CCRC-M38 antibody was positively correlated to the firmness of banana peel during fruit development and ripening. These results have provided new insights into the role of cell wall HGs in fruit development and ripening.

Flavor formation in citrus fruit is governed by complex and dynamic changes in primary and secondary metabolism during development and ripening. Here, we integrated metabolomic, hormonal, and transcriptomic analyses to elucidate the regulatory landscape underlying soluble sugar and organic acid metabolism in ‘Kiyomi’ citrus fruit. A total of 1679 metabolites were identified, revealing stage-specific reprogramming of metabolic pathways, including a sharp decline in citric acid after 90 days after flowering (DAF) and peak accumulation of sucrose at 180 DAF. Hormonal profiling showed that abscisic acid (ABA) progressively accumulated from 120 to 210 DAF, while 1-aminocyclopropane-1-carboxylic acid (ACC) peaked at 120 DAF and declined thereafter, suggesting distinct and temporally coordinated roles in ripening regulation. Transcriptomic profiling uncovered widespread temporal shifts in gene expression, with the most pronounced changes occurring between 180 and 210 DAF. Co-expression network analysis identified gene modules associated with sugar and acid accumulation, and highlighted transcription factors from the ERF, MYB, NAC, and HSF families as candidate regulators of ripening-related metabolic transitions. These findings provide a comprehensive framework for understanding the coordinated molecular and metabolic programs underlying flavor development in non-climacteric citrus fruit and offer candidate genes for the genetic improvement of fruit quality.

Plant melatonin is widely recognized as a pleiotropic regulator. As a growth-regulating hormone, it extensively participates in various growth and developmental processes and has significant functions in stress responses and disease resistance. Plant melatonin is synthesized primarily through the catalytic actions of five enzymes: TDC (tryptophan decarboxylase), T5H (tryptamine-5-hydroxylase), SNAT (serotonin N-acetyltransferase), ASMT (N-acetylserotonin methyltransferase), and COMT (caffeic acid-O-methyltransferase). There are multiple genes for each of these five enzymes in citrus genomes, however, with the exception of COMT5—whose function has recently been elucidated—and SNAT, which has only been preliminarily identified, the remaining genes have not been unequivocally characterized or functionally annotated. Hence, we carried out a genome-wide analysis of melatonin biosynthesis enzyme-related gene families in trifoliate orange (Poncirus trifoliata), one of the most common citrus rootstock varieties. Through bioinformatics approaches, we identified 96 gene family members encoding melatonin biosynthetic enzymes and characterized their protein sequence properties, phylogenetic relationships, gene structures, chromosomal distributions, and promoter cis-acting elements. Furthermore, by analyzing expression patterns in different tissues and under various stresses, we identified multiple stress-responsive melatonin synthase genes. These genes likely participate in melatonin synthesis under adverse conditions, thereby enhancing stress adaptation. Specifically, PtCOMT5, PtASMT11, and PtTDC9 were significantly induced by low temperature; PtSNAT1, PtSNAT14, PtSNAT18, and PtTDC10 were markedly responsive to drought; and PtASMT15, PtSNAT15, PtASMT16, and PtSNAT3 were strongly induced by ABA. Among them, PtASMT23 expression was induced up to 120-fold under low temperature, while PtSNAT18 showed over 100-fold upregulation under dehydration treatment. These findings strongly suggest that PtASMT23 and PtSNAT18 play critical roles in regulating melatonin biosynthesis in response to cold and drought stress, respectively. Collectively, these findings pinpoint novel genetic targets for enhancing stress resilience in citrus breeding programs and lay the foundation for the functional characterization of specific melatonin biosynthesis pathway gene family members in citrus and other horticultural crop species.