Soil salinization is a growing challenge for highbush blueberry (Vaccinium corymbosum) production, but knowledge about its physiological and molecular responses to salt stress remains limited. To address this, we performed Gene Ontology (GO) enrichment analysis and weighted gene co-expression network analysis (WGCNA) on differentially expressed genes from the roots and leaves of the salt-tolerant cultivar ‘Duke’ and the salt-sensitive cultivar ‘Sweetheart’. GO analysis revealed significant enrichment of ion transport and cellular ion homeostasis in the roots under salt stress. WGCNA identified a strong correlation between Na⁺ and K⁺ contents and the expression of hub genes VcHAK5, VcNHX1, and VcNHX2 under salt stress. These genes were significantly upregulated in the roots of the salt-tolerant cultivar. VcHAK5 encodes a K⁺-selective ion transporter in the plasma membrane, and VcNHX1 and VcNHX2 encode Na⁺/H⁺ and K⁺/H⁺ antiporters in the tonoplast. Knockdown mutants of these genes in blueberry calli showed hypersensitivity to salt stress. Furthermore, reciprocal grafting between salt-sensitive and salt-tolerant blueberry cultivars demonstrated that lower root Na⁺ content and Na⁺/K⁺ ratios are crucial for salt tolerance. This study provides the first comprehensive insights into blueberry responses to salt stress, identifies target genes and highlights the critical role of salt-resistant roots.

Sucrose transporters (SUTs) play a crucial role in carbon allocation from the source leaf to the sink end, and the function of SUTs varies among family members. However, the genome-wide identification of the SUT superfamily in Camellia oleifera is lacking, and their biological function remains elusive. In this study, four SUT genes - designated CoSUT1-4 - were identified in C. oleifera through a genome-wide analysis and classified into three subfamilies. We used a combination of cis-acting elements analysis, mRNA quantification, histochemical analysis, and heterologous transformation to evaluate the expression profiles and functions of these SUTs. A key finding is that CoSUT4, localized on the plasma membrane, is highly expressed in mature leaves and the early stage of seed development in C. oleifera. In vitro culture of C. oleifera seed revealed the responsiveness of CoSUT4 to various exogenous hormones such as ABA and GA. CoSUT4 was able to restore the growth of the yeast strain SUSY7/ura3 (a sucrose transport-deficient mutant) on sucrose-containing media and specifically contributed to sucrose translocation and tissue growth in CoSUT4-overexpressed apple calli. In situ hybridization identified chalazal nucellus and transfer cells as the action sites of CoSUT4 at the maternal-filial interface mediating sucrose transportation in oil tea seeds. CoSUT4 overexpression in Arabidopsis thaliana atsuc4 mutant restored the growth and seed yield deficiencies of the mutant, leading to an increase in filled seeds and oil content. Additionally, CoSUT4 overexpression enhanced the drought and salt stress tolerance by augmenting sugar content. Overall, these findings provide valuable insights into the function of SUTs and present promising candidates for the genetic enhancement of seed production in C. oleifera.

Phloem loading and transport of sugar from leaves to sink tissues such as fruits are crucial for yield formation. Camellia oleifera is an evergreen horticultural crop with high value; however, its low production limits the development of the C. oleifera industry. In this study, using a combination of ultrastructural observation, fluorescence loss in photobleaching (FLIP) and inhibitor treatment, we revealed that C. oleifera leaves mainly adopt a symplastic loading route from mesophyll cells to the surrounding vascular bundle cells in minor veins. HPLC assays showed that sucrose is the main sugar transported and only a small amount of raffinose or stachyose was detected in petioles, supporting a passive symplastic loading route in C. oleifera leaves. Compared to leaves grown this year (LT), the carbohydrate synthesis capacity in leaves grown last year (LL) was decreased while LL retained more soluble sugar, suggesting a decrease in transport capacity with leaf ageing. TEM and tissue staining showed that a reduction in plasmodesmata density leads to a decline in the degree of cellular coupling and is responsible for the weakening transport capacity in older leaves. RNA-seq revealed several differentially expressed genes (DEGs) including CoPDCB1-1, CoSUT1 and CoSWEET12, which are likely involved in the regulation of phloem loading and sugar transport. An expression correlation network is constructed between PD-callose binding protein genes, sugar transporter genes and senescence-associated genes. Collectively, this study provides the evidence of the passive symplastic phloem loading pathway in C. oleifera leaves and constructs the correlation between sugar transport and leaf ageing.