A CENH3 mutation promotes meiotic exit and restores fertility in SMG7-deficient Arabidopsis

Ultraviolet attenuates centromere-mediated meiotic genome stability and alters gametophytic ploidy consistency in flowering plants

Ultraviolet (UV) radiation influences development and genome stability in organisms; however, its impact on meiosis, a special cell division essential for the delivery of genetic information across generations in eukaryotes, has not yet been elucidated. In this study, by performing cytogenetic studies, we reported that UV radiation does not damage meiotic chromosome integrity but attenuates centromere-mediated chromosome stability and induces unreduced gametes in Arabidopsis thaliana. We showed that functional centromere-specific histone 3 (CENH3) is required for obligate crossover formation and plays a role in the protection of sister chromatid cohesion under UV stress. Moreover, we found that UV specifically alters the orientation and organization of spindles and phragmoplasts at meiosis II, resulting in meiotic restitution and unreduced gametes. We determined that UV-induced meiotic restitution does not rely on the UV Resistance Locus8-mediated UV perception and the Tapetal Development and Function1- and Aborted Microspores-dependent tapetum development, but possibly occurs via altered JASON function and downregulated Parallel Spindle1. This study provides evidence that UV radiation influences meiotic genome stability and gametophytic ploidy consistency in flowering plants.

Heat stress impairs centromere structure and segregation of meiotic chromosomes in Arabidopsis

Heat stress is a major threat to global crop production, and understanding its impact on plant fertility is crucial for developing climate-resilient crops. Despite the known negative effects of heat stress on plant reproduction, the underlying molecular mechanisms remain poorly understood. Here, we investigated the impact of elevated temperature on centromere structure and chromosome segregation during meiosis in Arabidopsis thaliana. Consistent with previous studies, heat stress leads to a decline in fertility and micronuclei formation in pollen mother cells. Our results reveal that elevated temperature causes a decrease in the amount of centromeric histone and the kinetochore protein BMF1 at meiotic centromeres with increasing temperature. Furthermore, we show that heat stress increases the duration of meiotic divisions and prolongs the activity of the spindle assembly checkpoint during meiosis I, indicating an impaired efficiency of the kinetochore attachments to spindle microtubules. Our analysis of mutants with reduced levels of centromeric histone suggests that weakened centromeres sensitize plants to elevated temperature, resulting in meiotic defects and reduced fertility even at moderate temperatures. These results indicate that the structure and functionality of meiotic centromeres in Arabidopsis are highly sensitive to heat stress, and suggest that centromeres and kinetochores may represent a critical bottleneck in plant adaptation to increasing temperatures.

The structure, function, and evolution of plant centromeres

Centromeres are essential regions of eukaryotic chromosomes responsible for the formation of kinetochore complexes, which connect to spindle microtubules during cell division. Notably, although centromeres maintain a conserved function in chromosome segregation, the underlying DNA sequences are diverse both within and between species and are predominantly repetitive in nature. The repeat content of centromeres includes high-copy tandem repeats (satellites), and/or specific families of transposons. The functional region of the centromere is defined by loading of a specific histone 3 variant (CENH3), which nucleates the kinetochore and shows dynamic regulation. In many plants, the centromeres are composed of satellite repeat arrays that are densely DNA methylated and invaded by centrophilic retrotransposons. In some cases, the retrotransposons become the sites of CENH3 loading. We review the structure of plant centromeres, including monocentric, holocentric, and metapolycentric architectures, which vary in the number and distribution of kinetochore attachment sites along chromosomes. We discuss how variation in CENH3 loading can drive genome elimination during early cell divisions of plant embryogenesis. We review how epigenetic state may influence centromere identity and discuss evolutionary models that seek to explain the paradoxically rapid change of centromere sequences observed across species, including the potential roles of recombination. We outline putative modes of selection that could act within the centromeres, as well as the role of repeats in driving cycles of centromere evolution. Although our primary focus is on plant genomes, we draw comparisons with animal and fungal centromeres to derive a eukaryote-wide perspective of centromere structure and function.

Haploid rhapsody: the molecular and cellular orchestra of in vivo haploid induction in plants

In planta haploid induction (HI), which reduces the chromosome number in the progeny after fertilization, has garnered increasing attention for its significant potential in crop breeding and genetic research. Despite the identification of several natural and synthetic HI systems in different plant species, the molecular and cellular mechanisms underlying these HI systems remain largely unknown. This review synthesizes the current understanding of HI systems in plants (with a focus on genes and molecular mechanisms involved), including the molecular and cellular interactions which orchestrate the HI process. As most HI systems can function across taxonomic boundaries, we particularly discuss the evidence for conserved mechanisms underlying the process. These include mechanisms involved in preserving chromosomal integrity, centromere function, gamete communication and/or fusion, and maintenance of karyogamy. While significant discoveries and advances on haploid inducer systems have arisen over the past decades, we underscore gaps in understanding and deliberate on directions for further research for a more comprehensive understanding of in vivo HI processes in plants.

ZmTDM1 encodes a tetratricopeptide repeat domain protein and is required for meiotic exit in maize

Elaborate cell-cycle control must be adopted to ensure the continuity of the meiotic second division and termination after that. Despite its importance, however, the genetic controls underlying the meiotic cell cycle have not been reported in maize. Here, we characterized a meiotic cell-cycle controller ZmTDM1, which is a homolog of Arabidopsis TDM1 and encodes a canonical tetratricopeptide repeat domain protein in maize. The Zmtdm1 homozygous plants exhibited complete male sterility and severe female abortion. In Zmtdm1 mutants, cell-cycle progression was almost identical to that of wild type from leptotene to anaphase II. However, chromosomes in the tetrad failed meiotic termination at the end of the second division and underwent additional divisions in succession without DNA replication, reducing the ploidy to less than haploid in the product. In addition, two ZmTDM1-like homologs (ZmTDML1 and ZmTDML2) were not functional in meiotic cell-cycle control. Moreover, ZmTDM1 interacted with RING-type E3 ubiquitin ligase, revealing that it acts as a subunit of the APC/C E3 ubiquitin ligase complex. Overall, our results identified a regulator of meiotic cell cycle in maize and demonstrated that ZmTDM1 is essential for meiotic exit after meiosis II.

Plant chromosome engineering - past, present and future

Spontaneous chromosomal rearrangements (CRs) play an essential role in speciation, genome evolution and crop domestication. To be able to use the potential of CRs for breeding, plant chromosome engineering was initiated by fragmenting chromosomes by X-ray irradiation. With the rise of the CRISPR/Cas system, it became possible to induce double-strand breaks (DSBs) in a highly efficient manner at will at any chromosomal position. This has enabled a completely new level of predesigned chromosome engineering. The genetic linkage between specific genes can be broken by inducing chromosomal translocations. Natural inversions, which suppress genetic exchange, can be reverted for breeding. In addition, various approaches for constructing minichromosomes by downsizing regular standard A or supernumerary B chromosomes, which could serve as future vectors in plant biotechnology, have been developed. Recently, a functional synthetic centromere could be constructed. Also, different ways of genome haploidization have been set up, some based on centromere manipulations. In the future, we expect to see even more complex rearrangements, which can be combined with previously developed engineering technologies such as recombinases. Chromosome engineering might help to redefine genetic linkage groups, change the number of chromosomes, stack beneficial genes on mini cargo chromosomes, or set up genetic isolation to avoid outcrossing.

Single-Cell Transcriptome Atlas and Regulatory Dynamics in Developing Cotton Anthers

Plant anthers are composed of different specialized cell types with distinct roles in plant reproduction. High temperature (HT) stress causes male sterility, resulting in crop yield reduction. However, the spatial expression atlas and regulatory dynamics during anther development and in response to HT remain largely unknown. Here, the first single-cell transcriptome atlas and chromatin accessibility survey in cotton anther are established, depicting the specific expression and epigenetic landscape of each type of cell in anthers. The reconstruction of meiotic cells, tapetal cells, and middle layer cell developmental trajectories not only identifies novel expressed genes, but also elucidates the precise degradation period of middle layer and reveals a rapid function transition of tapetal cells during the tetrad stage. By applying HT, heterogeneity in HT response is shown among cells of anthers, with tapetal cells responsible for pollen wall synthesis are most sensitive to HT. Specifically, HT shuts down the chromatin accessibility of genes specifically expressed in the tapetal cells responsible for pollen wall synthesis, such as QUARTET 3 (QRT3) and CYTOCHROME P450 703A2 (CYP703A2), resulting in a silent expression of these genes, ultimately leading to abnormal pollen wall and male sterility. Collectively, this study provides substantial information on anthers and provides clues for heat-tolerant crop creation.

Cytological and transcriptomic analyses provide insights into the pollen fertility of synthetic allodiploid Brassica juncea hybrids

KEY MESSAGE

Imbalanced chromosomes and cell cycle arrest, along with down-regulated genes in DNA damage repair and sperm cell differentiation, caused pollen abortion in synthetic allodiploid Brassica juncea hybrids. Interspecific hybridization is considered to be a major pathway for species formation and evolution in angiosperms, but the occurrence of pollen abortion in the hybrids is common, prompting us to recheck male gamete development in allodiploid hybrids after the initial combination of different genomes. Here, we investigated the several key meiotic and mitotic events during pollen development using the newly synthesised allodiploid B. juncea hybrids (AB, 2n = 2× = 18) as a model system. Our results demonstrated the partial synapsis and pairing of non-homologous chromosomes concurrent with chaotic spindle assembly, affected chromosome assortment and distribution during meiosis, which finally caused difference in genetic constitution amongst the final tetrads. The mitotic cell cycle arrest during microspore development resulted in the production of anucleate pollen cells. Transcription analysis showed that sets of key genes regulating cyclin (CYCA1;2 and CYCA2;3), DNA damage repair (DMC1, NBS1 and MMD1), and ubiquitin-proteasome pathway (SINAT4 and UBC) were largely downregulated at the early pollen meiosis stages, and those genes involved in sperm cell differentiation (DUO1, PIRL1, PIRL9 and LBD27) and pollen wall synthesis (PME48, VGDH11 and COBL10) were mostly repressed at the late pollen mitosis stages in the synthetic allodiploid B. juncea hybrids (AB). In conclusion, this study elucidated the related mechanisms affecting pollen fertility during male gametophyte development at the cytological and transcriptomic levels in the synthetic allodiploid B. juncea hybrids.

Female meiotic drive in plants: mechanisms and dynamics

Female meiosis is fundamentally asymmetric, creating an arena for genetic elements to compete for inclusion in the egg to maximize their transmission. Centromeres, as mediators of chromosomal segregation, are prime candidates to evolve via 'female meiotic drive'. According to the centromere-drive model, the asymmetry of female meiosis ignites a coevolutionary arms race between selfish centromeres and kinetochore proteins, the by-product of which is accelerated sequence divergence. Here, I describe and compare plant models that have been instrumental in uncovering the mechanistic basis of female meiotic drive (maize) and the dynamics of active selfish centromeres in nature (monkeyflowers). Then, I speculate on the mechanistic basis of drive in monkeyflowers, discuss how centromere strength influences chromosomal segregation in plants, and describe new insights into the evolution of plant centromeres.

High temperature increases centromere-mediated genome elimination frequency and enhances haploid induction in Arabidopsis

Double haploid production is the most effective way to create true-breeding lines in a single generation. In Arabidopsis, haploid induction via mutation of the centromere-specific histone H3 (cenH3) has been shown when the mutant is outcrossed to the wild-type, and the wild-type genome remains in the haploid progeny. However, factors that affect haploid induction are still poorly understood. Here, we report that a mutant of the cenH3 assembly factor Kinetochore Null2 (KNL2) can be used as a haploid inducer when pollinated by the wild-type. We discovered that short-term temperature stress of the knl2 mutant increased the efficiency of haploid induction 10-fold. We also demonstrated that a point mutation in the CENPC-k motif of KNL2 is sufficient to generate haploid-inducing lines, suggesting that haploid-inducing lines in crops can be identified in a naturally occurring or chemically induced mutant population, avoiding the generic modification (GM) approach at any stage. Furthermore, a cenh3-4 mutant functioned as a haploid inducer in response to short-term heat stress, even though it did not induce haploids under standard conditions. Thus, we identified KNL2 as a new target gene for the generation of haploid-inducer lines and showed that exposure of centromeric protein mutants to high temperature strongly increases their haploid induction efficiency.

A complex role of Arabidopsis CDKD;3 in meiotic progression and cytokinesis

Meiosis is a specialized cell division that halves the number of chromosomes in two consecutive rounds of chromosome segregation. In angiosperm plants is meiosis followed by mitotic divisions to form rudimentary haploid gametophytes. In Arabidopsis, termination of meiosis and transition to gametophytic development are governed by TDM1 and SMG7 that mediate inhibition of translation. Mutants deficient in this mechanism do not form tetrads but instead undergo multiple cycles of aberrant nuclear divisions that are likely caused by the failure to downregulate cyclin dependent kinases during meiotic exit. A suppressor screen to identify genes that contribute to meiotic exit uncovered a mutation in cyclin-dependent kinase D;3 (CDKD;3) that alleviates meiotic defects in smg7 deficient plants. The CDKD;3 deficiency prevents aberrant meiotic divisions observed in smg7 mutants or delays their onset after initiation of cytokinesis, which permits formation of functional microspores. Although CDKD;3 acts as an activator of cyclin-dependent kinase A;1 (CDKA;1), the main cyclin dependent kinase that regulates meiosis, cdkd;3 mutation appears to promote meiotic exit independently of CDKA;1. Furthermore, analysis of CDKD;3 interactome revealed enrichment for proteins implicated in cytokinesis, suggesting a more complex function of CDKD;3 in cell cycle regulation.

Meiotic exit in Arabidopsis is driven by P-body-mediated inhibition of translation

Meiosis, at the transition between diploid and haploid life cycle phases, is accompanied by reprograming of cell division machinery and followed by a transition back to mitosis. We show that, in Arabidopsis, this transition is driven by inhibition of translation, achieved by a mechanism that involves processing bodies (P-bodies). During the second meiotic division, the meiosis-specific protein THREE-DIVISION MUTANT 1 (TDM1) is incorporated into P-bodies through interaction with SUPPRESSOR WITH MORPHOGENETIC EFFECTS ON GENITALIA 7 (SMG7). TDM1 attracts eIF4F, the main translation initiation complex, temporarily sequestering it in P-bodies and inhibiting translation. The failure of tdm1 mutants to terminate meiosis can be overcome by chemical inhibition of translation. We propose that TDM1-containing P-bodies down-regulate expression of meiotic transcripts to facilitate transition of cell fates to postmeiotic gametophyte differentiation.