RESUMEN
TARGET OF RAPAMYCIN (TOR) is a conserved eukaryotic Ser/Thr protein kinase that coordinates growth and metabolism with nutrient availability. We conducted a medium-throughput functional genetic screen to discover essential genes that promote TOR activity in plants, and identified a critical regulatory enzyme, cytosolic phosphoribosyl pyrophosphate (PRPP) synthetase (PRS4). PRS4 synthesizes cytosolic PRPP, a key upstream metabolite in nucleotide synthesis and salvage pathways. We found that prs4 knockouts are embryo-lethal in Arabidopsis thaliana, and that silencing PRS4 expression in Nicotiana benthamiana causes pleiotropic developmental phenotypes, including dwarfism, aberrant leaf shape, and delayed flowering. Transcriptomic analysis revealed that ribosome biogenesis is among the most strongly repressed processes in prs4 knockdowns. Building on these results, we discovered that TOR activity is inhibited by chemical or genetic disruption of nucleotide biosynthesis, but that this effect can be reversed by supplying plants with nucleobases. Finally, we show that TOR transcriptionally promotes nucleotide biosynthesis to support the demands of ribosomal RNA synthesis. We propose that TOR coordinates ribosome biogenesis with nucleotide availability in plants to maintain metabolic homeostasis and support growth.
Asunto(s)
Proteínas de Arabidopsis/metabolismo , Arabidopsis/metabolismo , Nucleótidos/metabolismo , Fosfatidilinositol 3-Quinasas/metabolismo , Ribosomas/metabolismo , Arabidopsis/embriología , Arabidopsis/genética , Citosol/metabolismo , Silenciador del Gen , Genes de Plantas , Fósforo/metabolismo , Células Vegetales/metabolismo , Desarrollo de la Planta , Purinas/biosíntesis , Pirimidinas/biosíntesis , Nicotiana/metabolismo , Transcriptoma/genéticaRESUMEN
Narrow odd dwarf (nod) and Liguleless narrow (Lgn) are pleiotropic maize mutants that both encode plasma membrane proteins, cause similar developmental patterning defects, and constitutively induce stress signaling pathways. To investigate how these mutants coordinate maize development and physiology, we screened for protein interactors of NOD by affinity purification. LGN was identified by this screen as a strong candidate interactor, and we confirmed the NOD-LGN molecular interaction through orthogonal experiments. We further demonstrated that LGN, a receptor-like kinase, can phosphorylate NOD in vitro, hinting that they could act in intersecting signal transduction pathways. To test this hypothesis, we generated Lgn-R;nod mutants in two backgrounds (B73 and A619), and found that these mutations enhance each other, causing more severe developmental defects than either single mutation on its own, with phenotypes including very narrow leaves, increased tillering, and failure of the main shoot. Transcriptomic and metabolomic analyses of the single and double mutants in the two genetic backgrounds revealed widespread induction of pathogen defense genes and a shift in resource allocation away from primary metabolism in favor of specialized metabolism. These effects were similar in each single mutant and heightened in the double mutant, leading us to conclude that NOD and LGN act cumulatively in overlapping signaling pathways to coordinate growth-defense tradeoffs in maize.
Asunto(s)
Proteínas de Plantas , Zea mays , Zea mays/metabolismo , Proteínas de Plantas/genética , Proteínas de Plantas/metabolismo , Hojas de la Planta/metabolismo , Fenotipo , Mutación , Regulación de la Expresión Génica de las PlantasRESUMEN
Hypersensitive response (HR)-conferred resistance is associated with induction of programmed cell death and pathogen spread restriction in its proximity. The exact role of chloroplastic reactive oxygen species and its link with salicylic acid (SA) signaling in HR remain unexplained. To unravel this, we performed a detailed spatiotemporal analysis of chloroplast redox response in palisade mesophyll and upper epidermis to potato virus Y (PVY) infection in a resistant potato genotype and its transgenic counterpart with impaired SA accumulation and compromised resistance. Besides the cells close to the cell death zone, we detected individual cells with oxidized chloroplasts further from the cell death zone. These are rare in SA-deficient plants, suggesting their role in signaling for resistance. We confirmed that chloroplast redox changes play important roles in signaling for resistance, as blocking chloroplast redox changes affected spatial responses at the transcriptional level. Through spatiotemporal study of stromule induction after PVY infection, we show that stromules are induced by cell death and also as a response to PVY multiplication at the front of infection. Overall induction of stromules is attenuated in SA-deficient plants.
Asunto(s)
Potyvirus , Solanum tuberosum , Cloroplastos/metabolismo , Oxidación-Reducción , Comunicación Celular , Transducción de Señal , Apoptosis , Potyvirus/fisiología , Solanum tuberosum/genética , Enfermedades de las Plantas/genéticaRESUMEN
The coordinated redistribution of sugars from mature "source" leaves to developing "sink" leaves requires tight regulation of sugar transport between cells via plasmodesmata (PD). Although fundamental to plant physiology, the mechanisms that control PD transport and thereby support development of new leaves have remained elusive. From a forward genetic screen for altered PD transport, we discovered that the conserved eukaryotic glucose-TOR (TARGET OF RAPAMYCIN) metabolic signaling network restricts PD transport in leaves. Genetic approaches and chemical or physiological treatments to either promote or disrupt TOR activity demonstrate that glucose-activated TOR decreases PD transport in leaves. We further found that TOR is significantly more active in mature leaves photosynthesizing excess sugars than in young, growing leaves, and that this increase in TOR activity correlates with decreased rates of PD transport. We conclude that leaf cells regulate PD trafficking in response to changing carbohydrate availability monitored by the TOR pathway.
Asunto(s)
Proteínas de Arabidopsis/metabolismo , Arabidopsis/metabolismo , Fosfatidilinositol 3-Quinasas/metabolismo , Células Vegetales/metabolismo , Hojas de la Planta/metabolismo , Plasmodesmos/metabolismo , Arabidopsis/embriología , Arabidopsis/genética , Proteínas de Arabidopsis/genética , Transporte Biológico , Metabolismo de los Hidratos de Carbono , Perfilación de la Expresión Génica , Regulación del Desarrollo de la Expresión Génica , Regulación de la Expresión Génica de las Plantas , Técnicas de Silenciamiento del Gen , Silenciador del Gen , Hojas de la Planta/crecimiento & desarrollo , Transporte de Proteínas , Transducción de Señal , Nicotiana/genética , Nicotiana/metabolismoRESUMEN
mRNA translation is the growth rate-limiting step in genome expression. Target of rapamycin (TOR) evolved a central regulatory role in eukaryotes as a signaling hub that monitors nutrient availability to maintain homeostasis and promote growth, largely by increasing the rate of translation initiation and protein synthesis. The dynamic pathways engaged by TOR to regulate translation remain debated even in well-studied yeast and mammalian models, however, despite decades of intense investigation. Recent studies have firmly established that TOR also regulates mRNA translation in plants through conserved mechanisms, such as the TOR-LARP1-5'TOP signaling axis, and through pathways specific to plants. Here, we review recent advances in our understanding of the regulation of mRNA translation in plants by TOR.
Asunto(s)
Plantas , Sirolimus , Plantas/genética , Plantas/metabolismo , ARN Mensajero/metabolismo , Transducción de Señal/genética , Sirolimus/metabolismoRESUMEN
Liguleless narrow1 encodes a plasma membrane-localized receptor-like kinase required for normal development of maize (Zea mays) leaves, internodes, and inflorescences. The semidominant Lgn-R mutation lacks kinase activity, and phenotypic severity is dependent on inbred background. We created near isogenic lines and assayed the phenotype in multiple environments. Lgn-R plants that carry the B73 version of Sympathy for the ligule (Sol-B) fail to grow under hot conditions, but those that carry the Mo17 version (Sol-M) survive at hot temperatures and are significantly taller at cool temperatures. To identify Sol, we used recombinant mapping and analyzed the Lgn-R phenotype in additional inbred backgrounds. We identified amino acid sequence variations in GRMZM2G075262 that segregate with severity of the Lgn-R phenotypes. This gene is expressed at high levels in Lgn-R B73, but expression drops to nonmutant levels with one copy of Sol-M An EMS mutation solidified the identity of SOL as a maize homolog of Arabidopsis (Arabidopsis thaliana) ENHANCED DISEASE RESISTANCE4 (EDR4). SOL, like EDR4, is induced in response to pathogen-associated molecular patterns such as flg22. Integrated transcriptomic and phosphoproteomic analyses suggest that Lgn-R plants constitutively activate an immune signaling cascade that induces temperature-sensitive responses in addition to defects in leaf development. We propose that aspects of the severe Lgn-R developmental phenotype result from constitutive defense induction and that SOL potentially functions in repressing this response in Mo17 but not B73. Identification of LGN and its interaction with SOL provides insight into the integration of developmental control and immune responses.
Asunto(s)
Proteínas de Arabidopsis/metabolismo , Arabidopsis/metabolismo , Arabidopsis/genética , Proteínas de Arabidopsis/genética , Mutación/genética , Proteínas de Plantas/genética , Proteínas de Plantas/metabolismo , Zea mays/genética , Zea mays/metabolismoRESUMEN
Plasmodesmata (PD) are essential for plant development, but little is known about their regulation. Several studies have linked PD transport to chloroplast-centered signaling networks, but the physiological significance of this connection remains unclear. Here, we show that PD transport is strongly regulated by light and the circadian clock. Light promotes PD transport during the day, but light is not sufficient to increase rates of PD transport at night, suggesting a circadian gating mechanism. Silencing expression of the core circadian clock gene, LHY/CCA1, allows light to strongly promote PD transport during subjective night, confirming that the canonical plant circadian clock controls the PD transport light response. We conclude that PD transport is dynamically regulated during the day/night cycle. Due to the many roles of PD in plant biology, this discovery has strong implications for plant development, physiology, and pathogenesis.
Asunto(s)
Arabidopsis/fisiología , Relojes Circadianos/efectos de la radiación , Luz , Nicotiana/fisiología , Células Vegetales/metabolismo , Células Vegetales/efectos de la radiación , Plasmodesmos/metabolismo , Plasmodesmos/efectos de la radiación , Arabidopsis/efectos de la radiación , Transporte Biológico/efectos de la radiación , Fotoperiodo , Hojas de la Planta/crecimiento & desarrollo , Hojas de la Planta/efectos de la radiación , Nicotiana/efectos de la radiaciónRESUMEN
A fundamental mystery of plant cell biology is the occurrence of "stromules," stroma-filled tubular extensions from plastids (such as chloroplasts) that are universally observed in plants but whose functions are, in effect, completely unknown. One prevalent hypothesis is that stromules exchange signals or metabolites between plastids and other subcellular compartments, and that stromules are induced during stress. Until now, no signaling mechanisms originating within the plastid have been identified that regulate stromule activity, a critical missing link in this hypothesis. Using confocal and superresolution 3D microscopy, we have shown that stromules form in response to light-sensitive redox signals within the chloroplast. Stromule frequency increased during the day or after treatment with chemicals that produce reactive oxygen species specifically in the chloroplast. Silencing expression of the chloroplast NADPH-dependent thioredoxin reductase, a central hub in chloroplast redox signaling pathways, increased chloroplast stromule frequency, whereas silencing expression of nuclear genes related to plastid genome expression and tetrapyrrole biosynthesis had no impact on stromules. Leucoplasts, which are not photosynthetic, also made more stromules in the daytime. Leucoplasts did not respond to the same redox signaling pathway but instead increased stromule formation when exposed to sucrose, a major product of photosynthesis, although sucrose has no impact on chloroplast stromule frequency. Thus, different types of plastids make stromules in response to distinct signals. Finally, isolated chloroplasts could make stromules independently after extraction from the cytoplasm, suggesting that chloroplast-associated factors are sufficient to generate stromules. These discoveries demonstrate that chloroplasts are remarkably autonomous organelles that alter their stromule frequency in reaction to internal signal transduction pathways.
Asunto(s)
Cloroplastos/metabolismo , Nicotiana/metabolismo , Transducción de Señal , Secuencia de Bases , Benzoquinonas/farmacología , Cloroplastos/ultraestructura , Ritmo Circadiano/efectos de los fármacos , Diurona/farmacología , Silenciador del Gen/efectos de los fármacos , Proteínas Fluorescentes Verdes/metabolismo , Modelos Biológicos , Datos de Secuencia Molecular , NADP/metabolismo , Oxidación-Reducción/efectos de los fármacos , Fotosíntesis/efectos de los fármacos , Filogenia , Epidermis de la Planta/efectos de los fármacos , Epidermis de la Planta/metabolismo , Proteínas de Plantas/genética , Proteínas de Plantas/metabolismo , Especies Reactivas de Oxígeno/metabolismo , Transducción de Señal/efectos de los fármacos , Sacarosa/farmacología , Reductasa de Tiorredoxina-Disulfuro/metabolismo , Imagen de Lapso de Tiempo , Nicotiana/efectos de los fármacosRESUMEN
Plant genomes contain large numbers of cell surface leucine-rich repeat (LRR) and intracellular nucleotide binding (NB)-LRR immune receptors encoded by resistance (R) genes that recognize specific pathogen effectors and trigger resistance responses. The unregulated expression of NB-LRR genes can trigger autoimmunity in the absence of pathogen infection and inhibit plant growth. Despite the potential serious consequence on agricultural production, the mechanisms regulating R-gene expression are not well understood. We identified microRNA (miRNA) progenitor genes precursor transcripts, and two miRNAs [nta-miR6019 (22-nt) and nta-miR6020 (21-nt)] that guide cleavage of transcripts of the Toll and Interleukin-1 receptor-NB-LRR immune receptor N from tobacco that confers resistance to tobacco mosaic virus (TMV). We further showed that cleavage by nta-miR6019 triggers RNA-dependent RNA polymerase 6- and ribonuclease Dicer-like 4-dependent biogenesis of 21-nt secondary siRNAs "in phase" with the 22-nt miR6019 cleavage site. Furthermore, we found that processing of the 22-nt nta-miR6019 depended on an asymmetric bulge caused by mismatch in the nta-miR6019 precursor. Interestingly, coexpression of N with nta-miR6019 and nta-miR6020 resulted in attenuation of N-mediated resistance to TMV, indicating that these miRNAs have functional roles in NB-LRR regulation. Using a bioinformatics approach, we identified six additional 22-nt miRNA and two 21-nt miRNA families from three Solanaceae species-tobacco, tomato, and potato. We show that members of these miRNA families cleave transcripts of predicted functional R genes and trigger production of phased secondary 21-nt siRNAs. Our results demonstrate a conserved role for miRNAs and secondary siRNAs in NB-LRR/LRR immune receptor gene regulation and pathogen resistance in Solanaceae.
Asunto(s)
Inmunidad Innata , MicroARNs/fisiología , Nicotiana/fisiología , Receptores Inmunológicos/fisiología , Secuencia de Bases , Regulación de la Expresión Génica de las Plantas , Silenciador del Gen , MicroARNs/genética , Datos de Secuencia Molecular , Receptores Inmunológicos/genética , Homología de Secuencia de Ácido Nucleico , Nicotiana/inmunologíaRESUMEN
We use Arabidopsis thaliana embryogenesis as a model system for studying intercellular transport via plasmodesmata (PD). A forward genetic screen for altered PD transport identified increased size exclusion limit (ise) 1 and ise2 mutants with increased intercellular transport of fluorescent 10-kDa tracers. Both ise1 and ise2 exhibit increased formation of twinned and branched PD. ISE1 encodes a mitochondrial DEAD-box RNA helicase, whereas ISE2 encodes a DEVH-type RNA helicase. Here, we show that ISE2 foci are localized to the chloroplast stroma. Surprisingly, plastid development is defective in both ise1 and ise2 mutant embryos. In an effort to understand how RNA helicases that localize to different organelles have similar impacts on plastid and PD development/function, we performed whole-genome expression analyses. The most significantly affected class of transcripts in both mutants encode products that target to and enable plastid function. These results reinforce the importance of plastid-mitochondria-nucleus cross-talk, add PD as a critical player in the plant cell communication network, and thereby illuminate a previously undescribed signaling pathway dubbed organelle-nucleus-plasmodesmata signaling. Several genes with roles in cell wall synthesis and modification are also differentially expressed in both mutants, providing new targets for investigating PD development and function.
Asunto(s)
Núcleo Celular/metabolismo , Plasmodesmos/fisiología , Arabidopsis/fisiología , Comunicación Celular , Cloroplastos/metabolismo , Perfilación de la Expresión Génica , Regulación de la Expresión Génica de las Plantas , Silenciador del Gen , Genes de Plantas , Proteínas Fluorescentes Verdes/metabolismo , Microscopía Confocal/métodos , Modelos Genéticos , Mutación , Estructura Terciaria de Proteína , ARN Helicasas/metabolismo , Nicotiana/metabolismoRESUMEN
Recent studies suggest that intercellular transport via plasmodesmata (PD) is regulated by cellular redox state. Until now, this relationship has been unclear, as increased production of reactive oxygen species (ROS) has been associated with both increased and decreased intercellular transport via PD. Here, we show that silencing two genes that both increase transport via PD, INCREASED SIZE EXCLUSION LIMIT1 (ISE1) and ISE2, alters organelle redox state. Using redox-sensitive green fluorescent proteins targeted to the mitochondria or plastids, we show that, relative to wild-type leaves, plastids are more reduced in both ISE1- and ISE2-silenced leaves, whereas mitochondria are more oxidized in ISE1-silenced leaves. We further show that PD transport is positively regulated by ROS production in mitochondria following treatment with salicylhydroxamic acid but negatively regulated by an oxidative shift in both chloroplasts and mitochondria following treatment with paraquat. Thus, oxidative shifts in the mitochondrial redox state positively regulate intercellular transport in leaves, but oxidative shifts in the plastid redox state counteract this effect and negatively regulate intercellular transport. This proposed model reconciles previous contradictory evidence relating ROS production to PD transport and supports accumulating evidence that mitochondria and plastids are crucial regulators of PD function.
Asunto(s)
Arabidopsis/metabolismo , Mitocondrias/metabolismo , Plasmodesmos/metabolismo , Plastidios/metabolismo , Arabidopsis/genética , Proteínas de Arabidopsis/genética , Proteínas de Arabidopsis/metabolismo , Transporte Biológico/efectos de los fármacos , Cloroplastos/metabolismo , ARN Helicasas DEAD-box/genética , ARN Helicasas DEAD-box/metabolismo , Silenciador del Gen , Oxidación-Reducción , Paraquat/farmacología , Hojas de la Planta/efectos de los fármacos , Hojas de la Planta/genética , Hojas de la Planta/metabolismo , ARN Helicasas/genética , ARN Helicasas/metabolismo , Especies Reactivas de Oxígeno/metabolismo , Salicilamidas/farmacologíaRESUMEN
Higher plants generate new leaves from shoot meristems throughout their vegetative lifespan. The tempo of leaf initiation is dynamically regulated by physiological cues, but little is known about the underlying genetic signaling pathways that coordinate this rate. Two maize (Zea mays) mutants, terminal ear1 (te1) and phytochrome B1;phytochrome B2 (phyB1;phyB2), oppositely affect leaf initiation rates and total leaf number at the flowering time: te1 mutants make leaves faster whereas phyB1;phyB2 mutants make leaves slower than wild-type plants. To test whether PhyB1, PhyB2, and TE1 act in overlapping or distinct pathways to regulate leaf initiation, we crossed te1 and phyB1;phyB2 created an F2 population segregating for these three mutations and quantified various phenotypes among the resulting genotypes, including leaf number, leaf initiation rate, plant height, leaf length, leaf width, number of juvenile leaves, stalk diameter, and dry shoot biomass. Leaf number and initiation rate in phyB1;phyB2;te1 plants fell between the extremes of the two parents, suggesting an additive genetic interaction between te1 and phyB1;phyB2 rather than epistasis. Therefore, we conclude that PhyB1, PhyB2, and TE1 likely control leaf initiation through distinct signaling pathways.
Asunto(s)
Fitocromo B , Proteínas de Plantas , Proteínas de Unión al ARN , Zea mays , Mutación , Fitocromo B/genética , Fitocromo B/metabolismo , Hojas de la Planta/genética , Hojas de la Planta/metabolismo , Zea mays/fisiología , Proteínas de Plantas/metabolismo , Proteínas de Unión al ARN/metabolismoRESUMEN
Robustness is the invariant development of phenotype despite environmental changes and genetic perturbations. In the Arabidopsis flower bud, four sepals initiate at robust positions and times and grow to equal size to enclose and protect the inner floral organs. We previously characterized the mutant development related myb-like1 (drmy1), where 3-5 sepals initiate at irregular positions and variable times and grow to different sizes, compromising their protective function. The molecular mechanism underlying this loss of robustness was unclear. Here, we show that drmy1 has reduced TARGET OF RAPAMYCIN (TOR) activity, ribosomal content, and translation. Translation reduction decreases the protein level of ARABIDOPSIS RESPONSE REGULATOR7 (ARR7), a rapidly synthesized and degraded cytokinin signaling inhibitor. The resultant upregulation of cytokinin signaling disrupts the robust positioning of auxin signaling, causing variable sepal initiation. Our work shows that the homeostasis of translation, a ubiquitous cellular process, is crucial for the robust spatiotemporal patterning of organogenesis.
RESUMEN
Eukaryotic cells monitor and regulate metabolism through the atypical protein kinase target of rapamycin (TOR) regulatory hub. TOR is activated by amino acids in animals and fungi through molecular signaling pathways that have been extensively defined in the past ten years. Very recently, several studies revealed that TOR is also acutely responsive to amino acid metabolism in plants, but the mechanisms of amino acid sensing are not yet established. In this review, we summarize these discoveries, emphasizing the diversity of amino acid sensors in human cells and highlighting pathways that are indirectly sensitive to amino acids, i.e., how TOR monitors changes in amino acid availability without a bona fide amino acid sensor. We then discuss the relevance of these model discoveries to plant biology. As plants can synthesize all proteinogenic amino acids from inorganic precursors, we focus on the possibility that TOR senses both organic metabolites and inorganic nutrients. We conclude that an evolutionary perspective on nutrient sensing by TOR benefits both agricultural and biomedical science, contributing to ongoing efforts to generate crops for a sustainable agricultural future.
Asunto(s)
Aminoácidos , Sirolimus , Aminoácidos/metabolismo , Animales , Eucariontes/metabolismo , Plantas/metabolismo , Transducción de Señal , TransductoresRESUMEN
Plasmodesmata (PD) are membrane-lined channels that cross the cell wall to connect the cytosol of adjacent plant cells, permitting diverse cytosolic molecules to move between cells. PD are essential for plant multicellularity, and the regulation of PD transport contributes to metabolism, developmental patterning, abiotic stress responses, and pathogen defenses, which has sparked broad interest in PD among diverse plant biologists. Here, we present a straightforward method to reproducibly quantify changes in the rate of PD transport in leaves. Individual cells are transformed with Agrobacterium to express fluorescent proteins, which then move beyond the transformed cell via PD. Forty-eight to 72 h later, the extent of GFP movement is monitored by confocal fluorescence microscopy. This assay is versatile and may be combined with transient gene overexpression, virus-induced gene silencing, physiological treatments, or pharmaceutical treatments to test how PD transport responds to specific conditions. We expect that this improved method for monitoring PD transport in leaves will be broadly useful for plant biologists working in diverse fields.
Asunto(s)
Hojas de la Planta , Plasmodesmos , Agrobacterium/genética , Células Vegetales/metabolismo , Hojas de la Planta/metabolismo , Plantas , Plasmodesmos/metabolismoRESUMEN
The plant hormone salicylic acid plays an important role in balancing plant immunity and growth. In this issue of Cell Host & Microbe, Liu et al. (2022) discovered that a long non-coding RNA, lncSABC1, promotes growth in uninfected plants and unleashes defenses when pathogens attack by transcriptionally regulating salicylic acid biosynthesis.
Asunto(s)
ARN Largo no Codificante , Regulación de la Expresión Génica de las Plantas , Enfermedades de las Plantas/genética , Reguladores del Crecimiento de las Plantas , Inmunidad de la Planta , Plantas/genética , ARN Largo no Codificante/genética , Ácido SalicílicoRESUMEN
Development is coordinated by dozens of signals that act in overlapping pathways to orchestrate multicellular growth. Understanding how signaling pathways intersect and diverge at a molecular level is critical to predicting how organisms will react to dynamic environmental conditions. In plants, two antagonistic signaling hubs are strictly required to sense and respond to many nutrients and hormones: TARGET OF RAPAMYCIN (TOR) and ETHYLENE INSENSITIVE 2 (EIN2). In this Landmark report, Fu et al. discover that TOR and EIN2 directly interact to choreograph growth and define an unexpected molecular mechanism at the intersection of hormonal and metabolic signaling networks1.
RESUMEN
Plant cells are connected by plasmodesmata (PD), nanoscopic channels in cell walls that allow diverse cytosolic molecules to move between neighboring cells. PD transport is tightly coordinated with physiology and development, although the range of signaling pathways that influence PD transport has not been comprehensively defined. Several plant hormones, including salicylic acid (SA) and auxin, are known to regulate PD transport, but the effects of other hormones have not been established. In this study, we provide evidence that cytokinins promote PD transport in leaves. Using a green fluorescent protein (GFP) movement assay in the epidermis of Nicotiana benthamiana, we have shown that PD transport significantly increases when leaves are supplied with exogenous cytokinins at physiologically relevant concentrations or when a positive regulator of cytokinin responses, ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 5 (AHP5), is overexpressed. We then demonstrated that silencing cytokinin receptors, ARABIDOPSIS HISTIDINE KINASE 3 (AHK3) or AHK4 or overexpressing a negative regulator of cytokinin signaling, AAHP6, significantly decreases PD transport. These results are supported by transcriptomic analysis of mutants with increased PD transport (ise1-4), which show signs of enhanced cytokinin signaling. We concluded that cytokinins contribute to dynamic changes in PD transport in plants, which will have implications in several aspects of plant biology, including meristem patterning and development, regulation of the sink-to-source transition, and phytohormone crosstalk.
RESUMEN
For plants, light is the source of energy and the most relevant regulator of growth and adaptations to the environment by inducing changes in gene expression at various levels, including alternative splicing. Light-triggered chloroplast retrograde signals control alternative splicing in Arabidopsis thaliana. Here, we provide evidence that light regulates the expression of a core set of splicing-related factors in roots. Alternative splicing responses in roots are not directly caused by light but are instead most likely triggered by photosynthesized sugars. The target of rapamycin (TOR) kinase plays a key role in this shoot-to-root signaling pathway. Knocking down TOR expression or pharmacologically inhibiting TOR activity disrupts the alternative splicing responses to light and exogenous sugars in roots. Consistently, splicing decisions are modulated by mitochondrial activity in roots. In conclusion, by activating the TOR pathway, sugars act as mobile signals to coordinate alternative splicing responses to light throughout the whole plant.