Litchi downy blight (LDB) caused by Peronophythora litchii destroys 20–30% of litchi fruit every year and causes significant economic losses. Some Exiguobacterium strains exhibit considerable promise in both agricultural and industrial sectors. E. acetylicum SI17, isolated from the litchi fruit carposphere, demonstrated significant biocontrol activity against LDB through pre-harvest treatment. To elucidate its underlying regulatory mechanisms, the genome of SI17 was sequenced and analyzed, revealing a circular chromosome spanning 3,157,929 bp and containing 3541 protein-coding genes and 101 RNA genes. Notably, 94 genes were implicated in the production of secondary metabolites. Among the 29 Exiguobacterium strains so far sequenced, SI17 possessed the largest genome. In the phylogenomic analysis encompassing the entire genome, SI17 was clustered into Group I. Treating litchi fruit with SI17 before harvesting resulted in a decrease in H₂O₂ content in the fruit peel and an increase in superoxide dismutase activity, thus enhancing resistance to LDB. Interestingly, SI17 did not display plate antagonism against Peronophythora litchii SC18. It can be inferred that SI17 generates secondary metabolites, which enhance litchi’s resistance to LDB. This study represents the first documentation of an Exiguobacterium strain exhibiting a role in litchi plant disease and showcasing significant potential for the biological control of LDB.

SO₂ fumigation treatment (commonly known as sulfur treatment, ST) is a key method in the postharvest preservation of imported and exported fresh longan fruits, effectively reducing pericarp browning and enhancing color. Nonetheless, distinctive aromas, often referred to as “sulfur flavor”, may develop in the aril during the extended preservation period. This study employed “Caopu” longan as the test material and patented SO₂-releasing paper (ZL201610227848.7) as a treatment to perform a 35-day low-temperature (5 °C) storage of the fruit. The changes in glucosinolates (GSLs) and associated metabolites in the aril of treated fruit (ST) were examined utilizing ultra-high-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) detection and widely targeted metabolomics technology. The findings indicated that following 35 days of storage, nearly all control (CK) fruit pericarp turned to brown, resulting in an edible fruit rate of 75.41% and a commercial fruit rate of 0%. In contrast, the treated (ST) fruit demonstrated an edible fruit rate and a commercial rate of 99.44%, while the pericarp color changed from dark yellow-brown to light earthy yellow. The sulfur-containing metabolites identified in longan fruit aril predominantly consist of amino acids and their derivatives (60.44%), followed by alkaloids (15.38%), nucleotides and their derivatives (1.10%), and other types (23.08%), which include GSLs. SO₂ treatment significantly reduced the content of oxidized glutathione in fruit aril but increased the content of GSLs and related amino acids and their derivatives. Via screening, 19 differential sulfur-containing metabolites were obtained between ST and CK, including 11 GSLs. The identified differential metabolites of GSLs were all increased, primarily comprising aliphatic GSLs, such as 1-hydroxymethyl glucosinolate, 2-Propenyl glucosinolate (Sinigrin), and 4-Methylsulfinylbutyl glucosinolate (Glucoraphanin). Pathway analysis showed that these differential metabolites were mainly involved in coenzyme factor synthesis, cysteine and methionine metabolism, and amino acid synthesis, among other pathways. To the best of our knowledge, this is the first study to reveal the causes of the special flavor of longan aril after SO₂ treatment, which is a great concern for longan consumers. Moreover, this study provides a scientific basis for exploring the reasons and mechanisms for the development of the sulfur flavor in the SO₂-treated fruits during postharvest storage.

Longan (Dimocarpus longan Lour.) is an important tropical and subtropical fruit, and most of its cultivars bloom once a year (once-flowering, OF). Dimocarpus longan cv. ‘Sijimi’ (SJ) is a tropical ecotype variety that blooms several times a year (continuous-flowering, CF) without the need for low-temperature induction. Several studies have focused on the mechanism of continuous flowering in SJ longan; however, none used leaves as research material. As leaves are a key organ in sensing floral-induction signals, we compared gene-expression differences between mature leaves of CF (SJ) and OF (D. longan cv. ‘Shixia’ (SX) and D. longan cv. ‘Chuliang’ (CL)) longan by transcriptome sequencing. An average of 47,982,637, 43,833,340 and 54,441,291 clean reads were obtained for SJ, SX and CL respectively, and a total of 6745 differentially expressed genes (DEGs) were detected. Following Metabolic pathways, Plant-pathogen interaction and Biosynthesis of secondary metabolites, most of the other genes were assigned to the KEGG classifications of MAPK signaling pathway- plant, Plant hormone signal transduction, Amino sugar and nucleotide sugar metabolism and Starch and sucrose metabolism. WGCNA analysis clustered genes into 27 modules, among which bisque4 and darkorange2 module genes specifically were expressed at low and high levels in SJ, respectively. Different gene-expression patterns were detected between CF and OF longan in bisque4 and darkorange2 modules, especially the high levels of transcription factor (TF) expression and the large number of gibberellic acid (GA)-signaling-pathway-specific genes expressed at high levels in CF longan (SJ). Floral-induction-gene expression levels in CF longan, such as levels of GA-signaling-related and FT genes, were always high. In CF longan, after vegetative-growth accumulation, flowers could be directly induced, thereby eliminating the need for low-temperature induction.

Longan (Dimocarpus longan Lour.) exhibits vigorous vegetative growth and strong fruit setting ability but suffers from alternate bearing. The role of endogenous hormones in mediating the effects of fruiting load remains unclear. This study investigated how the initial fruiting branch rate (IFBR) and initial fruit number per cluster (INFC) regulate endogenous hormones in the aril and leaflets of mature ‘Shixia’ longan. Key findings reveal the aril as the hormonal sink, accumulating auxin (IAA) and abscisic acid (ABA), while leaves retain IAA precursors (TRP) and conjugates. Higher IFBR and INFC increased the demand for IAA in the aril to support expansion but simultaneously elevated ABA levels in leaves. Notably, IFBR exerted a stronger influence than INFC. These hormonal changes were significantly correlated with the fruit shedding rate and soluble solid content. Overall, the endogenous hormone profile was optimized by maintaining a moderate IFBR of approximately 60% and an INFC within the range of 60 to 80, achieving balance between fruit expansion, leaf vitality, and yield. The results provide a hormonal basis for precise crop load management in longan cultivation.

The HB (homeobox) transcription factor family plays important roles in plant growth, development, morphogenesis, and stress responses; however, its involvement in longan (Dimocarpus longan Lour.) fruit energy metabolism remains unclear. In this study, 32 DlHB family members were identified in the longan genome, and their physicochemical properties, phylogenetic relationships, gene structures, conserved motifs, conserved domains, tissue-specific expression patterns, promoter cis-acting elements, and collinearity relationships were systematically analyzed. The DlHB family was classified into four subfamilies, HD-ZIP I–IV, with substantial divergence in structural composition, expression patterns, and putative regulatory features. Our previous work showed that 1.5% chitosan (CTS) treatment improved postharvest longan fruit quality through modulation of energy metabolism, and the corresponding CTS-treatment transcriptome was therefore used here to screen energy-metabolism-associated DlHB candidates. DlHB22 was selected as a representative candidate, and its CTS-responsive expression was independently confirmed by qRT-PCR. Exogenous ATP treatment was then used as an independent physiological validation of the relationship between energy metabolism and postharvest storability; ATP-treated fruit showed reduced deterioration together with higher ATP, ADP, and AMP contents and higher activities of H+-ATPase, Ca²+-ATPase, cytochrome c oxidase (CCO), and succinate dehydrogenase (SDH) at 15 d, although adenylate energy charge (AEC) was lower than in the control. DlHB22 localized predominantly to the nucleus. Heterologous overexpression of DlHB22 in tomato accelerated fruit color transition and ripening progression and altered ATP, ADP, and AMP contents, AEC, and the activities of H+-ATPase, Ca²+-ATPase, CCO, and SDH. Transcriptome analysis of DlHB22-overexpressing tomato fruit revealed broad transcriptional changes in pathways associated with central carbon metabolism, energy metabolism, glutathione metabolism, hormone signaling, and MAPK signaling, and qRT-PCR validation of six representative DEGs was consistent with the RNA-seq trends. Because tomato is climacteric whereas longan is non-climacteric, the heterologous tomato results demonstrate the regulatory potential of DlHB22 but do not establish an identical native ripening pathway in longan. Overall, DlHB22 is best regarded as a candidate transcription factor associated with longan fruit energy metabolism, whose native regulatory mechanism requires direct validation in longan.