The Arabidopsis thaliana checkpoint kinase WEE1 protects against premature vascular differentiation during replication stress

The Arabidopsis SIAMESE-RELATED cyclin-dependent kinase inhibitors SMR5 and SMR7 regulate the DNA damage checkpoint in response to reactive oxygen species

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.

Zinc finger artificial transcription factor-based nearest inactive analogue/nearest active analogue strategy used for the identification of plant genes controlling homologous recombination

In previous work, we selected a particular transcription factor, designated VP16-HRU, from a pool of zinc finger artificial transcription factors (ZF-ATFs) used for genome interrogation. When expressed in Arabidopsis thaliana under control of the ribosomal protein S5A promoter, the RPS5A::VP16-HRU construct led to a 200- to 300-fold increase in the frequency of somatic intrachromosomal homologous recombination (iHR). Because the expression of each ZF-ATF leads to a large number of transcriptional changes, we designed a strategy employing a collection of structurally similar ZF-ATFs to filter out the transcriptional changes relevant to the phenotype by deep sequencing. In that manner, 30 transcripts were found to be consistently induced in plants with enhanced homologous recombination (HR). For 25 of the cognate genes, their effect on the HR process was assessed using cDNA/gDNA expression constructs. For three genes, ectopic expression indeed led to enhanced iHR frequencies, albeit much lower than the frequency observed when a HR-inducing ZF-ATF was present. Altogether, our data demonstrate that despite the large number of transcriptional changes brought about by individual ZF-ATFs, causal changes can be identified. In our case, the picture emerged that a natural regulatory switch for iHR does not exist but that ZF-ATFs-like VP16-HRU act as an ectopic master switch, orchestrating the timely expression of a set of plant genes that each by themselves only have modest effects, but when acting together support an extremely high iHR frequency.

A SCARECROW-RETINOBLASTOMA protein network controls protective quiescence in the Arabidopsis root stem cell organizer

Ben Scheres and colleagues report that in the growing tip of plant roots, a gene regulatory network that includes the plant homologue of Retinoblastoma regulates the divisions of long-term stem cells to replenish tissue and to protect the root stem cell niche.

Quiescent long-term somatic stem cells reside in plant and animal stem cell niches. Within the Arabidopsis root stem cell population, the Quiescent Centre (QC), which contains slowly dividing cells, maintains surrounding short-term stem cells and may act as a long-term reservoir for stem cells. The RETINOBLASTOMA-RELATED (RBR) protein cell-autonomously reinforces mitotic quiescence in the QC. RBR interacts with the stem cell transcription factor SCARECROW (SCR) through an LxCxE motif. Disruption of this interaction by point mutation in SCR or RBR promotes asymmetric divisions in the QC that renew short-term stem cells. Analysis of the in vivo role of quiescence in the root stem cell niche reveals that slow cycling within the QC is not needed for structural integrity of the niche but allows the growing root to cope with DNA damage.

In the plant Arabidposis thaliana, root meristems (in the growing tip of the root) contain slowly dividing cells that act as an organizing center for the root stem cells that surround them. This centre is called the quiescent centre (QC). In this study, we show that the slow rate of division in the QC is regulated by the interaction between two proteins: Retinoblastoma homolog (RBR) and SCARECROW (SCR), a transcription factor that controls stem cell maintenance. RBR and SCR regulate quiescence in the QC by repressing an asymmetric cell division that generates short-term stem cells. Here we genetically manipulate the cells in the QC to alter their quiescence by regulating the RBR/SCR interaction to demonstrate that quiescence is not needed for the organizing capacity of the QC but instead provides cells with a higher resistance to genotoxic stress, allowing stem cells in the QC to survive even if more rapidly cycling stem cells are damaged. A role for mitotic quiescence has been reported in animal stem cells, in which Rb has been implicated. These findings indicate that it might serve a similar role in plant stem cells.

Identification of a Wee1-like kinase gene essential for procyclic Trypanosoma brucei survival

Regulation of eukaryotic cell cycle progression requires sequential activation and inactivation of cyclin-dependent kinases (CDKs). Activation of the cyclin B-cdc2 kinase complex is a pivotal step in mitotic initiation and the tyrosine kinase Wee1 is a key regulator of cell cycle sequence during G2/M transition and inhibits mitotic entry by phosphorylating the inhibitory tyrosine 15 on the cdc2 M-phase-inducing kinase. Wee1 degradation is essential for the exit from the G2 phase. In trypanosomatids, little is known about the genes that regulate cyclin B-cdc2 complexes at the G2/M transition of their cell cycle. Although canonical tyrosine kinases are absent in the genome of trypanosomatids, phosphorylation on protein tyrosine residues has been reported in Trypanosoma brucei. Here, we characterized a Wee1-like protein kinase gene from T. brucei. Expression of TbWee1 in a Schizosaccharomyces pombe strain null for Wee1 inhibited cell division and caused cell elongation. This demonstrates the lengthening of G2, which provided cells with extra time to grow before dividing. The Wee1-like protein kinase was expressed in the procyclic and bloodstream proliferative slender forms of T. brucei and the role of Wee1 in cell cycle progression was analyzed by generating RNA interference cell lines. In the procyclic form of T. brucei, the knock-down of TbWee1 expression by RNAi led to inhibition of parasite growth. Abnormal phenotypes showing an increase in the percentage of cells with 1N0K, 0N1K and 2N1K were observed in these RNAi cell lines. Using parasites with a synchronized cell cycle, we demonstrated that TbWee1 is linked to the G2/M phase. We also showed that TbWee1 is an essential gene necessary for proper cell cycle progression and parasite growth in T. brucei. Our results provide evidence for the existence of a functional Wee1 in T. brucei with a potential role in cell division at G2/M.

ATM/ATR-dependent Tyr15 phosphorylation of cyclin-dependent kinases in response to hydroxyurea in Vicia faba root meristem cells

DNA damage or stalled replication forks activate cell cycle checkpoints. However, the regulation of metabolic pathways that are responsible for maintenance of genome integrity in plants is still largely unknown. Present research on Vicia faba root meristem cells indicates that inhibitory phosphorylation of cyclin-dependent kinases (Cdks) at Tyr15 plays a prominent role during blockage of cell cycle in response to genotoxic stress. Phosphorylation of P-loop in Cdks takes place in ATM/ATR-dependent manner. Although, Tyr15 phosphorylation upon hydroxyurea (HU) treatment was found in most cells classified to G1 and S phase, interestingly, the number of phoshpo-Tyr15-positive cells decreases in G2 phase. Presented data confirm much similarity in regulation of Cdks functions under genotoxic stress between plants and animals; however, they may also substantiate evolutionarily developed differences especially in regulation of G2/M transition between these two kingdoms.

From Zinnia to Arabidopsis: approaching the involvement of peroxidases in lignification

Zinnia elegans constitutes one of the most useful model systems for studying xylem differentiation, which simultaneously involves secondary cell wall synthesis, cell wall lignification, and programmed cell death. Likewise, the in vitro culture system of Z. elegans has been the best characterized as the differentiation of mesophyll cells into tracheary elements allows study of the biochemistry and physiology of xylogenesis free from the complexity that heterogeneous plant tissues impose. Moreover, Z. elegans has emerged as an excellent plant model to study the involvement of peroxidases in cell wall lignification. This is due to the simplicity and duality of the lignification pattern shown by the stems and hypocotyls, and to the basic nature of the peroxidase isoenzyme. This protein is expressed not only in hypocotyls and stems but also in mesophyll cells transdifferentiating into tracheary elements. Therefore, not only does this peroxidase fulfil all the catalytic requirements to be involved in lignification overcoming all restrictions imposed by the polymerization step, but also its expression is inherent in lignification. In fact, its basic nature is not exceptional since basic peroxidases are differentially expressed during lignification in other model systems, showing unusual and unique biochemical properties such as oxidation of syringyl moieties. This review focuses on the experiments which led to a better understanding of the lignification process in Zinnia, starting with the basic knowledge about the lignin pattern in this plant, how lignification takes place, and how a sole basic peroxidase with unusual catalytic properties is involved and regulated by hormones, H2O2, and nitric oxide.

Control of the meiotic cell division program in plants

While the question of why organisms reproduce sexually is still a matter of controversy, it is clear that the foundation of sexual reproduction is the formation of gametes with half the genomic DNA content of a somatic cell. This reduction in genomic content is accomplished through meiosis that, in contrast to mitosis, comprises two subsequent chromosome segregation steps without an intervening S phase. In addition, meiosis generates new allele combinations through the compilation of new sets of homologous chromosomes and the reciprocal exchange of chromatid segments between homologues. Progression through meiosis relies on many of the same, or at least homologous, cell cycle regulators that act in mitosis, e.g., cyclin-dependent kinases and the anaphase-promoting complex/cyclosome. However, these mitotic control factors are often differentially regulated in meiosis. In addition, several meiosis-specific cell cycle genes have been identified. We here review the increasing knowledge on meiotic cell cycle control in plants. Interestingly, plants appear to have relaxed cell cycle checkpoints in meiosis in comparison with animals and yeast and many cell cycle mutants are viable. This makes plants powerful models to study meiotic progression and allows unique modifications to their meiotic program to develop new plant-breeding strategies.

Identification of MAIN, a factor involved in genome stability in the meristems of Arabidopsis thaliana

Stem cells in the root and shoot apical meristem provide the descendant cells required for growth and development throughout the lifecycle of a plant. We found that mutations in the Arabidopsis MAINTENANCE OF MERISTEMS (MAIN) gene led to plants with distorted stem cell niches in which stem cells are not maintained and undergo premature differentiation or cell death. The malfunction of main meristems leads to short roots, mis-shaped leaves, reduced fertility and partial fasciation of stems. MAIN encodes a nuclear-localized protein and is a member of a so far uncharacterized plant-specific gene family. As main mutant plants are hypersensitive to DNA-damaging agents, expression of genes involved in DNA repair is induced and dead cells with damaged DNA accumulate in the mutant meristems, we propose that MAIN is required for meristem maintenance by sustaining genome integrity in stem cells and their descendants cells.

Hormone interactions in xylem development: a matter of signals

Xylem provides long-distance transport of water and nutrients as well as structural support in plants. The development of the xylem tissues is modulated by several internal signals. In the last decades, the bloom of genetic and genomic tools has led to increased understanding of the molecular mechanisms underlying the function of the traditional plant hormones in xylem specification and differentiation. Critical functions have been assigned to novel signaling molecules, such as thermospermine. These signals do not function independently, but interact in a manner we are only now beginning to understand. We review the current knowledge of hormone signaling pathways and their crosstalk in cambial cell initiation and maintenance, and in xylem specification and differentiation.

Plant WEE1 kinase is cell cycle regulated and removed at mitosis via the 26S proteasome machinery

In yeasts and animals, premature entry into mitosis is prevented by the inhibitory phosphorylation of cyclin-dependent kinase (CDK) by WEE1 kinase, and, at mitosis, WEE1 protein is removed through the action of the 26S proteasome. Although in higher plants WEE1 function has been confirmed in the DNA replication checkpoint, Arabidopsis wee1 insertion mutants grow normally, and a role for the protein in the G2/M transition during an unperturbed plant cell cycle is yet to be confirmed. Here data are presented showing that the inhibitory effect of WEE1 on CDK activity in tobacco BY-2 cell cultures is cell cycle regulated independently of the DNA replication checkpoint: it is high during S-phase but drops as cells traverse G2 and enter mitosis. To investigate this mechanism further, a yeast two-hybrid screen was undertaken to identify proteins interacting with Arabidopsis WEE1. Three F-box proteins and a subunit of the proteasome complex were identified, and bimolecular fluorescence complementation confirmed an interaction between AtWEE1 and the F-box protein SKP1 interacting partner 1 (SKIP1). Furthermore, the AtWEE1-green fluorescent protein (GFP) signal in Arabidopsis primary roots treated with the proteasome inhibitor MG132 was significantly increased compared with mock-treated controls. Expression of AtWEE1-YFP(C) (C-terminal portion of yellow fluorescent protein) or AtWEE1 per se in tobacco BY-2 cells resulted in a premature increase in the mitotic index compared with controls, whereas co-expression of AtSKIP1-YFP(N) negated this effect. These data support a role for WEE1 in a normal plant cell cycle and its removal at mitosis via the 26S proteasome.

ATR and MKP1 play distinct roles in response to UV-B stress in Arabidopsis

Ultraviolet-B (UV-B) stress activates MAP kinases (MAPKs) MPK3 and MPK6 in Arabidopsis. MAPK activity must be tightly controlled in order to ensure an appropriate cellular outcome. MAPK phosphatases (MKPs) effectively control MAPKs by dephosphorylation of phosphothreonine and phosphotyrosine in their activation loops. Arabidopsis MKP1 is an important regulator of MPK3 and MPK6, and mkp1 knockout mutants are hypersensitive to UV-B stress, which is associated with reduced inactivation of MPK3 and MPK6. Here, we demonstrate that MPK3 and MPK6 are hyperactivated in response to UV-B in plants that are deficient in photorepair, suggesting that UV-damaged DNA is a trigger of MAPK signaling. This is not due to a block in replication, as, in contrast to atr, the mkp1 mutant is not hypersensitive to the replication-inhibiting drug hydroxyurea, hydroxyurea does not activate MPK3 and MPK6, and atr is not impaired in MPK3 and MPK6 activation in response to UV-B. We further show that mkp1 leaves and roots are UV-B hypersensitive, whereas atr is mainly affected at the root level. Tolerance to UV-B stress has been previously associated with stem cell removal and CYCB1;1 accumulation. Although UV-B-induced stem cell death and CYCB1;1 expression are not altered in mkp1 roots, CYCB1;1 expression is reduced in mkp1 leaves. We conclude that the MKP1 and ATR pathways operate in parallel, with primary roles for ATR in roots and MKP1 in leaves.

Diversity, classification and function of the plant protein kinase superfamily

Eukaryotic protein kinases belong to a large superfamily with hundreds to thousands of copies and are components of essentially all cellular functions. The goals of this study are to classify protein kinases from 25 plant species and to assess their evolutionary history in conjunction with consideration of their molecular functions. The protein kinase superfamily has expanded in the flowering plant lineage, in part through recent duplications. As a result, the flowering plant protein kinase repertoire, or kinome, is in general significantly larger than other eukaryotes, ranging in size from 600 to 2500 members. This large variation in kinome size is mainly due to the expansion and contraction of a few families, particularly the receptor-like kinase/Pelle family. A number of protein kinases reside in highly conserved, low copy number families and often play broadly conserved regulatory roles in metabolism and cell division, although functions of plant homologues have often diverged from their metazoan counterparts. Members of expanded plant kinase families often have roles in plant-specific processes and some may have contributed to adaptive evolution. Nonetheless, non-adaptive explanations, such as kinase duplicate subfunctionalization and insufficient time for pseudogenization, may also contribute to the large number of seemingly functional protein kinases in plants.

Defective root growth triggered by oxidative stress is controlled through the expression of cell cycle-related genes

Reactive oxygen species (ROS) have many functions in aerobic organisms. High levels of ROS can have a negative impact on plant cells leading to senescence and cell death. ROS accumulates in cells subjected to environmental stress and induces a cellular response to this external stimulus. To protect cells from the negative impacts of excess ROS, plants also possess a ROS detoxifying system to maintain normal ROS levels. The regulation of ROS levels is particularly important as ROS also functions as an important signal molecule and can regulate plant growth by modulating gene expression. Despite the functional importance of ROS signaling, little is known about the molecular mechanisms involved in the regulation of gene expression through ROS. Therefore, the present study investigated the effect of hydrogen peroxide (H(2)O(2)), a ROS compound, on cell cycle-related gene expression. Gene expression analyses coupled with microdissected sections of the developmental zone of Arabidopsis root tips revealed that H(2)O(2) affects the expression of cell cycle-related genes. Additionally, ROS scavenging enzymes were found to play an important role in the root growth phenotype induced by H(2)O(2). Specifically, root growth inhibition by H(2)O(2) was diminished in transgenic Arabidopis overexpressing peroxidase but increased in a catalase2 (cat2) mutant. The strong root growth inhibition observed in the cat2 mutant upon H(2)O(2) treatment indicated that CAT2 has an essential role in maintaining root meristem activity in the presence of oxidative stress. Overall, these results confirm that ROS function not only as stress-related compounds but that they also function as signaling molecules to regulate the progression of the cell cycle in root tips.

A general G1/S-phase cell-cycle control module in the flowering plant Arabidopsis thaliana

The decision to replicate its DNA is of crucial importance for every cell and, in many organisms, is decisive for the progression through the entire cell cycle. A comparison of animals versus yeast has shown that, although most of the involved cell-cycle regulators are divergent in both clades, they fulfill a similar role and the overall network topology of G1/S regulation is highly conserved. Using germline development as a model system, we identified a regulatory cascade controlling entry into S phase in the flowering plant Arabidopsis thaliana, which, as a member of the Plantae supergroup, is phylogenetically only distantly related to Opisthokonts such as yeast and animals. This module comprises the Arabidopsis homologs of the animal transcription factor E2F, the plant homolog of the animal transcriptional repressor Retinoblastoma (Rb)-related 1 (RBR1), the plant-specific F-box protein F-BOX-LIKE 17 (FBL17), the plant specific cyclin-dependent kinase (CDK) inhibitors KRPs, as well as CDKA;1, the plant homolog of the yeast and animal Cdc2⁺/Cdk1 kinases. Our data show that the principle of a double negative wiring of Rb proteins is highly conserved, likely representing a universal mechanism in eukaryotic cell-cycle control. However, this negative feedback of Rb proteins is differently implemented in plants as it is brought about through a quadruple negative regulation centered around the F-box protein FBL17 that mediates the degradation of CDK inhibitors but is itself directly repressed by Rb. Biomathematical simulations and subsequent experimental confirmation of computational predictions revealed that this regulatory circuit can give rise to hysteresis highlighting the here identified dosage sensitivity of CDK inhibitors in this network.

Evidence for a role of Arabidopsis CDT1 proteins in gametophyte development and maintenance of genome integrity

Meristems retain the ability to divide throughout the life cycle of plants, which can last for over 1000 years in some species. Furthermore, the germline is not laid down early during embryogenesis but originates from the meristematic cells relatively late during development. Thus, accurate cell cycle regulation is of utmost importance to avoid the accumulation of mutations during vegetative growth and reproduction. The Arabidopsis thaliana genome encodes two homologs of the replication licensing factor CDC10 Target1 (CDT1), and overexpression of CDT1a stimulates DNA replication. Here, we have investigated the respective functions of Arabidopsis CDT1a and CDT1b. We show that CDT1 proteins have partially redundant functions during gametophyte development and are required for the maintenance of genome integrity. Furthermore, CDT1-RNAi plants show endogenous DNA stress, are more tolerant than the wild type to DNA-damaging agents, and show constitutive induction of genes involved in DNA repair. This DNA stress response may be a direct consequence of reduced CDT1 accumulation on DNA repair or may relate to the ability of CDT1 proteins to form complexes with DNA polymerase ε, which functions in DNA replication and in DNA stress checkpoint activation. Taken together, our results provide evidence for a crucial role of Arabidopsis CDT1 proteins in genome stability.

Xylem cell death: emerging understanding of regulation and function

Evolutionary, as well as genetic, evidence suggests that vascular development evolved originally as a cell death programme that allowed enhanced movement of water in the extinct protracheophytes, and that secondary wall formation in the water-conducting cells evolved afterwards, providing mechanical support for effective long-distance transport of water. The extant vascular plants possess a common regulatory network to coordinate the different phases of xylem maturation, including secondary wall formation, cell death, and finally autolysis of the cell contents, by the action of recently identified NAC domain transcription factors. Consequently, xylem cell death is an inseparable part of the xylem maturation programme, making it difficult to uncouple cell death mechanistically from secondary wall formation, and thus identify the key factors specifically involved in regulation of cell death. Current knowledge suggests that the necessary components for xylem cell death are produced early during xylem differentiation, and cell death is prevented through the action of inhibitors and storage of hydrolytic enzymes in inactive forms in compartments such as the vacuole. Bursting of the central vacuole triggers autolytic hydrolysis of the cell contents, which ultimately leads to cell death. This cascade of events varies between the different xylem cell types. The water-transporting tracheary elements rely on a rapid cell death programme, with hydrolysis of cell contents taking place for the most part, if not entirely, after vacuolar bursting, while the xylem fibres disintegrate cellular contents at a slower pace, well before cell death. This review includes a detailed description of cell morphology, function of plant growth regulators, such as ethylene and thermospermine, and the action of hydrolytic nucleases and proteases during cell death of the different xylem cell types.

Integrated genomic analyses identify WEE1 as a critical mediator of cell fate and a novel therapeutic target in acute myeloid leukemia

Acute myeloid leukemia (AML) remains a therapeutic challenge despite increasing knowledge about the molecular origins of the disease, as the mechanisms of AML cell escape from chemotherapy remain poorly defined. We hypothesized that AML cells are addicted to molecular pathways in the context of chemotherapy and used complementary approaches to identify these addictions. Using novel molecular and computational approaches, we performed genome-wide shRNA screens to identify proteins that mediate AML cell fate after cytarabine exposure, gene expression profiling of AML cells exposed to cytarabine to identify genes with induced expression in this context, and examination of existing gene expression data from primary patient samples. The integration of these independent analyses strongly implicates cell cycle checkpoint proteins, particularly WEE1, as critical mediators of AML cell survival after cytarabine exposure. Knockdown of WEE1 in a secondary screen confirmed its role in AML cell survival. Pharmacologic inhibition of WEE1 in AML cell lines and primary cells is synergistic with cytarabine. Further experiments demonstrate that inhibition of WEE1 prevents S-phase arrest induced by cytarabine, broadening the functions of WEE1 that may be exploited therapeutically. These data highlight the power of integrating functional and descriptive genomics, and identify WEE1 as potential therapeutic target in AML.

Novel insights into maintaining genomic integrity: Wee1 regulating Mus81/Eme1

Maintenance of genomic integrity is essential for cell survival. Specifically, during DNA replication cells use a complex network of mechanisms that prevents genomic instability. Recently, we and others identified Wee1, a serine/threonine and tyrosine kinase, as a new modulator of the genomic stability during S phase. Loss of its activity causes a general DNA damage response activation and a decrease in replication fork speed. These effects are counteracted by the downregulation of the endonuclease complex Mus81-Eme1, showing a new link between this endonuclease and Wee1 during DNA replication. Here we discuss the function of Wee1 in genomic stability and its relationship with the Mus81-Eme1 complex.

Repairing breaks in the plant genome: the importance of keeping it together

DNA damage threatens the integrity of the genome and has potentially lethal consequences for the organism. Plant DNA is under continuous assault from endogenous and environmental factors and effective detection and repair of DNA damage are essential to ensure the stability of the genome. One of the most cytotoxic forms of DNA damage are DNA double-strand breaks (DSBs) which fragment chromosomes. Failure to repair DSBs results in loss of large amounts of genetic information which, following cell division, severely compromises daughter cells that receive fragmented chromosomes. This review will survey recent advances in our understanding of plant responses to chromosomal breaks, including the sources of DNA damage, the detection and signalling of DSBs, mechanisms of DSB repair, the role of chromatin structure in repair, DNA damage signalling and the link between plant recombination pathways and transgene integration. These mechanisms are of critical importance for maintenance of plant genome stability and integrity under stress conditions and provide potential targets for the improvement of crop plants both for stress resistance and for increased precision in the generation of genetically improved varieties.

Molecular control and function of endoreplication in development and physiology

Endoreplication, also called endoreduplication, is a cell cycle variant of multicellular eukaryotes in which mitosis is skipped and cells repeatedly replicate their DNA, resulting in cellular polyploidy. In recent years, research results have shed light on the molecular mechanism of endoreplication control, but the function of this cell-cycle variant has remained elusive. However, new evidence is at last providing insight into the biological relevance of cellular polyploidy, demonstrating that endoreplication is essential for developmental processes, such as cell fate maintenance, and is a prominent response to physiological conditions, such as pathogen attack or DNA damage. Thus, endoreplication is being revealed as an important module in plant growth that contributes to the robustness of plant life.

Gene regulation in response to DNA damage

To deal with different kinds of DNA damages, there are a number of repair pathways that must be carefully orchestrated to guarantee genomic stability. Many proteins that play a role in DNA repair are involved in multiple pathways and need to be tightly regulated to conduct the functions required for efficient repair of different DNA damage types, such as double strand breaks or DNA crosslinks caused by radiation or genotoxins. While most of the factors involved in DNA repair are conserved throughout the different kingdoms, recent results have shown that the regulation of their expression is variable between different organisms. In the following paper, we give an overview of what is currently known about regulating factors and gene expression in response to DNA damage and put this knowledge in context with the different DNA repair pathways in plants. This article is part of a Special Issue entitled: Plant gene regulation in response to abiotic stress.