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Survival of living organisms is fully dependent on their maintenance of genome integrity, being permanently threatened by replication stress in proliferating cells. Although the plant DNA damage response (DDR) regulator SOG1 has been demonstrated to cope with replicative defects, accumulating evidence points to other pathways functioning independently of SOG1. Here, we have studied the role of the Arabidopsis E2FA and EF2B transcription factors, two well-characterized regulators of DNA replication, in the response to replication stress. Through a combination of reverse genetics and chromatin-immunoprecipitation approaches, we show that E2FA and E2FB share many target genes with SOG1, providing evidence for their involvement in the DDR. Analysis of double and triple mutant combinations revealed that E2FB, rather than E2FA, plays the most prominent role in sustaining growth in the presence of replicative defects, either operating antagonistically or synergistically with SOG1. Reversely, SOG1 aids in overcoming the replication defects of E2FA/E2FB-deficient plants. Our data reveal a complex transcriptional network controlling the replication stress response, in which both E2Fs and SOG1 act as key regulatory factors.ONE-SENTENCE SUMMARYThe Arabidopsis E2FA and EF2B transcription factors differently contribute to the plant’s response to DNA replication defects in a cooperative way with the DNA damage response regulator SOG1.

The anaphase-promoting complex/cyclosome (APC/C) marks key cell cycle proteins for proteasomal breakdown, thereby ensuring unidirectional progression through the cell cycle. Its target recognition is temporally regulated by activating subunits, one of which is called CELL CYCLE SWITCH 52 A2 (CCS52A2). We sought to expand the knowledge on the APC/C by using the severe growth phenotypes of CCS52A2-deficient Arabidopsis (Arabidopsis thaliana) plants as a readout in a suppressor mutagenesis screen, resulting in the identification of the previously undescribed gene called PIKMIN1 (PKN1). PKN1 deficiency rescues the disorganized root stem cell phenotype of the ccs52a2-1 mutant, whereas an excess of PKN1 inhibits the growth of ccs52a2-1 plants, indicating the need for control of PKN1 abundance for proper development. Accordingly, the lack of PKN1 in a wild-type background negatively impacts cell division, while its systemic overexpression promotes proliferation. PKN1 shows a cell cycle phase-dependent accumulation pattern, localizing to microtubular structures, including the preprophase band, the mitotic spindle, and the phragmoplast. PKN1 is conserved throughout the plant kingdom, with its function in cell division being evolutionarily conserved in the liverwort Marchantia polymorpha. Our data thus demonstrate that PKN1 represents a novel, plant-specific protein with a role in cell division that is likely proteolytically controlled by the CCS52A2-activated APC/C.

During plant development, a precise balance of cytokinin is crucial for correct growth and patterning, but it remains unclear how this is achieved across different cell types and in the context of a growing organ. Here we show that in the root apical meristem, the TMO5/LHW complex increases active cytokinin levels via two cooperatively acting enzymes. By profiling the transcriptomic changes of increased cytokinin at single-cell level, we further show that this effect is counteracted by a tissue-specific increase in CYTOKININ OXIDASE 3 expression via direct activation of the mobile transcription factor SHORTROOT. In summary, we show that within the root meristem, xylem cells act as a local organizer of vascular development by non-autonomously regulating cytokinin levels in neighbouring procambium cells via sequential induction and repression modules.

InArabidopsis thaliana, the negative brassinosteroid (BR) signalling regulator, BR INSENSITIVE2 (BIN2) promotes and restricts stomatal asymmetric cell division (ACD) depending on its subcellular localization, which is regulated by the stomatal lineage-specific scaffolding protein POLAR. BRs inactivate BIN2, but how they govern stomatal development remains unclear. Mapping the single-cell transcriptome of stomatal lineages with either exogenous BRs or the specific BIN2 inhibitor revealed that the two modes of BR activation triggered spatiotemporally distinct transcriptional responses. We established that when in a complex with POLAR and its closest homolog POLAR-LIKE1, BIN2 is insulated from BR-mediated inactivation, nevertheless, it remains sensitive to the inhibitor. Subsequently, BR signalling is attenuated in ACD precursors, whereas it remains active in epidermal cells that would undergo differentiation. Our study demonstrates how scaffold proteins contribute to cellular signal specificity of hormonal responses in plants.

Single-cell approaches are quickly changing our view on biological systems by increasing the spatiotemporal resolution of our analyses to the level of the individual cell. The field of plant biology has fully embraced single-cell transcriptomics and is rapidly expanding the portfolio of available technologies and applications. In this review, we give an overview of the main advances in plant single-cell transcriptomics over the past few years and provide the reader with an accessible guideline covering all steps, from sample preparation to data analysis. We end by offering a glimpse of how these technologies will shape and accelerate plant-specific research in the near future.

Contains fulltext : 222330.pdf (Publisher’s version ) (Open Access) Plants show remarkable regenerative abilities, allowing them to recover from wounds and organ loss. This regenerative capacity is controlled in part by auxin, the most promiscuous plant hormone controlling organogenesis and tissue patterning. We show that stem cell death diverges the auxin flow, much like rocks in a stream, resulting in an auxin accumulation in the tissues surrounding the wound. We demonstrate that within these tissues, wound-induced expression of the plant-specific transcription factor ERF115 works synergistically with the change in auxin accumulation, thereby specifying stem cell identity in the cells surrounding the damaged stem cells. This gain of stem cell identity drives formative divisions, allowing replacement of the lost stem cells and thus successful regeneration.Plants are known for their outstanding capacity to recover from various wounds and injuries. However, it remains largely unknown how plants sense diverse forms of injury and canalize existing developmental processes into the execution of a correct regenerative response. Auxin, a cardinal plant hormone with morphogen-like properties, has been previously implicated in the recovery from diverse types of wounding and organ loss. Here, through a combination of cellular imaging and in silico modeling, we demonstrate that vascular stem cell death obstructs the polar auxin flux, much alike rocks in a stream, and causes it to accumulate in the endodermis. This in turn grants the endodermal cells the capacity to undergo periclinal cell division to repopulate the vascular stem cell pool. Replenishment of the vasculature by the endodermis depends on the transcription factor ERF115, a wound-inducible regulator of stem cell division. Although not the primary inducer, auxin is required to maintain ERF115 expression. Conversely, ERF115 sensitizes cells to auxin by activating ARF5/MONOPTEROS, an auxin-responsive transcription factor involved in the global auxin response, tissue patterning, and organ formation. Together, the wound-induced auxin accumulation and ERF115 expression grant the endodermal cells stem cell activity. Our work provides a mechanistic model for wound-induced stem cell regeneration in which ERF115 acts as a wound-inducible stem cell organizer that interprets wound-induced auxin maxima. 14 juli 2020

The anaphase-promoting complex/cyclosome (APC/C) marks key cell cycle proteins for proteasomal breakdown, thereby ensuring unidirectional progression through the cell cycle. Its target recognition is temporally regulated by activating subunits, one of which is called CELL CYCLE SWITCH 52 A2 (CCS52A2). We sought to expand the knowledge of identified APC/C targets by using the severe growth phenotypes of CCS52A2-deficient Arabidopsis thaliana plants as a readout in a suppressor mutagenesis screen, resulting in the identification of the previously undescribed gene called PIKMIN1 (PKN1). PKN1 deficiency rescues the disorganized root stem cell phenotype of the ccs52a2-1 mutant, whereas an excess of PKN1 inhibits growth of ccs52a2-1 plants, indicating the importance of PKN1 abundance for proper development. Accordingly, the lack of PKN1 in a wild-type background negatively impacts cell division, while its ectopic expression promotes proliferation. PKN1 shows a cell cycle phase-dependent accumulation pattern, localizing to microtubular structures, including the preprophase band, the mitotic spindle, and phragmoplast. PKN1 is conserved throughout the plant kingdom, with its function in cell division being evolutionary conserved in the liverwort Marchantia polymorpha. Our data thus demonstrate that PKN1 represents a novel, plant-specific gene with a rate-limiting role in cell division, which is proteolytically controlled by the CCS52A2-activated APC/C.One-Sentence SummaryPKN1 is a conserved plant-specific protein that is rate-limiting for cell division, likely through its interaction with microtubuli, and is proteolytically controlled by APC/CCCS52A2.

During DNA replication, the WEE1 kinase is responsible for safeguarding genomic integrity by phosphorylating and thus inhibiting cyclin-dependent kinases (CDKs), which are the driving force of the cell cycle. Consequentially, wee1 mutant plants fail to respond properly to problems arising during DNA replication and are hypersensitive to replication stress. Here, we report the identification of the polα-2 mutant, mutated in the catalytic subunit of DNA polymerase α, as a suppressor mutant of wee1. The mutated protein appears to be less stable, causing a loss of interaction with its subunits and resulting in a prolonged S-phase.

Depuis le début des années 2000, le continent africain connaît un cycle de forte croissance économique, mais les transformations structurelles, gages d’émergence de ses économies, sont plus lentes. Les emplois de qualité demeurent rares, et l’informel reste la norme sur le marché du travail. L’hétérogénéité de cette économie informelle rend sa définition complexe et représente un obstacle à sa prise en compte par les politiques publiques. Objectifs de soutien et de modernisation des unités de production, et objectifs de formalisation et de mise en conformité avec un certain nombre de normes et de règlementations sont les deux faces complémentaires de toute politique économique visant à relever le défi de l’informalité dans un objectif d’émergence., Since the early 2000s, the African continent has been experiencing a cycle of strong economic growth, but the structural transformations needed for its economies to truly emerge have been slower in coming. Quality jobs remain scarce, and informal employment remains the norm in the labour market. The heterogeneity of this informal economy makes it complex to define and harder to be factored into public policy. The objectives of supporting and modernizing production units and of formalizing and bringing them into line with a certain number of standards and regulations are two complementary sides of any economic policy aimed at tackling the challenges that such informality poses for emerging economies., Desde principios de los años 2000, el continente africano ha experimentado un ciclo de fuerte crecimiento económico, pero las transformaciones estructurales, que son condiciones para la emergencia de sus economías, son más lentas. Los empleos de calidad siguen siendo escasos, y el empleo informal continúa siendo la norma en el mercado laboral. La heterogeneidad de esta economía informal hace compleja su definición y representa un obstáculo para que se tome en consideración en las políticas públicas. Los objetivos de apoyo y modernización de las unidades de producción, así como los objetivos de formalización y adaptación a una serie de normas y reglamentos, son los dos aspectos complementarios de toda política económica destinada a hacer frente al reto de la informalidad con vistas a la emergencia.

Aluminum (Al) toxicity and inorganic phosphate (Pi) limitation are widespread chronic abiotic and mutually enhancing stresses that profoundly affect crop yield. Both stresses cause a strong inhibition of root growth, resulting from a progressive exhaustion of the stem cell niche. Here, we report on a casein kinase 2 (CK2) inhibitor identified by its capability to maintain a functional root stem cell niche under Al toxic conditions. CK2 operates through phosphorylation of the cell cycle checkpoint activator SUPPRESSOR OF GAMMA RADIATION1 (SOG1), priming its activity under DNA-damaging conditions. In addition to yielding Al tolerance, CK2 and SOG1 inactivation prevents meristem exhaustion under Pi starvation, revealing the existence of a low Pi-induced cell cycle checkpoint that depends on the DNA damage activator ATAXIA-TELANGIECTASIA MUTATED. Overall, our data reveal an important physiological role for the plant DNA damage response pathway under agriculturally limiting growth conditions, opening new avenues to cope with Pi limitation. ONE-SENTENCE SUMMARY Casein kinase 2 and DNA damage response regulators play a pivotal role in the control of Arabidopsis root growth in response to Al toxicity and phosphate limitation.

The effects of brassinosteroid signaling on shoot and root development have been characterized in great detail but did not identify a simple consistent positive or negative impact on a basic cellular parameter that would comprehensively explain the phenotype of brassinosteroid-related mutants. Here we combined digital 3D single-cell shape analysis and single-cell mRNA sequencing to characterize root meristems and mature root segments of brassinosteroid-blind mutants and wildtype. These data demonstrate that brassinosteroid signaling neither affects cell volume nor cell proliferation capacity. Instead, brassinosteroid signaling is essential for the precise orientation of cell division planes and the extent and timing of anisotropic cell expansion. Moreover, we found that the cell-aligning effects of brassinosteroid signaling can propagate to normalize the anatomy of both adjacent and distant brassinosteroid-blind cells through non-cell-autonomous functions, which are sufficient to restore overall root growth vigor. Finally, single-cell transcriptome data discern directly brassinosteroid-responsive genes from genes that can react to non-cell-autonomous brassinosteroid-dependent signals and highlight arabinogalactans as sentinels of brassinosteroid-dependent anisotropic cell expansion.

The effects of brassinosteroid signaling on shoot and root development have been characterized in great detail but a simple consistent positive or negative impact on a basic cellular parameter was not identified. In this study, we combined digital 3D single-cell shape analysis and single-cell mRNA sequencing to characterize root meristems and mature root segments of brassinosteroid-blind mutants and wild type. The resultant datasets demonstrate that brassinosteroid signaling affects neither cell volume nor cell proliferation capacity. Instead, brassinosteroid signaling is essential for the precise orientation of cell division planes and the extent and timing of anisotropic cell expansion. Moreover, we found that the cell-aligning effects of brassinosteroid signaling can propagate to normalize the anatomy of both adjacent and distant brassinosteroid-blind cells through non-cell-autonomous functions, which are sufficient to restore growth vigor. Finally, single-cell transcriptome data discern directly brassinosteroid-responsive genes from genes that can react non-cell-autonomously and highlight arabinogalactans as sentinels of brassinosteroid-dependent anisotropic cell expansion., This study combines digital 3D single-cell shape analysis and single-cell mRNA sequencing to characterize roots of brassinosteroid-blind mutants. The data demonstrate that brassinosteroid signaling is essential for precise cell division plane orientation and the extent and timing of anisotropic cell expansion. These cell-aligning brassinosteroid effects can propagate non-cell-autonomously, manifesting in arabinogalactan expression as sentinels of brassinosteroid-dependent anisotropic cell expansion.

The ataxia-telangiectasia mutated (ATM) and ATM and Rad3-related (ATR) kinases coordinate the DNA damage response. The roles described for Arabidopsis thaliana ATR and ATM are assumed to be conserved over other plant species, but molecular evidence is scarce. Here, we demonstrate that the functions of ATR and ATM are only partially conserved between Arabidopsis and maize (Zea mays). In both species, ATR and ATM play a key role in DNA repair and cell cycle checkpoint activation, but whereas Arabidopsis plants do not suffer from the absence of ATR under control growth conditions, maize mutant plants accumulate replication defects, likely due to their large genome size. Moreover, contrarily to Arabidopsis, maize ATM deficiency does not trigger meiotic defects, whereas the ATR kinase appears to be crucial for the maternal fertility. Strikingly, ATR is required to repress premature endocycle onset and cell death in the maize endosperm. Its absence results in a reduction of kernel size, protein and starch content, and a stochastic death of kernels, a process being counteracted by ATM. Additionally, while Arabidopsis atr atm double mutants are viable, no such mutants could be obtained for maize. Therefore, our data highlight that the mechanisms maintaining genome integrity may be more important for vegetative and reproductive development than previously anticipated.

The WEE1 and ATM AND RAD3-RELATED (ATR) kinases are important regulators of the plant intra-S-phase checkpoint; consequently, WEE1KO and ATRKO roots are hypersensitive to replication-inhibitory drugs. Here, we report on a loss-of-function mutant allele of the FASCIATA1 (FAS1) subunit of the chromatin assembly factor 1 (CAF-1) complex that suppresses the phenotype of WEE1- or ATR-deficient Arabidopsis (Arabidopsis thaliana) plants. We demonstrate that lack of FAS1 activity results in the activation of an ATAXIA TELANGIECTASIA MUTATED (ATM)- and SUPPRESSOR OF GAMMA-RESPONSE 1 (SOG1)-mediated G2/M-arrest that renders the ATR and WEE1 checkpoint regulators redundant. This ATM activation accounts for the telomere erosion and loss of ribosomal DNA that are described for fas1 plants. Knocking out SOG1 in the fas1 wee1 background restores replication stress sensitivity, demonstrating that SOG1 is an important secondary checkpoint regulator in plants that fail to activate the intra-S-phase checkpoint.

Plants are known for their outstanding capacity to recover from various wounds and injuries. However, it remains largely unknown how plants sense diverse forms of injury and canalize existing developmental processes into the execution of a correct regenerative response. Auxin, a cardinal plant hormone with morphogen-like properties, has been previously implicated in the recovery from diverse types of wounding and organ loss. Here, through a combination of cellular imaging and in silico modeling, we demonstrate that vascular stem cell death obstructs the polar auxin flux, much alike rocks in a stream, and causes it to accumulate in the endodermis. This in turn grants the endodermal cells the capacity to undergo periclinal cell division to repopulate the vascular stem cell pool. Replenishment of the vasculature by the endodermis depends on the transcription factor ERF115, a wound-inducible regulator of stem cell division. Although not the primary inducer, auxin is required to maintain

The anaphase promoting complex/cyclosome (APC/C) controls unidirectional progression through the cell cycle by marking key cell cycle proteins for proteasomal turnover. Its activity is temporally regulated by the docking of different activating subunits, known in plants as CELL DIVISION PROTEIN20 (CDC20) and CELL CYCLE SWITCH52 (CCS52). Despite the importance of the APC/C during cell proliferation, the number of identified targets in the plant cell cycle is limited. Here, we used the growth and meristem phenotypes of Arabidopsis (Arabidopsis thaliana) CCS52A2-deficient plants in a suppressor mutagenesis screen to identify APC/C(CCS52A2) substrates or regulators, resulting in the identification of a mutant cyclin CYCA3;4 allele. CYCA3;4 deficiency partially rescues the ccs52a2-1 phenotypes, whereas increased CYCA3;4 levels enhance the scored ccs52a2-1 phenotypes. Furthermore, whereas the CYCA3;4 protein is promptly broken down after prophase in wild-type plants, it remains present in later stages of mitosis in ccs52a2-1 mutant plants, marking it as a putative APC/C(CCS52A2) substrate. Strikingly, increased CYCA3;4 levels result in aberrant root meristem and stomatal divisions, mimicking phenotypes of plants with reduced RETINOBLASTOMA-RELATED PROTEIN1 (RBR1) activity. Correspondingly, RBR1 hyperphosphorylation was observed in CYCA3;4 gain-of-function plants. Our data thus demonstrate that an inability to timely destroy CYCA3;4 contributes to disorganized formative divisions, possibly in part caused by the inactivation of RBR1.

UMR BFP - Equipe OrFE; International audience; Galls induced by plant-parasitic nematodes involve a hyperactivation of the plant mitotic and endocycle machinery for their profit. Dedifferentiation of host root cells includes drastic cellular and molecular readjustments. In such background, potential DNA damage in the genome of gall cells is eminent. We questioned if DNA damage checkpoints activation followed by DNA repair occurred, or was eventually circumvented, in nematode-induced galls. Galls display transcriptional activation of the DNA damage checkpoint kinase WEE1, correlated with its protein localization in the nuclei. The promoter of the stress marker gene SMR7 was evaluated under the WEE1-knockout background. Drugs inducing DNA damage and a marker for DNA repair, PARP1 were used to understand mechanisms that might cope with DNA damage in galls. Our functional study revealed that gall cells lacking WEE1 conceivably entered mitosis prematurely disturbing the cell cycle despite the loss of genome integrity. The disrupted nuclei phenotype in giant cells hinted to the accumulation of mitotic defects. As well, WEE1-knockout in Arabidopsis and downregulation in tomato repressed infection and reproduction of root-knot nematodes. Together with data on DNA damaging drugs, we suggest a conserved function for WEE1 controlling a G1/S cell cycle arrest in response to replication defect in galls.

The anaphase promoting complex/cyclosome (APC/C) controls unidirectional progression through the cell cycle by marking key cell cycle proteins for proteasomal turnover. Its activity is temporally regulated by the docking of different activating subunits, known in plants as CELL DIVISION PROTEIN20 (CDC20) and CELL CYCLE SWITCH52 (CCS52). Despite the importance of the APC/C during cell proliferation, the number of identified targets in the plant cell cycle is limited. Here, we used the growth and meristem phenotypes of Arabidopsis (Arabidopsis thaliana) CCS52A2-deficient plants in a suppressor mutagenesis screen to identify APC/CCCS52A2 substrates or regulators, resulting in the identification of a mutant cyclin CYCA3;4 allele. CYCA3;4 deficiency partially rescues the ccs52a2-1 phenotypes, whereas increased CYCA3;4 levels enhance the scored ccs52a2-1 phenotypes. Furthermore, whereas the CYCA3;4 protein is promptly broken down after prophase in wild-type plants, it remains present in later stages of mitosis in ccs52a2-1 mutant plants, marking it as a putative APC/CCCS52A2 substrate. Strikingly, increased CYCA3;4 levels result in aberrant root meristem and stomatal divisions, mimicking phenotypes of plants with reduced RETINOBLASTOMA-RELATED PROTEIN1 (RBR1) activity. Correspondingly, RBR1 hyperphosphorylation was observed in CYCA3;4 gain-of-function plants. Our data thus demonstrate that an inability to timely destroy CYCA3;4 contributes to disorganized formative divisions, possibly in part caused by the inactivation of RBR1.

International audience; In sub-Saharan Africa, the vast majority of businesses belong to the so-called “informal” sector. As yet, little is known about their take up of the mobile phone, which suddenly and massively irrupted into their daily operations within less than a decade. This article, which is based on an original study of 500 entrepreneurs in the informal sector in Dakar, combines quantitative and qualitative methods. Its contribution is threefold. First, the study provides broad and detailed empirical insight into the adoption and use of mobile phones by informal entrepreneurs in sub-Saharan Africa. Second, the article draws on an original survey and analysis methodology to propose a robust and comprehensive typology of these entrepreneurs’ business uses of mobile phones. Finally, it provides a statistical analysis of the relationship between mobile phone uses and economic performance. Ultimately, this article seeks to contribute to a better understanding of the informal sector and its specific dynamics.; En Afrique subsaharienne, l’immense majorité des entreprises appartiennent au secteur dit « informel ». On sait encore peu de choses sur la façon dont elles se sont emparées du téléphone mobile, qui a fait une brusque et massive irruption dans leur quotidien en moins d’une décennie. S’appuyant sur une enquête originale mêlant méthodes quantitatives et qualitatives, auprès de 500 entrepreneurs du secteur informel de Dakar, l’article apporte trois contributions principales. Premièrement, l’enquête apporte un éclairage empirique ample et détaillé sur l’adoption et les usages du mobile par les entrepreneurs informels d’Afrique subsaharienne. Deuxièmement, l’article propose une typologie robuste et compréhensive des usages professionnels du mobile par ces entrepreneurs, à partir d’une méthodologie d’enquête et d’analyse originale. Enfin, l’article restitue une analyse statistique des relations entre usages du mobile et performances économiques. Il s’agit in fine de contribuer à une meilleure connaissance du secteur informel et de ses dynamiques propres.

The endocycle represents a modified mitotic cell cycle that in plants is often coupled to cell enlargement and differentiation. Endocycle onset is controlled by activity of the Anaphase Promoting Complex/Cyclosome (APC/C), a multisubunit E3 ubiquitin ligase targeting cell-cycle factors for destruction. CELL CYCLE SWITCH52 (CCS52) proteins represent rate-limiting activator subunits of the APC/C. In Arabidopsis (Arabidopsis thaliana), mutations in either CCS52A1 or CCS52A2 activators result in a delayed endocycle onset, whereas their overexpression triggers increased DNA ploidy levels. Here, the relative contribution of the APC/C-CCS52A1 and APC/C-CCS52A2 complexes to different developmental processes was studied through analysis of their negative regulators, being the ULTRAVIOLET-B-INSENSITIVE4 protein and the DP-E2F-Like1 transcriptional repressor, respectively. Our data illustrate cooperative activity of the APC/C-CCS52A1 and APC/C-CCS52A2 complexes during root and trichome development, but functional interdependency during leaf development. Furthermore, we found APC/C-CCS52A1 activity to control CCS52A2 expression. We conclude that interdependency of CCS52A-controlled APC/C activity is controlled in a tissue-specific manner.

Although aluminum (Al) toxicity represents a global agricultural problem, the biochemical targets for Al remain elusive. Recently identified Arabidopsis mutants with increased Al tolerance provide evidence of DNA as one of the main targets of Al. This insight could lead the way for novel strategies to generate Al-tolerant crop plants.

Cadmium (Cd) exposure causes an oxidative challenge and inhibits cell cycle progression, ultimately impacting plant growth. Stress-induced effects on the cell cycle are often a consequence of activation of the DNA damage response (DDR). The main aim of this study was to investigate the role of the transcription factor SUPPRESSOR OF GAMMA RESPONSE 1 (SOG1) and three downstream cyclin-dependent kinase inhibitors of the SIAMESE-RELATED (SMR) family in the Cd-induced DDR and oxidative challenge in leaves of Arabidopsis thaliana. Effects of Cd on plant growth, cell cycle regulation and the expression of DDR genes were highly similar between the wildtype and smr4/5/7 mutant. In contrast, sog1-7 mutant leaves displayed a much lower Cd sensitivity within the experimental time-frame and significantly less pronounced upregulations of DDR-related genes, indicating the involvement of SOG1 in the Cd-induced DDR. Cadmium-induced responses related to the oxidative challenge were disturbed in the sog1-7 mutant, as indicated by delayed Cd-induced increases of hydrogen peroxide and glutathione concentrations and lower upregulations of oxidative stress-related genes. In conclusion, our results attribute a novel role to SOG1 in regulating the oxidative stress response and connect oxidative stress to the DDR in Cd-exposed plants.

International audience; Somatic polyploidy caused by endoreplication is observed in arthropods, molluscs, and vertebrates but is especially prominent in higher plants, where it has been postulated to be essential for cell growth and fate maintenance. However, a comprehensive understanding of the physiological significance of plant endopolyploidy has remained elusive. Here, we modeled and experimentally verified a high-resolution DNA endoploidy map of the developing Arabidopsis thaliana root, revealing a remarkable spatiotemporal control of DNA endoploidy levels across tissues. Fitting of a simplified model to publicly available data sets profiling root gene expression under various environmental stress conditions suggested that this root endoploidy patterning may be stress-responsive. Furthermore, cellular and transcriptomic analyses revealed that inhibition of endoreplication onset alters the nuclear-to-cellular volume ratio and the expression of cell wall-modifying genes, in correlation with the appearance of cell structural changes. Our data indicate that endopolyploidy might serve to coordinate cell expansion with structural stability and that spatiotemporal endoreplication pattern changes may buffer for stress conditions, which may explain the widespread occurrence of the endocycle in plant species growing in extreme or variable environments.

Whereas our knowledge about the diverse pathways aiding DNA repair upon genome damage is steadily increasing, little is known about the molecular players that adjust the plant cell cycle in response to DNA stress. By a meta-analysis of DNA stress microarray data sets, three family members of the SIAMESE/SIAMESE-RELATED (SIM/SMR) class of cyclin-dependent kinase inhibitors were discovered that react strongly to genotoxicity. Transcriptional reporter constructs corroborated specific and strong activation of the three SIM/SMR genes in the meristems upon DNA stress, whereas overexpression analysis confirmed their cell cycle inhibitory potential. In agreement with being checkpoint regulators, SMR5 and SMR7 knockout plants displayed an impaired checkpoint in leaf cells upon treatment with the replication inhibitory drug hydroxyurea (HU). Surprisingly, HU-induced SMR5/SMR7 expression depends on ATAXIA TELANGIECTASIA MUTATED (ATM) and SUPPRESSOR OF GAMMA RESPONSE1, rather than on the anticipated replication stress-activated ATM AND RAD3-RELATED kinase. This apparent discrepancy was explained by demonstrating that, in addition to its effect on replication, HU triggers the formation of reactive oxygen species (ROS). ROS-dependent transcriptional activation of the SMR genes was confirmed by different ROS-inducing conditions, including high-light treatment. We conclude that the identified SMR genes are part of a signaling cascade that induces a cell cycle checkpoint in response to ROS-induced DNA damage.

Because of their sessile lifestyle, plants have developed extensive mechanisms to safeguard their genetic information from one generation to the next. The WEE1 kinase is one of the guardians of genome integrity, being important during S-phase progression under replication stress. Knock-out plants for WEE1 (WEE1(KO)) show a hypersensitive response when grown on replication-inhibiting drugs. Recently, we reported the identification of a mutant in the RNase H2A gene that could partially complement this replication phenotype. Here, we present the identification of a second member of the RNase H2 complex, RNase H2B, being able to complement the root growth phenotype of WEE1(KO) plants. We additionally show that deletion of a conserved domain in RNase H2B leads to loss of interaction with the RNase H2C subunit, likely explaining the loss of activity of the RNase H2 complex.