A role for TOR signaling at every stage of plant life

Cucumber (Cucumis sativus L.) translationally controlled tumor protein interacts with CsRab11A and promotes activation of target of rapamycin in response to Podosphaera xanthii

The target of rapamycin (TOR) kinase serves as a central regulator that integrates nutrient and energy signals to orchestrate cellular and organismal physiology in both animals and plants. Despite significant advancements having been made in understanding the molecular and cellular functions of plant TOR kinases, the upstream regulators that modulate TOR activity are not yet fully elucidated. In animals, the translationally controlled tumor protein (TCTP) is recognized as a key player in TOR signaling. This study reveals that two TCTP isoforms from Cucumis sativus, when introduced into Arabidopsis, are instrumental in balancing growth and defense mechanisms against the fungal pathogen Golovinomyces cichoracearum. We hypothesize that plant TCTPs act as upstream regulators of TOR in response to powdery mildew caused by Podosphaera xanthii in Cucumis. Our research further uncovers a stable interaction between CsTCTP and a small GTPase, CsRab11A. Transient transformation assays indicate that CsRab11A is involved in the defense against P. xanthii and promotes the activation of TOR signaling through CsTCTP. Moreover, our findings demonstrate that the critical role of TOR in plant disease resistance is contingent upon its regulated activity; pretreatment with a TOR inhibitor (AZD-8055) enhances cucumber plant resistance to P. xanthii, while pretreatment with a TOR activator (MHY-1485) increases susceptibility. These results suggest a sophisticated adaptive response mechanism in which upstream regulators, CsTCTP and CsRab11A, coordinate to modulate TOR function in response to P. xanthii, highlighting a novel aspect of plant-pathogen interactions.

TOR in plants: Multidimensional regulators of plant growth and signaling pathways

Target Of Rapamycin (TOR) represents a ubiquitous kinase complex that has emerged as a central regulator of cell growth and metabolism in nearly all eukaryotic organisms. TOR is an evolutionarily conserved protein kinase, functioning as a central signaling hub that integrates diverse internal and external cues to regulate a multitude of biological processes. These processes collectively exert significant influence on plant growth, development, nutrient assimilation, photosynthesis, fruit ripening, and interactions with microorganisms. Within the plant domain, the TOR complex comprises three integral components: TOR, RAPTOR, and LST8. This comprehensive review provides insights into various facets of the TOR protein, encompassing its origin, structure, function, and the regulatory and signaling pathways operative in photosynthetic organisms. Additionally, we explore future perspectives related to this pivotal protein kinase.

Effect of interfering TOR signaling pathway on the biosynthesis of terpenoids in Salvia miltiorrhiza Bge

The TOR (Target of Rapamycin) signaling pathway, which takes TOR kinase as the core, regulates the absorption, distribution, and recycling of nutrients by integrating metabolic network and other signaling pathways, thus participating in the plant growth-defense trade-off. While terpenoids play an important role in plant growth, development, stress response, and signal transduction. The effect of the TOR signaling pathway on terpenoid biosynthesis in plants has yet to be studied in detail. In this study, the tissue culture seedlings of Salvia miltiorrhiza were treated with the TOR inhibitor AZD8055. The results show that the roots of the control group had begun to grow on the 8th day, while the seedlings treated with AZD8055 had no rooting signs. Combined with the expression changes of genes related to the TOR signaling pathway in the first 8 days, samples on the 3rd, 6th, and 8th days were selected for RNA-Seq analysis. Through RNA-Seq analysis, a total of 50,689 unigenes were obtained from the samples of these three periods, of which 4088 unigenes showed differential expression. The function enrichment and time-series analysis of differentially expressed genes (DEGs) showed that the main influence of the TOR signal pathway on plant growth-related processes was gradually transmitted with treatment time after TOR was inhibited. Pathway enrichment analysis of DEGs showed that the genes in the biosynthesis of terpenoids, such as diterpenoid and carotenoid biosynthetic pathways, could be regulated. Compared with other stages, DEGs related to terpenoid biosynthesis were mainly regulated in the S2 stage. In addition, the genes involved in terpenoid skeleton biosynthesis was also considerably enriched in the S2 stage, according to the results of gene set enrichment analysis (GSEA) of unigenes. Inhibition of the TOR signaling pathway may affect the biosynthesis of terpenoid signaling molecules, inhibit gibberellin's biosynthesis, and promote abscisic acid's biosynthesis. This study has discussed the effect of interfering with the TOR pathway on terpenoid biosynthesis in S. miltiorrhiza from the perspective of omics and provides new insight into the interaction between the terpenoid biosynthesis pathway and the growth-defense trade-off of medicinal plants.

Target of rapamycin (TOR) plays a role in regulating ROS-induced chloroplast damage during cucumber (Cucumis sativus) leaf senescence

In cucumber production, delaying leaf senescence is crucial for improving cucumber yield and quality. Target of rapamycin (TOR) is a highly conserved serine/threonine protein kinase in eukaryotes, which can integrate exogenous and endogenous signals (such as cell energy state levels) to stimulate cell growth, proliferation, and differentiation. However, no studies have yet examined the regulatory role of TOR signalling in cucumber leaf senescence. In this study, the effects of TOR signalling on dark-induced cucumber leaf senescence were investigated using the TOR activator MHY1485 and inhibitor AZD8055 combined with transient transformation techniques. The results indicate that TOR responds to dark-induced leaf senescence, and alterations in TOR activity/expression influence cucumber leaf resistance to dark-induced senescence. Specifically, in plants with elevated TOR activity/expression, we observed reduced expression of senescence-related genes, less membrane lipid damage, decreased cell apoptosis, lower levels of reactive oxygen species production, and less damage to the photosynthetic system compared to the control. In contrast, in plants with reduced TOR activity/expression, we observed higher expression of senescence-related genes, increased membrane lipid damage, enhanced cell apoptosis, elevated levels of reactive oxygen species production, and more damage to the photosynthetic system. These comprehensive results underscore the critical role of TOR in regulating dark-induced cucumber leaf senescence. These findings provide a foundation for controlling premature leaf senescence in cucumber production and offer insights for further exploration of leaf senescence mechanisms and the development of more effective control methods.

Phosphoproteomics analysis of the effect of target of rapamycin kinase inhibition on Cucumis sativus in response to Podosphaera xanthii

Target of rapamycin (TOR) kinase is a conserved sensor of cell growth in yeasts, plants, and mammals. Despite the extensive research on the TOR complex in various biological processes, large-scale phosphoproteomics analysis of TOR phosphorylation events upon environmental stress are scarce. Powdery mildew caused by Podosphaera xanthii poses a major threat to the quality and yield of cucumber (Cucumis sativus L.). Previous studies concluded that TOR participated in abiotic and biotic stress responses. Hence, studying the underlying mechanism of TOR-P. xanthii infection is particularly important. In this study, we performed a quantitative phosphoproteomics studies of Cucumis against P. xanthii attack under AZD-8055 (TOR inhibitor) pretreatment. A total of 3384 phosphopeptides were identified from the 1699 phosphoproteins. The Motif-X analysis showed high sensitivity and specificity of serine sites under AZD-8055-treatment or P. xanthii stress, and TOR exhibited a unique preference for proline at +1 position and glycine at -1 position to enhance the phosphorylation response to P. xanthii. The functional analysis suggested that the unique responses were attributed to proteins related to plant hormone signaling, mitogen-activated protein kinase cascade signaling, phosphatidylinositol signaling system, and circadian rhythm; and calcium signaling- and defense response-related proteins. Our results provided rich resources for understanding the molecular mechanism of how the TOR kinase controlled plant growth and stress adaptation.

TOR kinase, a GPS in the complex nutrient and hormonal signaling networks to guide plant growth and development

To survive and sustain growth, sessile plants have developed sophisticated internal signalling networks that respond to various external and internal cues. Despite the central roles of nutrient and hormone signaling in plant growth and development, how hormone-driven processes coordinate with metabolic status remains largely enigmatic. Target of rapamycin (TOR) kinase is an evolutionarily conserved master regulator that integrates energy, nutrients, growth factors, hormones, and stress signals to promote growth in all eukaryotes. Inspired by recent comprehensive systems, chemical, genetic, and genomic studies on TOR in plants, this review discusses a potential role of TOR as a 'global positioning system' that directs plant growth and developmental programs both temporally and spatially by integrating dynamic information in the complex nutrient and hormonal signaling networks. We further evaluate and depict the possible functional and mechanistic models for how a single protein kinase, TOR, is able to recognize, integrate, and even distinguish a plethora of positive and negative input signals to execute appropriate and distinct downstream biological processes via multiple partners and effectors.

Changes in physiological traits and expression of key genes involved in sugar signaling pathway in rice under high temperature stress

Efficient assimilate partitioning between the source and sink organs to achieve increased grain weight is coordinated by the sugar signaling mechanism. The expression of the genes involved in sugar signaling mainly hexokinases 2 (OsHXK2), Sucrose-nonfermentation1-related protein kinase1 (OsSnRK1), trehalose-6-phosphate synthase 1 (OsTPS1) and target of rapamycin (OsTOR) under high temperature stress was examined in tolerant (NL-44) and susceptible (Vandana) varieties of rice. The photosynthetic rate, stomatal conductance, water-use efficiency, photochemical efficiency (Fv/Fm), quantum yield (ϕPSII), pollen viability, spikelet fertility and 1000 grain weight were significantly higher in NL-44 compared to Vandana under stress. The difference in the gene expression levels in the vegetative and grain-filling phases as well as between the tolerant and susceptible varieties, revealed unique pathways of sugar signaling under heat stress. In the vegetative phase, the expression of OsTOR seems to be the difference between NL-44 and Vandana for their differed heat stress tolerance whereas, in the grain-filling phase, the difference between the varieties lay in the regulation of OsHXK2. The comparative changes in the expression levels between the genes under the varying conditions indicate the sugar status in the source and sink organs that are available for translocation or remobilization.

Nitric oxide-an antidote to seed aging modifies meta-tyrosine content and expression of aging-linked genes in apple embryos

Short-term (3 h) treatment of embryos isolated from accelerated aged apple seeds (Malus domestica Borkh.) with nitric oxide (NO) partially reduced the effects of aging. The study aimed to investigate the impact of the short-term NO treatment of embryos isolated from apple seeds subjected to accelerated aging on the expression of genes potentially linked to the regulation of seed aging. Apple seeds were artificially aged for 7, 14, or 21 days. Then, the embryos were isolated from the seeds, treated with NO, and cultured for 48 h. Progression of seed aging was associated with the decreased transcript levels of most of the analyzed genes (Lea1, Lea2a, Lea4, Hsp70b, Hsp20a, Hsp20b, ClpB1, ClpB4, Cpn60a, Cpn60b, Raptor, and Saur). The role of NO in the mitigation of seed aging depended on the duration of the aging. After 7 and 14 days of seed aging, a decreased expression of genes potentially associated with the promotion of aging (Tor, Raptor, Saur) was noted. NO-dependent regulation of seed aging was associated with the stimulation of the expression of genes encoding chaperones and proteins involved in the repair of damaged proteins. After NO application, the greatest upregulation of ClpB, Pimt was noted in the embryos isolated from seeds subjected to 7-day long accelerated aging, Hsp70b, Hsp70c, and Cpn in the embryos of seeds aged for 14 days, and Lea2a in the embryos of seeds after 21 days of aging. We also demonstrated the increased meta-tyrosine concentration depending or in respect the progression of artificial aging, and the NO-induced increased phenylalanine content in seeds artificially aged for 21 days. In the NO-treated embryos of seeds aged for 7 and 21 days, the level of tyrosine was almost doubled compared to the aged tissue. Our data confirmed the usage of meta-tyrosine as a marker of seed aging and indicated that the increased meta-tyrosine/tyrosine ratio could be related to the loss of seed viability.

Roles of TOR signaling in nutrient deprivation and abiotic stress

In living organisms, nutrient, energy, and environmental stimuli sensing and signaling are considered as the most primordial regulatory networks governing growth and development. Target of Rapamycin (TOR) is a diversified Serine/Threonine protein kinase existing in all eukaryotes that regulates distinct salient growth and developmental signaling pathways. TOR signaling acts as a central hub in plants that allows a variety of nutrients, energy, hormones, and environmental stimuli to be integrated. TOR is activated by several nutrients and promotes energy-consuming processes such as cell division, protein translation, mRNA translation and ribosome biogenesis. We summarized the recent findings on the TOR function in regulating the dynamic networks of nutrients, including sugar, sulfur, nitrogen, carbon, phosphorus, potassium, and amino acids. TOR's role in abiotic stress was discussed, in which TOR orchestrating stress signaling, including heat, cold, salt, and osmotic stress, to regulate transcriptional and metabolic reprogramming, as well as growth and development. The interconnections between TOR and SnRK1 kinase were discussed in controlling nutrient deprivation and abiotic stress.

Target of rapamycin (TOR) regulates the response to low nitrogen stress via autophagy and hormone pathways in Malus hupehensis

Target of rapamycin (TOR) is a highly conserved master regulator in eukaryotes; it regulates cell proliferation and growth by integrating different signals. However, little is known about the function of TOR in perennial woody plants. Different concentrations of AZD8055 (an inhibitor of TOR) were used in this study to investigate the role of TOR in the response to low nitrogen (N) stress in the wild apple species Malus hupehensis. Low N stress inhibited the growth of M. hupehensis plants, and 1 μM AZD alleviated this effect. Plants supplied with 1 μM AZD had higher photosynthetic capacity, which promoted the accumulation of biomass, as well as higher contents of N and anthocyanins and lower content of starch. Exogenous application of 1 μM AZD also promoted the development of the root system. Plants supplied with at least 5 μM AZD displayed early leaf senescence. RNA-seq analysis indicated that TOR altered the expression of genes related to the low N stress response, such as genes involved in photosystem, starch metabolism, autophagy, and hormone metabolism. Further analysis revealed altered autophagy in plants supplied with AZD under low N stress; the metabolism of plant hormones also changed following AZD supplementation. In sum, our findings revealed that appropriate inhibition of TOR activated autophagy and jasmonic acid signaling in M. hupehensis, which allowed plants to cope with low N stress. Severe TOR inhibition resulted in the excessive accumulation of salicylic acid, which probably led to programmed cell death in M. hupehensis.

Sugars and the speed of life-Metabolic signals that determine plant growth, development and death

Plant growth and development depend on the availability of carbohydrates synthesised in photosynthesis (source activity) and utilisation of these carbohydrates for growth (sink activity). External conditions, such as temperature, nutrient availability and stress, can affect source as well as sink activity. Optimal utilisation of resources is under circadian clock control. This molecular timekeeper ensures that growth responses are adjusted to different photoperiod and temperature settings by modulating starch accumulation and degradation accordingly. For example, during the night, starch degradation is required to provide sugars for growth. Under favourable growth conditions, high sugar availability stimulates growth and development, resulting in an overall accelerated life cycle of annual plants. Key signalling components include trehalose-6-phosphate (Tre6P), which reflects sucrose availability and stimulates growth and branching when the conditions are favourable. Under sink limitation, Tre6P does, however, inhibit night-time starch degradation. Tre6P interacts with Sucrose-non-fermenting1-Related Kinase1 (SnRK1), a protein kinase that inhibits growth under starvation and stress conditions and delays development (including flowering and senescence). Tre6P inhibits SnRK1 activity, but SnRK1 increases the Tre6P to sucrose ratio under favourable conditions. Alongside Tre6P, Target of Rapamycin (TOR) stimulates processes such as protein synthesis and growth when sugar availability is high. In annual plants, an accelerated life cycle results in early leaf and plant senescence, thus shortening the lifespan. While the availability of carbohydrates in the form of sucrose and other sugars also plays an important role in seasonal life cycle events (phenology) of perennial plants, the sugar signalling pathways in perennials are less well understood.

Auxin and Target of Rapamycin Spatiotemporally Regulate Root Organogenesis

The programs associated with embryonic roots (ERs), primary roots (PRs), lateral roots (LRs), and adventitious roots (ARs) play crucial roles in the growth and development of roots in plants. The root functions are involved in diverse processes such as water and nutrient absorption and their utilization, the storage of photosynthetic products, and stress tolerance. Hormones and signaling pathways play regulatory roles during root development. Among these, auxin is the most important hormone regulating root development. The target of rapamycin (TOR) signaling pathway has also been shown to play a key role in root developmental programs. In this article, the milestones and influential progress of studying crosstalk between auxin and TOR during the development of ERs, PRs, LRs and ARs, as well as their functional implications in root morphogenesis, development, and architecture, are systematically summarized and discussed.

Contributions of TOR Signaling on Photosynthesis

The target of rapamycin (TOR) protein kinase is an atypical Ser/Thr protein kinase and evolutionally conserved among yeasts, plants, and mammals. TOR has been established as a central hub for integrating nutrient, energy, hormone, and environmental signals in all the eukaryotes. Despite the conserved functions across eukaryotes, recent research has shed light on the multifaceted roles of TOR signaling in plant-specific functional and mechanistic features. One of the most specific features is the involvement of TOR in plant photosynthesis. The recent development of tools for the functional analysis of plant TOR has helped to uncover the involvement of TOR signaling in several steps preceding photoautotrophy and maintenance of photosynthesis. Here, we present recent novel findings relating to TOR signaling and its roles in regulating plant photosynthesis, including carbon nutrient sense, light absorptions, and leaf and chloroplast development. We also provide some gaps in our understanding of TOR function in photosynthesis that need to be addressed in the future.

Convergence and Divergence of Sugar and Cytokinin Signaling in Plant Development

Plants adjust their growth and development through a sophisticated regulatory system integrating endogenous and exogenous cues. Many of them rely on intricate crosstalk between nutrients and hormones, an effective way of coupling nutritional and developmental information and ensuring plant survival. Sugars in their different forms such as sucrose, glucose, fructose and trehalose-6-P and the hormone family of cytokinins (CKs) are major regulators of the shoot and root functioning throughout the plant life cycle. While their individual roles have been extensively investigated, their combined effects have unexpectedly received little attention, resulting in many gaps in current knowledge. The present review provides an overview of the relationship between sugars and CKs signaling in the main developmental transition during the plant lifecycle, including seed development, germination, seedling establishment, root and shoot branching, leaf senescence, and flowering. These new insights highlight the diversity and the complexity of the crosstalk between sugars and CKs and raise several questions that will open onto further investigations of these regulation networks orchestrating plant growth and development.

Target of Rapamycin in Control of Autophagy: Puppet Master and Signal Integrator

The target of rapamycin (TOR) is an evolutionarily-conserved serine/threonine kinase that senses and integrates signals from the environment to coordinate developmental and metabolic processes. TOR senses nutrients, hormones, metabolites, and stress signals to promote cell and organ growth when conditions are favorable. However, TOR is inhibited when conditions are unfavorable, promoting catabolic processes such as autophagy. Autophagy is a macromolecular degradation pathway by which cells degrade and recycle cytoplasmic materials. TOR negatively regulates autophagy through phosphorylation of ATG13, preventing activation of the autophagy-initiating ATG1-ATG13 kinase complex. Here we review TOR complex composition and function in photosynthetic and non-photosynthetic organisms. We also review recent developments in the identification of upstream TOR activators and downstream effectors of TOR. Finally, we discuss recent developments in our understanding of the regulation of autophagy by TOR in photosynthetic organisms.

TOR kinase activity in Chlamydomonas reinhardtii is modulated by cellular metabolic states

Target of rapamycin (TOR) kinase is a sensor and a central integrator of internal and external metabolic cues. However, in algae and in higher plants, the components of TOR kinase signaling are yet to be characterized. Here, we establish an assay system to study TOR kinase activity in Chlamydomonas reinhardtii using the phosphorylation status of its putative downstream target, CrS6K. Using this assay, we probe the modulation of cellular TOR kinase activity under various physiological states such as photoautotrophy, heterotrophy, mixotrophy, and nitrogen (N) starvation. Importantly, we uncover that excess acetate in the medium leads to high cellular reactive oxygen species levels, triggering autophagy and a concomitant drop in TOR kinase activity in a dose-dependent manner, thus leading to a N-starvation-like cellular phenotype, even when nitrogen is present.

Reciprocal regulation of photosynthesis and mitochondrial respiration by TOR kinase in Chlamydomonas reinhardtii

While the role of TOR kinase in the chloroplast biogenesis and transcriptional regulation of photosynthesis is well documented in Arabidopsis, the functional relevance of this metabolic sensor kinase in chloroplast-mitochondria cross talk is unknown. Using Chlamydomonas reinhardtii as the model system, we demonstrate the role of TOR kinase in the regulation of chloroplast and mitochondrial functions: We show that TOR kinase inhibition impairs the maintenance of high ETR associated with PSII and low NPQ and inhibits efficient state transitions between PSII and PSI. While compromised photosynthetic functions are observed in TOR kinase inhibited cells, same conditions lead to augmentation in mitochondrial basal respiration rate by twofold and concomitantly a rise in ATP production. Interestingly, such upregulated mitochondrial functions in TOR-inhibited cells are mediated by fragmented mitochondria via upregulating COXIIb and downregulating Hxk1 and AOX1 protein levels. We propose that TOR kinase may act as a sensor that counter-regulates chloroplast versus mitochondrial functions in a normal C. reinhardtii cell.