All multicellular organisms require intercellular communication, which has pivotal functions in facing environmental challenges and in coordinating development. Plant intercellular communication ...occurs via symplasmic and apoplasmic pathways. In the symplasmic route, plasmodesmata (PD), the plant-specic microchannels, penetrate cell walls, thus providing a route for communication between cells. PD allow trafcking of nutrients, hormones, and other informational molecules between neighboring cells and through distantly located tissues.
Multicellular organisms regulate cell numbers and cell fate by using asymmetric cell division (ACD) and symmetric cell division (SCD) during their development and to adapt to unfavorable ...environmental conditions. A stem cell self-renews and generates differentiated cells. In plants, various types of cells are produced by ACD or SCD; however, the molecular mechanisms of ACD or SCD and the cell division mode switch are largely unknown. The moss Physcomitrium (Physcomitrella) patens is a suitable model to study plant stem cells due to its simple anatomy. Here, we report the cell division mode switch induced by abscisic acid (ABA) in P. patens. ABA is synthesized in response to abiotic stresses and induces round-shape cells, called brood cells, from cylindrical protonemal cells. Although two daughter cells with distinct sizes were produced by ACD in a protonemal stem cell on ABA-free media, the sizes of two daughter cells became similar with ABA treatment. Actin microfilaments were spatially localized on the apices of apical stem cells in protonemata on ABA-free media, but the polar accumulation was lost under the condition of ABA treatment. Moreover, ABA treatment conferred an identical cell fate to the daughter cells in terms of cell division activity. Collectively, the results indicate ABA may suppress the ACD characteristics but evoke SCD in cells. We also noticed that ABA-induced brood cells not only self-renewed but regenerated protonemal cells when ABA was removed from the media, suggesting that brood cells are novel stem cells that are induced by environmental signals in P. patens.
Plants have evolved and grown under the selection pressure of gravitational force at 1
g
on Earth. In response to this selection pressure, plants have acquired gravitropism to sense gravity and ...change their growth direction. In addition, plants also adjust their morphogenesis in response to different gravitational forces in a phenomenon known as gravity resistance. However, the gravity resistance phenomenon in plants is poorly understood due to the prevalence of 1
g
gravitational force on Earth: not only it is difficult to culture plants at gravity > 1
g
(hypergravity) for a long period of time but it is also impossible to create a < 1
g
environment (μ
g
, micro
g
) on Earth without specialized facilities. Despite these technical challenges, it is important to understand how plants grow in different gravity conditions in order to understand land plant adaptation to the 1
g
environment or for outer space exploration. To address this, we have developed a centrifugal device for a prolonged duration of plant culture in hypergravity conditions, and a project to grow plants under the μ
g
environment in the International Space Station is also underway. Our plant material of choice is
Physcomitrium (Physcomitrella) patens
, one of the pioneer plants on land and a model bryophyte often used in plant biology. In this review, we summarize our latest findings regarding
P. patens
growth response to hypergravity, with reference to our on-going “Space moss” project. In our ground-based hypergravity experiments, we analyzed the morphological and physiological changes and found unexpected increments of chloroplast size and photosynthesis rate, which might underlie the enhancement of growth and increase in the number of gametophores and rhizoids. We further discussed our approaches at the cellular level and compare the gravity resistance in mosses and that in angiosperms. Finally, we highlight the advantages and perspectives from the space experiments and conclude that research with bryophytes is beneficial to comprehensively and precisely understand gravitational responses in plants.
Summary
High‐throughput RNA sequencing (RNA‐seq) has recently become the method of choice to define and analyze transcriptomes. For the model moss Physcomitrella patens, although this method has been ...used to help analyze specific perturbations, no overall reference dataset has yet been established. In the framework of the Gene Atlas project, the Joint Genome Institute selected P. patens as a flagship genome, opening the way to generate the first comprehensive transcriptome dataset for this moss. The first round of sequencing described here is composed of 99 independent libraries spanning 34 different developmental stages and conditions. Upon dataset quality control and processing through read mapping, 28 509 of the 34 361 v3.3 gene models (83%) were detected to be expressed across the samples. Differentially expressed genes (DEGs) were calculated across the dataset to permit perturbation comparisons between conditions. The analysis of the three most distinct and abundant P. patens growth stages – protonema, gametophore and sporophyte – allowed us to define both general transcriptional patterns and stage‐specific transcripts. As an example of variation of physico‐chemical growth conditions, we detail here the impact of ammonium supplementation under standard growth conditions on the protonemal transcriptome. Finally, the cooperative nature of this project allowed us to analyze inter‐laboratory variation, as 13 different laboratories around the world provided samples. We compare differences in the replication of experiments in a single laboratory and between different laboratories.
Significance statement
This work presents the first large scale RNA‐seq dataset for the moss model Physcomitrella patens. The sampling covers 34 experiments, ensuring a good coverage of the organisms’ life cycle under standard laboratory growing conditions as well as a panel of treatments. The detection of differentially expressed genes allows a clear contrast between these experiments and permits insight in the biological mechanisms underlining the observed differences.
Endogenous brassinosteroids (BRs) in non-flowering land plants were analyzed. BRs were found in a liverwort (Marchantia polymorpha), a moss (Physcomitrella patens), lycophytes (Selaginella ...moellendorffii and S. uncinata) and 13 fern species. A biologically active BR, castasterone (CS), was identified in most of these non-flowering plants but another biologically active BR, brassinolide, was not. It may be distinctive that levels of CS in non-flowering plants were orders of magnitude lower than those in flowering plants. 22-Hydroxycampesterol and its metabolites were identified in most of the non-flowering plants suggesting that the biosynthesis of BRs via 22-hydroxylation of campesterol occurs as in flowering plants. Phylogenetic analyses indicated that M. polymorpha, P. patens and S. moellendorffii have cytochrome P450s in the CYP85 clans which harbors BR biosynthesis enzymes, although the P450 profiles are simpler as compared with Arabidopsis and rice. Furthermore, these basal land plants were found to have multiple P450s in the CYP72 clan which harbors enzymes to catabolize BRs. These findings indicate that green plants were able to synthesize and inactivate BRs from the land-transition stage.
Castasterone, 22-hydroxycampesterol, and other brassinosteroids were identified and quantified in non-flowering plants. Homologs of Arabidopsis P450 enzymes involved in brassinosteroid biosynthesis and catabolism are phylogenetically analyzed in a liverwort, moss and lycophyte. Display omitted
•Brassinosteroids occur in liverwort, moss, lycophyte and fern species examined.•Castasterone may be a biologically active brassinosteroid in these plants.•Castasterone may be synthesized from 22-hydroxycampesterol as in flowering plants.•Liverwort, moss and lycophyte species here have cytochrome P450s in the CYP85 and CYP72 clans.•The P450s in the CYP85 clan in the lower plants are at middle stages of evolution.
The production of yellow-skinned apples has increased in recent years; however, they are less acceptable to consumers than red-skinned apples. Therefore, the production of high-quality fruit is ...essential. In the present study, we applied a girdling treatment at 1) different times (June or August), and 2) different widths (2 cm, 5 cm, and 8 cm). We compared the effect on tree growth, flower bud formation and fruit quality over a 4-year field trial using vigorous yellow-skinned ‘Koukou’ apple trees. The August girdling reduced the shoot growth by about 6 to 10 cm per shoot and this reduction also restricted secondary extension of the shoots. The August treatment improved the fruit quality parameters, including the soluble solids content and skin color index, more effectively than the June treatment. Compared with the girdling timing, the effect of girdling widths in August was less obvious, but even the 2 cm treatment in August was enough to reduce tree vigor and improve fruit quality. Apical flower bud formation was accelerated by the August girdling in both the treatment year and the following year. Regardless of the girdling width, the August girdling improved the fruit quality parameters: e.g. flesh firmness, soluble solids content, and skin color index. The August girding fruit also had higher sorbitol and sucrose concentrations than the control. Moreover, both the girdling treatments accelerated the incidence of watercore, which is a preferred condition for the Asian market. In conclusion, we found that all widths of girdling in August improved the harvested fruit qualities, including the sugar (sucrose) content, as well as watercore development in the treatment year and effectively controlled tree vigor, increased flower bud formation and increased yields.
Cell polarity is fundamentally important to growth and development in higher plants, from pollen tubes to root hairs. Basal land plants (mosses and ferns) also have cell polarity, developing ...protonemal apical cells that show polar tip growth. Flowering plants have a distinct group of Rho GTPases that regulate polarity in polarized cell growth. Rop/RAC signaling module components have been identified in non-flowering plants, but their roles remain unclear. To understand the importance and evolution of Rop/RAC signaling in polarity regulation in land plants, we examined the functions of PpRop and PpRopGEF in protonemal apical cells of the moss Physcomitrella patens. Inducible overexpression of PpRop2 or PpRopGEF3 caused depolarized growth of tip-growing apical cells. PpRop2 overexpression also caused aberrant cross wall formation. Fluorescent protein-tagged PpRop2 localized to the plasma membrane, including the cross wall membrane, and fluorescent-tagged PpRopGEF3 showed polarized localization to the tip region in apical cells. Thus, our results suggest common functions of PpRop and PpRopGEF in the tip-growing apical cells and the importance of a conserved Rop/RAC signaling module in the control of cell polarity in land plants.
•We analyzed the functions of Rops and RopGEFs in the moss Physcomitrella patens.•PpRop2 or PpRopGEF3 overexpression disrupted tip growth in protonemal apical cells.•PpRop2 overexpression caused abnormal cross wall formation.•Localization of PpRop2 and PpRopGEF3 overlapped in the tip region in apical cells.•Conserved function of Rop/RAC module in the control of cell polarity in land plants.
Arabinogalactan proteins are functionally diverse cell wall structural glycoproteins that have been implicated in cell wall remodeling, although the mechanistic actions remain elusive. Here, we ...identify and characterize two AGP glycoproteins, SLEEPING BEAUTY (SB) and SB-like (SBL), that negatively regulate the gametophore bud initiation in Physcomitrium patens by dampening cell wall loosening/softening. Disruption of SB and SBL led to accelerated gametophore formation and altered cell wall compositions. The function of SB is glycosylation dependent and genetically connected with the class C auxin response factor (ARF) transcription factors PpARFC1B and PpARFC2. Transcriptomics profiling showed that SB upregulates PpARFC2, which in turn suppresses a range of cell wall-modifying genes that are required for cell wall loosening/softening. We further show that PpARFC2 binds directly to multiple AuxRE motifs on the cis-regulatory sequences of PECTIN METHYLESTERASE to suppress its expression. Hence, our results demonstrate a mechanism by which the SB modulates the strength of intracellular auxin signaling output, which is necessary to fine-tune the timing of gametophore initials formation.
Abscisic acid (ABA)-mediated abiotic stress tolerance causes plant growth inhibition. Under such stress conditions, some mosses generate
stress-resistant stem cells, also called brood cells or ...brachycytes, that do not exist under normal conditions. However, the cell physiological basis of the growth inhibition and the stem cell formation is not well understood. Here, we show that the ABA-induced growth inhibition of the moss
apical protonemal cells (protonemal stem cells) is mediated through a shift from asymmetric to symmetric cell division. This change of the cell division mode, and consequently change of stem cell activity, is substantiated by dampening cell polarity and cell proliferative activity through the altered distribution of cytoskeletal elements, the mitotic spindle and the vacuole, which results in the production of stress-resistant stem cells. Alteration of the cell physiological data is supported by the results of RNAseq analysis indicating rapid changes in both cell polarity and cell cycle regulation, while long-term treatments with ABA for 5 to 10 days impact mainly the transcriptional and translational regulation. The regulation of cell polarity and cell cycle genes suggests growth arrest mediated by small GTPases (ROPs) and their guanine exchange factors (ROPGEFs) and by cyclin and cyclin-dependent-kinase complex, respectively. Our data suggest that a tradeoff relationship between growth ability and abiotic stress response in the moss is substantiated by ABA signaling to suppress cell polarity and asymmetric cell growth and may play a pivotal role in stem cell fate conversion to newly produced stress-resistant stem cells.
Chloroplast division is a critical process for the maintenance of appropriate chloroplast number in plant cells. It is known that in some plant species and cell types, environmental stresses can ...affect chloroplast division, differentiation and morphology, however the significance and regulation of these processes are largely unknown. Here we investigated the regulation of salt stress-induced chloroplast division in protonemal cells of the moss,
Physcomitrella patens
, and found that, salt stress as one of the major abiotic stresses, induced chloroplast division and resulted in increased chloroplast numbers. We further identified three APETALA2/ETHYLENE RESPONSIVE FACTOR (AP2/ERF) transcription factors (TFs) that were responsible for this regulation. These AP2/ERF genes were up-regulated under salt stress, and amino acid sequences and phylogenetic analyses indicated that all TFs possess only one conserved AP2 domain and likely belong to the same subgroup of ERF-B3 in the AP2/ERF superfamily. Overexpression of these TFs significantly increased the chloroplast number even in the absence of NaCl stress. On the contrary, inducible overexpression of the dominant repressor form of these TFs suppressed salt stress-induced chloroplast division. Thus, our results suggest that salt stress induced-chloroplast division is regulated through members of the AP2/ERF TF superfamily.
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