The role of the Arabidopsis E2FB transcription factor in regulating auxin-dependent cell division

Dwarfs standing tall: breeding towards the 'Yellow revolution' through insights into plant height regulation

High oilseed production is an exigency due to the increasing edible oil consumption of the growing population. Rapeseed and mustard are cultivated worldwide and contribute significantly to the world's total oilseed production. Already a plateau is reached in terms of area and yield in most of the existing cultivars. Most of the commercially cultivated high yielding rapeseed and mustard varieties are tall, mainly due to a wider use of heterosis. However, they are susceptible to lodging and consequent yield losses. Plant yield is strongly dependent upon its architecture; therefore, 'ideotype breeding' is the key approach adopted to develop new varieties with enhanced yield potential, which is less explored in these crops. Dwarf/ semi dwarf plant type varieties has shown its improved yield potential over tall plant type in cereals which further leads to 'Green revolution' in Asian countries. Although, many induced dwarf mutants in rapeseed and mustard were isolated, unlike dwarf green-revolution varieties of cereals, most of them had undesirable plant types with defects including extreme dwarfism and sterility, leading to poor yield potential. Understanding the genetic and molecular mechanisms governing plant height and its correlation with yield and yield contributing characters is crucial. In this review, recent insights into genetic, molecular, and anatomical regulation of plant height have been discussed. The role of hormones, their crosstalk, and hormonal control for cell division and expansion have been delineated with respect to plant architecture. Many dwarfing genes are identified as being part of various phytohormone pathways. Parallelly, molecular links between plant height and flowering time have been explored. The overall synthesis of the review points out some key target pathways and genes that will be useful for plant breeders as well as biotechnologists for targeted genome editing for improving plant architecture without a yield penalty.

RETINOBLASTOMA-RELATED Has Both Canonical and Noncanonical Regulatory Functions During Thermo-Morphogenic Responses in Arabidopsis Seedlings

Warm temperatures accelerate plant growth, but the underlying molecular mechanism is not fully understood. Here, we show that increasing the temperature from 22°C to 28°C rapidly activates proliferation in the apical shoot and root meristems of wild-type Arabidopsis seedlings. We found that one of the central regulators of cell proliferation, the cell cycle inhibitor RETINOBLASTOMA-RELATED (RBR), is suppressed by warm temperatures. RBR became hyper-phosphorylated at a conserved CYCLIN-DEPENDENT KINASE (CDK) site in young seedlings growing at 28°C, in parallel with the stimulation of the expressions of the regulatory CYCLIN D/A subunits of CDK(s). Interestingly, while under warm temperatures ectopic RBR slowed down the acceleration of cell proliferation, it triggered elongation growth of post-mitotic cells in the hypocotyl. In agreement, the central regulatory genes of thermomorphogenic response, including PIF4 and PIF7, as well as their downstream auxin biosynthetic YUCCA genes (YUC1-2 and YUC8-9) were all up-regulated in the ectopic RBR expressing line but down-regulated in a mutant line with reduced RBR level. We suggest that RBR has both canonical and non-canonical functions under warm temperatures to control proliferative and elongation growth, respectively.

Allelic variations of WAK106-E2Fa-DPb1-UGT74E2 module regulate fibre properties in Populus tomentosa

Wood formation, intricately linked to the carbohydrate metabolism pathway, underpins the capacity of trees to produce renewable resources and offer vital ecosystem services. Despite their importance, the genetic regulatory mechanisms governing wood fibre properties in woody plants remain enigmatic. In this study, we identified a pivotal module comprising 158 high-priority core genes implicated in wood formation, drawing upon tissue-specific gene expression profiles from 22 Populus samples. Initially, we conducted a module-based association study in a natural population of 435 Populus tomentosa, pinpointing PtoDPb1 as the key gene contributing to wood formation through the carbohydrate metabolic pathway. Overexpressing PtoDPb1 led to a 52.91% surge in cellulose content, a reduction of 14.34% in fibre length, and an increment of 38.21% in fibre width in transgenic poplar. Moreover, by integrating co-expression patterns, RNA-sequencing analysis, and expression quantitative trait nucleotide (eQTN) mapping, we identified a PtoDPb1-mediated genetic module of PtoWAK106-PtoDPb1-PtoE2Fa-PtoUGT74E2 responsible for fibre properties in Populus. Additionally, we discovered the two PtoDPb1 haplotypes that influenced protein interaction efficiency between PtoE2Fa-PtoDPb1 and PtoDPb1-PtoWAK106, respectively. The transcriptional activation activity of the PtoE2Fa-PtoDPb1 haplotype-1 complex on the promoter of PtoUGT74E2 surpassed that of the PtoE2Fa-PtoDPb1 haplotype-2 complex. Taken together, our findings provide novel insights into the regulatory mechanisms of fibre properties in Populus, orchestrated by PtoDPb1, and offer a practical module for expediting genetic breeding in woody plants via molecular design.

Protein post-translational modifications in auxin signaling

Protein post-translational modifications (PTMs), such as ubiquitination, phosphorylation, and small ubiquitin-like modifier (SUMO)ylation, are crucial for regulating protein stability, activity, subcellular localization, and binding with cofactors. Such modifications remarkably increase the variety and complexity of proteomes, which are essential for regulating numerous cellular and physiological processes. The regulation of auxin signaling is finely tuned in time and space to guide various plant growth and development. Accumulating evidence indicates that PTMs play critical roles in auxin signaling regulations. Thus, a thorough and systematic review of the functions of PTMs in auxin signal transduction will improve our profound comprehension of the regulation mechanism of auxin signaling and auxin-mediated various processes. This review discusses the progress of protein ubiquitination, phosphorylation, histone acetylation and methylation, SUMOylation, and S-nitrosylation in the regulation of auxin signaling.

Modelling how plant cell-cycle progression leads to cell size regulation

Populations of cells typically maintain a consistent size, despite cell division rarely being precisely symmetrical. Therefore, cells must possess a mechanism of "size control", whereby the cell volume at birth affects cell-cycle progression. While size control mechanisms have been elucidated in a number of other organisms, it is not yet clear how this mechanism functions in plants. Here, we present a mathematical model of the key interactions in the plant cell cycle. Model simulations reveal that the network of interactions exhibits limit-cycle solutions, with biological switches underpinning both the G1/S and G2/M cell-cycle transitions. Embedding this network model within growing cells, we test hypotheses as to how cell-cycle progression can depend on cell size. We investigate two different mechanisms at both the G1/S and G2/M transitions: (i) differential expression of cell-cycle activator and inhibitor proteins (with synthesis of inhibitor proteins being independent of cell size), and (ii) equal inheritance of inhibitor proteins after cell division. The model demonstrates that both these mechanisms can lead to larger daughter cells progressing through the cell cycle more rapidly, and can thus contribute to cell-size control. To test how these features enable size homeostasis over multiple generations, we then simulated these mechanisms in a cell-population model with multiple rounds of cell division. These simulations suggested that integration of size-control mechanisms at both G1/S and G2/M provides long-term cell-size homeostasis. We concluded that while both size independence and equal inheritance of inhibitor proteins can reduce variations in cell size across individual cell-cycle phases, combining size-control mechanisms at both G1/S and G2/M is essential to maintain size homeostasis over multiple generations. Thus, our study reveals how features of the cell-cycle network enable cell-cycle progression to depend on cell size, and provides a mechanistic understanding of how plant cell populations maintain consistent size over generations.

The canonical E2Fs together with RETINOBLASTOMA-RELATED are required to establish quiescence during plant development

Maintaining stable and transient quiescence in differentiated and stem cells, respectively, requires repression of the cell cycle. The plant RETINOBLASTOMA-RELATED (RBR) has been implicated in stem cell maintenance, presumably by forming repressor complexes with E2F transcription factors. Surprisingly we find that mutations in all three canonical E2Fs do not hinder the cell cycle, but similarly to RBR silencing, result in hyperplasia. Contrary to the growth arrest that occurs when exit from proliferation to differentiation is inhibited upon RBR silencing, the e2fabc mutant develops enlarged organs with supernumerary stem and differentiated cells as quiescence is compromised. While E2F, RBR and the M-phase regulatory MYB3Rs are part of the DREAM repressor complexes, and recruited to overlapping groups of targets, they regulate distinct sets of genes. Only the loss of E2Fs but not the MYB3Rs interferes with quiescence, which might be due to the ability of E2Fs to control both G1-S and some key G2-M targets. We conclude that collectively the three canonical E2Fs in complex with RBR have central roles in establishing cellular quiescence during organ development, leading to enhanced plant growth.

SNF1-related protein kinase 1 represses Arabidopsis growth through post-translational modification of E2Fa in response to energy stress

Cellular sugar starvation and/or energy deprivation serves as an important signaling cue for the live cells to trigger the necessary stress adaptation response. When exposed to cellular energy stress (ES) conditions, the plants reconfigure metabolic pathways and rebalance energy status while restricting vegetative organ growth. Despite the vital importance of this ES-induced growth restriction, the regulatory mechanism underlying the response remains largely elusive in plants. Using plant cell- and whole plant-based functional analyses coupled with extended genetic validation, we show that cellular ES-activated SNF1-related protein kinase 1 (SnRK1.1) directly interacts with and phosphorylates E2Fa transcription factor, a critical cell cycle regulator. Phosphorylation of E2Fa by SnRK1.1 leads to its proteasome-mediated protein degradation, resulting in S-phase repression and organ growth restriction. Our findings show that ES-dependently activated SnRK1.1 adjusts cell proliferation and vegetative growth for plants to cope with constantly fluctuating environments.

GhBOP1 as a Key Factor of Ribosomal Biogenesis: Development of Wrinkled Leaves in Upland Cotton

Block of proliferation 1 (BOP1) is a key protein that helps in the maturation of ribosomes and promotes the progression of the cell cycle. However, its role in the leaf morphogenesis of cotton remains unknown. Herein, we report and study the function of GhBOP1 isolated from Gossypium hirsutum. The sequence alignment revealed that BOP1 protein was highly conserved among different species. The yeast two-hybrid experiments, bimolecular fluorescence complementation, and luciferase complementation techniques revealed that GhBOP1 interact with GhPES and GhWDR12. Subcellular localization experiments revealed that GhBOP1, GhPES and GhWDR12 were localized at the nucleolus. Suppression of GhBOP1 transcripts resulted in the uneven bending of leaf margins and the presence of young wrinkled leaves by virus-induced gene silencing assay. Abnormal palisade arrangements and the presence of large upper epidermal cells were observed in the paraffin sections of the wrinkled leaves. Meanwhile, a jasmonic acid-related gene, GhOPR3, expression was increased. In addition, a negative effect was exerted on the cell cycle and the downregulation of the auxin-related genes was also observed. These results suggest that GhBOP1 plays a critical role in the development of wrinkled cotton leaves, and the process is potentially modulated through phytohormone signaling.

The chromosome-level genome provides insight into the molecular mechanism underlying the tortuous-branch phenotype of Prunus mume

Plant with naturally twisted branches is referred to as a tortuous-branch plant, which have extremely high ornamental value due to their zigzag shape and the natural twisting of their branches. Prunus mume is an important woody ornamental plant. However, the molecular mechanism underlying this unique trait in Prunus genus is unknown. Here, we present a chromosome-level genome assembly of the cultivated P. mume var. tortuosa created using Oxford Nanopore combined with Hi-C scaffolding, which resulted in a 237.8 Mb genome assembly being anchored onto eight pseudochromosomes. Molecular dating indicated that P. mume is the most recently differentiated species in Prunus. Genes associated with cell division, development and plant hormones play essential roles in the formation of tortuous branch trait. A putative regulatory pathway for the tortuous branch trait was constructed based on gene expression levels. Furthermore, after transferring candidate PmCYCD genes into Arabidopsis thaliana, we found that seedlings overexpressing these genes exhibited curled rosette leaves. Our results provide insights into the evolutionary history of recently differentiated species in Prunus genus, the molecular basis of stem morphology, and the molecular mechanism underlying the tortuous branch trait and highlight the utility of multi-omics in deciphering the properties of P. mume plant architecture.

Development of microfluidic chip for entrapping tobacco BY-2 cells

The tobacco BY-2 cell line has been used widely as a model system in plant cell biology. BY-2 cells are nearly transparent, which facilitates cell imaging using fluorescent markers. As cultured cells are drifted in the medium, therefore, it was difficult to observe them for a long period. Hence, we developed a microfluidic device that traps BY-2 cells and fixes their positions to allow monitoring the physiological activity of cells. The device contains 112 trap zones, with parallel slots connected in series at three levels in the flow channel. BY-2 cells were cultured for 7 days and filtered using a sieve and a cell strainer before use to isolate short cell filaments consisting of only a few cells. The isolated cells were introduced into the flow channel, resulting in entrapment of cell filaments at 25 out of 112 trap zones (22.3%). The cell numbers increased through cell division from 1 to 4 days after trapping with a peak of mitotic index on day 2. Recovery experiments of fluorescent proteins after photobleaching confirmed cell survival and permeability of plasmodesmata. Thus, this microfluidic device and one-dimensional plant cell samples allowed us to observe cell activity in real time under controllable conditions.

Jasmonates and Histone deacetylase 6 activate Arabidopsis genome-wide histone acetylation and methylation during the early acute stress response

BACKGROUND

Jasmonates (JAs) mediate trade-off between responses to both biotic and abiotic stress and growth in plants. The Arabidopsis thaliana HISTONE DEACETYLASE 6 is part of the CORONATINE INSENSITIVE1 receptor complex, co-repressing the HDA6/COI1-dependent acetic acid-JA pathway that confers plant drought tolerance. The decrease in HDA6 binding to target DNA mirrors histone H4 acetylation (H4Ac) changes during JA-mediated drought response, and mutations in HDA6 also cause depletion in the constitutive repressive marker H3 lysine 27 trimethylation (H3K27me3). However, the genome-wide effect of HDA6 on H4Ac and much of the impact of JAs on histone modifications and chromatin remodelling remain elusive.

RESULTS

We performed high-throughput ChIP-Seq on the HDA6 mutant, axe1-5, and wild-type plants with or without methyl jasmonate (MeJA) treatment to assess changes in active H4ac and repressive H3K27me3 histone markers. Transcriptional regulation was investigated in parallel by microarray analysis in the same conditions. MeJA- and HDA6-dependent histone modifications on genes for specialized metabolism; linolenic acid and phenylpropanoid pathways; and abiotic and biotic stress responses were identified. H4ac and H3K27me3 enrichment also differentially affects JAs and HDA6-mediated genome integrity and gene regulatory networks, substantiating the role of HDA6 interacting with specific families of transposable elements in planta and highlighting further specificity of action as well as novel targets of HDA6 in the context of JA signalling for abiotic and biotic stress responses.

CONCLUSIONS

The findings demonstrate functional overlap for MeJA and HDA6 in tuning plant developmental plasticity and response to stress at the histone modification level. MeJA and HDA6, nonetheless, maintain distinct activities on histone modifications to modulate genetic variability and to allow adaptation to environmental challenges.

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.

Genomic prediction of preliminary yield trials in chickpea: Effect of functional annotation of SNPs and environment

Achieving yield potential in chickpea (Cicer arietinum L.) is limited by many constraints that include biotic and abiotic stresses. Combining next-generation sequencing technology with advanced statistical modeling has the potential to increase genetic gain efficiently. Whole genome resequencing data was obtained from 315 advanced chickpea breeding lines from the Australian chickpea breeding program resulting in more than 298,000 single nucleotide polymorphisms (SNPs) discovered. Analysis of population structure revealed a distinct group of breeding lines with many alleles that are absent from recently released Australian cultivars. Genome-wide association studies (GWAS) using these Australian breeding lines identified 20 SNPs significantly associated with grain yield in multiple field environments. A reduced level of nucleotide diversity and extended linkage disequilibrium suggested that some regions in these chickpea genomes may have been through selective breeding for yield or other traits. A large introgression segment that introduced from C. echinospermum for phytophthora root rot resistance was identified on chromosome 6, yet it also has unintended consequences of reducing yield due to linkage drag. We further investigated the effect of genotype by environment interaction on genomic prediction of yield. We found that the training set had better prediction accuracy when phenotyped under conditions relevant to the targeted environments. We also investigated the effect of SNP functional annotation on prediction accuracy using different subsets of SNPs based on their genomic locations: regulatory regions, exome, and alternative splice sites. Compared with the whole SNP dataset, a subset of SNPs did not significantly decrease prediction accuracy for grain yield despite consisting of a smaller number of SNPs.

E2Fb and E2Fa transcription factors independently regulate the DNA damage response after ultraviolet B exposure in Arabidopsis

Ultraviolet (UV)B radiation affects plant growth inhibiting cell proliferation. This inhibition is in part controlled by the activity of transcription factors from the E2F family. In particular, the participation of E2Fc and E2Fe in UV-B responses in Arabidopsis plants was previously reported. However, the E2Fa and E2Fb contribution to these processes has still not been investigated. Thus, in this work, we provide evidence that, in Arabidopsis, both E2Fa and E2Fb control leaf size under UV-B conditions without participating in the repair of cyclobutane pyrimidine dimers in the DNA. Nevertheless, in UV-B-exposed seedlings, E2Fa, but not E2Fb, regulates primary root elongation, cell proliferation, and programmed cell death in the meristematic zone. Using e2fa mutants that overexpress E2Fb, we showed that the role of E2Fa in the roots could not be replaced by E2Fb. Finally, our results show that E2Fa and E2Fb differentially regulate the expression of genes that activate the DNA damage response and cell cycle progression, both under conditions without UV-B and after exposure. Overall, we showed that both E2Fa and E2Fb have different and non-redundant roles in developmental and DNA damage responses in Arabidopsis plants exposed to UV-B.

Genetic Conservation of CBS Domain Containing Protein Family in Oryza Species and Their Association with Abiotic Stress Responses

Crop Wild Relatives (CWRs) form a comprehensive gene pool that can answer the queries related to plant domestication, speciation, and ecological adaptation. The genus ‘Oryza’ comprises about 27 species, of which two are cultivated, while the remaining are wild. Here, we have attempted to understand the conservation and diversification of the genes encoding Cystathionine β-synthase (CBS) domain-containing proteins (CDCPs) in domesticated and CWRs of rice. Few members of CDCPs were previously identified to be stress-responsive and associated with multiple stress tolerance in rice. Through genome-wide analysis of eleven rice genomes, we identified a total of 36 genes encoding CDCPs in O. longistaminata, 38 in O. glaberrima, 39 each in O. rufipogon, O. glumaepatula, O. brachyantha, O. punctata, and O. sativa subsp. japonica, 40 each in O. barthii and O. meridionalis, 41 in O. nivara, and 42 in O. sativa subsp. indica. Gene duplication analysis as well as non-synonymous and synonymous substitutions in the duplicated gene pairs indicated that this family is shaped majorly by the negative or purifying selection pressure through the long-term evolution process. We identified the presence of two additional hetero-domains, namely TerCH and CoatomerE (specifically in O. sativa subsp. indica), which were not reported previously in plant CDCPs. The in silico expression analysis revealed some of the members to be responsive to various abiotic stresses. Furthermore, the qRT-PCR based analysis identified some members to be highly inducive specifically in salt-tolerant genotype in response to salinity. The cis-regulatory element analysis predicted the presence of numerous stress as well as a few phytohormone-responsive elements in their promoter region. The data presented in this study would be helpful in the characterization of these CDCPs from rice, particularly in relation to abiotic stress tolerance.

Role of sugar and auxin crosstalk in plant growth and development

Under the natural environment, nutrient signals interact with phytohormones to coordinate and reprogram plant growth and survival. Sugars are important molecules that control almost all morphological and physiological processes in plants, ranging from seed germination to senescence. In addition to their functions as energy resources, osmoregulation, storage molecules, and structural components, sugars function as signaling molecules and interact with various plant signaling pathways, such as hormones, stress, and light to modulate growth and development according to fluctuating environmental conditions. Auxin, being an important phytohormone, is associated with almost all stages of the plant's life cycle and also plays a vital role in response to the dynamic environment for better growth and survival. In the previous years, substantial progress has been made that showed a range of common responses mediated by sugars and auxin signaling. This review discusses how sugar signaling affects auxin at various levels from its biosynthesis to perception and downstream gene activation. On the same note, the review also highlights the role of auxin signaling in fine-tuning sugar metabolism and carbon partitioning. Furthermore, we discussed the crosstalk between the two signaling machineries in the regulation of various biological processes, such as gene expression, cell cycle, development, root system architecture, and shoot growth. In conclusion, the review emphasized the role of sugar and auxin crosstalk in the regulation of several agriculturally important traits. Thus, engineering of sugar and auxin signaling pathways could potentially provide new avenues to manipulate for agricultural purposes.

Transient Overexpression of E2Fb to Induce Cell Divisions in Nicotiana benthamiana Pavement Cells

The availability of a fast and controlled mitotic model system that could simplify the generation of genetic material and reduce the experimental time from months to days would largely benefit research in plant cell division. In this protocol, we propose the use of pavement cells of Nicotiana benthamiana leaves to study cell division, which is artificially induced by Agrobacterium-mediated transient overexpression of the transcription factor E2Fb. The cell division-inducing overexpression of E2Fb can be combined with the expression of fluorescent protein-tagged proteins of interest or with dyes, which could be visualized throughout the cell cycle under the microscope. This simple and affordable method enables the study of cell cycle regulation and cell division in plants, from genome replication to cell wall formation, in a fast and controlled manner, and can be used for functional studies when coupled with chemical inhibitors or reverse genetic approaches.

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.

Regulation of the Plant Cell Cycle in Response to Hormones and the Environment

Developmental and environmental signals converge on cell cycle machinery to achieve proper and flexible organogenesis under changing environments. Studies on the plant cell cycle began 30 years ago, and accumulated research has revealed many links between internal and external factors and the cell cycle. In this review, we focus on how phytohormones and environmental signals regulate the cell cycle to enable plants to cope with a fluctuating environment. After introducing key cell cycle regulators, we first discuss how phytohormones and their synergy are important for regulating cell cycle progression and how environmental factors positively and negatively affect cell division. We then focus on the well-studied example of stress-induced G2 arrest and view the current model from an evolutionary perspective. Finally, we discuss the mechanisms controlling the transition from the mitotic cycle to the endocycle, which greatly contributes to cell enlargement and resultant organ growth in plants.

Alterations in hormonal signals spatially coordinate distinct responses to DNA double-strand breaks in Arabidopsis roots

Plants have a high ability to cope with changing environments and grow continuously throughout life. However, the mechanisms by which plants strike a balance between stress response and organ growth remain elusive. Here, we found that DNA double-strand breaks enhance the accumulation of cytokinin hormones through the DNA damage signaling pathway in the Arabidopsis root tip. Our data showed that activation of cytokinin signaling suppresses the expression of some of the PIN-FORMED genes that encode efflux carriers of another hormone, auxin, thereby decreasing the auxin signals in the root tip and causing cell cycle arrest at G₂ phase and stem cell death. Elevated cytokinin signaling also promotes an early transition from cell division to endoreplication in the basal part of the root apex. We propose that plant hormones spatially coordinate differential DNA damage responses, thereby maintaining genome integrity and minimizing cell death to ensure continuous root growth.

The F-box protein MIO1/SLB1 regulates organ size and leaf movement in Medicago truncatula

The size of leaf and seed organs, determined by the interplay of cell proliferation and expansion, is closely related to the final yield and quality of forage and crops. Yet the cellular and molecular mechanisms underlying organ size modulation remain poorly understood, especially in legumes. Here, MINI ORGAN1 (MIO1), which encodes an F-box protein SMALL LEAF AND BUSHY1 (SLB1) recently reported to control lateral branching in Medicago truncatula, was identified as a key regulator of organ size. We show that loss-of-function of MIO1/SLB1 severely reduced organ size. Conversely, plants overexpressing MIO1/SLB1 had enlarged organs. Cellular analysis revealed that MIO1/SLB1 controlled organ size mainly by modulating primary cell proliferation during the early stages of leaf development. Biochemical analysis revealed that MIO1/SLB1 could form part of SKP1/Cullin/F-box (SCF) E3 ubiquitin ligase complex, to target BIG SEEDS1 (BS1), a repressor of primary cell division, for degradation. Interestingly, we found that MIO1/SLB1 also played a key role in pulvinus development and leaf movement by modulating cell proliferation of the pulvinus as leaves developed. Our study not only demonstrates a conserved role of MIO1/SLB1 in the control of organ size in legumes, but also sheds light on the novel function of MIO1/SLB1 in leaf movement.

Molecular Analysis of the E2F/DP Gene Family of Daucus carota and Involvement of the DcE2F1 Factor in Cell Proliferation

E2F transcription factors are key components of the RB/E2F pathway that, through the action of cyclin-dependent kinases, regulates cell cycle progression in both plants and animals. Moreover, plant and animal E2Fs have also been shown to regulate other cellular functions in addition to cell proliferation. Based on structural and functional features, they can be divided into different classes that have been shown to act as activators or repressors of E2F-dependent genes. Among the first plant E2F factors to be reported, we previously described DcE2F1, an activating E2F which is expressed in cycling carrot (Daucus carota) cells. In this study, we describe the identification of the additional members of the E2F/DP family of D. carota, which includes four typical E2Fs, three atypical E2F/DEL genes, and three related DP genes. Expression analyses of the carrot E2F and DP genes reveal distinctive patterns and suggest that the functions of some of them are not necessarily linked to cell proliferation. DcE2F1 was previously shown to transactivate an E2F-responsive promoter in transient assays but the functional role of this protein in planta was not defined. Sequence comparisons indicate that DcE2F1 could be an ortholog of the AtE2FA factor of Arabidopsis thaliana. Moreover, ectopic expression of the DcE2F1 cDNA in transgenic Arabidopsis plants is able to upregulate AtE2FB and promotes cell proliferation, giving rise to polycotyly with low frequency, effects that are highly similar to those observed when over-expressing AtE2FA. These results indicate that DcE2F1 is involved in the control of cell proliferation and plays important roles in the regulation of embryo and plant development.

The genetic control of succulent leaf development

Succulent leaves have long intrigued biologists; much research has been done to define succulence, understand the evolutionary trajectory and implications of leaf succulence, and contextualize the ecological importance of water storage for plants inhabiting dry habitats, particularly those using CAM photosynthesis. Surprisingly little is understood about the molecular regulation of leaf succulence, despite advances in our understanding of the molecular foundation of leaf architecture in model systems. Moreover, leaf succulence is a drought avoidance trait, one that has yet to be fully used for crop improvement. Here, connections between disparate literatures are highlighted: research on the regulation of cell size, the determination of vascular patterning, and water transport between cells have direct implications for our understanding of leaf succulence. Connecting functional genomics of leaf patterning with knowledge of the evolution and ecology of succulent species will guide future research on the determination and maintenance of leaf succulence.

Plant cell cycle regulators: Mitogen-activated protein kinase, a new regulating switch?

Cell cycle is essential for the maintenance of genetic material and continuity of a species. Its regulation involves a complex interplay between multiple proteins with diverse molecular functions such as the kinases, transcription factors, proteases and phosphatases. Every step of this cycle requires a certain combination of these protein regulators which paves the way for the next stage. It is now evident that plants have their own unique features in the context of cell cycle regulation. Cell cycle in plants is not only necessary for maintenance of its physio-morphological parameter but it also regulates traits important for mankind like grain or fruit size. This makes it even more important to understand how plants regulate its cell cycle amidst various a/biotic stresses it is subjected to during its lifetime. The association of MAPK signaling pathways with every major developmental and stress response pathways in plants raises the question of its potential role in cell cycle regulation. There are number of cell cycle regulating proteins with putative sites for MAPK phosphorylation. The MAPK signaling pathway may directly or in a parallel pathway regulate the plant cell cycle. Unraveling the role of MAPK in cell cycle will open up new arenas to explore.

Analysis of Spatio-Temporal Transcriptome Profiles of Soybean (Glycine max) Tissues during Early Seed Development

Soybean (Glycine max) is an important crop providing oil and protein for both human and animal consumption. Knowing which biological processes take place in specific tissues in a temporal manner will enable directed breeding or synthetic approaches to improve seed quantity and quality. We analyzed a genome-wide transcriptome dataset from embryo, endosperm, endothelium, epidermis, hilum, outer and inner integument and suspensor at the global, heart and cotyledon stages of soybean seed development. The tissue specificity of gene expression was greater than stage specificity, and only three genes were differentially expressed in all seed tissues. Tissues had both unique and shared enriched functional categories of tissue-specifically expressed genes associated with them. Strong spatio-temporal correlation in gene expression was identified using weighted gene co-expression network analysis, with the most co-expression occurring in one seed tissue. Transcription factors with distinct spatiotemporal gene expression programs in each seed tissue were identified as candidate regulators of expression within those tissues. Gene ontology (GO) enrichment of orthogroup clusters revealed the conserved functions and unique roles of orthogroups with similar and contrasting expression patterns in transcript abundance between soybean and Arabidopsis during embryo proper and endosperm development. Key regulators in each seed tissue and hub genes connecting those networks were characterized by constructing gene regulatory networks. Our findings provide an important resource for describing the structure and function of individual soybean seed compartments during early seed development.

The genetic framework of shoot regeneration in Arabidopsis comprises master regulators and conditional fine-tuning factors

Clonal propagation and genetic engineering of plants requires regeneration, but many species are recalcitrant and there is large variability in explant responses. Here, we perform a genome-wide association study using 190 natural Arabidopsis accessions to dissect the genetics of shoot regeneration from root explants and several related in vitro traits. Strong variation is found in the recorded phenotypes and association mapping pinpoints a myriad of quantitative trait genes, including prior candidates and potential novel regeneration determinants. As most of these genes are trait- and protocol-specific, we propose a model wherein shoot regeneration is governed by many conditional fine-tuning factors and a few universal master regulators such as WUSCHEL, whose transcript levels correlate with natural variation in regenerated shoot numbers. Potentially novel genes in this last category are AT3G09925, SUP, EDA40 and DOF4.4. We urge future research in the field to consider multiple conditions and genetic backgrounds.

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.

A conserved role for γ-tubulin as a regulator of E2F transcription factors

This article comments on:

Kállai BM, Kourová H, Chumová J, Papdi C, Trögelová L, Kofroňová O, Hozák P, Filimonenko V, Mészáros T, Magyar Z, Bögre L, Binarová P. 2020. γ-Tubulin interacts with E2F transcription factors to regulate proliferation and endocycling in Arabidopsis. Journal of Experimental Botany 71, 1265–1277.

Time-Series Single-Cell RNA-Seq Data Reveal Auxin Fluctuation during Endocycle

Appropriate cell cycle regulation is crucial for achieving coordinated development and cell differentiation in multicellular organisms. In Arabidopsis, endoreduplication is often observed in terminally differentiated cells and several reports have shown its molecular mechanisms. Auxin is a key factor for the mode transition from mitotic cell cycle to endocycle; however, it remains unclear if and how auxin maintains the endocycle mode. In this study, we reanalyzed root single-cell transcriptome data and reconstructed cell cycle trajectories of the mitotic cell cycle and endocycle. With progression of the endocycle, genes involved in auxin synthesis, influx and efflux were induced at the specific cell phase, suggesting that auxin concentration fluctuated dynamically. Such induction of auxin-related genes was not observed in the mitotic cell cycle, suggesting that the auxin fluctuation plays some roles in maintaining the endocycle stage. In addition, the expression level of CYCB1;1, which is required for cell division in the M phase, coincided with the expected amount of auxin and cell division. Our analysis also provided a set of genes expressed in specific phases of the cell cycle. Taking these findings together, reconstruction of single-cell transcriptome data enables us to identify properties of the cell cycle more accurately.

ErbB-3 BINDING PROTEIN 1 Regulates Translation and Counteracts RETINOBLASTOMA RELATED to Maintain the Root Meristem

The ErbB-3 BINDING PROTEIN 1 (EBP1) drives growth, but the mechanism of how it acts in plants is little understood. Here, we show that EBP1 expression and protein abundance in Arabidopsis (Arabidopsis thaliana) are predominantly confined to meristematic cells and are induced by sucrose and partially dependent on TARGET OF RAPAMYCIN (TOR) kinase activity. Consistent with being downstream of TOR, silencing of EBP1 restrains, while overexpression promotes, root growth, mostly under sucrose-limiting conditions. Inducible overexpression of RETINOBLASTOMA RELATED (RBR), a sugar-dependent transcriptional repressor of cell proliferation, depletes meristematic activity and causes precocious differentiation, which is attenuated by EBP1. To understand the molecular mechanism, we searched for EBP1- and RBR-interacting proteins by affinity purification and mass spectrometry. In line with the double-stranded RNA-binding activity of EBP1 in human (Homo sapiens) cells, the overwhelming majority of EBP1 interactors are part of ribonucleoprotein complexes regulating many aspects of protein synthesis, including ribosome biogenesis and mRNA translation. We confirmed that EBP1 associates with ribosomes and that EBP1 silencing hinders ribosomal RNA processing. We revealed that RBR also interacts with a set of EBP1-associated nucleolar proteins as well as factors that function in protein translation. This suggests EBP1 and RBR act antagonistically on common processes that determine the capacity for translation to tune meristematic activity in relation to available resources.

E2FB Interacts with RETINOBLASTOMA RELATED and Regulates Cell Proliferation during Leaf Development

Cell cycle entry and quiescence are regulated by the E2F transcription factors in association with RETINOBLASTOMA-RELATED (RBR). E2FB is considered to be a transcriptional activator of cell cycle genes, but its function during development remains poorly understood. Here, by studying E2FB-RBR interaction, E2F target gene expression, and epidermal cell number and shape in e2fb mutant and overexpression lines during leaf development in Arabidopsis (Arabidopsis thaliana), we show that E2FB in association with RBR plays a role in the inhibition of cell proliferation to establish quiescence. In young leaves, both RBR and E2FB are abundant and form a repressor complex that is reinforced by an autoregulatory loop. Increased E2FB levels, either by expression driven by its own promoter or ectopically together with DIMERIZATION PARTNER A, further elevate the amount of this repressor complex, leading to reduced leaf cell number. Cell overproliferation in e2fb mutants and in plants overexpressing a truncated form of E2FB lacking the RBR binding domain strongly suggested that RBR repression specifically acts through E2FB. The increased number of small cells below the guard cells and of fully developed stomata indicated that meristemoids preferentially hyperproliferate. As leaf development progresses and cells differentiate, the amount of RBR and E2FB gradually declined. At this stage, elevation of E2FB level can overcome RBR repression, leading to reactivation of cell division in pavement cells. In summary, E2FB in association with RBR is central to regulating cell proliferation during organ development to determine final leaf cell number.

Mepiquat chloride promotes cotton lateral root formation by modulating plant hormone homeostasis

BACKGROUND

Mepiquat chloride (MC), a plant growth regulator, enhances root growth by promoting lateral root formation in cotton. However, the underlying molecular mechanisms of this phenomenon is still unknown.

METHODS

In this study, we used 10 cotton (Gossypium hirsutum Linn.) cultivars to perform a seed treatment with MC to investigate lateral root formation, and selected a MC sensitive cotton cultivar for dynamic monitor of root growth and transcriptome analysis during lateral root development upon MC seed treatment.

RESULTS

The results showed that MC treated seeds promotes the lateral root formation in a dosage-depended manner and the effective promotion region is within 5 cm from the base of primary root. MC treated seeds induce endogenous auxin level by altering gene expression of both gibberellin (GA) biosynthesis and signaling and abscisic acid (ABA) signaling. Meanwhile, MC treated seeds differentially express genes involved in indole acetic acid (IAA) synthesis and transport. Furthermore, MC-induced IAA regulates the expression of genes related to cell cycle and division for lateral root development.

CONCLUSIONS

Our data suggest that MC orchestrates GA and ABA metabolism and signaling, which further regulates auxin biosynthesis, transport, and signaling to promote the cell division responsible for lateral root formation.

Transient overexpression of E2Fb triggers cell divisions in pavement cells of Nicotiana benthamiana leaves

KEY MESSAGE

Agrobacterium-mediated transient overexpression of E2Fb triggers new cell divisions in pavement cells of Nicotiana benthamiana leaves. Transient transformation in Nicotiana benthamiana enables the study of multiple biological processes in a simple and fast manner. Here, we describe that, upon A. tumefaciens-mediated transient overexpression of the cell cycle regulator E2Fb from either Arabidopsis thaliana or N. benthamiana, cell divisions occur in epidermal pavement cells in N. benthamiana leaves, following a sequence of events that encompasses the nucleus taking a central position and being surrounded by chloroplasts, nuclear division, and formation of a new wall that divides the initial cell in two. Our results indicate that transient expression in N. benthamiana can be used to study cell division in plants, from DNA replication to cell wall formation, in a simple, controlled, and rapid manner.

A Molecular Framework for the Control of Adventitious Rooting by TIR1/AFB2-Aux/IAA-Dependent Auxin Signaling in Arabidopsis

In Arabidopsis thaliana, canonical auxin-dependent gene regulation is mediated by 23 transcription factors from the AUXIN RESPONSE FACTOR (ARF) family that interact with auxin/indole acetic acid repressors (Aux/IAAs), which themselves form co-receptor complexes with one of six TRANSPORT INHIBITOR1/AUXIN-SIGNALLING F-BOX (TIR1/AFB) proteins. Different combinations of co-receptors drive specific sensing outputs, allowing auxin to control a myriad of processes. ARF6 and ARF8 are positive regulators of adventitious root initiation upstream of jasmonate, but the exact auxin co-receptor complexes controlling the transcriptional activity of these proteins has remained unknown. Here, using loss-of-function mutants we show that three Aux/IAA genes, IAA6, IAA9, and IAA17, act additively in the control of adventitious root (AR) initiation. These three IAA proteins interact with ARF6 and/or ARF8 and likely repress their activity in AR development. We show that TIR1 and AFB2 are positive regulators of AR formation and TIR1 plays a dual role in the control of jasmonic acid (JA) biosynthesis and conjugation, as several JA biosynthesis genes are up-regulated in the tir1-1 mutant. These results lead us to propose that in the presence of auxin, TIR1 and AFB2 form specific sensing complexes with IAA6, IAA9, and/or IAA17 to modulate JA homeostasis and control AR initiation.

Gap 2 phase: making the fundamental decision to divide or not

The Gap phases of the cell cycle are essential to perceive internal and external signals and control cell division and differentiation. However, our knowledge of molecular mechanisms underlying G2 progression in plants remains quite limited. In this review, we summarize recent findings about core G2-phase regulators, such as B-type cyclin-dependent kinases (CDKs) and R1R2R3-type MYB transcription factors. We highlight developmental and stress signals that regulate expression and accumulation of the G2-phase regulators, and discuss how they fine-tune mitotic CDK activity and control cell proliferation, endoreplication and cell cycle checkpoints. A particular focus is on DNA damage-induced G2 arrest, which is prerequisite for maintenance of genome stability.

Loss of Wood Formation Genes in Monocot Genomes

Woodiness (secondary xylem derived from vascular cambium) has been gained and lost multiple times in the angiosperms, but has been lost ancestrally in all monocots. Here, we investigate the conservation of genes involved in xylogenesis in fully sequenced angiosperm genomes, hypothesizing that monocots have lost some essential orthologs involved in this process. We analyzed the conservation of genes preferentially expressed in the developing secondary xylem of two eudicot trees in the sequenced genomes of 26 eudicot and seven monocot species, and the early diverging angiosperm Amborella trichopoda. We also reconstructed a regulatory model of early vascular cambial cell identity and differentiation and investigated the conservation of orthologs across the angiosperms. Additionally, we analyzed the genome of the aquatic seagrass Zostera marina for additional losses of genes otherwise essential to, especially, secondary cell wall formation. Despite almost complete conservation of orthology within the early cambial differentiation gene network, we show a clear pattern of loss of genes preferentially expressed in secondary xylem in the monocots that are highly conserved across eudicot species. Our study provides candidate genes that may have led to the loss of vascular cambium in the monocots, and, by comparing terrestrial angiosperms to an aquatic monocot, highlights genes essential to vasculature on land.

Cell cycle control by the target of rapamycin signalling pathway in plants

Cells need to ensure a sufficient nutrient and energy supply before committing to proliferate. In response to positive mitogenic signals, such as light, sugar availability, and hormones, the target of rapamycin (TOR) signalling pathway promotes cell growth that connects to the entry and passage through the cell division cycle via multiple signalling mechanisms. Here, we summarize current understanding of cell cycle regulation by the RBR-E2F regulatory hub and the DREAM-like complexes, and highlight possible functional relationships between these regulators and TOR signalling. A genetic screen recently uncovered a downstream signalling component to TOR that regulates cell proliferation, YAK1, a member of the dual specificity tyrosine phosphorylation-regulated kinase (DYRK) family. YAK1 activates the plant-specific SIAMESE-RELATED (SMR) cyclin-dependent kinase inhibitors and therefore could be important to regulate both the CDKA-RBR-E2F pathway to control the G1/S transition and the mitotic CDKB1;1 to control the G2/M transition. TOR, as a master regulator of both protein synthesis-driven cell growth and cell proliferation is also central for cell size homeostasis. We conclude the review by briefly highlighting the potential applications of combining TOR and cell cycle knowledge in the context of ensuring future food security.

Transcriptomic analysis of contrasting inbred lines and F2 segregant of Chinese cabbage provides valuable information on leaf morphology

BACKGROUND

Leaf morphology influences plant growth and productivity and is controlled by genetic and environmental cues. The various morphotypes of Brassica rapa provide an excellent resource for genetic and molecular studies of morphological traits.

OBJECTIVE

This study aimed to identify genes regulating leaf morphology using segregating B. rapa p F₂ population.

METHODS

Phenotyping and transcriptomic analyses were performed on an F₂ population derived from a cross between Rapid cycling B. rapa (RCBr) and B. rapa ssp. penkinensis, inbred line Kenshin. Analyses focused on four target traits: lamina (leaf) length (LL), lamina width (LW), petiole length (PL), and leaf margin (LM).

RESULTS

All four traits were controlled by multiple QTLs, and expression of 466 and 602 genes showed positive and negative correlation with leaf phenotypes, respectively. From this microarray analysis, large numbers of genes were putatively identified as leaf morphology-related genes. The Gene Ontology (GO) category containing the highest number of differentially expressed genes (DEGs) was "phytohormones". The sets of genes enriched in the four leaf phenotypes did not overlap, indicating that each phenotype was regulated by a different set of genes. The expression of BrAS2, BrAN3, BrCYCB1;2, BrCYCB2;1,4, BrCYCB3;1, CrCYCBD3;2, BrULT1, and BrANT seemed to be related to leaf size traits (LL and LW), whereas BrCUC1, BrCUC2, and BrCUC3 expression for LM trait.

CONCLUSION

An analysis integrating the results of the current study with previously published data revealed that Kenshin alleles largely determined LL and LW but LM resulted from RCBr alleles. Genes identified in this study could be used to develop molecular markers for use in Brassica breeding projects and for the dissection of gene function.

Transcriptome Analysis Reveals New Insights into the Bacterial Wilt Resistance Mechanism Mediated by Silicon in Tomato

Bacterial wilt is a devastating disease of tomato caused by soilborne pathogenic bacterium Ralstonia solanacearum. Previous studies found that silicon (Si) can increase tomato resistance against R. solanacearum, but the exact molecular mechanism remains unclear. RNA sequencing (RNA-Seq) technology was used to investigate the dynamic changes of root transcriptome profiles between Si-treated (+Si) and untreated (−Si) tomato plants at 1, 3, and 7 days post-inoculation with R. solanacearum. The contents of salicylic acid (SA), ethylene (ET), and jasmonic acid (JA) and the activity of defense-related enzymes in roots of tomato in different treatments were also determined. The burst of ET production in roots was delayed, and SA and JA contents were altered in Si treatment. The transcriptional response to R. solanacearum infection of the +Si plants was quicker than that of the untreated plants. The expression levels of differentially-expressed genes involved in pathogen-associated molecular pattern-triggered immunity (PTI), oxidation resistance, and water-deficit stress tolerance were upregulated in the Si-treated plants. Multiple hormone-related genes were differentially expressed in the Si-treated plants. Si-mediated resistance involves mechanisms other than SA- and JA/ET-mediated stress responses. We propose that Si-mediated tomato resistance to R. solanacearum is associated with activated PTI-related responses and enhanced disease resistance and tolerance via several signaling pathways. Such pathways are mediated by multiple hormones (e.g., SA, JA, ET, and auxin), leading to diminished adverse effects (e.g., senescence, water-deficit, salinity and oxidative stress) normally caused by R. solanacearum infection. This finding will provide an important basis to further characterize the role of Si in enhancing plant resistance against biotic stress.

Transcriptional profiling identifies critical steps of cell cycle reprogramming necessary for Plasmodiophora brassicae-driven gall formation in Arabidopsis

Plasmodiophora brassicae is a soil-borne biotroph whose life cycle involves reprogramming host developmental processes leading to the formation of galls on its underground parts. Formation of such structures involves modification of the host cell cycle leading initially to hyperplasia, increasing the number of cells to be invaded, followed by overgrowth of cells colonised by the pathogen. Here we show that P. brassicae infection stimulates formation of the E2Fa/RBR1 complex and upregulation of MYB3R1, MYB3R4 and A- and B-type cyclin expression. These factors were previously described as important regulators of the G2-M cell cycle checkpoint. As a consequence of this manipulation, a large population of host hypocotyl cells are delayed in cell cycle exit and maintained in the proliferative state. We also report that, during further maturation of galls, enlargement of host cells invaded by the pathogen involves endoreduplication leading to increased ploidy levels. This study characterises two aspects of the cell cycle reprogramming efforts of P. brassicae: systemic, related to the disturbance of host hypocotyl developmental programs by preventing cell cycle exit; and local, related to the stimulation of cell enlargement via increased endocycle activity.

E2FA and E2FB transcription factors coordinate cell proliferation with seed maturation

The E2F transcription factors and the RETINOBLASTOMA-RELATED repressor protein are principal regulators coordinating cell proliferation with differentiation, but their role during seed development is little understood. We show that in fully developed Arabidopsis thaliana embryos, cell number was not affected either in single or double mutants for the activator-type E2FA and E2FB. Accordingly, these E2Fs are only partially required for the expression of cell cycle genes. In contrast, the expression of key seed maturation genes LEAFY COTYLEDON 1/2 (LEC1/2), ABSCISIC ACID INSENSITIVE 3, FUSCA 3 and WRINKLED 1 is upregulated in the e2fab double mutant embryo. In accordance, E2FA directly regulates LEC2, and mutation at the consensus E2F-binding site in the LEC2 promoter de-represses its activity during the proliferative stage of seed development. In addition, the major seed storage reserve proteins, 12S globulin and 2S albumin, became prematurely accumulated at the proliferating phase of seed development in the e2fab double mutant. Our findings reveal a repressor function of the activator E2Fs to restrict the seed maturation programme until the cell proliferation phase is completed.

Highlighted Article: During seed and embryo development the E2FA and E2FB transcription factors coordinate cell proliferation with differentiation and accumulation of seed reserves; however, they are not essential for sustaining cell proliferation.

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

Retinoblastoma (pRb) is a multifunctional regulator, which was likely present in the last common ancestor of all eukaryotes. The Arabidopsis pRb homolog RETINOBLASTOMA RELATED 1 (RBR1), similar to its animal counterparts, controls not only cell proliferation but is also implicated in developmental decisions, stress responses and maintenance of genome integrity. Although most functions of pRb-type proteins involve chromatin association, a genome-wide understanding of RBR1 binding sites in Arabidopsis is still missing. Here, we present a plant chromatin immunoprecipitation protocol optimized for genome-wide studies of indirectly DNA-bound proteins like RBR1. Our analysis revealed binding of Arabidopsis RBR1 to approximately 1000 genes and roughly 500 transposable elements, preferentially MITES. The RBR1-decorated genes broadly overlap with previously identified targets of two major transcription factors controlling the cell cycle, i.e. E2F and MYB3R3 and represent a robust inventory of RBR1-targets in dividing cells. Consistently, enriched motifs in the RBR1-marked domains include sequences related to the E2F consensus site and the MSA-core element bound by MYB3R transcription factors. Following up a key role of RBR1 in DNA damage response, we performed a meta-analysis combining the information about the RBR1-binding sites with genome-wide expression studies under DNA stress. As a result, we present the identification and mutant characterization of three novel genes required for growth upon genotoxic stress.

The Retinoblastoma (pRb) tumor suppressor is a master regulator of the cell cycle and its inactivation is associated with many types of cancer. Since pRb’s first description as a transcriptional repressor of genes important for cell cycle progression, many more functions have been elucidated, e.g. in developmental decisions and genome integrity. Homologs of human pRb have been identified in most eukaryotes, including plants, indicating an ancient evolutionary origin of pRb-type proteins. We describe here the first genome-wide DNA-binding study for a plant pRb protein, i.e. RBR1, the only pRb homolog in Arabidopsis thaliana. We see prominent binding of RBR1 to the 5’ region of genes involved in cell cycle regulation, chromatin organization and DNA repair. Moreover, we also reveal extensive binding of RBR1 to specific classes of DNA transposons. Since RBR1 is involved in a plethora of processes, our dataset provides a valuable resource for researches from different fields. As an example, we used our dataset to successfully identify new genes necessary for growth upon DNA damage exerted by drugs such as cisplatin or the environmentally prevalent metal aluminum.

Genome-wide identification and characterization of long non-coding RNAs responsive to Dickeya zeae in rice

Plant long non-coding RNA (lncRNA) is a type of newly emerging epigenetic regulator playing a critical role in plant growth, development, and biotic stress responses. However, it is unknown whether lncRNAs are involved in resistance responses between rice and Dickeya zeae, a bacterial agent causing rice foot rot disease. In this study, RNA-seq was performed to uncover the co-expression regulating networks mediated by D. zeae responsive lncRNAs and their candidate target genes. Of the 4709 lncRNAs identified, 2518 and 2191 were up- and down-regulated in response to D. zeae infection, respectively. Expression changes of 17 selected lncRNAs and their predicted targets with a potential role in defense response were investigated by qPCR. The expression levels of five lncRNAs were up-regulated while their cognate candidate target genes were down-regulated upon D. zeae infection. In addition, several lncRNAs were predicted to be target mimics of osa-miR396 and osa-miR156. These results suggest that lncRNAs might play a role in response to D. zeae infection by regulating the transcript levels of their targets and miRNAs in rice.

Rhizobia promote the growth of rice shoots by targeting cell signaling, division and expansion

The growth-promotion of rice seedling following inoculation with Sinorhizobium meliloti 1021 was a cumulative outcome of elevated expression of genes that function in accelerating cell division and enhancing cell expansion. Various endophytic rhizobacteria promote the growth of cereal crops. To achieve a better understanding of the cellular and molecular bases of beneficial cereal-rhizobia interactions, we performed computer-assisted microscopy and transcriptomic analyses of rice seedling shoots (Oryza sativa) during early stages of endophytic colonization by the plant growth-promoting Sinorhizobium meliloti 1021. Phenotypic analyses revealed that plants inoculated with live rhizobia had increased shoot height and dry weight compared to control plants inoculated with heat-killed cells of the same microbe. At 6 days after inoculation (DAI) with live cells, the fourth-leaf sheaths showed significant cytological differences including their enlargement of parenchyma cells and reduction in shape complexity. Transcriptomic analysis of shoots identified 2,414 differentially-expressed genes (DEGs) at 1, 2, 5 and 8 DAI: 195, 1390, 1025 and 533, respectively. Among these, 46 DEGs encoding cell-cycle functions were up-regulated at least 3 days before the rhizobia ascended from the roots to the shoots, suggesting that rhizobia are engaged in long-distance signaling events during early stages of this plant-microbe interaction. DEGs involved in phytohormone production, photosynthetic efficiency, carbohydrate metabolism, cell division and wall expansion were significantly elevated at 5 and 8 DAI, consistent with the observed phenotypic changes in rice cell morphology and shoot growth-promotion. Correlation analysis identified 104 height-related DEGs and 120 dry-weight-related DEGs that represent known quantitative-trait loci for seedling vigor and increased plant height. These findings provide multiple evidences of plant-microbe interplay that give insight into the growth-promotion processes associated with this rhizobia-rice beneficial association.

A Protoplast Transient Expression System to Enable Molecular, Cellular, and Functional Studies in Phalaenopsis orchids

The enigmatic nature of the specialized developmental programs of orchids has fascinated plant biologists for centuries. The recent releases of orchid genomes indicate that orchids possess new gene families and family expansions and contractions to regulate a diverse suite of developmental processes. However, the extremely long orchid life cycle and lack of molecular toolkit have hampered the advancement of orchid biology research. To overcome the technical difficulties and establish a platform for rapid gene regulation studies, in this study, we developed an efficient protoplast isolation and transient expression system for Phalaenopsis aphrodite. This protocol was successfully applied to protein subcellular localization and protein-protein interaction studies. Moreover, it was confirmed to be useful in delineating the PaE2F/PaDP-dependent cell cycle pathway and studying auxin response. In summary, the established orchid protoplast transient expression system provides a means to functionally characterize orchid genes at the molecular level allowing assessment of transcriptome responses to transgene expression and widening the scope of molecular studies in orchids.

Transcriptional profiling of the CAM plant Agave salmiana reveals conservation of a genetic program for regeneration

In plants, the best characterized plant regeneration process is de novo organogenesis. This type of regeneration is characterized by the formation of a multicellular structure called callus. Calli are induced via phytohormone treatment of plant sections. The callus formation in plants like Agave species with Crassulacean Acid Metabolism (CAM) is poorly studied. In this study, we induced callus formation from Agave salmiana leaves and describe cell arrangement in this tissue. Moreover, we determined and analyzed the transcriptional program of calli, as well as those of differentiated root and leaf tissues, by using RNA-seq. We were able to reconstruct 170,844 transcripts of which 40,644 have a full Open Reading Frame (ORF). The global profile obtained by Next Generation Sequencing (NGS) reveals that several callus-enriched protein coding transcripts are orthologs of previously reported factors highly expressed in Arabidopsis calli. At least 62 genes were differentially expressed in Agave calli, 50 of which were up-regulated. Several of these are actively involved in the perception of, and response to, auxin and cytokinin. Not only are these the first results for the A. salmiana callus, but they provide novel data from roots and leaves of this Agave species, one of the largest non-tree plants in nature.

Negative regulator of E2F transcription factors links cell cycle checkpoint and DNA damage repair

DNA damage poses a serious threat to genome integrity and greatly affects growth and development. To maintain genome stability, all organisms have evolved elaborate DNA damage response mechanisms including activation of cell cycle checkpoints and DNA repair. Here, we show that the DNA repair protein SNI1, a subunit of the evolutionally conserved SMC5/6 complex, directly links these two processes in Arabidopsis SNI1 binds to the activation domains of E2F transcription factors, the key regulators of cell cycle progression, and represses their transcriptional activities. In turn, E2Fs activate the expression of SNI1, suggesting that E2Fs and SNI1 form a negative feedback loop. Genetically, overexpression of SNI1 suppresses the phenotypes of E2F-overexpressing plants, and loss of E2F function fully suppresses the sni1 mutant, indicating that SNI1 is necessary and sufficient to inhibit E2Fs. Altogether, our study revealed that SNI1 is a negative regulator of E2Fs and plays dual roles in DNA damage responses by linking cell cycle checkpoint and DNA repair.

The Canonical E2Fs Are Required for Germline Development in Arabidopsis

A number of cell fate determinations, including cell division, cell differentiation, and programmed cell death, intensely occur during plant germline development. How these cell fate determinations are regulated remains largely unclear. The transcription factor E2F is a core cell cycle regulator. Here we show that the Arabidopsis canonical E2Fs, including E2Fa, E2Fb, and E2Fc, play a redundant role in plant germline development. The e2fa e2fb e2fc (e2fabc) triple mutant is sterile, although its vegetative development appears normal. On the one hand, the e2fabc microspores undergo cell death during pollen mitosis. Microspores start to die at the bicellular stage. By the tricellular stage, the majority of the e2fabc microspores are degenerated. On the other hand, a wild type ovule often has one megaspore mother cell (MMC), whereas the majority of e2fabc ovules have two to three MMCs. The subsequent female gametogenesis of e2fabc mutant is aborted and the vacuole is severely impaired in the embryo sac. Analysis of transmission efficiency showed that the canonical E2Fs from both male and female gametophyte are essential for plant gametogenesis. Our study reveals that the canonical E2Fs are required for plant germline development, especially the pollen mitosis and the archesporial cell (AC)-MMC transition.

SiSTL2 Is Required for Cell Cycle, Leaf Organ Development, Chloroplast Biogenesis, and Has Effects on C4 Photosynthesis in Setaria italica (L.) P. Beauv

Deoxycytidine monophosphate deaminase (DCD) is a key enzyme in the de novo dTTP biosynthesis pathway. Previous studies have indicated that DCD plays key roles in the maintenance of the balance of dNTP pools, cell cycle progression, and plant development. However, few studies have elucidated the functions of the DCD gene in Panicoideae plants. Setaria has been proposed as an ideal model of Panicoideae grasses, especially for C₄ photosynthesis research. Here, a Setaria italica stripe leaf mutant (sistl2) was isolated from EMS-induced lines of "Yugu1," the wild-type parent. The sistl2 mutant exhibited semi-dwarf, striped leaves, abnormal chloroplast ultrastructure, and delayed cell cycle progression compared with Yugu1. High-throughput sequencing and map-based cloning identified the causal gene SiSTL2, which encodes a DCD protein. The occurrence of a single-base G to A substitution in the fifth intron introduced alternative splicing, which led to the early termination of translation. Further physiological and transcriptomic investigation indicated that SiSTL2 plays an essential role in the regulation of chloroplast biogenesis, cell cycle, and DNA replication, which suggested that the gene has conserved functions in both foxtail millet and rice. Remarkably, in contrast to DCD mutants in C₃ rice, sistl2 showed a significant reduction in leaf cell size and affected C₄ photosynthetic capacity in foxtail millet. qPCR showed that SiSTL2 had a similar expression pattern to typical C₄ genes in response to a low CO₂ environment. Moreover, the loss of function of SiSTL2 resulted in a reduction of leaf ¹³C content and the enrichment of DEGs in photosynthetic carbon fixation. Our research provides in-depth knowledge of the role of DCD in the C₄ photosynthesis model S. italica and proposed new directions for further study of the function of DCD.

Evidence for a Strong Correlation Between Transcription Factor Protein Disorder and Organismic Complexity

Studies of diverse phylogenetic lineages reveal that protein disorder increases in concert with organismic complexity but that differences nevertheless exist among lineages. To gain insight into this phenomenology, we analyzed all of the transcription factor (TF) families for which sequences are known for 17 species spanning bacteria, yeast, algae, land plants, and animals and for which the number of different cell types has been reported in the primary literature. Although the fraction of disordered residues in TF sequences is often moderately or poorly correlated with organismic complexity as gauged by cell-type number (r² < 0.5), an unbiased and phylogenetically broad analysis shows that organismic complexity is positively and strongly correlated with the total number of TFs, the number of their spliced variants and their total disordered residues content (r² > 0.8). Furthermore, the correlation between the fraction of disordered residues and cell-type number becomes stronger when confined to the TF families participating in cell cycle, cell size, cell division, cell differentiation, or cell proliferation, and other important developmental processes. The data also indicate that evolutionarily simpler organisms allow for the detection of subtle differences in the conserved IDRs of TFs as well as changes in variable IDRs, which can influence the DNA recognition and multifunctionality of TFs through direct or indirect mechanisms. Although strong correlations cannot be taken as evidence for cause-and-effect relationships, we interpret our data to indicate that increasing TF disorder likely was an important factor contributing to the evolution of organismic complexity and not merely a concurrent unrelated effect of increasing organismic complexity.