Tropic Responses Research Articles

Nonspecific phospholipases C3 and C4 interact with PIN-FORMED2 to regulate growth and tropic responses in Arabidopsis.

The dynamic changes in membrane phospholipids affect membrane biophysical properties and cell signaling, thereby influencing numerous biological processes. Nonspecific phospholipase C (NPC) enzymes hydrolyze common phospholipids to release diacylglycerol (DAG), which is converted to phosphatidic acid (PA) and other lipids. In this study, 2 Arabidopsis (Arabidopsis thaliana) tandemly arrayed genes, NPC3 and NPC4, were identified as critical factors modulating auxin-controlled plant growth and tropic responses. Moreover, NPC3 and NPC4 were shown to interact with the auxin efflux transporter PIN-FORMED2 (PIN2). The loss of NPC3 and NPC4 enhanced the endocytosis and vacuolar degradation of PIN2, which disrupted auxin gradients and slowed gravitropic and halotropic responses. Furthermore, auxin-triggered activation of NPC3 and NPC4 is required for the asymmetric PA distribution that controls PIN2 trafficking dynamics and auxin-dependent tropic responses. Collectively, our study reveals an NPC-derived PA signaling pathway in Arabidopsis auxin fluxes that is essential for fine-tuning the balance between root growth and environmental responses.

Waste valorization as low-cost media engineering for auxin production from the newly isolated Streptomyces rubrogriseus AW22: Model development

Indole-3-acetic acid (IAA) represents a crucial phytohormone regulating specific tropic responses in plants and functions as a chemical signal between plant hosts and their symbionts. The Actinobacteria strain of AW22 with high IAA production ability was isolated in Algeria for the first time and was characterized as Streptomyces rubrogriseus through chemotaxonomic analysis and 16 S rDNA sequence alignment. The suitable medium for a maximum IAA yield was engineered in vitro and in silico using machine learning-assisted modeling. The primary low-cost feedstocks comprised various concentrations of spent coffee grounds (SCGs) and carob bean grounds (CBGs) extracts. Further, we combined the Box-Behnken design from response surface methodology (BBD-RSM) with artificial neural networks (ANNs) coupled with the genetic algorithm (GA). The critical process parameters screened via Plackett-Burman design (PBD) served as BBD and ANN-GA inputs, with IAA yield as the output variable. Analysis of the putative IAA using thin-layer chromatography (TLC) and (HPLC) revealed Rf values equal to 0.69 and a retention time of 3.711 min, equivalent to the authentic IAA. AW 22 achieved a maximum IAA yield of 188.290 ± 0.38 μg/mL using the process parameters generated by the ANN-GA model, consisting of L-Trp, 0.6%; SCG, 30%; T°, 25.8 °C; and pH 9, after eight days of incubation. An R2 of 99.98%, adding to an MSE of 1.86 × 10−5 at 129 epochs, postulated higher reliability of ANN-GA-approach in predicting responses, compared with BBD-RSM modeling exhibiting an R2 of 76.28%. The validation experiments resulted in a 4.55-fold and 4.46-fold increase in IAA secretion, corresponding to ANN-GA and BBD-RSM models, respectively, confirming the validity of both models.

Root twisting drives halotropism via stress-induced microtubule reorientation.

Plants have evolved signaling mechanisms that guide growth away from adverse environments that can cause yield losses. Root halotropism is a sodium-specific negative tropism that is crucial for surviving and thriving under high salinity. Although root halotropism was discovered some years ago, the underlying molecular and cellular mechanisms remain unknown. Here, we show that abscisic acid (ABA)-mediated root twisting determines halotropism in Arabidopsis. An ABA-activated SnRK2 protein kinase (SnRK2.6) phosphorylates the microtubule-associated protein SP2L at Ser406, which induces a change in the anisotropic cell expansion at the root transition zone and is required for root twisting during halotropism. Salt stress triggers SP2L-mediated cortical microtubule reorientation, which guides cellulose microfibril patterns. Our findings thus outline the molecular mechanism of root halotropism and indicate that anisotropic cell expansion through microtubule reorientation and microfibril deposition has a central role in mediating tropic responses.

Bending to auxin: fast acid growth for tropisms.

The phytohormone auxin is the major growth regulator governing tropic responses including gravitropism. Auxin build-up at the lower side of stimulated shoots promotes cell expansion, whereas in roots it inhibits growth, leading to upward shoot bending and downward root bending, respectively. Yet it remains an enigma how the same signal can trigger such opposite cellular responses. In this review, we discuss several recent unexpected insights into the mechanisms underlying auxin regulation of growth, challenging several existing models. We focus on the divergent mechanisms of apoplastic pH regulation in shoots and roots revisiting the classical Acid Growth Theory and discuss coordinated involvement of multiple auxin signaling pathways. From this emerges a more comprehensive, updated picture how auxin regulates growth.

Agency, teleonomy and signal transduction in plant systems

AbstractPlants are the basis of nearly all food chains. The evolutionary response to inevitable predatory and other contingent hazards has provided plants with remarkable regenerative and plastic capabilities. Teleonomy has been characterized as purposive, adaptive and goal-directed behaviour. The evidence that plants are agents, that they act purposefully, is indicated by numerous behaviours, most notably plasticity. Through recurrent information exchange, growing roots construct a dynamic niche with bacterial and fungal symbionts. Purposeful shoot plasticity enables adaptive responses to abiotic and biotic hazards, with the goal of survival. Transgenerational inheritance furthers that goal for sibling survival. Teleonomic goals in shoot growth use proprioception to ensure successful tropic responses. Unlike animals that acquire energy biotically, nearly all plants are dependent solely on the physical environment. Convergent evolution is common and may result from the constraints of unchanging physical laws. Phenotypic plasticity initially provides a direction for evolutionary change. Our article indicates that there are features to evolutionary change in plants additional to those indicated by the modern synthesis and thus supports the extended evolutionary hypothesis.

Involvement of auxin in growth and stress response of cucumber

Cucumber (<italic>Cucumis sativus</italic> L.), an important vegetable, popular worldwide for its crispy texture and special flavor. Plant hormones such as auxin stands out for its dominating function in morpho- and organogenic processes, formation of organs as well as regulation of tropic responses. These developmental processes are entirely or partially dependent on auxin biosynthesis, transport, and signal transduction. In cucumber, auxin not only fine-tunes its morphogenesis but also response to environmental stress. The role of auxin in regulating different organs (root, hypocotyl, shoot, leaf, tendril, flower, and fruit) development in cucumber was reviewed in the present paper. Moreover, the role of auxin in cucumber response to biotic stresses (powdery mildew, downy mildew, and nematode infections) and abiotic stresses (heat, iron, waterlogging, cold, salinity, drought, and heavy metal stresses) was discussed. Finally, we pointed out the blind spots and future research thoughts to extend our understanding of this myriad molecule in cucumber growth and stress biology.

Interaction of gravitropism and phototropism in roots of Brassica oleracea

Gravitropism and phototropism play a primary role in orienting root growth. Tropistic responses of roots mediated by gravity and light have been extensively investigated, and a complex mutual interaction occurs between these two tropisms. To date, most studies have been conducted in 1 g, microgravity, or simulated microgravity, whereas no studies investigated root phototropism in hypergravity. Therefore, we studied the effects of several gravity treatments with those of different light wavelengths on root growth orientation. Here, we report growth and curvature of Brassica oleracea roots under different g levels, from simulated microgravity up to 20 g, and unilateral illumination with different spectral treatments provided by light emitting diodes. Microgravity was simulated with a random positioning machine whereas hypergravity conditions were obtained using the Large Diameter Centrifuge at the laboratories of the European Space Agency in the Netherlands. Four light treatments (white light, blue light, red light, and dark) were used in this study. Overall, roots of seedlings grown in the dark were longer than those developed under unilateral light treatments, regardless of the gravity level. Unilateral blue light or white light stimulated a negative phototropism of roots under all g levels, and root curvature was not affected by either hypergravity or simulated microgravity compared to 1 g. Results also confirmed previous findings on the effect of light intensity on root curvature and highlighted the relevance of blue-light photon flux density in root phototropism. Roots illuminated with red light showed a weak curvature in simulated microgravity but not in hypergravity. Moreover, root curvature under red light was similar to dark-grown roots in all g levels, suggesting a possible involvement of surface-dependent phenomena in root skewing under either red light or dark conditions. Further studies can confirm phototropic responses of B. oleracea in the weightless environment of orbiting spacecraft. Nevertheless, according to our findings, directional lighting represents an effective stimulus to guide root growth in a wide range of gravity conditions.

Mutually opposing activity of PIN7 splicing isoforms is required for auxin‐mediated tropic responses in Arabidopsis thaliana

Advanced transcriptome sequencing has revealed that the majority of eukaryotic genes undergo alternative splicing (AS). Nonetheless, little effort has been dedicated to investigating the functional relevance of particular splicing events, even those in the key developmental and hormonal regulators. Combining approaches of genetics, biochemistry and advanced confocal microscopy, we describe the impact of alternative splicing on the PIN7 gene in the model plant Arabidopsis thaliana. PIN7 encodes a polarly localized transporter for the phytohormone auxin and produces two evolutionarily conserved transcripts, PIN7a and PIN7b. PIN7a and PIN7b, differing in a four amino acid stretch, exhibit almost identical expression patterns and subcellular localization. We reveal that they are closely associated and mutually influence each other's mobility within the plasma membrane. Phenotypic complementation tests indicate that the functional contribution of PIN7b per se is minor, but it markedly reduces the prominent PIN7a activity, which is required for correct seedling apical hook formation and auxin-mediated tropic responses. Our results establish alternative splicing of the PIN family as a conserved, functionally relevant mechanism, revealing an additional regulatory level of auxin-mediated plant development.

PIN-mediated polar auxin transport regulations in plant tropic responses.

Tropisms, growth responses to environmental stimuli such as light or gravity, are spectacular examples of adaptive plant development. The plant hormone auxin serves as a major coordinative signal. The PIN auxin exporters, through their dynamic polar subcellular localizations, redirect auxin fluxes in response to environmental stimuli and the resulting auxin gradients across organs underlie differential cell elongation and bending. In this review, we discuss recent advances concerning regulations of PIN polarity during tropisms, focusing on PIN phosphorylation and trafficking. We also cover how environmental cues regulate PIN actions during tropisms, as well as the crucial role of auxin feedback on PIN polarity during bending termination. Finally, the interactions between different tropisms are reviewed to understand plant adaptive growth in the natural environment.

Genome-Wide Identification and Characterization of PIN-FORMED (PIN) Gene Family Reveals Role in Developmental and Various Stress Conditions in Triticum aestivum L.

PIN-FORMED (PIN) genes play a crucial role in regulating polar auxin distribution in diverse developmental processes, including tropic responses, embryogenesis, tissue differentiation, and organogenesis. However, the role of PIN-mediated auxin transport in various plant species is poorly understood. Currently, no information is available about this gene family in wheat (Triticum aestivum L.). In the present investigation, we identified the PIN gene family in wheat to understand the evolution of PIN-mediated auxin transport and its role in various developmental processes and under different biotic and abiotic stress conditions. In this study, we performed genome-wide analysis of the PIN gene family in common wheat and identified 44 TaPIN genes through a homology search, further characterizing them to understand their structure, function, and distribution across various tissues. Phylogenetic analyses led to the classification of TaPIN genes into seven different groups, providing evidence of an evolutionary relationship with Arabidopsis thaliana and Oryza sativa. A gene exon/intron structure analysis showed a distinct evolutionary path and predicted the possible gene duplication events. Further, the physical and biochemical properties, conserved motifs, chromosomal, subcellular localization, transmembrane domains, and three-dimensional (3D) structure were also examined using various computational approaches. Cis-elements analysis of TaPIN genes showed that TaPIN promoters consist of phytohormone, plant growth and development, and stress-related cis-elements. In addition, expression profile analysis also revealed that the expression patterns of the TaPIN genes were different in different tissues and developmental stages. Several members of the TaPIN family were induced during biotic and abiotic stress. Moreover, the expression patterns of TaPIN genes were verified by qRT-PCR. The qRT-PCR results also show a similar expression with slight variation. Therefore, the outcome of this study provides basic genomic information on the expression of the TaPIN gene family and will pave the way for dissecting the precise role of TaPINs in plant developmental processes and different stress conditions.

Plant thermotropism: an underexplored thermal engagement and avoidance strategy.

Various strategies evolved in plants to adjust the position of organs relative to the prevailing temperature condition, which allows optimal plant growth and performance. Such responses are classically separated into nastic and tropic responses. During plant thermotropic responses, organs move towards (engage) or away (avoid) from a directional temperature cue. Despite thermotropism being a classic botanical concept, the underlying ecological function and molecular and biophysical mechanisms remain poorly understood to this day. This contrasts to the relatively well-studied thermonastic movements (hyponasty) of e.g., rosette leaves. In this review, we provide an update on the current knowledge on plant thermotropisms and propose directions for future research and application.

Cell kinetics of auxin transport and activity in Arabidopsis root growth and skewing

Auxin is a key regulator of plant growth and development. Local auxin biosynthesis and intercellular transport generates regional gradients in the root that are instructive for processes such as specification of developmental zones that maintain root growth and tropic responses. Here we present a toolbox to study auxin-mediated root development that features: (i) the ability to control auxin synthesis with high spatio-temporal resolution and (ii) single-cell nucleus tracking and morphokinetic analysis infrastructure. Integration of these two features enables cutting-edge analysis of root development at single-cell resolution based on morphokinetic parameters under normal growth conditions and during cell-type-specific induction of auxin biosynthesis. We show directional auxin flow in the root and refine the contributions of key players in this process. In addition, we determine the quantitative kinetics of Arabidopsis root meristem skewing, which depends on local auxin gradients but does not require PIN2 and AUX1 auxin transporter activities. Beyond the mechanistic insights into root development, the tools developed here will enable biologists to study kinetics and morphology of various critical processes at the single cell-level in whole organisms.

On the Evolutionary Origins of Land Plant Auxin Biology.

Indole-3-acetic acid, that is, auxin, is a molecule found in a broad phylogenetic distribution of organisms, from bacteria to eukaryotes. In the ancestral land plant auxin was co-opted to be the paramount phytohormone mediating tropic responses and acting as a facilitator of developmental decisions throughout the life cycle. The evolutionary origins of land plant auxin biology genes can now be traced with reasonable clarity. Genes encoding the two enzymes of the land plant auxin biosynthetic pathway arose in the ancestral land plant by a combination of horizontal gene transfer from bacteria and possible neofunctionalization following gene duplication. Components of the auxin transcriptional signaling network have their origins in ancestral alga genes, with gene duplication and neofunctionalization of key domains allowing integration of a portion of the preexisting transcriptional network with auxin. Knowledge of the roles of orthologous genes in extant charophycean algae is lacking, but could illuminate the ancestral functions of both auxin and the co-opted transcriptional network.

Plant tropisms as a window on plant computational processes.

Plants are living information-processing organisms with highly adaptive behavior, allowing them to prosper in a harsh and fluctuating environment in spite of being sessile. Lacking a central nervous system, plants are distributed systems orchestrating complex computational processes performed at the tissue level. Here I consider plant tropisms as a useful input-output system boasting a robust mathematical description, naturally permitting a dialogue between mathematical modeling and biological observations. I propose tropisms as an ideal framework for the study of plant computational processes, allowing us to infer the relationship between observed tropic responses and known stimuli. I concentrate on macroscopic models, and elucidate this approach by presenting recent examples focusing on computational processes involved at different hierarchical levels of interactions: a plant's interaction with itself and its internal state, with the abiotic environment, and with neighboring plants.

A General 3D Model for Growth Dynamics of Sensory-Growth Systems: From Plants to Robotics.

In recent years, there has been a rise in interest in the development of self-growing robotics inspired by the moving-by-growing paradigm of plants. In particular, climbing plants capitalize on their slender structures to successfully negotiate unstructured environments while employing a combination of two classes of growth-driven movements: tropic responses, growing toward or away from an external stimulus, and inherent nastic movements, such as periodic circumnutations, which promote exploration. In order to emulate these complex growth dynamics in a 3D environment, a general and rigorous mathematical framework is required. Here, we develop a general 3D model for rod-like organs adopting the Frenet-Serret frame, providing a useful framework from the standpoint of robotics control. Differential growth drives the dynamics of the organ, governed by both internal and external cues while neglecting elastic responses. We describe the numerical method required to implement this model and perform numerical simulations of a number of key scenarios, showcasing the applicability of our model. In the case of responses to external stimuli, we consider a distant stimulus (such as sunlight and gravity), a point stimulus (a point light source), and a line stimulus that emulates twining of a climbing plant around a support. We also simulate circumnutations, the response to an internal oscillatory cue, associated with search processes. Lastly, we also demonstrate the superposition of the response to an external stimulus and circumnutations. In addition, we consider a simple example illustrating the possible use of an optimal control approach in order to recover tropic dynamics in a way that may be relevant for robotics use. In all, the model presented here is general and robust, paving the way for a deeper understanding of plant response dynamics and also for novel control systems for newly developed self-growing robots.

Butylated Hydroxytoluene (BHT) Inhibits PIN1 Exocytosis From BFA Compartments in Arabidopsis Roots.

The activity of polarly localized PIN-FORMED (PIN) auxin efflux carriers contributes to the formation of auxin gradients which guide plant growth, development, and tropic responses. Both the localization and abundance of PIN proteins in the plasma membrane depend on the regulation of PIN trafficking through endocytosis and exocytosis and are influenced by many external and internal stimuli, such as reactive oxygen species, auxin transport inhibitors, flavonoids and plant hormones. Here, we investigated the regulation of endosomal PIN cycling by using a Brefeldin A (BFA) assay to study the effect of a phenolic antioxidant ionol, butylated hydroxytoluene (BHT), on the endocytosis and exocytosis of PIN1 and PIN2. BHT is one of the most widely used antioxidants in the food and feed industries, and as such is commonly released into the environment; however, the effect of BHT on plants remains poorly characterized. Preincubation of Arabidopsis seedlings with BHT before BFA treatment strongly enhanced the internalization of PIN1 into BFA compartments. After the simultaneous application of BHT and NAA, the NAA effect dominated PIN internalization suggesting the BHT effect occurred downstream to that of NAA. Washing seedlings with BHT after BFA treatment prevented the release of PIN1 from BFA compartments back to the plasma membrane, indicating that BHT application inhibited PIN1 exocytosis. Overall rates of PIN2 internalization were less pronounced than those of PIN1 in seedlings pre-incubated with BHT before BFA treatment, and PIN2 exocytosis was not inhibited by BHT, indicating a specific activity of BHT on PIN1 exocytosis. Comparison of BHT activity with other potential stimuli of PIN1 and PIN2 trafficking [e.g., H2O2 (ROS), salt stress, reduced glutathione (GSH), dithiothreitol (DTT), and flavonoids] showed that BHT has a new activity distinct from the activities of other regulators of PIN trafficking. The findings support BHT as a potentially interesting pharmacological tool for dissecting PIN trafficking and auxin transport.

The Asymmetric Expression of SAUR Genes Mediated by ARF7/19 Promotes the Gravitropism and Phototropism of Plant Hypocotyls

The asymmetric distribution of auxin leads to the bending growth of hypocotyls during gravitropic and phototropic responses, but the signaling events downstream of auxin remain unclear. Here, we identify many SAUR genes showing asymmetric expression in soybean hypocotyls during gravistimulation and then study their homologs in Arabidopsis. SAUR19 subfamily genes have asymmetric expression in Arabidopsis hypocotyls during gravitropic and phototropic responses, induced by the lateral redistribution of auxin. Both the mutation of SAUR19 subfamily genes and the ectopic expression of SAUR19 weaken these tropic responses, indicating the critical role of their asymmetric expression. The auxin-responsive transcription factor ARF7 may directly bind the SAUR19 promoter and activate SAUR19 expression asymmetrically in tropic responses. Taken together, our results reveal that a gravity- or light-triggered asymmetric auxin distribution induces the asymmetric expression of SAUR19 subfamily genes by ARF7 and ARF19 in the hypocotyls, which leads to bending growth during gravitropic and phototropic responses.

The change of gravity vector induces short-term phosphoproteomic alterations in Arabidopsis

Plants can sense the gravitational force. When plants perceive a change in this natural force, they tend to reorient their organs with respect to the direction of the gravity vector, i.e., the shoot stem curves up. In the present study, we performed a 4C quantitative phosphoproteomics to identify those altered protein phosphosites resulting from 150 s of reorientation of Arabidopsis plants on earth. A total of 5556 phosphopeptides were identified from the gravistimulated Arabidopsis. Quantification based on the 15N-stable isotope labeling in Arabidopsis (SILIA) and computational analysis of the extracted ion chromatogram (XIC) of phosphopeptides showed eight and five unique PTM peptide arrays (UPAs) being up- and down-regulated, respectively, by gravistimulation. Among the 13 plant reorientation-responsive protein groups, many are related to the cytoskeleton dynamic and plastid movement. Interestingly, the most gravistimulation-responsive phosphosites are three serine residues, S350, S376, and S410, of a blue light receptor Phototropin 1 (PHOT1). The immunoblots experiment confirmed that the change of gravity vector indeed affected the phosphorylation level of S410 in PHOT1. The functional role of PHOT1 in gravitropic response was further validated with gravicurvature measurement in the darkness of both the loss-of-function double mutant phot1phot2 and its complementary transgenic plant PHOT1/phot1phot2. SignificanceThe organs of sessile organisms, plants, are able to move in response to environmental stimuli, such as gravity vector, touch, light, water, or nutrients, which is termed tropism. For instance, the bending of plant shoots to the light source is called phototropism. Since all plants growing on earth are continuously exposed to the gravitational field, plants receive the mechanical signal elicited by the gravity vector change and convert it into plant morphogenesis, growth, and development. Past studies have resulted in various hypotheses for gravisensing, but our knowledge about how the signal of gravity force is transduced in plant cells is still minimal.In the present study, we performed a SILIA-based 4C quantitative phosphoproteomics on 150-s gravistimulated Arabidopsis seedlings to explore the phosphoproteins involved in the gravitropic response. Our data demonstrated that such a short-term reorientation of Arabidopsis caused changes in phosphorylation of cytoskeleton structural proteins like Chloroplast Unusual Positioning1 (CHUP1), Patellin3 (PATL3), and Plastid Movement Impaired2 (PMI2), as well as the blue light receptor Phototropin1 (PHOT1). These results suggested that protein phosphorylation plays a crucial role in gravisignaling, and two primary tropic responses of plants, gravitropism and phototropism, may share some common components and signaling pathways.We expect that the phosphoproteins detected from this study will facilitate the subsequent molecular and cellular studies on the mechanism underlying the signal transduction in plant gravitropic response.

Root Tropisms: Investigations on Earth and in Space to Unravel Plant Growth Direction.

Root tropisms are important responses of plants, allowing them to adapt their growth direction. Research on plant tropisms is indispensable for future space programs that envisage plant-based life support systems for long-term missions and planet colonization. Root tropisms encompass responses toward or away from different environmental stimuli, with an underexplored level of mechanistic divergence. Research into signaling events that coordinate tropistic responses is complicated by the consistent coincidence of various environmental stimuli, often interacting via shared signaling mechanisms. On Earth the major determinant of root growth direction is the gravitational vector, acting through gravitropism and overruling most other tropistic responses to environmental stimuli. Critical advancements in the understanding of root tropisms have been achieved nullifying the gravitropic dominance with experiments performed in the microgravity environment. In this review, we summarize current knowledge on root tropisms to different environmental stimuli. We highlight that the term tropism must be used with care, because it can be easily confused with a change in root growth direction due to asymmetrical damage to the root, as can occur in apparent chemotropism, electrotropism, and magnetotropism. Clearly, the use of Arabidopsis thaliana as a model for tropism research contributed much to our understanding of the underlying regulatory processes and signaling events. However, pronounced differences in tropisms exist among species, and we argue that these should be further investigated to get a more comprehensive view of the signaling pathways and sensors. Finally, we point out that the Cholodny-Went theory of asymmetric auxin distribution remains to be the central and unifying tropistic mechanism after 100 years. Nevertheless, it becomes increasingly clear that the theory is not applicable to all root tropistic responses, and we propose further research to unravel commonalities and differences in the molecular and physiological processes orchestrating root tropisms.

A Positive Tropism of Rice Roots toward a Nutrient Source.

Plants take up water and nutrients through roots, and uptake efficiency depends on root behavior. Roots recognize the moisture gradient in the soil and grow toward the direction of high moisture. This phenomenon is called hydrotropism, and it contributes to efficient water uptake. As nutrients in soil are also unevenly distributed, it is beneficial for plants to grow their roots in the direction of increasing nutrient concentrations, but such a phenomenon has not been demonstrated. Here, we describe the directional growth of roots in response to a nutrient gradient. Using our assay system, the gradient of a nitrogen nutrient, NH4+, was sufficient to stimulate positive tropic responses of rice lateral roots. This phenomenon is a tropism of plant roots to nutrients; hence, we propose the name 'nutritropism'. As well as other tropisms, differential cell elongation was observed before the elongation zone during nutritropism, but the pattern promoting cell elongation preferentially on the non-stimulated side was opposite to those in root hydrotropism and gravitropism. Our evaluation of the NH4+ gradient suggested that the root tips responded to a sub-micromolar difference in NH4+ concentration on both sides of the root. Hydrotropism, gravitropism and phototropism were described in plants as the 'power of movement' by Charles and Francis Darwin in 1880, and these three tropisms have attracted the attention of plant scientists for more than 130 years. Our discovery of nutritropism represents the fourth 'power of movement' in plants and provides a novel root behavioral property used by plants to acquire nutrients efficiently.