Kip-related protein 3 is required for control of endoreduplication in the shoot apical meristem and leaves of Arabidopsis

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.

HECT-type ubiquitin ligase KAKTUS mediates the proteasome-dependent degradation of cyclin-dependent kinase inhibitor KRP2 during trichome morphogenesis in Arabidopsis

Trichome development is a fascinating model to elaborate the plant cell differentiation and growth processes. A wealth of information has pointed to the contributions of the components associated with cell cycle control and ubiquitin/26S proteasome system (UPS) to trichome morphogenesis, but how these two pathways are connected remains obscure. Here, we report that HECT‐type ubiquitin ligase KAKTUS (KAK) targets the cyclin‐dependent kinase (CDK) inhibitor KRP2 (for kip‐related protein 2) for proteasome‐dependent degradation during trichome branching in Arabidopsis. We show that over‐expression of KRP2 promotes trichome branching and endoreduplication which is similar to kak loss of function mutants. KAK directly interacts with KRP2 and mediates KRP2 degradation. Mutation of KAK results in the accumulation of steady‐state KRP2. Consistently, in kak pKRP2:KRP2‐GFP plants, the trichome branching is further induced compared with the single mutant. Taken together, our studies bridge the cell cycle control and UPS pathways during trichome development and underscore the importance of post‐translational control in epidermal differentiation.

Leaf-size control beyond transcription factors: Compensatory mechanisms

Plant leaves display abundant morphological richness yet grow to characteristic sizes and shapes. Beginning with a small number of undifferentiated founder cells, leaves evolve via a complex interplay of regulatory factors that ultimately influence cell proliferation and subsequent post-mitotic cell enlargement. During their development, a sequence of key events that shape leaves is both robustly executed spatiotemporally following a genomic molecular network and flexibly tuned by a variety of environmental stimuli. Decades of work on Arabidopsis thaliana have revisited the compensatory phenomena that might reflect a general and primary size-regulatory mechanism in leaves. This review focuses on key molecular and cellular events behind the organ-wide scale regulation of compensatory mechanisms. Lastly, emerging novel mechanisms of metabolic and hormonal regulation are discussed, based on recent advances in the field that have provided insights into, among other phenomena, leaf-size regulation.

Identification of growth regulators using cross-species network analysis in plants

With the need to increase plant productivity, one of the challenges plant scientists are facing is to identify genes that play a role in beneficial plant traits. Moreover, even when such genes are found, it is generally not trivial to transfer this knowledge about gene function across species to identify functional orthologs. Here, we focused on the leaf to study plant growth. First, we built leaf growth transcriptional networks in Arabidopsis (Arabidopsis thaliana), maize (Zea mays), and aspen (Populus tremula). Next, known growth regulators, here defined as genes that when mutated or ectopically expressed alter plant growth, together with cross-species conserved networks, were used as guides to predict novel Arabidopsis growth regulators. Using an in-depth literature screening, 34 out of 100 top predicted growth regulators were confirmed to affect leaf phenotype when mutated or overexpressed and thus represent novel potential growth regulators. Globally, these growth regulators were involved in cell cycle, plant defense responses, gibberellin, auxin, and brassinosteroid signaling. Phenotypic characterization of loss-of-function lines confirmed two predicted growth regulators to be involved in leaf growth (NPF6.4 and LATE MERISTEM IDENTITY2). In conclusion, the presented network approach offers an integrative cross-species strategy to identify genes involved in plant growth and development.

AtELP4 a subunit of the Elongator complex in Arabidopsis, mediates cell proliferation and dorsoventral polarity during leaf morphogenesis

The Elongator complex in eukaryotes has conserved tRNA modification functions and contributes to various physiological processes such as transcriptional control, DNA replication and repair, and chromatin accessibility. ARABIDOPSIS ELONGATOR PROTEIN 4 (AtELP4) is one of the six subunits (AtELP1-AtELP6) in Arabidopsis Elongator. In addition, there is an Elongator-associated protein, DEFORMED ROOTS AND LEAVES 1 (DRL1), whose homolog in yeast (Kti12) binds tRNAs. In this study, we explored the functions of AtELP4 in plant-specific aspects such as leaf morphogenesis and evolutionarily conserved ones between yeast and Arabidopsis. ELP4 comparison between yeast and Arabidopsis revealed that plant ELP4 possesses not only a highly conserved P-loop ATPase domain but also unknown plant-specific motifs. ELP4 function is partially conserved between Arabidopsis and yeast in the growth sensitivity toward caffeine and elevated cultivation temperature. Either single Atelp4 or drl1-102 mutants and double Atelp4 drl1-102 mutants exhibited a reduction in cell proliferation and changed the adaxial-abaxial polarity of leaves. In addition, the single Atelp4 and double Atelp4 drl1-102 mutants showed remarkable downward curling at the whole part of leaf blades in contrast to wild-type leaf blades. Furthermore, our genetic study revealed that AtELP4 might epistatically act on DRL1 in the regulation of cell proliferation and dorsoventral polarity in leaves. Taken together, we suggest that AtELP4 as part of the plant Elongator complex may act upstream of a regulatory pathway for adaxial-abaxial polarity and cell proliferation during leaf development.

Functional analysis of the eTM-miR171-SCL6 module regulating somatic embryogenesis in Lilium pumilum DC. Fisch

Somatic embryogenesis (SE) is of great significance in Lilium bulb production, germplasm preservation and genetic improvement. miRNAs are important regulators of plant growth and development at the transcriptional level. Previous research by our group has shown that lpu-miR171 and its target gene SCARECROW-LIKE 6 (SCL6) play an important regulatory role in lily SE, and we predicted and identified that endogenous target mimics (eTMs) can regulate lpu-miR171. However, the associated mechanism and internal regulatory network are not yet clear. In the present study, lpu-miR171 was used as an entry point to explore the regulatory network between its upstream eTMs and its downstream target gene LpSCL6, as well as to identify the mechanism of this regulatory network in Lilium SE. Tobacco transient transformation confirmed that miRNA171 significantly inhibited the expression of LpSCL6. On this basis, the Lilium stable genetic transformation system was used to demonstrate that silencing lpu-miR171a and lpu-miR171b and overexpressing LpSCL6-II and LpSCL6-I promoted starch accumulation in calli and the expression of key cell cycle genes, thus providing energy to meet preconditions for SE and accelerate the formation and development of Lilium somatic embryos. LpSCL6-II and LpSCL6-I are nuclear proteins with self-activation activity in yeast cells. In addition, we confirmed in Lilium that lpu-eTM171 is the eTM of lpu-miR171 that binds lpu-miR171 to prevent cleavage of the target gene LpSCL6, thereby promoting SE. Therefore, the present study established a new mechanism whereby the eTM-miR171-SCL6 module regulates SE in Lilium pumilum DC. Fisch. and provided new insights clarifying the mechanism of SE.

CYTOKININ-RESPONSIVE GROWTH REGULATOR regulates cell expansion and cytokinin-mediated cell cycle progression

The cytokinin (CK) phytohormones have long been known to activate cell proliferation in plants. However, how CKs regulate cell division and cell expansion remains unclear. Here, we reveal that a basic helix-loop-helix transcription factor, CYTOKININ-RESPONSIVE GROWTH REGULATOR (CKG), mediates CK-dependent regulation of cell expansion and cell cycle progression in Arabidopsis thaliana. The overexpression of CKG increased cell size in a ploidy-independent manner and promoted entry into the S phase of the cell cycle, especially at the seedling stage. Furthermore, CKG enhanced organ growth in a pleiotropic fashion, from embryogenesis to reproductive stages, particularly of cotyledons. In contrast, ckg loss-of-function mutants exhibited smaller cotyledons. CKG mainly regulates the expression of genes involved in the regulation of the cell cycle including WEE1. We propose that CKG provides a regulatory module that connects cell cycle progression and organ growth to CK responses.

An Artificial Conversion of Roots into Organs with Shoot Stem Characteristics by Inducing Two Transcription Factors

Somatic plant cells can regenerate shoots and/or roots or adventitious embryonic calluses, which may induce organ formation under certain conditions. Such regenerations occur via dedifferentiation of somatic cells, induction of organs, and their subsequent outgrowth. Despite recent advances in understanding of plant regeneration, many details of shoot induction remain unclear. Here, we artificially induced shoot stem-like green organs (SSOs) in Arabidopsis thaliana roots via simultaneous induction of two transcription factors (TFs), ARABIDOPSIS THALIANA HOMEOBOX PROTEIN 25 (ATHB25, At5g65410) and the B3 family transcription factor REPRODUCTIVE MERISTEM 7 (REM7, At3g18960). The SSOs exhibited negative gravitropism and differentiated vascular bundle phenotypes. The ATHB25/REM7 induced the expression of genes controlling shoot stem characteristics by ectopic expression in roots. Intriguingly, the restoration of root growth was seen in the consecutive and adjacent parts of the SSOs under gene induction conditions. Our findings thus provide insights into the development and regeneration of plant shoot stems.

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Co-induction of ATHB25 and REM7 produces shoot stem-like organs (SSOs) in roots

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SSOs exhibit negative gravitropism and differentiated vascular bundles

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Shoot- and root-specific genes are up- and down-regulated, respectively, in SSOs

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The restoration of normal root growth follows the SSO formation

ORESARA15 Acts Synergistically with ANGUSTIFOLIA3 and Separately from AINTEGUMENTA to Promote Cell Proliferation during Leaf Growth

Developing leaves undergo sequential coordinated cell proliferation and cell expansion to determine their final size and shape. Although several important regulators of cell proliferation have been reported, the gene network regulating leaf developmental processes remains unclear. Previously, we showed that ORESARA15 (ORE15) positively regulates the rate and duration of cell proliferation by promoting the expression of direct targets, GROWTH-REGULATING FACTOR (GRF) transcription factors, during leaf growth. In the current study, we examined the spatiotemporal patterns of ORE15 expression and determined that ORE15 expression partially overlapped with AN3/GIF1 and ANT expression along the midvein in the proximal region of the leaf blade in young leaves. Genetic analysis revealed that ORE15 may function synergistically with AN3 to control leaf growth as a positive regulator of cell proliferation. Our molecular and genetic studies are the first to suggest the importance of functional redundancies between ORE15 and AN3, and between AN3 and ANT in cell proliferation regulatory pathway during leaf growth.

Multiple mechanisms explain how reduced KRP expression increases leaf size of Arabidopsis thaliana

Although cell number generally correlates with organ size, the role of cell cycle control in growth regulation is still largely unsolved. We studied kip related protein (krp) 4, 6 and 7 single, double and triple mutants of Arabidopsis thaliana to understand the role of cell cycle inhibitory proteins in leaf development. We performed leaf growth and seed size analysis, kinematic analysis, flow cytometery, transcriptome analysis and mathematical modeling of G1/S and G2/M checkpoint progression of the mitotic and endoreplication cycle. Double and triple mutants progressively increased mature leaf size, because of elevated expression of cell cycle and DNA replication genes stimulating progression through the division and endoreplication cycle. However, cell number was also already increased before leaf emergence, as a result of an increased cell number in the embryo. We show that increased embryo and seed size in krp4/6/7 results from seed abortion, presumably reducing resource competition, and that seed size differences contribute to the phenotype of several large-leaf mutants. Our results provide a new mechanistic understanding of the role of cell cycle regulation in leaf development and highlight the contribution of the embryo to the development of leaves after germination in general.

ORESARA15, a PLATZ transcription factor, mediates leaf growth and senescence in Arabidopsis

Plant leaves undergo a series of developmental changes from leaf primordium initiation through growth and maturation to senescence throughout their life span. Although the mechanisms underlying leaf senescence have been intensively elucidated, our knowledge of the interrelationship between early leaf development and senescence is still fragmentary. We isolated the oresara15-1Dominant (ore15-1D) mutant, which had an extended leaf longevity and an enlarged leaf size, from activation-tagged lines of Arabidopsis. Plasmid rescue identified that ORE15 encodes a PLANT A/T-RICH SEQUENCE- AND ZINC-BINDING PROTEIN family transcription factor. Phenotypes of ore15-1D and ore15-2, a loss-of-function mutant, were evaluated through physiological and anatomical analyses. Microarray, quantitative reverse transcription polymerase chain reaction, and chromatin immunoprecipitation as well as genetic analysis were employed to reveal the molecular mechanism of ORE15 in the regulation of leaf growth and senescence. ORE15 enhanced leaf growth by promoting the rate and duration of cell proliferation in the earlier stage and suppressed leaf senescence in the later stage by modulating the GROWTH-REGULATING FACTOR (GRF)/GRF-INTERACTING FACTOR regulatory pathway. Our study highlighted a molecular conjunction through ORE15 between growth and senescence, which are two temporally separate developmental processes during leaf life span.

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

The plant cell cycle is tightly regulated by factors that integrate endogenous cues and environmental signals to adapt plant growth to changing conditions. Under drought, cell division in young leaves is blocked by an active mechanism, reducing the evaporative surface and conserving energy resources. The molecular function of cyclin-dependent kinase-inhibitory proteins (CKIs) in regulating the cell cycle has already been well studied, but little is known about their involvement in cell cycle regulation under adverse growth conditions. In this study, we show that the transcript of the CKI gene SIAMESE-RELATED1 (SMR1) is quickly induced under moderate drought in young Arabidopsis (Arabidopsis thaliana) leaves. Functional characterization further revealed that SMR1 inhibits cell division and affects meristem activity, thereby restricting the growth of leaves and roots. Moreover, we demonstrate that SMR1 is a short-lived protein that is degraded by the 26S proteasome after being ubiquitinated by a Cullin-RING E3 ubiquitin ligase. Consequently, overexpression of a more stable variant of the SMR1 protein leads to a much stronger phenotype than overexpression of the native SMR1. Under moderate drought, both the SMR1 transcript and SMR1 protein accumulate. Despite this induction, smr1 mutants do not show overall tolerance to drought stress but do show less growth inhibition of young leaves under drought. Surprisingly, the growth-repressive hormone ethylene promotes SMR1 induction, but the classical drought hormone abscisic acid does not.

Exploiting cell cycle inhibitor genes of the KRP family to control root-knot nematode induced feeding sites in plants

Cell cycle control in galls provoked by root-knot nematodes involves the activity of inhibitor genes like the Arabidopsis ICK/KRP members. Ectopic KRP1, KRP2 and KRP4 expression resulted in decreased gall size by inhibiting mitotic activity, whereas KRP6 induces mitosis in galls. Herein, we investigate the role of KRP3, KRP5 and KRP7 during gall development and compared their role with previously studied members of this class of cell cycle inhibitors. Overexpression of KRP3 and KRP7 culminated in undersized giant cells, with KRP3^OE galls presenting peculiar elongated giant cells. Nuclei in KRP3^OE and KRP5^OE lines presented a convoluted and apparently connected phenotype. This appearance may be associated with the punctuated protein nuclear localization driven by specific common motifs. As well, ectopic expression of KRP3^OE and KRP5^OE affected nematode development and offspring. Decreased mitotic activity in galls of KRP3^OE and KRP7^OE lines led to a reduced gall size which presented distinct shapes - from more elongated like in the KRP3^OE line to small rounded like in the KRP7^OE line. Results presented strongly support the idea that induced expression of cell cycle inhibitors such as KRP3 and KRP7 in galls can be envisaged as a conceivable strategy for nematode feeding site control in crop species attacked by phytopathogenic nematodes.

Plant Cyclin-Dependent Kinase Inhibitors of the KRP Family: Potent Inhibitors of Root-Knot Nematode Feeding Sites in Plant Roots

Root-knot nematodes (RKN), Meloidogyne spp., are distributed worldwide and impose severe economic damage to many agronomically important crops. The plant cell cycle machinery is considered one of the pivotal components for the formation of nematode feeding sites (NFSs) or galls. These feeding sites contain five to nine hypertrophied giant cells (GC) resulting from developmental reprogramming of host root cells by this pathogen. GC undergo synchronous waves of mitotic activity uncoupled from cytokinesis giving rise to large multinucleate cells. As development of the NFS progresses, multiple rounds of DNA synthesis occur in the nuclei of GC, coupled with nuclear and cellular expansion. These cells are highly metabolically active and provide the nematode with nutrients necessary for its development and completion of its life cycle. In Arabidopsis seven cyclin dependent kinase inhibitors (CKIs) belonging to the interactors/inhibitors of the cyclin dependent kinases (ICK) family, also referred as Kip-Related Proteins (KRPs) have been identified. Interactions of KRPs with CDK/Cyclin complexes decrease CDK activity, affecting both cell cycle progression and DNA content in a concentration-dependent manner. We performed the functional analysis of all Arabidopsis KRP gene members during RKN interaction in Arabidopsis to obtain more insight into their role during gall development. We demonstrated that three members of this family (KRP2, KRP5, and KRP6) were highly expressed in galls and were important for cell cycle regulation during NFS development as shown by their different modes of action. We also pointed out that cell cycle inhibition through overexpression of all members of the KRP family can affect NFS development and consequently compromise the nematode's life cycle. In this review we summarized our recent understanding of the KRP family of genes, and their role in controlling cell cycle progression at the RKN feeding site.

The Arabidopsis glutamate receptor-like gene GLR3.6 controls root development by repressing the Kip-related protein gene KRP4

In Arabidopsis, 20 genes encode putative glutamate receptor-like proteins (AtGLRs). However, the functions of most genes are unknown. In this study, our results revealed that loss of function of AtGLR3.6 (atglr3.6-1) leads to reduced primary root growth and fewer lateral roots, whereas AtGLR3.6 overexpression induced both primary and lateral root growth. The glr3.6-1 mutant exhibited a smaller root meristem size compared with the wild type, indicating that AtGLR3.6 controls root meristem size. In addition, atglr3.6-1 roots show a decreased mitotic activity accounting for the reduced root meristem size. Furthermore, expression of a gene encoding a cell cycle inhibitor, the cyclin-dependent kinase (CDK) inhibitor Kip-related protein 4 (KRP4), was significantly up-regulated in the mutant and down-regulated in AtGLR3.6-overexpressing roots, suggesting a role for KRP4 in AtGLR3.6-mediated root meristem maintenance. Importantly, the atglr3.6-1 mutant recovered most of its root growth when KRP4 expression is down-regulated, whereas elevated KRP4 expression in AtGLR3.6-overexpressing plants phenocopied the wild-type root growth, implying an underlying relationship between AtGLR3.6 and KRP4 genes. Cytosolic Ca(2+) elevation is reduced in atglr3.6-1 roots, suggesting impaired calcium signaling. Moreover, calcium treatment reduced the level of KRP4 and hence induced root growth. Collectively, we reveal that AtGLR3.6 is required for primary and lateral root development, and KRP4 functions as a downstream signaling element in Arabidopsis thaliana.

Identification of Drought Tolerant Mechanisms in Maize Seedlings Based on Transcriptome Analysis of Recombination Inbred Lines

Zea mays is an important crop that is sensitive to drought stress, but survival rates and growth status remain strong in some drought-tolerant lines under stress conditions. Under drought conditions, many biological processes, such as photosynthesis, carbohydrate metabolism and energy metabolism, are suppressed, while little is known about how the transcripts of genes respond to drought stress in the genome-wide rang in the seedling stage. In our study, the transcriptome profiles of two maize recombination inbred lines (drought-tolerant RIL70 and drought-sensitive RIL93) were analyzed at different drought stages to elucidate the dynamic mechanisms underlying drought tolerance in maize seedlings during drought conditions. Different numbers of differentially expressed genes presented in the different stages of drought stress in the two RILs, for the numbers of RIL93 vs. RIL70 were: 9 vs. 358, 477 vs. 103, and 5207 vs. 152 respectively in DT1, DT2, and DT5. Gene Ontology enrichment analysis revealed that in the initial drought-stressed stage, the primary differentially expressed genes involved in cell wall biosynthesis and transmembrane transport biological processes were overrepresented in RIL70 compared to RIL93. On the contrary, differentially expressed genes profiles presented at 2 and 5 day-treatments, the primary differentially expressed genes involved in response to stress, protein folding, oxidation-reduction, photosynthesis and carbohydrate metabolism, were overrepresented in RIL93 compared to RIL70. In addition, the transcription of genes encoding key members of the cell cycle and cell division processes were blocked, but ABA- and programmed cell death-related processes responded positively in RIL93. In contrast, the expression of cell cycle genes, ABA- and programmed cell death-related genes was relatively stable in RIL70. The results we obtained supported the working hypothesis that signaling events associated with turgor homeostasis, as established by cell wall biosynthesis regulation- and aquaporin-related genes, responded early in RIL70, which led to more efficient detoxification signaling (response to stress, protein folding, oxidation-reduction) during drought stress. This energy saving response at the early stages of drought should facilitate more cell activity under stress conditions and result in drought tolerance in RIL70.

Cell cycling with the SEB: a personal view

This review, written from a personal perspective, traces firstly the development of plant cell cycle research from the 1970s onwards, with some focus on the work of the author and of Dr Dennis Francis. Secondly there is a discussion of the support for and discussion of plant cell cycle research in the SEB, especially through the activities of the Cell Cycle Group within the Society's Cell Biology Section. In the main part of the review, selected aspects of DNA replication that have of been of special interest to the author are discussed. These are DNA polymerases and associated proteins, pre-replication events, regulation of enzymes and other proteins, nature and activation of DNA replication origins, and DNA endoreduplication. For all these topics, there is mention of the author's own work, followed by a brief synthesis of current understanding and a look to possible future developments.

The cell-cycle interactome: a source of growth regulators?

When plants develop, cell proliferation and cell expansion are tightly controlled in order to generate organs with a determinate final size such as leaves. Several studies have demonstrated the importance of the cell proliferation phase for leaf growth, illustrating that cell-cycle regulation is crucial for correct leaf development. A large and complex set of interacting proteins that constitute the cell-cycle interactome controls the transition from one cell-cycle phase to another. Here, we review the current knowledge on cell-cycle regulators from this interactome affecting final leaf size when their expression is altered, mainly in Arabidopsis. In addition to the description of mutants of CYCLIN-DEPENDENT KINASES (CDKs), CYCLINS (CYCs), and their transcriptional and post-translational regulators, a phenotypic analysis of gain- and loss-of-function mutants for 27 genes encoding proteins that interact with cell-cycle proteins is presented. This compilation of information shows that when cell-cycle-related genes are mis-expressed, leaf growth is often altered and that, seemingly, three main trends appear to be crucial in the regulation of final organ size by cell-cycle-related genes: (i) cellular compensation; (ii) gene dosage; and (iii) correct transition through the G2/M phase by ANAPHASE PROMOTING COMPLEX/CYCLOSOME (APC/C) activation. In conclusion, this meta-analysis shows that the cell-cycle interactome is enriched in leaf growth regulators, and illustrates the potential to identify new leaf growth regulators among putative new cell-cycle regulators.

The Cyclin-Dependent Kinase Inhibitor KRP6 Induces Mitosis and Impairs Cytokinesis in Giant Cells Induced by Plant-Parasitic Nematodes in Arabidopsis

In Arabidopsis thaliana, seven cyclin-dependent kinase (CDK) inhibitors have been identified, designated interactors of CDKs or Kip-related proteins (KRPs). Here, the function of KRP6 was investigated during cell cycle progression in roots infected by plant-parasitic root-knot nematodes. Contrary to expectations, analysis of Meloidogyne incognita-induced galls of KRP6-overexpressing lines revealed a role for this particular KRP as an activator of the mitotic cell cycle. In accordance, KRP6-overexpressing suspension cultures displayed accelerated entry into mitosis, but delayed mitotic progression. Likewise, phenotypic analysis of cultured cells and nematode-induced giant cells revealed a failure in mitotic exit, with the appearance of multinucleated cells as a consequence. Strong KRP6 expression upon nematode infection and the phenotypic resemblance between KRP6 overexpression cell cultures and root-knot morphology point toward the involvement of KRP6 in the multinucleate and acytokinetic state of giant cells. Along these lines, the parasite might have evolved to manipulate plant KRP6 transcription to the benefit of gall establishment.

Enhanced cell expansion in a KRP2 overexpressor is mediated by increased V-ATPase activity

Decreased cell numbers during leaf development often trigger increased cell size, a phenomenon called compensation. In compensation-exhibiting mutants, the unusually high cell expansion activity occurs through two different mechanisms during the post-mitotic stage of leaf development, except in the KIP-RELATED PROTEIN 2-overexpressing line (KRP2 o/e), whose cell sizes are 2-fold greater during proliferative growth. However, the molecular basis of compensated cell expansion (CCE) has not been characterized. The det3-1 mutant has a mutation in the C-subunit of the vacuolar-type H(+)-ATPase (V-ATPase) complex that causes a 50% decrease in its activity and cell size. To determine the contribution of V-ATPase activity to CCE, the cellular phenotypes of double mutants between det3-1 and compensation-exhibiting fugu5-1, an3-4, fas1-5 and KRP2 o/e were analyzed in detail. Interestingly, while decreased V-ATPase activity caused by det3-1 did not suppress CCE in fugu5-1, fas1-5 and an3-4, CCE in KRP2 o/e was totally suppressed. Furthermore, measurements revealed that the activity and quantity of the A-subunit of the V-ATPase complex were significantly increased in the shoots of KRP2 o/e plants. Importantly, the unusually increased size of actively dividing KRP2 o/e cells was restored to normal in the det3-1 background. Taken together, our data strongly suggest that CCE in KRP2 o/e, but not in other compensation-exhibiting mutants, occurs exclusively through the increase of V-ATPase activity.

Class III compensation, represented by KRP2 overexpression, depends on V-ATPase activity in proliferative cells

Compensation refers to an increase in cell size when the cell number is significantly decreased due to the mutation or gain of function of a gene that negatively affects the cell cycle. Given the importance of coordinated growth during organogenesis in both animal and plant systems, compensation is important to understand the mechanism of size regulation. In leaves, cell division precedes cell differentiation (which involves cell expansion); therefore, a decrease in cell number triggers enhanced cell expansion (compensated cell expansion; hereafter, CCE). Functional analyses of genes for which a loss or gain of function triggers compensation have increased our understanding of the molecular mechanisms underlying the decrease in cell number. Nevertheless, the mechanisms that induce enhanced cell expansion (the link between cell cycling and expansion), as well as the cellular machinery mediating CCE, have not been characterized. We recently characterized an important pathway involved in cell enlargement in KRP2-overexpressing plants. Here, we discuss the potential axial role of plant KRPs in triggering enlargement in cells with meristematic features.