SIAMESE-RELATED1 Is Regulated Posttranslationally and Participates in Repression of Leaf Growth under Moderate Drought

The SIAMESE family of cell-cycle inhibitors in the response of plants to environmental stresses

Environmental abiotic constraints are known to reduce plant growth. This effect is largely due to the inhibition of cell division in the leaf and root meristems caused by perturbations of the cell cycle machinery. Progression of the cell cycle is regulated by CDK kinases whose phosphorylation activities are dependent on cyclin proteins. Recent results have emphasized the role of inhibitors of the cyclin-CDK complexes in the impairment of the cell cycle and the resulting growth inhibition under environmental constraints. Those cyclin-CDK inhibitors (CKIs) include the KRP and SIAMESE families of proteins. This review presents the current knowledge on how CKIs respond to environmental changes and on the role played by one subclass of CKIs, the SIAMESE RELATED proteins (SMRs), in the tolerance of plants to abiotic stresses. The SMRs could play a central role in adjusting the balance between growth and stress defenses in plants exposed to environmental stresses.

Leaf growth - complex regulation of a seemingly simple process

Understanding the underlying mechanisms of plant development is crucial to successfully steer or manipulate plant growth in a targeted manner. Leaves, the primary sites of photosynthesis, are vital organs for many plant species, and leaf growth is controlled by a tight temporal and spatial regulatory network. In this review, we focus on the genetic networks governing leaf cell proliferation, one major contributor to final leaf size. First, we provide an overview of six regulator families of leaf growth in Arabidopsis: DA1, PEAPODs, KLU, GRFs, the SWI/SNF complexes, and DELLAs, together with their surrounding genetic networks. Next, we discuss their evolutionary conservation to highlight similarities and differences among species, because knowledge transfer between species remains a big challenge. Finally, we focus on the increase in knowledge of the interconnectedness between these genetic pathways, the function of the cell cycle machinery as their central convergence point, and other internal and environmental cues.

A plant's perception of growth-promoting bacteria and their metabolites

Many recent studies have highlighted the importance of plant growth-promoting (rhizo)bacteria (PGPR) in supporting plant's development, particularly under biotic and abiotic stress. Most focus on the plant growth-promoting traits of selected strains and the latter's effect on plant biomass, root architecture, leaf area, and specific metabolite accumulation. Regarding energy balance, plant growth is the outcome of an input (photosynthesis) and several outputs (i.e., respiration, exudation, shedding, and herbivory), frequently neglected in classical studies on PGPR-plant interaction. Here, we discuss the primary evidence underlying the modifications triggered by PGPR and their metabolites on the plant ecophysiology. We propose to detect PGPR-induced variations in the photosynthetic activity using leaf gas exchange and recommend setting up the correct timing for monitoring plant responses according to the specific objectives of the experiment. This research identifies the challenges and tries to provide future directions to scientists working on PGPR-plant interactions to exploit the potential of microorganisms' application in improving plant value.

The response of Arabidopsis to the apocarotenoid β-cyclocitric acid reveals a role for SIAMESE-RELATED 5 in root development and drought tolerance

New regulatory functions in plant development and environmental stress responses have recently emerged for a number of apocarotenoids produced by enzymatic or nonenzymatic oxidation of carotenoids. β-Cyclocitric acid (β-CCA) is one such compound derived from β-carotene, which triggers defense mechanisms leading to a marked enhancement of plant tolerance to drought stress. We show here that this response is associated with an inhibition of root growth affecting both root cell elongation and division. Remarkably, β-CCA selectively induced cell cycle inhibitors of the SIAMESE-RELATED (SMR) family, especially SMR5, in root tip cells. Overexpression of the SMR5 gene in Arabidopsis induced molecular and physiological changes that mimicked in large part the effects of β-CCA. In particular, the SMR5 overexpressors exhibited an inhibition of root development and a marked increase in drought tolerance which is not related to stomatal closure. SMR5 up-regulation induced changes in gene expression that strongly overlapped with the β-CCA-induced transcriptomic changes. Both β-CCA and SMR5 led to a down-regulation of many cell cycle activators (cyclins, cyclin-dependent kinases) and a concomitant up-regulation of genes related to water deprivation, cellular detoxification, and biosynthesis of lipid biopolymers such as suberin and lignin. This was correlated with an accumulation of suberin lipid polyesters in the roots and a decrease in nonstomatal leaf transpiration. Taken together, our results identify the β-CCA-inducible and drought-inducible SMR5 gene as a key component of a stress-signaling pathway that reorients root metabolism from growth to multiple defense mechanisms leading to drought tolerance.

Endoreduplication in plant organogenesis: a means to boost fruit growth

Endoreduplication is the major source of somatic endopolyploidy in higher plants, and leads to variation in cell ploidy levels due to iterative rounds of DNA synthesis in the absence of mitosis. Despite its ubiquitous occurrence in many plant organs, tissues, and cells, the physiological meaning of endoreduplication is not fully understood, although several roles during plant development have been proposed, mostly related to cell growth, differentiation, and specialization via transcriptional and metabolic reprogramming. Here, we review recent advances in our knowledge of the molecular mechanisms and cellular characteristics of endoreduplicated cells, and provide an overview of the multi-scale effects of endoreduplication on supporting growth in plant development. In addition, the effects of endoreduplication in fruit development are discussed, since it is highly prominent during fruit organogenesis where it acts as a morphogenetic factor supporting rapid fruit growth, as illustrated by case of the model fleshy fruit, tomato (Solanum lycopersicum).

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.

SIAMESE-RELATED1 imposes differentiation of stomatal lineage ground cells into pavement cells

The leaf epidermis represents a multifunctional tissue consisting of trichomes, pavement cells and stomata, the specialized cellular pores of the leaf. Pavement cells and stomata both originate from regulated divisions of stomatal lineage ground cells (SLGCs), but whereas the ontogeny of the stomata is well characterized, the genetic pathways activating pavement cell differentiation remain relatively unexplored. Here, we reveal that the cell cycle inhibitor SIAMESE-RELATED1 (SMR1) is essential for timely differentiation of SLGCs into pavement cells by terminating SLGC self-renewal potency, which depends on CYCLIN A proteins and CYCLIN-DEPENDENT KINASE B1. By controlling SLGC-to-pavement cell differentiation, SMR1 determines the ratio of pavement cells to stomata and adjusts epidermal development to suit environmental conditions. We therefore propose SMR1 as an attractive target for engineering climate-resilient plants.

SCARECROW-LIKE28 modulates organ growth in Arabidopsis by controlling mitotic cell cycle exit, endoreplication, and cell expansion dynamics

The processes that contribute to plant organ morphogenesis are spatial-temporally organized. Within the meristem, mitosis produces new cells that subsequently engage in cell expansion and differentiation programs. The latter is frequently accompanied by endoreplication, being an alternative cell cycle that replicates the DNA without nuclear division, causing a stepwise increase in somatic ploidy. Here, we show that the Arabidopsis SCL28 transcription factor promotes organ growth by modulating cell expansion dynamics in both root and leaf cells. Gene expression studies indicated that SCL28 regulates members of the SIAMESE/SIAMESE-RELATED (SIM/SMR) family, encoding cyclin-dependent kinase inhibitors with a role in promoting mitotic cell cycle (MCC) exit and endoreplication, both in response to developmental and environmental cues. Consistent with this role, mutants in SCL28 displayed reduced endoreplication, both in roots and leaves. We also found evidence indicating that SCL28 co-expresses with and regulates genes related to the biogenesis, assembly, and remodeling of the cytoskeleton and cell wall. Our results suggest that SCL28 controls, not only cell proliferation as reported previously but also cell expansion and differentiation by promoting MCC exit and endoreplication and by modulating aspects of the biogenesis, assembly, and remodeling of the cytoskeleton and cell wall.

Characterization of the soybean KRP gene family reveals a key role for GmKRP2a in root development

Kip-related proteins (KRPs), as inhibitory proteins of cyclin-dependent kinases, are involved in the growth and development of plants by regulating the activity of the CYC-CDK complex to control cell cycle progression. The KRP gene family has been identified in several plants, and several KRP proteins from Arabidopsis thaliana have been functionally characterized. However, there is little research on KRP genes in soybean, which is an economically important crop. In this study, we identified nine GmKRP genes in the Glycine max genome using HMM modeling and BLASTP searches. Protein subcellular localization and conserved motif analysis showed soybean KRP proteins located in the nucleus, and the C-terminal protein sequence was highly conserved. By investigating the expression patterns in various tissues, we found that all GmKRPs exhibited transcript abundance, while several showed tissue-specific expression patterns. By analyzing the promoter region, we found that light, low temperature, an anaerobic environment, and hormones-related cis-elements were abundant. In addition, we performed a co-expression analysis of the GmKRP gene family, followed by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) set enrichment analysis. The co-expressing genes were mainly involved in RNA synthesis and modification and energy metabolism. Furthermore, the GmKRP2a gene, a member of the soybean KRP family, was cloned for further functional analysis. GmKRP2a is located in the nucleus and participates in root development by regulating cell cycle progression. RNA-seq results indicated that GmKRP2a is involved in cell cycle regulation through ribosome regulation, cell expansion, hormone response, stress response, and plant pathogen response pathways. To our knowledge, this is the first study to identify and characterize the KRP gene family in soybean.

Exogenous spermidine improved drought tolerance in Ilex verticillata seedlings

Winterberry (Ilex verticillata (L.) A. Gray) is a recently introduced ornamental tree species in China that has not been closely investigated for its drought resistance. In this study, we used two-year-old cuttings from I. verticillata (L.) A. Gray and two representative varieties derived from it, I. verticillata 'Oosterwijk' and I. verticillata 'Jim Dandy', as materials to investigate how this plant responds to drought stress and whether exogenous spermidine (SPD) can alleviate the negative effects caused by drought stress. The results showed that as the degree of drought stress increased, the leaves of winterberry seedlings became chlorotic, and their edges became dry. Similarly, the relative water content, specific leaf weight, chlorophyll content, leaf nitrogen content, net photosynthetic rate, stomatal conductance and transpiration rate were significantly reduced, whereas the content of malondialdehyde continuously increased with the degree of drought stress. The activities of superoxide dismutase, peroxidase, and catalase increased under moderate drought stress and then decreased under severe drought stress. The levels of soluble sugar and abscisic acid continued to increase, while those of auxin and gibberellic acid decreased. When compared with individual drought stress, an increase in the amount of external SPD clearly alleviated the effect of drought stress on winterberry seedlings. The combined phenotypes and physiological indices of the winterberry leaves under drought stress conditions revealed that the drought resistance of the native species was significantly higher than its two varieties. This finding serves as an important theoretical foundation for the popularization and application of I. verticillata (L.) A. Gray and the two varieties.

ERF4 interacts with and antagonizes TCP15 in regulating endoreduplication and cell growth in Arabidopsis

Endoreduplication is prevalent during plant growth and development, and is often correlated with large cell and organ size. Despite its prevalence, the transcriptional regulatory mechanisms underlying the transition from mitotic cell division to endoreduplication remain elusive. Here, we characterize ETHYLENE-RESPONSIVE ELEMENT BINDING FACTOR 4 (ERF4) as a positive regulator of endoreduplication through its function as a transcriptional repressor. ERF4 was specifically expressed in mature tissues in which the cells were undergoing expansion, but was rarely expressed in young organs. Plants overexpressing ERF4 exhibited much larger cells and organs, while plants that lacked functional ERF4 displayed smaller organs than the wild-type. ERF4 was further shown to regulate cell size by controlling the endopolyploidy level in the nuclei. Moreover, ERF4 physically associates with the class I TEOSINTE BRANCHED 1/CYCLOIDEA/PCF (TCP) protein TCP15, a transcription factor that inhibits endoreduplication by activating the expression of a key cell-cycle gene, CYCLIN A2;3 (CYCA2;3). A molecular and genetic analysis revealed that ERF4 promotes endoreduplication by directly suppressing the expression of CYCA2;3. Together, this study demonstrates that ERF4 and TCP15 function as a module to antagonistically regulate each other's activity in regulating downstream genes, thereby controlling the switch from the mitotic cell cycle to endoreduplication during leaf development. These findings expand our understanding of how the control of the cell cycle is fine-tuned by an ERF4-TCP15 transcriptional complex.

Members of SIAMESE-RELATED Class Inhibitor Proteins of Cyclin-Dependent Kinase Retard G2 Progression and Increase Cell Size in Arabidopsis thaliana

Cell size requires strict and flexible control as it significantly impacts plant growth and development. Unveiling the molecular mechanism underlying cell size control would provide fundamental insights into plants’ nature as sessile organisms. Recently, a GRAS family transcription factor SCARECROW-LIKE28 (SCL28) was identified as a determinant of cell size in plants; specifically, SCL28 directly induces a subset of SIAMESE-RELATED (SMR) family genes encoding plant-specific inhibitors of cyclin-dependent kinases (i.e., SMR1, SMR2, SMR6, SMR8, SMR9, SMR13, and SMR14), thereby slowing down G2 phase progression to provide the time to increase cell volume. Of the SMR genes regulated by SCL28, genetic analysis has demonstrated that SMR1, SMR2, and SMR13 cooperatively regulate the cell size downstream of SCL28 in roots and leaves, whereas other SMR members’ contribution remains unexplored. This study shows that in root meristematic cells, SMR9 redundantly participates in cell size control with SMR1, SMR2, and SMR13. Moreover, our cell cycle analysis provides the first experimental evidence that SMR proteins inhibit the G2 progression of proliferating cells. Overall, these findings illuminate the diverse yet overlapping roles of SMR proteins in cell cycle regulation while reinforcing that SMRs are essential downstream effectors of SCL28 to modulate G2 progression and cell size.

RADA-dependent branch migration has a predominant role in plant mitochondria and its defect leads to mtDNA instability and cell cycle arrest

Mitochondria of flowering plants have large genomes whose structure and segregation are modulated by recombination activities. The post-synaptic late steps of mitochondrial DNA (mtDNA) recombination are still poorly characterized. Here we show that RADA, a plant ortholog of bacterial RadA/Sms, is an organellar protein that drives the major branch-migration pathway of plant mitochondria. While RadA/Sms is dispensable in bacteria, RADA-deficient Arabidopsis plants are severely impacted in their development and fertility, correlating with increased mtDNA recombination across intermediate-size repeats and accumulation of recombination-generated mitochondrial subgenomes. The radA mutation is epistatic to recG1 that affects the additional branch migration activity. In contrast, the double mutation radA recA3 is lethal, underlining the importance of an alternative RECA3-dependent pathway. The physical interaction of RADA with RECA2 but not with RECA3 further indicated that RADA is required for the processing of recombination intermediates in the RECA2-depedent recombination pathway of plant mitochondria. Although RADA is dually targeted to mitochondria and chloroplasts we found little to no effects of the radA mutation on the stability of the plastidial genome. Finally, we found that the deficient maintenance of the mtDNA in radA apparently triggers a retrograde signal that activates nuclear genes repressing cell cycle progression.

In flowering plants, the mitochondrial genome is very large and dynamic, and its stability influences plant fitness and development. Rearrangements by recombination drive its very rapid evolution and can lead to valuable agronomic traits such as cytoplasmic sterility, used by breeders for the production of hybrid seeds. Here we describe RADA, a DNA helicase essential for the stability of the mitochondrial DNA in Arabidopsis. We demonstrate that RADA has branch migrating activity, accelerating the processing of recombination intermediates. radA mutants are severely affected in development and fertility. They display mitochondrial genome instability that results in uncoordinated replication of subgenomes created by recombination. Furthermore, we found that an important component of the growth defects of radA mutants is apparently a cellular response triggered by the sensing of damages to the mitochondrial genome, resulting in the activation of genes that suppress the progression of the cell cycle. Our results underline the importance of better understanding the plant mitochondrial recombination pathways and their cross-talk with nuclear gene expression.

Genome-Wide Identification of Direct Targets of ZjVND7 Reveals the Putative Roles of Whole-Genome Duplication in Sour Jujube in Regulating Xylem Vessel Differentiation and Drought Tolerance

The effects of whole-genome duplication span multiple levels. Previous study reported that the autotetraploid sour jujube exhibited superior drought tolerance than diploid. However, the difference in water transport system between diploids and autotetraploids and its mechanism remain unclear. Here, we found the number of xylem vessels and parenchyma cells in autotetraploid sour jujube increased to nearly twice that of diploid sour jujube, which may be closely related to the differences in xylem vessel differentiation-related ZjVND7 targets between the two ploidy types. Although the five enriched binding motifs are different, the most reliable motif in both diploid and autotetraploid sour jujube was CTTNAAG. Additionally, ZjVND7 targeted 236 and 321 genes in diploids and autotetraploids, respectively. More identified targeted genes of ZjVND7 were annotated to xylem development, secondary wall synthesis, cell death, cell division, and DNA endoreplication in autotetraploids than in diploids. SMR1 plays distinct roles in both proliferating and differentiated cells. Under drought stress, the binding signal of ZjVND7 to ZjSMR1 was stronger in autotetraploids than in diploids, and the fold-changes in the expression of ZjVND7 and ZjSMR1 were larger in the autotetraploids than in the diploids. These results suggested that the targeted regulation of ZjVND7 on ZjSMR1 may play valuable roles in autotetraploids in the response to drought stress. We hypothesized that the binding of ZjVND7 to ZjSMR1 might play a role in cell division and transdifferentiation from parenchyma cells to vessels in the xylem. This regulation could prolong the cell cycle and regulate endoreplication in response to drought stress and abscisic acid, which may be stronger in polyploids.

Spatial differences in stoichiometry of EGR phosphatase and Microtubule-associated Stress Protein 1 control root meristem activity during drought stress

During moderate severity drought and low water potential (ψw) stress, poorly understood signaling mechanisms restrict both meristem cell division and subsequent cell expansion. We found that the Arabidopsis thaliana Clade E Growth-Regulating 2 (EGR2) protein phosphatase and Microtubule-Associated Stress Protein 1 (MASP1) differed in their stoichiometry of protein accumulation across the root meristem and had opposing effects on root meristem activity at low ψw. Ectopic MASP1 or EGR expression increased or decreased, respectively, root meristem size and root elongation during low ψw stress. This, along with the ability of phosphomimic MASP1 to overcome the EGR-mediated suppression of root meristem size and the observation that ectopic EGR expression had no effect on unstressed plants, indicated that during low ψw EGR activation and attenuation of MASP1 phosphorylation in their overlapping zone of expression determines root meristem size and activity. Ectopic EGR expression also decreased root cell size at low ψw. Conversely, both the egr1-1 egr2-1 and egr1-1 egr2-1 masp1-1 mutants had similarly increased root cell size but only egr1-1egr2-1 had increased cell division. These observations demonstrated that EGRs affect meristem activity via MASP1 but affect cell expansion via other mechanisms. Interestingly, EGR2 was highly expressed in the root cortex, a cell type important for growth regulation and environmental response.

Inter-tissue and inter-organ signaling in drought stress response and phenotyping of drought tolerance

Plant response to drought stress includes systems for intracellular regulation of gene expression and signaling, as well as inter-tissue and inter-organ signaling, which helps entire plants acquire stress resistance. Plants sense water-deficit conditions both via the stomata of leaves and roots, and transfer water-deficit signals from roots to shoots via inter-organ signaling. Abscisic acid is an important phytohormone involved in the drought stress response and adaptation, and is synthesized mainly in vascular tissues and guard cells of leaves. In leaves, stress-induced abscisic acid is distributed to various tissues by transporters, which activates stomatal closure and expression of stress-related genes to acquire drought stress resistance. Moreover, the stepwise stress response at the whole-plant level is important for proper understanding of the physiological response to drought conditions. Drought stress is sensed by multiple types of sensors as molecular patterns of abiotic stress signals, which are transmitted via separate parallel signaling networks to induce downstream responses, including stomatal closure and synthesis of stress-related proteins and metabolites. Peptide molecules play important roles in the inter-organ signaling of dehydration from roots to shoots, as well as signaling of osmotic changes and reactive oxygen species/Ca²⁺ . In this review, we have summarized recent advances in research on complex plant drought stress responses, focusing on inter-tissue signaling in leaves and inter-organ signaling from roots to shoots. We have discussed the mechanisms via which drought stress adaptations and resistance are acquired at the whole-plant level, and have proposed the importance of quantitative phenotyping for measuring plant growth under drought conditions.

Increasing yield on dry fields: molecular pathways with growing potential

Drought stress constitutes one of the major constraints to agriculture all over the world, and its devastating effect is only expected to increase in the following years due to climate change. Concurrently, the increasing food demand in a steadily growing population requires a proportional increase in yield and crop production. In the past, research aimed to increase plant resilience to severe drought stress. However, this often resulted in stunted growth and reduced yield under favorable conditions or moderate drought. Nowadays, drought tolerance research aims to maintain plant growth and yield under drought conditions. Overall, recently deployed strategies to engineer drought tolerance in the lab can be classified into a 'growth-centered' strategy, which focuses on keeping growth unaffected by the drought stress, and a 'drought resilience without growth penalty' strategy, in which the main aim is still to boost drought resilience, while limiting the side effects on plant growth. In this review, we put the scope on these two strategies and some molecular players that were successfully engineered to generate drought-tolerant plants: abscisic acid, brassinosteroids, cytokinins, ethylene, ROS scavenging genes, strigolactones, and aquaporins. We discuss how these pathways participate in growth and stress response regulation under drought. Finally, we present an overview of the current insights and future perspectives in the development of new strategies to improve drought tolerance in the field.

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

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

Plant CDKs-Driving the Cell Cycle through Climate Change

In a growing population, producing enough food has become a challenge in the face of the dramatic increase in climate change. Plants, during their evolution as sessile organisms, developed countless mechanisms to better adapt to the environment and its fluctuations. One important way is through the plasticity of their body and their forms, which are modulated during plant growth by accurate control of cell divisions. A family of serine/threonine kinases called cyclin-dependent kinases (CDK) is a key regulator of cell divisions by controlling cell cycle progression. In this review, we compile information on the primary response of plants in the regulation of the cell cycle in response to environmental stresses and show how the cell cycle proteins (mainly the cyclin-dependent kinases) involved in this regulation can act as components of environmental response signaling cascades, triggering adaptive responses to drive the cycle through climate fluctuations. Understanding the roles of CDKs and their regulators in the face of adversity may be crucial to meeting the challenge of increasing agricultural productivity in a new climate.

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.

The CDK Inhibitor SIAMESE Targets Both CDKA;1 and CDKB1 Complexes to Establish Endoreplication in Trichomes

Endoreplication, also known as endoreduplication, is a modified cell cycle in which DNA is replicated without subsequent cell division. Endoreplication plays important roles in both normal plant development and in stress responses. The SIAMESE (SIM) gene of Arabidopsis (Arabidopsis thaliana) encodes a cyclin-dependent kinase (CDK) inhibitor that plays a central role in establishing endoreplication, and is the founding member of the SIAMESE-RELATED (SMR) family of plant-specific CDK inhibitor genes. However, there has been conflicting evidence regarding which specific cyclin/CDK complexes are inhibited by SIM in vivo. In this work, we use genetic evidence to show that SIM likely inhibits both CDKA;1- and CDKB1-containing CDK complexes in vivo, thus promoting endoreplication in developing Arabidopsis trichomes. We also show that SIM interacts with CYCA2;3, a binding partner of CDKB1;1, via SIM motif A, which we previously identified as a CDK-binding motif. By contrast, SIM motif C, which has been indicated as a cyclin binding motif in other contexts, appears to be relatively unimportant for interaction between SIM and CYCA2;3. Together with earlier results, our work suggests that SIM and other SMRs likely have a multivalent interaction with CYC/CDK complexes.

Endoreplication - a means to an end in cell growth and stress response

Endoreplication, also called endoreduplication or endopolyploidization, is a cell cycle variant in which the genome is re-replicated in the absence of mitosis causing cellular polyploidization. Despite the common occurrence of endoreplication in plants and the tremendous extent in specific tissues and cell types such as the endosperm, the underlying molecular regulation and the physiological consequences have only now started to be understood. Endoreplication is often associated with cell differentiation and withdrawal from mitotic cycles. Recent studies have underlined the importance of endoreplication as a stress response and we summarize here this progress with particular focus on future perspectives offered by the recent advances in genomics and biotechnology.

Plant growth under suboptimal water conditions: early responses and methods to study them

Drought stress forms a major environmental constraint during the life cycle of plants, often decreasing plant yield and in extreme cases threatening survival. The molecular and physiological responses induced by drought have been the topic of extensive research during the past decades. Because soil-based approaches to studying drought responses are often challenging due to low throughput and insufficient control of the conditions, osmotic stress assays in plates were developed to mimic drought. Addition of compounds such as polyethylene glycol, mannitol, sorbitol, or NaCl to controlled growth media has become increasingly popular since it offers the advantage of accurate control of stress level and onset. These osmotic stress assays enabled the discovery of very early stress responses, occurring within seconds or minutes following osmotic stress exposure. In this review, we construct a detailed timeline of early responses to osmotic stress, with a focus on how they initiate plant growth arrest. We further discuss the specific responses triggered by different types and severities of osmotic stress. Finally, we compare short-term plant responses under osmotic stress versus in-soil drought and discuss the advantages, disadvantages, and future of these plate-based proxies for drought.

Endoreplication as a potential driver of cell wall modifications

Endoreplication represents a variant of the mitotic cell cycle during which cells replicate their DNA without mitosis and/or cytokinesis, resulting in an increase in the cells' ploidy level. This process is especially prominent in higher plants, where it has been correlated with cell differentiation, metabolic output and rapid cell growth. However, different reports argue against a ploidy-dependent contribution to cell growth. Here, we review accumulating data suggesting that endocycle onset might exert an effect on cell growth through transcriptional control of cell wall-modifying genes to drive cell wall changes required to accommodate turgor-driven rapid cell expansion, consistent with the idea that vacuolar expansion rather than a ploidy-driven increase in cellular volume represents the major force driving cell growth.

Identification of TCP13 as an Upstream Regulator of ATHB12 during Leaf Development

Leaves grow by distinct phases controlled by gene regulatory networks including many transcription factors. Arabidopsis thaliana homeobox 12 (ATHB12) promotes leaf growth especially during the cell expansion phase. In this study, we identify TCP13, a member of the TCP transcription factor family, as an upstream inhibitor of ATHB12. Yeast one-hybrid screening using a 1.2-kb upstream region of ATHB12 resulted in the isolation of TCP13 as well as other transcription factors. Transgenic plants constitutively expressing TCP13 displays a significant reduction in leaf cell size especially during the cell expansion period, while repression of TCP13 and its paralogs (TCP5 and TCP17) result in enlarged leaf cells, indicating that TCP13 and its paralogs inhibit leaf development, mainly at the cell expansion phase. Its expression pattern during leaf expansion phase is opposite to ATHB12 expression. Consistently, the expression of ATHB12 and its downstream genes decreases when TCP13 was overexpressed, and increases when the expression of TCP13 and its paralogs is repressed. In chromatin immunoprecipitation assays using TCP13-GFP plants, a fragment of the ATHB12 upstream region that contains the consensus sequence for TCP binding is strongly enriched. Taken together, these findings indicate that TCP13 and its paralogs inhibit leaf growth by repressing ATHB12 expression.

Source-Sink Regulation in Crops under Water Deficit

To meet the food demands of an increasing world population, it is necessary to improve crop production; a task that is made more challenging by the changing climate. Several recent reports show that increasing the capacity of plants to assimilate carbon (source strength), or to tap into the internal carbon reservoir (sink strength), has the potential to improve plant productivity in the field under water-deficit conditions. Here, we review the effects of water deficit on the source-sink communication, as well as the respective regulatory mechanisms underpinning plant productivity. We also highlight stress-tolerant traits that can contribute to harness source and sink strengths towards producing high-yielding and drought-tolerant crops, depending on the drought scenario.

Mechanisms Underlying the Environmentally Induced Plasticity of Leaf Morphology

The primary function of leaves is to provide an interface between plants and their environment for gas exchange, light exposure and thermoregulation. Leaves have, therefore a central contribution to plant fitness by allowing an efficient absorption of sunlight energy through photosynthesis to ensure an optimal growth. Their final geometry will result from a balance between the need to maximize energy uptake while minimizing the damage caused by environmental stresses. This intimate relationship between leaf and its surroundings has led to an enormous diversification in leaf forms. Leaf shape varies between species, populations, individuals or even within identical genotypes when those are subjected to different environmental conditions. For instance, the extent of leaf margin dissection has, for long, been found to inversely correlate with the mean annual temperature, such that Paleobotanists have used models based on leaf shape to predict the paleoclimate from fossil flora. Leaf growth is not only dependent on temperature but is also regulated by many other environmental factors such as light quality and intensity or ambient humidity. This raises the question of how the different signals can be integrated at the molecular level and converted into clear developmental decisions. Several recent studies have started to shed the light on the molecular mechanisms that connect the environmental sensing with organ-growth and patterning. In this review, we discuss the current knowledge on the influence of different environmental signals on leaf size and shape, their integration as well as their importance for plant adaptation.

Functional Analysis of Short Linear Motifs in the Plant Cyclin-Dependent Kinase Inhibitor SIAMESE

Endoreplication, a modified cell cycle in which DNA is replicated without subsequent cell division, plays an important but poorly understood role in plant growth and in plant responses to biotic and abiotic stress. The Arabidopsis (Arabidopsis thaliana) SIAMESE (SIM) gene encodes the first identified member of the SIAMESE-RELATED (SMR) family of cyclin-dependent kinase inhibitors. SIM controls endoreplication during trichome development, and sim mutant trichomes divide several times instead of endoreplicating their DNA. The SMR family is defined by several short linear amino acid sequence motifs of largely unknown function, and family members have little sequence similarity to any known protein functional domains. Here, we investigated the roles of the conserved motifs in SIM site-directed Arabidopsis mutants using several functional assays. We identified a potential cyclin-dependent kinase (CDK)-binding site, which bears no resemblance to other known CDK interaction motifs. We also identified a potential site of phosphorylation and two redundant nuclear localization sequences. Surprisingly, the only motif with similarity to the other family of plant CDK inhibitors, the INHIBITOR/INTERACTOR OF CDC2 KINASE/KIP-RELATED PROTEIN proteins, is not required for SIM function in vivo. Because even highly divergent members of the SMR family are able to replace SIM function in Arabidopsis trichomes, it is likely that the results obtained here for SIM will apply to other members of this plant-specific family of CDK inhibitors.