Modulation of plant root growth by nitrogen source-defined regulation of polar auxin transport
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ABSTRACT: Availability of the essential macronutrient nitrogen in soil plays a critical role in plant growth, development and impacts agricultural productivity. Plants have evolved different strategies for sensing and responding to heterogeneous nitrogen distribution. Modulation of root system architecture, including primary root growth and branching, is among the most essential plant adaptions to ensure adequate nitrogen acquisition. However, the immediate molecular pathways coordinating the adjustment of root growth in response to distinct nitrogen sources, such as nitrate or ammonium, are poorly understood. Here, we show that growth as manifested by cell division and elongation is synchronized by coordinated auxin flux between two adjacent outer tissue layers of the root. This coordination is achieved by nitrate-dependent dephosphorylation of the PIN2 auxin efflux carrier at a previously uncharacterized phosphorylation site, leading to subsequent PIN2 lateralization and thereby regulating auxin flow between adjacent tissues. A dynamic computer model based on our experimental data successfully recapitulates experimental observations. Our study provides mechanistic insights broadening our understanding of root growth mechanisms in dynamic environments.
Project description:Availability of the essential macronutrient nitrogen in soil plays a critical role in plant growth, development, and impacts agricultural productivity. Plants have evolved different strategies for sensing and responding to heterogeneous nitrogen distribution. Modulation of root system architecture, including primary root growth and branching, is among the most essential plant adaptions to ensure adequate nitrogen acquisition. However, the immediate molecular pathways coordinating the adjustment of root growth in response to distinct nitrogen sources, such as nitrate or ammonium, are poorly understood. Here, we show that growth as manifested by cell division and elongation is synchronized by coordinated auxin flux between two adjacent outer tissue layers of the root. This coordination is achieved by nitrate-dependent dephosphorylation of the PIN2 auxin efflux carrier at a previously uncharacterized phosphorylation site, leading to subsequent PIN2 lateralization and thereby regulating auxin flow between adjacent tissues. A dynamic computer model based on our experimental data successfully recapitulates experimental observations. Our study provides mechanistic insights broadening our understanding of root growth mechanisms in dynamic environments.
Project description:Plant development is governed by signaling molecules called phytohormones. Typically, in certain developmental processes more than 1 hormone is implicated and, thus, coordination of their overlapping activities is crucial for correct plant development. However, molecular mechanisms underlying the hormonal crosstalk are only poorly understood. Multiple hormones including cytokinin and auxin have been implicated in the regulation of root development. Here we dissect the roles of cytokinin in modulating growth of the primary root. We show that cytokinin effect on root elongation occurs through ethylene signaling whereas cytokinin effect on the root meristem size involves ethylene-independent modulation of transport-dependent asymmetric auxin distribution. Exogenous or endogenous modification of cytokinin levels and cytokinin signaling lead to specific changes in transcription of several auxin efflux carrier genes from the PIN family having a direct impact on auxin efflux from cultured cells and on auxin distribution in the root apex. We propose a novel model for cytokinin action in regulating root growth: Cytokinin influences cell-to-cell auxin transport by modification of expression of several auxin transport components and thus modulates auxin distribution important for regulation of activity and size of the root meristem.
Project description:Flooding is a severe limitation for crop production worldwide. Unlike other crop plants, rice (Oryza sativa L.) is well adapted to partial submergence rendering it a suitable crop plant to understand flooding tolerance. Formation of adventitious roots (ARs), that support or replace the main root system, is a characteristic response to flooding. In rice, AR emergence is induced by ethylene and in the dark where roots grow upward. We used the synthetic auxins 2,4-D and α-NAA, and the auxin transport inhibitor naphthylphtalamic acid (NPA) to study emergence, growth rate and growth angle of ARs. While α-NAA had no effect, NPA and 2,4-D reduced the root elongation rate and the angle with a stronger effect on root angle in the dark than in the light. Furthermore, NPA delayed emergence of AR primordia suggesting that efflux carrier-mediated auxin transport is required for all aspects of directed AR growth. Expression analysis using OsPIN:GUS reporter lines revealed that OsPIN1b and OsPIN1c promoters were active in the stele and root cap in accord with their predicted role in acropetal auxin transport. OsPIN2 was expressed at the root tip and was reduced in the presence of NPA. Auxin activity, detected with DR5:VENUS, increased in primordia following growth induction. By contrast, auxin activity was high in epidermal cells above primordia and declined following growth induction suggesting that auxin levels are antagonistically regulated in AR primordia and in epidermal cells above AR primordia suggesting that auxin signaling contributes to the coordinated processes of epidermal cell death and AR emergence.
Project description:Recovery of the root system following physical damage is an essential issue for plant survival. An injured root system is able to regenerate by increases in lateral root (LR) number and acceleration of root growth. The horticultural technique of root pruning (root cutting) is an application of this response and is a common garden technique for controlling plant growth. Although root pruning is widely used, the molecular mechanisms underlying the subsequent changes in the root system are poorly understood. In this study, root pruning was employed as a model system to study the molecular mechanisms of root system regeneration. Notably, LR defects in wild-type plants treated with inhibitors of polar auxin transport (PAT) or in the auxin signaling mutant auxin/indole-3-acetic acid19/massugu2 were recovered by root pruning. Induction of IAA19 following root pruning indicates an enhancement of auxin signaling by root pruning. Endogenous levels of IAA increased after root pruning, and YUCCA9 was identified as the primary gene responsible. PAT-related genes were induced after root pruning, and the YUCCA inhibitor yucasin suppressed root regeneration in PAT-related mutants. Therefore, we demonstrate the crucial role of YUCCA9, along with other redundant YUCCA family genes, in the enhancement of auxin biosynthesis following root pruning. This further enhances auxin transport and activates downstream auxin signaling genes, and thus increases LR number.
Project description:Source-to-sink carbon (C) allocation driven by the sink strength, i.e., the ability of a sink organ to import C, plays a central role in tissue growth and biomass productivity. However, molecular drivers of sink strength have not been thoroughly characterized in trees. Auxin, as a major plant phytohormone, regulates the mobilization of photoassimilates in source tissues and elevates the translocation of carbohydrates toward sink organs, including roots. In this study, we used an ‘auxin-stimulated carbon sink’ approach to understand the molecular processes involved in the long-distance source-sink C allocation in poplar. Poplar cuttings were foliar sprayed with polar auxin transport modulators, including auxin enhancers (AE) (i.e., IBA and IAA) and auxin inhibitor (AI) (i.e., NPA), followed by a comprehensive analysis of leaf, stem, and root tissues using biomass evaluation, phenotyping, C isotope labeling, metabolomics, and transcriptomics approaches. Auxin modulators altered root dry weight and branching pattern, and AE increased photosynthetically fixed C allocation from leaf to root tissues. The transcriptome analysis identified highly expressed genes in root tissue under AE condition including transcripts encoding polygalacturonase and β-amylase that could increase the sink size and activity. Metabolic analyses showed a shift in overall metabolism including an altered relative abundance levels of galactinol, and an opposite trend in citrate levels in root tissue under AE and AI conditions. In conclusion, we postulate a model suggesting that the source-sink C relationships in poplar could be fueled by mobile sugar alcohols, starch metabolism-derived sugars, and TCA-cycle intermediates as key molecular drivers of sink strength.
Project description:Source-to-sink carbon (C) allocation driven by the sink strength, i.e., the ability of a sink organ to import C, plays a central role in tissue growth and biomass productivity. However, molecular drivers of sink strength have not been thoroughly characterized in trees. Auxin, as a major plant phytohormone, regulates the mobilization of photoassimilates in source tissues and elevates the translocation of carbohydrates toward sink organs, including roots. In this study, we used an 'auxin-stimulated carbon sink' approach to understand the molecular processes involved in the long-distance source-sink C allocation in poplar. Poplar cuttings were foliar sprayed with polar auxin transport modulators, including auxin enhancers (AE) (i.e., IBA and IAA) and auxin inhibitor (AI) (i.e., NPA), followed by a comprehensive analysis of leaf, stem and root tissues using biomass evaluation, phenotyping, C isotope labeling, metabolomics and transcriptomics approaches. Auxin modulators altered root dry weight and branching pattern, and AE increased photosynthetically fixed C allocation from leaf to root tissues. The transcriptome analysis identified highly expressed genes in root tissue under AE condition including transcripts encoding polygalacturonase and β-amylase that could increase the sink size and activity. Metabolic analyses showed a shift in overall metabolism including an altered relative abundance levels of galactinol, and an opposite trend in citrate levels in root tissue under AE and AI conditions. In conclusion, we postulate a model suggesting that the source-sink C relationships in poplar could be fueled by mobile sugar alcohols, starch metabolism-derived sugars and TCA-cycle intermediates as key molecular drivers of sink strength.
Project description:Melatonin (N-acetyl-5-methoxytryptamine) plays important roles in regulating both biotic and abiotic stress tolerance, biological rhythms, plant growth and development. Sharing the same substrate (tryptophan) for the biosynthesis, melatonin and auxin also have similar effects in plant development. However, the specific function of melatonin in modulating plant root growth and the relationship between melatonin and auxin as well as underlying mechanisms are still unclear. In this study, we found high concentration of melatonin remarkably inhibited root growth in Arabidopsis by reducing root meristem size. Further studies showed that melatonin negatively regulated auxin biosynthesis, the expression of PINFORMED (PIN) proteins as well as auxin response in Arabidopsis. Moreover, the root growth of the triple mutant pin1pin3pin7 was more tolerant than that of wild-type in response to melatonin treatment, suggesting the essential role of PIN1/3/7 in melatonin-mediated root growth. Combination treatment of melatonin and 5-Triiodobenzoic acid (TIBA) did not enhance melatonin-mediated reduction of root meristem size, indicating that polar auxin transport (PAT) may be necessary for the regulation of root meristem size by melatonin treatment. Taken together, this study indicates that melatonin regulates root growth in Arabidopsis, through auxin synthesis and polar auxin transport, at least partially.
Project description:The interplay between myosin- and auxin-mediated processes was investigated by following root development in the triple myosin knockout mutant xi-k xi-1 xi-2 (3KO). It was found that the 3KO plants generated significantly more lateral and adventitious roots than the wild-type plants or the rescued plant line expressing functional myosin XI-K:yellow fluorescent protein (YFP; 3KOR). Using the auxin-dependent reporter DR5:venus, a significant change in the auxin gradient toward the root tip was found in 3KO plants, which correlated with the loss of polar localization of the auxin transporter PIN1 in the stele and with the increased number of stele cells with oblique cell walls. Interestingly, myosin XI-K:YFP was localized to the cell division apparatus in the root and shoot meristems. In anaphase and early telophase, XI-K:YFP was concentrated in the midzone and the forming cell plate. In late telophase, XI-K:YFP formed a ring that overlapped with the growing phragmoplast. Myosin receptors MyoB1 and MyoB2 that are highly expressed throughout the plant were undetectable in dividing cells, suggesting that the myosin function in cell division relies on distinct adaptor proteins. These results suggest that myosin XIs are involved in orchestrating root organogenesis via effects on polar distribution of auxin responses and on cell division.
Project description:The flattened leaf form is an important adaptation for efficient photosynthesis, and the developmental process of flattened leaves has been intensively studied. Classic microsurgery studies in potato and tomato suggest that the shoot apical meristem (SAM) communicates with the leaf primordia to promote leaf blade formation. More recently, it was found that polar auxin transport (PAT) could mediate this communication. However, it is unclear how the expression of leaf patterning genes is tailored by PAT routes originating from SAM. By combining experimental observations and computer model simulations, we show that microsurgical incisions and local inhibition of PAT in tomato interfere with auxin transport toward the leaf margins, reducing auxin response levels and altering the leaf blade shape. Importantly, oval auxin responses result in the bipolar expression of SlLAM1 that determines leaf blade formation. Furthermore, wounding caused by incisions promotes degradation of SlREV, a known regulator of leaf polarity. Additionally, computer simulations suggest that local auxin biosynthesis in early leaf primordia could remove necessity for external auxin supply originating from SAM, potentially explaining differences between species. Together, our findings establish how PAT near emerging leaf primordia determines spatial auxin patterning and refines SlLAM1 expression in the leaf margins to guide leaf flattening.
Project description:Maize is a globally important food, feed crop and raw material for the food and energy industry. Plant architecture optimization plays important roles in maize yield improvement. PIN-FORMED (PIN) proteins are important for regulating auxin spatiotemporal asymmetric distribution in multiple plant developmental processes. In this study, ZmPIN1a overexpression in maize increased the number of lateral roots and inhibited their elongation, forming a developed root system with longer seminal roots and denser lateral roots. ZmPIN1a overexpression reduced plant height, internode length and ear height. This modification of the maize phenotype increased the yield under high-density cultivation conditions, and the developed root system improved plant resistance to drought, lodging and a low-phosphate environment. IAA concentration, transport capacity determination and application of external IAA indicated that ZmPIN1a overexpression led to increased IAA transport from shoot to root. The increase in auxin in the root enabled the plant to allocate more carbohydrates to the roots, enhanced the growth of the root and improved plant resistance to environmental stress. These findings demonstrate that maize plant architecture can be improved by root breeding to create an ideal phenotype for further yield increases.