Developmental stage-dependent response ofPilobolus crystallinus sporangiophores to gravitative and centrifugal stimulation

Gravitropic response of sporangiophores ofPilobolus crystallinus was studied by successive microscopic observation of the sporangiophores horizontally placed in the dark (red light) and by analysis of sporangiophore response to centrifugal stimulation. Negative tropism against the gravitative and also centrifugal stimulation was found only in mature sporangiophores after development of sporangium and after the resumption of elongation beneath the fully-developed subsporangial vescle, but there was no response in younger sporangiophores, implying that the gravitative perception system of the sporangiophores is dependent on their developmental stages.     

Spiral growth in the radially-expanding piloboloid mutants ofPhycomyces blakesleeanus

The growth zone of the sporangiophore of a piloboloid mutant,pil, ofPhycomyces expands radially at an increased rate until the growth zone becomes nearly spherical, in sharp contrast to that of the wild-type sporangiophore which exhibits longitudinal elongation only and is conical. The rotation of thepil sporangiophore reverses its direction from clockwise (CW) to counterclockwise (CCW) during the period of increased radial expansion, and the CCW rotation continues as long as does the radial expansion. The direction of rotation and the time of reversal are correlated with the relative rates of cell-wall expansion in the longitudinal and transverse directions. The CCW rotation of the sporangiophore of this mutant can be explained by the behavior of the microfibrils, as previously proposed to explain the rotation of the wild-type sporangiophore.Abbreviations CW clockwise - CCW counterclockwise — both as viewed from above     

Phycomyces: MODIFICATION OF LIGHT-INDUCED SPIRAL GROWTH AFTER MECHANICAL CONDITIONING OF THE CELL WALL

Mature stage IVb Phycomyces sporangiophores show left-hand spiral growth; that is, viewed from above, the sporangium rotates clockwise. It has been shown that mechanical conditioning (strain-hardening) of the cell wall by the Instron technique increases the ratio of rotation to the elongation growth rate compared to nonmechanically conditioned controls. It is reported that the addition of a saturating light stimulus to these sporangiophores causes a decrease in the ratio of rotation to elongation growth rate. This result is in agreement with the fibril slippage model, i.e. the counterclockwise rotation of stage IVa is a result of parallel fibrils lying in a right-handed spiral configuration slipping by one another. It is suggested that a light stimulus added to a mechanically conditioned stage IVb sporangiophore activates one or more cell wall-loosening enzymes which act by decreasing the number of intermolecular bonds between parallel fibrils causing fibril slippage, resulting in counterclockwise rotation. It is precisely this counterclockwise contribution that decreases the rotation to elongation growth ratio of mechanically conditioned and then light-stimulated stage IVb sporangiophores.     

Effects of microbeam light on growth and phototropism ofPilobolus crystallinus sporangiophores

A small blue-light beam (50 μm in diam) was used to examine light-growth response and phototropism inPilobolus crystallinus sporangiophores. Continuous irradiation by microbeam of a region 100–150 μm from the apex promoted the growth of a dark-adapted sporangiophore for about 15 min after a lag period of 1–2 min. After the promotion, the growth rate fell below that before the irradiation. Irradiation of the apex of sporangiophore slightly promoted the growth but strongly inhibited the growth after the promotion. A smaller light beam (10 μm in diam) applied continuously at grazing incidence along one side of the sporangiophore caused bending toward the shaded side, implying that the irradiated side grew more rapidly than the shaded side and that the lens effect is involved in the phototropism of young sporangiophores ofP. crystallinus. The involvement of the lens effect was confirmed by the fact that a carotenoid-less mutant was 1.5–2 times more sensitive to unilateral blue light than the wild type, probably because of a smaller intracellular light attenuation during passage through the mutant cell.     

The influence of phosphorus limitation of the diatomAsterionella Formosa on the zoospore production of its fungal parasiteRhizophydium Planktonicum

The influence of phosphorus limitedAsterionella on the zoospore production of its fungal parasiteRhizophydium planktonicum was measured, using laboratory cultures of host and parasite. At saturated phosphorus concentrations the host reached a specific growth rate of 0.95.d^–1. Growing on these host cells, the mean parasite zoospore production was 26 spores per sporangium, and the mean development time of a sporangium was 45 hours. Growing on phosphorus limited hosts, the zoospore production decreased to less then 9 spores per sporangium, and the development time decreased to 40 hours. On phosphorus limited hosts, zoospores were produced at a slower rate. The algal growth rate was reduced to a greater extent than the fungal growth rate. Therefore, it could be concluded that phosphorus limitation ofAsterionella will facilitate the development of an epidemic of its parasiteRhizophidium, at least at high diatom densities, when possible differences in infectability of the algae play a minor role.     

“Apical dominance” in the sporangiophore of the fungus Phycomyces

Summary Control, by the spores, over the elongation and the branching of the sporangiophore is described. If the sporangium is removed from a sporangiophore, the sporangiophore stops growing within a few hours. 6–16h later a branch grows from the defunct growing zone. This branch forms a new sporangium and spores, and then starts to elongate again. The original growing zone can be rescued by replacing the sporangium with another sporangium or a sporangium-sized drop of spores. If the original growing zone is rescued, then it continues growing and the sporangiophore does not form a branch. It can only be rescued, however, within the first 60 min after the original sporangium is removed.This work was submitted in partial fulfillment of the requirements for the Ph. D. degree.     

The Distribution of Cytoplasmic Vesicles, Multivesicular Bodies and Paramural Bodies in Elongating Sporangiophores and Swelling Sporangia of Thamnidium elegans Link

In Thamnidium elegans Link, cytoplasmic vesicles of variablesize were present in large numbers in sporangiophore apicesin agar and in smaller numbers in sporangiophore apices in air.Golgi-like cisternae were associated with small, sub-apicalclusters of vesicles. Apices in agar contained a nucleus andmitochondrion-free tip zone in which vesicles were concentrated.This zone was almost completely absent from apices in air. Inswelling sporangia, vesicles were sparce and were not concentratedagainst the wall. Rates of surface area increase were similarfor sporangiophores elongating, respectively, in agar and inair. Rates of surface area increase during sporangium swellingwere equal to or greater than rates of surface area increaseduring sporangiophore elongation. Vesicles were associated withformation of a secondary wall layer in swollen sporangia. Paramuralbodies and multivesicular bodies were present at all stagesof sporangiophore elongation and sporangium swelling. Isolatedhalo bodies (apical corpuscles) were present in walls at sporangiophoretips, and clusters of similar bodies were present in side wallsof sporangiophores.     

The analysis of spiral growth in phycomyces using a novel optical method

A conical mirror was designed and used to measure simultaneously the elongational and rotational displacement of a number of markers on the growing zone of the sporangiophore of Phycomyces. The results obtained by this new optical method demonstrate that the rotational rate is roughly proportional to the elongational rate, except in the lower region of the growing zone where a significant amount of rotation occurs without measurable elongation. From the data presented in this report, we have constructed a model that appears to explain the mechanism responsible for the left-handed spiral growth of the developing sporangiophore.     

中华水韭孢子囊发育研究

采用半薄切片法,连续观察了极度濒危级(CR)植物中华水韭大小孢子囊的发育过程,以期从无性生殖的角度,为探讨其濒危原因提供直观可靠的理论根据。结果显示:(1)中华水韭的大小孢子叶相间排列,无混生孢子囊。(2)隔丝为孢子供给营养,其体积直接影响孢子的大小、产量和育性。(3)大小孢子囊都近半数败育,小孢子囊为整齐发育,大孢子囊为不整齐发育。(4)大小孢子囊均无柄,且都不存在开裂结构,只有孢子囊壁腐烂后才能散播孢子。研究认为,中华水韭的濒危与孢子囊的发育特征密切相关,孢子囊的高频率败育、没有开裂结构以及对环境的依赖,是造成中华水韭濒危的重要因素之一;通过与近缘类群孢子囊的比较,发现仅水韭孢子的散播借助外力,对生境要求较高,即验证了水韭古老的系统学地位,同时说明水韭更具有监测生境地区环境指标的能力。     

Phycomyces: growth responses of the sporangium

During the development of the sporangiophore of the fungus Phycomyces blakesleeanus there occurs a period of several hours when the sporangiophore does not elongate; instead, its “growth” is diverted into the formation of a sporangium at its top. This period of head formation is called stage II. Clearly, growth has not ceased but rather the geometry of the growing area has changed from that of a cylinder to a sphere. The growing sphere is found to have properties similar to the stage IV growing zone in that it functions as a sensory receptor and effector. The growing sporangium responds to both light (light head response) and humidity (wet head response). A model is presented giving a possible mechanism by which the ultimate size of the sporangium is regulated.     

Time course of phytochrome-mediated changes in growth gradients on coleoptiles of Triticum aestivum L.

The length of parenchyma cells along the axis of dark-grown coleoptiles of Triticum aestivum L. and the pattern of competence for red-light-(R-) induced stimulation or inhibition of cell elongation in the course of coleoptile development were determined by microscopic measurements in a file of 240 cells from the tip to the base. On the basis of these measurements distinct zones (responding in different ways to R) were selected for studying the early time course of phytochrome-mediated growth-rate changes in intact coleoptiles by use of a sensitive transducer system. Between 2 d and 4 d after sowing dark-grown coleoptiles showed a graded incline in cell growth activity from the apex to the base (growth gradient). Whereas cell elongation in the coleoptile base ceased 4 d after sowing, cell elongation speeded up in the tip and middle region at that time. Those cells that grew slowly in darkness (tip and middle region between 2d and 3 d after sowing) were stimulated in growth by R-pulse irradiation (1 min R, 660 nm, 1000 J · m^–2). In contrast, the growth of fast-growing cells (base between 2 d and 4 d after sowing, tip and middle region between 4 d and 5 d after sowing) was inhibited by R. However, the starting time for R-induced growth changes was different for different coleoptile zones. The respective data point to the storage of a phytochrome-mediated signal in the cells of the middle region, until these cells become competent to respond to it; alternatively, Pfr, the far-red-light-absorbing form of phytochrome, may be stored in a stable form. Continuous recordings on the effect of R, far-red (FR) and R/FR on the zonal growth responses were made on intact coleoptiles, selected 3 d after sowing. During a 5-h investigation period the R-induced changes in growth rate could be divided into two phases: (i) A transient growth inhibition which started approx. 15 min after R. This response was qualitatively the same in all coleoptile zones investigated (tip, middle region, base). (ii) Zonal-specific growth responses which became measurable approx. 2.5 h after R, i.e. growth promotion in the tip, growth inhibition in the base and an adaptation of growth rate to the dark control level in the middle region. The R-induced growth rate changes were reversible by FR for both phases. Additional growth experiments on excised coleoptile segments under R and auxin application indicated that the zonal-specific growth promotion or inhibition may be not mediated by an influence of R on the auxin level.Abbreviations FR far-red light - Pfr far-red-light-absorbing form of phytochrome - R red light The technical assistance of Mrs. B. Liebe is gratefully acknowledged.     

REGIONS OF CELL DIVISION AND ELONGATION DURING STEM GROWTH OF XANTHIUM

Regions of cell division and cell elongation were established during stem growth of Xanthium pensylvanicum (cocklebur). From percent mitosis it was determined that the region of cell division in a Xanthium stem of Plastochron Index (PI) 13.89 was 20 mm long, starting from the stem apex and proceeding in a basipetal direction. Measurements of cortical cell lengths demonstrated that mature cell length in the stem of the same plant was reached at about 55 mm distance from the stem apex. Between 20 and 55 mm distance from the apex, shoot growth occurred by cell elongation alone. The maximum rate of change in cell length (dC/dX) was at about 15 mm distance from the stem apex. The length of the apical growth region was a function of the age of the plants. The older the plant, the longer its apical region of elongation. Growth of Xanthium stems was due primarily to elongation of internodes; the nodal regions did not seem to elongate.     

Distribution and redistribution of extension growth along vertical and horizontal gravireacting maize roots

Horizontal primary roots of Zea mays L. were photographed during the course of their gravireaction and during a preceding growth period in the vertical orientation. The displacement, by root elongation, of marker particles on the root surface was recorded. The particle-displacement rates were used to estimate the distribution of elemental elongation rates along opposite sides of the growing root apex. In the temperature range 21–25°C there was a stimulation of local elongation rates along the upper side of a gravireacting root and a reduction (and sometimes a cessation) of elongation along the lower side. Elemental elongation rates have been related to the development of root curvature, and the magnitude of the differential growth between upper and lower sides required for a particular rate of bending has also been estimated. The results complement, and are compatible with, findings relating to the distribution of certain endogenous growth regulators believed to participate in the gravireaction.Abbreviation RELEL relative elemental rate of elongation     

Negative phototropism in the piloboloid mutants of Phycomyces blakesleeanus Bgff.

The sporangiophore (spph) of a piloboloid mutant, genotype pil, of Phycomyces ceases elongation and expands radially in the growth zone shortly after reaching the developmental stage IV b. The pil spph is always negatively phototropic to unilateral visible light when its diameter exceeds 210 m. Photoinduction of spph initiation, light-growth response, threshold of light energy fluence rate for the negative phototropism, avoidance and gravitropism in the pil mutant are all normal. In liquid paraffin, the pil spph shows negative phototropism as does the wild-type spph. Genetic analyses indicate that the negative phototropism of the pil mutant is governed by the phenotypic characteristics of pil but not by specific gene(s) responsible for negative phototropism. These facts imply that the reverse phototropism of the pil mutant results from a loss of the convergent lens effect of the cell because of the increase in cell diameter.Abbreviations spph(s) sporangiophore(s) - wt(s) wild type(s)     

Alternative fruiting pathways in phycomyces

Developmental distinctions between giant and dwarf fruiting bodies of Phycomyces blakesleeanus (Burgeff) were studied by means of size measurements and growth analyses. Histograms of fruiting body lengths showed a bimodal distribution, with peaks around 0.3 millimeter (dwarfs) and 30 millimeters (giants). Individual cultures contain both giants and dwarfs. Differences between giants and dwarfs appear in the first phase of development; the apex of the giant is tapered, whereas the dwarf apex is dome-shaped. Probable cytological distinctions at this stage are cited in discussion. The dwarfs terminate enlargement upon expansion of the sporangium, thus lacking the subsequent phase of rapid elongation (stage IV) that contributes 90% of the length in the case of giants. It was concluded that P. blakesleeanus maintains two developmental patterns for asexual fruiting, with dwarfs and giants differing radically in growth regulation.     

Postmitotic ‘isodiametric’ cell growth in the maize root apex

The onset of rapid cell elongation occurred at different distances from the apex in various tissues of the primary root of maize (Zea mays L.). Furthermore, the comparison of these distances with those determined for the cessation of mitotic divisions revealed a considerable discrepancy. The onset of rapid cell elongation was realized much farther from the root apex than the cessation of cell divisions and therefore a distinct region could be distinguished in every examined maize root tissue. This region was denoted the region of postmitotic isodiametric cell growth. Cells in this region grew in width as well as in length and obtained approximately a square-isodiametric shape. They were also characterized, as are cells in the meristem, by intense nucleic-acid metabolism. This prominent postmitotic isodiametric cell growth was observed in both polyploid and diploid tissues, and indicates that postmitotic isodiametric cell growth, like mitotic division and cell elongation growth, represents an important developmental stage in plant cell ontogeny.The authors dedicate this paper to Dr. M. Luxová on the occasion of her 65th birthday     

Spatial distribution of growth rates and of epidermal cell lengths in the elongation zone during leaf development in Lolium perenne L.

Relative elemental growth rates (REGR) and lengths of epidermal cells along the elongation zone of Lolium perenne L. leaves were determined at four developmental stages ranging from shortly after emergence of the leaf tip to shortly before cessation of leaf growth. Plants were grown at constant light and temperature. At all developmental stages the length of epidermal cells in the elongation zone of both the blade and sheath increased from 12 m at the leaf base to about 550 m at the distal end of the elongation zone, whereas the length of epidermal cells within the joint region only increased from 12 to 40 m. Throughout the developmental stages elongation was confined to the basal 20 to 30 mm of the leaf with maximum REGR occurring near the center of the elongation zone. Leaf elongation rate (LER) and the spatial distributions of REGR and epidermal cell lengths were steady to a first approximation between emergence of the leaf tip and transition from blade to sheath growth. Elongation of epidermal cells in the sheath started immediately after the onset of elongation of the most proximal blade epidermal cells. During transition from blade to sheath growth the length of the blade and sheath portion of the elongation zone decreased and increased, respectively, with the total length of the elongation zone and the spatial distribution of REGR staying near constant, with exception of the joint region which elongated little during displacement through the elongation zone. Leaf elongation rate decreased rapidly during the phase when only the sheath was growing. This was associated with decreasing REGR and only a small decrease in the length of the elongation zone. Data on the spatial distributions of growth rates and of epidermal cell lengths during blade elongation were used to derive the temporal pattern of epidermal cell elongation. These data demonstrate that the elongation rate of an epidermal cell increased for days and that cessation of epidermal cell elongation was an abrupt event with cell elongation rate declining from maximum to zero within less than 10 h.Abbreviations LER leaf elongation rate - REGR relative elemental growth rates     

Colchicine-induced expansion ofVaucheria cell apex. Alteration from isotropic to transversally anisotropic growth

The tip-growth ofVaucheria geminata was analyzed. The elemental rate of surface expansion (RERE) at the very apex of this alga cell reaches ca. 100% min⁻¹. The expansion is almost isotropic; i.e. the both meridional and latitudinal components of RERE are almost equal. An antimicrotubular reagent, colchicine, caused expansion at the actively growing cell apex of this alga. This drug did not change the surface expansion rate, but altered the polarity of cell wall expansion from isotropic to transversally anisotropic. The orientation of cell wall microfibrils is random at the apex but axial at the basal cylindrical part of the cell. Colchicine did not change the fluence-response relationship for the first positive phototropism.     

Gravitropism in Phycomyces: a role for sedimenting protein crystals and floating lipid globules

To elucidate the graviperception of the unicellular fungus, Phycomycesblakesleeanus, sporangiophores were inspected for intracellular structures which relocate with respect to gravity. Two structures, paracrystalline proteins (so-called octahedral crystals) and an aggregate of lipid globules, were identified which showed redistribution upon reorientation of the sporangiophore. Octahedral crystals occur throughout the sporangiophore, including the apical growing zone, and are localized inside vacuoles in which they reside singly or in clusters of up to 40 loosely associated individuals. Upon a 90° reorientation of sporangiophores, crystal clusters sedimented in approximately 50–200 s from the upper to the lower side, corresponding to a speed of 0.5–2 μm s⁻¹. Stage-4 sporangiophores (with sporangium) of three mutants which lack the crystals displayed anormal kinetics of gravitropism and substantially reduced bending angles in comparison to sporangiophores of the wild type. While horizontally placed wild-type sporangiophores reached the vertical position after 10–12 h, the crystal-lacking mutants bent maximally 40°–50° upward. In stage-1 sporangiophores a conspicuous aggregate of lipid globules is positioned about 50 μm below the apex. The globules floated upwards when the sporangiophore was placed horizontally forming in this way a cap-like aggregate. It is proposed that both the sedimenting protein crystals and the upward-floating globules are involved in gravisensing. Received: 23 March 1999 / Accepted: 27 May 1999     

Possible Involvement of Cytokinin in Nitrate-mediated Root Growth in Maize

Response of root system architecture to nutrient availability in soils is an essential way for plants to adapt to soil environments. Nitrate can affect root development either as a result of changes in the external concentration, or through changes in the internal nutrient status of the plant. Nevertheless, less is known about the physiological mechanisms. In the present study, two maize (Zea mays L.) inbred lines (478 and Wu312) were used to study a possible role of cytokinin in nitrate-mediated root growth in nutrient solutions. Root elongation of 478 was more sensitive to high nitrate supply than that of Wu312. Medium high nitrate (5 mM) inhibited root elongation in 478, while, root elongation in Wu312 was only inhibited at high NO ₃ ⁻ supply (20 mM). Under high nitrate supply, the root elongation zone in 478 became swollen and the site of lateral root elongation was close towards the root tip. Both of the phenomena are typical of root growth induced by exogenous cytokinin treatments. Correspondingly, zeatin and zeatin nucleotide (Z + ZR) concentrations were increased at higher nitrate supply in 478, whereas they were constant in Wu312. Furthermore, exogenous cytokinin 6-benzylaminopurine (6-BA) completely reversed the stimulatory effect of low nitrate on root elongation. Therefore, it is supposed that the inhibitory effect of high concentration of nitrate on root elongation is, at least in part, mediated by increased cytokinin level in roots. High nitrate supply may have negative influences on root apex activity by affecting cytokinin metabolism so that root apical dominance is weakened and, therefore, root elongation is suppressed and lateral roots grow closer to the root apex. Nitrate suppressed lateral root elongation in Wu312 at concentration higher than 5 mM. In 478, however, this phenomenon was not significant even at 20 mM nitrate. Although exogenous 6-BA (20 nM) could suppress lateral root elongation as well, the inhibitory effect of high NO ₃ ⁻ concentration of nitrate on lateral root growth cannot be explained by changes in endogenous cytokinin alone.