Evaluation of Crossability between Nicotiana benthamiana and Nicotiana excelsior

Wild tobacco species in the Nicotiana section Suaveolentes are promising genetic resources to introduce their disease resistance to cultivated tobacco, Nicotiana tabacum. However, hybrid lethality is observed in hybrid seedlings from crosses between most Suaveolentes species and N. tabacum. In particular, N. benthamiana belonging to the section Suaveolentes produces only viable hybrids after crossing with N. tabacum. In the present study, crossability between N. benthamiana and N. excelsior (section Suaveolentes) was investigated to test the possible usefulness of N. benthamiana as the bridge parent to transfer desirable genes of N. excelsior to N. tabacum via bridge crossing. After reciprocal crosses using three accessions of N. benthamiana and N. excelsior each, several crossing barriers such as cross-incompatibility, seed abortion, and male and female hybrid sterility were observed. Although reciprocal hybrids between N. benthamiana and N. excelsior showed a high degree of chromosome pairing in meiosis, univalents and multivalents, as well as chromosome bridges and lagging chromosomes, were observed. These meiotic abnormalities were thought to cause hybrid sterility. The possible usefulness of reciprocal hybrids between N. benthamiana and N. excelsior is discussed.

Transcriptomic Analysis Reveals Key Candidate Genes Related to Seed Abortion in Chinese Jujube (Ziziphus jujuba Mill.)

Background: Seed abortion is a common phenomenon in Chinese jujube that seriously hinders the process of cross-breeding. However, the molecular mechanisms of seed abortion remain unclear in jujube. Methods: Here, we performed transcriptome sequencing using eight flower and fruit tissues at different developmental stages in Ziziphus jujuba Mill. ‘Zhongqiusucui’ to identify key genes related to seed abortion. Histological analysis revealed a critical developmental process of embryo abortion after fertilization. Results: Comparisons of gene expression revealed a total of 14,012 differentially expressed genes. Functional enrichment analyses of differentially expressed genes between various sample types uncovered several important biological processes, such as embryo development, cellular metabolism, and stress response, that were potentially involved in the regulation of seed abortion. Furthermore, gene co-expression network analysis revealed a suite of potential key genes related to ovule and seed development. We focused on three types of candidate genes, agamous subfamily genes, plant ATP-binding cassette subfamily G transporters, and metacaspase enzymes, and showed that the expression profiles of some members were associated with embryo abortion. Conclusions: This work generates a comprehensive gene expression data source for unraveling the molecular mechanisms of seed abortion and aids future cross-breeding efforts in jujube.

Genome-scale Transcriptomic Insights Into the Gene-regulatory Network of Seed Abortion in Triploid Siraitia Grosvenorii

BackgroundSiraitia grosvenorii (Swingle) C. Jeffrey, also known as Luohanguo or monk fruit, is a famous traditional Chinese medicine ingredient with important medicinal value and broad development prospects. Diploid S. grosvenorii has too many seeds. Thus, studying the molecular mechanism of seed abortion in triploid S. grosvenorii, identifying the abortion-related genes, and regulating their expression will be a new direction to obtain seedless S. grosvenorii. Herein, we examined the submicroscopic structure of triploid S. grosvenorii seeds during abortion. ResultsBy measuring the content of endogenous hormones, we found that abscisic acid (ABA) and trans-zeatin (ZR) levels were significantly downregulated at days 15 and 20 after flowering. RNA-seq of triploid seeds at different developmental stages was performed to identify the key genes regulating abortion of triploid S. grosvenorii seeds. Multiple differentially expressed genes between adjacent stages were identified; seven genes were differentially expressed across all stages. Weight gene co-expression network analysis revealed that the enhancement of monoterpene and terpene metabolic processes might lead to seed abortion by reducing substrate flow to ABA and ZR.ConclusionsThese findings provide insights into the gene-regulatory network of seed abortion in triploid S. grosvenorii from different perspectives, thereby facilitating the innovation of the breeding technology of S. grosvenorii.

Comparative Transcriptome Profiling Analysis to Unravel the Potential Mechanism of Seed Abortion in Lumnitzera littorea

Lumnitzera littorea (Jack) Voigt. is a species of mangrove plant belonging to the family Combretaceae. Natural reproduction of L. littorea is extremely difficult due to its seed abortion in China. To reveal the molecular mechanism of seed abortion, we performed transcriptome to analyze the seeds of L. littorea-T (normal seeds) and L. littorea-S (abortive seeds). After analysis of the raw data, 64,868 transcripts (mean length = 658 bp) were assembled. Among these transcripts, 39,779 were functionally annotated. Then, differentially expressed genes (DEGs) were screened, and 23,513 transcripts were identified that were likely involved in seed abortion. About 207 DEGs assigned to Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, and approximately 12.1% fell into reproduction categories. Genes involved in response to sucrose and starch metabolism, and phytohormone biosynthesis showed significant different expression levels between normal and abortive seeds of L. littorea. Further expressions patterns of key genes contribute to seed development were analyzed using quantitative real-time PCR, and the results were consistent with high-throughput sequencing data. The information obtained in this study will aid in the research of the mechanism of seed abortion in further molecular studies of L. littorea. © 2021 Friends Science Publishers

Embryo CHH hypermethylation is mediated by RdDM and is autonomously directed in Brassica rapa

BackgroundRNA directed DNA methylation (RdDM) initiates cytosine methylation in all contexts, and maintains asymmetric CHH methylation (where H is any base other than G). Mature plant embryos show one of the highest levels of CHH methylation, and it has been suggested that RdDM is responsible for this hypermethylation. Because loss of RdDM in Brassica rapa causes seed abortion, embryo methylation might play a role in seed development. RdDM is required in the maternal sporophyte, suggesting that small RNAs from the maternal sporophyte might translocate to the developing embryo, triggering DNA methylation that prevents seed abortion. This raises the question whether embryo hypermethylation is autonomously regulated by the embryo itself or influenced by the maternal sporophyte.ResultsHere, we demonstrate that B. rapa embryos are hypermethylated in both euchromatin and heterochromatin and that this process requires RdDM. Contrary to current models, B. rapa embryo hypermethylation is not correlated with demethylation of the endosperm. We also show that maternal somatic RdDM is not sufficient for global embryo hypermethylation, and we find no compelling evidence for maternal somatic influence over embryo methylation at any locus. Decoupling of maternal and zygotic RdDM leads to successful seed development despite loss of embryo CHH hypermethylation.ConclusionsWe conclude that embryo CHH hypermethylation is conserved, autonomously controlled, and not required for embryo development. Furthermore, maternal somatic RdDM, while required for seed development, does not directly influence embryo methylation patterns.

Pollination, Fertilization, and Embryo Development in Southern China Fresh-eating Jujube

To explore the reasons for seed abortion in southern China fresh-eating jujube, improve its reproductive biology, and provide a theoretical basis for the crossbreeding of jujube, we carried out self-pollination and cross-pollination experiments with Ziziphus jujuba Mill. ‘Zhongqiusucui’ as the female parent. We observed the process of pollen tube growth in pistil and embryo development by fluorescence microscopy and paraffin section methods. The results show there were self- and cross-incompatibilities during pollination and fertilization, and there were no significant differences in pollen germination and pollen tube growth between self-pollination and cross-pollination. It took at least 4 hours for pollen and stigma to recognize each other, 6 hours for pollen to germinate on the stigma, and 12 hours for the pollen tube to penetrate the mastoid cells of the stigma. After 48 hours of pollination, the pollen tube reached one third of the style. The pollen tube remained stagnant 72 to 120 hours after pollination, and remained at one third of the stylar canal. Simultaneously, the pollen tubes on the stigma twisted and interacted with each other, and expanded into a spherical shape. A few pollen tubes reached the ovary and completed fertilization. However, some early globular embryos degenerated before forming into globular embryos and resulted in the formation of empty embryo sacs, which leads to seed abortion. In conclusion, the poor pollination and fertilization, and the blocked development of the embryo resulted in seed abortion in Z. jujuba ‘Zhongqiusucui’.