Lateral Meristems Research Articles

Lateral Meristems Responsible for Secondary Growth of the Monocotyledons: A Survey of the State of the Art.

This review highlights key historical works and the recent research on the monocot lateral meristems. It discusses the terminological issues (elucidating the terminological inconsistency found in the literature concerned), origination of secondary meristems, their morphology and characteristic features of the derivative tissues. Also the monocot cambium response to hormonal and gravitational stimuli is discussed. The summarized inputs in the present note are believed to renew interest in this field, which is important for a more comprehensive understanding of the abnormal secondary growth in the monocotyledons.

HYBRID AND SELFED SEEDLING PROGENIES OF VITIS HYBRID 'ROGER'S RED' GRAPE SEGREGATE FOR TENDRIL DISTRIBUTION

Grapevines of most species and cultivars produce lateral meristems (tendrils and inflorescences) on at least two successive nodes in three. Lateral meristems complicate rootstock cutting production, since tendrils are removed from rootstock cuttings prior to propagation and since tendrils clinging to wires, canes, or other tendrils make collecting cuttings more time consuming and expensive. To develop improved grapevine rootstocks with reduced lateral meristems, an elite nematode resistant rootstock selection ((Vitis berlandieri × V. riparia) × V. biformis) was crossed with ?Roger?s Red? (Vitis hybrid), and ?Roger?s Red? was also self-pollinated. Seedlings from controlled pollinations were grown in a greenhouse and trained up a stake. Starting at the first node in alternate phyllotaxy (that is, a node with a lateral meristem) and continuing for twelve nodes, the presence or absence of lateral meristems was observed and the type of lateral meristem was reported. There were 141 cross-pollinated seedlings. There were 83 seedlings from ?Roger?s Red? self-pollination. The expected pattern of lateral meristem distribution was wild type: two successive lateral meristem bearing nodes, followed by a single lateral meristem-free node. However, 43 of 83 seedlings from ?Roger?s Red? self-pollination showed two or more successive lateral meristem-free nodes (flanked by lateral meristem-bearing nodes), and 12 of these 43 showed at least two sets of two successive lateral meristem-free nodes. One self-pollinated seedling showed three successive lateral meristem-free nodes in the alternate phyllotaxy. In contrast, in the hybrid population only 33 of 141 seedlings showed the non-wild type pattern of two successive lateral meristem-free nodes, and only 6 of these 33 showed at least two sets of two successive lateral meristem-free nodes. Tendril distribution is under genetic control and the production of extra tendril-free nodes was segregating in these populations. Dominant alleles at two genes are required for the mutant phenotype. A genetic model and the gene names Successive Tendril free Node-1 and Successive Tendril free Node-2 are proposed.

Effects of Osmolytic Agents on Somatic Embryogenesis of Saffron (<i>Crocus sativus </i>L.)

A protocol for callus induction from meristem tissues and subsequent somatic embryo formation were established in this study. Explants were taken from apical and lateral meristems of saffron and these explants were cultured on MS medium supplemented with combinations of 2.4-dichlorophenoxyacetic acid (2.4-D) and Kinetin (Kn). The effects of osmotic agents such as abscisic acid (ABA), polyethylene glycol (PEG) and Gelrite on somatic embryogenesis were also investigated. After 45 and 60 days of culture, calli were induced from apical and lateral meristems, respectively. The apical meristems yielded higher quality calli when compared to the lateral meristems. The highest frequency of callogenesis and the growth rate of callus were achieved from apical meristems on Murashige and Skoogs (MS) medium supplemented with 2.4-D (2 mg/l) and Kinetin (0.5 mg/l). After 45 days of subculture, the segments of nodular calli were transferred to plant growth regulator (PGR)- free media for induction of pre-embryogenesis embryo formation. Pre-matured embryos were cultured on MS medium supplemented with different osmotic agents such as Gelrite, ABA and PEG to study their effects on embryo maturation. Both PEG and ABA proved more effective for somatic embryo maturation as compared to Gelrite.

Molecular control of wood formation in trees.

Wood (also termed secondary xylem) is the most abundant biomass produced by plants, and is one of the most important sinks for atmospheric carbon dioxide. The development of wood begins with the differentiation of the lateral meristem, vascular cambium, into secondary xylem mother cells followed by cell expansion, secondary wall deposition, programmed cell death, and finally heartwood formation. Significant progress has been made in the past decade in uncovering the molecular players involved in various developmental stages of wood formation in tree species. Hormonal signalling has been shown to play critical roles in vascular cambium cell proliferation and a peptide-receptor-transcription factor regulatory mechanism similar to that controlling the activity of apical meristems is proposed to be involved in the maintenance of vascular cambium activity. It has been demonstrated that the differentiation of vascular cambium into xylem mother cells is regulated by plant hormones and HD-ZIP III transcription factors, and the coordinated activation of secondary wall biosynthesis genes during wood formation is mediated by a transcription network encompassing secondary wall NAC and MYB master switches and their downstream transcription factors. Most genes encoding the biosynthesis enzymes for wood components (cellulose, xylan, glucomannan, and lignin) have been identified in poplar and a number of them have been functionally characterized. With the availability of genome sequences of tree species from both gymnosperms and angiosperms, and the identification of a suite of wood-associated genes, it is expected that our understanding of the molecular control of wood formation in trees will be greatly accelerated.

Особенности динамики радиального прироста дуба черешчатого в условиях радиоактивного загрязнения лесов Центральной лесостепи

It is shown that the overall thrusts of processes of xylogeneses of oak in conditions of radioac-tive contamination are consistent with pre-viously established researchers for oak forests of Central forest steppe. Irradiation, mostly with short-lived isotopes of iodine, caused changes in the activity of the lateral meristem of woody plants - cambium. The impact had relatively short time interval and affected mainly xylogeneses of early wood regardless of the age of forming layer of wood.

Class I KNOX transcription factors promote differentiation of cambial derivatives into xylem fibers in the Arabidopsis hypocotyl.

The class I KNOX transcription factors SHOOT MERISTEMLESS (STM) and KNAT1 are important regulators of meristem maintenance in shoot apices, with a dual role of promoting cell proliferation and inhibiting differentiation. We examined whether they control stem cell maintenance in the cambium of Arabidopsis hypocotyls, a wood-forming lateral meristem, in a similar fashion as in the shoot apical meristem. Weak loss-of-function alleles of KNAT1 and STM led to reduced formation of xylem fibers - highly differentiated cambial derivatives - whereas cell proliferation in the cambium was only mildly affected. In a knat1;stm double mutant, xylem fiber differentiation was completely abolished, but residual cambial activity was maintained. Expression of early and late markers of xylary cell differentiation was globally reduced in the knat1;stm double mutant. KNAT1 and STM were found to act through transcriptional repression of the meristem boundary genes BLADE-ON-PETIOLE 1 (BOP1) and BOP2 on xylem fiber differentiation. Together, these data indicate that, in the cambium, KNAT1 and STM, contrary to their function in the shoot apical meristem, promote cell differentiation through repression of BOP genes.

Investigation of morphogenesis of inflorescence and determination of the nature of inheritance of “supernumerary spikelets” trait of bread wheat (Triticum aestivum L.) mutant line

Using C-banding and FISH methods, the karyotype of MC1611 induced mutant of bread wheat, which develop additional spikelets at a rachis node (trait "supernumerary spikelets") was characterized. It was determined that the mutant phenotype is not associated with aneuploidy and major chromosomal rearrangements. The results of genetic analysis showed that supernumerary spikelets of the line are caused by a mutation of the single bh-D. 1 gene, influenced by the genetic background. The mutation causes abnormalities of inflorescence morphogenesis associated with the development of ectopic spikelet meristems in place of floral meristems in the basal part of the spikelets, causing the appearance of additional spikes at a rachis node. The mutant phenotype suggests that the Bh-D gene determines the fate of the lateral meristem in ear, which develops as floral meristem and gives rise to floral organs in wild-type inflorescences. In the Bh-D. 1 mutant, the establishing identity is impaired. The characterized mutant can be used in further studies on molecular genetic basis of development of wheat inflorescence.

Lateral meristems of higher plants: Phytohormonal and genetic control

Lateral meristems (pericycle, procambium and cambium, phellogen) are positioned in parallel to the lateral surface of the organ, where they are present, and produce concentric layers of undifferentiated cells. Primary lateral meristems, procambium and pericycle, arise during embryogenesis; secondary lateral meristems, cambium and phellogen, — during post embryonic development. Pericycle is most pluripotent plant meristem, as it may give rise to a variety of other types of meristems: lateral meristems (cambium, phellogen), apical meristems of lateral roots, and also shoot meristems during plant in vitro regeneration. Procambium and cambium developing from it give rise to the vascular tissues of the stems and roots, ensuring their thickening. The review considers the genetic control of lateral meristem development and the role of phytohormones in the control of their activities.

GENETIC CONTROL OF PHYLLOTAXY PHASE SHIFT IN JUVENILE VINES IN A ROOTSTOCK HYBRID POPULATION

Grapevine seedlings initially display spiral phyllotaxy of true leaves, then undergo a shift to alternate phyllotaxy with the production of the first lateral meristems (typically tendrils). The node at which the shift from spiral to alternate phyllotaxy occurs varies from about the 4th to about the 12th node on the vine. To investigate the genetic control of the transition from spiral phyllotaxy to alternate phyllotaxy, a population segregating for this trait was developed and screened. The population derived from four female parents and six male parents crossed in a Design 2 mating array (all female parents crossed to all male parents). The female parents were the pistillate flowered rootstock varieties 1613 Couderc, 93-5 Couderc (California clone), Vitis rupestris 187G, and Fercal. The male parents were staminate flowered grape rootstock germplasm, species, and species hybrid selections with diverse backgrounds, including accessions from the USDA ARS National Clonal Germplasm Repository, Davis, California (denoted with DVIT accession numbers): IAC 572, Vitis labrusca Y137 DVIT 1392, Vitis hybrid Y93 DVIT 1519, Vitis hybrid Q126 DVIT 1456, Vitis hybrid Q130 DVIT 1466, and Vitis hybrid R127 DVIT 1490. The species background of the male parents includes V. labrusca, V. mustangensis, V. riparia, V. tiliifolia, and V. rupestris. Seedlings from controlled crosses were grown in individual pots in a greenhouse with artificial illumination to provide 24 h day length. The node number of the first observed lateral meristem was recorded; the goal was 50 seedlings per population for each of 24 populations, although some populations showed poor seed germination. Male and female parents differ from one another at P =0.01, with male parents falling into three groups and female parents into two groups based on the mean node of phyllotaxy shift in their seedlings. Narrow sense heritability of the first tendril bearing node is estimated at 0.23 based on breeding value of male parents and at 0.45 based on breeding value of female parents.

Cytokinin pathway mediates APETALA1 function in the establishment of determinate floral meristems in Arabidopsis.

In angiosperms, after the floral transition, the inflorescence meristem produces floral meristems (FMs). Determinate growth of FMs produces flowers of a particular size and form. This determinate growth requires specification of floral organs and termination of stem-cell divisions. Establishment of the FM and specification of outer whorl organs (sepals and petals) requires the floral homeotic gene APETALA1 (AP1). To determine FM identity, AP1 also prevents the formation of flowers in the axils of sepals. The mechanisms underlying AP1 function in the floral transition and in floral organ patterning have been studied extensively, but how AP1 terminates sepal axil stem-cell activities to suppress axillary secondary flower formation remains unclear. Here we show that AP1 regulates cytokinin levels by directly suppressing the cytokinin biosynthetic gene LONELY GUY1 and activating the cytokinin degradation gene CYTOKININ OXIDASE/DEHYDROGENASE3. Restoring the expression of these genes to wild-type levels in AP1-expressing cells or suppressing cytokinin signaling inhibits indeterminate inflorescence meristem activity caused by ap1 mutation. We conclude that suppression of cytokinin biosynthesis and activation of cytokinin degradation mediates AP1 function in establishing determinate FM. A deeper understanding of axil-lateral meristem activity provides crucial information for enhancing yield by engineering crops that produce more elaborated racemes.

Inflorescence development in tomato: gene functions within a zigzag model.

Tomato is a major crop plant and several mutants have been selected for breeding but also for isolating important genes that regulate flowering and sympodial growth. Besides, current research in developmental biology aims at revealing mechanisms that account for diversity in inflorescence architectures. We therefore found timely to review the current knowledge of the genetic control of flowering in tomato and to integrate the emerging network into modeling attempts. We developed a kinetic model of the tomato inflorescence development where each meristem was represented by its “vegetativeness” (V), reflecting its maturation state toward flower initiation. The model followed simple rules: maturation proceeded continuously at the same rate in every meristem (dV); floral transition and floral commitment occurred at threshold levels of V; lateral meristems were initiated with a gain of V (ΔV) relative to the V level of the meristem from which they derived. This last rule created a link between successive meristems and gave to the model its zigzag shape. We next exploited the model to explore the diversity of morphotypes that could be generated by varying dV and ΔV and matched them with existing mutant phenotypes. This approach, focused on the development of the primary inflorescence, allowed us to elaborate on the genetic regulation of the kinetic model of inflorescence development. We propose that the lateral inflorescence meristem fate in tomato is more similar to an immature flower meristem than to the inflorescence meristem of Arabidopsis. In the last part of our paper, we extend our thought to spatial regulators that should be integrated in a next step for unraveling the relationships between the different meristems that participate to sympodial growth.

Transcriptional Analysis of Tendril and Inflorescence Development in Grapevine (Vitis vinifera L.)

In grapevine (Vitis vinifera L.), the lateral meristem can give rise to either tendrils or inflorescences which are determined organs. To get insights into the processes of tendril and inflorescence development, we characterized the transcriptional variation taking place in both organs. The results of the global transcriptional analyses along tendril and inflorescence development suggested that these two homologous organs initially share a common transcriptional program related to cell proliferation and growth functions. In later developmental stages they showed organ specific gene expression programs related to the particular differentiation processes taking place in each organ. In this way, tendrils showed higher transcription of genes related to photosynthesis, hormone signaling and secondary metabolism than inflorescences, while inflorescences displayed higher transcriptional activity for genes encoding transcription factors, mainly those belonging to the MADS-box gene family. The expression profiles of selected transcription factors related with inflorescence and flower meristem identity and with flower organogenesis were generally conserved with respect to their homologs in model species. Regarding tendrils, it was interesting to find that genes related with reproductive development in other species were also recruited for grapevine tendril development. These results suggest a role for those genes in the regulation of basic cellular mechanisms common to both developmental processes.

The morphogenesis of lupine root nodules during infection by Rhizobium lupini

The development of root nodules in <em>Lupinus luteus</em> infected by <em>Rhizobium lupini</em> was studied using cytological methods. The results obtained from examination of material sampled 6, 9, 13, 15, 20, 29 and 60 days after infection are given. The successive stages of development are described and the cytological characteristics of the tissue are presented. The mitotic divisions of the root cortex parenchyma cells, which initiated the formation of the nodule primordium, were accompanied by structural changes in the root hairs and divisions in the root pericycle. The development of the nodule was associated with the activity of the lateral meristems, which encompass both the infected cells and cells not containing bacteroids Characteristics of bacteria found in the symplast and apoplast of the bacteroid tissue are given.

An Activated Form of UFO Alters Leaf Development and Produces Ectopic Floral and Inflorescence Meristems

Plants are unique in their ability to continuously produce new meristems and organ primordia. In Arabidopsis, the transcription factor LEAFY (LFY) functions as a master regulator of a gene network that is important for floral meristem and organ specification. UNUSUAL FLORAL ORGANS (UFO) is a co-activator of LEAFY and is required for proper activation of APETALA3 in the floral meristem during the specification of stamens and petals. The ufo mutants display defects in other parts of the flower and the inflorescence, suggestive of additional roles. Here we show that the normal determinacy of the developing Arabidopsis leaves is affected by the expression of a gain-of-function UFO fusion protein with the VP16 transcriptional activator domain. In these lines, the rosette and cauline leaf primordia exhibit reiterated serration, and upon flowering produce ectopic meristems that develop into flowers, bract leaves and inflorescences. These striking phenotypes reveal that developing leaves maintain the competency to initiate flower and inflorescence programs. Furthermore, the gain-of-function phenotypes are dependent on LFY and the SEPALLATA (SEP) MADS-box transcription factors, indicative of their functional interactions with UFO. The findings of this study also suggest that UFO promotes the establishment of the lateral meristems and primordia in the peripheral zone of the apical and floral meristems by enhancing the activity of LFY. These novel phenotypes along with the mutant phenotypes of UFO orthologs in other plant species suggest a broader function for UFO in plants.

Characterization of cytokinin signaling and homeostasis gene families in two hardwood tree species: Populus trichocarpa and Prunus persica

BackgroundThrough the diversity of cytokinin regulated processes, this phytohormone has a profound impact on plant growth and development. Cytokinin signaling is involved in the control of apical and lateral meristem activity, branching pattern of the shoot, and leaf senescence. These processes influence several traits, including the stem diameter, shoot architecture, and perennial life cycle, which define the development of woody plants. To facilitate research about the role of cytokinin in regulation of woody plant development, we have identified genes associated with cytokinin signaling and homeostasis pathways from two hardwood tree species.ResultsTaking advantage of the sequenced black cottonwood (Populus trichocarpa) and peach (Prunus persica) genomes, we have compiled a comprehensive list of genes involved in these pathways. We identified genes belonging to the six families of cytokinin oxidases (CKXs), isopentenyl transferases (IPTs), LONELY GUY genes (LOGs), two-component receptors, histidine containing phosphotransmitters (HPts), and response regulators (RRs). All together 85 Populus and 45 Prunus genes were identified, and compared to their Arabidopsis orthologs through phylogenetic analyses.ConclusionsIn general, when compared to Arabidopsis, differences in gene family structure were often seen in only one of the two tree species. However, one class of genes associated with cytokinin signal transduction, the CKI1-like family of two-component histidine kinases, was larger in both Populus and Prunus than in Arabidopsis.

N-MYC DOWN-REGULATED-LIKE Proteins Regulate Meristem Initiation by Modulating Auxin Transport and MAX2 Expression

BackgroundN-MYC DOWN-REGULATED-LIKE (NDL) proteins interact with the Gβ subunit (AGB1) of the heterotrimeric G protein complex and play an important role in AGB1-dependent regulation of lateral root formation by affecting root auxin transport, auxin gradients and the steady-state levels of mRNA encoding the PIN-FORMED 2 and AUXIN 1 auxin transport facilitators. Auxin transport in aerial tissue follows different paths and utilizes different transporters than in roots; therefore, in the present study, we analyzed whether NDL proteins play an important role in AGB1-dependent, auxin-mediated meristem development.Methodology/Principal FindingsExpression levels of NDL gene family members need to be tightly regulated, and altered expression (both over-expression and down-regulation) confers ectopic growth. Over-expression of NDL1 disrupts vegetative and reproductive organ development. Reduced expression of the NDL gene family members results in asymmetric leaf emergence, twinning of rosette leaves, defects in leaf formation, and abnormal silique distribution. Reduced expression of the NDL genes in the agb1-2 (null allele) mutant rescues some of the abnormal phenotypes, such as silique morphology, silique distribution, and peduncle angle, suggesting that proper levels of NDL proteins are maintained by AGB1. We found that all of these abnormal aerial phenotypes due to altered NDL expression were associated with increases in basipetal auxin transport, altered auxin maxima and altered MAX2 expression within the inflorescence stem.Conclusion/SignificanceNDL proteins, together with AGB1, act as positive regulators of meristem initiation and branching. AGB1 and NDL1 positively regulate basipetal inflorescence auxin transport and modulate MAX2 expression in shoots, which in turn regulates organ and lateral meristem formation by the establishment and maintenance of auxin gradients.

Sweetpotato plant regeneration via an improved somatic embryogenesis protocol

Genetic transformation holds considerable promise for sweetpotato enhancement, but is limited by the low regeneration efficiency of transformed cells. Somatic embryogenesis is the most efficient method for regenerating genetically transformed sweetpotato plants, but is still limited by low efficiency and strong genotype dependency. In this study, based on a review of previous work, various growth regulators and gelling agents were assessed for an optimized three-stage protocol. Lateral meristems of sweetpotato cultivars ‘Jonathan’ and ‘Jewel’ were used to investigate: (i) the influence of the auxins 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) on the induction of sweetpotato somatic embryogenesis, (ii) the effect of the abscisic acid (ABA) and gibberellic acid (GA3) on the development and maturation of somatic embryos, (iii) the effect of GA3 on development of mature embryos into plantlets, and (iv) the influence of two gelling agents (agar and Gelrite) on different embryogenic phases during all stages of the protocol. Among the treatments evaluated, best results were obtained when growing the explants on Murashige and Skoog (MS) medium with 1.3mg/l 2,4,5-T (in the dark) during the first stage, transferring the calli to media with 1mg/l ABA at the second stage and then cultivating them on 0.424mg/l GA3 for plant development. However, no significant differences were found between gelling agents.

Does warming affect growth rate and biomass production of shrubs in the High Arctic?

Few studies have assessed directly the impact of warming on plant growth and biomass production in the High Arctic. Here, we aimed to investigate the impact of 7 years of warming (open greenhouses) on the aboveground relative growth rate (RGR) of Cassiope tetragona and Salix arctica in North-Eastern Greenland. RGR was assessed for apical (leaves, stem, reproductive organs) and lateral meristems (secondary growth of stem and branches) and accompanied by measures of gross ecosystem production (GEP), branching and tissue carbon (C) concentration. Measurements were based on harvest and biometric methods (for RGR and branching) and gas exchange and chemical analysis (for GEP and C concentration). Warming nearly doubled the apical RGR of Cassiope, whereas it did not affect the apical RGR of Salix. Similarly, secondary growth increased for Cassiope but not for Salix. In particular, warming enhanced the secondary growth of old stem segments of Cassiope formed before the treatment began. The increase in Cassiope RGR was associated with an increase in gross photosynthetic uptake, branching and C concentration in old green tissues. Overall, the different growth measures consistently indicated that temperature limits the growth of Cassiope but not that of Salix in North-Eastern Greenland. Summer warming thus has the potential to stimulate biomass production in the High Arctic but major species-specific differences are expected.

Stem cell function during plant vascular development

The plant vascular system, composed of xylem and phloem, evolved to connect plant organs and transport various molecules between them. During the post-embryonic growth, these conductive tissues constitutively form from cells that are derived from a lateral meristem, commonly called procambium and cambium. Procambium/cambium contains pluripotent stem cells and provides a microenvironment that maintains the stem cell population. Because vascular plants continue to form new tissues and organs throughout their life cycle, the formation and maintenance of stem cells are crucial for plant growth and development. In this decade, there has been considerable progress in understanding the molecular control of the organization and maintenance of stem cells in vascular plants. Noticeable advance has been made in elucidating the role of transcription factors and major plant hormones in stem cell maintenance and vascular tissue differentiation. These studies suggest the shared regulatory mechanisms among various types of plant stem cell pools. In this review, we focus on two aspects of stem cell function in the vascular cambium, cell proliferation and cell differentiation.

Comparison of Skin-set and Periderm Maturation in ‘Red Norland’ Potatoes Grown in Two Soil Types in Wisconsin

Skin-set is a necessary prerequisite for potato storability, but there is little published information on the effect of cultural conditions on the development of skin-set. The phellogen layer acts as a lateral meristem for the peri- derm and while active, fracture of its thin radial walls leads directly to skinning injury during harvest and handling into storage. During maturation of the periderm, these radial walls thicken. Red-skinned cultivars are often grown in muck soils in Wisconsin to improve skin condition and color. We tested skin-set and phellogen radial wall thickness for 'Red Norland' tubers grown on either loamy sand or muck soils in central Wisconsin. Skin-set varied between years and soil types, but there was no consistent effect of soil type on skin-set. Phellogen radial walls in Red Norland tubers doubled in thickness between bulking and vine-kill, but did not change in the three week period following vine- kill even when skin-set improved. Physiological processes other than cell wall thickening must be responsible skin-set during the three weeks following vine death.