Phenylalanine ammonia-lyase and phenolic compounds are related to hybrid lethality in the cross Nicotiana suaveolens×N. tabacum

Morpho-histology of co-occurrence of somatic embryos, shoots, and inflorescences within a callus of Limonium 'Misty Blue'

This is the first attempt to report the co-occurrence of somatic embryos, shoots, and inflorescences and their sequential development from stem cell niches of an individual callus mass through morpho-histological study of any angiosperm. In the presence of a proper auxin/cytokinin combination, precambial stem cells from the middle layer of a compact callus, which was derived from the thin cell layer of the inflorescence rachis of Limonium, expressed the highest level of totipotency and pluripotency and simultaneously developed somatic embryos, shoots, and inflorescences. This study also proposed the concept of programmed cell death during bipolar somatic embryo and unipolar shoot bud pattern formation. The unique feature of this research was the stepwise histological description of in vitro racemose inflorescence development. Remarkably, during the initiation of inflorescence development, either a unipolar structure with open vascular elements or an independent bipolar structure with closed vascular elements were observed. The protocol predicted the production of 6.6 ± 0.24 and 7.4 ± 0.24 somatic embryos and shoots, respectively, from 400 mg of callus, which again multiplied, rooted, and acclimatised. The plants' ploidy level and genetic fidelity were assessed randomly before acclimatisation by flow cytometry and inter simple sequence repeats (ISSR) marker analysis. Finally, the survivability and flower quality of the regenerated plants were evaluated in the field.

Genetic Cause of Hybrid Lethality Observed in Reciprocal Interspecific Crosses between Nicotiana simulans and N. tabacum

Hybrid lethality, a type of postzygotic reproductive isolation, is an obstacle to wide hybridization breeding. Here, we report the hybrid lethality that was observed in crosses between the cultivated tobacco, Nicotiana tabacum (section Nicotiana), and the wild tobacco species, Nicotiana simulans (section Suaveolentes). Reciprocal hybrid seedlings were inviable at 28 °C, and the lethality was characterized by browning of the hypocotyl and roots, suggesting that hybrid lethality is due to the interaction of nuclear genomes derived from each parental species, and not to a cytoplasmic effect. Hybrid lethality was temperature-sensitive and suppressed at 36 °C. However, when hybrid seedlings cultured at 36 °C were transferred to 28 °C, all of them showed hybrid lethality. After crossing between an N. tabacum monosomic line missing one copy of the Q chromosome and N. simulans, hybrid seedlings with or without the Q chromosome were inviable and viable, respectively. These results indicated that gene(s) on the Q chromosome are responsible for hybrid lethality and also suggested that N. simulans has the same allele at the Hybrid Lethality A1 (HLA1) locus responsible for hybrid lethality as other species in the section Suaveolentes. Haplotype analysis around the HLA1 locus suggested that there are at least six and two haplotypes containing Hla1-1 and hla1-2 alleles, respectively, in the section Suaveolentes.

Understanding and overcoming hybrid lethality in seed and seedling stages as barriers to hybridization and gene flow

Hybrid lethality is a type of reproductive isolation barrier observed in two developmental stages, hybrid embryos (hybrid seeds) and hybrid seedlings. Hybrid lethality has been reported in many plant species and limits distant hybridization breeding including interspecific and intergeneric hybridization, which increases genetic diversity and contributes to produce new germplasm for agricultural purposes. Recent studies have provided molecular and genetic evidence suggesting that underlying causes of hybrid lethality involve epistatic interaction of one or more loci, as hypothesized by the Bateson-Dobzhansky-Muller model, and effective ploidy or endosperm balance number. In this review, we focus on the similarities and differences between hybrid seed lethality and hybrid seedling lethality, as well as methods of recovering seed/seedling activity to circumvent hybrid lethality. Current knowledge summarized in our article will provides new insights into the mechanisms of hybrid lethality and effective methods for circumventing hybrid lethality.

The hybrid lethality of interspecific F1 hybrids of Nicotiana: a clue to understanding hybrid inviability-a major obstacle to wide hybridization and introgression breeding of plants

Reproductive isolation poses a major obstacle to wide hybridization and introgression breeding of plants. Hybrid inviability in the postzygotic isolation barrier inevitably reduces hybrid fitness, consequently causing hindrances in the establishment of novel genotypes from the hybrids among genetically divergent parents. The idea that the plant immune system is involved in the hybrid problem is applicable to the intra- and/or interspecific hybrids of many different taxa. The lethality characteristics and expression profile of genes associated with the hypersensitive response of the hybrids, along with the suppression of causative genes, support the deleterious epistatic interaction of parental NB-LRR protein genes, resulting in aberrant hyper-immunity reactions in the hybrid. Moreover, the cellular, physiological, and biochemical reactions observed in hybrid cells also corroborate this hypothesis. However, the difference in genetic backgrounds of the respective hybrids may contribute to variations in lethality phenotypes among the parental species combinations. The mixed state in parental components of the chaperone complex (HSP90-SGT1-RAR1) in the hybrid may also affect the hybrid inviability. This review article discusses the facts and hypothesis regarding hybrid inviability, alongside the findings of studies on the hybrid lethality of interspecific hybrids of the genus Nicotiana. A possible solution for averting the hybrid problem has also been scrutinized with the aim of improving the wide hybridization and introgression breeding program in plants.

Antiviral agent fTDP stimulates the SA signaling pathway and enhances tobacco defense against tobacco mosaic virus

TEER-decreasing protein (TDP) from Flammulina velutipes was antiviral resource against tobacco mosaic virus (TMV). However, the resistance mechanisms have not been clarified. In this study, the fTDP (fusion teer-decreasing protein), obtained by prokaryotic fusion expression system, exhibited obvious protective efficacy against TMV and significantly suppressed the reproduction of TMV in tobacco. Transcriptomics and proteomics analysis showed that fTDP may interact with a receptor, activate the mitogen-activated protein kinase (MAPK) pathway and NB-ARC and increase the content of reactive oxygen species (ROS) and salicylic acid (SA), which promoted the hypersensitive response (HR) and system acquired resistance (SAR). SAR caused increased expression of catalase (CAT), pathogenesis-related protein 1 (PR1), phenylalanine ammonia lyase (PAL) and other proteins involved in pathogen defense, such as chalcone-dihydroflavone isomerase (CHI) and cytochrome P450. In conclusion, SAR was induced by fTDP to protect tobacco from TMV infection and alleviate the symptoms caused by the virus. The study provided a theoretical basis for the application of the TDP protein, which may represent a potential biopesticide.

Temperature-dependent sugar accumulation in interspecific Capsicum F1 plants showing hybrid weakness

In plants, F₁ hybrids showing hybrid weakness exhibit weaker growth than their parents. The phenotypes of hybrid weakness are often suppressed at certain temperatures. However, it is unclear whether hybrid weakness in Capsicum annuum × C. chinense is temperature-dependent or not. Our study showed that Capsicum hybrid weakness was suppressed at 30 and 35 °C and was induced at 15, 20, and 25 °C. Moreover, we investigated the time course of hybrid weakness in cell death, metabolite content, and gene expression in leaves of plants transferred to 20 °C after growing at 30 °C for 21 days. The expression of pathogen defense-related genes was upregulated at 1 day after transfer to 20 °C (DAT). Cell death was detected at 7 DAT, plant growth had almost stopped since 14 DAT, and sugars were accumulated at 42 DAT in hybrid plants. The study revealed that some sugar transporter genes, which had been upregulated since 7 DAT, were involved in sugar accumulation in Capsicum hybrid weakness. Thus, our results demonstrated that gene expression changes occur first, followed by physiological and morphological changes after induction of hybrid weakness. These responses observed in this study in Capsicum hybrid weakness are likely to be owed to plant defense responses-like reactions.

Genetic Mapping of the HLA1 Locus Causing Hybrid Lethality in Nicotiana Interspecific Hybrids

Hybrid lethality, a postzygotic mechanism of reproductive isolation, is a phenomenon that causes the death of F₁ hybrid seedlings. Hybrid lethality is generally caused by the epistatic interaction of two or more loci. In the genus Nicotiana, N. debneyi has the dominant allele Hla1-1 at the HLA1 locus that causes hybrid lethality in F₁ hybrid seedlings by interaction with N. tabacum allele(s). Here, we mapped the HLA1 locus using the F₂ population segregating for the Hla1-1 allele derived from the interspecific cross between N. debneyi and N. fragrans. To map HLA1, several DNA markers including random amplified polymorphic DNA, amplified fragment length polymorphism, and simple sequence repeat markers, were used. Additionally, DNA markers were developed based on disease resistance gene homologs identified from the genome sequence of N. benthamiana. Linkage analysis revealed that HLA1 was located between two cleaved amplified polymorphic sequence markers Nb14-CAPS and NbRGH1-CAPS at a distance of 10.8 and 10.9 cM, respectively. The distance between these markers was equivalent to a 682 kb interval in the genome sequence of N. benthamiana.

Analysis of the possible cytogenetic mechanism for overcoming hybrid lethality in an interspecific cross between Nicotiana suaveolens and Nicotiana tabacum

Hybrid lethality is a type of reproductive isolation in which hybrids die before maturation, due to the interaction between the two causative genes derived from each of the hybrid parents. The interspecific hybrid of Nicotiana suaveolens × Nicotiana tabacum is a model plant used in studies on hybrid lethality. While most of the progeny produced from such a cross die, some individuals grow normally and mature. Separately, a technique for producing mature hybrids by artificial culture has been developed. However, the mechanism by which hybrids overcome lethality, either spontaneously or by artificial culture, remains unclear. In the present study, we found that some hybrids that overcome lethality, either spontaneously or by artificial culture, lack the distal part of the Q chromosome, a region that includes the gene responsible for lethality. Quantitative polymerase chain reaction results suggested that the distal deletion of the Q chromosome, detected in some hybrid seedlings that overcome lethality, is caused by reciprocal translocations between homoeologous chromosomes. The results showed that chromosomal instability during meiosis in amphidiploid N. tabacum as well as during artificial culturing of hybrid seedlings is involved in overcoming hybrid lethality in interspecific crosses of the genus Nicotiana.