Wei Liu

Biochar, a highly effective amendment, is widely used for soil improvement and environmental remediation. However, research on its application in litchi (Litchi chinensis) cultivation is relatively scarce, particularly regarding its potential to enhance the rhizospheric soil ecological environment. In this study, a pot experiment was conducted to investigate the effects of biochar derived from maize (Zea mays) and rice (Oryza sativa), applied at different rates (3%, 6%, 10%), on the physical and chemical properties, enzyme activities, and microbial community structure and diversity in the rhizospheric soil of litchi seedlings. The results showed that biochar application significantly (p < 0.05) improved soil nutrient conditions, including total nitrogen (TN), total phosphorus (TP), available phosphorus (AP), available potassium (AK), pH, and soil sucrase (SC) enzyme activity. Notably, treatment with 10% maize biochar exhibited the most pronounced improvement across all parameters, barring AP. Furthermore, biochar application stimulated the proliferation of specific bacterial taxa (Acidobacteriota, Bacteroidota, and Chloroflexota) and fungal phyla (Ascomycota and Mortierellomycota), increasing bacterial diversity while decreasing fungal diversity and richness. Correlation analysis further revealed the close relationships between soil microbial communities and fertility factors. This study provides substantial evidence regarding the efficacy and feasibility of biochar in improving the rhizospheric soil ecological environment of litchi. It offers a theoretical foundation for the scientific application of biochar in orchard soil management.

Superoxide dismutase (SOD) serves as a critical enzyme that is involved in plant development and abiotic stresses by effectively detoxifying reactive oxygen species (ROS). Though the SOD gene family has been reported across various plant species, its specific members and functional roles in litchi (Litchi chinensis Sonn.) remain poorly understood. In this study, a total of seven SOD (christened LcSOD) genes were identified from the litchi genome and classified into three groups across six chromosomes. Notably, genes from the same evolutionary branch had more similar structures and motif distributions. The LcSOD genes were confirmed to have a stronger collinearity with dicotyledons than with monocotyledons. Cis-acting elements analysis indicated that the LcSOD gene family was deeply involved in orchestrating growth, development, and responses to multiple phytohormones and diverse stresses. Expression patterns of the LcSOD genes across different tissues revealed universal and specific expressions. In leaves, expression levels of the LcSOD genes were induced by cold, heat, drought, and salt stresses, and transcript levels correlated positively with concomitant changes in key physiological parameters under the same conditions. In addition, the LcSOD genes were characterized for their physicochemical properties, subcellular localizations, secondary and tertiary structures, gene ontology (GO) annotations, and protein-protein interactions. Our findings offer comprehensive insights into the LcSOD gene family, enriching genetic resources. They provide a framework for functional characterization and the development of stress-resistant cultivars, driving both basic research and applied breeding programs in litchi.

Class III peroxidases (CIII PRXs) are plant-specific enzymes with high activity that play key roles in the catalysis of oxidation-reduction reactions. In plants, CIII PRXs can reduce hydrogen peroxide to catalyze oxidation–reduction reactions, thereby affecting plant growth, development, and stress responses. To date, no systematic analysis of the CIII PRX gene family in litchi (Litchi chinensis Sonn.) has been documented, although the genome has been reported. In this study, a total of 77 CIII PRX (designated LcPRX) gene family members were predicted in the litchi genome to provide a reference for candidate genes in the responses to abiotic stresses during litchi growth and development. All of these LcPRX genes had different numbers of highly conserved PRX domains and were unevenly distributed across fifteen chromosomes. They were further clustered into eight clades using a phylogenetic tree, and almost every clade had its own unique gene structure and motif distribution. Collinearity analysis confirmed that there were eleven pairs of duplicate genes among the LcPRX members, and segmental duplication (SD) was the main driving force behind the LcPRX gene expansion. Tissue-specific expression profiles indicated that the expression levels of all the LcPRX family members in different tissues of the litchi tree were significantly divergent. After different abiotic stress treatments, quantitative real-time PCR (qRT-PCR) analysis revealed that the LcPRX genes responded to various stresses and displayed differential expression patterns. Physicochemical properties, transmembrane domains, subcellular localization, secondary structures, and cis-acting elements were also analyzed. These findings provide insights into the characteristics of the LcPRX gene family and give valuable information for further elucidating its molecular function and then enhancing abiotic stress tolerance in litchi through molecular breeding.

The MADS-box protein is an important transcription factor in plants and plays an important role in regulating the plant abiotic stress response. In this study, a total of 94 MADS-box genes were predicted in the litchi genome, and these genes were widely distributed on all the chromosomes. The LcMADS-box gene family was divided into six subgroups (Mα, Mβ, Mγ, Mδ, MIKC, and UN) based on their phylogenetical relationships with Arabidopsis, and the closely linked subgroups exhibited more similarity in terms of motif distribution and intron/exon numbers. Transcriptome analysis indicated that LcMADS-box gene expression varied in different tissues, which can be divided into universal expression and specific expression. Furthermore, we further validated that LcMADS-box genes can exhibit different responses to various stresses using quantitative real-time PCR (qRT-PCR). Moreover, physicochemical properties, subcellular localization, collinearity, and cis-acting elements were also analyzed. The findings of this study provide valuable insights into the MADS-box gene family in litchi, specifically in relation to stress response. The identification of hormone-related and stress-responsive cis-acting elements in the MADS-box gene promoters suggests their involvement in stress signaling pathways. This study contributes to the understanding of stress tolerance mechanisms in litchi and highlights potential regulatory mechanisms underlying stress responses.

Flower color variations have increasingly been recognized as playing an important role in the adaptation to UV-B radiation; however, the underlying mechanism is poorly understood in perennial fruit trees. Litchi is an important fruit tree, and extremely early maturing (EEM) and middle-to-late-maturing (MLM) cultivars that originated from regions with high and low UV-B radiation have dark brown and light yellow flower buds, respectively, while their hybrid early-maturing (EM) cultivars have an intermediate brown flower bud. This study comprehensively analyzed the metabolome and transcriptome of flower buds of litchi EEM, EM and MLM cultivars to explore the mechanism underlying flower color variation during the adaptation to UV-B radiation for the first time. Metabolomic analysis identified 72 flavonoids in litchi flower buds, among which a higher accumulation of flavonol glycosides was responsible for darker flower buds of EEM cultivars. And transcriptome analysis revealed key structural genes, including LcCHI, LcFLS and seven UGTs, together with two transcription factors (LcMYB12 and LcMYB111), which could be directly up-regulated by UV-B radiation, playing critical roles in regulating the differential accumulation of flavonol glycosides. These results provide new insights into the molecular mechanism underlying adaptation to UV-B radiation and provide a genetic basis for future breeding of stress-tolerant cultivars of litchi.

Flavonol synthase (FLS) is the crucial enzyme of the flavonol biosynthetic pathways, and its expression is tightly regulated in plants. In our previous study, two alleles of LcFLS, LcFLS-A and LcFLS-B, have been identified in litchi, with extremely early-maturing (EEM) cultivars only harboring LcFLS-A, while middle-to-late-maturing (MLM) cultivars only harbor LcFLS-B. Here, we overexpressed both LcFLS alleles in tobacco, and transgenic tobacco produced lighter-pink flowers and showed increased flavonol levels while it decreased anthocyanin levels compared to WT. Two allelic promoters of LcFLS were identified, with EEM cultivars only harboring proLcFLS-A, while MLM cultivars only harbor proLcFLS-B. One positive and three negative R2R3-MYB transcription regulators of LcFLS expression were identified, among which only positive regulator LcMYB111 showed a consistent expression pattern with LcFLS, which both have higher expression in EEM than that of MLM cultivars. LcMYB111 were further confirmed to specifically activate proLcFLS-A with MYB-binding element (MBE) while being unable to activate proLcFLS-B with mutated MBE (MBEm). LcHY5 were also identified and can interact with LcMYB111 to promote LcFLS expression. Our study elucidates the function of LcFLS and its differential regulation in different litchi cultivars for the first time.

Litchi (Litchi chinensis Sonn.) is a highly valuable fruit crop that is widely grown in tropical and subtropical areas of the world. Studying its genetic diversity and population structure is critical for effective conservation and breeding programs. In this study, we developed 150 single-nucleotide polymorphism (SNP) markers that were evenly spaced across litchi genome and applied them to the evaluation of the genetic diversity of 84 litchi accessions, including old cultivars, modern cultivars, hybrids from known parents and wild accessions. Ninety-one SNP markers, showing high levels of polymorphism and high genotyping success rates, were used for further analysis. The newly developed SNP markers captured a relatively higher level of genetic diversity (He = 0.364) in litchi cultivars and could be successfully applied for the identification of synonymous cultivars and hybrids with close genetic backgrounds. Cluster analysis grouped all genotypes into three clusters that showed perfect association with their fruit maturation period, among which wild accessions clustered with their corresponding domesticated cultivars, and hybrids from different parent combinations showed different inheritance tendencies. Our study not only provided a set of efficient SNP markers for future genetic research, but also laid an important foundation for the conservation and genetic breeding of litchi.

Lychee is an exotic tropical fruit with a distinct flavor. The genome of cultivar ‘Feizixiao’ was assembled into 15 pseudochromosomes, totaling ~470 Mb. High heterozygosity (2.27%) resulted in two complete haplotypic assemblies. A total of 13,517 allelic genes (42.4%) were differentially expressed in diverse tissues. Analyses of 72 resequenced lychee accessions revealed two independent domestication events. The extremely early maturing cultivars preferentially aligned to one haplotype were domesticated from a wild population in Yunnan, whereas the late-maturing cultivars that mapped mostly to the second haplotype were domesticated independently from a wild population in Hainan. Early maturing cultivars were probably developed in Guangdong via hybridization between extremely early maturing cultivar and late-maturing cultivar individuals. Variable deletions of a 3.7 kb region encompassed by a pair of CONSTANS-like genes probably regulate fruit maturation differences among lychee cultivars. These genomic resources provide insights into the natural history of lychee domestication and will accelerate the improvement of lychee and related crops.

Litchi is an important fruit cultivated in tropical and subtropical areas with high nutritious and delicious flavor and the pulp is the main part of the fruit consumed. Previous studies found that litchi had high total phenol content and antioxidant activity, but most of them focused on the identification of single or a few phenolic components with a low throughput test, and the metabolic differences of cultivars are still unknown to a some extent. In this study we used widely targeted metabolome based on ultra-performance liquid chromatography coupled with mass spectrometry (UPLC-MS/MS) to analyze the polyphenol metabolites of five different genotypes of mature litchi fruit. A total of 126 polyphenol metabolites in eight categories were identified to reveal the composition and differences of polyphenol; 15 common differential metabolites and 20 specific differential metabolites to each cultivar were found for the first time. The results infer that flavonoids, flavonols, hydroxycinnamoyls and catechins are the main polyphenol metabolites of litchi pulp. Cluster analysis showed that there were three groups of polyphenols from high to low; early maturing Feizhixiao is a kind of high polyphenol content cultivars, especially in catechins, anthocyanins, flavonols, quinic acids and hydroxycinnamoyls. The polyphenols in the flesh of mature litchi are rich, and there are significant differences among cultivars; there was a level of correlation between the contents of phenolics and the maturity of litchi cultivars; the content of phenolics in early maturing litchi cultivars appeared higher than those of mid- to late-maturing cultivars. This experiment will provide significant reference information for cultivation, breeding, processing and consumption.