Endopolyploidy occurs when DNA replication takes place without subsequent mitotic nuclear division, resulting in cell-specific ploidy levels within tissues. In plants, endopolyploidy plays an important role in sustaining growth and development, but only a few studies have demonstrated a role in abiotic stress response. In this study, we investigated the function of ploidy level and nuclear and cell size in leaf expansion throughout development and tracked cell type-specific ploidy in the halophyte In addition to developmental endopolyploidy, we examined the effects of salinity stress on ploidy level. We focused specifically on epidermal bladder cells (EBC), which are modified balloon-like trichomes, due to their large size and role in salt accumulation. Our results demonstrate that ploidy increases as the leaves expand in a similar manner for each leaf type, and ploidy levels up to 512C were recorded for nuclei in EBC of leaves of adult plants. Salt treatment led to a significant increase in ploidy levels in the EBC, and these cells showed spatially related differences in their ploidy and nuclear and cell size depending on the positions on the leaf and stem surface. Transcriptome analysis highlighted salinity-induced changes in genes involved in DNA replication, cell cycle, endoreduplication, and trichome development in EBC. The increase in cell size and ploidy observed in under salinity stress may contribute to salt tolerance by increasing the storage capacity for sodium sequestration brought about by higher metabolic activity driving rapid cell enlargement in the leaf tissue and EBC.

The use of extremophyte models to select growth promoting traits during environmental stresses is a recognized yet an underutilized strategy to design stress-resilient plants. Schrenkiella parvula, a leading extremophyte model in Brassicaceae, can grow and complete its life cycle under multiple environmental stresses, including high salinity. While S. parvula is equipped with foundational genomic resources to identify genetic clues that potentially lead to stress adaptations at the phenome level, a comprehensive physiological and structural characterization of salt stress responses throughout its lifecycle is absent. We aimed to identify the influential traits that lead to resilient growth and strategic decisions to ensure survival of the species in an extreme environment, and examined salt-induced changes in the physiology and anatomy of S. parvula throughout its life cycle across multiple tissues. We found that S. parvula maintains or even enhances growth during various developmental stages at salt stress levels known to inhibit growth in Arabidopsis thaliana and most crops. The resilient growth of S. parvula was associated with key traits synergistically allowing continued primary root growth, expansion of xylem vessel elements across the root-shoot continuum, and the high capacity to maintain tissue water levels by developing larger and thicker leaves while facilitating continued photosynthesis during salt stress. In turn, the stress-resilient growth during the vegetative phase of S. parvula allowed a successful transition to a reproductive phase via early flowering followed by the development of larger siliques with viable seeds on salt-treated plants. Additionally, the success of self-fertilization in early flowering stages was dependent on salt-induced filament elongation. Our results suggest that the maintenance of leaf water status and enhancement of selfing in early flowers to ensure reproductive success are among the most influential traits that contribute to the extremophilic lifestyle of S. parvula in its natural habitat.

The rapid invasion of the non‐native Phragmites australis (Poaceae, subfamily Arundinoideae) is a major threat to native wetland ecosystems in North America and elsewhere. We describe the first reference genome for P. australis and compare invasive (ssp. australis) and native (ssp. americanus) genotypes collected from replicated populations across the Laurentian Great Lakes to deduce genomic bases driving its invasive success. Here, we report novel genomic features including a Phragmites lineage‐specific whole genome duplication, followed by gene loss and preferential retention of genes associated with transcription factors and regulatory functions in the remaining duplicates. Comparative transcriptomic analyses revealed that genes associated with biotic stress and defence responses were expressed at a higher basal level in invasive genotypes, but native genotypes showed a stronger induction of defence responses when challenged by a fungal endophyte. The reference genome and transcriptomes, combined with previous ecological and environmental data, add to our understanding of mechanisms leading to invasiveness and support the development of novel, genomics‐assisted management approaches for invasive Phragmites.

Alternative splicing extends the coding potential of genomes by creating multiple isoforms from one gene. Isoforms can render transcript specificity and diversity to initiate multiple responses required during transcriptome adjustments in stressed environments. Although the prevalence of alternative splicing is widely recognized, how diverse isoforms facilitate stress adaptation in plants that thrive in extreme environments are unexplored. Here we examine how an extremophyte model, Schrenkiella parvula, coordinates alternative splicing in response to high salinity compared to a salt-stress sensitive model, Arabidopsis thaliana. We use Iso-Seq to generate full length reference transcripts and RNA-seq to quantify differential isoform usage in response to salinity changes. We find that single-copy orthologs where S. parvula has a higher number of isoforms than A. thaliana as well as S. parvula genes observed and predicted using machine learning to have multiple isoforms are enriched in stress associated functions. Genes that showed differential isoform usage were largely mutually exclusive from genes that were differentially expressed in response to salt. S. parvula transcriptomes maintained specificity in isoform usage assessed via a measure of expression disorderdness during transcriptome reprogramming under salt. Our study adds a novel resource and insight to study plant stress tolerance evolved in extreme environments.

35The multicellular embryo, and ultimately the entire organism, is a derivative of the fertilized 36 egg cell. Unlike in animals, transcription factor networks orchestrating faithful egg 37 development are still largely unknown in plants. We have identified that egg cell 38 differentiation in Arabidopsis require interplay between evolutionarily conserved onco-protein 39 homologs RETINOBLASTOMA-RELATED (RBR) and redundant MYB proteins 40 MYB64/MYB119. RBR physically interacts with the MYBs; and with plant-specific 41 transcription factors belonging to the RWP-RK-domain (RKD) family and LEAFY 42 COTYLEDON1 (LEC1), which participate in development of egg cells and inherent stress 43 response. RBR binds to most of these egg cell-expressed loci at the DNA level, partially 44 overlapping with sites of histone methylation H3K27me3. Since deregulation of RKDs 45 phenocopies mutants of RBR and the MYBs in terms of cell proliferation in the egg cell 46 spatial domain, all the corresponding proteins are likely required to restrict parthenogenetic 47 cell divisions of the egg cells. Cross-talk among these transcription factors, and direct 48 regulation by RBR, govern egg cell development and expression of egg-to-zygotic polarity 49 factors of the WUSCHEL RELATED HOMEOBOX family. Together, a network of RBR-50 centric transcription factors underlies egg cell development and stress response, possibly, in 51 combination with several other predicted nodes. 52 53 54 55 Key words 56 egg cell | transcription factor | RETINOBLASTOMA RELATED | MYB | RKD | stress | 57 parthenogenesis 58 59 4 Author summary 60 61The RETINOBLASTOMA protein is one of the core components of the Eukaryotic 62 cell cycle, and corresponding evolutionary homologs have been implicated not only 63 to repress cell division but also to control differentiation and development. How 64 RETINOBLASTOMA RELATED (RBR) associate with other higher order regulators 65 to control faithful egg cell development in sexual plants is pivotal for manipulation of 66 successful reproduction in general, and engineering of parthenogenesis when 67 asexual or apomictic seed progeny are desirable over sexual plants. Using a suite of 68 molecular methods, we show that a RBR-associated transcription factor network 69 operates to specify egg cells in Arabidopsis. Complex cross-regulation within these 70 transcription factors seems to be necessary for successful maternal egg cell to 71 zygotic transition and reproductive stress response. Detailed genetic analysis 72 implicate that RBR and its interactive partners belonging to MYB and RWP-RK 73 transcription factor families are possibly required to prevent parthenogenesis of the 74 sexual egg cells. Novel RBR networks and stress nodes explained in this study 75 might help to improve our understanding of sexual and asexual reproduction. 76 77 78 79 80 Proper differentiation of the egg cells is pivotal for sexual reproduction as well as 81 parthenogenesis. In flowering plants, the egg cells are terminally differentiated within 82 the miniature ...