Tropic Responses Research Articles

Root hydrotropism: An update

While water shortage remains the single-most important factor influencing world agriculture, there are very few studies on how plants grow in response to water potential, i.e., hydrotropism. Terrestrial plant roots dwell in the soil, and their ability to grow and explore underground requires many sensors for stimuli such as gravity, humidity gradients, light, mechanical stimulations, temperature, and oxygen. To date, extremely limited information is available on the components of such sensors; however, all of these stimuli are sensed in the root cap. Directional growth of roots is controlled by gravity, which is fixed in direction and intensity. However, other environmental factors, such as water potential gradients, which fluctuate in time, space, direction, and intensity, can act as a signal for modifying the direction of root growth accordingly. Hydrotropism may help roots to obtain water from the soil and at the same time may participate in the establishment of the root system. Current genetic analysis of hydrotropism in Arabidopsis has offered new players, mainly AHR1, NHR1, MIZ1, and MIZ2, which seem to modulate how root caps sense and choose to respond hydrotropically as opposed to other tropic responses. Here we review the mechanism(s) by which these genes and the plant hormones abscisic acid and cytokinins coordinate hydrotropism to counteract the tropic responses to gravitational field, light or touch stimuli. The biological consequence of hydrotropism is also discussed in relation to water stress avoidance.

Plant tropisms: From Darwin to the International Space Station

Plant tropisms play a fundamental role in shaping the growth form of plants, and these fascinating movements are the focus of this thematic issue of the American Journal of Botany. The issue includes 16 reviews of the current literature and eight original manuscripts written by a diverse group of international experts in their respective fields. This special issue emphasizes tropistic responses to three fundamental stimuli governing plant growth: water, light, and gravity. We hope this issue will inform the current generation and inspire the next generation of plant biologists.

PIN auxin efflux carriers are necessary for pulse-induced but not continuous light-induced phototropism in Arabidopsis.

Auxin efflux carrier PIN-FORMED (PIN) proteins are thought to have central roles in regulating asymmetrical auxin translocation during tropic responses, including gravitropism and phototropism, in plants. Although PIN3 is known to be involved in phototropism in Arabidopsis (Arabidopsis thaliana), no severe defects of phototropism in any of the pin mutants have been reported. We show here that the pulse-induced, first positive phototropism is impaired partially in pin1, pin3, and pin7 single mutants, and severely in triple mutants. In contrast, such impairment was not observed in continuous-light-induced second positive phototropism. Analysis with an auxin-reporter gene demonstrated that PIN3-mediated auxin gradients participate in pulse-induced phototropism but not in continuous-light-induced phototropism. Similar functional separation was also applicable to PINOID, a regulator of PIN localization. Our results strongly suggest the existence of functionally distinct mechanisms i.e. a PIN-dependent mechanism in which transient stimulation is sufficient to induce phototropism, and a PIN-independent mechanism that requires continuous stimulation and does not operate in the former phototropism process. Although a previous study has proposed that blue-light photoreceptors, the phototropins, control PIN localization through the transcriptional down-regulation of PINOID, we could not detect this blue-light-dependent down-regulation event, suggesting that other as yet unknown mechanisms are involved in phototropin-mediated phototropic responses.

A phytochrome–phototropin light signaling complex at the plasma membrane

Phytochromes are red/far-red photochromic photoreceptors central to regulating plant development. Although they are known to enter the nucleus upon light activation and, once there, regulate transcription, this is not the complete picture. Various phytochrome effects are manifested much too rapidly to derive from changes in gene expression, whereas others seem to occur without phytochrome entering the nucleus. Phytochromes also guide directional responses to light, excluding a genetic signaling route and implying instead plasma membrane association and a direct cytoplasmic signal. However, to date, no such association has been demonstrated. Here we report that a phytochrome subpopulation indeed associates physically with another photoreceptor, phototropin, at the plasma membrane. Yeast two-hybrid methods using functional photoreceptor molecules showed that the phytochrome steering growth direction in Physcomitrella protonemata binds several phototropins specifically in the photoactivated Pfr state. Split-YFP studies in planta showed that the interaction occurs exclusively at the plasma membrane. Coimmunoprecipitation experiments provided independent confirmation of in vivo phy-phot binding. Consistent with this interaction being associated with a cellular signal, we found that phytochrome-mediated tropic responses are impaired in Physcomitrella phot(-) mutants. Split-YFP revealed a similar interaction between Arabidopsis phytochrome A and phototropin 1 at the plasma membrane. These associations additionally provide a functional explanation for the evolution of neochrome photoreceptors. Our results imply that the elusive phytochrome cytoplasmic signal arises through binding and coaction with phototropin at the plasma membrane.

An Auxin Influx Transporter Regulates Vascular Patterning in Cotyledons

From embryogenesis to fruit set, auxin governs many important aspects of plant growth and development. Through its ability to promote cell division and determine the direction of growth, auxin participates in tropic responses and dictates the layout of the plant. The spatial distribution of auxin

The SAUR19 subfamily of SMALL AUXIN UP RNA genes promote cell expansion

The plant hormone auxin controls numerous aspects of plant growth and development by regulating the expression of hundreds of genes. SMALL AUXIN UP RNA (SAUR) genes comprise the largest family of auxin-responsive genes, but their function is unknown. Although prior studies have correlated the expression of some SAUR genes with auxin-mediated cell expansion, genetic evidence implicating SAURs in cell expansion has not been reported. The Arabidopsis SAUR19, SAUR20, SAUR21, SAUR22, SAUR23, and SAUR24 (SAUR19-24) genes encode a subgroup of closely related SAUR proteins. We demonstrate that these SAUR proteins are highly unstable in Arabidopsis. However, the addition of an N-terminal GFP or epitope tag dramatically increases the stability of SAUR proteins. Expression of these stabilized SAUR fusion proteins in Arabidopsis confers numerous auxin-related phenotypes indicative of increased and/or unregulated cell expansion, including increased hypocotyl and leaf size, defective apical hook maintenance, and altered tropic responses. Furthermore, seedlings expressing an artificial microRNA targeting multiple members of the SAUR19-24 subfamily exhibit short hypocotyls and reduced leaf size. Together, these findings demonstrate that SAUR19-24 function as positive effectors of cell expansion. This regulation may be achieved through the modulation of auxin transport, as SAUR gain-of-function and loss-of-function seedlings exhibit increased and reduced basipetal indole-3-acetic acid transport, respectively. Consistent with this possibility, SAUR19-24 proteins predominantly localize to the plasma membrane.

Light and abscisic acid signalling are integrated by MIZ1 gene expression and regulate hydrotropic response in roots of Arabidopsis thaliana

Plant roots undergo tropic growth in response to environmental cues, and each tropic response is affected by several environmental stimuli. Even its importance, molecular regulation of hydrotropism has not been largely uncovered. Tropic responses including hydrotropism were impacted by other environmental signal. We found that hydrotropism was reduced in dark-grown seedling. Moreover, we found that the expression of MIZ1, an essential gene for hydrotropism, was regulated by light signal. From our genetic analysis, phytochrome A (phyA)-, phyB- and HY5-mediated blue-light signalling play curial roles in light-mediated induction of MIZ1 and hydrotropism. In addition, we found that abscisic acid (ABA) also induced MIZ1 expression. ABA treatment could recover weak hydrotropism and MIZ1 expression level of hy5, and ABA synthesis inhibitor, abamineSG, further reduced hydrotropic curvature of hy5. In contrast, ABA treatment did not affect ahydrotropic phenotype of miz1. These results suggest that ABA signalling regulates MIZ1 expression independently from light signalling. Our results demonstrate that environmental signals, such as light and stresses mediated by ABA signalling, are integrated into MIZ1 expression and thus regulate hydrotropism. These machineries will allow plants to acquire sufficient amounts of water.

GOLVEN Secretory Peptides Regulate Auxin Carrier Turnover during Plant Gravitropic Responses

Growth and development are coordinated by an array of intercellular communications. Known plant signaling molecules include phytohormones and hormone peptides. Although both classes can be implicated in the same developmental processes, little is known about the interplay between phytohormone action and peptide signaling within the cellular microenvironment. We show that genes coding for small secretory peptides, designated GOLVEN (GLV), modulate the distribution of the phytohormone auxin. The deregulation of the GLV function impairs the formation of auxin gradients and alters the reorientation of shoots and roots after a gravity stimulus. Specifically, the GLV signal modulates the trafficking dynamics of the auxin efflux carrier PIN-FORMED2 involved in root tropic responses and meristem organization. Our work links the local action of secretory peptides with phytohormone transport.

Stiff mutant genes of phycomyces affect turgor pressure and wall mechanical properties to regulate elongation growth rate.

Regulation of cell growth is paramount to all living organisms. In plants, algae and fungi, regulation of expansive growth of cells is required for development and morphogenesis. Also, many sensory responses of stage IVb sporangiophores of Phycomyces blakesleeanus are produced by regulating elongation growth rate (growth responses) and differential elongation growth rate (tropic responses). “Stiff” mutant sporangiophores exhibit diminished tropic responses and are found to be defective in at least five genes; madD, E, F, G, and J. Prior experimental research suggests that the defective genes affect growth regulation, but this was not verified. All the growth of the single-celled stalk of the stage IVb sporangiophore occurs in a short region termed the “growth zone.” Prior experimental and theoretical research indicates that elongation growth rate of the stage IVb sporangiophore can be regulated by controlling the cell wall mechanical properties within the growth zone and the magnitude of the turgor pressure. A quantitative biophysical model for elongation growth rate is required to elucidate the relationship between wall mechanical properties and turgor pressure during growth regulation. In this study, it is hypothesized that the mechanical properties of the wall within the growth zone of stiff mutant sporangiophores are different compared to wild type (WT). A biophysical equation for elongation growth rate is derived for fungal and plant cells with a growth zone. Two strains of stiff mutants are studied, C149 madD120 (−) and C216 geo- (−). Experimental results demonstrate that turgor pressure is larger but irreversible wall deformation rates within the growth zone and growth zone length are smaller for stiff mutant sporangiophores compared to WT. These findings can explain the diminished tropic responses of the stiff mutant sporangiophores. It is speculated that the defective genes affect the amount of wall-building material delivered to the inner cell wall.

Tropic responses of potato single-node explant cultures

A special in-vitro protocol was elaborated enabling the production of potato single-node explant plantlets that can be used as objects for tropic studies. In light-grown plantlets, achievement of a full (90?) phototropic (PT) curvature required 75 to 120 min of continuous unilateral blue light irradiation or 120-135 min of gravitropic stimulation (GT). Time-lapse photography revealed that the curves describing PT and GT bending have a sigmoid shape. Continuous BL irradiation was necessary for the induction of continuous PT bending. If the BL was turned off after 30-50 min of PT stimulation, the bending gradually decreased and stopped in darkness after 25.0 ? 2.0 min. Within this period, curvature increased by 15.5 ? 1.5?. When the BL was turned off upon completion of PT bending (when the plantlets reached an angle of 90?), the plantlets entered the phase of fast straightening. The 90? PT curvature was significantly exaggerated in darkness by turning the jars from a vertical to horizontal position providing 120.74 ? 2.5? as the final curvature angle after two more hours in darkness.

Division of labor: multiple and specialized controls of vegetative growth and development in a poplar tree

Plants size, shape and adaptability are determined in large by their capacity to sustain, spur, redirect or arrest localized and whole-plant growth. We show that GA signaling and metabolism determines the level and extent of shoot and root growth. Increase of GA concentrations or signaling leads to increased shoot growth but suppresses root development. These GA-related responses are underpinned by sets of highly specialized in their functions enzymes and signaling factors and cross-talk with other hormonal pathways. The differential effects of GA on root and shoot growth and development are likely associated with a regulatory mechanism responding to optimum and stress conditions. Ongoing work in the laboratory employs genomics and genetics approaches to more thoroughly understand poplar growth and development under stress conditions including nitrogen and water limitations. We use genetic networks analysis and forward genetics approaches to identify key regulators of poplar roots response to stress. Besides overall growth, trees show incredible repertoire of spatiotemporal regulation of growth in relation to control of organ size, growth periodicity and tropic responses. We have identified and characterized novel regulators of these processes in poplar.

A Hormonal Regulatory Module That Provides Flexibility to Tropic Responses

Plants orient their growth depending on directional stimuli such as light and gravity, in a process known as tropic response. Tropisms result from asymmetrical accumulation of auxin across the responding organ relative to the direction of the stimulus, which causes differential growth rates on both sides of the organ. Here, we show that gibberellins (GAs) attenuate the gravitropic reorientation of stimulated hypocotyls of dark-grown Arabidopsis (Arabidopsis thaliana) seedlings. We show that the modulation occurs through induction of the expression of the negative regulator of auxin signaling INDOLE-3-ACETIC ACID INDUCIBLE19/MASSUGU2. The biological significance of this regulatory mechanism involving GAs and auxin seems to be the maintenance of a high degree of flexibility in tropic responses. This notion is further supported by observations that GA-deficient seedlings showed a much lower variance in the response to gravity compared to wild-type seedlings and that the attenuation of gravitropism by GAs resulted in an increased phototropic response. This suggests that the interplay between auxin and GAs may be particularly important for plant orientation under competing tropic stimuli.

Development and Application of the Teaching Programs for the Pre-service Teachers Regarding Tropic Responses in Arabidopsis thaliana Based on Creative Problem Solving Model of Life Science

Development and Application of the Teaching Programs for the Pre-service Teachers Regarding Tropic Responses in Arabidopsis thaliana Based on Creative Problem Solving Model of Life Science

Alkoxy-auxins Are Selective Inhibitors of Auxin Transport Mediated by PIN, ABCB, and AUX1 Transporters

Polar auxin movement is a primary regulator of programmed and plastic plant development. Auxin transport is highly regulated at the cellular level and is mediated by coordinated transport activity of plasma membrane-localized PIN, ABCB, and AUX1/LAX transporters. The activity of these transporters has been extensively analyzed using a combination of pharmacological inhibitors, synthetic auxins, and knock-out mutants in Arabidopsis. However, efforts to analyze auxin-dependent growth in other species that are less tractable to genetic manipulation require more selective inhibitors than are currently available. In this report, we characterize the inhibitory activity of 5-alkoxy derivatives of indole 3-acetic acid and 7-alkoxy derivatives of naphthalene 1-acetic acid, finding that the hexyloxy and benzyloxy derivatives act as potent inhibitors of auxin action in plants. These alkoxy-auxin analogs inhibit polar auxin transport and tropic responses associated with asymmetric auxin distribution in Arabidopsis and maize. The alkoxy-auxin analogs inhibit auxin transport mediated by AUX1, PIN, and ABCB proteins expressed in yeast. However, these analogs did not inhibit or activate SCF(TIR1) auxin signaling and had no effect on the subcellular trafficking of PIN proteins. Together these results indicate that alkoxy-auxins are inactive auxin analogs for auxin signaling, but are recognized by PIN, ABCB, and AUX1 auxin transport proteins. Alkoxy-auxins are powerful new tools for analyses of auxin-dependent development.

Inflorescence stems of the mdr1 mutant display altered gravitropism and phototropism

Auxins are a major group of growth regulators that are involved in all stages of plant growth and development, and these molecules play a key role in the response phase of both phototropism and gravitropism. Polar auxin transport within tissues is mediated by auxin influx and auxin efflux carriers that are asymmetrically distributed across cell membranes. The MDR ( multi drug resistance) family of proteins has been identified in plants and is involved in the transport of auxin. In these studies, we examined tropistic responses in the mdr1 (= abcb19) mutant of Arabidopsis thaliana using conventional reorientation studies as well as a computer-based feedback system to investigate the kinetics of root gravitropism and phototropism. Time course of curvature studies show that tropisms were not affected in light-grown seedlings of mdr1 mutants, with the exception of an enhancement of gravitropism in hypocotyls. In mature plants, mdr1 inflorescence stems exhibit attenuated gravitropism but have enhanced phototropic curvature in response to unidirectional blue illumination. Thus, the decreased auxin flow in the mdr1 mutant has a more profound effect in inflorescence stems compared to hypocotyls of seedlings, possibly due to alterations in membrane trafficking pathways in gravity-perceiving endodermal cells of stems. Our data add support to the hypothesis that hypocotyls and stems differ in their cellular mechanisms of gravitropism.

Activation tagging is an effective gene tagging system in Populus

Knowledge of the functional relationship between genes and organismal phenotypes in perennial plants is extremely limited. Using a population of 627 independent events, we assessed the feasibility of activation tagging as a forward genetics tool for Populus. Mutant identification after 2 years of field testing was nearly sevenfold (6.5%) higher than in greenhouse studies that employed Arabidopsis and identical transformation vectors. Approximately two thirds of all mutant phenotypes were not seen in vitro and in the greenhouse; they were discovered only after the second year of field assessment. The trees’ large size (5-10 m in height), perennial growth, and interactions with the natural environment are factors that are thought to have contributed to the high rate of observable phenotypes in the field. The mutant phenotypes affected a variety of morphological and physiological traits, including leaf size and morphology, crown architecture, stature, vegetative dormancy, and tropic responses. Characterization of the insertion in more than 100 events with and without mutant phenotypes showed that tags predominantly (70%) inserted in a 13-Kbp region up- and downstream of the genes’ coding regions with approximately even distribution among the 19 chromosomes. Transcriptional activation was observed in many proximal genes studied. Successful phenotype recapitulation was observed in 10 of 12 retransformed genes tested, indicating true tagging and a functional relationship between the genes and observed phenotypes for most activation lines. Our studies indicate that in addition to associating mapping and QTL approaches, activation tagging can be used successfully as an effective forward gene discovery tool in Populus.

Signaling in plants

This book provides detailed coverage of a wide range of aspects of signalling and emphasizes the critical importance of such signalling to plants, which as sessile organisms are at the mercy of their biotic and abiotic environment. The book provides insight into how plants can respond and overcome such challenges. The dynamism of plant signalling pathways are clearly demonstrated, for example in cell wall composition, in which mutants are described that result in different wall compositions and relative amounts of wall components can be altered, indicating a sensing-and-response network to this pathway. The book breaks plant cells down into their varying components and provides individual chapters describing how different aspects of the cell relate to whole-plant physiology. This provides the reader with the opportunity to focus on specific aspects of signalling, or to get to grips with various aspects in manageable pieces. Given that the book fragments the different signalling mechanisms into ‘bite-sized pieces’ it also achieves good integration of these pieces. The authors link very well between the different chapters with citations of relevant sections from other chapters covering related topics. This makes following links between different responses and stimuli easier. Chapters are included that provide detailed analysis of specific signalling molecules, with particular focus paid to auxin and calcium, and they also cover situations where perception of changing environments are critical, such as tropic responses and stress responses. The fundamental role of auxin is described in a number of chapters. This is particularly evident in the chapter focusing on the specific role of auxin signalling, for example the role of auxin associated with modifications to the cell wall and gravitropism. The book provides a valuable perspective to the dynamic nature of plants. It clearly demonstrates the complexity of the response networks and shows how plants are able to respond to the changing environments they face, both on a daily and seasonal basis, but also under conditions of stress, including pathogen attack, oxygen deprivation and also in symbiotic associations, such as with rhizobial bacteria. The book also provides a good perspective on how different aspects of cells interact with each other, and the dynamism of the actin cytoskeleton in accommodating growth and development is an excellent example of this. This chapter illustrates how the actin cytoskeleton serves as a signalling hub that regulates numerous activities, such as subcellular compartmentation, organelle and vesicle mobility and structural reinforcement. The book provides an excellent historical perspective to signalling in plants; this is put into context with recent developments and the use of molecular genetic analysis to confirm hypotheses that were made by physiological observations. In addition, most of the chapters also discuss recent developments and provide observations on where the key targets for future research currently are. This provides the reader with a focused outlook on how the fields are developing and what are the major questions that can be addressed with the new post-genomics tools. The illustrations are of high quality; although some chapters have only a limited number of figures and pathways included, these provide valuable information and aid the reader in understanding the complex interactions and pathways that are being discussed. Overall this volume provides a very focused discussion of key topics of plant signalling and provides the reader with good opportunities to follow the specific areas in more detail. It will serve as an excellent resource for advanced graduate students and post-graduates who wish to obtain a detailed insight into plant cell signalling.

Understanding phototropism: from Darwin to today

Few individuals have had the lasting impact on such a breadth of science as Charles Darwin. While his writings about time aboard the HMS Beagle, his study of the Galapagos islands (geology, fauna, and flora), and his theories on evolution are well known, less appreciated are his studies on plant growth responses to a variety of environmental stimuli. In fact, Darwin, together with the help of his botanist son Francis, left us an entire book, 'The power of movements in plants', describing his many, varied, and insightful observations on this topic. Darwin's findings have provided an impetus for an entire field of study, the study of plant tropic responses, or differential growth (curvature) of plant organs in response to directional stimuli. One tropic response that has received a great deal of attention is the phototropic response, or curvature response to directional light. This review summarizes many of the most significant advancements that have been made in our understanding of this response and place these recent findings in the context of Darwin's initial observations.

The PIN-FORMED (PIN) protein family of auxin transporters

A review of the PIN auxin-efflux transporters, which have important roles in plant development.

Hormonal interactions during root tropic growth: hydrotropism versus gravitropism

Terrestrial plants have evolved remarkable morphological plasticity that enables them to adapt to their surroundings. One of the most important traits that plants have acquired is the ability to sense environmental cues and use them as a basis for governing their growth orientation. The directional growth of plant organs relative to the direction of environmental stimuli is a tropism. The Cholodny-Went theory proposes that auxin plays a key role in several tropisms. Recent molecular genetic studies have strongly supported this hypothesis for gravitropism. However, the molecular mechanisms of other tropisms are far less clear. Hydrotropism is the response of roots to a moisture gradient. Since its re-discovery in 1985, root hydrotropism has been shown to be common among higher plant species. Additionally, in some species, gravitropism interferes with hydrotropism, suggesting that both shared and divergent mechanisms mediating the two tropisms exist. This hypothesis has been supported by recent studies, which provide an understanding of how roots sense multiple environmental cues and exhibit different tropic responses. In this review, we focus on the overlapping and unique mechanisms of the hormonal regulation underlying gravitropism and hydrotropism in roots.