Litchi has great economic significance as a global fruit crop. However, the advancement of litchi functional genomics has encountered substantial obstacles due to its recalcitrance to stable transformation. Here, we present an efficacious Agrobacterium tumefaciens-mediated transformation system in somatic embryos of ‘Heiye’ litchi. This system was developed through the optimization of key variables encompassing explant selection, A. tumefaciens strain delineation, bacterium concentration, infection duration, and infection methodology. The subsequent validation of the transformation technique in litchi was realized through the ectopic expression of LcMYB1, resulting in the generation of transgenic calli. However, the differentiation of transgenic calli into somatic embryos encountered substantial challenges. To delineate the intricate molecular underpinnings of LcMYB1's inhibitory role in somatic embryo induction, a comprehensive transcriptome analysis was conducted that encompassed embryogenic calli (C), globular embryos (G), and transgenic calli (TC). A total of 1,166 common differentially expressed genes (DEGs) were identified between C-vs.-G and C-vs.-TC. Gene Ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that these common DEGs were mostly related to plant hormone signal transduction pathways. Furthermore, RT-qPCR corroborated the pronounced down-regulation of numerous genes that are associated with somatic embryo induction within the transgenic calli. The development of this transformation system provides valuable support for functional genomics research in litchi.

Litchi chinensis Sonn. is an important economic fruit tree in tropical and subtropical regions. Regrettably, the efficiency of plant regeneration via somatic embryogenesis in litchi is typically low due to the poor conversion of embryos to plants. The purpose of this study was to establish a regeneration system via somatic embryogenesis from immature embryos explants in ‘Heiye’ cultivar of litchi. Our results demonstrated that MS medium supplemented with 2.0 mg L⁻¹ 2,4-D was optimal for callus induction. For somatic embryo (SE) induction, MS medium containing 0.5 g L⁻¹ activated charcoal (AC) was the most effective, while the use of zeatin (ZT) and thidiazuron (TDZ) resulted in abnormal somatic embryos. The rooting and regeneration rate of 2.15% and 17.5%, respectively, were achieved using MS medium supplemented with 0.5 g L⁻¹ AC. Furthermore, transcriptome analysis was performed on embryogenic callus (EC), globular embryo (GE), and heart embryo (HE) to explore the molecular mechanisms of early somatic embryogenesis. 2,587 common differentially expressed genes (DEGs) between EC_vs_GE and EC_vs_HE were identified, and the expression patterns of these common DEGs were separated into twelve major clusters. GO annotation and KEGG pathway analysis revealed that these common DEGs were implicated in plant hormone signal transduction, auxin-activated signaling pathway, and other biological processes. Additionally, differentially expressed transcription factors were identified, and the function of LcBBM2 which is specifically highly expressed during early somatic embryogenesis was verified. Overexpression of LcBBM2 in tomato promotes callus and shoot formation. Therefore, this study can provide a theoretical basis and technical support for genetic breeding improvement of litchi.

Sucrose synthase (SUS, EC 2.4.1.13) is widely considered as a key enzyme involved in plant sucrose metabolism, and the gene family encoding different SUS isozymes has been identified and characterized in several plant species. However, to date scant information about the SUS genes is available in Litchi chinensis Sonn. Here, we identified five SUS genes in litchi. These LcSUSs shared high levels of similarity in both nucleotide and amino acid sequences. Their gene structure, phylogenetic relationships, and expression profiles were characterized. Gene structure analysis indicated that the LcSUSs have similar exon-intron structures. Phylogenetic analysis revealed that the five members could be classified into three groups (LcSUS1 and LcSUS2 in SUS Ⅱ, LcSUS4 and LcSUS5 in SUS Ⅲ, and LcSUS3 in SUS Ⅰ), demonstrating evolutionary conservation in the SUS family across litchi and other plant species. The expression levels of LcSUSs were investigated via real-time PCR in various tissues and different developmental stages of aril. For tissues and organs, LcSUSs exhibited distinct but partially redundant expression profiles in litchi, being predominantly expressed in young leaves (sink). During aril development, the expression pattern of LcSUS1 was consistent with the trend of sugar accumulation, indicating it may play important roles in determination of sink strength in aril. Moreover, transcript levels of LcSUS2, LcSUS4, and LcSUS5 varied between cultivars with different hexose/sucrose ratios, which may regulate the sugar composition in aril. Our results provide insights into physiological functions of SUS genes in litchi, especially roles in regulating sugar accumulation in aril.