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The discovery of the CRISPR/Cas genome-editing system has revolutionized our understanding of the plant genome. CRISPR/Cas has been used for over a decade to modify plant genomes for the study of specific genes and biosynthetic pathways as well as to speed up breeding in many plant species, including both model and non-model crops. Although the CRISPR/Cas system is very efficient for genome editing, many bottlenecks and challenges slow down further improvement and applications. In this review we discuss the challenges that can occur during tissue culture, transformation, regeneration, and mutant detection. We also review the opportunities provided by new CRISPR platforms and specific applications related to gene regulation, abiotic and biotic stress response improvement, and de novo domestication of plants., Competing Interests: Declaration of interests The authors declare no conflicts of interest., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Industrial chicory ( Cichorium intybus var. sativum ) and witloof ( C. intybus var. foliosum ) are crops with an important economic value, mainly cultivated for inulin production and as a leafy vegetable, respectively. Both crops are rich in nutritionally relevant specialized metabolites with beneficial effects for human health. However, their bitter taste, caused by the sesquiterpene lactones (SLs) produced in leaves and taproot, limits wider applications in the food industry. Changing the bitterness would thus create new opportunities with a great economic impact. Known genes encoding enzymes involved in the SL biosynthetic pathway are GERMACRENE A SYNTHASE (GAS), GERMACRENE A OXIDASE (GAO) , COSTUNOLIDE SYNTHASE (COS) and KAUNIOLIDE SYNTHASE ( KLS ). In this study, we integrated genome and transcriptome mining to further unravel SL biosynthesis. We found that C. intybus SL biosynthesis is controlled by the phytohormone methyl jasmonate (MeJA). Gene family annotation and MeJA inducibility enabled the pinpointing of candidate genes related with the SL biosynthetic pathway. We specifically focused on members of subclade CYP71 of the cytochrome P450 family. We verified the biochemical activity of 14 C . intybus CYP71 enzymes transiently produced in Nicotiana benthamiana and identified several functional paralogs for each of the GAO , COS and KLS genes, pointing to redundancy in and robustness of the SL biosynthetic pathway. Gene functionality was further analyzed using CRISPR/Cas9 genome editing in C. intybus . Metabolite profiling of mutant C. intybus lines demonstrated a successful reduction in SL metabolite production. Together, this study increases our insights into the C. intybus SL biosynthetic pathway and paves the way for the engineering of C. intybus bitterness., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 De Bruyn, Ruttink, Lacchini, Rombauts, Haegeman, De Keyser, Van Poucke, Desmet, Jacobs, Eeckhaut, Goossens and Van Laere.)

Industrial chicory ( Cichorium intybus var. sativum ) is a biannual crop mostly cultivated for extraction of inulin, a fructose polymer used as a dietary fiber. F1 hybrid breeding is a promising breeding strategy in chicory but relies on stable male sterile lines to prevent self-pollination. Here, we report the assembly and annotation of a new industrial chicory reference genome. Additionally, we performed RNA-Seq on subsequent stages of flower bud development of a fertile line and two cytoplasmic male sterile (CMS) clones. Comparison of fertile and CMS flower bud transcriptomes combined with morphological microscopic analysis of anthers, provided a molecular understanding of anther development and identified key genes in a range of underlying processes, including tapetum development, sink establishment, pollen wall development and anther dehiscence. We also described the role of phytohormones in the regulation of these processes under normal fertile flower bud development. In parallel, we evaluated which processes are disturbed in CMS clones and could contribute to the male sterile phenotype. Taken together, this study provides a state-of-the-art industrial chicory reference genome, an annotated and curated candidate gene set related to anther development and male sterility as well as a detailed molecular timetable of flower bud development in fertile and CMS lines., Competing Interests: Authors OM and CN are employed by Chicoline, a division of Cosucra Groupe Warcoing. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. This study received funding from Cosucra Groupe Warcoing S.A. Belgium as scholarship to EW and JV. The funder had the following involvement in the study: Cosucra Groupe Warcoing S.A. Belgium was partner in the DGA project Grant D31-1221 with the Walloon region DGARNE Belgium that initiated genome sequencing. ND and OM, affiliated with Cosucra Groupe Warcoing, created and provided the inbred line L8001., (Copyright © 2023 Waegneer, Rombauts, Baert, Dauchot, De Keyser, Eeckhaut, Haegeman, Liu, Maudoux, Notté, Staelens, Van der Veken, Van Laere and Ruttink.)

Celeriac F 1 hybrid seed production is currently complicated due to the instability of cytoplasmic male sterile lines. To develop alternative alloplasmic CMS lines, an asymmetric protoplast fusion and hybrid screening methodology was established. Celeriac suspension cells protoplasts were used as the acceptor and carrot, coriander and white celery mesophyll protoplasts as the donor for protoplast fusion experiments. Acceptor cytoplasmic inheritance was inhibited by iodoacetamide treatment and donor nuclear genome inheritance was prevented by UV exposure. Protoplasts were selectively stained and fused using electroporation and polyethylene glycol, and candidate hybrid shoots were obtained. One chloroplast and three mitochondrial markers that could distinguish acceptor and donors organelles were used to characterize over 600 plants obtained after fusion events, without identifying any cybrid. In order to increase the testing efficiency a high number of micro plantlets were pooled and hence the presence of the carrot specific Atp1 marker in one of the pooled samples was detected. We demonstrated that fusion took place between celeriac and a carrot indicating that the creation of viable hybrids is possible although at a very low frequency. These findings open the path for new cytoplasmic hybridization and the isolation of novel CMS lines of celeriac.

Cichorium intybus var. foliosum (witloof) is an economically important crop with a high nutritional value thanks to many specialized metabolites, such as polyphenols and terpenoids. However, witloof plants are rich in sesquiterpene lactones (SL) which are important for plant defense but also impart a bitter taste, thus limiting industrial applications. Inactivating specific genes in the SL biosynthesis pathway could lead to changes in the SL metabolite content and result in altered bitterness. In this study, a CRISPR/Cas9 genome editing workflow was implemented for witloof, starting with polyethylene glycol (PEG) mediated protoplast transfection for CRISPR/Cas9 vector delivery, followed by whole plant regeneration and mutation analysis. Protoplast transfection efficiencies ranged from 20 to 26 %. A CRISPR/Cas9 vector targeting the first exon of the phytoene desaturase ( CiPDS ) gene was transfected into witloof protoplasts and resulted in the knockout of CiPDS , giving rise to an albino phenotype in 23% of the regenerated plants. Further implementing our protocol, the SL biosynthesis pathway genes germacrene A synthase (GAS), germacrene A oxidase (GAO) , and costunolide synthase (COS) were targeted in independent experiments. Highly multiplex (HiPlex) amplicon sequencing of the genomic target loci revealed plant mutation frequencies of 27.3, 42.7, and 98.3% in regenerated plants transfected with a CRISPR/Cas9 vector targeting CiGAS, CiGAO , and CiCOS , respectively. We observed different mutation spectra across the loci, ranging from consistently the same +1 nucleotide insertion in CiCOS across independent mutated lines, to a complex set of 20 mutation types in CiGAO across independent mutated lines. These results demonstrate a straightforward workflow for genome editing based on transfection and regeneration of witloof protoplasts and subsequent HiPlex amplicon sequencing. Our CRISPR/Cas9 workflow can enable gene functional research and faster incorporation of novel traits in elite witloof lines in the future, thus facilitating the development of novel industrial applications for witloof., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2020 De Bruyn, Ruttink, Eeckhaut, Jacobs, De Keyser, Goossens and Van Laere.)

Chrysanthemum × morifolium protoplasts were isolated and regenerated to assess possible protoclonal variation in the regenerants. After a preliminary screening of the potential of different regeneration systems for protoplast regeneration, we produced a series of cut chrysanthemum 'Arjuna' leaf protoplast regenerants through liquid culture. Regenerants (54) were vegetatively propagated and grown under a commercial production system in 2 different seasons. All screened regenerants were significantly affected with regard to either flower number, flower size, flower weight, leaf weight, stalk weight, or plant size. A significant plant size reduction in 43/52 and 48/49 regenerants for both seasons was the most recorded effect. Also a reduction in flowering induction time up to 10 days, altered flower types and colors were observed. Differences between growing seasons were notable. Possible molecular backgrounds including genome size variation and commercial applications in breeding of chrysanthemum are discussed., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2020 Eeckhaut, Van Houtven, Bruznican, Leus and Van Huylenbroeck.)

Rhizobium rhizogenes infects and transforms a wide range of plant species. It thereby introduces new genes located on transfer-DNA of the root inducing plasmid (pRi) into the plant genome and one of its abilities is to alter the host root system. Explants from pRi transformed roots from Sinningia speciosa were regenerated to create naturally transgenic Ri lines. The presence of rol and aux genes in the Ri lines was linked with altered growth characteristics: shorter peduncles, wrinkled leaves, delayed flowering and enhanced root growth. The potential of Ri lines for breeding was evaluated through consecutive backcrossing with the original host genotype. The progeny of reciprocal crosses showed non-Mendelian inheritance suggesting partial transmission of the of the aux and rol genes. The typical Ri phenotype observed in the primary Ri line was partially inherited. These results revealed that the Ri phenotype is a complex trait influenced by the genetic background of the Ri line., (Copyright © 2020 Desmet, Dhooghe, De Keyser, Quataert, Eeckhaut, Van Huylenbroeck and Geelen.)

Cultivated for the crispy petioles and round, fleshy, and flavored hypocotyl celery and celeriac have over two centuries of breeding history in Europe. In this review paper we summarized the most recent advances touching when necessary the historical context of celery and celeriac breeding. In the post genomic era of research, the genome sequence of celery is only partially available. We comprised however in this paper the most important aspects of celery genetics that are available today and have applicability in celery modern cultivars development. We discussed the problems and traits that drive the main celery and celeriac breeding goals, like hybrid seed production, disease resistance, and interesting enlarged hypocotyl and petiole characteristics. Besides the classical breeding traits we covered the potential of integration of existing cultivars as sources for consumer oriented traits like nutraceuticals and health promoting substances. Sustainability is a subject that is continuously growing in popularity and we looked at the genetic base of celery and celeriac that makes them sources for abiotic stress resistance and candidates for phytoremediation. We explored the fundamental concepts gained in various fields of celery and related species research, as resources for future improvement of celery and celeriac germplasm. We forecast what the next years will bring to Apium breeding., (Copyright © 2020 Bruznican, De Clercq, Eeckhaut, Van Huylenbroeck and Geelen.)

Karyotype analysis and FISH mapping using 45S rDNA sequences on 6 economically important plant species Anthuriumandraeanum Linden ex André, 1877, Monsteradeliciosa Liebmann, 1849, Philodendronscandens Koch & Sello, 1853, Spathiphyllumwallisii Regel, 1877, Syngoniumauritum (Linnaeus, 1759) Schott, 1829 and Zantedeschiaelliottiana (Knight, 1890) Engler, 1915 within the monocotyledonous family Araceae (aroids) were performed. Chromosome numbers varied between 2n=2x=24 and 2n=2x=60 and the chromosome length varied between 15.77 µm and 1.87 µm. No correlation between chromosome numbers and genome sizes was observed for the studied genera. The chromosome formulas contained only metacentric and submetacentric chromosomes, except for Philodendronscandens in which also telocentric and subtelocentric chromosomes were observed. The highest degree of compaction was calculated for Spathiphyllumwallisii (66.49Mbp/µm). B-chromosome-like structures were observed in Anthuriumandraeanum. Their measured size was 1.87 times smaller than the length of the shortest chromosome. After FISH experiments, two 45S rDNA sites were observed in 5 genera. Only in Zantedeschiaelliottiana, 4 sites were seen. Our results showed clear cytogenetic differences among genera within Araceae, and are the first molecular cytogenetics report for these genera. These chromosome data and molecular cytogenetic information are useful in aroid breeding programmes, systematics and evolutionary studies.

In this review we focus on recent progress in protoplast regeneration, symmetric and asymmetric hybridization and novel technology developments. Regeneration of new species and improved culture techniques opened new horizons for practical breeding in a number of crops. The importance of protoplast sources and embedding systems is discussed. The study of reactive oxygen species effects and DNA (de)condensation, along with thorough phytohormone monitoring, are in our opinion the most promising research topics in the further strive for rationalization of protoplast regeneration. Following, fusion and fragmentation progress is summarized. Genomic, transcriptomic and proteomic studies have led to better insights in fundamental processes such as cell wall formation, cell development and chromosome rearrangements in fusion products, whether or not obtained after irradiation. Advanced molecular screening methods of both genome and cytoplasmome facilitate efficient screening of both symmetric and asymmetric fusion products. We expect that emerging technologies as GISH, high resolution melting and next generation sequencing will pay major contributions to our insights of genome creation and stabilization, mainly after asymmetric hybridization. Finally, we demonstrate agricultural valorization of somatic hybridization through enumerating recent introgression of diverse traits in a number of commercial crops.

Key Message: A standard method has been developed with which we are able to fully regenerate protoplasts of different Cichorium species. For the first time, endive protoplasts have been regenerated into plantlets. Protoplast regeneration is essential for somatic hybridizations. In this study, a standard method for plantlet regeneration from Cichorium protoplasts was developed. We evaluated the effect of the low melting point agarose (LMPA) bead technique on the regeneration capacity of protoplasts of seven C. intybus and four C. endivia genotypes. The LMPA bead technique was more efficient than culture in liquid or solid medium and allowed us to obtain plating efficiencies up to 4.9 % in C. intybus genotypes and efficiencies of up to 0.7 % in C. endivia genotypes. Moreover, the LMPA bead technique offers great advantages over liquid and solid culture systems: the media can be readily refreshed, protoplasts can be monitored separately, and microcalli can easily be removed from the beads. This increased efficiency was observed for all of the 11 Cichorium genotypes tested. Shoot formation was induced more efficiently when using 0.5 mg l(-1) indole-3-acetic acid-enriched medium (up to 87.5 % of the protoplast-derived calli started shoot development) compared to 1-naphthaleneacetic acid-enriched medium. The LMPA bead technique optimized in this study enabled for the first time the full plantlet regeneration from protoplasts of C. endivia genotypes and increased the protoplast regenerating ability in other Cichorium species. This fine-tuned LMPA bead technique can therefore be applied for protoplast regeneration after protoplast fusions of the genus Cichorium.

Methods for in vitro initiation and multiplication and general culture practices of Rhododendron are presented. Also acclimatization procedures are described. Protocols for callus, shoot, and root induction are described. Several protocols for breeding applications are highlighted in more detail.

The genome sizes of a Begonia collection comprising 37 species and 23 hybrids of African, Asiatic, Middle American, and South American origin were screened using flow cytometry. Within the collection, 1C values varied between 0.23 and 1.46 pg DNA. Genome sizes were, in most cases, not positively correlated with chromosome number, but with pollen size. A 12-fold difference in mean chromosome size was found between the genotypes with the largest and smallest chromosomes. In general, chromosomes from South American genotypes were smaller than chromosomes of African, Asian, or Middle American genotypes, except for B. boliviensis and B. pearcei. Cytological chromosome studies in different genotypes showed variable chromosome numbers, length, width, and total chromosome volume, which confirmed the diversity in genome size. Large secondary constrictions were present in several investigated genotypes. These data show that chromosome number and structure exhibit a great deal of variation within the genus Begonia, and likely help to explain the large number of taxa found within the genus.

To introduce yellow, blue or orange pigmentation in the Rhododendron subgenus Tsutsusi (including R. simsii hybrids) by classical breeding, intersubgeneric pollination experiments involving Hymenanthes, Pentanthera and Rhododendron (including Vireya) species and hybrids were started. Prefertilization research by pollen tube staining revealed a different behaviour between bilateral crosses. Ovule culture was applied as a means to save a substantially larger amount of hybrid embryos from abortion. Though flower/seed capsule abortion eliminated many crosses in an early developmental stage, ovules of many crosses could be initiated in vitro on various time intervals after pollination. Germination was achieved on basal Rhododendron medium + 50 mg/l GA3. Green seedlings were obtained after Tsutsusi x Hymenanthes, Hymenanthes x Tsutsusi, Tsutsusi x Rhododendron, Rhododendron x Tsutsusi and Vireya x Tsutsusi pollination. Many seedlings have little growth vigour and next to green seedlings albino seedlings occur frequently, depending on the pollination type. The first true leaves of many assumed hybrids obviously lack chlorophyll. The ploidy level of Vireya x Tsutsusi and Tsutsusi x Pentanthera seedlings is intermediate, which strongly suggests their hybrid origin. Molecular analysis through SSR confirmed the interspecific charachter of seedlings in half of the crosses tested.