Differences In Cell Length Research Articles

Natural variation in OsGASR7 regulates grain length in rice.

Natural variation in OsGASR7 regulates grain length in rice.

Dissection of two quantitative trait loci with pleiotropic effects on plant height and spike length linked in coupling phase on the short arm of chromosome 2D of common wheat (Triticum aestivum L.)

Key messageTwo QTL with pleiotropic effects on plant height and spike length linked in coupling phase on chromosome 2DS were dissected, and diagnostic marker for each QTL was developed.Plant height (PHT) is a crucial trait related to plant architecture and yield potential, and dissection of its underlying genetic basis would help to improve the efficiency of designed breeding in wheat. Here, two quantitative trait loci (QTL) linked in coupling phase on the short arm of chromosome 2D with pleiotropic effects on PHT and spike length, QPht/Sl.cau-2D.1 and QPht/Sl.cau-2D.2, were separated and characterized. QPht/Sl.cau-2D.1 is a novel QTL located between SNP makers BS00022234_51 and BobWhite_rep_c63957_1472. QPht/Sl.cau-2D.2 is mapped between two SSR markers, SSR-2062 and Xgwm484, which are located on the same genomic interval as Rht8. Moreover, the diagnostic marker tightly linked with each QTL was developed for the haplotype analysis using diverse panels of wheat accessions. The frequency of the height-reduced allele of QPht/Sl.cau-2D.1 is much lower than that of QPht/Sl.cau-2D.2, suggesting that this novel QTL may be an attractive target for genetic improvement. Consistent with a previous study of Rht8, a significant difference in cell length was observed between the NILs of QPht/Sl.cau-2D.2. By contrast, there was no difference in cell length between NILs of QPht/Sl.cau-2D.1, indicating that the underlying molecular mechanism for these two QTL may be different. Collectively, these data provide a new example of QTL dissection, and the developed diagnostic markers will be useful in marker-assisted pyramiding of QPht/Sl.cau-2D.1 and/or QPht/Sl.cau-2D.2 with the other genes in wheat breeding.

Dissection of two quantitative trait loci with pleiotropic effects on plant height and spike length linked in coupling phase on the short arm of chromosome 2D of common wheat (Triticum aestivum L.).

Two QTL with pleiotropic effects on plant height and spike length linked in coupling phase on chromosome 2DS were dissected, and diagnostic marker for each QTL was developed. Plant height (PHT) is a crucial trait related to plant architecture and yield potential, and dissection of its underlying genetic basis would help to improve the efficiency of designed breeding in wheat. Here, two quantitative trait loci (QTL) linked in coupling phase on the short arm of chromosome 2D with pleiotropic effects on PHT and spike length, QPht/Sl.cau-2D.1 and QPht/Sl.cau-2D.2, were separated and characterized. QPht/Sl.cau-2D.1 is a novel QTL located between SNP makers BS00022234_51 and BobWhite_rep_c63957_1472. QPht/Sl.cau-2D.2 is mapped between two SSR markers, SSR-2062 and Xgwm484, which are located on the same genomic interval as Rht8. Moreover, the diagnostic marker tightly linked with each QTL was developed for the haplotype analysis using diverse panels of wheat accessions. The frequency of the height-reduced allele of QPht/Sl.cau-2D.1 is much lower than that of QPht/Sl.cau-2D.2, suggesting that this novel QTL may be an attractive target for genetic improvement. Consistent with a previous study of Rht8, a significant difference in cell length was observed between the NILs of QPht/Sl.cau-2D.2. By contrast, there was no difference in cell length between NILs of QPht/Sl.cau-2D.1, indicating that the underlying molecular mechanism for these two QTL may be different. Collectively, these data provide a new example of QTL dissection, and the developed diagnostic markers will be useful in marker-assisted pyramiding of QPht/Sl.cau-2D.1 and/or QPht/Sl.cau-2D.2 with the other genes in wheat breeding.

Contrasting lengths of Pelargonium floral nectar tubes result from late differences in rate and duration of growth.

Much of morphological evolution in flowers has arisen from pollinator-mediated selection, often manifest as a match between the length of the pollinator's proboscis and the depth of tubular corollas or spurs. We investigate development, growth and homology of the unique nectar tube of Pelargonium, frequently described as 'a spur adnate to the pedicel'. We focused on two species. The nectar tube of P. ionidiflorum is three times longer than that of P. odoratissimum. Light and scanning electron microscopy were carried out, and daily growth measurements were used to compare nectar tube development and vascular patterns. Nectar tubes in both species are initiated centripetally to the dorsal sepal in a space created by lateral displacement of two antepetalous stamens. The cavity deepens through subsequent intercalary growth of the receptacle that proceeds at the same rate in both species until tubes reach approx. 10 mm in length. Differences in final nectar tube lengths arise via an increase in the rate and duration of growth of the receptacle that begins just before anthesis (floral opening) and continues for several days past anthesis in P. ionidiflorum but does not occur in P. odoratissimum. Epidermal cells of the dorsal surface of the nectar tube in P. ionidiflorum are approx. 1.6 times longer than those in P. odoratissimum. Histological sections show no evidence that the nectar tube is a spur that became evolutionarily fused to the pedicel. Nectar tubes in Pelargonium are localized cavities that form in the receptacle via intercalary growth. Differences in the rate and duration of growth just prior to and following anthesis underlie differences in final tube lengths. Because differences in cell lengths do not fully account for differences in nectar tube lengths, evolutionary diversification must involve changes in both cell cycle and cell expansion.

Lichen secondary metabolites affect growth of Physcomitrella patens by allelopathy.

Lichen secondary metabolites can function as allelochemicals and affect the development and growth of neighboring bryophytes, fungi, vascular plants, microorganisms, and even other lichens. Lichen overgrowth on bryophytes is frequently observed in nature even though mosses grow faster than lichens, but there is still little information on the interactions between lichens and bryophytes.In the present study, we used extracts from six lichen thalli containing secondary metabolites like usnic acid, protocetraric acid, atranorin, lecanoric acid, nortistic acid, and thamnolic acid. To observe the influence of these metabolites on bryophytes, the moss Physcomitrella patens was cultivated for 5weeks under laboratory conditions and treated with lichen extracts. Toxicity of natural mixtures of secondary metabolites was tested at three selected doses (0.001, 0.01, and 0.1%). When the mixture contained substantial amounts of usnic acid, we observed growth inhibition of protonemata and reduced development of gametophores. Significant differences in cell lengths and widths were also noticed. Furthermore, usnic acid had a strong effect on cell division in protonemata suggesting a strong impact on the early stages of bryophyte development by allelochemicals contained in the lichen secondary metabolites.Biological activities of lichen secondary metabolites were confirmed in several studies such as antiviral, antibacterial, antitumor, antiherbivore, antioxidant, antipyretic, and analgetic action or photoprotection. This work aimed to expand the knowledge on allelopathic effects on bryophyte growth.

Arabidopsis VILLIN2 and VILLIN3 are required for the generation of thick actin filament bundles and for directional organ growth.

In plant cells, actin filament bundles serve as tracks for myosin-dependent organelle movement and play a role in the organization of the cytoplasm. Although virtually all plant cells contain actin filament bundles, the role of the different actin-bundling proteins remains largely unknown. In this study, we investigated the role of the actin-bundling protein villin in Arabidopsis (Arabidopsis thaliana). We used Arabidopsis T-DNA insertion lines to generate a double mutant in which VILLIN2 (VLN2) and VLN3 transcripts are truncated. Leaves, stems, siliques, and roots of vln2 vln3 double mutant plants are twisted, which is caused by local differences in cell length. Microscopy analysis of the actin cytoskeleton showed that in these double mutant plants, thin actin filament bundles are more abundant while thick actin filament bundles are virtually absent. In contrast to full-length VLN3, truncated VLN3 lacking the headpiece region does not rescue the phenotype of the vln2 vln3 double mutant. Our results show that villin is involved in the generation of thick actin filament bundles in several cell types and suggest that these bundles are involved in the regulation of coordinated cell expansion.

The mechanism of stamen movement in Chimonanthus praecox (Calycanthaceae): differential cell growth rates on the adaxial and abaxial surfaces of filaments after flower opening

In flowers of Chimonanthus (Calycanthaceae), stamen movements have been known for 150 years. Stamens are initially recurved when flowers open. Subsequently, within approximately 48 h, the stamens gradually turn upright and eventually enclose the gynoecium. However, the exact mechanism involved in this process is still unresolved. In this study, we found that in recurved stamens at the female stage the cell lengths in the epidermal and hypodermal tissue of the abaxial surfaces were significantly smaller than those of the adaxial surfaces. In erect stamens, however, no significant difference in cell length between the hypodermal tissues of both surfaces was found. Although the difference in cell length between the epidermal tissues for erect stamens was significant, it was smaller than that in recurved filaments. We found no variations in cell number between the epidermal tissues or between the hypodermal tissues among recurved and erect stamens. Thus, we conclude that in Chimonanthus the differential cell growth rates between the adaxial and abaxial surfaces of filaments could account for the gradual inward stamen movement following flower opening rather than cell division. Furthermore, application of indole-3-acetic acid (IAA) and gibberellic acid (GA3) to intact flowers in vitro or the wounding of stigmas significantly promoted the stamen erection rate. Surprisingly, we did not observe any effect on this movement following hand-pollination. Different patterns of stamen movement in Calycanthaceae and their implications for reproductive biology are also discussed.

Strigolactones Negatively Regulate Mesocotyl Elongation in Rice during Germination and Growth in Darkness

Strigolactones (SLs) are newly discovered plant hormones that regulate plant growth and development including shoot branching. They also stimulate symbiosis with arbuscular mycorrhizal fungi. Rice has at least three genes that are involved in SL synthesis (D10, D17/HTD1 and D27) and at least two genes that are involved in SL signaling (D3) and SL signaling or downstream metabolism (D14/D88/HTD2). We observed that mesocotyl elongation in darkness was greater in rice mutants defective in these genes than in the wild type. Exogenous application of a synthetic SL analog, GR24, rescued the phenotype of mesocotyl elongation in the SL-deficient mutants, d10-1, d17-1 and d27-1, in a dose-dependent manner, but did not affect mesocotyl lengths of the SL-insensitive mutants, d3-1 and d14-1. No significant differences in cell length were found between the d mutants and the wild type, except for some cells on the lower half of the d3-1 mesocotyl that were shortened. On the other hand, the number of cells in the mesocotyls was 3- to 6-fold greater in the d mutants than in the wild type. Treatment with GR24 reduced the number of cells in the d10-1 mesocotyl to the wild-type level, but did not affect the number of cells in the d3-1 and d14-1 mesocotyls. These findings indicate that SLs negatively regulate cell division, but not cell elongation, in the mesocotyl during germination and growth of rice in darkness.

Function of a Chemotaxis-Like Signal Transduction Pathway in Modulating Motility, Cell Clumping, and Cell Length in the Alphaproteobacterium Azospirillum brasilense

A chemotaxis signal transduction pathway (hereafter called Che1) has been previously identified in the alphaproteobacterium Azospirillum brasilense. Previous experiments have demonstrated that although mutants lacking CheB and/or CheR homologs from this pathway are defective in chemotaxis, a mutant in which the entire chemotaxis pathway has been mutated displayed a chemotaxis phenotype mostly similar to that of the parent strain, suggesting that the primary function of this Che1 pathway is not the control of motility behavior. Here, we report that mutants carrying defined mutations in the cheA1 (strain AB101) and the cheY1 (strain AB102) genes and a newly constructed mutant lacking the entire operon [Delta(cheA1-cheR1)::Cm] (strain AB103) were defective, but not null, for chemotaxis and aerotaxis and had a minor defect in swimming pattern. We found that mutations in genes of the Che1 pathway affected the cell length of actively growing cells but not their growth rate. Cells of a mutant lacking functional cheB1 and cheR1 genes (strain BS104) were significantly longer than wild-type cells, whereas cells of mutants impaired in the cheA1 or cheY1 genes, as well as a mutant lacking a functional Che1 pathway, were significantly shorter than wild-type cells. Both the modest chemotaxis defects and the observed differences in cell length could be complemented by expressing the wild-type genes from a plasmid. In addition, under conditions of high aeration, cells of mutants lacking functional cheA1 or cheY1 genes or the Che1 operon formed clumps due to cell-to-cell aggregation, whereas the mutant lacking functional CheB1 and CheR1 (BS104) clumped poorly, if at all. Further analysis suggested that the nature of the exopolysaccharide produced, rather than the amount, may be involved in this behavior. Interestingly, mutants that displayed clumping behavior (lacking cheA1 or cheY1 genes or the Che1 operon) also flocculated earlier and quantitatively more than the wild-type cells, whereas the mutant lacking both CheB1 and CheR1 was delayed in flocculation. We propose that the Che1 chemotaxis-like pathway modulates the cell length as well as clumping behavior, suggesting a link between these two processes. Our data are consistent with a model in which the function of the Che1 pathway in regulating these cellular functions directly affects flocculation, a cellular differentiation process initiated under conditions of nutritional imbalance.

What the Distribution of Cell Lengths in the Root Meristem Does and Does Not Reveal About Cell Division.

Roots have long been realized to be useful material for studies of cell division. Despite this long history of use, the behavior of cells in the meristem is often misinterpreted. A common error is to argue that differences in cell length reflect differences in cell division rate. In this article we explain the fallacy behind this argument and show how the analysis of cell length distribution can lead to insight about the root meristem. These observations support a model for the root meristem where cells of various tissues grow at the same relative growth rate and divide at the same frequency, indicating that these growth parameters are built into the cells at a fundamental level. The differences in cell length between various tissues appear to arise at their formation, first at the tissue initials and ultimately in the seed. Length differences among mature cells may be enhanced by differences in the location within the meristem where division ceases. Discovering mechanisms regulating the length of initial cells and the position where cells cease division requires a realistic understanding of how growth constrains the division behavior of dividing cells.

Varied diazotrophies, morphologies, and toxicities of genetically similar isolates of Cylindrospermopsis raciborskii (nostocales, cyanophyceae) from Northern Australia.

The potentially toxic freshwater cyanobacterium Cylindrospermopsis raciborskii has become increasingly prevalent in tropical and temperate water bodies worldwide. This paper investigates the effects of different nitrogen sources (NO3-, NH4+, and omission of a fixed form of nitrogen) on the growth rates, morphologies, and cylindrospermopsin (CYL) concentrations (expressed as a percentage of the freeze-dried weight) of seven C. raciborskii isolates obtained from a range of water bodies in northern Australia and grown in batch culture. In general, growth rates were lowest in the absence of a fixed-nitrogen source and highest with NH4+ as the nitrogen source. Conversely, the highest concentrations of CYL were recorded in cultures grown in the absence of a fixed-nitrogen source and the lowest were found in cultures supplied with NH4+. Cultures supplied with NO3- were intermediate with respect to both CYL concentration and growth rate. Different nitrogen sources resulted in significant differences in the morphology of C. raciborskii trichomes. Most notable were the loss of heterocysts and the tapering of end cells in cultures supplied with NH4+ and the statistically significant increase in vegetative cell length (nitrogen depleted < NO3- < NH4+). The morphological changes induced by different nitrogen sources were consistent for all isolates, despite measurable differences in vegetative-cell and heterocyst dimensions among isolates. Such induced morphological variation has implications for Cylindrospermopsis taxonomy, given that distinctions between species are based on minor and overlapping differences in cell lengths and widths. The close phylogenetic association among all seven isolates was confirmed by the high level (>99.8%) of similarity of their 16S rRNA gene sequences. Another genetic technique, analysis of the HIP1 octameric-palindrome repeated sequence, showed greater heterogeneity among the isolates and appears to be a useful method for distinguishing among isolates of C. raciborskii.

Morphology and life history of four Japanese species of Elachista (Elachistaceae, Phaeophyceae), including a new record of E. fucicola

SUMMARY Elachista fucicola (Velley) Areschoug (Elachistaceae, Phaeophyceae) is newly recorded from Japan, and compared with three previously described species, Elachista coccophorae Takamatsu, Elachista mollis Takamatsu and Elachista okamurae Yoshida. All species showed direct-type life histories in culture without sexual fusion. Prostrate filaments of E. fucicola formed globular plurilocular zoidangia similar to those reported from Atlantic isolates of this species. However, these were different from uniseriate plurilocular zoidangia of the three Japanese species. Furthermore, the position of a meristematic region in assimilatory filaments and the morphology of paraphyses can easily distinguish these from E. fucicola. Previously emphasized differences in cell length : width ratios in assimilatory filaments showed only slight differences. Elachista coccophorae is characterized by thick-walled assimilatory filaments and curved paraphyses, and the absence of downwardly growing rhizoidal filaments. Elachista mollis and E. okamurae are very similar. However, germlings from plurizoids formed on erect filaments of E. mollis showed characteristic pseudodiscoid growth in culture. Halothrix coccophorae Ohta and Elachista zosterae Noda are reduced to synonyms of E. coccophorae and E. mollis, respectively.

The mechanism of floral heliotropism in the snow buttercup, Ranunculus adoneus

We designed field experiments using solar‐tracking Ranunculus adoneus flowers to determine where photoreception occurred, which organs responded, and how movement was achieved. Flower peduncles bend eastward in the morning and gradually unbend over the course of the day. Peduncles were found to bend significantly more frequently in the middle region near the floral bracts, 1–3 cm below the flower, than elsewhere on the peduncle. Because the peduncle tip continued to track the sun even after the flower itself was removed, our experiments concentrated on shielding (or conversely, exposing) various portions of peduncles from (or to) sunlight. Photoreception occurred primarily in the portion of the stem just beneath the floral receptacle. By following the position of landmarks applied to the stem, we found that 40% more growth occurred on the shaded side of bent peduncles, compared to the sunlit side. In contrast, top‐shielded peduncles did not solar track well and grew only 25% more on the shaded side than on the sunlit side. This growth differential corresponded to differences in cell length on the two sides of bent peduncles, with significantly longer epidermal cells occurring on the shaded side than on the sunlit side.

Analysis of cell division and elongation underlying the developmental acceleration of root growth in Arabidopsis thaliana.

To investigate the relation between cell division and expansion in the regulation of organ growth rate, we used Arabidopsis thaliana primary roots grown vertically at 20 degreesC with an elongation rate that increased steadily during the first 14 d after germination. We measured spatial profiles of longitudinal velocity and cell length and calculated parameters of cell expansion and division, including rates of local cell production (cells mm-1 h-1) and cell division (cells cell-1 h-1). Data were obtained for the root cortex and also for the two types of epidermal cell, trichoblasts and atrichoblasts. Accelerating root elongation was caused by an increasingly longer growth zone, while maximal strain rates remained unchanged. The enlargement of the growth zone and, hence, the accelerating root elongation rate, were accompanied by a nearly proportionally increased cell production. This increased production was caused by increasingly numerous dividing cells, whereas their rates of division remained approximately constant. Additionally, the spatial profile of cell division rate was essentially constant. The meristem was longer than generally assumed, extending well into the region where cells elongated rapidly. In the two epidermal cell types, meristem length and cell division rate were both very similar to that of cortical cells, and differences in cell length between the two epidermal cell types originated at the apex of the meristem. These results highlight the importance of controlling the number of dividing cells, both to generate tissues with different cell lengths and to regulate the rate of organ enlargement.

Characterization of the Leaf Epidermis of Barley (Hordeum vulgareL. 'Himalaya')

The cell types of mature leaves of barley ( Hordeum vulgare L. ‘Himalaya’) are described. Blade and sheath epidermal cell types were characterized according to their position relative to the veins, stomatal rows, and sclerenchyma cells. Cells over veins were further classified according to the size of vein. Cell lengths of the approx. 15 different cell types ranged from approximately 50 μm to over 2 mm. The principal difference in cell length between the abaxial and adaxial surfaces was seen for cells lying between the veins; on the adaxial surface these cells (bulliform cells) were about 200 μm long whereas those on the abaxial surface were over 2 mm in length. Total file number across the blade width was the same on the abaxial and adaxial surfaces. However, there were more cells lying over veins than between veins on the adaxial surface, and vice versa for the abaxial surface. The detailed description of leaf epidermal cell types of barley in this study provides the basis for comparison with mutants which differ in leaf length (Wenzel et al ., Annals of Botany79 : 45–50, 1997).

Survival of, and induced stress resistance in, carbon-starved Pseudomonas fluorescens cells residing in soil.

We investigated the survival, cell length, and development of general stress resistance in populations of Pseudomonas fluorescens R2f and its rifampin-resistant mutant, R2f Rpr, following exposure to carbon starvation conditions in liquid cultures and residence in two different soils, Flevo silt loam (FSL) and Ede loamy sand (ELS). In much the same way as was recently shown for P. putida KT2442, carbon-starved P. fluorescens R2f populations revealed enhanced resistance to otherwise lethal treatments, such as exposure to ethanol, high temperature, osmotic tension, and oxidative stress. A large population of nonculturable P. fluorescens R2f Rpr cells arose shortly after their introduction into ELS soil, whereas the formation of nonculturable cells was not observed in FSL soil. Also, the inoculant cell (based on immunofluorescence) and CFU counts decreased faster in ELS soil than in FSL soil. Introduction of carbon-starved instead of exponential-growth-phase R2f Rpr cells into ELS soil did not affect bacterial survival. The inoculant cell length decreased in soil, and no large differences in cell length in the two soil types were observed. Addition of glucose to ELS soil resulted in a stable cell length of R2f Rpr cells, whereas carbon-starved cells introduced into ELS soil remained small. Exponentially growing R2f Rpr cells developed enhanced resistance to ethanol, high temperature, osmotic tension, and oxidative stress within 1 day in both soils, whereas cells introduced into ELS soil amended with glucose showed decreased resistance. Cells that were carbon starved prior to introduction into ELS soil showed unchanged stress resistance levels upon residence in soil.

Redefinition of the Genera Diophrys and Paradiophrys and Establishment of the Genus Diophryopsis N. G. (Ciliophora, Hypotrichida): Implication for the Species Problem

ABSTRACT. Species belonging to the genus Diophrys Dujardin, 1841, are easily recognized due to possession of the usual complement of approximately seven frontoventral cirri, five transverse cirri, two left marginal cirri, and three large caudal cirri. Separation of these species has been based upon differences in cell length and width, the number and arrangement of cilia in dorsal kinetics, the configuration of the adoral zone of oral polykinetids, the number and distribution of cirri within cirral groups, and the number and arrangement of macronuclei. Jankowski used some of these characteristics to divide the genus into two genera, Diophrys and Paradiophrys, with several subgenera [Jankowski, A. W. 1978. Systematic revision of the class Polyhymenophora (Spirotricha), morphology, systematics and evolution. Tezisy Dokl. Zool. Inst. Akad. Nauk SSSR, 197839‐40. (in Russian); Jankowski, A. W. 1979. Systematics and phylogeny of the order Hypotrichida Stein, 1859 (Protozoa, Ciliophora). Trudy. Zool. Inst. Akad. Nauk SSSR, 86:48–85. (in Russian with English summary)]. Data obtained from light microscopic examination of stained (nigrosin‐butanol, Chatton‐Lwoff, and Protargol) cells in interphase or division supports and modifies the use of particular structural features of these ciliates for the purpose of taxonomic classification. The structural variability within and among populations of different species within the genera Diophrys (D. appendiculata, D. oligothrix, and D. scutum) and Paradiophrys (P. irmgard and P. multinucleata) is described. D. hystrix is redescribed as the type of the new genus Diophryopsis n. g. Comparative information on the cortical morphogenesis of division of selected species within each genus is reviewed. Two taxonomic classifications of these hypotrichs are discussed: 1) a listing of diagnoses and synonymies and 2) a binary key for identification of all species at the light microscope level. An alternative evolutionary explanation of variations among isolates is presented.

Gibberellic acid-induced cell elongation in cotton suspension cultures

Gibberellic acid (GA3) causes cell elongation in cotton suspension cultures derived from cotton ovule callus tissue of both auxin-dependent and-independent lines. Cell elongation was more pronounced in auxin-dependent cultures. Cells were cultured for a period of 14 days but differences in cell lengths could be detected after 6 days in culture. Cell elongation took place in cultures in which GA3 was present throughout the culture period or only for the first 3 days. Auxins and cytokinin alone or in the presence of GA3 did not promote cotton cell elongation above the value for the treatment with GA3 alone.

Comparison of photosynthetic carbon reduction (Kranz) cells having different ontogenetic origins in the C4 NADP – malic enzyme grass Arundinella hirta

The C4 grass Arundinella hirta is characterized by unusual leaf blade anatomy: photosynthetic carbon reduction takes place both within the chlorenchymatous bundle sheath cells of the longitudinal veins and within longitudinal strands of "distinctive cells" that form part of the leaf mesophyll and are often completely isolated from vascular tissue. Although they are equivalent physiologically, these two cell types have different ontogenetic origins: bundle sheath cells are delimited from procambium early in leaf development, whereas distinctive cells differentiate from ground meristem at a later developmental stage. Although the two cell types share numerous cytological features (large chloroplasts with reduced grana, thick cell walls with a suberin lamella), we also found significant differences in cell lengths, length to width ratios, cell cross-sectional areas, organelle numbers per cell cross section, phenol content of the cell walls, and numbers of pit fields in the longitudinal cell walls. The size and shape of bundle sheath cells are likely a direct consequence of procambial origin. The thicker walls of bundle sheath cells (in major veins) and their greater lignification may reflect the inductive effect of cell differentiation in the proximity of sclerenchyma and vascular tissues. Differences between major and minor vein bundle sheath cells may reflect differences in the timing of initiation of procambial strands. Our analysis of cell wall characteristics has also shown the presence of numerous primary pit fields in the transverse walls between adjacent distinctive cells in a file; plasmodesmata in these pit fields form a pathway for longitudinal symplastic transport not previously known to exist.

Effects of Low Water Potential on Cortical Cell Length in Growing Regions of Maize Roots

Roots growing under low water potential commonly exhibit a marked decrease in growth rate and in diameter. Using median longitudinal sections of fixed maize (Zea mays L. cv WF9 x Mo 17) seedling roots, we investigated the cellular basis for these effects. Cortical cells in the shortened elongation zone of water stressed roots were longer than cortical cells in the comparable location of well-watered roots. Nearly twofold differences in cell length were seen in the region 2 to 4 millimeters behind the root apex. The shortened growth zone, however, leads to a final mean cortical cell length approximately 30% shorter in the stressed roots. These differences were present regardless of the age of the control roots. These data, and the slower growth rate seen in water-stressed roots, suggest that the water deficit causes a significant reduction in the rate of cell supply to the cortical cell files.