Gravitropism: interaction of sensitivity modulation and effector redistribution

Homoeologous exchanges contribute to branch angle variations in rapeseed: Insights from transcriptome, QTL-seq and gene functional analysis

Branch angle (BA) is a critical morphological trait that significantly influences planting density, light interception and ultimately yield in plants. Despite its importance, the regulatory mechanism governing BA in rapeseed remains poorly understood. In this study, we generated 109 transcriptome data sets for 37 rapeseed accessions with divergent BA phenotypes. Relative to adaxial branch segments, abaxial segments accumulated higher levels of auxin and exhibited lower expression of six TCP1 homologues and one GA20ox3. A co-expression network analysis identified two modules highly correlated with BA. The modules contained homologues to known BA control genes, such as FUL, YUCCA6, TCP1 and SGR3. Notably, a homoeologous exchange (HE), occurring at the telomeres of A09, was prevalent in large BA accessions, while an A02-C02 HE was common in small BA accessions. In their corresponding regions, these HEs explained the formation of hub gene hotspots in the two modules. QTL-seq analysis confirmed that the presence of a large A07-C06 HE (~8.1 Mb) was also associated with a small BA phenotype, and BnaA07.WRKY40.b within it was predicted as candidate gene. Overexpressing BnaA07.WRKY40.b in rapeseed increased BA by up to 20°, while RNAi- and CRISPR-mediated mutants (BnaA07.WRKY40.b and BnaC06.WRKY40.b) exhibited decreased BA by up to 11.4°. BnaA07.WRKY40.b was exclusively localized to the nucleus and exhibited strong expression correlations with many genes related to gravitropism and plant architecture. Taken together, our study highlights the influence of HEs on rapeseed plant architecture and confirms the role of WRKY40 homologues as novel regulators of BA.

Not so hidden anymore: Advances and challenges in understanding root growth under water deficits

Limited water availability is a major environmental factor constraining plant development and crop yields. One of the prominent adaptations of plants to water deficits is the maintenance of root growth that enables sustained access to soil water. Despite early recognition of the adaptive significance of root growth maintenance under water deficits, progress in understanding has been hampered by the inherent complexity of root systems and their interactions with the soil environment. We highlight selected milestones in the understanding of root growth responses to water deficits, with emphasis on founding studies that have shaped current knowledge and set the stage for further investigation. We revisit the concept of integrated biophysical and metabolic regulation of plant growth and use this framework to review central growth-regulatory processes occurring within root growth zones under water stress at subcellular to organ scales. Key topics include the primary processes of modifications of cell wall-yielding properties and osmotic adjustment, as well as regulatory roles of abscisic acid and its interactions with other hormones. We include consideration of long-recognized responses for which detailed mechanistic understanding has been elusive until recently, for example hydrotropism, and identify gaps in knowledge, ongoing challenges, and opportunities for future research.

Expression of auxin transporter genes in flax (Linum usitatissimum) fibers during gravity response

Gravitropism is an adaptive reaction of plants associated with the ability of various plant organs to be located and to grow in a certain direction relative to the gravity vector, while usually the asymmetric distribution of the phytohormone auxin is a necessary condition for the gravitropical bending of plant organs. Earlier, we described significant morphological changes in phloem fibers with a thickened cell wall located on different sides of the stem in the area of the gravitropic curvature. The present study is the first work devoted to the identification of genes encoding auxin transporters in cells at different stages of development and during gravity response. In this study, the flax genes encoding the AUX1/LAX, PIN-FORMED, PIN-LIKES, and ABCB auxin transporters were identified. A comparative analysis of the expression of these genes in flax phloem fibers at different stages of development revealed increased expression of some of these genes at the stage of intrusive growth (LusLAX2 (A, B), LuxPIN1-D, LusPILS7 (C, D)), at the early stage of tertiary cell wall formation (LusAUX1 (A, D), LusABCB1 (A, B), LusABCB15-A, LusPIN1 (A, B), LusPIN4-A, and LusPIN5-A), and at the late stage of tertiary cell wall development (LusLAX3 (A, B)). It was shown that in the course of gravitropism, the expression of many genes, including those responsible for the influx of auxin in cells (LusAUX1-D), in the studied families increased. Differential expression of auxin transporter genes was revealed during gravity response in fibers located on different sides of the stem (upper (PUL) and lower (OPP)). The difference was observed due to the expression of genes, the products of which are responsible for auxin intracellular transport (LusPILS3, LusPILS7-A) and its efflux (LusABCB15-B, LusABCB19-B). It was noted that the increased expression of PIN genes and ABCB genes was more typical of fibers on the opposite side. The results obtained allow us to make an assumption about the presence of differential auxin content in the fibers of different sides of gravistimulated flax plants, which may be determined by an uneven outflow of auxin. This study gives an idea of auxin carriers in flax and lays the foundation for further studies of their functions in the development of phloem fiber and in gravity response.

Plants in Microgravity: Molecular and Technological Perspectives

Plants are vital components of our ecosystem for a balanced life here on Earth, as a source of both food and oxygen for survival. Recent space exploration has extended the field of plant biology, allowing for future studies on life support farming on distant planets. This exploration will utilize life support technologies for long-term human space flights and settlements. Such longer space missions will depend on the supply of clean air, food, and proper waste management. The ubiquitous force of gravity is known to impact plant growth and development. Despite this, we still have limited knowledge about how plants can sense and adapt to microgravity in space. Thus, the ability of plants to survive in microgravity in space settings becomes an intriguing topic to be investigated in detail. The new knowledge could be applied to provide food for astronaut missions to space and could also teach us more about how plants can adapt to unique environments. Here, we briefly review and discuss the current knowledge about plant gravity-sensing mechanisms and the experimental possibilities to research microgravity-effects on plants either on the Earth or in orbit.

Whole genome duplication and dispersed duplication characterize the evolution of the plant PINOID gene family across plant species

PINOID is a kinase belonging to the AGCVIII family, which regulates the polar distribution of PIN proteins and plays an important role in plant geotropism. However, the origin and evolutionary history of this gene family is not fully known. In this study, we identified 79 similar sequences across 17 plant species genomes (PINOID, D6PK, PINOID2, "hypothetical kinase"). Our results show that the AGCVIII kinase family may have originated from related "Hypothetical Kinases" that come out sister to the rest of the gene family members. These kinases differentiated their functions are found in different plant classes: D6PK in moss and PINOID and PINOID2 evolving in angiosperms including the pioneer plant Amborella trichopoda. Our study investigates the evolution of PINOID kinases from a phylogenetic perspective giving us insight into how this important plant signal transduction network switch evolved to play a fundamental and important function in plant growth and development. We highlight the importance of whole genome duplications and dispersed duplications as opposed to tandem duplications in the evolution of this gene family.

Root architecture simulation improves the inference from seedling root phenotyping towards mature root systems

Root phenotyping provides trait information for plant breeding. A shortcoming of high-throughput root phenotyping is the limitation to seedling plants and failure to make inferences on mature root systems. We suggest root system architecture (RSA) models to predict mature root traits and overcome the inference problem. Sixteen pea genotypes were phenotyped in (i) seedling (Petri dishes) and (ii) mature (sand-filled columns) root phenotyping platforms. The RSA model RootBox was parameterized with seedling traits to simulate the fully developed root systems. Measured and modelled root length, first-order lateral number, and root distribution were compared to determine key traits for model-based prediction. No direct relationship in root traits (tap, lateral length, interbranch distance) was evident between phenotyping systems. RootBox significantly improved the inference over phenotyping platforms. Seedling plant tap and lateral root elongation rates and interbranch distance were sufficient model parameters to predict genotype ranking in total root length with an RSpearman of 0.83. Parameterization including uneven lateral spacing via a scaling function substantially improved the prediction of architectures underlying the differently sized root systems. We conclude that RSA models can solve the inference problem of seedling root phenotyping. RSA models should be included in the phenotyping pipeline to provide reliable information on mature root systems to breeding research.

Effect of modeled microgravity on radiation-induced adaptive response of root growth in Arabidopsis thaliana

Space particles have an inevitable impact on organisms during space missions; radio-adaptive response (RAR) is a critical radiation effect due to both low-dose background and sudden high-dose radiation exposure during solar storms. Although it is relevant to consider RAR within the context of microgravity, another major space environmental factor, there is no existing evidence as to its effects on RAR. In the present study, we established an experimental method for detecting the effects of gamma-irradiation on the primary root growth of Arabidopsis thaliana, in which RAR of root growth was significantly induced by several dose combinations. Microgravity was simulated using a two-dimensional rotation clinostat. It was shown that RAR of root growth was significantly inhibited under the modeled microgravity condition, and was absent in pgm-1 plants that had impaired gravity sensing in root tips. These results suggest that RAR could be modulated in microgravity. Time course analysis showed that microgravity affected either the development of radio-resistance induced by priming irradiation, or the responses of plants to challenging irradiation. After treatment with the modeled microgravity, attenuation in priming irradiation-induced expressions of DNA repair genes (AtKu70 and AtRAD54), and reduced DNA repair efficiency in response to challenging irradiation were observed. In plant roots, the polar transportation of the phytohormone auxin is regulated by gravity, and treatment with an exogenous auxin (indole-3-acetic acid) prevented the induction of RAR of root growth, suggesting that auxin might play a regulatory role in the interaction between microgravity and RAR of root growth.

Pin1At regulates PIN1 polar localization and root gravitropism

Root gravitropism allows plants to establish root systems and its regulation depends on polar auxin transport mediated by PIN-FORMED (PIN) auxin transporters. PINOID (PID) and PROTEIN PHOSPHATASE 2A (PP2A) act antagonistically on reversible phosphorylation of PINs. This regulates polar PIN distribution and auxin transport. Here we show that a peptidyl-prolyl cis/trans isomerase Pin1At regulates root gravitropism. Downregulation of Pin1At suppresses root agravitropic phenotypes of pp2aa and 35S:PID, while overexpression of Pin1At affects root gravitropic responses and enhances the pp2aa agravitropic phenotype. Pin1At also affects auxin transport and polar localization of PIN1 in stele cells, which is mediated by PID and PP2A. Furthermore, Pin1At catalyses the conformational change of the phosphorylated Ser/Thr-Pro motifs of PIN1. Thus, Pin1At mediates the conformational dynamics of PIN1 and affects PID- and PP2A-mediated regulation of PIN1 polar localization, which correlates with the regulation of root gravitropism.

An ABC transporter B family protein, ABCB19, is required for cytoplasmic streaming and gravitropism of the inflorescence stems

A significant feature of plant cells is the extensive motility of organelles and the cytosol, which was originally defined as cytoplasmic streaming. We suggested previously that a three-way interaction between plant-specific motor proteins myosin XIs, actin filaments, and the endoplasmic reticulum (ER) was responsible for cytoplasmic streaming. (1) Currently, however, there are no reports of molecular components for cytoplasmic streaming other than the actin-myosin-cytoskeleton and ER-related proteins. In the present study, we found that elongated cells of inflorescence stems of Arabidopsis thaliana exhibit vigorous cytoplasmic streaming. Statistical analysis showed that the maximal velocity of plastid movements is 7.26 µm/s, which is much faster than the previously reported velocities of organelles. Surprisingly, the maximal velocity of streaming in the inflorescence stem cells was significantly reduced to 1.11 µm/s in an Arabidopsis mutant, abcb19-101, which lacks ATP BINDING CASSETTE SUBFAMILY B19 (ABCB19) that mediates the polar transport of the phytohormone auxin together with PIN-FORMED (PIN) proteins. Polar auxin transport establishes the auxin concentration gradient essential for plant development and tropisms. Deficiency of ABCB19 activity eventually caused enhanced gravitropic responses of the inflorescence stems and abnormally flexed inflorescence stems. These results suggest that ABCB19-mediated auxin transport plays a role not only in tropism regulation, but also in cytoplasmic streaming.

NITRIC OXIDE-ASSOCIATED PROTEIN1 (AtNOA1) is essential for salicylic acid-induced root waving in Arabidopsis thaliana

Root waving responses have been attributed to both environmental and genetics factors, but the potential inducers and transducers of root waving remain elusive. Thus, the identification of novel signal elements related to root waving is an intriguing field of research. Genetic, physiological, cytological, live cell imaging, and pharmacological approaches provide strong evidence for the involvement of Arabidopsis thaliana NITRIC OXIDE-ASSOCIATED PROTEIN1 (AtNOA1) in salicylic acid (SA)-induced root waving. SA specially induced root waving, with an overall decrease in root elongation in A. thaliana, and this SA-induced response was disrupted in the Atnoa1 mutant, as well as in nonexpresser of pathogenesis-related genes 1 (npr1), which is defective in SA-mediated plant defense signal transduction, but not in npr3/4 single and double mutants. The expression assays revealed that the abundance of AtNOA1 was significantly increased by application of SA. Genetic and pharmacological analyses showed that SA-induced root waving involved an AtNOA1-dependent Ca(2+) signal transduction pathway, and PIN-FORMED2 (PIN2) -based polar auxin transport possibly plays a crucial role in this process. Our work suggests that SA signaling through NPR1 and AtNOA1 is involved in the control of root waving, which provides new insights into the mechanisms that control root growth behavior on a hard agar surface.

Strategies of seedlings to overcome their sessile nature: auxin in mobility control

Plants are sessile organisms that are permanently restricted to their site of germination. To compensate for their lack of mobility, plants evolved unique mechanisms enabling them to rapidly react to ever changing environmental conditions and flexibly adapt their postembryonic developmental program. A prominent demonstration of this developmental plasticity is their ability to bend organs in order to reach the position most optimal for growth and utilization of light, nutrients, and other resources. Shortly after germination, dicotyledonous seedlings form a bended structure, the so-called apical hook, to protect the delicate shoot meristem and cotyledons from damage when penetrating through the soil. Upon perception of a light stimulus, the apical hook rapidly opens and the photomorphogenic developmental program is activated. After germination, plant organs are able to align their growth with the light source and adopt the most favorable orientation through bending, in a process named phototropism. On the other hand, when roots and shoots are diverted from their upright orientation, they immediately detect a change in the gravity vector and bend to maintain a vertical growth direction. Noteworthy, despite the diversity of external stimuli perceived by different plant organs, all plant tropic movements share a common mechanistic basis: differential cell growth. In our review, we will discuss the molecular principles underlying various tropic responses with the focus on mechanisms mediating the perception of external signals, transduction cascades and downstream responses that regulate differential cell growth and consequently, organ bending. In particular, we highlight common and specific features of regulatory pathways in control of the bending of organs and a role for the plant hormone auxin as a key regulatory component.

Identification of gravitropic response indicator genes in Arabidopsis inflorescence stems

Differential organ growth during gravitropic response is caused by differential accumulation of auxin, that is, relative higher auxin concentration in lower flanks than in upper flanks of responding organs. Auxin responsive reporter systems such as DR5::GUS and DR5::GFP have usually been used as indicators of gravitropic response in roots and hypocotyls of Arabidopsis. However, in the inflorescence stems, the reporter systems don't work well to monitor gravitropic response. Here, we aim to certify appropriate gravitropic response indicators (GRIs) in inflorescence stems. We performed microarray analysis comparing gene expression profiles between upper and lower flanks of Arabidopsis inflorescence stems after gravistimulation. Thirty genes showed > 2-fold differentially increased expression in lower flanks at 30 min, of which 19 were auxin response genes. We focused on IAA5 and IAA2 and verified whether they are appropriate GRIs by real-time qRT-PCR analyses. Transcript levels of IAA5 and IAA2 were remarkably higher in lower flanks than in upper flanks after gravistimulation. The biased IAA5 or IAA2 expression is disappeared in sgr2-1 mutant which is defective in gravity perception, indicating that gravity perception process is essential for formation of the biased gene expression during gravitropism. IAA5 expression was remarkably increased in lower flanks at 30 min after gravistimulation, whereas IAA2 expression was gradually decreased in upper flanks in a time-dependent manner. Therefore, we conclude that IAA5 is a sensitive GRI to monitor asymmetric auxin signaling caused by gravistimulation in Arabidopsis inflorescence stems.

Gravistimulation changes the accumulation pattern of the CsPIN1 auxin efflux facilitator in the endodermis of the transition zone in cucumber seedlings

Cucumber (Cucumis sativus) seedlings grown in a horizontal position develop a specialized protuberance (or peg) on the lower side of the transition zone between the hypocotyl and the root. This occurs by suppressing peg formation on the upper side via a decrease in auxin resulting from a gravitational response. However, the gravity-stimulated mechanism of inducing asymmetric auxin distribution in the transition zone is poorly understood. The gravity-sensing tissue responsible for regulating auxin distribution in the transition zone is thought to be the endodermal cell. To characterize the gravity-stimulated mechanism, the auxin efflux facilitator PIN-FORMED1 (CsPIN1) in the endodermis was identified and the localization of CsPIN1 proteins during the gravimorphogenesis of cucumber seedlings was examined. Immunohistochemical analysis revealed that the accumulation pattern of CsPIN1 protein in the endodermal cells of the transition zone of cucumber seedlings grown horizontally differed from that of plants grown vertically. Gravistimulation for 30 min prompted changes in the accumulation pattern of CsPIN1 protein in the endodermis as well as the asymmetric distribution of auxin in the transition zone. Furthermore, 2,3,5-triiodobenzoic acid inhibited the differential distribution of auxin as well as changes in the accumulation pattern of CsPIN1 in the endodermis of the transition zone during gravistimulation. These results suggest that the altered pattern of CsPIN1 accumulation in the endodermis in response to gravistimulation influences lateral auxin transport through the endodermis, resulting in asymmetric auxin distribution in the transition zone.

Physiological interactions of antiauxins with auxin in roots

The compound 4,4,4-trifluoro-3-(indole-3-)butyric acid (TFIBA) typically promotes root elongation but inhibits hypocotyl elongation and therefore can be described as antiauxin. We compared the mode of action of TFIBA with the classical antiauxin p-chlorophenoxyisobutyric acid (PCIB). TFIBA, more than PCIB, promoted primary root elongation in young flax (Linum usitatissimum) roots on plain agar, but inhibited root growth in older seedlings in the presence of nutrients. The root content of indole-3-acetic acid (IAA) after TFIBA and PCIB treatment increased almost two-fold. Abscisic acid was affected only by supraoptimal TFIBA, but increased after PCIB application. TFIBA inhibited acropetal auxin transport at concentrations higher than optimal for root elongation while PCIB had no effect. Basipetal auxin transport was promoted at less than 0.1mM but inhibited at 1mM TFIBA. In contrast, PCIB promoted basipetal auxin transport between 0.1 and 0.5mM; higher concentrations had no effect. Gravitropism was promoted by TFIBA at concentrations optimal for growth, but inhibited by higher concentrations. PCIB inhibited root gravitropism in a concentration dependent manner. The selective effect of TFIBA on IAA but not ABA and the interference with auxin transport and gravicurvature indicate that the mode of action of TFIBA is different from that of PCIB despite similar functions.

Role of xyloglucan in gravitropic bending of azuki bean epicotyl

The mechanism of the gravitropic bending was studied in azuki bean epicotyls. The cell wall extensibility of the lower side became higher than that of the upper side in the epicotyl bending upward. The contents of matrix polysaccharides of the cell wall (pectin and xyloglucan in hemicellulose-II) in the lower side became smaller than those in the upper side. The molecular mass of xyloglucans in the lower side decreased. After an epicotyl was fixed to a metal rod to prevent the bending, gravistimulation was applied. Fundamentally the same results were obtained with respect to rheological and chemical characteristics of the cell wall as those of epicotyls showing gravitropic bending. The present results suggested that the initial gravitropic bending was caused by the increase in extensibility of the lower side and the decrease in extensibility of the upper side via the change of the cell wall matrix, especially xyloglucans.

A major factor in gravitropism in radish hypocotyls is the suppression of growth on the upper side of hypocotyls

The changes in length on the two opposite sides of etiolated radish (Raphanus sativus) hypocotyls prior to, and following gravitropic stimulation, were measured using an infrared-imaging system. It was observed that the growth suppression on the upper side began first at least 10 min after the onset of gravitropic stimulation, and after 30 min the acceleration in growth on the lower side started. The gravitropic curvature was steadily induced from 10 min. When radish hypocotyls were switched from a vertical to horizontal position for different durations and then replaced to the vertical position, the growth suppression on the gravity-stimulated (upper) side was observed in all cases, but the acceleration in growth on the opposite (lower) side appeared only in continuously gravity-stimulated seedlings, although it occurred later than the growth suppression on the upper side. These results suggest that the suppression in growth on the upper side of the hypocotyls is a direct effect of gravitropic stimulation, but not the acceleration on the lower side. When 4-methylthio-3-butenyl isothiocyanate (4-MTBI), which has an inhibitory activity against radish hypocotyl growth, was applied on the one side of radish hypocotyls and then the 4-MTBI-applied side or opposite side was placed in a horizontal position, the former showed greater bending than the control, suggesting that the growth suppression on the upper side is enhanced and maintained with MTBI application there. In the latter case, the seedlings showed less bending than the control, suggesting a decrease in growth on the lower side with MTBI application. All the results suggest that gravitropism of radish hypocotyls may be caused by an increase in growth-inhibiting substance(s) induced with gravitropic stimulation in the upper side, inducing growth inhibition there.

Differential accumulation of the mRNA of the auxin-repressed gene CsGRP1 and the auxin-induced peg formation during gravimorphogenesis of cucumber seedlings

When cucumber seeds are germinated horizontally, an outgrowth (peg) develops on the lower side of the transition zone between the hypocotyl and the root for pulling the cotyledons and plumule out of the seed coat. We previously suggested that gravistimulation suppresses peg formation on the upper side of the transition zone when placed in a horizontal position. In the gravistimulated transition zone, auxin and the mRNAs of auxin-inducible genes are more abundant in the lower side than in the upper side. Here, using fluorescent differential display, we identified Cucumis sativus glycine-rich protein1(CsGRP1) as a gene whose mRNA accumulated more abundantly on the upper side than on the lower side of the transition zone in response to gravistimulation. Auxin starvation increased CsGRP1 mRNA in segments of the transition zone, and inhibition of polar auxin transport with 2,3,5-triiodobenzoic acid (TIBA) prevented the asymmetric accumulation of CsGRP1 mRNA. These results suggest that gravistimulation increases not only the expression of auxin-inducible genes on the lower side of the transition zone, but also the expression of auxin-repressed genes, such as CsGRP1, on the upper side of cucumber seedlings. In the hypocotyls of 3-day-old seedlings, neither gravistimulation nor changes in auxin level influenced the accumulation of CsGRP1 mRNA. These results suggest that the transition zone responds to gravistimulation in a specific manner by an asymmetric expression of CsGRP1 gene during regulation of peg formation.

Modelling of auxin transport affected by gravity and differential radial growth

When a tree stem deviates from verticality, as a result of different environmental factors, patterns of differential radial growth appear. Higher rates of wood production have been observed on the lower side of the tree and lower rates in the opposite side. Biological studies on plant hormones have shown that the concentration of auxin induces radial growth. They also have demonstrated the redistribution of auxin transport in response to gravity. Auxin is then designated as a mediator for differential growth. This paper presents a model for three-dimensional (3-D) auxin transport in conifer trees, which includes gravity dependence. We obtain realistic heterogeneous patterns of auxin distribution over the tree. Then, we propose a law of growth based on auxin concentration to simulate successive differential radial growths. The predicted growths are compared with experimental results of reconstruction of 3-D annual growth of Radiata pine.

Microelectrode ion and O2 fluxes measurements reveal differential sensitivity of barley root tissues to hypoxia

Hypoxia-induced changes in net H+, K+ and O2 fluxes across the plasma membrane (PM) of epidermal root cells were measured using the non-invasive microelectrode ion flux measurement (MIFE) system in elongation, meristem and mature root zones of two barley (Hordeum vulgare L.) varieties contrasting in their waterlogging (WL) tolerance. The ultimate goal of this study was to shed light on the mechanisms underlying effects of WL on plant nutrient acquisition and mechanisms of WL tolerance in barley. Our measurements revealed that functionally different barley root zones have rather different O2 requirements, with the highest O2 influx being in the elongation zone of the root at about 1 mm from the tip. Oxygen deprivation has qualitatively different effects on the activity of PM ion transporters in mature and elongation zones. In the mature zone, hypoxic treatment caused a very sharp decline in K+ uptake in the WL sensitive variety Naso Nijo, but did not reduce K+ influx in the WL tolerant TX9425 variety. In the elongation zone, onset of hypoxia enhanced K+ uptake from roots of both cultivars. Pharmacological experiments suggested that hypoxia-induced K+ flux responses are likely to be mediated by both K(+) -inward- (KIR) and non-selective cation channels (NSCC) in the elongation zone, while in the mature zone K(+) -outward- (KOR) channels are the key contributors. Overall, our results suggest that oxygen deprivation has an immediate and substantial effect on root ion flux patterns, and that this effect is different in WL-sensitive and WL-tolerant cultivars. To what extent this difference in ion flux response to hypoxia is a factor conferring WL tolerance in barley remains to be answered in future studies.

Gravitropic bending and plant hormones

Gravitropism is a complex multistep process that redirects the growth of roots and various above-ground organs in response to changes in the direction of the gravity vector. The anatomy and morphology of these graviresponding organs indicates a certain spatial separation between the sensing region and the responding one, a situation that strongly suggests the requirement of phytohormones as mediators to coordinate the process. The Cholodny-Went hypothesis suggested auxin as the main mediator of gravitropism. So far, ample evidence has been gathered with regard to auxin asymmetrical detection, polar and lateral transport involving influx and efflux carriers, response signaling pathway, and possible modes of action in differential cell elongation, supports its major role in gravitropism at least in roots. However, it is becoming clear that the participation of other hormones, acting in concert with auxin, is necessary as well. Of particular importance is the role of ethylene in shoot gravitropism, possibly associated with the modulation of auxin transport or sensitivity, and the key role implicated for cytokinin as the putative root cap inhibitor that controls early root gravitropism. Therefore, the major advances in the understanding of transport and signaling of auxin, ethylene, and cytokinin may shed light on the possibly tight and complicated interactions between them in gravitropism. Not much convincing evidence has been accumulated regarding the participation of other phytohormones, such as gibberellins, abscisic acid, brassinosteroids, jasmonates, and salicylic acid, in gravitropism. However, the emerging concept of cooperative hormone action opens new possibilities for a better understanding of the complex interactions of all phytohormones and their possible synergistic effects and involvement in the gravitropic bending process.

Mechanotransduction in root gravity sensing cells

The analysis of the dose-response curve of the gravitropic reaction of lentil seedling roots has shown that these organs are more sensitive when they have been grown in microgravity than when they have been grown on a 1 g centrifuge in space before gravistimulation. This difference of gravisensitivity is not due to the volume or the density of starch grains of statoliths, which are about the same in both conditions (1 g or microgravity). However, the distribution of statoliths within the statocyte may be responsible for this differential sensitivity, since the dispersion of these organelles is greater in microgravity than in 1 g. When lentil roots grown in microgravity or in 1 g are stimulated at 0.93 g for 22 min, the amyloplasts sediment following two different trajectories. They move from the proximal half of the statocytes toward the lower longitudinal wall in the microgravity grown sample and from the distal half toward the longitudinal wall in the 1 g grown sample. At the end of the stimulation, they reach a similar position within the statocytes. If the roots of both samples are left in microgravity for 3 h, the amyloplasts move toward the cell centre in a direction that makes an average angle of 40 degrees with respect to the lower longitudinal wall. The actin filaments, which are responsible for this movement, may have an overall orientation of 40 degrees with respect to this wall. Thus, when roots grown in microgravity are stimulated on the minicentrifuge the amyloplasts slide on the actin filaments, whereas they move perpendicular to them in 1 g grown roots. Our results suggest that greater sensitivity of seedling roots grown in microgravity should be due to greater dispersion of statoliths, to better contacts between statoliths and the actin network and to greater number of activated mechanoreceptors. One can hypothesize that stretch activated ion channels (SACs) located in the plasma membrane are responsible for the transduction of gravistimulus. These SACs may be connected together by elements of the cytoskeleton lining the plasma membrane and to the actin filaments. They could be stimulated by the action of statoliths on the actin network and/or on these elements of the cytoskeleton which link the mechanoreceptors (SACs).

Isolation and functional analysis of a Brassica juncea gene encoding a component of auxin efflux carrier

Polar auxin transport plays a divergent role in plant growth and developmental processes including root and embryo development, vascular pattern formation and cell elongation. Recently isolated Arabidopsis pin gene family was believed to encode a component of auxin efflux carrier (Gälweiler et al, 1998). Based on the Arabidopsis pin1 sequence we have isolated a Brassica juncea cDNA (designated Bjpinl), which encoded a 70-kDa putative auxin efflux carrier. Deduced BjPIN1 shared 65% identities at protein level with AtPIN1 and was highly homologous to other putative PIN proteins of Arabidopsis (with highest homology to AtPIN3). Hydrophobic analysis showed similar structures between BjPIN1 and AtPIN proteins. Presence of 6 exons (varying in size between 65 bp and 1229 bp) and 5 introns (sizes between 89 bp and 463 bp) in the genomic fragment was revealed by comparing the genomic and cDNA sequences. Northern blot analysis indicated that Bjpinl was expressed in most of the tissues tested, with a relatively higher level of transcript in flowers and a lower level in root tissues. Promoter-reporter gene fusion studies further revealed the expression of Bjpinl in the mature pollen grains, young seeds, root tip, leaf vascular tissue and trace bundle, stem epidermis, cortex and vascular cells. BjPIN1 was localized on the plasma membrane as demonstrated through fusion expression of green fluorescent protein (GFP). Auxin efflux carrier activity was elevated in transgenic Arabidopsis expressing BjPIN1.

Channelling auxin action: modulation of ion transport by indole-3-acetic acid

The growth hormone auxin is a key regulator of plant cell division and elongation. Since plants lack muscles, processes involved in growth and movements rely on turgor formation, and thus on the transport of solutes and water. Modern electrophysiological techniques and molecular genetics have shed new light on the regulation of plant ion transporters in response to auxin. Guard cells, hypocotyls and coleoptiles have advanced to major model systems in studying auxin action. This review will therefore focus on the molecular mechanism by which auxin modulates ion transport and cell expansion in these model cell types.

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.

The dose-response curve of the gravitropic reaction: a re-analysis

The dose-response curve of the gravitropic reaction is often used to evaluate the gravisensing of plant organs. It has been proposed (Larsen 1957) that the response (curvature) varies linearly as a function of the logarithm of the dose of gravistimulus. As this model fitted correctly most of the data obtained in the literature, the presentation time (tp, minimal duration of stimulation in the gravitational field to induce a response) or the presentation dose (dp, minimal quantity in g.s of stimulation to induce a response) were estimated by extrapolating down to zero curvature the straight line representing the response as a function of the logarithm of the stimulus. This method was preferred to a direct measurement of dp or tp with minute stimulations, since very slight gravitropic response cannot be distinguished from the background oscillations of the extremity of the organs. In the present review, it is shown that generally the logarithmic model (L) does not fit the experimental data published in the literature as well as the hyperbolic model (H). The H model in its simplest form is related to a response in which a ligand-receptor system is the limiting phase in the cascade of events leading to the response (Weyers et al. 1987). However, it is demonstrated that the differential growth, responsible for the curvature (and the angle of curvature), would vary as a hyperbolic function of the dose of stimulation, even if several steps involving ligand-receptor systems are responsible for the gravitropic curvature. In the H model, there is theoretically no presentation time (or presentation dose) since the curve passes through the origin. The value of the derivative of the H function equals a/b and represents the slope of the cune at the origin. It could be therefore used to estimate gravisensitivity. This provides a measurement of graviresponsiveness for threshold doses of stimulation. These results imply that the presentation time (or presentation dose) derived from the L model cannot be used anymore as an estimate of gravisensitivity. On the contrary, the perception time (minimal duration of a repeated stimulation which induces a response), which is less than 1 s, should be related to the perception of gravity. The consequences of these results on the mode of action and the nature of graviperception are discussed.

Gravity-stimulated changes in auxin and invertase gene expression in maize pulvinal cells

Maize (Zea mays) stem gravitropism involves differential elongation of cells within a highly specialized region, the stem internodal pulvinus. In the present study, we investigated factors that control gravitropic responses in this system. In the graviresponding pulvinus, hexose sugars (D-Glc and D-Fru) accumulated asymmetrically across the pulvinus. This correlated well with an asymmetric increase in acid invertase activity across the pulvinus. Northern analyses revealed asymmetric induction of one maize acid invertase gene, Ivr2, consistent with transcriptional regulation by gravistimulation. Several lines of evidence indicated that auxin redistribution, as a result of polar auxin transport, is necessary for gravity-stimulated Ivr2 transcript accumulation and differential cell elongation across the maize pulvinus. First, the auxin transport inhibitor, N-1-naphthylphthalamic acid, inhibited gravistimulated curvature and Ivr2 transcript accumulation. Second, a transient gradient of free indole-3-acetic acid (IAA) across the pulvinus was apparent shortly after initiation of gravistimulation. This temporarily free IAA gradient appears to be important for differential cell elongation and Ivr2 transcript accumulation. This is based on the observation that N-1-naphthylphthalamic acid will not inhibit gravitropic responses when applied to pulvinus tissue after the free IAA gradient peak has occurred. Third, IAA alone can stimulate Ivr2 transcript accumulation in non-gravistimulated pulvini. The gravity- and IAA-stimulated increase in Ivr2 transcripts was sensitive to the protein synthesis inhibitor, cycloheximide. Based on these results, a two-phase model describing possible relationships between gravitropic curvature, IAA redistribution, and Ivr2 expression is presented.

Root gravitropism: an experimental tool to investigate basic cellular and molecular processes underlying mechanosensing and signal transmission in plants

The ability of plant organs to use gravity as a guide for growth, named gravitropism, has been recognized for over two centuries. This growth response to the environment contributes significantly to the upward growth of shoots and the downward growth of roots commonly observed throughout the plant kingdom. Root gravitropism has received a great deal of attention because there is a physical separation between the primary site for gravity sensing, located in the root cap, and the site of differential growth response, located in the elongation zones (EZs). Hence, this system allows identification and characterization of different phases of gravitropism, including gravity perception, signal transduction, signal transmission, and curvature response. Recent studies support some aspects of an old model for gravity sensing, which postulates that root-cap columellar amyloplasts constitute the susceptors for gravity perception. Such studies have also allowed the identification of several molecules that appear to function as second messengers in gravity signal transduction and of potential signal transducers. Auxin has been implicated as a probable component of the signal that carries the gravitropic information between the gravity-sensing cap and the gravity-responding EZs. This has allowed the identification and characterization of important molecular processes underlying auxin transport and response in plants. New molecular models can be elaborated to explain how the gravity signal transduction pathway might regulate the polarity of auxin transport in roots. Further studies are required to test these models, as well as to study the molecular mechanisms underlying a poorly characterized phase of gravitropism that is independent of an auxin gradient.

Polar auxin transport: controlling where and how much

Auxin is transported through plant tissues, moving from cell to cell in a unique polar manner. Polar auxin transport controls important growth and developmental processes in higher plants. Recent studies have identified several proteins that mediate polar auxin transport and have shown that some of these proteins are asymmetrically localized, paving the way for studies of the mechanisms that regulate auxin transport. New data indicate that reversible protein phosphorylation can control the amount of auxin transport, whereas protein secretion through Golgi-derived vesicles and interactions with the actin cytoskeleton might regulate the localization of auxin efflux complexes.

[Microgravity and root gravitropism]

Space experiments permit to understand better some phases of the gravitropic reaction which occurs when the orientation of the root changes in the gravitational field. In gravisensing cells (statocytes in the root cap), the nucleus is attached to the cell periphery, close to the plasma membrane, by actin filaments. The location of the amyloplasts (statoliths) depends also greatly on these elements of the cytoskeleton. A short period in microgravity (5 min.) modifies the location of the nucleus and of the amyloplasts in the statocytes. The tensions exerted by these very dense organelles on the actin network disappear and this network undergoes a relaxation. The kinetics of gravitropic curvature is also better understood. In fact, gravitropic reaction is regulated by a mechanism depending on gravity. In roots grown in space, then stimulated for 1 h on a 1 g centrifuge, and replaced in microgravity, the regulation limiting the curvature does not occur. It is hypothesized that the sedimentation of the amyloplasts on the endoplasmic reticulum placed at the basal pole of the statocytes could be responsible for this regulation. The contacts between these two organelles should have also a role in root growth. This hypothesis will be tested in our next space experiment (July 94). The experiments in near weightlessness also permit to determine the presentation time which is the duration of stimulation necessary to evoke a slight but significant curvature. Presentation time is 27 s. This short period allows a slight movement of the amyloplasts only (around 0.45 micrometer). The sequence of events leading to the curvature of the root is now well established: the first signal is the separation of the endoplasmic reticulum and the amyloplasts, when the root is subjected to a change in orientation. It is followed by the pressure of these organelles on the actin network which transmits this mechanical effect to the plasma membrane. The transduction of the effect occurs then by the activation of the ions channels (Ca++) and the carrier of a growth inhibitor (auxin), both located in the plasma membrane. This growth inhibitor provokes an asymmetrical growth in the distal part of the meristem and the proximal part of the cell elongation zone. At last, when the root tip reaches the direction of gravity, the amyoloplasts sediment on the endoplasmic reticulum and induce a signal of termination of the curavature.

Cytoplasmic pH dynamics in maize pulvinal cells induced by gravity vector changes

In maize (Zea mays) and other grasses, changes in orientation of stems are perceived by pulvinal tissue, which responds to the stimulus by differential growth resulting in upward bending of the stem. The amyloplast-containing bundle sheath cells are the sites of gravity perception, although the initial steps of gravity perception and transmission remain unclear. In columella cells of Arabidopsis roots, we previously found that cytoplasmic pH (pH(c)) is a mediator in early gravitropic signaling (A.C. Scott, N.S. Allen [1999] Plant Physiol 121: 1291-1298). The question arises whether pH(c) has a more general role in signaling gravity vector changes. Using confocal ratiometric imaging and the fluorescent pH indicator carboxy seminaphtorhodafluor acetoxymethyl ester acetate, we measured pH(c) in the cells composing the maize pulvinus. When stem slices were gravistimulated and imaged on a horizontally mounted confocal microscope, pH(c) changes were only apparent within the bundle sheath cells, and not in the parenchyma cells. After turning, cytoplasmic acidification was observed at the sides of the cells, whereas the cytoplasm at the base of the cells where plastids slowly accumulated became more basic. These changes were most apparent in cells exhibiting net amyloplast sedimentation. Parenchyma cells and isolated bundle sheath cells did not show any gravity-induced pH(c) changes although all cell types responded to external stimuli in the predicted way: Propionic acid and auxin treatments induced acidification, whereas raising the external pH caused alkalinization. The results suggest that pH(c) has an important role in the early signaling pathways of maize stem gravitropism.

Lateral redistribution of auxin is not the means for gravitropic differential growth of coleoptiles: A new model

Gravicurvature in water- and auxin (IAA)-incubated coleoptiles of rye (Secale cereale L.) is similar, despite a general strongly enhancing effect of exogenous IAA on the overall (cell) elongation of these organs. Longitudinally split coleoptiles or isolated longitudinally halved coleoptiles (horizontally positioned as upper or lower halves) respond gravitropically in the same way as water-incubated intact coleoptiles, irrespective of whether the halves are incubated in distilled water or IAA. A new model for the principal mechanism of regulation of gravitropic growth is proposed which depends on, yet does not involve, the redistribution of IAA as the means for gravistimulated differential growth, as postulated by the Cholodny-Went hypothesis (CWH). It is based on a gravimediated temporarily restrained infiltration of IAA-induced wall-loosening factors into the growth-limiting outer epidermal walls of the concave organ flank.

Basipetal auxin transport is required for gravitropism in roots of Arabidopsis

Auxin transport has been reported to occur in two distinct polarities, acropetally and basipetally, in two different root tissues. The goals of this study were to determine whether both polarities of indole-3-acetic acid (IAA) transport occur in roots of Arabidopsis and to determine which polarity controls the gravity response. Global application of the auxin transport inhibitor naphthylphthalamic acid (NPA) to roots blocked the gravity response, root waving, and root elongation. Immediately after the application of NPA, the root gravity response was completely blocked, as measured by an automated video digitizer. Basipetal [(3)H]IAA transport in Arabidopsis roots was inhibited by NPA, whereas the movement of [(14)C]benzoic acid was not affected. Inhibition of basipetal IAA transport by local application of NPA blocked the gravity response. Inhibition of acropetal IAA transport by application of NPA at the root-shoot junction only partially reduced the gravity response at high NPA concentrations. Excised root tips, which do not receive auxin from the shoot, exhibited a normal response to gravity. The Arabidopsis mutant eir1, which has agravitropic roots, exhibited reduced basipetal IAA transport but wild-type levels of acropetal IAA transport. These results support the hypothesis that basipetally transported IAA controls root gravitropism in Arabidopsis.

PIN-pointing the molecular basis of auxin transport

Significant advances in the genetic dissection of the auxin transport pathway have recently been made. Particularly relevant is the molecular analysis of mutants impaired in auxin transport and the subsequent cloning of genes encoding candidate proteins for the elusive auxin efflux carrier. These studies are thought to pave the way to the detailed understanding of the molecular basis of several important facets of auxin action.

Root gravitropism: a complex response to a simple stimulus?

Roots avoid depleting their immediate environment of essential nutrients by continuous growth. Root growth is directed by environmental cues, including gravity. Gravity sensing occurs mainly in the columella cells of the root cap. Upon reorientation within the gravity field, the root-cap amyloplasts sediment, generating a physiological signal that promotes the development of a curvature at the root elongation zones. Recent molecular genetic studies in Arabidopsis have allowed the identification of genes that play important roles in root gravitropism. Among them, the ARG1 gene encodes a DnaJ-like protein involved in gravity signal transduction, whereas the AUX1 and AGR1 genes encode proteins involved in polar auxin transport. These studies have important implications for understanding the intra- and inter-cellular signaling processes that underlie root gravitropism.

Plant graviperception and gravitropism: a newcomer's view

Gravitropism is an adaptable mechanism corresponding to the directed growth by which plants orient in response to the gravity vector. The overall process is generally divided into three distinct stages: graviperception, gravitransduction, and asymmetric growth response. The phenomenology of these different steps has been described by using refined cell biology approaches combined with formal and molecular genetics. To date, it clearly appears that the cellular organization plays crucial roles in gravisensing and that gravitropism is genetically different between organs. Moreover, while interfering with other physical or chemical stimuli and sharing probably some common intermediary steps in the transduction pathway, gravity has its own perception and transduction systems. The intimate mechanisms involved in these processes have to be unveiled at the molecular level and their biological relevance addressed at the cellular and whole plant levels under normal and microgravitational conditions. gravitropism: a newcomer's view.

AUX1 regulates root gravitropism in Arabidopsis by facilitating auxin uptake within root apical tissues

Plants employ a specialized transport system composed of separate influx and efflux carriers to mobilize the plant hormone auxin between its site(s) of synthesis and action. Mutations within the permease-like AUX1 protein significantly reduce the rate of carrier-mediated auxin uptake within Arabidopsis roots, conferring an agravitropic phenotype. We are able to bypass the defect within auxin uptake and restore the gravitropic root phenotype of aux1 by growing mutant seedlings in the presence of the membrane-permeable synthetic auxin, 1-naphthaleneacetic acid. We illustrate that AUX1 expression overlaps that previously described for the auxin efflux carrier, AtPIN2, using transgenic lines expressing an AUX1 promoter::uidA (GUS) gene. Finally, we demonstrate that AUX1 regulates gravitropic curvature by acting in unison with the auxin efflux carrier to co-ordinate the localized redistribution of auxin within the Arabidopsis root apex. Our results provide the first example of a developmental role for the auxin influx carrier within higher plants and supply new insight into the molecular basis of gravitropic signalling.

The microtubule cytoskeleton does not integrate auxin transport and gravitropism in maize roots

The Cholodny-Went hypothesis of gravitropism suggests that the graviresponse is controlled by the distribution of auxin. However, the mechanism of auxin transport during the graviresponse of roots is still unresolved. To determine whether the microtubule (MT) cytoskeleton is participating in auxin transport, the cytoskeleton was examined and the movement of 3H-IAA measured in intact and excised taxol, oryzalin, and naphthylphthalamic acid (NPA)-treated roots of Zea mays cv. Merit. Taxol and oryzalin did not inhibit the graviresponse of roots but the auxin transport inhibitor NPA greatly inhibited both auxin transport and graviresponse. NPA had no effect on MT organization in vertical roots, but caused MT reorientation in horizontally placed roots. Regardless of treatment, the organization of MTs in intact roots differed from that in root segments. The MT inhibitors, taxol and oryzalin had opposite effects on the MTs, namely, depolymerization (oryzalin) and stabilization and thickening (taxol), but both treatments caused swelling of the roots. The data indicate that the MT cytoskeleton does not directly interfere with auxin transport or auxin-mediated growth responses in maize roots.

AtPIN2 defines a locus of Arabidopsis for root gravitropism control

The molecular mechanisms underlying gravity perception and signal transduction which control asymmetric plant growth responses are as yet unknown, but are likely to depend on the directional flux of the plant hormone auxin. We have isolated an Arabidopsis mutant of the AtPIN2 gene using transposon mutagenesis. Roots of the Atpin2::En701 null-mutant were agravitropic and showed altered auxin sensitivity, a phenotype characteristic of the agravitropic wav6-52 mutant. The AtPIN2 gene was mapped to chromosome 5 (115.3 cM) corresponding to the WAV6 locus and subsequent genetic analysis indicated that wav6-52 and Atpin2::En701 were allelic. The AtPIN2 gene consists of nine exons defining an open reading frame of 1944 bp which encodes a 69 kDa protein with 10 putative transmembrane domains interrupted by a central hydrophilic loop. The topology of AtPIN2p was found to be similar to members of the major facilitator superfamily of transport proteins. We have shown that the AtPIN2 gene was expressed in root tips. The AtPIN2 protein was localized in membranes of root cortical and epidermal cells in the meristematic and elongation zones revealing a polar localization. These results suggest that AtPIN2 plays an important role in control of gravitropism regulating the redistribution of auxin from the stele towards the elongation zone of roots.

Proton and calcium flux oscillations in the elongation region correlate with root nutation

The involvement of Ca2+ and H+ flux oscillations in root nutation was studied for decapped roots of corn (Zea mays L. cv. Aussie Gold) placed horizontally. Net ion fluxes were measured around the elongation and meristematic regions using a microelectrode ion flux measuring system. High correlation between H+ flux oscillations and root nutations was found in the elongation region. Two oscillatory components of H+ flux, with periods of about 90 min and 7 min, correlated with root circumnutations and micronutations, respectively. The periods of H+ flux oscillations and rhythmical root movements in this region could be modified similarly by external factors including pH. In the meristematic region no association between ion flux behaviour and nutation was apparent. Ion flux oscillations and nutations both decreased in amplitude as the growth rate at the measured location decreased. Possible involvement of ion flux oscillations in root circumnutation is discussed. It is concluded that a model involving an internal oscillator must be developed to explain the H+ flux involvement in root nutations.

Reduced naphthylphthalamic acid binding in the tir3 mutant of Arabidopsis is associated with a reduction in polar auxin transport and diverse morphological defects

Polar auxin transport plays a key role in the regulation of plant growth and development. To identify genes involved in this process, we have developed a genetic procedure to screen for mutants of Arabidopsis that are altered in their response to auxin transport inhibitors. We recovered a total of 16 independent mutants that defined seven genes, called TRANSPORT INHIBITOR RESPONSE (TIR) genes. Recessive mutations in one of these genes, TIR3, result in altered responses to transport inhibitors, a reduction in polar auxin transport, and a variety of morphological defects that can be ascribed to changes in indole-3-acetic acid distribution. Most dramatically, tir3 seedlings are strongly deficient in lateral root production, a process that is known to depend on polar auxin transport from the shoot into the root. In addition, tir3 plants display a reduction in apical dominance as well as decreased elongation of siliques, pedicels, roots, and the inflorescence. Biochemical studies indicate that tir3 plants have a reduced number of N-1-naphthylphthalamic (NPA) binding sites, suggesting that the TIR3 gene is required for expression, localization, or stabilization of the NPA binding protein (NBP). Alternatively, the TIR3 gene may encode the NBP. Because the tir3 mutants have a substantial defect in NPA binding, their phenotype provides genetic evidence for a role for the NBP in plant growth and development.

Changes in IAA responsiveness in the elongation region of graviresponding mung bean roots

IAA responsiveness of sections of root tissue taken from the top and bottom of mung bean roots was assessed prior to and at varying times following gravistimulation. Prior to gravistimulation, root tissue sections from the sides of the elongation zone responded similarly to IAA. After gravistimulation (within 5 min), root sections from the bottom of the elongation zone became more responsive to IAA than sections collected from the upper side of the elongation zone. The change in IAA responsiveness of these tissue sections was transient with root sections from both the top and bottom of the elongation zone again exhibiting similar responsiveness to IAA following 15 minutes of gravistimulation. These studies also examined if the root tip is required for the gravity-induced shift in IAA responsiveness in the tissues of the elongation zone. The IAA responsiveness of top and bottom sections of the elongation zone from decapped mung bean roots was assessed at varying times following gravistimulation. The responsiveness to IAA of top and bottom sections changed rapidly in decapped roots, just as had been previously found for intact roots. Although the alteration in responsiveness was transient in decapped roots (just as intact roots), the time it took for the sections to recover previous responsiveness to IAA was extended. The results suggest that the initial growth response of graviresponding roots may be due to a change in the IAA responsiveness of tissues in the elongation zone and not an asymmetric accumulation of IAA on the lower side of the elongation zone. The results also indicate that the gravity-induced shift in IAA responsiveness in the elongation zone occurs independently of the root cap, suggesting that the cells in the elongation region can perceive and respond to gravity independently of the root cap during the initial phases of the gravity response.

Plant tropisms: the ins and outs of auxin

The Cholodny-Went hypothesis holds that gravitropic curvature of a growing plant organ depends on regulated transport of the plant hormone auxin; new studies of the agravitropic mutant aux1 of Arabidopsis provide strong evidence in support of this hypothesis.

Transgenic white clover. Studies with the auxin-responsive promoter, GH3, in root gravitropism and lateral root development

We report improved method for white clover (Trifolium repens) transformation using Agrobacterium tumefaciens. High efficiencies of transgenic plant production were achieved using cotyledons of imbibed mature seed. Transgenic plants were recovered routinely from over 50% of treated cotyledons. The bar gene and phosphinothricin selection was shown to be a more effective selection system than nptII (kanamycin selection) or aadA (spectinomycin selection). White clover was transformed with the soybean auxin responsive promoter, GH3, fused to the GUS gene (beta-glucuronidase) to study the involvement of auxin in root development. Analysis of 12 independent transgenic plants showed that the location and pattern of GUS expression was consistent but the levels of expression varied. The level of GH3:GUS expression in untreated plants was enhanced specifically by auxin-treatment but the pattern of expression was not altered. Expression of the GH3:GUS fusion was not enhanced by other phytohormones. A consistent GUS expression pattern was evident in untreated plants presumably in response to endogenous auxin or to differences in auxin sensitivity in various clover tissues. In untreated plants, the pattern of GH3:GUS expression was consistent with physiological responses which are regarded as being auxin-mediated. For the first time it is shown that localised spots of GH3:GUS activity occurred in root cortical tissue opposite the sites where lateral roots subsequently were initiated. Newly formed lateral roots grew towards and through these islands of GH3:GUS expression, implying the importance of auxin in controlling lateral root development. Similarly, it is demonstrated for the first time that gravistimulated roots developed a rapid (within 1 h) induction of GH3:GUS activity in tissues on the non-elongating side of the responding root and this induction occurred concurrently with root curvature. These transgenic plants could be useful tools in determining the physiological and biochemical changes that occur during auxin-mediated responses.

The response to auxin of rapeseed (Brassica napus L.) roots displaying reduced gravitropism due to transformation by Agrobacterium rhizogenes

It has recently been documented that, compared to untransformed controls, the roots of oilseed rape (Brassica napus L. CV CrGC5) seedlings transformed by Agrobacterium rhizogenes A4 show a reduced gravitropic reaction (Legue et al. 1994, Physiol Plant 91: 559-566). After stimulation at 90 degrees C or 135 degrees, the transformed root tips curve. but never reach a vertical orientation. In the present study, we investigated the causes of reduced gravitropic bending observed in stimulated transformed root tips. First, we localized the gravitropic curvature in normal and in transformed roots after 1.5 h of stimulation. The cells involved in root curvature (target cells) corresponded at the cellular level to the apical part of the zone of increasing cell length. In transformed roots grown in the vertical position, these cells showed a reduction in cell length compared to controls. Because auxin is considered to be the gravitropic mediator, the response of normal and transformed roots to exogenous auxin was studied. Indole-3-acetic acid (IAA) was applied along the first 3 mm using resin beads loaded with the hormone. In comparison to normal roots, transformed roots showed reduced bending toward the bead at all points of bead application. Moreover, the cells which responded to IAA corresponded to the target cells involved in the gravitropic reaction. The level of endogenous IAA was lower in transformed roots. Thus, it was concluded that the modified behavior of transformed roots during gravitropic stimulation could be due to differences either in IAA levels or in reactivity of the target cells to the message from the cap.

Arabidopsis AUX1 gene: a permease-like regulator of root gravitropism

The plant hormone auxin regulates various developmental processes including root formation, vascular development, and gravitropism. Mutations within the AUX1 gene confer an auxin-resistant root growth phenotype and abolish root gravitropic curvature. Polypeptide sequence similarity to amino acid permeases suggests that AUX1 mediates the transport of an amino acid-like signaling molecule. Indole-3-acetic acid, the major form of auxin in higher plants, is structurally similar to tryptophan and is a likely substrate for the AUX1 gene product. The cloned AUX1 gene can restore the auxin-responsiveness of transgenic aux1 roots. Spatially, AUX1 is expressed in root apical tissues that regulate root gravitropic curvature.

The behaviour of normal and agravitropic transgenic roots of rapeseed (Brassica napus L.) under microgravity conditions

In the TRANSFORM experiment for IML-2 on the Space Shuttle Columbia, normal (wild type = WT) and genetically transformed agravitropic rapeseed roots were tested under microgravity conditions. The aim of the experiment was to determine if the wild-type roots behaved differently (growth, morphology, gravitropical sensitivity) from the transgenic roots. The appearance of the organelles and distribution of statoliths (i.e. amyloplasts with starch grains) in the gravitropic reactive cells (statocytes) under weightlessness was compared for the two types of roots. Attempts have also been made to regenerate new plants from the root material tested in space. Both the WT and the transgenic root types showed the expected increase in length during 36 h of photorecording. Contrary to the results of the ground controls, no significant difference in elongation rates was found between the WT and transgenic roots grown in orbit. However, there are indications that the total growth both in the WT and the transgenic roots was higher in the ground control than for roots in orbit. After a 60 min 1 x g stimulation of the roots on board the Shuttle, no detectable curvatures were obtained in either the transgenic or the WT roots. However, it cannot be excluded that a minute curvature development occurs in the root tips but was not detected due to technical reasons. The ultrastructure was well preserved in both the WT and the transgenic roots, despite the fact that the tissue was kept in the prefixative for over 3 weeks. No marked differences in ultrastructure were observed between the transformed root statocyte cells and the equivalent cells in the wild type. There were no obvious differences in root morphology during the orbital period. Light micrographs and morphometrical analysis indicate that the amyloplasts of both the wild type and transformed root statocytes are randomly distributed over the cells kept under micro-g conditions for 37 h after a 14 h stimulation on the 1 x g centrifuge. The main scientific conclusion from the TRANSFORM experiment is that the difference in growth found in the ground control between the WT and the transgenic root types seems to be eliminated under weightlessness. Explanations for this behaviour cannot be found in the root ultrastructure or in root morphology.

Effect of phosphorus deficiency on growth angle of basal roots in Phaseolus vulgaris

Root architectural plasticity might be an important factor in the acquisition by plants of immobile nutrients such as phosphorus (P). In this study, we examined the effect of P availability on the orientation of basal roots with respect to gravity, and thereby on the growth angle of these roots of common bean (Phaseolus vulgaris L.). In one set of studies the growth angle of basal roots of bean seedlings was measured over time. Sixteen bean genotypes were examined; six showed a decrease in root orientation with respect to gravity in low P media, one increased orientation, and nine showed no difference within 5 d of basal root emergence. Bean taproots also showed decreased root orientation with respect to gravity in low P. Growth angle after 5 d was correlated with field performance of contrasting genotypes in low P tropical soils. Mineral deficiencies other than P did not cause changes in root angle. In a split pouch system that provided high or low P solution to different parts of the root system, the decrease in root angle in low P was found to be a response to global P availability, and not local to the portion of the root system in low P. Effects of P availability on root angle were associated with reduced shoot P concentration, but preceded effects on plant biomass accumulation and leaf area expansion. Results from growth pouches for genotype G 19833 were confirmed using a solid-phase buffered sand-culture system supplying P at three levels. Pea (Pisum sativum), soybean (Glycine max Williams), chickpea (Cicer arietinum), lima bean (Phaseolus lunatus), and lentil (Lens culinaris) were grown with and without P; soybean and pea also showed decreased basal root angles in low P.

Possible use of a 3-D clinostat to analyze plant growth processes under microgravity conditions

A three-dimensional (3-D) clinostat equipped with two rotation axes placed at right angles was constructed, and various growth processes of higher plants grown on this clinostat were compared with ground controls, with plants grown on the conventional horizontal clinostat, and with those under real microgravity in space. On the 3-D clinostat, cress roots developed a normal root cap and the statocytes showed the typical polar organization except a random distribution of statoliths. The structural features of clinostatted statocytes were fundamentally similar to those observed under real microgravity. The graviresponse of cress roots grown on the 3-D clinostat was the same as the control roots. On the 3-D clinostat, shoots and roots exhibited a spontaneous curvature as well as an altered growth direction. Such an automorphogenesis was sometimes exaggerated when plants were subjected to the horizontal rotation, whereas the curvature was suppressed on the vertical rotation. These discrepancies in curvature between the 3-D clinostat and the conventional ones appear to be brought about by the centrifugal force produced. Thus, the 3-D clinostat was proven as a useful device to simulate microgravity.