CONCLUSIONS: Developing bryophytes differentially modify their plasmodesmata structure and function. Secondary plasmodesmata formation via twinning appears to be an ancestral trait. Plasmodesmata networks in hornwort sporophyte meristems resemble those of angiosperms. All land-plant taxa use plasmodesmata (PD) cell connections for symplasmic communication. In angiosperm development, PD networks undergo an extensive remodeling by structural and functional PD modifications, and by postcytokinetic formation of additional secondary PD (secPD). Since comparable information on PD dynamics is scarce for the embryophyte sister groups, we investigated maturating tissues of Anthoceros agrestis (hornwort), Physcomitrium patens (moss), and Marchantia polymorpha (liverwort). As in angiosperms, quantitative electron microscopy revealed secPD formation via twinning in gametophytes of all model bryophytes, which gives rise to laterally adjacent PD pairs or to complex branched PD. This finding suggests that PD twinning is an ancient evolutionary mechanism to adjust PD numbers during wall expansion. Moreover, all bryophyte gametophytes modify their existing PD via taxon-specific strategies resembling those of angiosperms. Development of type II-like PD morphotypes with enlarged diameters or formation of pit pairs might be required to maintain PD transport rates during wall thickening. Similar to angiosperm leaves, fluorescence redistribution after photobleaching revealed a considerable reduction of the PD permeability in maturating P. patens phyllids. In contrast to previous reports on monoplex meristems of bryophyte gametophytes with single initials, we observed targeted secPD formation in the multi-initial basal meristems of A. agrestis sporophytes. Their PD networks share typical features of multi-initial angiosperm meristems, which may hint at a putative homologous origin. We also discuss that monoplex and multi-initial meristems may require distinct types of PD networks, with or without secPD formation, to control maintenance of initial identity and positional signaling.

Germ cells (GCs) serve as indispensable carriers in both animals and plants, ensuring genetic continuity across generations. While it is generally acknowledged that the timing of germline segregation differs significantly between animals and plants, ongoing debates persist as new evidence continues to emerge. In this review, we delve into studies focusing on male germ cell specifications in plants, and we summarize the core gene regulatory circuits in germ cell specification, which show remarkable parallels to those governing meristem homeostasis. The similarity in germline establishment between animals and plants is also discussed.

Gene drive elements promote the spread of linked traits and can be used to change the composition or fate of wild populations. Cleave and Rescue (ClvR) drive elements sit at a fixed chromosomal position and include a DNA sequence-modifying enzyme such as Cas9/gRNAs that disrupts endogenous versions of an essential gene and a recoded version of the essential gene resistant to cleavage. ClvR spreads by creating conditions in which those lacking ClvR die because they lack functional versions of the essential gene. Here we demonstrate the essential features of the ClvR gene drive in the plant Arabidopsis thaliana through killing of gametes that fail to inherit a ClvR that targets the essential gene YKT61. Resistant alleles, which can slow or prevent drive, were not observed. Modelling shows plant ClvRs are robust to certain failure modes and can be used to rapidly drive population modification or suppression. Possible applications are discussed.

Heterosis boosts crop yield; however, harnessing additional progressive heterosis in polyploids is challenging for breeders. We bioengineered a \'mitosis instead of meiosis\' (MiMe) system that generates unreduced, clonal gametes in three hybrid tomato genotypes and used it to establish polyploid genome design. Through the hybridization of MiMe hybrids, we generated \'4-haplotype\' plants that encompassed the complete genetics of their four inbred grandparents, providing a blueprint for exploiting polyploidy in crops.

Gametogenesis, which is essential to the sexual reproductive system, has drastically changed during plant evolution. Bryophytes, lycophytes and ferns develop reproductive organs called gametangia-antheridia and archegonia for sperm and egg production, respectively. However, the molecular mechanism of early gametangium development remains unclear. Here we identified a \'non-canonical\' type of BZR/BES transcription factor, MpBZR3, as a regulator of gametangium development in a model bryophyte, Marchantia polymorpha. Interestingly, overexpression of MpBZR3 induced ectopic gametangia. Genetic analysis revealed that MpBZR3 promotes the early phase of antheridium development in male plants. By contrast, MpBZR3 is required for the late phase of archegonium development in female plants. We demonstrate that MpBZR3 is necessary for the successful development of both antheridia and archegonia but functions in a different manner between the two sexes. Together, the functional specialization of this \'non-canonical\' type of BZR/BES member may have contributed to the evolution of reproductive systems.

The conquest of land by plants was concomitant with, and possibly enabled by, the evolution of three-dimensional (3D) growth. The moss Physcomitrium patens provides a model system for elucidating molecular mechanisms in the initiation of 3D growth. Here, we investigate whether the phytohormone ethylene, which is believed to have been a signal before land plant emergence, plays a role in 3D growth regulation in P. patens. We report ethylene controls 3D gametophore formation, based on results from exogenously applied ethylene and genetic manipulation of PpEIN2, which is a central component in the ethylene signaling pathway. Overexpression (OE) of PpEIN2 activates ethylene responses and leads to earlier formation of gametophores with fewer gametophores produced thereafter, phenocopying ethylene-treated wild-type. Conversely, Ppein2 knockout mutants, which are ethylene insensitive, show initially delayed gametophore formation with more gametophores produced later. Furthermore, pharmacological and biochemical analyses reveal auxin levels are decreased in the OE lines but increased in the knockout mutants. Our results suggest that evolutionarily, ethylene and auxin molecular networks were recruited to build the plant body plan in ancestral land plants. This might have played a role in enabling ancient plants to acclimate to the continental surfaces of the planet.

CONCLUSIONS: Fiber-like cells with thickened cell walls of specific structure and polymer composition that includes (1 → 4)-β-galactans develop in the outer stem cortex of several moss species gametophytes. The early land plants evolved several specialized cell types and tissues that did not exist in their aquatic ancestors. Of these, water-conducting elements and reproductive organs have received most of the research attention. The evolution of tissues specialized to fulfill a mechanical function is by far less studied despite their wide distribution in land plants. For vascular plants following a homoiohydric trajectory, the evolutionary emergence of mechanical tissues is mainly discussed starting with the fern-like plants with their hypodermal sterome or sclerified fibers that have xylan and lignin-based cell walls. However, mechanical challenges were also faced by bryophytes, which lack lignified cell-walls. To characterize mechanical tissues in the bryophyte lineage, following a poikilohydric trajectory, we used six wild moss species (Polytrichum juniperinum, Dicranum sp., Rhodobryum roseum, Eurhynchiadelphus sp., Climacium dendroides, and Hylocomium splendens) and analyzed the structure and composition of their cell walls. In all of them, the outer stem cortex of the leafy gametophytic generation had fiber-like cells with a thickened but non-lignified cell wall. Such cells have a spindle-like shape with pointed tips. The additional thick cell wall layer in those fiber-like cells is composed of sublayers with structural evidence for different cellulose microfibril orientation, and with specific polymer composition that includes (1 → 4)-β-galactans. Thus, the basic cellular characters of the cells that provide mechanical support in vascular plant taxa (elongated cell shape, location at the periphery of a primary organ, the thickened cell wall and its peculiar composition and structure) also exist in mosses.

Macroalgal populations often consist of free-living haploid (gametophyte) and diploid (sporophyte) stages. Various ecological studies have been conducted to examine the demographic diversity of haploid-diploid populations with regard to the dominant stage. Here, I relaxed the assumption of classical research that the life history parameters of haploids and diploids are identical and developed a generalized haploid-diploid model that explicitly accounts for population density dependence and asexual reproduction. Analysis of this model yielded an exact solution for the abundance ratio of haploids to diploids in a population in which the ratio is determined by the balance of four demographic forces: sexual reproduction by haploids, sexual reproduction by diploids, asexual reproduction by haploids, and asexual reproduction by diploids. Furthermore, the persistence of a haploid-diploid population and its total biomass are shown to be determined by the basic reproductive number (R0), which is shown to be a function of these four demographic forces. When R0 is greater than one, the haploid-diploid population stably persists, and the ploidy ratio obtained by the analytical solution is realized.

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Marine life is populated by a huge diversity of organisms with an incredible range of colour. While structural colour mechanisms and functions are usually well studied in marine animal species, there is a huge knowledge gap regarding the marine macroalgae (red, green and brown seaweeds) that have structural coloration and the biological significance of this phenomenon in these photosynthetic organisms. Here we show that structural colour in the gametophyte life history phase of the red alga Chondrus crispus plays an important role as a photoprotective mechanism in synergy with the other pigments present. In particular, we have demonstrated that blue structural coloration attenuates the more energetic light while simultaneously favouring green and red light harvesting through the external antennae (phycobilisomes) which possess an intensity-dependent photoprotection mechanism. These insights into the relationship between structural colour and photosynthetic light management further our understanding of the mechanisms involved.