Initiation Of Leaf Primordia Research Articles

Advancements in Rice Leaf Development Research

Rice leaf morphology is a pivotal component of the ideal plant architecture, significantly impacting rice yield. The process of leaf development unfolds through three distinct stages: the initiation of leaf primordia, the establishment and maintenance of polarity, and leaf expansion. Genes regulating leaf morphology encompass transcription factors, hormones, and miRNAs. An in-depth synthesis and categorization of genes associated with leaf development, particularly those successfully cloned, hold paramount importance in unraveling the complexity of rice leaf development. Furthermore, it provides valuable insights into the potential for molecular-level manipulation of rice leaf types. This comprehensive review consolidates the stages of rice leaf development, the genes involved, molecular regulatory pathways, and the influence of plant hormones. Its objective is to establish a foundational understanding of the creation of ideal rice leaf forms and their practical application in molecular breeding.

Transcriptomic analyses to summarize gene expression patterns that occur during leaf initiation of Chinese cabbage.

In Chinese cabbage, rosette leaves expose their adaxial side to the light converting light energy into chemical energy, acting as a source for the growth of the leafy head. In the leafy head, the outer heading leaves expose their abaxial side to the light while the inner leaves are shielded from the light and have become a sink organ of the growing Chinese cabbage plant. Interestingly, variation in several ad/abaxial polarity genes is associated with the typical leafy head morphotype. The initiation of leaf primordia and the establishment of leaf ad/abaxial polarity are essential steps in the initiation of marginal meristem activity leading to leaf formation. Understanding the molecular genetic mechanisms of leaf primordia formation, polar differentiation, and leaf expansion is thus relevant to understand leafy head formation. As Brassica's are mesa-hexaploids, many genes have multiple paralogues, complicating analysis of the genetic regulation of leaf development. In this study, we used laser dissection of Chinese cabbage leaf primordia and the shoot apical meristem (SAM) to compare gene expression profiles between both adaxial and abaxial sides and the SAM aiming to capture transcriptome changes underlying leaf primordia development. We highlight genes with roles in hormone pathways and transcription factors. We also assessed gene expression gradients along expanded leaf blades from the same plants to analyze regulatory links between SAM, leaf primordia and the expanding rosette leaf. The catalogue of differentially expressed genes provides insights in gene expression patterns involved in leaf development and form a starting point to unravel leafy head formation.

Leaf-based induction of protocorm-like bodies, their encapsulation, storage and post-storage germination with genetic fidelity in Mokara Sayan × Ascocenda Wangsa gold

• First report on direct induction of PLBs Mokara Sayan × Ascocenda Wangsa gold. • Earliest and maximum PLB induction occurred on MS medium fortified with 3 mg L –1 TDZ. • Synthetic seeds were developed encapsulating PLBs with 3 % Na-alginate and 75 mM CaCl 2 . • Better storage of synthetic seeds up to 180 days was attained at 25°C in comparison to 4°C. • RAPD marker-based analysis ensured genetic integrity of synthetic seed-derived plantlets. An innovative and accelerated in vitro mass propagation protocol via direct induction of protocorm-like bodies (PLBs) from leaf sections of orchid hybrid Mokara Sayan × Ascocenda Wangsa gold was developed for the first time. Initially, leaf sections were transferred into Murashige and Skoog (MS) medium supplemented with two cytokinin sources namely, thidiazuron (TDZ) and zeatin with variable concentrations. The earliest (∼8 days) induction of PLBs with maximum number (34 PLBs cm –2 leaf section) and induction frequency (82.8 %) were recorded in MS medium fortified with 3 mg L –1 TDZ. The same medium formulation resulted in the highest growth rate (∼93.7 mg day –1 ) with maximum diameter of PLBs (3.4 mm) when compared with other treatments. Observations on proliferating PLBs under variable pressure scanning electron microscopy illustrated their distinctive growth phases wherein proliferating PLBs exhibited globular shape, typical epidermal texture with a rough surface that steadily became more wrinkled, consisting of basal absorbing hair-bodies and the apical zone forming shoot primordia. Stereo microscopic observations revealed gradual maturation of PLBs, initiation of leaf and shoot primordia. PLBs were encapsulated with calcium-alginate for short-term storage and germplasm exchange. Lower concentrations of sodium alginate (1 and 2 %) produced synthetic seeds that were soft to grip and extremely irregular in shape, whereas higher concentration (4 %) resulted in stiffness of synthetic seeds and reduced germination frequency. The most suitable gel matrix to form uniform spherical beads was optimized at 3 % sodium alginate and 75 mM calcium chloride. Synthetic seeds were effectively stored at 25 °C for up to 180 days. The germination efficiency of synthetic seeds gradually declined after storage at both 4 °C and 25 °C, with synthetic seeds stored at 25 °C being more tolerant than those stored at 4 °C. Even after 180 days of storage, the synthetic seeds that were stored at 25 °C exhibited 71.2 % germination frequency and ∼14 days to germination. Synthetic seeds stored at 4 °C deteriorated rapidly and degenerated completely within 180 days. Random amplified polymorphic DNA marker-based analysis carried out with randomly selected synthetic seed-derived plantlets showed 100 % monomorphism with a total 512 bands of molecular size ranging between 130—2100 bp, ensuring post-storage genetic fidelity.

The Formation of Shapes: Interplay of Genes during Leaf Development Processes

Leaf shape, as one of the clearest manifestations of plant morphology, shows considerable variation owing to genetics and the environment. Leaf initiation occurs in the peripheral zone of the SAM and goes through the three overlapping phases of leaf primordium initiation, leaf dorsiventral development, and leaf marginal meristem establishment. Transcription factors, such as KNOX, WOX, and CUC; hormone-regulating genes, such as GA2ox, GA20ox, and PIN1; and miRNAs such as miR164/165 are tightly involved in leaf shaping through the generation of intricate cooperative networks in different temporal phases and specific tissue zones. Here, we briefly discuss the critical interplay occurring between certain genes and the pivotal role these play in the leaf developmental network and phytohormone regulation, including AS1/AS2–KNOX–GA20ox–GA, miR164–NAM/CUC–PIN1–auxin, and CUC–BAS1/CYP734A–BR, and we attempt to summarize several basic insights into the mechanisms of leaf shape regulation.

Roles of Phytohormones and Their Signaling Pathways in Leaf Development and Stress Responses.

Phytohormones participate in various processes over the course of a plant's lifecycle. In addition to the five classical phytohormones (auxins, cytokinins, gibberellins, abscisic acid, and ethylene), phytohormones such as brassinosteroids, jasmonic acid, salicylic acid, strigolactones, and peptides also play important roles in plant growth and stress responses. Given the highly interconnected nature of phytohormones during plant development and stress responses, it is challenging to study the biological function of a single phytohormone in isolation. In the current Review, we describe the combined functions and signaling cascades (especially the shared points and pathways) of various phytohormones in leaf development, in particular, during leaf primordium initiation and the establishment of leaf polarity and leaf morphology as well as leaf development under various stress conditions. We propose a model incorporating the roles of multiple phytohormones in leaf development and stress responses to illustrate the underlying combinatorial signaling pathways. This model provides a reference for breeding stress-resistant crops.

A single-cell analysis of the Arabidopsis vegetative shoot apex.

The shoot apical meristem allows for reiterative formation of new aerial structures throughout the life cycle of a plant. We use single-cell RNA sequencing to define the cellular taxonomy of the Arabidopsis vegetative shoot apex at the transcriptome level. We find that the shoot apex is composed of highly heterogeneous cells, which can be partitioned into 7 broad populations with 23 transcriptionally distinct cell clusters. We delineate cell-cycle continuums and developmental trajectories of epidermal cells, vascular tissue, and leaf mesophyll cells and infer transcription factors and gene expression signatures associated with cell fate decisions. Integrative analysis of shoot and root apical cell populations further reveals common and distinct features of epidermal and vascular tissues. Our results, thus, offer a valuable resource for investigating the basic principles underlying cell division and differentiation in plants at single-cell resolution.

Shoot Apical Meristem Growth in Saguaro (Carnegiea gigantea, Cactaceae) Is Episodic and Radially Asymmetrical Rather Than Uniform

Premise of research. As a shoot of Carnegiea gigantea (saguaro, columnar cactus) grows longer, its shoot apical meristem (SAM) occasionally adds new ribs, presumably because the SAM is increasing in size, giving it a larger basal circumference (the leaf primordium initiation zone). A new rib, which is a phyllotactic orthostichy, can be initiated when the SAM basal circumference has increased by the width of a leaf primordium. Rib initiation sites are markers that indicate when and where an SAM has enlarged. If SAMs grow uniformly, steadily, and symmetrically, the initiation sites of new ribs should occur at uniform intervals upward along the shoot and should also be symmetrical around the shoot’s circumference.Methodology. In the field, 18 mature plants were examined, and the height and circumferential position of the start point of every new rib were mapped.Pivotal results. Two to five new ribs would be initiated in rapid succession longitudinally, followed by a considerable period of shoot growth during which no new ribs would be initiated. Circumferentially, new ribs tended to be located laterally close to a previous new rib rather than being located symmetrically or randomly.Conclusions. SAMs of C. gigantea grow episodically and asymmetrically.

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.

Dynamics of leaf and spikelet primordia initiation in wheat as affected by Ppd-1a alleles under field conditions.

Wheat adaptation is affected by Ppd genes, but the role of these alleles in the rates of leaf and spikelet initiation has not been properly analysed. Twelve near isogenic lines (NILs) combining Ppd-1a alleles from different donors introgressed in A, B, and/or D genomes were tested under field conditions during two growing seasons together with the wild type, Paragon. Leaf initiation rate was unaffected by Ppd-1a alleles so the final leaf number (FLN) was reduced in parallel with reductions in the duration of the vegetative phase. Spikelet primordia initiation was accelerated and consequently the effect on spikelets per spike was less than proportional to the effect on the duration of spikelet initiation. The magnitude of these effects on spikelet plastochron depended on the doses of Ppd-1 homoeoalleles and the specific insensitivity alleles carried. Double ridge was consistently later than floral initiation, but the difference between them was not affected by Ppd-1a alleles. These findings have potential for selecting the best combinations from the Ppd-1 homoeoallelic series for manipulating adaptation taking into consideration particular effects on spikelet number.

Functional Genomics and Genetic Control of Compound Leaf Development in Medicago truncatula: An Overview.

Diverse forms of leaves are present in nature. However, the regulatory mechanisms that underpin the development of diverse leaf forms remain enigmatic. The initiation of leaf primordia from the periphery of shoot apical meristem (SAM) requires downregulation of the class 1 knotted-like homeobox KNOXI proteins. In plants with simple leaves, this downregulation is permanent, consistent with leaves being determinant organs. In most of plants with compound leaves, the KNOXI proteins are reactivated in developing leaf primordia, and this reactivation is required for the development of compound leaves in these plants. Surprisingly, in Medicago truncatula and pea (Pisum sativum) that belong to the so-called inverted repeat-lacking clade (IRLC) of legume plants, the KNOXI proteins are not reactivated in leaf primordia and therefore not likely involved in the development of compound leaves in these plants. Instead, the legume FLORICAULA/LEAFY orthologues, UNIFOLIATA (UNI) and SINGLE LEAFLET1 (SGL1), are required for the initiation and development of lateral leaflet primordia in pea and M. truncatula plants, respectively. On the other hand, PALMATE-LIKE PENTAFOLIATA1 (PALM1) encoding a novel Cys(2)His(2) zinc finger transcription factor is required to suppress a morphogenetic activity at the leaf margin by negatively regulating SGL1 gene expression, and FUSED COMPOUND LEAF1 (FCL1) encoding a class M KNOX protein is required for the development of the leaf proximo-distal axis and organ boundary separation in M. truncatula. Thus, these recent studies have shown that SGL1/UNI, FCL1, and PALM1 provide a genetic framework for our understanding of compound leaf development in the legume plants.

Metabolic shift in sugars and amino acids regulates sprouting in Saffron corm

Saffron is one of the most expensive spices of the world. Since this spice is triploid and meiosis is unusual, it cannot reproduce sexually like other plants; rather, it is propagated vegetatively via an underground corm, which can withstand a long dry dormant period before sprouting. Thus, corms are indispensable to saffron propagation. To identify and analyse signature metabolites associated with the ‘dormancy-sprouting’ process, non-targeted GC-MS was performed at different stages of corm development. Comparative metabolite profiling reflected dissimilar profiles among the stages as portrayed by differential cluster patterns of metabolites in the PCA and PLS-DA analysis. Correlation analysis revealed the interdependencies of individual metabolites and metabolic pathway. At the onset of stage 2, characterized by the initiation and differentiation of leaf primordia, a shift from dormancy to active metabolism occurred as derived from the increased abundance of sugars and other metabolites involved in the tricarboxylic acid cycle, glycolytic, amino acid and fatty acid pathways. These changes contribute to sprouting and vegetative growth of the corm. The present study provides new insights into saffron corm composition and metabolite changes associated with various stages of corm development and may pave the way for achieving agronomical improvements in this economically important spice.

Phyllotaxis

Leaves and flowers are arranged in regular patterns around the stem of a plant, a phenomenon known as phyllotaxis. Different arrangements occur, such as distichous, decussate or spiral (Figure 1). Most prevalent in nature are spirals in which the average divergence angles between successive organs are close to 137.5°, the so-called 'golden angle'. It is this exact number that has given phyllotaxis its special flavor as a quantitative developmental problem, and over the centuries, it has enjoyed the attention of scientists far beyond botany. In the 1830s mathematicians described the spirals as they related to the Fibonacci numbers, and in the 1860s improved microscopes made it possible for botanists to observe the initiation of leaf and flower primordia in a diversity of plants. This descriptive work led to the conclusion that new organ primordia form in the first available space between existing primordia, a conclusion still valid today. But how does it work? Ideas from the early 20th century suggested that an inhibitor produced by existing primordia diffuses towards the shoot apical meristem: where the concentration of the inhibitor falls below a threshold value, an organ is initiated. Other models dating back to the 1870s have tried to explain phyllotactic patterning by applying the laws of mechanics. Such models went through a long period of marginal interest, but have experienced a remarkable renaissance over the past 20 years. In this Primer I will give a broad overview of phyllotaxis, its emergence from the shoot apical meristem, how auxin and its transporter function as a 'pattern generator', and the role of tissue mechanics and computational modeling.

Coordination of auxin-triggered leaf initiation by tomato LEAFLESS

Lateral plant organs, particularly leaves, initiate at the flanks of the shoot apical meristem (SAM) following auxin maxima signals; however, little is known about the underlying mechanisms. Here, we show that tomato leafless (lfs) mutants fail to produce cotyledons and leaves and grow a naked pin while maintaining an active SAM. A similar phenotype was observed among pin-like shoots induced by polar auxin transport inhibitors such as 2,3,5-triiodobenzoic acid (TIBA). Both types of pin-like shoots showed reduced expression of primordia markers as well as abnormal auxin distribution, as evidenced by expression of the auxin reporters pPIN1:PIN1:GFP and DR5:YFP Upon auxin microapplication, both lfs meristems and TIBA-pin apices activated DR5:YFP expression with similar kinetics; however, only lfs plants failed to concurrently initiate leaf primordia. We found that LFS encodes the single tomato ortholog of Arabidopsis DORNRONSCHEN (DRN) and DRN-like (DRNL) genes and is transiently expressed at incipient and young primordia, overlapping with auxin response maxima. LFS is rapidly induced by auxin application, implying feed-forward activity between LFS and auxin signals. However, driving LFS at auxin response maxima sites using the DR5 promoter fails to fully rescue lfs plants, suggesting that additional, auxin-independent regulation is needed. Indeed, extended GCC-box elements upstream of LFS drove primordia-specific expression in a LFS-dependent but auxin-independent manner. We thus suggest that LFS transiently acts at the site of primordia initiation, where it provides a specific context to auxin response maxima culminating in leaf primordia initiation.

Primordium initiation drives tree growth

Tree growth is driven by leaf primordium initiation at shoot apices through a series of growth and developmental events largely mediated hormonally and often determinate in nature. This repeating pattern keeps trees alive and reproductive and conserves the phenotype.

Changes in distribution of zeatin and indole-3-acetic acid in cells during callus induction and organogenesis in vitro in immature embryo culture of wheat

Plant organogenesis remains one of the most essential questions of plant developmental biology. Callus tissue in vitro is a valuable tool for the studies on hormonal aspects of plant organogenesis, especially its early events, and immunohistochemical analysis is one of the few approaches offering information on the localization and role of hormones during organ development. The localization of endogenous zeatin and indole-3-acetic acid was investigated during simultaneous bud and root formation in calluses derived from immature embryos of wheat (Triticum aestivum L.). Calluses were induced on Murashige and Skoog (MS) medium supplemented with 2.0 mg L−1 2,4-dichlorophenoxyacetic acid. To stimulate simultaneous bud and root formation, calluses were transferred onto MS medium supplemented with 0.2 mg L−1 kinetin and 0.2 mg L−1 indoleacetic acid. Strong immunostaining for both hormones was detected in proliferating callus tissue, in developing meristematic centers and meristematic zones (whose cells were shown to be involved in organ formation), and at the sites of shoot and root apex initiation. During further development, shoot apexes with leaf primordia were heavily immunostained for zeatin, while immunostaining for indole-3-acetic acid was more intense at the sites of leaf primordia initiation and incipient primordia themselves. In the developing roots, immunostaining for both hormones reached a maximum in the root apex and gradually declined with increasing distance from the apex. Cells of developing procambial strands were also strongly stained for both zeatin and indole-3-acetic acid. These data suggest considerable similarity between patterns of hormone distribution in organs in vitro and in vivo. Thus, callus culture is a convenient and useful model for the study of fundamental biological questions such as how hormones regulate development.

Behavior of Leaf Meristems and Their Modification.

A major source of diversity in flowering plant form is the extensive variability of leaf shape and size. Leaf formation is initiated by recruitment of a handful of cells flanking the shoot apical meristem (SAM) to develop into a complex three-dimensional structure. Leaf organogenesis depends on activities of several distinct meristems that are established and spatiotemporally differentiated after the initiation of leaf primordia. Here, we review recent findings in the gene regulatory networks that orchestrate leaf meristem activities in a model plant Arabidopsis thaliana. We then discuss recent key studies investigating the natural variation in leaf morphology to understand how the gene regulatory networks modulate leaf meristems to yield a substantial diversity of leaf forms during the course of evolution.

Extended expression ofMaKN1contributes to the leaf morphology in aquatic forms ofMyriophyllum aquaticum

Myriophyllum aquaticum (Vell.) Verdc. is heterophyllous in nature with highly dissected simple leaves consisting of several lobes. KNOX (KNOTTED1-LIKE HOMEOBOX) genes are believed to have played an important role in the evolution of leaf diversity. Up-regulation of KNOX during leaf primordium initiation can lead to leaf dissection in plants with simple leaves and, if overexpressed, can produce ectopic meristems on leaves. A previous study on KNOX gene expression in the aerial form of this species showed that this gene is expressed in the shoot apical meristem (SAM), as well as in leaf primordia P0 to P8. Based on these results, it was hypothesized that the prolonged expression of the MaKN1 (Myriophyllum aquaticum Knotted1-like homeobox) gene beyond P8, might play an important role in the generation of more lobes, longer lobes, and hydathode formation in the aquatic leaves of M. aquaticum. The technique of in situ hybridization was carried out using a previously sequenced 300 bp fragment of MaKN1 to determine the expression patterns of this gene in the shoot of aquatic forms of the plant. Expression patterns of MaKN1 revealed that the SAM and leaf primordia of aquatic forms of M. aquaticum at levels P0 (youngest) to P4 were distributed throughout these structures. The level of expression of this MaKN1 gene progressively became more localized to lobes in older leaf primordia (levels P5 to P12). Previous studies of aerial forms of this plant showed MaKN1 expression until P8. Our results with aquatic forms show that the highly dissected leaf morphology in aquatic forms was the result of the prolonged expression of MaKN1 beyond P8. This resulted in the formation of elongated and slightly more numerous lobes, and hydathodes in aquatic forms. These findings support the view that KNOX genes are important developmental regulators of leaf morphogenesis and have played an important role in the evolution of leaf forms in the plant kingdom.

Elevated air humidity modulates bud size and the frequency of bud break in fast-growing deciduous trees: silver birch (Betula pendula Roth.) and hybrid aspen (Populus tremula L. × P. tremuloides Michx.)

Elevated atmospheric humidity reduced bud size by restricting primordium growth and increased the frequency of bud break in fast-growing deciduous trees, but the responses are species-specific. The initiation, development, and growth potential of buds determine the structure of a tree crown. Climate change scenarios project increasing amounts of precipitation at high latitudes in the future, but the effects of a more humid climate on bud size and growth potential remain mostly unexplored. This study investigates the effects of atmospheric humidity on fast-growing deciduous trees, silver birch (Betula pendula Roth.) and hybrid aspen (Populus tremula L. × P. tremuloides Michx.), planted in a unique free-air experimental facility, designed for increasing air humidity during the growing season. Over four consecutive years (2009–2012), the size of overwintering buds was assessed in the upper crown of young trees, and additionally, annual height increments were assessed during the study period. Furthermore, bud contents, probabilities of bud death and break, and subsequent shoot growth were examined. In silver birch, misting effect on height growth depended on year, possibly due to acclimation or variable weather conditions, but bud size was consistently reduced. Nevertheless, misting restricted the growth rather than the initiation of leaf primordia in the bud, and decreased bud size did not translate into changes in the leaf area of future shoots. In contrast, the size of hybrid aspen buds was markedly reduced by misting only in 2009; however, increased humidity promoted bud break, reducing the proportion of dormant buds. The underlying mechanisms causing reduced bud size may involve interactions with shoot growth, but require further study. Although the effect of stimulated bud break is subtle in a given year, cumulative effects may modulate crown structure in the long term, facilitating the acclimation of tree growth to rising humidity in the future.

Characterization of a null allelic mutant of the rice NAL1 gene reveals its role in regulating cell division.

Leaf morphology is closely associated with cell division. In rice, mutations in Narrow leaf 1 (NAL1) show narrow leaf phenotypes. Previous studies have shown that NAL1 plays a role in regulating vein patterning and increasing grain yield in indica cultivars, but its role in leaf growth and development remains unknown. In this report, we characterized two allelic mutants of NARROW LEAF1 (NAL1), nal1-2 and nal1-3, both of which showed a 50% reduction in leaf width and length, as well as a dwarf culm. Longitudinal and transverse histological analyses of leaves and internodes revealed that cell division was suppressed in the anticlinal orientation but enhanced in the periclinal orientation in the mutants, while cell size remained unaltered. In addition to defects in cell proliferation, the mutants showed abnormal midrib in leaves. Map-based cloning revealed that nal1-2 is a null allelic mutant of NAL1 since both the whole promoter and a 404-bp fragment in the first exon of NAL1 were deleted, and that a 6-bp fragment was deleted in the mutant nal1-3. We demonstrated that NAL1 functions in the regulation of cell division as early as during leaf primordia initiation. The altered transcript level of G1- and S-phase-specific genes suggested that NAL1 affects cell cycle regulation. Heterogenous expression of NAL1 in fission yeast (Schizosaccharomyces pombe) further supported that NAL1 affects cell division. These results suggest that NAL1 controls leaf width and plant height through its effects on cell division.

Growth and development of shoot apex in barley II. Distribution of growth rates during vegetative phase

The changes in the volume of the apical dome and of the frusta indicate that the mean relative rate of volume growth of the whole apical dome is much higher than in the first frustum which, in turn, grows much faster than the next one. It was found in studies of the distribution of mitoses and of cell arrangement that the volume relative growth rate of the distal part of the apical dome is at least 5-10 times slower than that at the level of new leaf primordia initiation.