Abscisic acid signaling negatively regulates nitrate uptake via phosphorylation of NRT1.1 by SnRK2s in Arabidopsis

Multi-class plant hormone HILIC-MS/MS analysis coupled with high-throughput phenotyping to investigate plant-environment interactions

Plant hormones are chemical signals governing almost every aspect of a plant's life cycle and responses to environmental cues. They are enmeshed within complex signaling networks that can only be deciphered by using broad-scale analytical methods to capture information about several plant hormone classes simultaneously. Methods used for this purpose are all based on reversed-phase (RP) liquid chromatography and mass spectrometric detection. Hydrophilic interaction chromatography (HILIC) is an alternative chromatographic method that performs well in analyses of biological samples. We therefore developed and validated a HILIC method for broad-scale plant hormone analysis including a rapid sample preparation procedure; moreover, derivatization or fractionation is not required. The method enables plant hormone screening focused on polar and moderately polar analytes including cytokinins, auxins, jasmonates, abscisic acid and its metabolites, salicylates, indoleamines (melatonin), and 1-aminocyclopropane-1-carboxylic acid (ACC), for a total of 45 analytes. Importantly, the major pitfalls of ACC analysis have been addressed. Furthermore, HILIC provides orthogonal selectivity to conventional RP methods and displays greater sensitivity, resulting in lower limits of quantification. However, it is less robust, so procedures to increase its reproducibility were established. The method's potential is demonstrated in a case study by employing an approach combining hormonal analysis with phenomics to examine responses of three Arabidopsis ecotypes toward three abiotic stress treatments: salinity, low nutrient availability, and their combination. The case study showcases the value of the simultaneous determination of several plant hormone classes coupled with phenomics data when unraveling processes involving complex cross-talk under diverse plant-environment interactions.

CPK10 protein kinase regulates Arabidopsis tolerance to boron deficiency through phosphorylation and activation of BOR1 transporter

Boron (B) is crucial for plant growth and development. B deficiency can impair numerous physiological and metabolic processes, particularly in root development and pollen germination, seriously impeding crop growth and yield. However, the molecular mechanism underlying boron signal perception and signal transduction is rather limited. In this study, we discovered that CPK10, a calcium-dependent protein kinase in the CPK family, has the strongest interaction with the boron transporter BOR1. Mutations in CPK10 led to growth and root development defects under B-deficiency conditions, while constitutively active CPK10 enhanced plant tolerance to B deficiency. Furthermore, we found that CPK10 interacted with and phosphorylated BOR1 at the Ser689 residue. Through various biochemical analyses and complementation of B transport in yeast and plants, we revealed that Ser689 of BOR1 is important for its transport activity. In summary, these findings highlight the significance of the CPK10-BOR1 signaling pathway in maintaining B homeostasis in plants and provide targets for the genetic improvement of crop tolerance to B-deficiency stress.

Recent advances in local and systemic nitrate signaling in Arabidopsisthaliana

Nitrate is the most abundant form of inorganic nitrogen in aerobic soils, serving both as a nutrient and a signaling molecule. Central to nitrate signaling in higher plants is the intricate balance between local and systemic signaling and response pathways. The interplay between local and systemic responses allows plants to regulate their global gene expression, metabolism, physiology, growth, and development under fluctuating nitrate availability. This review offers an overview of recent discoveries regarding new players on nitrate sensing and signaling, in local and systemic contexts in Arabidopsis thaliana. Additionally, it addresses unanswered questions that warrant further investigation for a better understanding of nitrate signaling and responses in plants.

Integration of nitrate and abscisic acid signaling in plants

To meet the demands of the new Green Revolution and sustainable agriculture, it is important to develop crop varieties with improved yield, nitrogen use efficiency, and stress resistance. Nitrate is the major form of inorganic nitrogen available for plant growth in many well-aerated agricultural soils, and acts as a signaling molecule regulating plant development, growth, and stress responses. Abscisic acid (ABA), an important phytohormone, plays vital roles in integrating extrinsic and intrinsic responses and mediating plant growth and development in response to biotic and abiotic stresses. Therefore, elucidating the interplay between nitrate and ABA can contribute to crop breeding and sustainable agriculture. Here, we review studies that have investigated the interplay between nitrate and ABA in root growth modulation, nitrate and ABA transport processes, seed germination regulation, and drought responses. We also focus on nitrate and ABA interplay in several reported omics analyses with some important nodes in the crosstalk between nitrate and ABA. Through these insights, we proposed some research perspectives that could help to develop crop varieties adapted to a changing environment and to improve crop yield with high nitrogen use efficiency and strong stress resistance.

Function of hormone signaling in regulating nitrogen-use efficiency in plants

Nitrogen (N) is one of the most important nutrients for crop plant performance, however, the excessive application of nitrogenous fertilizers in agriculture significantly increases production costs and causes severe environmental problems. Therefore, comprehensively understanding the molecular mechanisms of N-use efficiency (NUE) with the aim of developing new crop varieties that combine high yields with improved NUE is an urgent goal for achieving more sustainable agriculture. Plant NUE is a complex trait that is affected by multiple factors, of which hormones are known to play pivotal roles. In this review, we focus on the interaction between the biosynthesis and signaling pathways of plant hormones with N metabolism, and summarize recent studies on the interplay between hormones and N, including how N regulates multiple hormone biosynthesis, transport and signaling and how hormones modulate root system architecture (RSA) in response to external N sources. Finally, we explore potential strategies for promoting crop NUE by modulating hormone synthesis, transport and signaling. This provides insights for future breeding of N-efficient crop varieties and the advancement of sustainable agriculture.

Transcriptome Profiling Reveals the Gene Network Responding to Low Nitrogen Stress in Wheat

As one of the essential nutrients for plants, nitrogen (N) has a major impact on the yield and quality of wheat worldwide. Due to chemical fertilizer pollution, it has become increasingly important to improve crop yield by increasing N use efficiency (NUE). Therefore, understanding the response mechanisms to low N (LN) stress is essential for the regulation of NUE in wheat. In this study, LN stress significantly accelerated wheat root growth, but inhibited shoot growth. Further transcriptome analysis showed that 8468 differentially expressed genes (DEGs) responded to LN stress. The roots and shoots displayed opposite response patterns, of which the majority of DEGs in roots were up-regulated (66.15%; 2955/4467), but the majority of DEGs in shoots were down-regulated (71.62%; 3274/4565). GO and KEGG analyses showed that nitrate reductase activity, nitrate assimilation, and N metabolism were significantly enriched in both the roots and shoots. Transcription factor (TF) and protein kinase analysis showed that genes such as MYB-related (38/38 genes) may function in a tissue-specific manner to respond to LN stress. Moreover, 20 out of 107 N signaling homologous genes were differentially expressed in wheat. A total of 47 transcriptome datasets were used for weighted gene co-expression network analysis (17,840 genes), and five TFs were identified as the potential hub regulatory genes involved in the response to LN stress in wheat. Our findings provide insight into the functional mechanisms in response to LN stress and five candidate regulatory genes in wheat. These results will provide a basis for further research on promoting NUE in wheat.

Integrated BSA-seq and RNA-seq analysis to identify candidate genes associated with nitrogen utilization efficiency (NUtE) in rapeseed (Brassica napus L.)

Rapeseed (Brassica napus L.) is one of the important oil crops, with a high demand for nitrogen (N). It is essential to explore the potential of rapeseed to improve nitrogen utilization efficiency (NUtE). Rapeseed is an allotetraploid crop with a relatively large and complex genome, and there are few studies on the mapping of genes related to NUtE regulation. In this study, we used the combination of bulk segregant analysis sequencing (BSA-Seq) and RNA sequencing (RNA-Seq) to analyze the N-efficient genotype 'Zheyou 18' and N-inefficient genotype 'Sollux', to identify the genetic regulatory mechanisms. Several candidate genes were screened, such as the high-affinity nitrate transporter gene NRT2.1 (BnaC08g43370D) and the abscisic acid (ABA) signal transduction-related genes (BnaC02g14540D, BnaA03g20760D, and BnaA05g01330D). BnaA05g01330D was annotated as ABA-INDUCIBLE bHLH-TYPE TRANSCRIPTION FACTOR (AIB/bHLH17), which was highly expressed in the root. The results showed that the primary root length of the ataib mutant was significantly longer than that of the wild type under low N conditions. Overexpression of BnaA5.AIB could reduce the NUtE under low N levels in Arabidopsis (Arabidopsis thaliana). Candidate genes identified in this study may be involved in the regulation of NUtE in rapeseed, and new functions of AIB in orchestrating N uptake and utilization have been revealed. It is indicated that BnaA5.AIB may be the key factor that links ABA to N signaling and a negative regulator of NUtE. It will provide a theoretical basis and application prospect for resource conservation, environmental protection, and sustainable agricultural development.

Calcium-dependent protein kinases CPK21 and CPK23 phosphorylate and activate the iron-regulated transporter IRT1 to regulate iron deficiency in Arabidopsis

Iron (Fe) is an essential micronutrient for all organisms. Fe availability in the soil is usually much lower than that required for plant growth, and Fe deficiencies seriously restrict crop growth and yield. Calcium (Ca2+) is a second messenger in all eukaryotes; however, it remains largely unknown how Ca2+ regulates Fe deficiency. In this study, mutations in CPK21 and CPK23, which are two highly homologous calcium-dependent protein kinases, conferredimpaired growth and rootdevelopment under Fe-deficient conditions, whereas constitutively active CPK21 and CPK23 enhanced plant tolerance to Fe-deficient conditions. Furthermore, we found that CPK21 and CPK23 interacted with and phosphorylated the Fe transporter IRON-REGULATED TRANSPORTER1 (IRT1) at the Ser149 residue. Biochemical analyses and complementation of Fe transport in yeast and plants indicated that IRT1 Ser149 is critical for IRT1 transport activity. Taken together, these findings suggest that the CPK21/23-IRT1 signaling pathway is critical for Fe homeostasis in plants and provides targets for improving Fe-deficient environments and breeding crops resistant to Fe-deficient conditions.

Finding Balance in Adversity: Nitrate Signaling as the Key to Plant Growth, Resilience, and Stress Response

To effectively adapt to changing environments, plants must maintain a delicate balance between growth and resistance or tolerance to various stresses. Nitrate, a significant inorganic nitrogen source in soils, not only acts as an essential nutrient but also functions as a critical signaling molecule that regulates multiple aspects of plant growth and development. In recent years, substantial advancements have been made in understanding nitrate sensing, calcium-dependent nitrate signal transmission, and nitrate-induced transcriptional cascades. Mounting evidence suggests that the primary response to nitrate is influenced by environmental conditions, while nitrate availability plays a pivotal role in stress tolerance responses. Therefore, this review aims to provide an overview of the transcriptional and post-transcriptional regulation of key components in the nitrate signaling pathway, namely, NRT1.1, NLP7, and CIPK23, under abiotic stresses. Additionally, we discuss the specificity of nitrate sensing and signaling as well as the involvement of epigenetic regulators. A comprehensive understanding of the integration between nitrate signaling transduction and abiotic stress responses is crucial for developing future crops with enhanced nitrogen-use efficiency and heightened resilience.

The CBL1/9-CIPK1 calcium sensor negatively regulates drought stress by phosphorylating the PYLs ABA receptor

The stress hormone, Abscisic acid (ABA), is crucial for plants to respond to changes in their environment. It triggers changes in cytoplasmic Ca2+ levels, which activate plant responses to external stresses. However, how Ca2+ sensing and signaling feeds back into ABA signaling is not well understood. Here we reveal a calcium sensing module that negatively regulates drought stress via modulating ABA receptor PYLs. Mutants cbl1/9 and cipk1 exhibit hypersensitivity to ABA and drought resilience. Furthermore, CIPK1 is shown to interact with and phosphorylate 7 of 14 ABA receptors at the evolutionarily conserved site corresponding to PYL4 Ser129, thereby suppressing their activities and promoting PP2C activities under normal conditions. Under drought stress, ABA impedes PYLs phosphorylation by CIPK1 to respond to ABA signaling and survive in unfavorable environment. These findings provide insights into a previously unknown negative regulatory mechanism of the ABA signaling pathway, which is mediated by CBL1/9-CIPK1-PYLs, resulting in plants that are more sensitive to drought stress. This discovery expands our knowledge about the interplay between Ca2+ signaling and ABA signaling.

Brassinosteroid transcription factor BES1 modulates nitrate deficiency by promoting NRT2.1 and NRT2.2 transcription in Arabidopsis

Nitrogen (N) is one of the most essential mineral elements for plants. Brassinosteroids (BRs) play key roles in plant growth and development. Emerging evidence indicates that BRs participate in the responses to nitrate deficiency. However, the precise molecular mechanism underlying the BR signaling pathway in regulating nitrate deficiency remains largely unknown. The transcription factor BES1 regulates the expression of many genes in response to BRs. Root length, nitrate uptake and N concentration of bes1-D mutants were higher than those of wild-type under nitrate deficiency. BES1 levels strongly increased under low nitrate conditions, especially in the non-phosphorylated (active) form. Furthermore, BES1 directly bound to the promoters of NRT2.1 and NRT2.2 to promote their expression under nitrate deficiency. Taken together, BES1 is a key mediator that links BR signaling under nitrate deficiency by modulating high affinity nitrate transporters in plants.

A hierarchical model of ABA-mediated signal transduction in tea plant revealed by systematic genome mining analysis and interaction validation

As a critical signaling molecule, ABA plays an important role in plant growth, development and stresses response. However, tea plant [Camellia sinensis (L.)], an important economical perennial woody plant, has not been systematically reported in response to ABA signal transduction in vivo. In this study, we mined and identified the gene structure of CsPYL/CsPP2C-A/CsSnRK gene families in the ABA signal transduction pathway through the genome-wide analysis of tea plants. Spatiotemporal expression and stress response (drought, salt, chilling) expression patterns were characterized. The results showed that most members of CsPYLs were conserved, and the gene structures of members of A-type CsPP2Cs were highly similar, whereas the gene structure of CsSnRK2s was highly variable. The transcription levels of different family members were differentially expressed with plant growth and development, and their response to stress signal patterns was highly correlated. The expression patterns of CsPYL/CsPP2C-A/CsSnRK2 gene family members in different tissues of tea plant cuttings after exogenous ABA treatment were detected by qRT-PCR, and the hierarchical model of ABA signaling was constructed by correlation analysis to preliminarily obtain three potential ABA-dependent signaling transduction pathways. Subsequently, the protein interaction of the CsPYL4/7-CsPP2C-A2-CsSnRK2.8 signaling pathway was verified by yeast two-hybrid and surface plasmon resonance experiments, indicating that there is specific selectivity in the ABA signaling pathway. Our results provided novel insights into the ABA-dependent signal transduction model in tea plant and information for future functional characterizations of stress tolerance genes in tea plant.

Maize PIMT2 repairs damaged 3-METHYLCROTONYL COA CARBOXYLASE in mitochondria, affecting seed vigor

PROTEIN l-ISOASPARTYL O-METHYLTRANSFERASE (PIMT) affects seed vigor by repairing damaged proteins. While PIMT is capable of isoaspartyl (isoAsp) repair in all proteins, those proteins most susceptible to isoAsp formation have not been well characterized, and the mechanisms by which PIMT affects seed vigor remain largely unknown. Using co-immunoprecipitation and LC-MS/MS, we found that maize (Zea mays) PIMT2 (ZmPIMT2) interacted predominantly with both subunits of maize 3-METHYLCROTONYL COA CARBOXYLASE (ZmMCC). ZmPIMT2 is specifically expressed in the maize embryo. Both mRNA and protein levels of ZmPIMT2 increased during seed maturation and declined during imbibition. Maize seed vigor was decreased in the zmpimt2 mutant line, while overexpression of ZmPIMT2 in maize and Arabidopsis thaliana increased seed vigor upon artificial aging. ZmPIMT2 was localized in the mitochondria, as determined by subcellular localization assays using maize protoplasts. ZmPIMT2 binding to ZmMCCα was confirmed by luciferase complementation tests in both tobacco (Nicotiana benthamiana) leaves and maize protoplasts. Knockdown of ZmMCCα decreased maize seed aging tolerance. Furthermore, overexpression of ZmPIMT2 decreased the accumulation of isoAsp of ZmMCCα protein in seed embryos that underwent accelerated aging treatment. Taken together, our results demonstrate that ZmPIMT2 binds ZmMCCα in mitochondria, repairs isoAsp damage, and positively affects maize seed vigor.

Ca2+-dependent TaCCD1 cooperates with TaSAUR215 to enhance plasma membrane H+-ATPase activity and alkali stress tolerance by inhibiting PP2C-mediated dephosphorylation of TaHA2 in wheat

Alkali stress is a major constraint for crop production in many regions of saline-alkali land. However, little is known about the mechanisms through which wheat responds to alkali stress. In this study, we identified a calcium ion-binding protein from wheat, TaCCD1, which is critical for regulating the plasma membrane (PM) H⁺-ATPase-mediated alkali stress response. PM H⁺-ATPase activity is closely related to alkali tolerance in the wheat variety Shanrong 4 (SR4). We found that two D-clade type 2C protein phosphatases, TaPP2C.D1 and TaPP2C.D8 (TaPP2C.D1/8), negatively modulate alkali stress tolerance by dephosphorylating the penultimate threonine residue (Thr926) of TaHA2 and thereby inhibiting PM H⁺-ATPase activity. Alkali stress induces the expression of TaCCD1 in SR4, and TaCCD1 interacts with TaSAUR215, an early auxin-responsive protein. These responses are both dependent on calcium signaling triggered by alkali stress. TaCCD1 enhances the inhibitory effect of TaSAUR215 on TaPP2C.D1/8 activity, thereby promoting the activity of the PM H⁺-ATPase TaHA2 and alkali stress tolerance in wheat. Functional and genetic analyses verified the effects of these genes in response to alkali stress, indicating that TaPP2C.D1/8 function downstream of TaSAUR215 and TaCCD1. Collectively, this study uncovers a new signaling pathway that regulates wheat responses to alkali stress, in which Ca²⁺-dependent TaCCD1 cooperates with TaSAUR215 to enhance PM H⁺-ATPase activity and alkali stress tolerance by inhibiting TaPP2C.D1/8-mediated dephosphorylation of PM H⁺-ATPase TaHA2 in wheat.

Insights on Phytohormonal Crosstalk in Plant Response to Nitrogen Stress: A Focus on Plant Root Growth and Development

Nitrogen (N) is a vital mineral component that can restrict the growth and development of plants if supplied inappropriately. In order to benefit their growth and development, plants have complex physiological and structural responses to changes in their nitrogen supply. As higher plants have multiple organs with varying functions and nutritional requirements, they coordinate their responses at the whole-plant level based on local and long-distance signaling pathways. It has been suggested that phytohormones are signaling substances in such pathways. The nitrogen signaling pathway is closely associated with phytohormones such as auxin (AUX), abscisic acid (ABA), cytokinins (CKs), ethylene (ETH), brassinosteroid (BR), strigolactones (SLs), jasmonic acid (JA), and salicylic acid (SA). Recent research has shed light on how nitrogen and phytohormones interact to modulate physiology and morphology. This review provides a summary of the research on how phytohormone signaling affects root system architecture (RSA) in response to nitrogen availability. Overall, this review contributes to identifying recent developments in the interaction between phytohormones and N, as well as serving as a foundation for further study.

Plant nitrogen availability and crosstalk with phytohormones signallings and their biotechnology breeding application in crops

Nitrogen (N), one of the most important nutrients, limits plant growth and crop yields in sustainable agriculture system, in which phytohormones are known to play essential roles in N availability. Hence, it is not surprising that massive studies about the crosstalk between N and phytohormones have been constantly emerging. In this review, with the intellectual landscape of N and phytohormones crosstalk provided by the bibliometric analysis, we trace the research story of best-known crosstalk between N and various phytohormones over the last 20 years. Then, we discuss how N regulates various phytohormones biosynthesis and transport in plants. In reverse, we also summarize how phytohormones signallings modulate root system architecture (RSA) in response to N availability. Besides, we expand to outline how phytohormones signallings regulate uptake, transport, and assimilation of N in plants. Further, we conclude advanced biotechnology strategies, explain their application, and provide potential phytohormones-regulated N use efficiency (NUE) targets in crops. Collectively, this review provides not only a better understanding on the recent progress of crosstalk between N and phytohormones, but also targeted strategies for improvement of NUE to increase crop yields in future biotechnology breeding of crops.

Ca2+-dependent phosphorylation of NRAMP1 by CPK21 and CPK23 facilitates manganese uptake and homeostasis in Arabidopsis

Homeostasis of the essential micronutrient manganese (Mn) is crucially determined through availability and uptake efficiency in all organisms. Mn deficiency of plants especially occurs in alkaline and calcareous soils, seriously restricting crop yield. However, the mechanisms underlying the sensing and signaling of Mn availability and conferring regulation of Mn uptake await elucidation. Here, we uncover that Mn depletion triggers spatiotemporally defined long-lasting Ca²⁺ oscillations in Arabidopsis roots. These Ca²⁺ signals initiate in individual cells, expand, and intensify intercellularly to transform into higher-order multicellular oscillations. Furthermore, through an interaction screen we identified the Ca²⁺-dependent protein kinases CPK21 and CPK23 as Ca²⁺ signal-decoding components that bring about translation of these signals into regulation of uptake activity of the high-affinity Mn transporter natural resistance associated macrophage proteins 1 (NRAMP1). Accordingly, a cpk21/23 double mutant displays impaired growth and root development under Mn-limiting conditions, while kinase overexpression confers enhanced tolerance to low Mn supply to plants. In addition, we define Thr498 phosphorylation within NRAMP1 as a pivot mechanistically determining NRAMP1 activity, as revealed by biochemical assays and complementation of yeast Mn uptake and Arabidopsis nramp1 mutants. Collectively, these findings delineate the Ca²⁺-CPK21/23-NRAMP1 axis as key for mounting plant Mn homeostasis.

Ca2+-dependent successive phosphorylation of vacuolar transporter MTP8 by CBL2/3-CIPK3/9/26 and CPK5 is critical for manganese homeostasis in Arabidopsis

Manganese (Mn) is an essential micronutrient for all living organisms. However, excess Mn supply that can occur in acid or waterlogged soils has toxic effects on plant physiology and development. Although a variety of Mn transporter families have been characterized, we have only a rudimentary understanding of how these transporters are regulated to uphold and adjust Mn homeostasis in plants. Here, we demonstrate that two calcineurin-B-like proteins, CBL2/3, and their interacting kinases, CIPK3/9/26, are key regulators of plant Mn homeostasis. Arabidopsis mutants lacking CBL2 and 3 or their interacting protein kinases CIPK3/9/26 exhibit remarkably high Mn tolerance. Intriguingly, CIPK3/9/26 interact with and phosphorylate the tonoplast-localized Mn and iron (Fe) transporter MTP8 primarily at Ser35, which is conserved among MTP8 proteins from various species. Mn transport complementation assays in yeast combined with multiple physiological assays indicate that CBL-CIPK-mediated phosphorylation of MTP8 negatively regulates its transport activity from the cytoplasm to the vacuole. Moreover, we show that sequential phosphorylation of MTP8, initially at Ser31/32 by the calcium-dependent protein kinase CPK5 and subsequently at Ser35 by CIPK26, provides an activation/deactivation fine-tuning mechanism for differential regulation of Mn transport. Collectively, our findings define a two-tiered calcium-controlled mechanism for dynamic regulation of Mn homeostasis under conditions of fluctuating Mn supply.

Genetic dissection of seed yield and yield-related traits in Brassica napus grown with contrasting nitrogen supplies

Oilseed rape (B. napus) is the main oil crop in China as well as in the world. Nitrogen (N) deficiency significantly reduces the seed yield of B. napus. However, a very few studies involved in the genetic mechanism of seed yield and SY-related traits of B. napus in response to N deficiency. In this study, plant height (PH), branch number per plant (BN), pod number per plant (PN), seed number per pod (SN), 1000-seed weight (SW), and seed yield per plant (SY) were investigated using a B. napus double haploid (BnaTNDH) population derived from a cross between cultivars "Tapidor" and "Ningyou7" grown at an optimal N (ON) and a low N (LN) supplies in three-year field trials. Great variations of SY and related traits were observed in BnaTNDH population under contrasting N supplies. A total of 106 and 110 significant quantitative trait loci (QTLs) were detected for six traits at ON and LN in three field trials, respectively. All of these significant QTLs for the same trait identified in two or three trials were integrated into 20 stable QTLs. A total of 50 consensus QTLs and 53 unique QTLs were obtained from 172 significant QTLs and 20 stable QTLs, including 35 ON-specific QTLs, 29 LN-specific QTLs and 39 constitutive QTLs detected at both ON and LN. cqA3l was integrated from four QTLs for PN, PH, SN, SY at LN, cqC9c was integrated from QTLs for BN, SY, PN at ON and LN. Both cqA3l and cqC9c were detected in three trials. In addition, a total of 194 epistatic interactions, inculding 15 pleiotropic epistatic interactions, were identified. Eight of the 15 pleiotropic epistatic interactions were detected to affect SY. This result may help to better understand the genetic mechanism of yield traits in response to low N and promote the breeding of N-efficient varieties.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01281-0.

ipa1 improves rice drought tolerance at seedling stage mainly through activating abscisic acid pathway

KEY MESSAGE: ipa1 enhances rice drought tolerance mainly through activating the ABA pathway. It endows rice seedlings with a more developed root system, smaller leaf stomata aperture, and enhanced carbon metabolism. Drought is a major abiotic stress to crop production. IPA1 (IDEAL PLANT ARCHITECTURE 1)/OsSPL14 encodes a transcription factor and has been reported to function in both rice ideal plant architecture and biotic resistance. Here, with a pair of IPA1 and ipa1-NILs (Near Iso-genic Lines), we found that ipa1 could significantly improve rice drought tolerance at seedling stage. The ipa1 plants had a better-developed root system and smaller leaf stomatal aperture. Analysis of carbon-nitrogen metabolism-associated enzyme activity, gene expression, and metabolic profile indicated that ipa1 could tip the carbon-nitrogen metabolism balance towards an increased carbon metabolism pattern. In both the control and PEG-treated conditions, ABA content in the ipa1 seedlings was significantly higher than that in the IPA1 seedlings. Expression of the ABA biosynthesis genes was detected to be up-regulated, whereas the expression of ABA catabolism genes was down-regulated in the ipa1 seedlings. In addition, based on yeast one-hybrid assay and dual-luciferase assay, IPA1 was found to directly activate the promoter activity of OsHOX12, a transcription factor promoting ABA biosynthesis, and OsNAC52, a positive regulator of the ABA pathway. The expression of OsHOX12 and OsNAC52 was significantly up-regulated in the ipa1 plants. Combined with the previous studies, our results suggested that ipa1 could improve rice seedling drought tolerance mainly through activating the ABA pathway and that regulation of the ipa1-mediated ABA pathway will be an important strategy for improving drought resistance of rice.

Identification of Genes Associated with Nitrogen Stress Responses in Apple Leaves

Nitrogen (N) is an essential macronutrient that regulates diverse physiological processes for plant survival and development. In apple orchards, inappropriate N conditions can cause imbalanced growth and subsequent physiological disorders in trees. In order to investigate the molecular basis underlying the physiological signals for N stress responses, we examined the metabolic signals responsive to contrasting N stress conditions (deficient/excessive) in apple leaves using transcriptome approaches. The clustering of differentially expressed genes (DEGs) showed the expression dynamics of genes associated with each N stress group. Functional analyses of gene ontology and pathway enrichments revealed the potential candidates of metabolic signals responsible for N-deficient/excessive stress responses. The functional interactions of DEGs in each cluster were further explored by protein-protein interaction network analysis. Our results provided a comprehensive insight into molecular signals responsive to N stress conditions, and will be useful in future research to enhance the nutrition tolerance of tree crops.

GTPase ROP6 negatively modulates phosphate deficiency through inhibition of PHT1;1 and PHT1;4 in Arabidopsis thaliana

Phosphorus, an essential macroelement for plant growth and development, is a major limiting factor for sustainable crop yield. The Rho of plant (ROP) GTPase is involved in regulating multiple signal transduction processes in plants, but potentially including the phosphate deficiency signaling pathway remains unknown. Here, we identified that the rop6 mutant exhibited a dramatic tolerant phenotype under Pi-deficient conditions, with higher phosphate accumulation and lower anthocyanin content. In contrast, the rop6 mutant was more sensitive to arsenate (As(V)) toxicity, the analog of Pi. Immunoblot analysis displayed that the ROP6 protein was rapidly degraded through ubiquitin/26S proteasome pathway under Pi-deficient conditions. In addition, pull-down assay using GST-RIC1 demonstrated that the ROP6 activity was decreased obviously under Pi-deficient conditions. Strikingly, protein-protein interaction and two-voltage clamping assays demonstrated that ROP6 physically interacted with and inhibited the key phosphate uptake transporters PHT1;1 and PHT1;4 in vitro and in vivo. Moreover, genetic analysis showed that ROP6 functioned upstream of PHT1;1 and PHT1;4. Thus, we conclude that GTPase ROP6 modulates the uptake of phosphate by inhibiting the activities of PHT1;1 and PHT1;4 in Arabidopsis.

NRT1.1 Dual-Affinity Nitrate Transport/Signalling and its Roles in Plant Abiotic Stress Resistance

NRT1.1 is the first nitrate transport protein cloned in plants and has both high- and low-affinity functions. It imports and senses nitrate, which is modulated by the phosphorylation on Thr101 (T101). Structural studies have revealed that the phosphorylation of T101 either induces dimer decoupling or increases structural flexibility within the membrane, thereby switching the NRT1.1 protein from a low- to high-affinity state. Further studies on the adaptive regulation of NRT1.1 in fluctuating nitrate conditions have shown that, at low nitrate concentrations, nitrate binding only at the high-affinity monomer initiates NRT1.1 dimer decoupling and priming of the T101 site for phosphorylation activated by CIPK23, which functions as a high-affinity nitrate transceptor. However, nitrate binding in both monomers retains the unmodified NRT1.1, maintaining the low-affinity mode. This NRT1.1-mediated nitrate signalling and transport may provide a key to improving the efficiency of plant nitrogen use. However, recent studies have revealed that NRT1.1 is extensively involved in plant tolerance of several adverse environmental conditions. In this context, we summarise the recent progress in the molecular mechanisms of NRT1.1 dual-affinity nitrate transport/signalling and focus on its expected and unexpected roles in plant abiotic stress resistance and their regulation processes.

Tonoplast-associated calcium signaling regulates manganese homeostasis in Arabidopsis

Manganese (Mn) is an essential micronutrient in plants. However, excessive Mn absorption in acidic and waterlogged soils can lead to Mn toxicity. Despite their essential roles in Mn homeostasis, transcriptional and post-transcriptional modifications of Mn transporters remain poorly understood. Here, we demonstrated that high-Mn stress induces an obvious Ca²⁺ signature in Arabidopsis. We identified four calcium-dependent protein kinases, CPK4/5/6/11, that interact with the tonoplast-localized Mn and iron (Fe) transporter MTP8 in vitro and in vivo. The cpk4/5/6/11 quadruple mutant displayed a dramatic high-Mn-sensitive phenotype similar to that of the mtp8 mutant. CPKs phosphorylated the N-terminal domain of MTP8 primarily at the Ser31 and Ser32 residues. Transport assays combined with multiple physiological experiments on phospho-dead variant MTP8^S31/32A and phospho-mimetic variant MTP8^S31/32D plants under different Mn and Fe conditions suggested that Ser31 and Ser32 are crucial for MTP8 function. In addition, genetic analysis showed that CPKs functioned upstream of MTP8. In summary, we identified a tonoplast-associated calcium signaling cascade that orchestrates Mn homeostasis and links Mn toxicity, Ca²⁺ signaling, and Mn transporters. These findings provide new insight into Mn homeostasis mechanisms and Ca²⁺ signaling pathways in plants, providing potential targets for engineering heavy metal toxicity-tolerant plants.

Beneficial Effects of Exogenous Melatonin and Dopamine on Low Nitrate Stress in Malus hupehensis

Malus hupehensis, as an apple rootstock, is an economically important tree species popular due to its excellent fruit yield and stress resistance. Nitrogen is one of the critical limiting factors of plant growth and fruit yield, so it is crucial to explore new methods to improve nitrogen use efficiency. Melatonin and dopamine, as multifunctional metabolites, play numerous physiological roles in plants. We analyzed the effects of exogenous melatonin and dopamine treatments on the growth, root system architecture, nitrogen absorption, and metabolism of M. hupehensis when seedlings were exposed to nitrate-deficient conditions. Under low nitrate stress, plant growth slowed, and chlorophyll contents and ¹⁵NO₃ ^- accumulation decreased significantly. However, the application of 0.1 μmol/L melatonin or 100 μmol/L exogenous dopamine significantly reduced the inhibition attributable to low nitrate levels during the ensuing period of stress treatment, and the effect of dopamine was more obvious. In addition to modifying the root system architecture of nitrate-deficient plants, exogenous melatonin and dopamine also changed the uptake, transport, and distribution of ¹⁵NO₃ ^-. Furthermore, both exogenous melatonin and dopamine enhanced tolerance to low nitrate stress by maintaining the activity of enzymes (NR, NiR, GS, Fd-GOGAT, and NADH-GOGAT) and the transcription levels of related genes involved in leaf and root nitrogen metabolism. We also found that exogenous melatonin and dopamine promoted the expression of nitrate transporter genes (NRT1.1, NRT2.4, NRT2.5, and NRT2.7) in nitrate-deficient plant leaves and roots. Our results suggest that both exogenous melatonin and dopamine can mitigate low nitrate stress by changing the root system architecture, promoting the absorption of nitrate, and regulating the expression of genes related to nitrogen transport and metabolism. However, according to a comprehensive analysis of the results, exogenous dopamine plays a more significant role than melatonin in improving plant nitrogen use efficiency.