Simulation of Growth and Hydrotropism of Maize Roots

A three-dimensional model simulating the formation of root system architecture of maize was designed using object oriented programming (OOP) techniques. The model has been used to simulate the growth of roots in contrasting water profiles with or without gravitropism, and the mechanism of hydrotropism of root system and its relationship with gravitropism has been studied. In this model, the frontier of root system was treated as a population of root tips, each member of which responded individually to its local environment, and only a few of them could branch. The results of simulation showed that hydrotropism of maize roots could arise through the control of the elongation rate of single root by its local soil water potential. The difference in growth rate caused by the gradient of water potential along the soil profile alone could cause the root system as a whole to grow predominantly downwards, resulting in a shift of root distribution towards deeper layers. Gravitropism enhanced the downward predominance of the growth of root system, but the mechanism was different from that of hydrotropism.     

Hydrotropism: the current state of our knowledge

The response of roots to a moisture gradient has been reexamined, and positive hydrotropism has been demonstrated in recent years. Agravitropic roots of a pea mutant have contributed to the studies on hydrotropism. The kinetics of hydrotropic curvature, interactions between hydrotropism and gravitropism, moisture gradients required for the induction of hydrotropism, the sensing site for moisture gradients, characteristics of hydrotropic signal and differential growth, and calcium involvement in signal transduction have been subjects of these studies. This review summarizes the current state of our knowledge on hydrotropism in roots.     

Hydrotropism in roots: sensing of a gradient in water potential by the root cap

Roots of the agravitropic pea (Pisum sativum L.) mutant, ageotropum, responded to a gradient in water potential as small as 0.5 MPa by growing toward the higher water potential. This positive response occurred when a sorbitol-containing agar block was unilaterally applied to the root cap but not when applied to the elongation region. Unilateral application of higher concentrations of sorbitol to the elongation region caused root curvature toward the sorbitol source, presumably because of growth reduction on the water-stressed side. The control blocks of plain agar applied to either the root cap or the elongation region did not cause significant curvature of the roots. These results demonstrate that hydrotropism in roots occurs following perception of a gradient in water potential by the root cap.     

Induction of hydrotropism in clinorotated seedling roots of Alaska pea,Pisum sativum L.

Roots of the agravitropic pea (Pisum sativum L.) mutantageotropum show positive hydrotropism, whereas roots of Alaska peas are hydrotropically almost non-responsive. When the gravitropic response was nullified by rotation on clinostats, however, roots of Alaska peas showed unequivocal positive hydrotropism in response to a water potential gradlent. These results suggest that roots of Alaska peas possess normal ability to respond hydrotropically and their weak hydrotropic response results from a counteracting effect of gravitropism.     

Effect of Hydrotropism on Root System Development in Soybean ( Glycine max): Growth Experiments and a Model Simulation

To observe root system development, soybean plants (Glycine max) were grown in root boxes that were set horizontally to reduce the effect of gravity. Along with the root system development, the two-dimensional distribution of soil water content in the root boxes was measured continuously by the time domain reflectometry (TDR) method. Root system development and its morphological architecture were strongly affected by the positions of the water supply. It is suggested that root hydrotropism plays the dominant role in root system development. In addition to root hydrotropism, the importance of root compensatory growth is suggested. A combined model of root system development and soil water flow considering root hydrotropism and compensatory growth was used to simulate root system development and soil water flow. The morphological architecture of root systems and the distribution of soil water content obtained in the experiment were successfully explained by the model simulation. These results confirmed that root hydrotropism and compensatory growth are dominant factors in root system development under a reduced effect of gravity. The validity of the model was confirmed, and its applications for various purposes were suggested.     

The Root Cap: Cell Dynamics, Cell Differentiation and Cap Function

The root cap is a universal feature of angiosperm, gymnosperm, and pteridophyte roots. Besides providing protection against abrasive damage to the root tip, the root cap is also involved in the simultaneous perception of a number of signals – pressure, moisture, gravity, and perhaps others – that modulate growth in the main body of the root. These signals, which originate in the external environment, are transduced by the cap and are then transported from the cap to the root. Root gravitropism is one much studied response to an external signal. In the present paper, consideration is given to the structure of the root cap and, in particular, to how the meristematic initial cells of both the central cap columella and the lateral portion of the cap which surrounds the columella are organized in relation to the production of new cells. The subsequent differentiation and development of these cells is associated with their displacement through the cap and their eventual release, as border cells, from the cap periphery. Mutations, particularly in Arabidopsis, are increasingly playing a part in defining not only the pattern of genetic activity within different cells of the cap but also in revealing how the corresponding wild-type proteins relate to the range of functions of the cap. Notable in this respect have been analyses of the early events of root gravitropism. The ability to image auxin and auxin permeases within the cap and elsewhere in the root has also extended our understanding of this growth response. Images of auxin distribution may, in addition, help extend ideas concerning the positional controls of cell division and cell differentiation within the cap. However, firm information relating to these controls is scarce, though there are intriguing suggestions of some kind of physiological link between the border cells surrounding the cap and mitotic activity in the cap meristem. Open questions concern the structure and functional interrelationships between the root and the cap which surmounts it, and also the means by which the cap transduces the environmental signals that are of critical importance for the growth of the individual roots, and collectively for the shaping of the root system. Current address (Peter W. Barlow): School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK     

Several aspects of current research into the role of calcium in plant physiology

Calcium plays a variety of significant roles in the life cycle of plants. This review describes a brief summary of several examples of such roles in an attempt to provide some common ground relevant to the roles of calcium, with emphasis on the coupling between various stimuli and their respective responses. The selected topics include the regulation of turgor pressure, tropic responses, the cell cycle, and cell motility.     

Gravitropism of the primary root of maize: a complex pattern of differential cellular growth in the cortex independent of the microtubular cytoskeleton

The spatio-temporal sequence of cellular growth within the post-mitotic inner and outer cortical tissue of the apex of the primary root of maize (Zea mays L.) was investigated during its orthogravitropic response. In the early phase (0–30 min) of the graviresponse there was a strong inhibition of cell lengthening in the outer cortex at the lower side of the root, whereas lengthening was only slightly impaired in the outer cortex at the upper side. Initially, inhibition of differential cell lengthening was less pronounced in the inner cortex indicating that tissue tensions which, in these circumstances, inevitably develop at the outer-inner cortex interface, might help to drive the onset of the root bending. At later stages of the graviresponse (60 min), when a root curvature had already developed, cells of the inner cortex then exhibited a prominent cell length differential between upper and lower sides, whereas the outer cortex cells had re-established similar lengths. Again, tissue tensions associated with the different patterns of cellular behaviour in the inner and outer cortex tissues, could be of relevance in terminating the root bending. The perception of gravity and the complex tissue-specific growth responses both proceeded normally in roots which were rendered devoid of microtubules by colchicine and oryzalin treatments. The lack of involvement of microtubules in the graviresponse was supported by several other lines of evidence. For instance, although taxol stabilized the cortical microtubules and prevented their re-orientation in post-mitotic cortical cells located at the lower side of gravistimulated roots, root bending developed normally. In contrast, when gravistimulated roots were physically prevented from bending, re-oriented arrays of cortical microtubules were seen in all post-mitotic cortical cells, irrespective of their position within the root.Abbreviations CMTs cortical microtubules - CW Cholodny-Went - FF form factor - MT microtubule The research was supported by a fellowship from the Alexander von Humboldt Stiftung (Bonn, Germany) to F.B. Financial support to AGRAVIS by Deutsche Agentur für Raumfahrtangelegenheiten (DARA, Bonn) and Ministerium für Wissenschaft und Forschung (Düsseldorf) is gratefully acknowledged. IACR receives grant-aided support from the Biotechnology and Biological Sciences Research Council of the United Kingdom.     

The induction of anhydrobiosis in the sleeping chironomid: current status of our knowledge

An African chironomid, Polypedilum vanderplanki, is the only insect known to be capable of extreme desiccation tolerance, or anhydrobiosis. In the 1950s and 1960s, Hinton strenuously studied anhydrobiosis in this insect from a physiological standpoint; however, nobody has afterward investigated the phenomenon. In 2000, research on mechanisms underlying anhydrobiosis was resumed due to successful establishment of a rearing system for P. vanderplanki. This review is focused on the latest findings on the physiological and molecular mechanisms underlying the induction of anhydrobiosis in P. vanderplanki. Early experiments demonstrated that the induction of anhydrobiosis was possible in isolated tissues and independent from the control of central nervous system. However, to achieve successful anhydrobiosis, larvae need a slow regime of desiccation, allowing them to synthesize molecules, which will protect cells and tissues against the deleterious effects of dehydration. Trehalose, a nonreducing disaccharide, which accumulates in P. vanderplanki larvae up to 20% of the dry body mass, is thought to replace the water in its tissues. Similarly, highly hydrophilic proteins called the late embryogenesis abundant (LEA) proteins are expressed in huge quantities and act as a molecular shield to protect biological molecules against aggregation and denaturation. This function is shared by heat shock proteins, which are also upregulated during the desiccation process. At the same time, desiccating larvae express various antioxidant molecules and enzymes, to cope with the massive oxidative stress, which is responsible for general damage to membranes, proteins, and DNA in dehydrating cells. Finally, specific water channels, called aquaporins, accelerate dehydration, and trehalose together with LEA proteins forms a glassy matrix, which protects the biological molecules and the structural integrity of larvae in the anhydrobiotic state.     

The genus Anguilla Schrank, 1798: current state of knowledge and questions

The freshwater eels of the genus AnguillaSchrank are widely distributed. They have beenconsidered for a long time to all have acatadromous spawning strategy. However, in arecent work Tsukamoto et al. (2002)considered that catadromy should be seen asfacultative. They all have a long oceaniclarval stage known as the leptophalus stage. Alarge number of studies on their ecology,biology, and physiology exist but littleattention has been focused on their systematicsand species-relationships. Ege (1939)described 19 species and sub-species based onmorphometric data. Castle and Williamson (1974)made A. ancestralis a synonym of A.celebesensis. Morphological characters,however, have limitations as taxonomiccharacters because they overlapped in mostspecies. Biochemical characters, such asmt-DNA, are more informative. Dijkstra andJellyman (1999) found no genetic differencesbetween A. australis australis and A. a. schmidtti, and now 15 species arewidely recognized. Phylogenetic studies(Aoyama and Tsukamoto, 1997) suggestdescendant-ancestor relationships betweenAtlantic and Pacific eels, respectively. Themost likely dispersal route for the Atlanticeels from the Pacific appears to have beenthrough the Tethys Sea. Two species are foundin the Atlantic: A. anguilla and A.rostrata. They both spawn in the Sargasso Sea,but differ in morphometric and genetic traits,and are considered as two separate species witha relatively recent evolutionary divergence.The purpose of this paper is to presentknowledge and questions about the life history,taxonomy, and evolution of this mysteriousgenus.     

The influence of protein folding on late stages of the secretion of α-amylases from Bacillus subtilis

A derivative of the α-amylase from Bacillus licheniformis (AmyL) engineered to give an active enzyme with increased net positive charge is secreted by Bacillus subtilis with a yield that is significantly lower than that of the native enzyme. This reduction in yield is the result of increased proteolysis during or shortly after translocation through the cytoplasmic membrane. When we compared the overall rate of folding of the engineered derivative (AmyLQS50.5) with that of AmyL it exhibited a greater dependency on Ca²⁺ ions for in vitro folding. When the concentration of Ca²⁺ in the growth medium was increased, so too did the relative yield of AmyLQS50.5. We discuss the importance of secretory protein folding at the membrane/cell wall interface with respect to the yield of native and heterologous proteins from B. subtilis.     

The role of extracellular free-calcium gradients in gravitropic signalling in maize roots

Gravitropism in roots has been proposed to depend on a downward redistribution of calcium across the root cap. However, because of the many calcium-binding sites in the apoplast, redistribution might not result in a physiologically effective change in the apoplasmic calcium activity. To test whether there is such a change, we measured the effect of gravistimulation on the calcium activity of statocyte cell walls with calcium-specific microelectrodes. Such a measurement must be made on a tissue with gravity sensing cells at the surface. To obtain such a tissue, decapped maize roots (Zea mays L. cv. Golden Cross Bantam) were grown for 31 h to regenerate gravitropic sensitivity, but not root caps. The calcium activity in the apoplasm surrounding the gravity-sensing cells could then be measured. The initial pCa was 2.60 ± 0.28 (approx 2.5 mM). The calcium activity on the upper side of the root tip remained constant for 10 min after gravistimulation, then decreased 1.7-fold. On the lower side, after a similar lag the calcium activity increased 1.6-fold. Control roots, which were decapped but measured before recovering gravisensitivity (19 h), showed no change in calcium activity. To test whether this gradient is necessary for gravitropic curvature, we eliminated the calcium activity gradient during gravitropism by applying a mobile calcium-binding site (di-nitro-BAPTA; 1,2-bis(2-amino-5-nitro-phenoxy)ethane-N,N,N,N-tetraacetic acid) to the root cap; this treatment eliminated gravicurvature. A calcium gradient may be formed by proton-induced calcium desorption if there is a proton gradient. Preventing the formation of apoplastic pH gradients, using 10 and 50 mM 2-(N-morpholino)ethanesulfonic acid (Mes) buffer or 10 mM fusicoccin to stimulate proton excretion maximally, did not inhibit curvature; therefore the calcium gradient is not a secondary effect of a proton gradient. We have found a distinct and rapid differential in the apoplasmic calcium activity between the upper and lower sides of gravistimulated maize root tips which is necessary for gravitropism.Abbreviations BAPTA 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid - FC fusicoccin - Mes 2-(N-morpholino)ethanesulfonic acid The authors thank Phyllis Woolwine for drawing Fig. 1, Dr. Sarbjit Virk for assistance with total calcium measurements, Dr. Paul Sampson for statistical advice, and Michael Newton for developing the EM algorithm to analyze the time-series data. This work was supported by NASA grant NAGW-1394 and by a NASA Research Associateship to T.B. through NASA grant NAGW-70.     

The influence of mechanical impedance on the growth of maize roots

Summary Maize roots were grown between 1 mm glass beads on which a pressure of 40 kPa was applied. The roots were supplied with a constant flow of aerated nutrient solution. Compared with roots grown in a nutrient solution, the impeded crown roots showed a reduction in length of about 75%, whereas the diameter was about 50% increased.These changes in root morphology have been attributed to changes in cell wall structure of the cortex cells, which also occur as a result of the influence of ethylene.It is suggested that ethylene acts as an intermediate factor in the effect of mechanical impedance on root growth.     

Control of the rate of cell enlargement: Excision,wall relaxation,and growth-induced water potentials

A new guillotine thermocouple psychrometer was used to make continuous measurements of water potential before and after the excision of elongating and mature regions of darkgrown soybean (Glycine max L. Merr.) stems. Transpiration could not occur, but growth took place during the measurement if the tissue was intact. Tests showed that the instrument measured the average water potential of the sampled tissue and responded rapidly to changes in water potential. By measuring tissue osmotic potential ( _s ), turgor pressure ( _p ) could be calculated. In the intact plant, _s and _p were essentially constant for the entire 22 h measurement, but _s was lower and _p higher in the elongating region than in the mature region. This caused the water potential in the elongating region to be lower than in the mature region. The mature tissue equilibrated with the water potential of the xylem. Therefore, the difference in water potential between mature and elongating tissue represented a difference between the xylem and the elongating region, reflecting a water potential gradient from the xylem to the epidermis that was involved in supplying water for elongation. When mature tissue was excised with the guillotine, _s and _p did not change. However, when elongating tissue was excised, water was absorbed from the xylem, whose water potential decreased. This collapsed the gradient and prevented further water uptake. Tissue _p then decreased rapidly (5 min) by about 0.1 MPa in the elongating tissue. The _p decreased because the cell walls relaxed as extension, caused by _p , continued briefly without water uptake. The _p decreased until the minimum for wall extension (Y) was reached, whereupon elongation ceased. This was followed by a slow further decrease in Y but no additional elongation. In elongating tissue excised with mature tissue attached, there was almost no effect on water potential or _p for several hours. Nevertheless, growth was reduced immediately and continued at a decreasing rate. In this case, the mature tissue supplied water to the elongating tissue and the cell walls did not relax. Based on these measurements, a theory is presented for simultaneously evaluating the effects of water supply and water demand associated with growth. Because wall relaxation measured with the psychrometer provided a new method for determining Y and wall extensibility, all the factors required by the theory could be evaluated for the first time in a single sample. The analysis showed that water uptake and wall extension co-limited elongation in soybean stems under our conditions. This co-limitation explains why elongation responded immediately to a decrease in the water potential of the xylem and why excision with attached mature tissue caused an immediate decrease in growth rate without an immediate change in _p Abbreviations and symbols L tissue conductance for water - m wall extensibility - Y average yield threshold (MPa) - _o water potential of the xylem - _p turgor pressure - _s osmotic potential - _w water potential of the elon gating tissue     

Change in XET activities,cell wall extensibility and hypocotyl elongation of soybean seedlings at low water potential

In dark-grown soybean (Glycine max [L.] Merr.) seedlings, exposing the roots to water-deficient vermiculite (_w=–0.36 MPa) inhibited hypocotyl (stem) elongation. The inhibition was associated with decreased extensibility of the cell walls in the elongation zone. A detailed spatial analysis showed xyloglucan endotransglucosylase (XET; EC 2.4.1.207) activity on the basis of unit cell wall dry weight was decreased in the elongation region after seedlings were transplanted to low _w. The decrease in XET activity was at least partially due to an accumulation of cell wall mass. Since cell number was only slightly altered, wall mass had increased per cell and probably led to increased wall thickness and decreased cell wall extensibility. Alternatively, an increase in cell wall mass may represent a mechanism for regulating enzyme activity in cell walls, XET in this case, and therefore cell wall extensibility. Hypocotyl elongation was partially recovered after seedlings were grown in low-_w vermiculate for about 80 h. The partial recovery of hypocotyl elongation was associated with a partial recovery of cell wall extensibility and an enhancement of XET activity in the hypocotyl elongation zone. Our results indicate XTH proteins may play an important role in regulating cell wall extensibility and thus cell elongation in soybean hypocotyls. Our results also showed an imperfect correlation of spatial elongation and XET activity along the hypocotyls. Other potential functions of XTH and their regulation in soybean hypocotyl growth are discussed.     

Autoreproductive cells and plant meristem construction: The case of the tomato cap meristem

Summary Root apical meristems are composed of two zones in which either formative or proliferative cell divisions occur. Within the formative zone, autoreproductive initial cells (a-cells) occupy distinctive locations. By means of graph-L-systems, the behavior of one such type of a-cells has been investigated, with particular reference to root caps within the developing primordia of lateral roots ofLycopersicon esculentum cultivated in vitro. Here, the a-cells constitute the protoderm initials, cells which are found also in the root cap of many angiosperm species. A set of cuboidal (i.e., six-sided) acells develops early in the ontogeny of a lateral-root primordium. Then, according to both anatomical observations and theoretical simulations obtained by the application of graph-L-systems, sequential production of descendents from each a-cell leads to the formation of a new autoreproductive cell (a), a cap columella initial (c), and two mother cells (e and f) whose respective descendents differentiate as root epidermis and cap flank cells. In this graph-L-system, there is specification of the location of sister cells with respect to the three orthogonal directions of a cuboidal. In the early stage of root cap formation, only a few rounds of these formative cell divisions by each a-cell and its four types of descendents are required to provide the basic set of cells necessary for full cap development. After the lateral root emerges from the parent root, there may be a temporary cessation of the formative divisions of the a-cells which give rise to columella initials. Columella production is then supported entirely by its own independent set of autoreproductive c-initials. At the same time, division of the autoreproductive protoderm initial cell is directed towards maintaining the cap flank and the epidermal cell files. The regulation of the types of formative division by the a-cell may be represented by means of a division counter which may be specific for a given species.Dedicated to Professor Brian E. S. Gunning on the occasion of his 65th birthday     

Adsorption and exchange of Ca,Mg and K-ions on the root cell walls of clover and rye-grass

Summary The ion exchange properties of clover and rye-grass root cell walls were studied quantitatively. Three sets of experiments were performed: adsorption of Ca, Mg and K ions versus pH, adsorption versus cation concentration and exchange isotherms Ca–Mg and Mg–K. A model has been developed. It allows the satisfactory prediction of results (except for pH curves) with the adjustment of a minimum of parameters. The total charge (RT), on its own, accounts for the difference between the ion exchange properties of the clover and rye grass cell walls. The selectivities observed on root material are very different from those observed on soil exchange complexes. The decreasing affinities of cell walls for cations follow the sequence: Ca>MgK for cell walls. These differences of selectivity are much larger than those usually observed for soil exchange complexes.     

The effect of nitrogen on plant water relations in tea (Camellia sinensis)

An experiment was carried out to study the effect of nitrogen deficiency on the water relations of tea (Camellia sinensis). The plants were grown in sand and nitrogen deficiency induced by witholding the supply of nitrogen. Nitrogen deficiency increased stomatal resistance and reduced transpiration. The capacity of the stomata to open fully in the morning was not impaired by nitrogen deficiency. Leaf water potential and probably root resistance were not affected by nitrogen deficiency. The sensitivity of transpiration and stomatal resistance to sand water stress was increased by nitrogen deficiency.     

The importance of colony structure versus shoot morphology for the water balance of 22 subarctic bryophyte species

Questions: What are the water economy strategies of the dominant subarctic bryophytes in terms of colony and shoot traits? Can colony water retention capacity be predicted from morphological traits of both colonies and separate shoots? Are suites of water retention traits consistently related to bryophyte habitat and phylogenetic position? Location: Abisko Research Station, North Sweden. Methods: We screened 22 abundant subarctic bryophyte species from diverse habitats for water economy traits of shoots and colonies, including desiccation rates, water content at field capacity, volume and density (mg cm^?3) of water‐saturated and oven‐dried patches, evaporation rate (g·m^?2·s^?1) and cell wall thickness. The relationships between these traits and shoot and colony desiccation rates were analysed with Spearman rank correlations. Subsequent multivariate (cluster followed by PCA) analyses were based on turf density, turf and shoot desiccation rate, cell wall thickness and amount of external and internal water. Results: Individual shoot properties, i.e. leaf cell wall properties, water retention capacity and desiccation rate, did not correspond with colony water retention capacity. Colony desiccation rate depended on density of water‐saturated colonies, and was marginally significantly negatively correlated with species individual shoot desiccation rate but not related to any other shoot or colony trait. Multivariate analyses based on traits assumed to determine colony desiccation rate revealed six distinct species groups reflecting habitat choice and phylogenetic relationships. Conclusions: General relationships between shoot and colony traits as determinants of water economy will help to predict and upscale changes in hydrological function of bryophyte‐dominated peatlands undergoing climate‐induced shifts in species abundance, and feedbacks of such species shifts on permafrost insulation and carbon sequestration functions.