The meiotic topoisomerase VI B subunit (MTOPVIB) is essential for meiotic DNA double-strand break formation in barley (Hordeum vulgare L.)

Apomixis at high resolution: unravelling diplospory in Asteraceae

Apomictic plants are able to produce clonal seeds. This reproductive system allows the one-step fixation of any valuable trait for subsequent generations and would pave the way for a revolution in the agricultural system. Despite this, the introduction of apomixis in sexually reproducing crops has been hampered due to the difficulty in characterizing its genetic regulation. In this study, we described the high-resolution characterization of apomeiosis in the apomictic model species Erigeron annuus, Chondrilla juncea, and Taraxacum officinale. We showed that apomeiosis differs from meiosis in a few critical steps, including homologous chromosome synapsis and segregation during meiosis I. We then compared megasporogenesis in three T. officinale genetic backgrounds, showing that diplospory is superimposed on the sexual pathway without severely altering the expression of crucial meiotic genes. Our findings will contribute to the identification of pivotal players controlling this intriguing asexual reproductive strategy.

The plant early recombinosome: a high security complex to break DNA during meiosis

KEY MESSAGE

The formacion of numerous unpredictable DNA Double Strand Breaks (DSBs) on chromosomes iniciates meiotic recombination. In this perspective, we propose a 'multi-key lock' model to secure the risky but necesary breaks as well as a 'one per pair of cromatids' model for the topoisomerase-like early recombinosome. During meiosis, homologous chromosomes recombine at few sites of crossing-overs (COs) to ensure correct segregation. The initiation of meiotic recombination involves the formation of DNA double strand breaks (DSBs) during prophase I. Too many DSBs are dangerous for genome integrity: if these DSBs are not properly repaired, it could potentially lead to chromosomal fragmentation. Too few DSBs are also problematic: if the obligate CO cannot form between bivalents, catastrophic unequal segregation of univalents lead to the formation of sterile aneuploid spores. Research on the regulation of the formation of these necessary but risky DSBs has recently advanced in yeast, mammals and plants. DNA DSBs are created by the enzymatic activity of the early recombinosome, a topoisomerase-like complex containing SPO11. This opinion paper reviews recent insights on the regulation of the SPO11 cofactors necessary for the introduction of temporally and spatially controlled DSBs. We propose that a 'multi-key-lock' model for each subunit of the early recombinosome complex is required to secure the formation of DSBs. We also discuss the hypothetical implications that the established topoisomerase-like nature of the SPO11 core-complex can have in creating DSB in only one of the two replicated chromatids of early prophase I meiotic chromosomes. This hypothetical 'one per pair of chromatids' DSB formation model could optimize the faithful repair of the self-inflicted DSBs. Each DSB could use three potential intact homologous DNA sequences as repair template: one from the sister chromatid and the two others from the homologous chromosomes.

Wheat Topoisomerase VI Positively Regulates the Biosynthesis of Cuticular Wax and Cutin

Lipophilic cuticles mainly composed of wax mixtures and cutin matrices seal the plant epidermis and control plant development and environmental adaptation. Although cuticle-associated traits have been selected in the breeding of agronomically important cereal bread wheat, the biosynthesis of wheat cuticular wax and cutin remains poorly understood. Herein, wheat topoisomerase VI was characterized as an essential activator of cuticular wax and cutin biosynthesis. Knock-down of wheat TaTOP6A, TaTOP6B, TaRHL1, or TaBIN4 gene encoding component of topoisomerase VI resulted in decreased loads of leaf cuticular wax and cutin, as well as increased leaf cuticle permeability. Moreover, TaCYP86A2 was identified as a key component of the wheat cutin biosynthetic machinery. Reduction of wheat TaCYP86A2 expression led to decreased cutin accumulation and enhanced cuticle permeability. In addition, TaTOP6A, TaTOP6B, TaRHL1, or TaBIN4 was shown to enrich at the promoter regions of the wax biosynthesis gene TaKCS1 and the cutin biosynthesis gene TaCYP86A2. Importantly, chromatin at TaKCS1 and TaCYP86A2 promoters is marked by high nucleosome occupancy and low histone acetylation in TaTOP6A-, TaTOP6B-, TaRHL1-, or TaBIN4-silenced wheat leaves. These results collectively support that wheat topoisomerase VI positively regulates the biosynthesis of cuticular wax and cutin probably via maintaining a permissive chromatin state at TaKCS1 and TaCYP86A2 genes.

Barley stripe mosaic virus-mediated somatic and heritable gene editing in barley (Hordeum vulgare L.)

Genome editing strategies in barley (Hordeum vulgare L.) typically rely on Agrobacterium-mediated genetic transformation for the delivery of required genetic reagents involving tissue culture techniques. These approaches are genotype-dependent, time-consuming, and labor-intensive, which hampers rapid genome editing in barley. More recently, plant RNA viruses have been engineered to transiently express short guide RNAs facilitating CRISPR/Cas9-based targeted genome editing in plants that constitutively express Cas9. Here, we explored virus-induced genome editing (VIGE) based on barley stripe mosaic virus (BSMV) in Cas9-transgenic barley. Somatic and heritable editing in the ALBOSTRIANS gene (CMF7) resulting in albino/variegated chloroplast-defective barley mutants is shown. In addition, somatic editing in meiosis-related candidate genes in barley encoding ASY1 (an axis-localized HORMA domain protein), MUS81 (a DNA structure-selective endonuclease), and ZYP1 (a transverse filament protein of the synaptonemal complex) was achieved. Hence, the presented VIGE approach using BSMV enables rapid somatic and also heritable targeted gene editing in barley.

Crossover interference mechanism: New lessons from plants

Plants are the source of our understanding of several fundamental biological principles. It is well known that Gregor Mendel discovered the laws of Genetics in peas and that maize was used for the discovery of transposons by Barbara McClintock. Plant models are still useful for the understanding of general key biological concepts. In this article, we will focus on discussing the recent plant studies that have shed new light on the mysterious mechanisms of meiotic crossover (CO) interference, heterochiasmy, obligatory CO, and CO homeostasis. Obligatory CO is necessary for the equilibrated segregation of homologous chromosomes during meiosis. The tight control of the different male and female CO rates (heterochiasmy) enables both the maximization and minimization of genome shuffling. An integrative model can now predict these observed aspects of CO patterning in plants. The mechanism proposed considers the Synaptonemal Complex as a canalizing structure that allows the diffusion of a class I CO limiting factor linearly on synapsed bivalents. The coarsening of this limiting factor along the SC explains the interfering spacing between COs. The model explains the observed coordinated processes between synapsis, CO interference, CO insurance, and CO homeostasis. It also easily explains heterochiasmy just considering the different male and female SC lengths. This mechanism is expected to be conserved in other species.

Synthetic apomixis: the beginning of a new era

Apomixis is a process of asexual reproduction that enables plants to bypass meiosis and fertilization to generate clonal seeds that are identical to the maternal genotype. Apomixis has tremendous potential for breeding plants with desired characteristics, given its ability to fix any elite genotype. However, little is known about the origin and dynamics of natural apomictic plant systems. The introgression of apomixis-related genes from natural apomicts has achieved limited success. Therefore, synthetic apomixis, engineered to include apomeiosis, autonomous embryo formation, and autonomous endosperm development, has been proposed as a promising platform to effectuate apomixis in any crop. In this study, we have summarized recent advances in the understanding of synthetic apomixis and discussed the limitations of current synthetic apomixis systems and ways to overcome them.

Novel resistance to the Bymovirus BaMMV established by targeted mutagenesis of the PDIL5-1 susceptibility gene in barley

The Potyviridae are the largest family of plant-pathogenic viruses. Members of this family are the soil-borne bymoviruses barley yellow mosaic virus (BaYMV) and barley mild mosaic virus (BaMMV), which, upon infection of young winter barley seedlings in autumn, can cause yield losses as high as 50%. Resistance breeding plays a major role in coping with these pathogens. However, some viral strains have overcome the most widely used resistance. Thus, there is a need for novel sources of resistance. In ancient landraces and wild relatives of cultivated barley, alleles of the susceptibility factor PROTEIN DISULFIDE ISOMERASE LIKE 5-1 (PDIL5-1) were identified to confer resistance to all known strains of BaYMV and BaMMV. Although the gene is highly conserved throughout all eukaryotes, barley is thus far the only species for which PDIL5-1-based virus resistance has been reported. Whereas introgression by crossing to the European winter barley breeding pool is tedious, time-consuming and additionally associated with unwanted linkage drag, the present study exemplifies an approach to targeted mutagenesis of two barley cultivars employing CRISPR-associated endonuclease technology to induce site-directed mutations similar to those described for PDIL5-1 alleles that render certain landraces resistant. Homozygous primary mutants were produced in winter barley, and transgene-free homozygous M₂ mutants were produced in spring barley. A variety of mutants carrying novel PDIL5-1 alleles were mechanically inoculated with BaMMV, by which all frameshift mutations and certain in-frame mutations were demonstrated to confer resistance to this virus. Under greenhouse conditions, virus-resistant mutants showed no adverse effects in terms of growth and yield.

Identification, characterization, and rescue of CRISPR/Cas9 generated wheat SPO11-1 mutants

Increasing crop yields through plant breeding is time consuming and laborious, with the generation of novel combinations of alleles being limited by chromosomal linkage blocks and linkage-drag. Meiotic recombination is essential to create novel genetic variation via the reshuffling of parental alleles. The exchange of genetic information between homologous chromosomes occurs at crossover (CO) sites but CO frequency is often low and unevenly distributed. This bias creates the problem of linkage-drag in recombination 'cold' regions, where undesirable variation remains linked to useful traits. In plants, programmed meiosis-specific DNA double-strand breaks, catalysed by the SPO11 complex, initiate the recombination pathway, although only ~5% result in the formation of COs. To study the role of SPO11-1 in wheat meiosis, and as a prelude to manipulation, we used CRISPR/Cas9 to generate edits in all three SPO11-1 homoeologues of hexaploid wheat. Characterization of progeny lines shows plants deficient in all six SPO11-1 copies fail to undergo chromosome synapsis, lack COs and are sterile. In contrast, lines carrying a single copy of any one of the three wild-type homoeologues are phenotypically indistinguishable from unedited plants both in terms of vegetative growth and fertility. However, cytogenetic analysis of the edited plants suggests that homoeologues differ in their ability to generate COs and in the dynamics of synapsis. In addition, we show that the transformation of wheat mutants carrying six edited copies of SPO11-1 with the TaSPO11-1B gene, restores synapsis, CO formation, and fertility and hence opens a route to modifying recombination in this agronomically important crop.