Genome-wide identification of RETINOBLASTOMA RELATED 1 binding sites in Arabidopsis reveals novel DNA damage regulators

The dual role of the RETINOBLASTOMA-RELATED protein in the DNA damage response is coordinated by the interaction with LXCXE-containing proteins

Living organisms possess mechanisms to safeguard genome integrity. To avoid spreading mutations, DNA lesions are detected and cell division is temporarily arrested to allow repair mechanisms. Afterward, cells either resume division or respond to unsuccessful repair by undergoing programmed cell death (PCD). How the success rate of DNA repair connects to later cell fate decisions remains incompletely known, particularly in plants. The Arabidopsis thaliana RETINOBLASTOMA-RELATED1 (RBR) protein and its partner E2FA, play both structural and transcriptional functions in the DNA damage response (DDR). Here we provide evidence that distinct RBR protein interactions with LXCXE motif-containing proteins guide these processes. Using the N849F substitution in the RBR B-pocket domain, which specifically disrupts binding to the LXCXE motif, we show that these interactions are dispensable in unchallenging conditions. However, N849F substitution abolishes RBR nuclear foci and promotes PCD and growth arrest upon genotoxic stress. NAC044, which promotes growth arrest and PCD, accumulates after the initial recruitment of RBR to foci and can bind non-focalized RBR through the LXCXE motif in a phosphorylation-independent manner, allowing interaction at different cell cycle phases. Disrupting NAC044-RBR interaction impairs PCD, but their genetic interaction points to opposite independent roles in the regulation of PCD. The LXCXE-binding dependency of the roles of RBR in the DDR suggests a coordinating mechanism to translate DNA repair success to cell survival. We propose that RBR and NAC044 act in two distinct DDR pathways, but interact to integrate input from both DDR pathways to decide upon an irreversible cell fate decision.

Zeocin-induced DNA damage response in barley and its dependence on ATR

DNA damage response (DDR) is an essential mechanism by which living organisms maintain their genomic stability. In plants, DDR is important also for normal growth and yield. Here, we explored the DDR of a temperate model crop barley (Hordeum vulgare) at the phenotypic, physiological, and transcriptomic levels. By a series of in vitro DNA damage assays using the DNA strand break (DNA-SB) inducing agent zeocin, we showed reduced root growth and expansion of the differentiated zone to the root tip. Genome-wide transcriptional profiling of barley wild-type and plants mutated in DDR signaling kinase ATAXIA TELANGIECTASIA MUTATED AND RAD3-RELATED (hvatr.g) revealed zeocin-dependent, ATR-dependent, and zeocin-dependent/ATR-independent transcriptional responses. Transcriptional changes were scored also using the newly developed catalog of 421 barley DDR genes with the phylogenetically-resolved relationships of barley SUPRESSOR OF GAMMA 1 (SOG1) and SOG1-LIKE (SGL) genes. Zeocin caused up-regulation of specific DDR factors and down-regulation of cell cycle and histone genes, mostly in an ATR-independent manner. The ATR dependency was obvious for some factors associated with DDR during DNA replication and for many genes without an obvious connection to DDR. This provided molecular insight into the response to DNA-SB induction in the large and complex barley genome.

Distinctive and complementary roles of E2F transcription factors during plant replication stress responses

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 replication defects, accumulating evidence points to other pathways functioning independent of SOG1. Here, we report the roles of the Arabidopsis E2FA and EF2B transcription factors, two well-characterized regulators of DNA replication, in plant 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 plant growth in the presence of replication defects, either operating antagonistically or synergistically with SOG1. Conversely, SOG1 aids in overcoming the replication defects of E2FA/E2FB-deficient plants. Collectively, our data reveal a complex transcriptional network controlling the replication stress response in which E2Fs and SOG1 act as key regulatory factors.

Historical Selection, Adaptation Signatures, and Ambiguity of Introgressions in Wheat

Wheat was one of the crops domesticated in the Fertile Crescent region approximately 10,000 years ago. Despite undergoing recent polyploidization, hull-to-free-thresh transition events, and domestication bottlenecks, wheat is now grown in over 130 countries and accounts for a quarter of the world's cereal production. The main reason for its widespread success is its broad genetic diversity that allows it to thrive in different environments. To trace historical selection and hybridization signatures, genome scans were performed on two datasets: approximately 113K SNPs from 921 predominantly bread wheat accessions and approximately 110K SNPs from about 400 wheat accessions representing all ploidy levels. To identify environmental factors associated with the loci, a genome-environment association (GEA) was also performed. The genome scans on both datasets identified a highly differentiated region on chromosome 4A where accessions in the first dataset were dichotomized into a group (n = 691), comprising nearly all cultivars, wild emmer, and most landraces, and a second group (n = 230), dominated by landraces and spelt accessions. The grouping of cultivars is likely linked to their potential ancestor, bread wheat cv. Norin-10. The 4A region harbored important genes involved in adaptations to environmental conditions. The GEA detected loci associated with latitude and temperature. The genetic signatures detected in this study provide insight into the historical selection and hybridization events in the wheat genome that shaped its current genetic structure and facilitated its success in a wide spectrum of environmental conditions. The genome scans and GEA approaches applied in this study can help in screening the germplasm housed in gene banks for breeding, and for conservation purposes.

RETINOBLASTOMA-RELATED interactions with key factors of the RNA-directed DNA methylation (RdDM) pathway and its influence on root development

MAIN CONCLUSION

Our study presents evidence for a novel mechanism for RBR function in transcriptional gene silencing by interacting with key players of the RdDM pathway in Arabidopsis and several plant clades. Transposable elements and other repetitive elements are silenced by the RNA-directed DNA methylation pathway (RdDM). In RdDM, POLIV-derived transcripts are converted into double-stranded RNA (dsRNA) by the activity of RDR2 and subsequently processed into 24 nucleotide short interfering RNAs (24-nt siRNAs) by DCL3. 24-nt siRNAs serve as guides to direct AGO4-siRNA complexes to chromatin-bound POLV-derived transcripts generated from the template/target DNA. The interaction between POLV, AGO4, DMS3, DRD1, RDM1 and DRM2 promotes DRM2-mediated de novo DNA methylation. The Arabidopsis Retinoblastoma protein homolog (RBR) is a master regulator of the cell cycle, stem cell maintenance, and development. We in silico predicted and explored experimentally the protein-protein interactions (PPIs) between RBR and members of the RdDM pathway. We found that the largest subunits of POLIV and POLV (NRPD1 and NRPE1), the shared second largest subunit of POLIV and POLV (NRPD/E2), RDR1, RDR2, DCL3, DRM2, and SUVR2 contain canonical and non-canonical RBR binding motifs and several of them are conserved since algae and bryophytes. We validated experimentally PPIs between Arabidopsis RBR and several of the RdDM pathway proteins. Moreover, seedlings from loss-of-function mutants in RdDM and RBR show similar phenotypes in the root apical meristem. We show that RdDM and SUVR2 targets are up-regulated in the 35S:AmiGO-RBR background.

The Arabidopsis transcription factor NLP2 regulates early nitrate responses and integrates nitrate assimilation with energy and carbon skeleton supply

Nitrate signaling improves plant growth under limited nitrate availability and, hence, optimal resource use for crop production. Whereas several transcriptional regulators of nitrate signaling have been identified, including the Arabidopsis thaliana transcription factor NIN-LIKE PROTEIN7 (NLP7), additional regulators are expected to fine-tune this pivotal physiological response. Here, we characterized Arabidopsis NLP2 as a top-tier transcriptional regulator of the early nitrate response gene regulatory network. NLP2 interacts with NLP7 in vivo and shares key molecular features such as nitrate-dependent nuclear localization, DNA-binding motif, and some target genes with NLP7. Genetic, genomic, and metabolic approaches revealed a specific role for NLP2 in the nitrate-dependent regulation of carbon and energy-related processes that likely influence plant growth under distinct nitrogen environments. Our findings highlight the complementarity and specificity of NLP2 and NLP7 in orchestrating a multitiered nitrate regulatory network that links nitrate assimilation with carbon and energy metabolism for efficient nitrogen use and biomass production.

KNO1‐mediated autophagic degradation of the Bloom syndrome complex component RMI1 promotes homologous recombination

Homologous recombination (HR) is a key DNA damage repair pathway that is tightly adjusted to the state of a cell. A central regulator of homologous recombination is the conserved helicase-containing Bloom syndrome complex, renowned for its crucial role in maintaining genome integrity. Here, we show that in Arabidopsis thaliana, Bloom complex activity is controlled by selective autophagy. We find that the recently identified DNA damage regulator KNO1 facilitates K63-linked ubiquitination of RMI1, a structural component of the complex, thereby triggering RMI1 autophagic degradation and resulting in increased homologous recombination. Conversely, reduced autophagic activity makes plants hypersensitive to DNA damage. KNO1 itself is also controlled at the level of proteolysis, in this case mediated by the ubiquitin-proteasome system, becoming stabilized upon DNA damage via two redundantly acting deubiquitinases, UBP12 and UBP13. These findings uncover a regulatory cascade of selective and interconnected protein degradation steps resulting in a fine-tuned HR response upon DNA damage.

Genome-Wide Identification and Expression Analysis of Cysteine-Rich Polycomb-like Protein (CPP) Gene Family in Tomato

The cysteine-rich polycomb-like protein (CPP) gene family is a class of transcription factors containing conserved cysteine-rich CRC structural domains that is involved in the regulation of plant growth and stress tolerance to adversity. Relative to other gene families, the CPP gene family has not received sufficient attention. In this study, six SlCPPs were identified for the first time using the most recent genome-wide identification data of tomato. Subsequently, a phylogenetic analysis classified SlCPPs into four subfamilies. The analysis of cis-acting elements in the promoter indicates that SlCPPs are involved in plant growth and development and also stress response. We present for the first time the prediction of the tertiary structure of these SlCPPs proteins using the AlphaFold2 artificial intelligence system developed by the DeepMind team. Transcriptome data analysis showed that SlCPPs were differentially expressed in different tissues. Gene expression profiling showed that all SlCPPs except SlCPP5 were up-regulated under drought stress; SlCPP2, SlCPP3 and SlCPP4 were up-regulated under cold stress; SlCPP2 and SlCPP5 were up-regulated under salt stress; all SlCPPs were up-regulated under inoculation with Cladosporium fulvum; and SlCPP1, SlCPP3, and SlCPP4 were up-regulated under inoculation with Stemphylium lycopersici. We performed a virus-induced gene silencing experiment on SlCPP3, and the results indicated that SlCPP3 was involved in the response to drought stress. Finally, we predicted the interaction network of the key gene SlCPP3, and there was an interaction relationship between SlCPP3 and 10 genes, such as RBR1 and MSI1. The positive outcome showed that SlCPPs responded to environmental stress. This study provides a theoretical and empirical basis for the response mechanisms of tomato in abiotic stresses.

How Do Plants Cope with DNA Damage? A Concise Review on the DDR Pathway in Plants

DNA damage is induced by many factors, some of which naturally occur in the environment. Because of their sessile nature, plants are especially exposed to unfavorable conditions causing DNA damage. In response to this damage, the DDR (DNA damage response) pathway is activated. This pathway is highly conserved between eukaryotes; however, there are some plant-specific DDR elements, such as SOG1-a transcription factor that is a central DDR regulator in plants. In general, DDR signaling activates transcriptional and epigenetic regulators that orchestrate the cell cycle arrest and DNA repair mechanisms upon DNA damage. The cell cycle halts to give the cell time to repair damaged DNA before replication. If the repair is successful, the cell cycle is reactivated. However, if the DNA repair mechanisms fail and DNA lesions accumulate, the cell enters the apoptotic pathway. Thereby the proper maintenance of DDR is crucial for plants to survive. It is particularly important for agronomically important species because exposure to environmental stresses causing DNA damage leads to growth inhibition and yield reduction. Thereby, gaining knowledge regarding the DDR pathway in crops may have a huge agronomic impact-it may be useful in breeding new cultivars more tolerant to such stresses. In this review, we characterize different genotoxic agents and their mode of action, describe DDR activation and signaling and summarize DNA repair mechanisms in plants.

Epigenetic regulation in the shoot apical meristem

Epigenetic mechanisms form the basis of cellular memory, developmental decisions, and the cellular immune system that defends against transposons and viruses. Organs develop from the shoot apical meristem (SAM) to shape the plant's areal phenotype, and stem cells in the SAM serve as a functional germline. While many details on the regulation of stem cell pool size, organ initiation, and patterning at the meristem periphery are known, we know surprisingly little about the molecular characteristics of SAM cells, including their epigenome and how it changes during development. Here, we summarize information on epigenetic regulation of selected genes necessary for stem cell maintenance. As recent evidence suggests that SAM stem cells might be a hotspot of transposon activation, we discuss this aspect of epigenetic control in the meristem and speculate on mechanisms that maintain the flexibility of SAM stem cells in response to developmental or environmental cues.

Transcriptomic and methylation analysis of susceptible and tolerant grapevine genotypes following Plasmopara viticola infection

Downy mildew, caused by the biotrophic oomycete Plasmopara viticola, is one of the most economically significant grapevine diseases worldwide. Current strategies to cope with this threat rely on the massive use of chemical compounds during each cultivation season. The economic costs and negative environmental impact associated with these applications increased the urge to search for sustainable strategies of disease control. Improved knowledge of plant mechanisms to counteract pathogen infection may allow the development of alternative strategies for plant protection. Epigenetic regulation, in particular DNA methylation, is emerging as a key factor in the context of plant-pathogen interactions associated with the expression modulation of defence genes. To improve our understanding of the genetic and epigenetic mechanisms underpinning grapevine response to P. viticola, we studied the modulation of both 5-mC methylation and gene expression at 6 and 24 h post-infection (hpi). Leaves of two table grape genotypes (Vitis vinifera), selected by breeding activities for their contrasting level of susceptibility to the pathogen, were analysed. Following pathogen infection, we found variations in the 5-mC methylation level and the gene expression profile. The results indicate a genotype-specific response to pathogen infection. The tolerant genotype (N23/018) at 6 hpi exhibits a lower methylation level compared to the susceptible one (N20/020), and it shows an early modulation (at 6 hpi) of defence and epigenetic-related genes during P. viticola infection. These data suggest that the timing of response is an important mechanism to efficiently counteract the pathogen attack.

Zebularine, a DNA Methylation Inhibitor, Activates Anthocyanin Accumulation in Grapevine Cells

Through its role in the regulation of gene expression, DNA methylation can participate in the control of specialized metabolite production. We have investigated the link between DNA methylation and anthocyanin accumulation in grapevine using the hypomethylating drug, zebularine and Gamay Teinturier cell suspensions. In this model, zebularine increased anthocyanin accumulation in the light, and induced its production in the dark. To unravel the underlying mechanisms, cell transcriptome, metabolic content, and DNA methylation were analyzed. The up-regulation of stress-related genes, as well as a decrease in cell viability, revealed that zebularine affected cell integrity. Concomitantly, the global DNA methylation level was only slightly decreased in the light and not modified in the dark. However, locus-specific analyses demonstrated a decrease in DNA methylation at a few selected loci, including a CACTA DNA transposon and a small region upstream from the UFGT gene, coding for the UDP glucose:flavonoid-3-O-glucosyltransferase, known to be critical for anthocyanin biosynthesis. Moreover, this decrease was correlated with an increase in UFGT expression and in anthocyanin content. In conclusion, our data suggest that UFGT expression could be regulated through DNA methylation in Gamay Teinturier, although the functional link between changes in DNA methylation and UFGT transcription still needs to be demonstrated.

Metabolic and DNA checkpoints for the enhancement of Al tolerance

Acidic soils are a major limiting factor for food production in many developing countries. High concentrations of soluble Al cations, particularly Al³⁺, inhibit cell division and root elongation in plants. Al³⁺ damages several biomolecules, including DNA, impairing gene expression and cell cycle progression. Notably, the loss-of-function mutants of DNA checkpoints may mediate Al tolerance. Furthermore, mitochondrial organic acids play key roles in neutralizing Al³⁺ within the cell and around the rhizosphere. Here, we provide knowledge synthesis on interactions between checkpoints related to mitochondrial organic acid homeostasis and DNA integrity mediating Al tolerance in land plants. These interactions, coupled with remarkable advances in tools related to metabolism and cell cycle, may facilitate the development of next-generation productive crops under Al toxicity.

A hierarchical transcriptional network activates specific CDK inhibitors that regulate G2 to control cell size and number in Arabidopsis

How cell size and number are determined during organ development remains a fundamental question in cell biology. Here, we identified a GRAS family transcription factor, called SCARECROW-LIKE28 (SCL28), with a critical role in determining cell size in Arabidopsis. SCL28 is part of a transcriptional regulatory network downstream of the central MYB3Rs that regulate G2 to M phase cell cycle transition. We show that SCL28 forms a dimer with the AP2-type transcription factor, AtSMOS1, which defines the specificity for promoter binding and directly activates transcription of a specific set of SIAMESE-RELATED (SMR) family genes, encoding plant-specific inhibitors of cyclin-dependent kinases and thus inhibiting cell cycle progression at G2 and promoting the onset of endoreplication. Through this dose-dependent regulation of SMR transcription, SCL28 quantitatively sets the balance between cell size and number without dramatically changing final organ size. We propose that this hierarchical transcriptional network constitutes a cell cycle regulatory mechanism that allows to adjust cell size and number to attain robust organ growth.

Cell cycle checkpoint control in response to DNA damage by environmental stresses

Being sessile organisms, plants are ubiquitously exposed to stresses that can affect the DNA replication process or cause DNA damage. To cope with these problems, plants utilize DNA damage response (DDR) pathways, consisting of both highly conserved and plant-specific elements. As a part of this DDR, cell cycle checkpoint control mechanisms either pause the cell cycle, to allow DNA repair, or lead cells into differentiation or programmed cell death, to prevent the transmission of DNA errors in the organism through mitosis or to its offspring via meiosis. The two major DDR cell cycle checkpoints control either the replication process or the G2/M transition. The latter is largely overseen by the plant-specific SOG1 transcription factor, which drives the activity of cyclin-dependent kinase inhibitors and MYB3R proteins, which are rate limiting for the G2/M transition. By contrast, the replication checkpoint is controlled by different players, including the conserved kinase WEE1 and likely the transcriptional repressor RBR1. These checkpoint mechanisms are called upon during developmental processes, in retrograde signaling pathways, and in response to biotic and abiotic stresses, including metal toxicity, cold, salinity, and phosphate deficiency. Additionally, the recent expansion of research from Arabidopsis to other model plants has revealed species-specific aspects of the DDR. Overall, it is becoming evidently clear that the DNA damage checkpoint mechanisms represent an important aspect of the adaptation of plants to a changing environment, hence gaining more knowledge about this topic might be helpful to increase the resilience of plants to climate change.

Cycling in a crowd: Coordination of plant cell division, growth, and cell fate

The reiterative organogenesis that drives plant growth relies on the constant production of new cells, which remain encased by interconnected cell walls. For these reasons, plant morphogenesis strictly depends on the rate and orientation of both cell division and cell growth. Important progress has been made in recent years in understanding how cell cycle progression and the orientation of cell divisions are coordinated with cell and organ growth and with the acquisition of specialized cell fates. We review basic concepts and players in plant cell cycle and division, and then focus on their links to growth-related cues, such as metabolic state, cell size, cell geometry, and cell mechanics, and on how cell cycle progression and cell division are linked to specific cell fates. The retinoblastoma pathway has emerged as a major player in the coordination of the cell cycle with both growth and cell identity, while microtubule dynamics are central in the coordination of oriented cell divisions. Future challenges include clarifying feedbacks between growth and cell cycle progression, revealing the molecular basis of cell division orientation in response to mechanical and chemical signals, and probing the links between cell fate changes and chromatin dynamics during the cell cycle.

Constitutively enhanced genome integrity maintenance and direct stress mitigation characterize transcriptome of extreme stress-adapted Arabidopsis halleri

Heavy metal-rich toxic soils and ordinary soils are both natural habitats of Arabidopsis halleri, a diploid perennial and obligate outcrosser in the sister clade of the genetic model plant Arabidopsis thaliana. The molecular divergence underlying survival in sharply contrasting environments is unknown. Here we comparatively address metal physiology and transcriptomes of A. halleri originating from the most highly heavy metal-contaminated soil in Europe, Ponte Nossa, Italy (Noss), and from non-metalliferous (NM) soils. Plants from Noss exhibit enhanced hypertolerance and attenuated accumulation of cadmium (Cd), and their transcriptomic Cd responsiveness is decreased, compared to plants of NM soil origin. Among the condition-independent transcriptome characteristics of Noss, the most highly overrepresented functional class of 'meiotic cell cycle' comprises 21 transcripts with elevated abundance in vegetative tissues, in particular Argonaute 9 (AGO9) and the synaptonemal complex transverse filament protein-encoding ZYP1a/b. Increased AGO9 transcript levels in Noss are accompanied by decreased long terminal repeat retrotransposon expression. Similar to Noss, plants from other highly metalliferous sites in Poland and Germany share elevated somatic AGO9 transcript levels in comparison to plants originating from NM soils in their respective geographic regions. Transcript levels of Iron-Regulated Transporter 1 (IRT1) are very low and transcript levels of Heavy Metal ATPase 2 (HMA2) are strongly elevated in Noss, which can account for its altered Cd handling. We conclude that in plants adapted to the most extreme abiotic stress, broadly enhanced functions comprise genes with likely roles in somatic genome integrity maintenance, accompanied by few alterations in stress-specific functional networks.

The DREAM complex represses growth in response to DNA damage in Arabidopsis

The DNA of all organisms is constantly damaged by physiological processes and environmental conditions. Upon persistent damage, plant growth and cell proliferation are reduced. Based on previous findings that RBR1, the only Arabidopsis homolog of the mammalian tumor suppressor gene retinoblastoma, plays a key role in the DNA damage response in plants, we unravel here the network of RBR1 interactors under DNA stress conditions. This led to the identification of homologs of every DREAM component in Arabidopsis, including previously not recognized homologs of LIN52. Interestingly, we also discovered NAC044, a mediator of DNA damage response in plants and close homolog of the major DNA damage regulator SOG1, to directly interact with RBR1 and the DREAM component LIN37B. Consistently, not only mutants in NAC044 but also the double mutant of the two LIN37 homologs and mutants for the DREAM component E2FB showed reduced sensitivities to DNA-damaging conditions. Our work indicates the existence of multiple DREAM complexes that work in conjunction with NAC044 to mediate growth arrest after DNA damage.

Connections between the Cell Cycle and the DNA Damage Response in Plants

Due to their sessile lifestyle, plants are especially exposed to various stresses, including genotoxic stress, which results in altered genome integrity. Upon the detection of DNA damage, distinct cellular responses lead to cell cycle arrest and the induction of DNA repair mechanisms. Interestingly, it has been shown that some cell cycle regulators are not only required for meristem activity and plant development but are also key to cope with the occurrence of DNA lesions. In this review, we first summarize some important regulatory steps of the plant cell cycle and present a brief overview of the DNA damage response (DDR) mechanisms. Then, the role played by some cell cycle regulators at the interface between the cell cycle and DNA damage responses is discussed more specifically.

Hydroxyurea and Caffeine Impact pRb-like Protein-Dependent Chromatin Architecture Profiles in Interphase Cells of Vicia faba

The survival of cells depends on their ability to replicate correctly genetic material. Cells exposed to replication stress can experience a number of problems that may lead to deregulated proliferation, the development of cancer, and/or programmed cell death. In this article, we have induced prolonged replication arrest via hydroxyurea (HU) treatment and also premature chromosome condensation (PCC) by co-treatment with HU and caffeine (CF) in the root meristem cells of Vicia faba. We have analyzed the changes in the activities of retinoblastoma-like protein (RbS807/811ph). Results obtained from the immunocytochemical detection of RbS807/811ph allowed us to distinguish five unique activity profiles of pRb. We have also performed detailed 3D modeling using Blender 2.9.1., based on the original data and some final conclusions. 3D models helped us to visualize better the events occurring within the nuclei and acted as a high-resolution aid for presenting the results. We have found that, despite the decrease in pRb activity, its activity profiles were mostly intact and clearly recognizable, with some local alterations that may correspond to the increased demand in transcriptional activity. Our findings suggest that Vicia faba’s ability to withstand harsh environments may come from its well-developed and highly effective response to replication stress.

Pickle Recruits Retinoblastoma Related 1 to Control Lateral Root Formation in Arabidopsis

Lateral root (LR) formation is an example of a plant post-embryonic organogenesis event. LRs are issued from non-dividing cells entering consecutive steps of formative divisions, proliferation and elongation. The chromatin remodeling protein PICKLE (PKL) negatively regulates auxin-mediated LR formation through a mechanism that is not yet known. Here we show that PKL interacts with RETINOBLASTOMA-RELATED 1 (RBR1) to repress the LATERAL ORGAN BOUNDARIES-DOMAIN 16 (LBD16) promoter activity. Since LBD16 function is required for the formative division of LR founder cells, repression mediated by the PKL–RBR1 complex negatively regulates formative division and LR formation. Inhibition of LR formation by PKL–RBR1 is counteracted by auxin, indicating that, in addition to auxin-mediated transcriptional responses, the fine-tuned process of LR formation is also controlled at the chromatin level in an auxin-signaling dependent manner.

MYB3R-mediated active repression of cell cycle and growth under salt stress in Arabidopsis thaliana

Under environmental stress, plants are believed to actively repress their growth to save resource and alter its allocation to acquire tolerance against the stress. Although a lot of studies have uncovered precise mechanisms for responding to stress and acquiring tolerance, the mechanisms for regulating growth repression under stress are not as well understood. It is especially unclear which particular genes related to cell cycle control are involved in active growth repression. Here, we showed that decreased growth in plants exposed to moderate salt stress is mediated by MYB3R transcription factors that have been known to positively and negatively regulate the transcription of G2/M-specific genes. Our genome-wide gene expression analysis revealed occurrences of general downregulation of G2/M-specific genes in Arabidopsis under salt stress. Importantly, this downregulation is significantly and universally mitigated by the loss of MYB3R repressors by mutations. Accordingly, the growth performance of Arabidopsis plants under salt stress is significantly recovered in mutants lacking MYB3R repressors. This growth recovery involves improved cell proliferation that is possibly due to prolonging and accelerating cell proliferation, which were partly suggested by enlarged root meristem and increased number of cells positive for CYCB1;1-GUS. Our ploidy analysis further suggested that cell cycle progression at the G2 phase was delayed under salt stress, and this delay was recovered by loss of MYB3R repressors. Under salt stress, the changes in expression of MYB3R activators and repressors at both the mRNA and protein levels were not significant. This observation suggests novel mechanisms underlying MYB3R-mediated growth repression under salt stress that are different from the mechanisms operating under other stress conditions such as DNA damage and high temperature.

Direct Regulation of DNA Repair by E2F and RB in Mammals and Plants: Core Function or Convergent Evolution?

Simple Summary

Retinoblastoma (RB) proteins and E2F transcription factors partner together to regulate the cell cycle in many eukaryotic organisms. In organisms that lack one or both of these proteins, other proteins have taken on the essential function of cell cycle regulation. RB and E2F also have important functions outside of the cell cycle, including DNA repair. This review summarizes the non-canonical functions of RB and E2F in maintaining genome integrity and raises the question of whether such functions have always been present or have evolved more recently.

Abstract

Members of the E2F transcription factor family regulate the expression of genes important for DNA replication and mitotic cell division in most eukaryotes. Homologs of the retinoblastoma (RB) tumor suppressor inhibit the activity of E2F factors, thus controlling cell cycle progression. Organisms such as budding and fission yeast have lost genes encoding E2F and RB, but have gained genes encoding other proteins that take on E2F and RB cell cycle-related functions. In addition to regulating cell proliferation, E2F and RB homologs have non-canonical functions outside the mitotic cell cycle in a variety of eukaryotes. For example, in both mammals and plants, E2F and RB homologs localize to DNA double-strand breaks (DSBs) and directly promote repair by homologous recombination (HR). Here, we discuss the parallels between mammalian E2F1 and RB and their Arabidopsis homologs, E2FA and RB-related (RBR), with respect to their recruitment to sites of DNA damage and how they help recruit repair factors important for DNA end resection. We also explore the question of whether this role in DNA repair is a conserved ancient function of the E2F and RB homologs in the last eukaryotic common ancestor or whether this function evolved independently in mammals and plants.

Mobility connects: transposable elements wire new transcriptional networks by transferring transcription factor binding motifs

Transposable elements (TEs) constitute major fractions of plant genomes. Their potential to be mobile provides them with the capacity to cause major genome rearrangements. Those effects are potentially deleterious and enforced the evolution of epigenetic suppressive mechanisms controlling TE activity. However, beyond their deleterious effects, TE insertions can be neutral or even advantageous for the host, leading to long-term retention of TEs in the host genome. Indeed, TEs are increasingly recognized as major drivers of evolutionary novelties by regulating the expression of nearby genes. TEs frequently contain binding motifs for transcription factors and capture binding motifs during transposition, which they spread through the genome by transposition. Thus, TEs drive the evolution and diversification of gene regulatory networks by recruiting lineage-specific targets under the regulatory control of specific transcription factors. This process can explain the rapid and repeated evolution of developmental novelties, such as C₄ photosynthesis and a wide spectrum of stress responses in plants. It also underpins the convergent evolution of embryo nourishing tissues, the placenta in mammals and the endosperm in flowering plants. Furthermore, the gene regulatory network underlying flower development has also been largely reshaped by TE-mediated recruitment of regulatory elements; some of them being preserved across long evolutionary timescales. In this review, we highlight the potential role of TEs as evolutionary toolkits in plants by showcasing examples of TE-mediated evolutionary novelties.

The Cyclin CYCA3;4 Is a Postprophase Target of the APC/CCCS52A2 E3-Ligase Controlling Formative Cell Divisions in Arabidopsis

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.

Roles of plant retinoblastoma protein: cell cycle and beyond

The human retinoblastoma (RB1) protein is a tumor suppressor that negatively regulates cell cycle progression through its interaction with members of the E2F/DP family of transcription factors. However, RB-related (RBR) proteins are an early acquisition during eukaryote evolution present in plant lineages, including unicellular algae, ancient plants (ferns, lycophytes, liverworts, mosses), gymnosperms, and angiosperms. The main RBR protein domains and interactions with E2Fs are conserved in all eukaryotes and not only regulate the G1/S transition but also the G2/M transition, as part of DREAM complexes. RBR proteins are also important for asymmetric cell division, stem cell maintenance, and the DNA damage response (DDR). RBR proteins play crucial roles at every developmental phase transition, in association with chromatin factors, as well as during the reproductive phase during female and male gametes production and embryo development. Here, we review the processes where plant RBR proteins play a role and discuss possible avenues of research to obtain a full picture of the multifunctional roles of RBR for plant life.

Functional Analysis of the Plant Chromosomal Passenger Complex

The Aurora B kinase, encoded by the AURORA 3 (AUR3) gene in Arabidopsis (Arabidopsis thaliana), is a key regulator of cell division in all eukaryotes. Aurora B has at least two central functions during cell division; it is essential for the correct, i.e. balanced, segregation of chromosomes in mitosis and meiosis by controlling kinetochore function, and it acts at the division plane, where it is necessary to complete cytokinesis. To accomplish these two spatially distinct functions, Aurora B in animals is guided to its sites of action by Borealin, inner centromere protein (INCENP), and Survivin, which, together with Aurora B, form the chromosome passenger complex (CPC). However, besides Aurora homologs, only a candidate gene with restricted homology to INCENP has been described in Arabidopsis, raising the question of whether a full complement of the CPC exists in plants and how Aurora homologs are targeted subcellularly. Here, we have identified and functionally characterized a Borealin homolog, BOREALIN RELATED (BORR), in Arabidopsis. Together with detailed localization studies including the putative Arabidopsis INCENP homolog, these results support the existence of a CPC in plants.

Beyond What Your Retina Can See: Similarities of Retinoblastoma Function between Plants and Animals, from Developmental Processes to Epigenetic Regulation

The Retinoblastoma protein (pRb) is a key cell cycle regulator conserved in a wide variety of organisms. Experimental analysis of pRb's functions in animals and plants has revealed that this protein participates in cell proliferation and differentiation processes. In addition, pRb in animals and its orthologs in plants (RBR), are part of highly conserved protein complexes which suggest the possibility that analogies exist not only between functions carried out by pRb orthologs themselves, but also in the structure and roles of the protein networks where these proteins are involved. Here, we present examples of pRb/RBR participation in cell cycle control, cell differentiation, and in the regulation of epigenetic changes and chromatin remodeling machinery, highlighting the similarities that exist between the composition of such networks in plants and animals.

The F-Box-Like Protein FBL17 Is a Regulator of DNA-Damage Response and Colocalizes with RETINOBLASTOMA RELATED1 at DNA Lesion Sites

In Arabidopsis (Arabidopsis thaliana), the F-box protein F-BOX-LIKE17 (FBL17) was previously identified as an important cell-cycle regulatory protein. FBL17 is required for cell division during pollen development and for normal cell-cycle progression and endoreplication during the diploid sporophyte phase. FBL17 was reported to control the stability of the CYCLIN-DEPENDENT KINASE inhibitor KIP-RELATED PROTEIN (KRP), which may underlie the drastic reduction in cell division activity in both shoot and root apical meristems observed in fbl17 loss-of-function mutants. However, whether FBL17 has other substrates and functions besides degrading KRPs remains poorly understood. Here we show that mutation of FBL17 leads not only to misregulation of cell cycle genes, but also to a strong upregulation of genes involved in DNA damage and repair processes. This phenotype is associated with a higher frequency of DNA lesions in fbl17 and increased cell death in the root meristem, even in the absence of genotoxic stress. Notably, the constitutive activation of DNA damage response genes is largely SOG1-independent in fbl17 In addition, through analyses of root elongation, accumulation of cell death, and occurrence of γH2AX foci, we found that fbl17 mutants are hypersensitive to DNA double-strand break-induced genotoxic stress. Notably, we observed that the FBL17 protein is recruited at nuclear foci upon double-strand break induction and colocalizes with γH2AX, but only in the presence of RETINOBLASTOMA RELATED1. Altogether, our results highlight a role for FBL17 in DNA damage response, likely by ubiquitylating proteins involved in DNA-damage signaling or repair.

Reproductive Multitasking: The Female Gametophyte

Fertilization of flowering plants requires the organization of complex tasks, many of which become integrated by the female gametophyte (FG). The FG is a few-celled haploid structure that orchestrates division of labor to coordinate successful interaction with the sperm cells and their transport vehicle, the pollen tube. As reproductive outcome is directly coupled to evolutionary success, the underlying mechanisms are under robust molecular control, including integrity check and repair mechanisms. Here, we review progress on understanding the development and function of the FG, starting with the functional megaspore, which represents the haploid founder cell of the FG. We highlight recent achievements that have greatly advanced our understanding of pollen tube attraction strategies and the mechanisms that regulate plant hybridization and gamete fusion. In addition, we discuss novel insights into plant polyploidization strategies that expand current concepts on the evolution of flowering plants.

DNA-Damaging Effectors: New Players in the Effector Arena

In animal cells, nuclear DNA is the target of genotoxins produced by bacterial pathogens that cause genomic mutations eventually leading to apoptosis, senescence, and carcinogenic development. In response to the insult, the DNA damage response (DDR) is activated to ensure lesion repair. Accumulation of DNA breaks is also detected in plants during microbial infection. In this opinion article we propose that phytopathogens can produce DNA-damaging effectors. The recent identification of a functional genotoxin in devastating eukaryotic plant pathogens, such as oomycetes, supports the concept that DNA-damaging effectors may contribute to pathogenicity. Additionally, this raises the question of how plants can perceive these damages and whether this perception can be connected to the plant immune system.

Linking cell cycle to stomatal differentiation

Stomatal differentiation manifests via several rounds of asymmetric cell division and a single symmetric cell division: the former, formative divisions amplify the number of epidermal cells, and the latter is essential for creating a functional guard cell pair. These cell division patterns are coordinated with progressive fate specification and cell-state transitional steps, which rely on the transcriptional regulation by a set of cell type-specific basic helix loop helix (bHLH) transcription factors. It has been proposed that the mechanisms underlying cell-fate decision and cell cycle progression are interconnected in a wide range of developmental processes. This review highlights the recent findings on how cell cycle regulators are transcriptionally regulated and contributing to each step of stomatal lineage progression.

The Plant DNA Damage Response: Signaling Pathways Leading to Growth Inhibition and Putative Role in Response to Stress Conditions

Maintenance of genome integrity is a key issue for all living organisms. Cells are constantly exposed to DNA damage due to replication or transcription, cellular metabolic activities leading to the production of Reactive Oxygen Species (ROS) or even exposure to DNA damaging agents such as UV light. However, genomes remain extremely stable, thanks to the permanent repair of DNA lesions. One key mechanism contributing to genome stability is the DNA Damage Response (DDR) that activates DNA repair pathways, and in the case of proliferating cells, stops cell division until DNA repair is complete. The signaling mechanisms of the DDR are quite well conserved between organisms including in plants where they have been investigated into detail over the past 20 years. In this review we summarize the acquired knowledge and recent advances regarding the DDR control of cell cycle progression. Studying the plant DDR is particularly interesting because of their mode of development and lifestyle. Indeed, plants develop largely post-embryonically, and form new organs through the activity of meristems in which cells retain the ability to proliferate. In addition, they are sessile organisms that are permanently exposed to adverse conditions that could potentially induce DNA damage in all cell types including meristems. In the second part of the review we discuss the recent findings connecting the plant DDR to responses to biotic and abiotic stresses.