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.

Cuticular wax plays an important role in the quality of kumquat (Fortunella crassifolia Swingle) fruit. In this study, the wax morphology, compositional profile of epi- and intracuticular wax, and crucial gene expression in ‘Rongan’ kumquat (RAK) and ‘Huapi’ kumquat (HPK) were analyzed during fruit development. The results showed that the surfaces of two kumquat fruits were covered with an amorphous wax layer containing a small number of platelets. Compared to RAK, HPK contained more abundant and larger wax crystals during fruit development. In two kumquat fruits, the epicuticular wax and its major compositions consistently displayed significantly higher levels than the intracuticular wax. Additionally, their main wax composition shifted from alkanes in the early developmental stages to triterpenoids at harvest in both layers, while aldehydes were specifically enriched in the epicuticular wax. During the fruit development from 90 to 180 DAF, HPK fruit exhibited significantly higher levels of epicuticular wax and its majority fractions than RAK fruit. Meanwhile, the intracuticular wax contents of HPK from 90 DAF to 150 DAF were significantly higher than those in RAK, with triterpenoids accounting for the largest proportion of this increase. qRT-PCR results indicated that the up-regulation of wax-related genes in HPK was linked to its increased epicuticular wax deposition during the development. Overall, this study provided a comprehensive overview of the morphology, composition, and biosynthesis of cuticular wax in kumquat fruit during development.

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.