Comparative analysis of the transcriptome, methylome, and metabolome during pollen abortion of a seedless citrus mutant.

KEY MESSAGE: Pollen abortion could be mainly attributed to abnormal meiosis in the mutant. Multiomics analysis uncovered significant epigenetic variations between the mutant and its wild type during the pollen abortion process. Male sterility caused by aborted pollen can result in seedless fruit. A seedless Ponkan mandarin mutant (bud sport) was used to compare the transcriptome, methylome, and metabolome with its progenitor to understand the mechanism of citrus pollen abortion. Cytological observations showed that the anther of the mutant could form microspore mother cells, although the microspores failed to develop fertile pollen at the anther dehiscence stage. Based on pollen phenotypic analysis, pollen abortion could be mainly attributed to abnormal meiosis in the mutant. A transcriptome analysis uncovered the molecular mechanisms underlying pollen abortion between the mutant and its wild type. A total of 5421 differentially expressed genes were identified, and some of these genes were involved in the meiosis, hormone biosynthesis and signaling, carbohydrate, and flavonoid pathways. A total of 50,845 differentially methylated regions corresponding to 15,426 differentially methylated genes in the genic region were found between the mutant and its wild type by the methylome analysis. The expression level of these genes was negatively correlated with their methylation level, especially in the promoter regions. In addition, 197 differential metabolites were identified between the mutant and its wild type based on the metabolome analysis. The transcription and metabolome analysis further indicated that the expression of genes in the flavonoid, carbohydrate, and hormone metabolic pathways was significantly modulated in the pollen of the mutant. These results indicated that demethylation may alleviate the silencing of carbohydrate genes in the mutant, resulting in excessive starch and sugar hydrolysis and thereby causing pollen abortion in the mutant.

Transcriptomic insights into citrus segment membrane's cell wall components relating to fruit sensory texture.

BACKGROUND: During fresh fruit consumption, sensory texture is one factor that affects the organoleptic qualities. Chemical components of plant cell walls, including pectin, cellulose, hemicellulose and lignin, play central roles in determining the textural qualities. To explore the genes and regulatory pathways involved in fresh citrus' perceived sensory texture, we performed mRNA-seq analyses of the segment membranes of two citrus cultivars, Shiranui and Kiyomi, with different organoleptic textures. RESULTS: Segment membranes were sampled at two developmental stages of citrus fruit, the beginning and end of the expansion period. More than 3000 differentially expressed genes were identified. The gene ontology analysis revealed that more categories were significantly enriched in 'Shiranui' than in 'Kiyomi' at both developmental stages. In total, 108 significantly enriched pathways were obtained, with most belonging to metabolism. A detailed transcriptomic analysis revealed potential critical genes involved in the metabolism of cell wall structures, for example, GAUT4 in pectin synthesis, CESA1, 3 and 6, and SUS4 in cellulose synthesis, CSLC5, XXT1 and XXT2 in hemicellulose synthesis, and CSE in lignin synthesis. Low levels, or no expression, of genes involved in cellulose and hemicellulose, such as CESA4, CESA7, CESA8, IRX9 and IRX14, confirmed that secondary cell walls were negligible or absent in citrus segment membranes. A chemical component analysis of the segment membranes from mature fruit revealed that the pectin, cellulose and lignin contents, and the segment membrane's weight (% of segment) were greater in 'Kiyomi'. CONCLUSION: Organoleptic quality of citrus is easily overlooked. It is mainly determined by sensory texture perceived in citrus segment membrane properties. We performed mRNA-seq analyses of citrus segment membranes to explore the genes and regulatory pathways involved in fresh citrus' perceived sensory texture. Transcriptomic data showed high repeatability between two independent biological replicates. The expression levels of genes involved in cell wall structure metabolism, including pectin, cellulose, hemicellulose and lignin, were investigated. Meanwhile, chemical component contents of the segment membranes from mature fruit were analyzed. This study provided detailed transcriptional regulatory profiles of different organoleptic citrus qualities and integrated insights into the mechanisms affecting citrus' sensory texture.

Mechanism of seedlessness in a new lemon cultivar 'Xiangshui' [Citrus limon (L.) Burm. F].

Seedlessness is an important economic trait of lemon. Understanding the cellular and molecular mechanisms of seedlessness in 'Xiangshui' lemon requires detailed data on pollen and embryo sac fertility, embryo development and compatibility mechanisms governing self- and cross-pollination. The results of the current study indicate that the fertility of pollen and mature embryo sac remains normal. When flowers were self- or cross-pollinated, pollen grains of 'Xiangshui' were able to germinate on the stigma. In the case of self-pollination, pollen tubes became twisted, tube tips enlarged and tubes ruptured in the bottom of stigma. Following cross-pollination, tubes were able to grow normally in the style and ovary and enter the embryo sac, where double fertilization took place. Embryonic development resulting from cross-pollination was normal. After cross-pollination, the zygote began to divide at 2 weeks post-pollination, with early globular embryos observed after 3 weeks, globular and heart-shaped embryos at 4 weeks, torpedo-shaped embryos at 5 weeks, cotyledonary embryos at 6 weeks and thereafter germinable seeds. After self-pollination, however, ovules began to abort at 2 weeks post-pollination, with ovules disappearing at 5 weeks, ultimately producing seedless fruits. Emasculated unpollinated flowers also developed into seedless fruits, indicating that seedlessness contributes to parthenocarpy. However, gametophytic self-incompatibility has a major role in seedlessness in 'Xiangshui' lemon by blocking fertilization at the bottom of the stigma.

[Descriptions and microscopy identification of Citrus grandis var. tomentosa Hort. and Citrus grandis Osbeck].

OBJECTIVE: To identify Citrus grandis var. tomentosa Hort. and Citrus grandis Osbeck. METHODS: Descriptions and microscopy identification. RESULTS: There are nonglandular hair in the stem epidermis, many starch grains in the stem cortex and many prismatic crystals of calcium oxalate in the phloem of C. grandis var. tomentosa Hart. The palisade tissues of the leave are thick. The outer layer palisade tissues contains prismatic crystals. The cortex of leaf vein is thick. The other characters are similar to C. grandis Osbeck. CONCLUSION: Above-mentioned characters can be used to identify C. grandis var. tomentosa Hort. and C. grandis Osbeck.

[The microscopic observed for cells in mesocarp of part medicinal plant in Citrus L].

OBJECTIVE: To illustrate the microscopic characters of cells in mesocap of some of medicinal plants in Citrus of Rutaceae. METHOD: Microscopic observation and photograph were carried out. RESULT: It has been found that many of pavenchymatous cells are branches. It is spongy tissue that has huge room between cells. The thickening of cell wall is not obvious. The uneven thickness of walls of cells in mesocarp which is reported in the old documents are not found.

[Histological identification of Chinese drug zhiqiao & zhishi].

After growing area investigating which covered 10 Provinces (or Cities) of China, we gathered 11 types Zhiqiao and 6 types Zhishi which consistently came from 12 species (or varieties). Histological studies on them have been taken. microscopical characters including transverse section and powder were described. These studies will provide scientific basis for the identification of Zhiqiao and Zhishi.