Cell Proliferation In Meristem Research Articles

AtHSPR functions in gibberellin-mediated primary root growth by interacting with KNAT5 and OFP1 in Arabidopsis.

AtHSPR forms a complex with KNAT5 and OFP1 to regulate primary root growth through GA-mediated root meristem activity. KNAT5-OFP1 functions as a negative regulator of AtHSPR in response to GA. Plant root growth is modulated by gibberellic acid (GA) signaling and depends on root meristem maintenance. ARABIDOPSIS THALIANA HEAT SHOCK PROTEIN-RELATED (AtHSPR) is a vital regulator of flowering time and salt stress tolerance. However, little is known about the role of AtHSPR in the regulation of primary root growth. Here, we report that athspr mutant exhibits a shorter primary root compared to wild type and that AtHSPR interacts with KNOTTED1-LIKE HOMEOBOX GENE 5 (KNAT5) and OVATE FAMILY PROTEIN 1 (OFP1). Genetic analysis showed that overexpression of KNAT5 or OFP1 caused a defect in primary root growth similar to that of the athspr mutant, but knockout of KNAT5 or OFP1 rescued the short root phenotype in the athspr mutant by altering root meristem activity. Further investigation revealed that KNAT5 interacts with OFP1 and that AtHSPR weakens the inhibition of GIBBERELLIN 20-OXIDASE 1 (GA20ox1) expression by the KNAT5-OFP1 complex. Moreover, root meristem cell proliferation and root elongation in 35S::KNAT5athspr and 35S::OFP1athspr seedlings were hypersensitive to GA3 treatment compared to the athspr mutant. Together, our results demonstrate that the AtHSPR-KNAT5-OFP1 module regulates root growth and development by impacting the expression of GA biosynthetic gene GA20ox1, which could be a way for plants to achieve plasticity in response to the environment.

Cell Division and Meristem Dynamics in Fern Gametophytes.

One of the most important questions in all multicellular organisms is how to define and maintain different cell fates during continuous cell division and proliferation. Plant meristems provide a unique research system to address this fundamental question because meristems dynamically maintain themselves and sustain organogenesis through balancing cell division and cell differentiation. Different from the gametophytes of seed plants that depend on their sporophytes and lack meristems, the gametophytes of seed-free ferns develop different types of meristems (including apical cell-based meristems and multicellular apical and marginal meristems) to promote independent growth and proliferation during the sexual gametophyte phase. Recent studies combining confocal time-lapse imaging and computational image analysis reveal the cellular basis of the initiation and proliferation of different types of meristems in fern gametophytes, providing new insights into the evolution of meristems in land plants. In this review, we summarize the recent progress in understanding the cell growth dynamics in fern gametophytes and discuss both conserved and diversified mechanisms underlying meristem cell proliferation in seed-free vascular plants.

Comprehensive toxicity screening of Pazarsuyu stream water containing heavy metals and protective role of lycopene

In this study, heavy metal pollution in the Pazarsuyu stream of Giresun province and the protective role of lycopene against the toxicity caused by this pollution were investigated using the Allium test. Germination percentage, root length and weight gain as physiological markers of toxicity; mitotic index (MI), micronucleus (MN) and chromosomal aberrations (CAs) as genetic markers of toxicity; malondialdehyde (MDA) level, superoxide dismutase (SOD) and catalase (CAT) activities as biochemical markers of toxicity, and meristematic cell damages were used as anatomical markers. For this aim Allium cepa L. bulbs were divided into six groups and germinated for 72 h with 215 mg/L and 430 mg/L doses of lycopene, tap water and stream water. Heavy metals pollution was analyzed with ICP-MS and Fe > Sr > Ba > Be > Mo > Li were determined according to the rate of presence in the water samples of Pazarsuyu. As a result, germination-related parameters and meristematic cell proliferation of bulbs germinated with Pazarsuyu water samples decreased significantly. Germination percentage, root length and weight gain of the group treated with Pazarsuyu water samples were decreased 50%, 73% and 68%, respectively compared to control. In addition, MN and CAs frequencies, indicating the genotoxic effects, were increased and significant abnormalities were detected in MDA, SOD and CAT levels, which indicate the deterioration of antioxidant/oxidant balance. CA observed with high frequency was also confirmed by DNA fragmentation determined by the Comet test. Stream water application promoted anatomical damages such as epidermis and cortex cell damage, accumulation of some substances in cortex cells, flattened cell nucleus and non-apparent appearance of conduction tissue in root tip meristem cells. All these abnormalities observed in A. cepa root tip cells were associated with the presence of heavy metals in the water samples. Simultaneous application of lycopene with stream water reduced the effects of heavy metals and resulted in a dose-dependent improvement in all parameters studied. Lycopene application showed a protective role by providing an increase in germination parameters and MI, decrease in MN and CAs frequencies, and improvements in MDA, SOD and CAT activities. As a result, heavy metals detected in the water samples of Pazarsuyu stream caused multiple toxicities in the bio-indicator plant, and lycopene reduced this toxicity and recorded a protective role.

Crosstalk of Cytokinin with Ethylene and Auxin for Cell Elongation Inhibition and Boron Transport in Arabidopsis Primary Root under Boron Deficiency

Several studies have shown the role of phytohormones in the regulation of root growth of Arabidopsis plants under boron (B) deficiency. Ethylene and auxin play an important role in the control of Arabidopsis primary root cell elongation under short-term B deprivation, whereas cytokinins regulate root growth inhibition under B deficiency by controlling meristem cell proliferation. In this work, we study the possible interaction among cytokinin, ethylene, and auxin in the primary root response to B-deprivation treatment, as well as their possible role in B uptake and transport. Wild type (WT) and two mutants related to auxin and ethylene (aux1 and acs11) Arabidopsis plants were grown in control (10 µM B) or B starvation (0 µM B) treatment, in the absence or presence of trans-zeatin, and their primary root growth was analyzed. The possible interaction between these hormones was also studied by analyzing AUX1 gene expression in the acs11 mutant and ACS11 gene expression in the aux1 mutant. The GUS reporter lines ARR5::GUS, IAA2::GUS, and EBS::GUS were used to observe changes in cytokinin, auxin, and ethylene levels in the root, respectively. The results of this work suggest that cytokinin inhibits root cell elongation under B deficiency through two different mechanisms: (i) an ethylene-dependent mechanism through increased expression of the ACS11 gene, which would lead to increased ethylene in the root, and (ii) an ethylene-independent mechanism through decreased expression of the AUX1 gene, which alters auxin signaling in the meristematic and elongation zones and stele. We also report that changes in the expression of several B transporters occur in response to auxin, ethylene, and cytokinin that may affect the plant B content.

Potassium Chloroaurate-Mediated In Vitro Synthesis of Gold Nanoparticles Improved Root Growth by Crosstalk with Sucrose and Nutrient-Dependent Auxin Homeostasis in Arabidopsis thaliana.

In a hydroponic system, potassium chloroaurate (KAuCl4) triggers the in vitro sucrose (Suc)-dependent formation of gold nanoparticles (AuNPs). AuNPs stimulate the growth of the root system, but their molecular mechanism has not been deciphered. The root system of Arabidopsis (Arabidopsis thaliana) exhibits developmental plasticity in response to the availability of various nutrients, Suc, and auxin. Here, we showed the roles of Suc, phosphorus (P), and nitrogen (N) in facilitating a AuNPs-mediated increase in root growth. Furthermore, the recuperating effects of KAuCl4 on the natural (IAA) auxin-mediated perturbation of the root system were demonstrated. Arabidopsis seedlings harboring the cell division marker CycB1;1::CDB-GUS provided evidence of the restoration efficacy of KAuCl4 on the IAA-mediated inhibitory effect on meristematic cell proliferation of the primary and lateral roots. Arabidopsis harboring synthetic auxin DR5rev::GFP exhibited a reinstating effect of KAuCl4 on IAA-mediated aberration in auxin subcellular localization in the root. KAuCl4 also exerted significant and differential recuperating effects on the IAA-mediated altered expression of the genes involved in auxin signaling and biosynthetic pathways in roots. Our results highlight the crosstalk between KAuCl4-mediated improved root growth and Suc and nutrient-dependent auxin homeostasis in Arabidopsis.

ABERRANT PANICLE ORGANIZATION2 controls multiple steps in panicle formation through common direct-target genes.

At the transition from vegetative to reproductive growth in rice (Oryza sativa), a developmental program change occurs, resulting in panicle (rice inflorescence) formation. The initial event of the transition is the change of the shoot apical meristem to an inflorescence meristem (IM), accompanied by a rapid increase in the meristem size. Suppression of leaf growth also occurs, resulting in the formation of bracts. The IM generates branch meristems (BMs), indeterminate meristems that reiteratively generate next-order meristems. All meristems eventually acquire a determinate spikelet meristem identity and terminate after producing a floret. ABERRANT PANICLE ORGANIZATION2 (APO2) is the rice ortholog of Arabidopsis (Arabidopsis thaliana) LEAFY (LFY), a plant-specific transcription factor (TF). APO2 is a positive regulator of panicle branch formation. Here, we show that APO2 is also required to increase the meristem size of the IM and suppress bract outgrowth. We identified genes directly and indirectly regulated by APO2 and identified APO2-binding sites. These analyses showed that APO2 directly controls known regulators of panicle development, including SQUAMOSA PROMOTER BINDING PROTEIN LIKE14 and NECK LEAF1. Furthermore, we revealed that a set of genes act as downstream regulators of APO2 in controlling meristem cell proliferation during reproductive transition, bract suppression, and panicle branch formation. Our findings indicate that APO2 acts as a master regulator of rice panicle development by regulating multiple steps in the reproductive transition through directly controlling a set of genes.

Cell growth dynamics in two types of apical meristems in fern gametophytes.

SUMMARYIn contrast to seed plants, the gametophytes of seed‐free plants develop pluripotent meristems, which promote and sustain their independent growth and development. To date, the cellular basis of meristem development in gametophytes of seed‐free ferns remains largely unknown. In this study, we used Woodsia obtusa, the blunt‐lobe cliff fern, to quantitatively determine cell growth dynamics in two different types of apical meristems – the apical initial centered meristem and the multicellular apical meristem in gametophytes. Through confocal time‐lapse live imaging and computational image analysis and quantification, we determined unique patterns of cell division and growth that sustain or terminate apical initials, dictate the transition from apical initials to multicellular apical meristems, and drive proliferation of apical meristems in ferns. Quantitative results showed that small cells correlated to active cell division in fern gametophytes. The marginal cells of multicellular apical meristems in fern gametophytes undergo division in both anticlinal and periclinal orientations, not only increasing cell numbers but also playing a dominant role in increasing cell layers during gametophyte development. All these findings provide insights into the function and regulation of meristems in gametophytes of seed‐free vascular plants, suggesting both conserved and diversified mechanisms underlying meristem cell proliferation across land plants.

RNA demethylation increases the yield and biomass of rice and potato plants in field trials.

RNA N6-methyladenosine (m6A) modifications are essential in plants. Here, we show that transgenic expression of the human RNA demethylase FTO in rice caused a more than threefold increase in grain yield under greenhouse conditions. In field trials, transgenic expression of FTO in rice and potato caused ~50% increases in yield and biomass. We demonstrate that the presence of FTO stimulates root meristem cell proliferation and tiller bud formation and promotes photosynthetic efficiency and drought tolerance but has no effect on mature cell size, shoot meristem cell proliferation, root diameter, plant height or ploidy. FTO mediates substantial m6A demethylation (around 7% of demethylation in poly(A) RNA and around 35% decrease of m6A in non-ribosomal nuclear RNA) in plant RNA, inducing chromatin openness and transcriptional activation. Therefore, modulation of plant RNA m6A methylation is a promising strategy to dramatically improve plant growth and crop yield.

Salt-induced inhibition of rice seminal root growth is mediated by ethylene-jasmonate interaction.

The phytohormones ethylene and jasmonate play important roles in the adaptation of rice plants to salt stress. However, the molecular interactions between ethylene and jasmonate on rice seminal root growth under salt stress are unknown. In this study, the effects of NaCl on the homeostasis of ethylene and jasmonate, and on rice seminal root growth were investigated. Our results indicate that NaCl treatment promotes ethylene biosynthesis by up-regulating the expression of ethylene biosynthesis genes, whereas NaCl-induced ethylene does not inhibit rice seminal root growth directly, but rather indirectly, by promoting jasmonate biosynthesis. NaCl treatment also promotes jasmonate biosynthesis through an ethylene-independent pathway. Moreover, NaCl-induced jasmonate reduces meristem cell number and cell division activity via down-regulated expression of Oryza sativa PLETHORA (OsPLT) and cell division-related genes, respectively. Additionally, NaCl-induced jasmonate inhibits seminal root cell elongation by down-regulating the expression of cell elongation-related genes. Overall, salt stress promotes jasmonate biosynthesis through ethylene-dependent and -independent pathways in rice seminal roots, and jasmonate inhibits rice seminal root growth by inhibiting root meristem cell proliferation and root cell elongation.

The Microtubule-Associated Protein CLASP Is Translationally Regulated in Light-Dependent Root Apical Meristem Growth.

The ability for plant growth to be optimized, either in the light or dark, depends on the intricate balance between cell division and differentiation in specialized regions called meristems. When Arabidopsis (Arabidopsis thaliana) seedlings are grown in the dark, hypocotyl elongation is promoted, whereas root growth is greatly reduced as a result of changes in hormone transport and a reduction in meristematic cell proliferation. Previous work showed that the microtubule-associated protein CLASP sustains root apical meristem size by influencing microtubule organization and by modulating the brassinosteroid signaling pathway. Here, we investigated whether CLASP is involved in light-dependent root growth promotion, since dark-grown seedlings have reduced root apical meristem activity, as observed in the clasp-1 null mutant. We showed that CLASP protein levels were greatly reduced in the root tips of dark-grown seedlings, which could be reversed by exposing plants to light. We confirmed that removing seedlings from the light led to a discernible shift in microtubule organization from bundled arrays, which are prominent in dividing cells, to transverse orientations typically observed in cells that have exited the meristem. Brassinosteroid receptors and auxin transporters, both of which are sustained by CLASP, were largely degraded in the dark. Interestingly, we found that despite the lack of protein, CLASP transcript levels were higher in dark-grown root tips. Together, these findings uncover a mechanism that sustains meristem homeostasis through CLASP, and they advance our understanding of how roots modulate their growth according to the amount of light and nutrients perceived by the plant.

Inoculation of maize seeds with Pseudomonas putida leads to enhanced seedling growth in combination with modified regulation of miRNAs and antioxidant enzymes

Bacteria that positively interact with plant roots are defined as plant growth promoting rhizobacteria (PGPR). Although the positive effect of PGPR on plant growth has been widely studied, their impact on genetic regulation during plant growth processes remains largely unknown. Therefore, this study aimed to gain a deeper understanding of the regulatory role of miRNA and redox enzymes in response to PGPR at the maize seedling stage in leaf growth zone which consists of the meristem, elongation and mature zones. For this purpose, growth of the third leaf was monitored in response to Pseudomonas putida (P. putida) KT2440 at phenotypic, physiological, cellular, kinematic, and transcriptional levels. This application resulted in an increase of 15% in shoot length, 56% in both shoot fresh/dry weight, 10% in chlorophyll amount, 8% in mature cell length, 15% in leaf elongation rate, and 7% in cell production; meanwhile final leaf length was unchanged, while leaf area and leaf width decreased by 22% and 16%, respectively. Ascorbate peroxidase and glutathione reductase activity increased throughout the leaf growth zone indicating a possible role in PGPR-plant interaction during transition between cell division, expansion and differentiation processes. The expression analysis of cell cycle check point marker genes revealed that CycA2_1 was mainly responsible for promoting cell proliferation in meristem. miR160, miR169 and miR408 were differentially expressed in the meristem, indicating their indirect regulatory roles in the cell division response to PGPR. In addition, miR160, miR319 and miR396 were downregulated in the elongation zone, which draws attention to their possible role in regulating cell elongation processes. In summary, cell cycle, redox and miRNA regulation in maize seedling growth zones in response to P. putida were investigated for the first time in this study.

Nitrogen Deficiency-Induced Decrease in Cytokinins Content Promotes Rice Seminal Root Growth by Promoting Root Meristem Cell Proliferation and Cell Elongation.

Rice (Oryza sativa L.) seedlings grown under nitrogen (N) deficiency conditions show a foraging response characterized by increased root length. However, the mechanism underlying this developmental plasticity is still poorly understood. In this study, the mechanism by which N deficiency influences rice seminal root growth was investigated. The results demonstrated that compared with the control (1 mM N) treatment, N deficiency treatments strongly promoted seminal root growth. However, the N deficiency-induced growth was negated by the application of zeatin, which is a type of cytokinin (CK). Moreover, the promotion of rice seminal root growth was correlated with a decrease in CK content, which was due to the N deficiency-mediated inhibition of CK biosynthesis through the down-regulation of CK biosynthesis genes and an enhancement of CK degradation through the up-regulation of CK degradation genes. In addition, the N deficiency-induced decrease in CK content not only enhanced the root meristem cell proliferation rate by increasing the meristem cell number via the down-regulation of OsIAA3 and up-regulation of root-expressed OsPLTs, but also promoted root cell elongation by up-regulating cell elongation-related genes, including root-specific OsXTHs and OsEXPs. Taken together, our data suggest that an N deficiency-induced decrease in CK content promotes the seminal root growth of rice seedlings by promoting root meristem cell proliferation and cell elongation.

PRR5, 7and 9 positively modulate TOR signaling-mediated root cell proliferation by repressing TANDEM ZINC FINGER 1 in Arabidopsis.

Circadian clock coordinates numerous plant growth and developmental processes including cell elongation in the hypocotyl, whether or not it modulates cell proliferation is largely unknown. Here we have found that Pseudo Response Regulators (PRRs), essential components of circadian core oscillators, affect root meristem cell proliferation mediated by Target Of Rapamycin (TOR) signaling. The null mutants of PRRs display much reduced sensitivities to sugar-activated TOR signaling. We have subsequently identified Tandem Zinc Finger 1, encoding a processing body localized RNA-binding protein, as a direct target repressed by PRRs in mediating TOR signaling. Multiple lines of biochemical and genetic evidence have demonstrated that TZF1 acts downstream of PRRs to attenuate TOR signaling. Furthermore, TZF1 could directly bind TOR mRNA via its tandem zinc finger motif to affect TOR mRNA stability. Our findings support a notion that PRR-TZF1-TOR molecular axis modulates root meristem cell proliferation by integrating both transcriptional and post-transcriptional regulatory mechanisms.

Cell cycle acceleration and changes in essential nuclear functions induced by simulated microgravity in a synchronized Arabidopsis cell culture

Zero gravity is an environmental challenge unknown to organisms throughout evolution on Earth. Nevertheless, plants are sensitive to altered gravity, as exemplified by changes in meristematic cell proliferation and growth. We found that synchronized Arabidopsis-cultured cells exposed to simulated microgravity showed a shortened cell cycle, caused by a shorter G2/M phase and a slightly longer G1 phase. The analysis of selected marker genes and proteins by quantitative polymerase chain reaction and flow cytometry in synchronic G1 and G2 subpopulations indicated changes in gene expression of core cell cycle regulators and chromatin-modifying factors, confirming that microgravity induced misregulation of G2/M and G1/S checkpoints and chromatin remodelling. Changes in chromatin-based regulation included higher DNA methylation and lower histone acetylation, increased chromatin condensation, and overall depletion of nuclear transcription. Estimation of ribosome biogenesis rate using nucleolar parameters and selected nucleolar genes and proteins indicated reduced nucleolar activity under simulated microgravity, especially at G2/M. These results expand our knowledge of how meristematic cells are affected by real and simulated microgravity. Counteracting this cellular stress is necessary for plant culture in space exploration.

Cytotoxic, genotoxic and mutagenic potential of UHT whole milk

Abstract This study evaluated the action at the cellular level of long life whole milk, full type of six renowned companies operating in the Brazilian market, as well as in other South American countries. The evaluation was performed using root meristem cells of Allium cepa L., at exposure times 24 and 48 hours, directly in milk products marketed. The results indicated that all the milk samples reduced root meristem cell proliferation, proving, in this study, a significant cytotoxic effect. Still, exposure to milk resulted in a significant frequency of mitotic spindle changes in meristem cells, characterizing these foods as genotoxic and mutagenic under the study conditions. It can be concluded that the long life milk samples caused significant genetic instability to cells of the examined tissue. The results obtained for cytotoxic, mutagenic and genotoxic action of these long life milks are of great relevance because, to date, there are no published toxicity studies on such foods and food additives present in the composition.

Cytokinins regulate root growth through its action on meristematic cell proliferation but not on the transition to differentiation.

Contrary to the wide-spread view that cytokinins change the rate of root growth and meristem size by regulating the cell transition to elongation (differentiation), our data showed that cytokinins affected the cell cycle duration in the meristem. The rate of meristematic cell transition to elongation itself is regulated by two groups of independent processes, through influence on (i) the life-span of cells in the meristem, and (ii) the cell proliferation rate in the meristem. Trans-zeatin slows down the root growth rate and the cell transition to elongation as a result of prolongation of mitotic cycles. The life-span of cells in the meristem does not change. The number of meristematic cells in one file decreases due to inhibition of cell proliferation but not to an acceleration of cell transition to elongation. Roots of triple mutant ipt3ipt5ipt7, in which cytokinin synthesis is slowed down, behave in an opposite way such that the rate of cell transition to elongation and cell proliferation is speeded up. Their peculiarity is that the life-span of cells in meristem becomes shorter than in control roots. In both cases, a change in concentration of endogenous cytokinin or in its signalling are associated with a change in mitotic cycle duration.

Identification of cyst nematode B-type CLE peptides and modulation of the vascular stem cell pathway for feeding cell formation.

Stem cell pools in the SAM (shoot apical meristem), RAM (root apical meristem) and vascular procambium/cambium are regulated by CLE-receptor kinase-WOX signaling modules. Previous data showed that cyst nematode CLE-like effector proteins delivered into host cells through a stylet, act as ligand mimics of plant A-type CLE peptides and are pivotal for successful parasitism. Here we report the identification of a new class of CLE peptides from cyst nematodes with functional similarity to the B-type CLE peptide TDIF (tracheary element differentiation inhibitory factor) encoded by the CLE41 and CLE44 genes in Arabidopsis. We further demonstrate that the TDIF-TDR (TDIF receptor)-WOX4 pathway, which promotes procambial meristem cell proliferation, is involved in beet cyst nematode Heterodera schachtii parasitism. We observed activation of the TDIF pathway in developing feeding sites, reduced nematode infection in cle41 and tdr-1 wox4-1 mutants, and compromised syncytium size in cle41, tdr-1, wox4-1 and tdr-1 wox4-1 mutants. By qRT-PCR and promoter:GUS analyses, we showed that the expression of WOX4 is decreased in a clv1-101 clv2-101 rpk2-5 mutant, suggesting that WOX4 is a potential downstream target of nematode CLEs. Exogenous treatment with both nematode A-type and B-type CLE peptides induced massive cell proliferation in wild type roots, suggesting that the two types of CLEs may regulate cell proliferation during feeding site formation. These findings highlight an important role of the procambial cell proliferation pathway in cyst nematode feeding site formation.

Effect of guaianolides in the meiosis reinitiation of amphibian oocytes.

Sesquiterpene lactones (STLs) are a large and structurally diverse group of plant metabolites generally found in the Asteraceae family. STLs exhibit a wide spectrum of biological activities and it is generally accepted that their major mechanism of action is the alkylation of the thiol groups of biological molecules. The guaianolides is one of various groups of STLs. Anti-tumour and anti-migraine effects, an allergenic agent, an inhibitor of smooth muscle cells and of meristematic cell proliferation are only a few of the most commonly reported activities of STLs. In amphibians, fully grown ovarian oocytes are arrested at the beginning of meiosis I. Under stimulus with progesterone, this meiotic arrest is released and meiosis progresses to metaphase II, a process known as oocyte maturation. There are previous records of the inhibitory effect of dehydroleucodin (DhL), a guaianolide lactone, on the progression of meiosis. It has been also shown that DhL and its 11,13-dihydroderivative (2H-DhL; a mixture of epimers at C-11) act as blockers of the resumption of meiosis in fully grown ovarian oocytes from the amphibian Rhinella arenarum (formerly classified as Bufo arenarum). The aim of this study was to analyze the effect of four closely related guaianolides, i.e., DhL, achillin, desacetoxymatricarin and estafietin as possible inhibitors of meiosis in oocytes of amphibians in vitro and discuss some structure-activity relationships. It was found that the inhibitory effect on meiosis resumption is greater when the lactone has two potentially reactive centres, either a α,β-α',β'-diunsaturated cyclopentanone moiety or an epoxide group plus an exo-methylene-γ-lactone function.

B-type cyclin modulation in response to carbon balance in callus of Populus alba

In plants, sucrose is the principal transported carbon compound. Sucrose and/or glucose and fructose are relevant in the regulation of meristematic cell proliferation. The aim was to define the sugar balance and the relationship between the osmoregulation processes and the cell division patterns in callus of Populus alba in response to different sucrose medium concentrations. Callus proliferation and metabolism were assessed through biometric measures, non-structural soluble carbohydrates quantification, osmotic potential determination, as well as by quantification of mRNA accumulation of B-type cyclin genes. Calli were cultured on proliferation medium without sucrose or supplemented with 10, 20 or 30 g l−1 of sucrose. A large amount of non-structural soluble carbohydrates was used to restore the osmotic balance between callus and medium; upon reaching the equilibrium, carbon was then used for cell division. The initial investment of carbon for osmoregulation processes can explain the differences in the lag phase duration in response to the decrease of medium sucrose concentration. However, calli cultured on medium added with 30 g l−1 of sucrose used carbon both to adjust their osmotic potential and to restore the growth whilst the calli on sucrose free medium entered in a quiescent state. The growth rates compared to the transcript accumulation trends suggested that a threshold effect, rather than a quantitative regulation model, governed the relation between CycB gene transcription and cell division. The findings showed that poplar calli used the carbon following a “hierarchical” model based on their physiological state and the sugar concentration available in the medium.

Gene NANA regulates cell proliferation in Arabidopsis thaliana shoot apical meristem without interaction with CLV1, CLV2, CLV3

A constancycy of stem cell pool in shoot apical meristem of Arabidopsis thaliana is provided by a genetic regulation system with negative feedback loop based on the interaction of the gene WUS, which maintains indeterminate state of cells, with CLV genes, which restrict the level of WUS expression and stem cell pool size. clv mutations lead to an increase in the pool of stem cells in the apical and floral meristems and wus mutation leads to the opposite effect. Mutation na (nana), like wus mutation, causes premature termination of shoot apical meristem function, although it does not affect the activity of the flower meristem. To elucidate the role of NA in the control of shoot apical meristem functioning, the interaction of NA with CLVgenes were investigated. Additive phenotype of double mutants na clv1, na clv1-1, anl na clv3-2 indicates that the NA gene makes an independent contribution to the functioning of the shoot apical meristem. It is assumed that the NA gene controls apical meristem cell proliferation during the transition to the reproductive phase of plant development, acting much later and independently of the genes WUS-CLV.