The U-box E3 ubiquitin ligase (PUB) gene family plays an important role in regulating plant responses to abiotic stress. Poncirus trifoliata (trifoliate orange), a citrus rootstock with notable cold, drought, and salt tolerance, serves as an excellent model for studying stress-responsive genes. In this study, a total of 47 PUB genes (PtrPUBs) were identified in the trifoliate orange genome. Chromosomal distribution analysis indicated that PtrPUB genes were unevenly distributed across nine trifoliate orange chromosomes. Phylogenetic tree analysis indicated that 170 PUB proteins from trifoliate orange, Arabidopsis thaliana, and tomato were clustered into five subfamilies. Gene structure, conserved domain, and motif analyses revealed diverse exon–intron and motif organizations of PtrPUB genes, suggesting potential functional differentiation among PtrPUBs. Cis-acting analysis indicated that the promoters of PtrPUB genes harbor elements related to hormone signaling (ABA, MeJA), drought stress, and low-temperature responses. Transcriptomic data and qRT-PCR results suggested that PtrPUB genes are responsive to ABA and dehydration treatments. This study provides a foundation for understanding the functional roles of PUB genes in trifoliate orange and offers insights for improving stress tolerance in citrus breeding programs.

Citrus creasing is a physiological rind disorder. Satsuma mandarin (Citrus unshiu Marc.) is the most widely cultivated mandarin variety worldwide and exhibits a high susceptibility to creasing. To investigate the physiological mechanisms underlying creasing, satsuma mandarin trees were treated with different drought stress during fruit expansion, then the relationship between the soil water content and creasing incidence was analyzed, while also examining the rind morphology, oil gland distribution in the flavedo, antioxidant enzyme activities, hormone concentrations, cell wall components, mineral content of creasing fruit, and the impact of creasing on fruit quality. Results showed that severe water stress (35% SRWC) increased the creasing incidence rate by 28% compared to well-irrigated treatments (80% SRWC). The creasing fruit oil gland diameter reduced by 35.7% and the density increased by 149.7% compared to healthy fruits. Simultaneously, the content of H₂O₂ and proline elevated by 47.1% and 8.3% respectively, and the activities of SOD, POD, and CAT of the creasing rind were enhanced significantly. Additionally, the content of IAA, ZR, and MeJA decreased by 17.2%, 7.8%, and 50.2%, respectively. Cell wall components such as cellulose, hemicellulose, and protopectin content reduced by 44.6%, 31.7%, and 33.1%, while soluble pectin increased by 36.3%. Significant alterations were observed in several minerals (Al, Fe, Na, Ni, V, Ga, Zn, Ba, Sn, Hg, Sc, Y, and La). However, fruit quality remained unaffected by creasing. These results demonstrate that drought is a key factor inducing creasing. Increased oil gland density, the degradation of cell wall components, elevated oxidative stress, reductions in phytohormones, and altered mineral element content work together to contribute to rind cells’ structural instability and lead to creasing in the satsuma mandarin.

In this study, we identified 34 HD-ZIP genes at the genome-wide level in P. trifoliata (Poncirus trifoliata (L.) Raf.), and conducted comprehensive bioinformatic analyses. The expression profiles of PtrHB genes in response to six phytohormones (ABA, SA, JA, BR, ETH, GA) and two abiotic stresses (dehydration, cold) were further determined. Phylogenetic analysis assigned all PtrHB proteins to four distinct subfamilies (I–IV) with highly conserved domain compositions, in line with the established classification system in Arabidopsis thaliana. Segmental duplication was revealed as the primary driver of PtrHB family expansion, whereas three tandem duplication events were also detected. Promoter cis-element profiling revealed subfamily-specific distribution of hormone- and stress-responsive elements. Several candidate genes with prominent stimulus-specific responses were highlighted, including PtrHB32 (strongly induced by dehydration, cold and ABA), PtrHB15 (JA-responsive) and PtrHB16 (SA-responsive). Collectively, this study provides the first systematic genome-wide analysis of the HD-ZIP transcription factor family in P. trifoliata, and offers valuable insights into the evolutionary expansion and functional divergence of this family. Our results also lay a theoretical basis for functional validation of candidate stress-regulatory genes and for molecular breeding of stress-tolerant citrus rootstocks.

Light regulates plant growth and secondary metabolism. To improve the nutritional quality and economic benefits of satsuma mandarin (Citrus unshiu Marc.) under greenhouse conditions with reduced natural light, we investigated the effects of supplementary lighting with different light qualities and intensities on fruit coloration, carotenoid accumulation, and fruit quality. Supplementary lighting was applied from 156 days after flowering using red, blue, red–blue combinations, and white light. All treatments significantly increased the color contribution index and total carotenoid content in both the rind and flesh compared with the control, with the red–blue light RB14 and white light W1 treatments being the most effective. β-Cryptoxanthin, the major carotenoid in satsuma mandarin, increased by 120.3% and 97.6% under RB14 and W1, respectively. Supplementary lighting also significantly increased the soluble sugar, total soluble solid, ascorbic acid, amino acid, flavonoid, and phenolic acid contents at most sampling time points. The leaf chlorophyll content increased, and chlorophyll degradation was delayed. qRT-PCR showed that RB14 and W1 upregulated the key carotenoid biosynthesis genes CitPSY, CitPDS, CitZDS, CitCRTISO, CitLCYE, CitLCYB, CitBCH2, CitZEP, and CitVDE and downregulated CitCCD4. These results suggest that supplementary lighting may enhance carotenoid accumulation and improve fruit quality by increasing the chlorophyll content and modulating carotenogenic gene expression, thereby providing a theoretical basis for citrus quality improvement.

Sugar transporters of the SWEET family play a crucial role in sugar partitioning and fruit quality, yet their functions remain uncharacterized in Chinese bayberry (Morella rubra). In this study, we present the first genome-wide identification and characterization of the SWEET gene family in species, revealing 15 MrSWEET genes distributed across eight chromosomes. Phylogenetic analysis classified these genes into four conserved clades with distinct tissue-specific expression patterns. During fruit ripening, transcripts of MrSWEET1, 2b, 4, and 15 accumulated progressively, with MrSWEET15 showing the strongest increase. Weighted gene co-expression network analysis (WGCNA) identified a “lightcyan” module that correlates strongly with fruit sugar content (r > 0.8) and contains MrSWEET15, three additional MrSWEET genes, and 47 transcription factors (Dof, MYB, NAC, ERF, MADS, WRKY). Promoter analysis of MrSWEET15 revealed the presence of light- and hormone-responsive cis-elements (MYB, MYC, HY5, bZIP, and Dof), and MYB1, HY5, and Dof1.5 expression profiles are synchronized with MrSWEET15, suggesting potential regulatory relationships. These findings establish MrSWEET15 as a priority candidate for sugar transport in Chinese bayberry and our understanding of the molecular basis of sugar transport and fruit quality formation.