Complex physiological and molecular processes underlying root gravitropism

Gravitropism allows plant organs to guide their growth in relation to the gravity vector. For most roots, this response to gravity allows downward growth into soil where water and nutrients are available for plant growth and development. The primary site for gravity sensing in roots includes the root cap and appears to involve the sedimentation of amyloplasts within the columella cells. This process triggers a signal transduction pathway that promotes both an acidification of the wall around the columella cells, an alkalinization of the columella cytoplasm, and the development of a lateral polarity across the root cap that allows for the establishment of a lateral auxin gradient. This gradient is then transmitted to the elongation zones where it triggers a differential cellular elongation on opposite flanks of the central elongation zone, responsible for part of the gravitropic curvature. Recent findings also suggest the involvement of a secondary site/mechanism of gravity sensing for gravitropism in roots, and the possibility that the early phases of graviresponse, which involve differential elongation on opposite flanks of the distal elongation zone, might be independent of this auxin gradient. This review discusses our current understanding of the molecular and physiological mechanisms underlying these various phases of the gravitropic response in roots.     

铵减弱拟南芥主根向地性及其相关作用途径

向地性是决定植物根系空间构型的主要因素之一,对植物锚定和水分养分吸收至关重要。除了重力,根系向地性还受土壤环境因子影响。本文采用琼脂培养方法,研究了铵对拟南芥主根向地性反应的影响及相关作用途径。结果表明：短期内,不同浓度（NH4）2SO4均显著抑制主根向地性弯曲,但随着时间的延长,根尖向地性角度逐渐变小。而等（NH4）2SO4浓度的NaCl对主根向地性抑制效应较小,不同浓度的甘露醇不阻碍主根向地性弯曲。纽织化学染色结果显示铵处理12h以内,Col-0根尖没有淀粉体的快速降解过程,并且铵对淀粉体缺失突变体pgm—1主根向地性的影响同Col-0相似。铵处理部分恢复生长素转运载体突变体auxl-22和eir1-1主根向地性缺失。这些结果表明,铵对拟南芥主根向地性的影响独立于根尖淀粉体参与的重力感应途径。     

Cytokinin: perception,signal transduction,and role in plant growth and development

Cytokinins are essential hormones for the proper growth and development of plants. They exert their actions through the phosphorylation of two-component signaling factors. The two-component elements in cytokinin signaling display not only overlapping, but also specific functions throughout a life cycle. These elements regulate the development of shoots, roots, and inflorescence meristems inArabidopsis; shoot meristems in rice; and nodule formation in the lotus. They are also involved in interactions between plants and pathogens. In this review, we examine the mechanism for signaling events initiated by cytokinins inArabidopsis.     

Site of graviperception in roots: a re-examination

Two lines of evidence have been cited to support the assertion that the root cap is the sole site of graviperception in the root. The first evidence is based on surgical removal of the cap, which abolishes the response to gravity. This is sufficient to conclude that the cap is involved in gravitropism, but not to conclude that the cap is the site of graviperception. The second is based on the results of centrifugation experiments, in which different parts of the plant are subjected to different centrifugal forces. The data from such experiments have been cited to support the conclusion that the perception of gravity is limited to the rootcap. However, these data actually support the conclusion that gravity is perceived throughout the root tip, and not only in the root cap. We believe that the data support the conclusion that the root cap is involved in root gravitropism, but that there is inadequate evidence to conclude that the cap is the sole site of graviperception.     

Touch modulates gravity sensing to regulate the growth of primary roots of Arabidopsis thaliana

Plants must sense and respond to diverse stimuli to optimize the architecture of their root system for water and nutrient scavenging and anchorage. We have therefore analyzed how information from two of these stimuli, touch and gravity, are integrated to direct root growth. In Arabidopsis thaliana, touch stimulation provided by a glass barrier placed across the direction of growth caused the root to form a step-like growth habit with bends forming in the central and later the distal elongation zones. This response led to the main root axis growing parallel to, but not touching the obstacle, whilst the root cap maintained contact with the barrier. Removal of the graviperceptive columella cells of the root cap using laser ablation reduced the bending response of the distal elongation zone. Similarly, although the roots of the gravisensing impaired pgm1-1 mutant grew along the barrier at the same average angle as wild-type, this angle became more variable with time. These observations imply a constant gravitropic re-setting of the root tip response to touch stimulation from the barrier. In wild-type plants, transient touch stimulation of root cap cells, but not other regions of the root, inhibited both subsequent gravitropic growth and amyloplast sedimentation in the columella. Taken together, these results suggest that the cells of the root cap sense touch stimuli and their subsequent signaling acts on the columella cells to modulate their graviresponse. This interaction of touch and gravity signaling would then direct root growth to avoid obstacles in the soil while generally maintaining downward growth.     

Microsurgical removal of epidermal and cortical cells: evidence that the gravitropic signal moves through the outer cell layers in primary roots of maize

There is general agreement that during root gravitropism some sort of growth-modifying signal moves from the cap to the elongation zone and that this signal ultimately induces the curvature that leads to reorientation of the root. However, there is disagreement regarding both the nature of the signal and the pathway of its movement from the root cap to the elongation zone. We examined the pathway of movement by testing gravitropism in primary roots of maize (Zea mays L.) from which narrow (0.5 mm) rings of epidermal and cortical tissue were surgically removed from various positions within the elongation zone. When roots were girdled in the apical part of the elongation zone gravitropic curvature occurred apical to the girdle but not basal to the girdle. Filling the girdle with agar allowed curvature basal to the girdle to occur. Shallow girdles, in which only two or three cell layers (epidermis plus one or two cortical cell layers) were removed, prevented or greatly delayed gravitropic curvature basal to the girdle. The results indicate that the gravitropic signal moves basipetally through the outermost cell layers, perhaps through the epidermis itself.     

Transport of [3H]IAA label in gravistimulated primary roots of maize

The curvature of roots in response to gravity is attributed to the development of a differential concentration gradient of IAA in the top and bottom of the elongation region of roots. The development of the IAA gradient has been attributed to the redistribution of IAA from the stele to cortical tissues in the elongation region. The gravistimulated redistribution of IAA was investigated by applying [³H]IAA to the cut surface of 5 mm apical primary root segments. The movement of label from the stele-associated [³H]IAA into the root, tip, root cap, and cortical tissues on the top and bottom of the elongation region was determined in vertically growing roots and gravistimulated roots. Label from the stele moved into the region of cell differentiation (root tip) prior to accumulating in the elongation region. Little label was observed in the root cap. Gravistimulation did not increase the amount of label moving from the stele; but gravistimulation did increase the amount of label accumulating in cortical tissues on the lower side of the elongation region, and decreased the amount of label accumulating in cortical tissues on the upper side of the elongation region. Removal of the cap prior to or immediately following gravity stimulation rendered the roots partially insensitive to gravity and also prevented gravity-induced asymmetric redistribution of label. However, removal of the root cap following 30 min of gravistimulation did not alter root curvature or the establishment of an IAA asymmetry across the region of root elongation. These results suggest that a signal originating in the root cap directs auxin redistribution in tissues behind the root cap, leading to the development of an asymmetry of IAA concentration in the elongation region that in turn causes the differential growth rate in the elongation region of a graviresponding root.     

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.     

Calcium-dependent asymmetric movement of 3H-indole-3-acetic acid across gravistimulated isolated root caps of maize

There is evidence that the cap is the initial site of lateral auxin redistribution during the gravitropic response of roots. We tested this further by comparing asymmetric auxin redistribution across the tips of gravistimulated intact roots, decapped roots, isolated root caps and isolated apical sections taken from decapped roots. Gravistimulation caused asymmetric (downward) auxin movement across the tips of intact roots and isolated root caps but not across the tips of decapped roots or across isolated apical root segments. Naphthylphthalamic acid and pyrenoylbenzoic acid, inhibitors of polar auxin transport, inhibited asymmetric auxin redistribution across gravistimulated isolated root caps and across the tips of gravistimulated intact roots. For intact roots there was a positive correlation between the extent of inhibition of assymmetric auxin redistribution by polar auxin transport inhibitors and the extent of inhibition of asymmetric calcium chelating agent, ethylene glycol-bis(-aminoethyl ether)-N,N,N,N-tetraacetic acid, also caused parallel inhibition of asymmetric auxin redistribution and gravitropic curvature and this effect was reversed by subsequent treatment with calcium. The results support the hypothesis that the cap is a site of early development of auxin asymmetry in gravistimulated roots and that calcium plays an important role in the development of lateral auxin redistribution.     

The Control of Lateral Root Development in Cultured Pea Seedlings. III. Spacing Intervals

The spacing of lateral root primordia in the primary root of Pisum sativum (cv. Alaska) seedlings is influenced by both predetermined lateral root initiation sites in the embryonic radicle and by factors present during seedling growth. When pea seeds were germinated in the presence of the mitotic inhibitor, colchicine, the triarch radicle produced three ranks of primordiomorphs indicating sites of embryonic lateral root primordia. The number of primordiomorphs was not the same along the three xylem strands in the radicle. Normally germinated seedling roots (5 days old) also showed a different number of lateral root primordia associated with the three strands. In both cases, the strand with the greatest number of primordia (or primordiomorphs) was associated with a cotyledonary trace. This indicated a possible role for the cotyledons in setting the pattern of lateral root distribution during radicle development. The spacing of lateral root primordia could be altered by the application of growth regulators. Seedling root tips (2 mm) were removed (? rt) and replaced with indoleacetic acid (+IAA), and in some instances seedlings were also treated with the auxin transport inhibitor, 3,3a-dihydro-2-(p-methoxyphenyl)-8H-pyrazolo[5, 1-α]isoindol-8-one (+DPX). In the growth regulator treatments, primary root elongation was inhibited, a greater number of lateral root primordia were initiated compared to controls, and the spacing intervals between primordia were greatly reduced. The — rt, +IAA, +DPX-treatment resulted in the closest possible spacing intervals (av. 0.4 ? 0.6 mm), but resulted in fused or fasciated laterals. The — rt, + IAA-treatment produced the shortest spacing intervals which resulted in “normal” lateral roots (0.8 ? 1.1 mm).     

Secondary messengers and phospholipase A2 in auxin signal transduction

Despite recent progress auxin signal transduction remains largely scetchy and enigmatic. A good body of evidence supports the notion that the ABP1 could be a functional receptor or part of a receptor, respectively, but this is not generally accepted. Evidence for other functional receptors is lacking, as is any clearcut evidence for a function of G proteins. Protons may serve as second messengers in guard cells but the existing evidence for a role of calcium remains to be clearified. Phospholipases C and D seem not to have a function in auxin signal transduction whereas the indications for a role of phospholipase A₂ in auxin signal transduction accumulated recently. Mitogen-activated protein kinase (MAPK) is modulated by auxin and the protein kinase PINOID has a role in auxin transport modulation even though their functional linkage to other signalling molecules is ill-defined. It is hypothesized that signal transduction precedes activation of early genes such as IAA genes and that ubiquitination and the proteasome are a mechanism to integrate signal duration and signal strength in plants and act as major regulators of hormone sensitivity.     

Characterization of changes of pain behavior and signal transduction system in food-deprived mice

Fasting in general causes several metabolic changes. In the present study, we examined the possible changes of several types of nociception during the food deprivation were investigated in mice. After the mice were forced into the fasting for 12, 24, or 48?h, the changes of nociception were measured by the tail-flick, writhing, formalin or von-frey tests. We found that the nociceptive behavior induced by intraperitoneally (i.p.) administered acetic acid (writhing response) or intraplantar injection of 5% formalin into the hind-paw were reduced in fasted group. In addition, the tail-flick response and threshold for nociception in mechanical von-frey test were also elevated in fasted group. Moreover, the p-CREB and p-ERK levels in the dorsal root ganglia (DRG) and the spinal cord were reduced in food-deprived group. Furthermore, p-AMPKα₁ expressions in DRG and the spinal cord were up-regulated, whereas p-mTOR in DRG and the spinal cord was down-regulated in food-deprived group. Our results suggest that the chemical, mechanical, and thermal nociceptions appear to be reduced in a food-deprived mouse group. Additionally, reduction of nociception in food-deprived group appears to be closely associated with the expressions of several signal transduction molecules such as ERK, CREB, AMPKα₁ and mTOR proteins in DRG and the spinal cord.     

Novel software for analysis of root gravitropism: comparative response patterns of Arabidopsis wild-type and axr1 seedlings

In an earlier study (Evans, Ishikawa & Estelle 1994, Planta 194, 215-222) we used a video digitizer system to compare the kinetics of auxin action on root elongation in wild-type seedlings and seedlings of auxin response mutants of Arabidopsis thaliana (L.) Heynh. We have since modified the system software to allow determination of elongation on opposite sides of vertical or gravistimulated roots and to allow continuous measurement of the angle of orientation of sequential subsections of the root during the response. We used this technology to compare the patterns of differential growth that generate curvature in roots of the Columbia ecotype and in the mutants axr1-3, axr1-12 and axr2, which show reduced gravitropic responsiveness and reduced sensitivity to inhibition by auxin. The pattern of differential growth during gravitropism differed in roots of wild-type and axr1 seedlings. In wild-type roots, initial curvature resulted from differential inhibition of elongation in the distal elongation zone (DEZ). This was followed by an acceleration of elongation along the top side of the DEZ. In roots of axr1-3, curvature resulted from differential stimulation of elongation whereas in roots of axr1-12 the response was variable. Roots of axr2 did not exhibit gravitropic curvature. The observation that the pattern of differential growth causing curvature is dramatically altered by a change in sensitivity to auxin is consistent with the classical Cholodny-Went theory of gravitropism which maintains that differential growth patterns induced by gravistimulation are mediated primarily by gravi-induced shifts in auxin distribution. The new technology introduced with this report allows automated determination of stimulus response patterns in the small but experimentally popular roots of Arabidopsis.     

Plant-derived smoke solutions stimulate the growth of Lycopersicon esculentum roots in vitro

Aqueous extracts of smoke, derived from Themeda triandra, a fire-climax grass, and Passerina vulgaris, a fynbos plant, stimulated the growth of primary root sections of tomato roots in suspension culture. The optimal dilution for both extracts was 1:2000. Several of the fractions obtained from TLC separation of the Themeda and the Passerina extracts significantly promoted primary root growth. The auxins naphthaleneacetic acid (NAA), indolebutyric acid (IBA) and indoleacetic acid (IAA) were found to stimulate the growth of the primary root axis, with IAA and NAA significantly promoting lateral root number. Similarly, the naturally occurring cytokinins, zeatin and its derivatives (zeatin-O-glucoside; dihydrozeatin and zeatin riboside) stimulated primary root length. Zeatin and dihydrozeatin promoted secondary root growth, but only at very low concentrations.     

Abscisic acid,xanthoxin and violaxanthin in the caps of gravistimulated maize roots

The occurrence and distribution of abscisic acid (ABA), xanthoxin (Xa) and the carotenoid violaxanthin (Va) were investigated in root tips of maize (Zea mays L. cv. Merit). In roots grown in the dark, Va and ABA were present in relatively high amounts in the root cap and in low amounts in the adjacent terminal 1.5 mm of the root. Xanthoxin was present in equal concentrations in both regions. In roots exposed to light, the ABA distribution was reversed, with relatively low levels in the root cap and high levels in the adjacent 1.5-mm segment. Light also caused a decrease in Va in both regions of the root and an increase in Xa, especially in the cap. In the maize cultivar used for this work, light is necessary for gravitropic curving. This response occurs within the same time frame as the light-induced ABA redistribution as well as the changes in the levels of Va and Xa. These data are consistent with a role for ABA in root gravitropism and support the proposal that Xa may arise from the turnover of Va.Abbreviations ABA abscisic acid - GC gas chromatography - HPLC high-performance liquid chromatography - GC-MS gas chromatography-mass spectroscopy - V_a violaxanthin - Xa xanthoxin     

Gravity perception in decapped roots of Zea mays

Time-lapse photography and light microscopy were used to determine whether or not sedimentation of the newly developed amyloplasts in the apex of Zea mays L. roots occurred at the time when geotropic responsiveness reappears following removal of the cap. All decapped roots exhibiting a geotropic response had some amyloplast sedimentation in the apical cortical cells. Exposing decapped roots to a centrifugal acceleration of 25 g for 4 h showed that amyloplasts of a similar size and development were not displaced within the cytoplasm when this treatment began 12 h after decapping, whereas displacement did occur when the treatment began 24 h after decapping. This finding indicates the occurrence of a change in the physical characteristics of the cytoplasm between 12 h and 24 h after removing of the cap, which allows amyloplast movement and thus restores gravity perception.     

Differential expression profiles of growth-related genes in the elongation zone of maize primary roots

Growth in the apical elongation zone of plant roots is central to the development of functional root systems. Rates of root segmental elongation change from accelerating to decelerating as cell development proceeds from newly formed to fully elongated status. One of the primary variables regulating these changes in elongation rates is the extensibility of the elongating cell walls. To help decipher the complex molecular mechanisms involved in spatially variable root growth, we performed a gene identification study along primary root tips of maize (Zea mays) seedlings using suppression subtractive hybridization (SSH) and candidate gene approaches. Using SSH we isolated 150 non-redundant cDNA clones representing root growth-related genes (RGGs) that were preferentially expressed in the elongation zone. Differential expression patterns were revealed by Northern blot analysis for 41 of the identified genes and several candidate genes. Many of the genes have not been previously reported to be involved in root growth processes in maize. Genes were classified into groups based on the predicted function of the encoded proteins: cell wall metabolism, cytoskeleton, general metabolism, signaling and unknown. In-situ hybridization performed for two selected genes, confirmed the spatial distribution of expression shown by Northern blots and revealed subtle differences in tissue localization. Interestingly, spatial profiles of expression for some cell wall related genes appeared to correlate with the profile of accelerating root elongation and changed appropriately under growth-inhibitory water deficit.     

Dynamics in PIN2 auxin carrier ubiquitylation in gravity-responding Arabidopsis roots

Plant tropisms are decisively influenced by dynamic adjustments in spatiotemporal distribution of the growth regulators auxin. Polar auxin transport requires activity of PIN-type auxin carrier proteins, with their distribution at the plasma membrane significantly contributing to the directionality of auxin flow. Control of PIN protein distribution involves regulation of their endocytosis and further sorting into the lytic vacuole for degradation and recently, protein ubiquitylation has been demonstrated to control degradative sorting of plasma membrane proteins in plants.1-6 Here we show dynamic adjustments in PIN2 ubiquitylation in gravity-stimulated roots, a response that coincides with establishment of a lateral PIN2 expression gradient. Our results imply that perception and transduction of gravity signals triggers differential ubiquitylation of PIN2, which might feed back on the coordination of auxin distribution in root meristems.     

油菜素内脂信号转导研究进展（综述）

介绍油菜素内酯（BR）信号转导研究的分子生物学方法及其应用,以及BR调节基因表达的机理;综述BR信号转导机制的研究进展。