Glucose modulates proliferation in root apical meristems via TOR in maize during germination

Characterizing seed dormancy in Epimedium brevicornu Maxim.: Development of novel chill models and determination of dormancy release mechanisms by transcriptomics

PURPOSE

Epimedium brevicornu Maxim. is a perennial persistent C3 plant of the genus Epimedium Linn. in the family Berberaceae that exhibits severe physiological and morphological seed dormancy.We placed mature E. brevicornu seeds under nine stratification treatment conditions and explored the mechanisms of influence by combining seed embryo growth status assessment with related metabolic pathways and gene co-expression analysis.

RESULTS

We identified 3.9 °C as the optimum cold-stratification temperature of E. brevicornu seeds via a chilling unit (CU) model. The best treatment was variable-temperature stratification (10/20 °C, 12/12 h) for 4 months followed by low-temperature stratification (4 °C) for 3 months (4-3). A total of 63801 differentially expressed genes were annotated to 2587 transcription factors (TFs) in 17 clusters in nine treatments (0-0, 0-3, 1-3, 2-3, 3-3, 4-3, 4-2, 4-1, 4-0). Genes specifically highly expressed in the dormancy release treatment group were significantly enriched in embryo development ending in seed dormancy and fatty acid degradation, indicating the importance of these two processes. Coexpression analysis implied that the TF GRF had the most reciprocal relationships with genes, and multiple interactions centred on zf-HD and YABBY as well as on MYB, GRF, and TCP were observed.

CONCLUSION

In this study, analyses of plant hormone signal pathways and fatty acid degradation pathways revealed changes in key genes during the dormancy release of E. brevicornu seeds, providing evidence for the filtering of E. brevicornu seed dormancy-related genes.

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.

Tissue and subcellular localization of CycD2 and KRPs are dissimilarly distributed by glucose and sucrose during early maize germination

In maize, immunoprecipitation assays have shown that CycD2;2 interacts with KRPs. However, evidence on CycD2;2 or KRPs localization and their possible interaction in specific tissues is lacking and its physiological consequence is still unknown. This work explores the spatiotemporal presence of CyclinD2s and KRPs, cell cycle regulators, during maize seed germination (18 and 36 h) after soaking on glucose or sucrose (120 mM). CyclinD2s are positive actors driving proliferation; KRPs are inhibitors of the main kinase controlling proliferation (a negative signal that slows down the cell cycle). Cell cycle proteins were analyzed by immunolocalization on longitudinal sections of maize embryo axis in seven different tissues or zones (with different proliferation or differentiation potential) and in the nucleus of their cells. Results showed a prevalence of these cell cycle proteins on embryo axes from dry seeds, particularly, their accumulation in nuclei of radicle cells. The absence of sugar caused the accumulation of these regulators in different proliferating zones. CyclinD2 abundance was reduced during germination in the presence of sucrose along the embryo axis, while there was an increase at 36 h on glucose. KRP proteins showed a slight increase at 18 h and a decrease at 36 h on both sugars. There was no correlation between cell cycle regulators/DNA co-localization on both sugars. Results suggest glucose induced a specific accumulation of each cell cycle regulator depending on the proliferation zone as well as nuclear localization which may reflect the differential morphogenetic program regarding the proliferation potential in each zone, while sucrose has a mild influence on both cell cycle proteins accumulation during germination. Whenever CycD2s were present in the nucleus, KRPs were absent after treatment with either sugar and at the two imbibition times analyzed, along the different embryo axe zones.

Transcription factor TERF1 promotes seed germination under osmotic conditions by activating gibberellin acid signaling

Seed germination is the first step of seedling establishment, which is particularly sensitive to drought stress. Elucidating the mechanism regulating seed germination under drought stress is of great importance. We showed that overexpressing Tomato Ethylene Responsive Factor 1 (TERF1), an ERF transcription factor in the ethylene signaling pathway, significantly reduced seed sensitivity to mannitol treatment during seed germination. Germination assay demonstrated that TERF1 could activate gibberellin acid (GA) signaling pathway independent on GA metabolism during germination. By comparative transcriptome analysis (mannitol vs normal germination condition, mannitol vs mannitol plus paclobutrazol (PAC, an inhibitor of GA biosynthesis)) we identified the genes regulated by TERF1 specifically under mannitol treatment and confirmed that TERF1 could activate GA signaling pathway independent on GA metabolism, which were consistent with the germination assay with mannitol and mannitol plus PAC treatment. Based on sugar, gene expression and germination analysis we proved that TERF1 promoted seed germination through glucose signaling pathway mediated by GA. Thus our study provides an underlying mechanism for activating GA signaling pathway by TERF1 during seed germination under osmotic conditions.

HEXOKINASE1 forms a nuclear complex with the PRC2 subunits CURLY LEAF and SWINGER to regulate glucose signaling

The glucose sensor HEXOKINASE1 (HXK1) integrates myriad external and internal signals to regulate gene expression and development in Arabidopsis thaliana. However, how HXK1 mediates glucose signaling in the nucleus remains unclear. Here, using immunoprecipitation-coupled mass spectrometry, we show that two catalytic subunits of Polycomb Repressive Complex 2, SWINGER (SWN) and CURLY LEAF (CLF), directly interact with catalytically active HXK1 and its inactive forms (HXK1^G104D and HXK1^S177A ) via their evolutionarily conserved SANT domains. HXK1, CLF, and SWN target common glucose-responsive genes to regulate glucose signaling, as revealed by RNA sequencing. The glucose-insensitive phenotypes of the Arabidopsis swn-1 and clf-50 mutants were similar to that of hxk1, and genetic analysis revealed that CLF, SWN, and HXK1 function in the same genetic pathway. Intriguingly, HXK1 is required for CLF- and SWN-mediated histone H3 lysine 27 (H3K27me3) deposition and glucose-mediated gene repression. Moreover, CLF and SWN affect the recruitment of HXK1 to its target chromatin. These findings support a model in which HXK1 and epigenetic modifiers form a nuclear complex to cooperatively mediate glucose signaling, thereby affecting the histone modification and expression of glucose-regulated genes in plants.

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.

Exogenous Application of Low-Concentration Sugar Enhances Brassinosteroid Signaling for Skotomorphogenesis by Promoting BIN2 Degradation

In plants, seedling growth is subtly controlled by multiple environmental factors and endogenous phytohormones. The cross-talk between sugars and brassinosteroid (BR) signaling is known to regulate plant growth; however, the molecular mechanisms that coordinate hormone-dependent growth responses with exogenous sucrose in plants are incompletely understood. Skotomorphogenesis is a plant growth stage with rapid elongation of the hypocotyls. In the present study, we found that low-concentration sugars could improve skotomorphogenesis in a manner dependent on BR biosynthesis and TOR activation. However, accumulation of BZR1 in bzr1-1D mutant plants partially rescued the defects of skotomorphogenesis induced by the TOR inhibitor AZD, and these etiolated seedlings displayed a normal phenotype like that of wild-type seedlings in response to both sucrose and non-sucrose treatments, thereby indicating that accumulated BZR1 sustained, at least partially, the sucrose-promoted growth of etiolated seedlings (skotomorphogenesis). Moreover, genetic evidence based on a phenotypic analysis of bin2-3bil1bil2 triple-mutant and gain-of-function bin2–1 mutant plant indicated that BIN2 inactivation was conducive to skotomorphogenesis in the dark. Subsequent biochemical and molecular analyses enabled us to confirm that sucrose reduced BIN2 levels via the TOR–S6K2 pathway in etiolated seedlings. Combined with a determination of the cellulose content, our results indicated that sucrose-induced BIN2 degradation led to the accumulation of BZR1 and the enhancement of cellulose synthesis, thereby promoting skotomorphogenesis, and that BIN2 is the converging node that integrates sugar and BR signaling.

TOR senses and regulates spermidine metabolism during seedling establishment and growth in maize and Arabidopsis

Spermidine (Spd) is a nitrogen sink and signaling molecule that plays pivotal roles in eukaryotic cell growth and must be finetuned to meet various energy demands. In eukaryotes, target of rapamycin (TOR) is a central nutrient sensor, especially N, and a master-regulator of growth and development. Here, we discovered that Spd stimulates the growth of maize and Arabidopsis seedlings through TOR signaling. Inhibition of Spd biosynthesis led to TOR inactivation and growth defects. Furthermore, disruption of a TOR complex partner RAPTOR1B abolished seedling growth stimulation by Spd. Strikingly, TOR activated by Spd promotes translation of key metabolic enzyme upstream open reading frame (uORF)-containing mRNAs, PAO and CuAO, by facilitating translation reinitiation and providing feedback to polyamine metabolism and TOR activation. The Spd-TOR relay protected young-age seedlings of maize from expeditious stress heat shock. Our results demonstrate Spd is an upstream effector of TOR kinase in planta and provide its potential application for crop protection.

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Spermidine (Spd) stimulates growth of maize and Arabidopsis by activating TOR signaling

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TOR stimulates translation efficiency of uORF-containing mRNAs involved in Spd catabolism

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TOR provides feedback to polyamine homeostasis in response to excess of Spd

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The Spd-TOR signaling axis protects maize seedlings from expeditious heat stress

Sugars promote graft union development in the heterograft of cucumber onto pumpkin

The use of heterografts is widely applied for the production of several important commercial crops, but the molecular mechanism of graft union formation remains poorly understood. Here, cucumber grafted onto pumpkin was used to study graft union development, and genome-wide tempo-spatial gene expression at the graft interface was comprehensively investigated. Histological analysis suggested that resumption of the rootstock growth occurred after both phloem and xylem reconnection, and the scion showed evident callus production compared with the rootstock 3 days after grafting. Consistently, transcriptome data revealed specific responses between the scion and rootstock in the expression of genes related to cambium development, the cell cycle, and sugar metabolism during both vascular reconnection and healing, indicating distinct mechanisms. Additionally, lower levels of sugars and significantly changed sugar enzyme activities at the graft junction were observed during vascular reconnection. Next, we found that the healing process of grafted etiolated seedlings was significantly delayed, and graft success, xylem reconnection, and the growth of grafted plants were enhanced by exogenous glucose. This demonstrates that graft union formation requires the correct sugar content. Furthermore, we also found that graft union formation was delayed with a lower energy charge by the target of rapamycin (TOR) inhibitor AZD-8055, and xylem reconnection and the growth of grafted plants were enhanced under AZD-8055 with exogenous glucose treatment. Taken together, our results reveal that sugars play a positive role in graft union formation by promoting the growth of cucumber/pumpkin and provide useful information for understanding graft union healing and the application of heterografting in the future.

The tip of the iceberg: emerging roles of TORC1, and its regulatory functions in plant cells

Target of Rapamycin (TOR) is an evolutionarily conserved protein kinase that plays a central role in coordinating cell growth with light availability, the diurnal cycle, energy availability, and hormonal pathways. TOR Complex 1 (TORC1) controls cell proliferation, growth, metabolism, and defense in plants. Sugar availability is the main signal for activation of TOR in plants, as it also is in mammals and yeast. Specific regulators of the TOR kinase pathway in plants are inorganic compounds in the form of major nutrients in the soils, and light inputs via their impact on autotrophic metabolism. The lack of TOR is embryo-lethal in plants, whilst dysregulation of TOR signaling causes major alterations in growth and development. TOR exerts control as a regulator of protein translation via the action of proteins such as S6K, RPS6, and TAP46. Phytohormones are central players in the downstream systemic physiological TOR effects. TOR has recently been attributed to have roles in the control of DNA methylation, in the abundance of mRNA splicing variants, and in the variety of regulatory lncRNAs and miRNAs. In this review, we summarize recent discoveries in the plant TOR signaling pathway in the context of our current knowledge of mammalian and yeast cells, and highlight the most important gaps in our understanding of plants 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.

The Plant Target of Rapamycin: A Conduc TOR of Nutrition and Metabolism in Photosynthetic Organisms

Living organisms possess many mechanisms to sense nutrients and favorable conditions, which allow them to grow and develop. Photosynthetic organisms are very diverse, from green unicellular algae to multicellular flowering plants, but most of them are sessile and thus unable to escape from the biotic and abiotic stresses they experience. The Target of Rapamycin (TOR) signaling pathway is conserved in all eukaryotes and acts as a central regulatory hub between growth and extrinsic factors, such as nutrients or stress. However, relatively little is known about the regulations and roles of this pathway in plants and algae. Although some features of the TOR pathway seem to have been highly conserved throughout evolution, others clearly differ in plants, perhaps reflecting adaptations to different lifestyles and the rewiring of this primordial signaling module to adapt to specific requirements. Indeed, TOR is involved in plant responses to a vast array of signals including nutrients, hormones, light, stresses or pathogens. In this review, we will summarize recent studies that address the regulations of TOR by nutrients in photosynthetic organisms, and the roles of TOR in controlling important metabolic pathways, highlighting similarities and differences with the other eukaryotes.