Arabidopsis nitrate reductase activity is stimulated by the E3 SUMO ligase AtSIZ1

Reciprocal modulation of responses to nitrate starvation and hypoxia in roots and leaves of Arabidopsis thaliana

The flooding of agricultural land leads to hypoxia and nitrate leaching. While understanding the plant's response to these conditions is essential for crop improvement, the effect of extended nitrate limitation on subsequent hypoxia has not been studied in an organ-specific manner. We cultivated Arabidopsis thaliana without nitrate for 1 week before inducing hypoxia by bubbling the hydroponic solution with nitrogen gas for 16 h. In the roots, the transcripts of two transcription factor genes (HRA1, HRE2) and three genes involved in fermentation (SUS4, PDC1, ADH1) were ~10- to 100-fold upregulated by simultaneous hypoxia and nitrate starvation compared to the control condition (replete nitrate and oxygen). In contrast, this hypoxic upregulation was ~5 to 10 times stronger when nitrate was available. The phytoglobin genes PGB1 and PGB2, involved in nitric oxide (NO) scavenging, were massively downregulated by nitrate starvation (~1000-fold and 105-fold, respectively), but only under ambient oxygen levels; this was reflected in a 2.5-fold increase in NO concentration. In the leaves, HRA1, SUS4, and RAP2.3 were upregulated ~20-fold by hypoxia under nitrate starvation, whereas this upregulation was virtually absent in the presence of nitrate. Our results highlight that the plant's responses to nitrate starvation and hypoxia can influence each other.

E3 SUMO ligase SIZ1 splicing variants localize and function according to external conditions

SIZ1 (SAP and MIZ1) is a member of the Siz/PIAS-type RING family of E3 SUMO (small ubiquitin-related modifier) ligases that play key roles in growth, development, and stress responses in plant and animal systems. Nevertheless, splicing variants of SIZ1 have not yet been characterized. Here, we identified four splicing variants of Arabidopsis (Arabidopsis thaliana) SIZ1, which encode three different protein isoforms. The SIZ1 gene encodes an 873-amino acid (aa) protein. Among the four SIZ1 splicing variants (SSVs), SSV1 and SSV4 encode identical 885 aa proteins; SSV2 encodes an 832 aa protein; and SSV3 encodes an 884 aa protein. SSV2 mainly localized to the plasma membrane, whereas SIZ1, SSV1/SSV4, and SSV3 localized to the nucleus. Interestingly, SIZ1 and all SSVs exhibited similar E3 SUMO ligase activities and preferred SUMO1 and SUMO2 for their E3 ligase activity. Transcript levels of SSV2 were substantially increased by heat treatment, while those of SSV1, SSV3, and SSV4 transcripts were unaffected by various abiotic stresses. SSV2 directly interacted with and sumoylated cyclic nucleotide-gated ion channel 6 (CNGC6), a positive thermotolerance regulator, enhancing the stability of CNGC6. Notably, transgenic siz1-2 mutants expressing SSV2 exhibited greater heat stress tolerance than wild-type plants, whereas those expressing SIZ1 were sensitive to heat stress. Furthermore, transgenic cngc6 plants overaccumulating a mutated mCNGC6 protein (K347R, a mutation at the sumoylation site) were sensitive to heat stress, similar to the cngc6 mutants, while transgenic cngc6 plants overaccumulating CNGC6 exhibited restored heat tolerance. Together, we propose that alternative splicing is an important mechanism that regulates the function of SSVs during development or under adverse conditions, including heat stress.

Physiological, molecular, and environmental insights into plant nitrogen uptake, and metabolism under abiotic stresses

Nitrogen (N) as an inorganic macronutrient is inevitable for plant growth, development, and biomass production. Many external factors and stresses, such as acidity, alkalinity, salinity, temperature, oxygen, and rainfall, affect N uptake and metabolism in plants. The uptake of ammonium (NH4 +) and nitrate (NO3 -) in plants mainly depends on soil properties. Under the sufficient availability of NO3 - (>1 mM), low-affinity transport system is activated by gene network NRT1, and under low NO3 - availability (<1 mM), high-affinity transport system starts functioning encoded by NRT2 family of genes. Further, under limited N supply due to edaphic and climatic factors, higher expression of the AtNRT2.4 and AtNRT2.5T genes of the NRT2 family occur and are considered as N remobilizing genes. The NH4 + ion is the final form of N assimilated by cells mediated through the key enzymes glutamine synthetase and glutamate synthase. The WRKY1 is a major transcription factor of the N regulation network in plants. However, the transcriptome and metabolite profiles show variations in N assimilation metabolites, including glycine, glutamine, and aspartate, under abiotic stresses. The overexpression of NO3 - transporters (OsNRT2.3a and OsNRT1.1b) can significantly improve the biomass and yield of various crops. Altering the expression levels of genes could be a valuable tool to improve N metabolism under the challenging conditions of soil and environment, such as unfavorable temperature, drought, salinity, heavy metals, and nutrient stress.

Small ubiquitin-like modifiers E3 ligases in plant stress

Plants regularly encounter various environmental stresses such as salt, drought, cold, heat, heavy metals and pathogens, leading to changes in their proteome. Of these, a post-translational modification, SUMOylation is particularly significant for its extensive involvement in regulating various plant molecular processes to counteract these external stressors. Small ubiquitin-like modifiers (SUMO) protein modification significantly contributes to various plant functions, encompassing growth, development and response to environmental stresses. The SUMO system has a limited number of ligases even in fully sequenced plant genomes but SUMO E3 ligases are pivotal in recognising substrates during the process of SUMOylation. E3 ligases play pivotal roles in numerous biological and developmental processes in plants, including DNA repair, photomorphogenesis, phytohormone signalling and responses to abiotic and biotic stress. A considerable number of targets for E3 ligases are proteins implicated in reactions to abiotic and biotic stressors. This review sheds light on how plants respond to environmental stresses by focusing on recent findings on the role of SUMO E3 ligases, contributing to a better understanding of how plants react at a molecular level to such stressors.

Design and Enantioselective Synthesis of Chiral Pyranone Fused Indole Derivatives with Antibacterial Activities against Xanthomonas oryzae pv oryzae for Protection of Rice

A new class of chiral pyranone fused indole derivatives were prepared by means of N-heterocyclic carbene (NHC) organocatalysis and demonstrated notable antibacterial activity against Xanthomonas oryzae pv oryzae (Xoo). Bioassays showed that compounds (3S,4R)-5b, (3S,4R)-5d, and (3S,4R)-5l exhibited promising in vitro efficacy against Xoo, with EC50 values of 9.05, 9.71, and 5.84 mg/L, respectively, which were superior to that of the positive controls with commercial antibacterial agents, bismerthiazol (BT, EC50 = 27.8 mg/L) and thiodiazole copper (TC, EC50 = 70.1 mg/L). Furthermore, single enantiomer (3S,4R)-5l was identified as an optimal structure displaying 55.3% and 52.0% curative and protective activities against Xoo in vivo tests at a concentration of 200 mg/L, which slightly surpassed the positive control with TC (curative and protective activities of 47.2% and 48.8%, respectively). Mechanistic studies through molecular docking analysis revealed preliminary insights into the distinct anti-Xoo activity of the two single enantiomers (3S,4R)-5l and (3R,4S)-5l, wherein the (3S,4R)-configured stereoisomer could form a more stable interaction with XooDHPS (dihydropteroate synthase). These findings underscore the significant anti-Xoo potential of these chiral pyranone fused indole derivatives, and shall inspire further exploration as promising lead structures for a novel class of bactericides to combat bacterial infections and other plant diseases.

Gene pyramiding for boosted plant growth and broad abiotic stress tolerance

Abiotic stresses such as salinity, heat and drought seriously impair plant growth and development, causing a significant loss in crop yield and ornamental value. Biotechnology approaches manipulating specific genes prove to be effective strategies in crop trait modification. The Arabidopsis vacuolar pyrophosphatase gene AVP1, the rice SUMO E3 ligase gene OsSIZ1 and the cyanobacterium flavodoxin gene Fld have previously been implicated in regulating plant stress responses and conferring enhanced tolerance to different abiotic stresses when individually overexpressed in various plant species. We have explored the feasibility of combining multiple favourable traits brought by individual genes to acquire superior plant performance. To this end, we have simultaneously introduced AVP1, OsSIZ1 and Fld in creeping bentgrass. Transgenic (TG) plants overexpressing these three genes performed significantly better than wild type controls and the TGs expressing individual genes under both normal and various abiotic stress conditions, exhibited significantly enhanced plant growth and tolerance to drought, salinity and heat stresses as well as nitrogen and phosphate starvation, which were associated with altered physiological and biochemical characteristics and delicately fine-tuned expression of genes involved in plant stress responses. Our results suggest that AVP1, OsSIZ1 and Fld function synergistically to regulate plant development and plant stress response, leading to superior overall performance under both normal and adverse environments. The information obtained provides new insights into gene stacking as an effective approach for plant genetic engineering. A similar strategy can be extended for the use of other beneficial genes in various crop species for trait modifications, enhancing agricultural production.

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.

Effects of Storage Temperatures on Nitrogen Assimilation and Remobilization during Post-Harvest Senescence of Pak Choi

In the agricultural industry, the post-harvest leafy vegetable quality and shelf life significantly influence market value and consumer acceptability. This study examined the effects of different storage temperatures on leaf senescence, nitrogen assimilation, and remobilization in Pak Choi (Brassica rapa subsp. chinensis). Mature Pak Choi plants were harvested and stored at two different temperatures, 4 °C and 25 °C. Senescence was tracked via chlorophyll content and leaf yellowing. Concurrently, alterations in the total nitrogen, nitrate, and protein content were quantified on days 0, 3, 6, and 9 in old, mid, and young leaves of Pak Choi plants. As expected, 4 °C alleviated chlorophyll degradation and delayed senescence of Pak Choi compared to 25 °C. Total nitrogen and protein contents were inversely correlated, while the nitrate content remained nearly constant across leaf groups at 25 °C. Additionally, the transcript levels of genes involved in nitrogen assimilation and remobilization revealed key candidate genes that were differentially expressed between 4 °C and 25 °C, which might be targeted to extend the shelf life of the leafy vegetables. Thus, this study provides pivotal insights into the molecular and physiological responses of Pak Choi to post-harvest storage conditions.

PYRIDOX(AM)INE 5'-PHOSPHATE OXIDASE3 of Arabidopsis thaliana maintains carbon/nitrogen balance in distinct environmental conditions

The identification of factors that regulate C/N utilization in plants can make a substantial contribution to optimization of plant health. Here, we explored the contribution of pyridox(am)ine 5'-phosphate oxidase3 (PDX3), which regulates vitamin B6 homeostasis, in Arabidopsis (Arabidopsis thaliana). Firstly, N fertilization regimes showed that ammonium application rescues the leaf morphological phenotype of pdx3 mutant lines but masks the metabolite perturbance resulting from impairment in utilizing soil nitrate as a source of N. Without fertilization, pdx3 lines suffered a C/N imbalance and accumulated nitrogenous compounds. Surprisingly, exploration of photorespiration as a source of endogenous N driving this metabolic imbalance, by incubation under high CO2, further exacerbated the pdx3 growth phenotype. Interestingly, the amino acid serine, critical for growth and N management, alleviated the growth phenotype of pdx3 plants under high CO2, likely due to the requirement of pyridoxal 5'-phosphate for the phosphorylated pathway of serine biosynthesis under this condition. Triggering of thermomorphogenesis by growth of plants at 28 °C (instead of 22 °C) did not appear to require PDX3 function, and we observed that the consequent drive toward C metabolism counters the C/N imbalance in pdx3. Further, pdx3 lines suffered a salicylic acid-induced defense response, probing of which unraveled that it is a protective strategy mediated by nonexpressor of pathogenesis related1 (NPR1) and improves fitness. Overall, the study demonstrates the importance of vitamin B6 homeostasis as managed by the salvage pathway enzyme PDX3 to growth in diverse environments with varying nutrient availability and insight into how plants reprogram their metabolism under such conditions.

Pseudomonas aeruginosa LasI-dependent plant growth promotion requires the host nitrate transceptor AtNRT1.1/CHL1 and the nitrate reductases NIA1 and NIA2

MAIN CONCLUSION

In P. aeruginosa, mutation of the gene encoding N-acyl-L-homoserine lactone synthase LasI drives defense and plant growth promotion, and this latter trait requires adequate nitrate nutrition. Cross-kingdom communication with bacteria is crucial for plant growth and productivity. Here, we show a strong induction of genes for nitrate uptake and assimilation in Arabidopsis seedlings co-cultivated with P. aeruginosa WT (PAO1) or ΔlasI mutants defective on the synthesis of the quorum-sensing signaling molecule N-(3-oxododecanoyl)-L-homoserine lactone. Along with differential induction of defense-related genes, the change from plant growth repression to growth promotion upon bacterial QS disruption, correlated with upregulation of the dual-affinity nitrate transceptor CHL1/AtNRT1/NPF6.3 and the nitrate reductases NIA1 and NIA2. CHL1-GUS was induced in Arabidopsis primary root tips after transfer onto P. aeruginosa ΔlasI streaks at low and high N availability, whereas this bacterium required high concentrations of nitrogen to potentiate root and shoot biomass production and to improve root branching. Arabidopsis chl1-5 and chl1-12 mutants and double mutants in NIA1 and NIA2 nitrate reductases showed compromised growth under low nitrogen availability and failed to mount an effective growth promotion and root branching response even at high NH4NO3. WT P. aeruginosa PAO1 and P. aeruginosa ΔlasI mutant promoted the accumulation of nitric oxide (NO) in roots of both the WT and nia1nia2 double mutants, whereas NO donors SNP or SNAP did not improve growth or root branching in nia1nia2 double mutants with or without bacterial cocultivation. Thus, inoculation of Arabidopsis roots with P. aeruginosa drives gene expression for improved nitrogen acquisition and this macronutrient is critical for the plant growth-promoting effects upon disruption of the LasI quorum-sensing system.

ZmSIZ1a and ZmSIZ1b play an indispensable role in resistance against Fusarium ear rot in maize

Fusarium ear rot (FER) is a destructive fungal disease of maize caused by Fusarium verticillioides. FER resistance is a typical complex quantitative trait controlled by micro-effect genes, leading to difficulty in identifying the host resistance genes. SIZ1 encodes a SUMO E3 ligase regulating a wide range of plant developmental processes and stress responses. However, the function of ZmSIZ1 remains poorly understood. In this study, we demonstrate that ZmSIZ1a and ZmSIZ1b possess SUMO E3 ligase activity, and that the Zmsiz1a/1b double mutant, but not the Zmsiz1a or Zmsiz1b single mutants, exhibits severely impaired resistance to FER. Transcriptome analysis showed that differentially expressed genes were significantly enriched in plant disease resistance-related pathways, especially in plant-pathogen interaction, MAPK signalling, and plant hormone signal transduction. Thirty-five candidate genes were identified in these pathways. Furthermore, the integration of the transcriptome and metabolome data revealed that the flavonoid biosynthesis pathway was induced by F. verticillioides infection, and that accumulation of flavone and flavonol was significantly reduced in the Zmsiz1a/1b double mutant. Collectively, our findings demonstrate that ZmSIZ1a and ZmSIZ1b play a redundant, but indispensable role against FER, and provide potential new gene resources for molecular breeding of FER-resistant maize cultivars.

A phospho-switch constrains BTL2-mediated phytocytokine signaling in plant immunity

Enabling and constraining immune activation is of fundamental importance in maintaining cellular homeostasis. Depleting BAK1 and SERK4, the co-receptors of multiple pattern recognition receptors (PRRs), abolishes pattern-triggered immunity but triggers intracellular NOD-like receptor (NLR)-mediated autoimmunity with an elusive mechanism. By deploying RNAi-based genetic screens in Arabidopsis, we identified BAK-TO-LIFE 2 (BTL2), an uncharacterized receptor kinase, sensing BAK1/SERK4 integrity. BTL2 induces autoimmunity through activating Ca2+ channel CNGC20 in a kinase-dependent manner when BAK1/SERK4 are perturbed. To compensate for BAK1 deficiency, BTL2 complexes with multiple phytocytokine receptors, leading to potent phytocytokine responses mediated by helper NLR ADR1 family immune receptors, suggesting phytocytokine signaling as a molecular link connecting PRR- and NLR-mediated immunity. Remarkably, BAK1 constrains BTL2 activation via specific phosphorylation to maintain cellular integrity. Thus, BTL2 serves as a surveillance rheostat sensing the perturbation of BAK1/SERK4 immune co-receptors in promoting NLR-mediated phytocytokine signaling to ensure plant immunity.

Identification and functional characterization of the SUMO system in sweet potato under salt and drought stress

Sumoylation is a crucial post-translation modification (PTM) that is the covalent attachment of SUMO molecules to the substrate catalyzed by enzyme cascade. Sumoylation is essential in almost every physiological process of plants, particularly in response to abiotic stress. However, little is known about sumoylation in sweet potato (Ipomoea batatas), the world's seventh most important food crop. In this study, 17 sweet potato SUMO system genes have been cloned and functionally characterized. Multiple sequence alignment and phylogenetic analysis showed sweet potato SUMO system proteins had conserved domains and activity sites. IbSUMOs, IbSAE1, and IbSCE1 were localized in the cytoplasm and nucleus. E3 SUMO ligases showed nuclear or punctate localization. In vitro sumoylation assay confirmed the catalytic activity of sweet potato SUMO system components. Heterologous expression of IbSIZ1 genes in Arabidopsis atsiz1 mutant rescued the defective germination and growth phenotype. IbSCE1a/b and IbSIZ1a/b/c were salt and drought responsive genes. Heterologous expression of IbSCE1a/b/c improved the drought tolerance of Arabidopsis thaliana, while IbSIZ1a/b/c significantly enhanced the salt and drought tolerance. Our findings define that the SUMO system in sweet potato shared with conserved function but also possessed specific characterization. The resources presented here would facilitate uncovering the significance of sumoylation in sweet potato.

Apple SUMO E3 ligase MdSIZ1 regulates cuticular wax biosynthesis by SUMOylating transcription factor MdMYB30

A key function of SUMOylation is the coordinated modification of numerous proteins to optimize plant growth and resistance to environmental stress. Plant cuticular wax is deposited on the surface of primary plant organs to form a barrier that provides protection against changes in terrestrial environments. Many recent studies have examined cuticular wax biosynthetic pathways and regulation. However, whether SUMOylation is involved in the regulation of cuticle wax deposition at the posttranslational level remains unclear. Here, we demonstrate that a small ubiquitin-like modifier (SUMO) E3 ligase, SAP AND MIZ1 DOMAIN CONTAINING LIGASE1 (MdSIZ1), regulates wax accumulation and cuticle permeability in apple (Malus domestica Borkh), SUMO E2 CONJUGATING ENZYME 1(MdSCE1) physically interacts with MdMYB30, a transcription factor involved in the regulation of cuticle wax accumulation. MdSIZ1 mediates the SUMOylation and accumulation of MdMYB30 by inhibiting its degradation through the 26S proteasome pathway. Furthermore, MdMYB30 directly binds to the β-KETOACYL-COA SYNTHASE 1 (MdKCS1) promoter to activate its expression and promote wax biosynthesis. These findings indicate that the MdSIZ1-MdMYB30-MdKCS1 module positively regulates cuticular wax biosynthesis in apples. Overall, the findings of our study provide insights into the regulation pathways involved in cuticular wax biosynthesis.

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.

Current perspectives of ubiquitination and SUMOylation in abiotic stress tolerance in plants

Post-translational modification (PTM) is a critical and rapid mechanism to regulate all the major cellular processes through the modification of diverse protein substrates. Substrate-specific covalent attachment of ubiquitin and Small Ubiquitin-Like Modifier (SUMO) with the target proteins, known as ubiquitination and SUMOylation, respectively, are crucial PTMs that regulate almost every process in the cell by modulating the stability and fidelity of the proteins. Ubiquitination and SUMOylation play a very significant role to provide tolerance to the plants in adverse environmental conditions by activating/deactivating the pre-existing proteins to a great extent. We reviewed the importance of ubiquitination and SUMOylation in plants, implicating its prospects in various abiotic stress regulations. An exhaustive study of molecular mechanisms of ubiquitination and SUMOylation of plant proteins and their role will contribute to the understanding of physiology underlying mitigation of the abiotic stresses and survival in plants. It will be helpful to strategize the improvement of crops for abiotic stress tolerance.

Pepino (Solanum muricatum) Metabolic Profiles and Soil Nutrient Association Analysis in Three Growing Sites on the Loess Plateau of Northwestern China

Different soil nutrients affect the accumulation characteristics of plant metabolites. To investigate the differences among the metabolites of pepino grown in greenhouses on the Loess Plateau in northwest China, we investigated the main soil nutrients and their correlation with metabolites. A total of 269 pepino metabolites were identified using UPLC-QTOF-MS to detect metabolites in fruits from three major pepino growing regions and analyze their differential distribution characteristics. A total of 99 of these substances differed among pepino fruits from the three areas, and the main classes of the differential metabolites were, in order of number: amino acids and derivatives, nucleotides and derivatives, organic acids, alkaloids, vitamins, saccharides and alcohols, phenolic acids, lipids and others. An environmental factor analysis identified soil nutrients as the most significant differentiator. Five soil nutrient indicators: TN (total nitrogen), TP (total phosphorus), AP (available phosphorus), AK (available potassium), and OM (organic matter), exhibited significant differences in three growing sites. Metabolite and soil nutrient association analysis using redundancy analysis (RDA) and the Mantel test indicated that TN and OM contributed to the accumulation of amino acids and derivatives, nucleotides and derivatives, and alkaloids while inhibiting organic acids, vitamins coagulation biosynthesis. Moreover, AP and TP were associated with the highest accumulation of saccharides and, alcohols, phenolic acids. Consequently, differences in soil nutrients were reflected in pepino metabolite variability. This study clarified the metabolite variability and the relationship between pepino and soil nutrients in the main planting areas of northwest China. It provides a theoretical basis for the subsequent development of Pepino’s nutritional value and cultivation management.

Ammonium and nitric oxide condition the establishment of Arabidopsis Ler/Kas-2 immune-related hybrid incompatibility

High ammonium suppresses hybrid incompatibility between Ler and Kas-2 accessions through lowering nitric oxide levels and nitrate reductase activity required for autoimmunity. The immune-related hybrid incompatibility (HI) between Landsberg erecta (Ler) and Kashmir-2 (Kas-2) accessions is due to a deleterious genetic interaction between the RPP1 (RECOGNITION OF PERONOSPORA PARASITICA1)-like Ler locus and Kas-2 alleles of the receptor-like kinase SRF3 (STRUBBELIG RECEPTOR FAMILY 3). The genetic incompatibility is temperature-dependent and leads to constitutive activation of the salicylic acid (SA) pathway, dwarfism and cell death at 14-16 °C. Here we investigated the effect of nutrition on the occurrence of Ler/Kas-2 HI and found that high ammonium suppresses Ler/Kas-2 incompatible phenotypes independently of the ammonium/nitrate ratio. Ammonium feeding leads to compromised disease resistance to Pseudomonas syringae pv. tomato DC3000, lower total SA, nitric oxide and nitrate reductase activity in Ler/Kas-2 incompatible hybrids. In addition, we find that Ler/Kas-2 incompatibility is dependent on NPR1 (NONEXPRESSER OF PR GENES 1) and nitric oxide production. Overall, this work highlights the effect of nutrition on the expression of incompatible phenotypes independently of temperature.

Arabidopsis SUMO E3 ligase SIZ1 enhances cadmium tolerance via the glutathione-dependent phytochelatin synthesis pathway

Sumoylation is a posttranslational modification (PTM) in which SUMO (small ubiquitin-like modifier) is covalently conjugated to protein substrates via a range of enzymes. SUMO E3 ligase SIZ1 is involved in mediating several essential or nonessential element-responsive SUMO conjugations in Arabidopsis. However, whether SIZ1 is involved in the cadmium (Cd) response remains to be identified. In this study, we found that SIZ1 positively regulates plant Cd tolerance. The loss-of-function siz1-2 mutant exhibited impaired resistance to Cd exposure and accumulated more reactive oxygen species (ROS). Moreover, the transcription of GSH1, GSH2, PCS1, and PCS2 was suppressed while the accumulation of Cd was enhanced in the siz1-2 mutant under Cd exposure. Further analysis revealed that the higher Cd sensitivity of the siz1-2 mutant was partially rescued by the overexpression of GSH1. Consistently, Cd stress stimulated the accumulation of SUMO1 conjugates in wild-type plants but not in the siz1-2 mutant. Together, our results demonstrate that Cd-induced SIZ1 activates GSH- and PC synthesis-related gene expression to increase the synthesis of GSH- and PCs, thereby leading to higher Cd tolerance in plants.

Nitrate Uptake and Use Efficiency: Pros and Cons of Chloride Interference in the Vegetable Crops

Over the past five decades, nitrogen (N) fertilization has been an essential tool for boosting crop productivity in agricultural systems. To avoid N pollution while preserving the crop yields and profit margins for farmers, the scientific community is searching for eco-sustainable strategies aimed at increasing plants' nitrogen use efficiency (NUE). The present article provides a refined definition of the NUE based on the two important physiological factors (N-uptake and N-utilization efficiency). The diverse molecular and physiological mechanisms underlying the processes of N assimilation, translocation, transport, accumulation, and reallocation are revisited and critically discussed. The review concludes by examining the N uptake and NUE in tandem with chloride stress and eustress, the latter being a new approach toward enhancing productivity and functional quality of the horticultural crops, particularly facilitated by soilless cultivation.

Nitrogen availability determines plant growth promotion and the induction of root branching by the probiotic fungus Trichoderma atroviride in Arabidopsis seedlings

Plant growth-promoting fungi are integral components of the root microbiome that help the host resist biotic and abiotic stress while improving nutrient acquisition. Trichoderma atroviride is a common inhabitant of the rhizosphere, which establishes a perdurable symbiosis with plants through the emission of volatiles, diffusible compounds, and robust colonization. Currently, little is known on how the environment influences the Trichoderma-plant interaction. In this report, we assessed plant growth and root architectural reconfiguration of Arabidopsis seedlings grown in physical contact with T. atroviride under contrasting nitrate and ammonium availability. The shoot and root biomass accumulation and lateral root formation triggered by the fungus required high nitrogen supplements and involved nitrate reduction via AtNIA1 and NIA2. Ammonium supplementation did not restore biomass production boosted by T. atroviride in nia1nia2 double mutant, but instead fungal inoculation increased nitric oxide accumulation in Arabidopsis primary root tips depending upon nitrate supplements. N deprived seedlings were largely resistant to the effects of nitric oxide donor SNP triggering lateral root formation. T. atroviride enhanced expression of CHL1:GUS in root tips, particularly under high N supplements and required an intact CHL1 nitrate transporter to promote lateral root formation in Arabidopsis seedlings. These data imply that the developmental programs strengthened by Trichoderma and the underlying growth promotion in plants are dependent upon adequate nitrate nutrition and may involve nitric oxide as a second messenger.

Nitric Oxide Signaling and Its Association with Ubiquitin-Mediated Proteasomal Degradation in Plants

Nitric oxide (NO) is a versatile signaling molecule with diverse roles in plant biology. The NO-mediated signaling mechanism includes post-translational modifications (PTMs) of target proteins. There exists a close link between NO-mediated PTMs and the proteasomal degradation of proteins via ubiquitylation. In some cases, ubiquitin-mediated proteasomal degradation of target proteins is followed by an NO-mediated post-translational modification on them, while in other cases NO-mediated PTMs can regulate the ubiquitylation of the components of ubiquitin-mediated proteasomal machinery for promoting their activity. Another pathway that links NO signaling with the ubiquitin-mediated degradation of proteins is the N-degron pathway. Overall, these mechanisms reflect an important mechanism of NO signal perception and transduction that reflect a close association of NO signaling with proteasomal degradation via ubiquitylation. Therefore, this review provides insight into those pathways that link NO-PTMs with ubiquitylation.

The Regulation of Nitrate Reductases in Response to Abiotic Stress in Arabidopsis

The two homologous genes, NIA1 and NIA2, encode nitrate reductases in Arabidopsis, which govern the reduction of nitrate to nitrite. This step is the rate-limiting step of the nitrate assimilation and utilization. Therefore, the regulation of NIA1 and NIA2 is important for plant development and growth. Although they are similar in sequence and structure, their regulations are different. Genetic analysis uncovers that NIA1, rather than NIA2, plays a predominant role in adopting to ABA stress. Although both long-term stress conditions can cause an improvement in NIA1 levels, a decrease in NIA1 levels under short-term treatments seems to be necessary for plants to switch from the growth status into the adopting status. Interestingly, the downregulation of the NR is distinct under different stress conditions. Under ABA treatment, the NR proteins are degraded via a 26S-proteasome dependent manner, while the transcriptional regulation is the main manner to rapidly reduce the NIA1 levels under nitrogen deficiency and NaCl stress conditions. These results indicate that under stress conditions, the regulation of NIA1 is complex, and it plays a key role in regulating the balance between growth and adaptation.

Nitrogen assimilation in plants: current status and future prospects

Nitrogen (N) is the driving force for crop yields, however, excessive N application in agriculture not only increases production cost, but also causes severe environmental problems. Therefore, comprehensively understanding the molecular mechanisms of N use efficiency (NUE) and breeding crops with higher NUE is essential to tackle these problems. NUE of crops is determined by N uptake, transport, assimilation, and remobilization. In the process of N assimilation, nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), and glutamine-2-oxoglutarate aminotransferase (GOGAT, also known as glutamate synthase) are the major enzymes. NR and NiR mediate the initiation of inorganic N utilization, and GS/GOGAT cycle converts inorganic N to organic N, playing a vital role in N assimilation and the final NUE of crops. Besides, asparagine synthetase (ASN), glutamate dehydrogenase (GDH), and carbamoylphosphate synthetase (CPSase) are also involved. In this review, we summarize the function and regulation of these enzymes reported in three major crops, rice, maize, wheat, also in the model plant Arabidopsis, and we highlight their application in improving NUE of crops via manipulating N assimilation. Anticipated challenges and prospects toward fully understanding the function of N assimilation and further exploring the potential for NUE improvement are discussed.

A mutation in the pPLA-IIα gene encoding PATATIN-RELATED PHOSPHOLIPASE a causes late flowering in Arabidopsis

Arabidopsis PATATIN-RELATED PHOSPHOLIPASE 2A (pPLA-IIα) participates in the responses to various growth conditions. The factors affecting pPLA-IIα gene expression and pPLA-IIα protein activity for gycerolipids have been studied thoroughly, but the role of pPLA-IIα during the reproductive phase remains unclear. The effect of pPLA-IIα on flowering time was therefore investigated. ppla-iiα mutants flowered later than wild-type plants under long day conditions. Expression of the floral stimulators FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) was downregulated in ppla-iiα mutants compared with their expression in wild-type plants, but expression of the floral repressor FLOWERING LOCUS C (FLC) was upregulated. In addition, expression levels of COLDAIR, a long intronic noncoding RNA, decreased in ppla-iiα mutants. Taken together, these data indicate that pPLA-IIα acts as a positive regulator of flowering time through repression of FLC expression.

NIN-like protein7 and PROTEOLYSIS6 functional interaction enhances tolerance to sucrose, ABA, and submergence

Nitrate (NO3) assimilation and signaling regulate plant growth through the relevant function of the transcription factor NIN-like Protein7 (NLP7). NO3 is also the main source for plants to produce nitric oxide (NO), which regulates growth and stress responses. NO-mediated regulation requires efficient sensing via the PROTEOLYSIS6 (PRT6)-mediated proteasome-triggered degradation of group VII of ethylene response transcription factors through the Cys/Arg N-degron pathway. The convergence of NO3 signaling and N-degron proteolysis on NO-mediated regulation remains largely unknown. Here, we investigated the functional interaction between NLP7 and PRT6 using Arabidopsis (Arabidopsis thaliana) double prt6 nlp7 mutant plants as well as complementation lines overexpressing NLP7 in different mutant genetic backgrounds. prt6 nlp7 mutant plants displayed several potentiated prt6 characteristic phenotypes, including slower vegetative growth, increased NO content, and diminished tolerance to abiotic stresses such as high-sucrose concentration, abscisic acid, and hypoxia-reoxygenation. Although NLP7 has an N-terminus that could be targeted by the N-degron proteolytic pathway, it was not a PRT6 substrate. The potential PRT6- and NO-regulated nucleocytoplasmic translocation of NLP7, which is likely modulated by posttranslational modifications, is proposed to act as a regulatory loop to control NO homeostasis and action.

Phytosulfokine α (PSKα) delays senescence and reinforces SUMO1/SUMO E3 ligase SIZ1 signaling pathway in cut rose flowers (Rosa hybrida cv. Angelina)

Roses are widely used as cut flowers worldwide. Petal senescence confines the decorative quality of cut rose flowers, an impressively considerable economic loss. Herein, we investigated the SUMO1/SUMO E3 ligase SIZ1 signaling pathway during bud opening, and petal senescence of cut rose flowers. Our results exhibited that the higher expression of SUMO1 and SUMO E3 ligase SIZ1 during bud opening was accompanied by lower endogenous H2O2 accumulation arising from higher expression and activities of SOD, CAT, APX, and GR, promoting proline accumulation by increasing P5CS expression and activity and enhancing GABA accumulation by increasing GAD expression and activity. In harvested flowers, lower expressions of SUMO1 and SUMO E3 ligase SIZ1 during petal senescence were associated with higher endogenous H2O2 accumulation due to lower expression and activities of SOD, CAT, APX, and GR. Therefore, promoting the activity of the GABA shunt pathway as realized by higher expression and activities of GABA-T and SSADH accompanied by increasing OAT expression and activity for sufficiently supply proline in rose flowers during petal senescence might serve as an endogenous antisenescence mechanism for slowing down petals senescence by avoiding endogenous H2O2 accumulation. Following phytosulfokine α (PSKα) application, postponing petal senescence in cut rose flowers could be ascribed to higher expression of SUMO1 and SUMO E3 ligase SIZ1 accompanied by higher expression and activities of SOD, CAT, APX, and GR, higher activity of GABA shunt pathway as realized by higher expression and activities of GAD, GABA-T, and SSADH, higher expression and activities of P5CS and OAT for supplying proline and higher expression of HSP70 and HSP90. Therefore, our results highlight the potential of the PSKα as a promising antisenescence signaling peptide in the floriculture industry for postponing senescence and extending the vase life of cut rose flowers.

Transcript profiling of stress-responsive genes and metabolic changes during salinity in indica and japonica rice exhibit distinct varietal difference

In the present study, we carried out comprehensive transcript profiling of diverse genes under salinity (200 mM NaCl) at different time points, accompanied by certain biochemical alterations of the indica (IR-64 and Pokkali) and japonica (Nipponbare and M-202) rice. The higher susceptibility of Nipponbare and IR-64 was reflected by lower relative water content, chlorophyll loss, higher malondialdehyde content, and accumulation of H₂ O₂ , and reduced nitrate reductase activity, compared to M-202 and Pokkali, where such changes were less pronounced. Enhanced levels of anthocyanins and reduced glutathione, together with elevated phenylalanine ammonia lyase activity, mainly conferred protection to Nipponbare and IR-64, while metabolites like phenolics, flavonoids, proline, and polyamines were more induced in M-202 and Pokkali. Varietal differences in the expression pattern of diverse groups of genes during different durations (6, 24, and 48 h) of stress were striking. A gene showing early induction for a particular variety exhibited a delayed induction in another variety or a gradually decreased expression with treatment time. Pokkali was clearly identified as the salt-tolerant genotype among the examined varieties based on increased antioxidant potential and enhanced expression of genes encoding for PAL, CHS, and membrane transporters like SOS3, NHX-1, and HKT-1. The results presented in this work provide insight into the complex varying regulation patterns for different genes across the investigated rice varieties in providing salt tolerance and highlights distinct differences in expression patterns between susceptible and tolerant indica and japonica rice.

Arabidopsis SUMO E3 Ligase SIZ1 Interacts with HDA6 and Negatively Regulates HDA6 Function during Flowering

The changes in histone acetylation mediated by histone deacetylases (HDAC) play a crucial role in plant development and response to environmental changes. Mammalian HDACs are regulated by post-translational modifications (PTM), such as phosphorylation, acetylation, ubiquitination and small ubiquitin-like modifier (SUMO) modification (SUMOylation), which affect enzymatic activity and transcriptional repression. Whether PTMs of plant HDACs alter their functions are largely unknown. In this study, we demonstrated that the Arabidopsis SUMO E3 ligase SAP AND MIZ1 DOMAIN-CONTAINING LIGASE1 (SIZ1) interacts with HISTONE DEACETYLASE 6 (HDA6) both in vitro and in vivo. Biochemical analyses indicated that HDA6 is not modified by SUMO1. Overexpression of HDA6 in siz1-3 background results in a decreased level of histone H3 acetylation, indicating that the activity of HDA6 is increased in siz1-3 plants. Chromatin immunoprecipitation (ChIP) assays showed that SIZ1 represses HDA6 binding to its target genes FLOWERING LOCUS C (FLC) and MADS AFFECTING FLOWERING 4 (MAF4), resulting in the upregulation of FLC and MAF4 by increasing the level of histone H3 acetylation. Together, these findings indicate that the Arabidopsis SUMO E3 ligase SIZ1 interacts with HDA6 and negatively regulates HDA6 function.

Posttranslational Modifications: Regulation of Nitrogen Utilization and Signaling

Nitrogen is the most important macroelement required for the composition of key molecules, such as nucleic acids, proteins and other organic compounds. As sessile organisms, plants have evolved sophisticated mechanisms to acquire nitrogen for their normal growth and development. Besides the transcriptional and translational regulation of nitrogen uptake, assimilation, remobilization and signal transduction, posttranslational modifications (PTMs) are shown to participate in these processes in plants. In addition to alterations in protein abundance, PTMs may dramatically increase the complexity of the proteome without the concomitant changes in gene transcription and have emerged as an important type of protein regulation in terms of protein function, subcellular localization and protein activity and stability. Herein, we briefly summarize recent advances on the posttranslational regulation of nitrogen uptake, assimilation, remobilization and nitrogen signaling and discuss the underlying mechanisms of PTMs as well as the signal output of such PTMs. Understanding these regulation mechanisms will provide novel insights for improving the nitrogen use efficiency of plants.

Strategies for engineering improved nitrogen use efficiency in crop plants via redistribution and recycling of organic nitrogen

Global use of nitrogen (N) fertilizers has increased sevenfold from 1960 to 1995 but much of the N applied is lost to the environment. Modifying the temporal and spatial distribution of organic N within the plant can lead to improved grain yield and/or grain protein content for the same or reduced N fertilizer inputs. Biotechnological approaches to modify whole plant distribution of amino acids and ureides has proven successful in several crop species. Manipulating selective autophagy pathways in crops has also improved N remobilization efficiency to sink tissues whilst the contribution of ribophagy, RNA and purine catabolism to N recycling in crops is still too early to foretell. Improved recycling and remobilization of N must exploit N-stress responsive transcriptional regulators, N-sensing or phloem-localized promotors and genetic variation for N-responsive traits.

Ammonium-mediated reduction in salicylic acid content and recovery of plant growth in Arabidopsis siz1 mutants is modulated by NDR1 and NPR1

The siz1 mutants exhibit high SA accumulation and consequently severe dwarfism. Although siz1 mutants exhibit growth recovery upon exogenous ammonium supply, the underlying mechanism remains unknown. Here, we investigated the effect of ammonium on SA level and plant growth in SA-accumulating mutants. The growth of siz1-2 and siz1-3 mutants was recovered to wild-type (WT) levels upon exogenous ammonium supply, but that of siz1-3 ndr1 (non-race-specific disease resistance 1) and siz1-3 npr1 (non-expressor of pathogenesis related gene 1) double mutants was unaffected. The SA level was decreased by exogenous ammonium application in siz1-3 ndr1, siz1-3 npr1, and siz1-3 mutants. The level of nitrate reductase (NR) was almost the same in all genotypes (WT, siz1-3, ndr1, npr1, siz1-3 ndr1, and siz1-3 npr1), regardless of the ammonium treatment, suggesting that exogenous ammonium supply to ndr1 siz1-3 and npr1 siz1-3 double mutants does not have any effect on their growth and NR levels, but decreases the SA level. Taken together, these results indicate that ammonium acts as a signaling molecule to regulate the SA amount, and NDR1 and NPR1 play a positive role in the ammonium-mediated growth recovery of siz1 mutants.

Lessons from Comparison of Hypoxia Signaling in Plants and Mammals

Hypoxia is an important stress for organisms, including plants and mammals. In plants, hypoxia can be the consequence of flooding and causes important crop losses worldwide. In mammals, hypoxia stress may be the result of pathological conditions. Understanding the regulation of responses to hypoxia offers insights into novel approaches for crop improvement, particularly for the development of flooding-tolerant crops and for producing better therapeutics for hypoxia-related diseases such as inflammation and cancer. Despite their evolutionary distance, plants and mammals deploy strikingly similar mechanisms to sense and respond to the different aspects of hypoxia-related stress, including low oxygen levels and the resulting energy crisis, nutrient depletion, and oxidative stress. Over the last two decades, the ubiquitin/proteasome system and the ubiquitin-like protein SUMO have been identified as key regulators that act in concert to regulate core aspects of responses to hypoxia in plants and mammals. Here, we review ubiquitin and SUMO-dependent mechanisms underlying the regulation of hypoxia response in plants and mammals. By comparing and contrasting these mechanisms in plants and mammals, this review seeks to pinpoint conceptually similar mechanisms but also highlight future avenues of research at the junction between different fields of research.

Blue and UV-B light synergistically induce anthocyanin accumulation by co-activating nitrate reductase gene expression in Anthocyanin fruit (Aft) tomato

The tomato accession LA1996, which carries a dominant allele of anthocyanin fruit (Aft) locus, accumulates anthocyanins in the epidermis of fruits when exposed to sunlight. The involvement of blue, UV-A, UV-B and a combination of these wavelengths on anthocyanin accumulation and the molecular mechanism of their regulation was investigated in LA1996. The most effective treatment for inducing anthocyanin biosynthesis in Aft fruits was co-irradiation with blue and UV-B (blue + UV-B) light. Finding the correlated genes is an important approach towards understanding their molecular mechanisms. In the present study, the nitrate reductase (NR) gene SlNIA was isolated using RNA-seq profiling of Aft fruits given different light treatments. The functions of NR-mediated anthocyanin induction by blue + UV-B were confirmed using a series of chemical treatments, followed by assessment of NR activity and nitric oxide (NO) detection. The expression of NR was highly induced by blue + UV-B, and this specificity was also confirmed with the enzyme activity of NR and the NO concentration. The NR inhibitors, which reduce NO generation, the expression levels of anthocyanin related genes and decreased anthocyanin accumulation in LA1996. Our results suggest that NR plays a key role in blue + UV-B-mediated anthocyanin accumulation in LA1996 fruits.

Towards doubling fibre yield for cotton in the semiarid agricultural area by increasing tolerance to drought, heat and salinity simultaneously

Abiotic stresses such as extreme temperatures, water-deficit and salinity negatively affect plant growth and development, and cause significant yield losses. It was previously shown that co-overexpression of the Arabidopsis vacuolar pyrophosphatase gene AVP1 and the rice SUMO E3 ligase gene OsSIZ1 in Arabidopsis significantly increased tolerance to multiple abiotic stresses and led to increased seed yield for plants grown under single or multiple abiotic stress conditions. It was hypothesized that there might be synergistic effects between AVP1 overexpression and OsSIZ1 overexpression, which could lead to substantially increased yields if these two genes are co-overexpressed in real crops. To test this hypothesis, AVP1 and OsSIZ1 were co-overexpressed in cotton, and the impact of OsSIZ1/AVP1 co-overexpression on cotton's performance under normal growth and multiple stress conditions were analysed. It was found that OsSIZ1/AVP1 co-overexpressing plants performed significantly better than AVP1-overexpressing, OsSIZ1-overexpressing and wild-type cotton plants under single, as well as under multiple stress conditions in laboratory and field conditions. Two field studies showed that OsSIZ1/AVP1 co-overexpressing plants produced 133% and 81% more fibre than wild-type cotton in the dryland conditions of West Texas. This research illustrates that co-overexpression of AVP1 and OsSIZ1 is a viable strategy for engineering abiotic stress-tolerant crops and could substantially improve crop yields in low input or marginal environments, providing a solution for food security for countries in arid and semiarid regions of the world.

Nitrogen and Stem Development: A Puzzle Still to Be Solved

High crop yields are generally associated with high nitrogen (N) fertilizer rates. A growing tendency that is urgently demanding the adoption of precision technologies that manage N more efficiently, combined with the advances of crop genetics to meet the needs of sustainable farm systems. Among the plant traits, stem architecture has been of paramount importance to enhance harvest index in the cereal crops. Nonetheless, the reduced stature also brought undesirable effect, such as poor N-uptake, which has led to the overuse of N fertilizer. Therefore, a better understanding of how N signals modulate the initial and late stages of stem development might uncover novel semi-dwarf alleles without pleiotropic effects. Our attempt here is to review the most recent advances on this topic.

Post-Translational Modifications of Nitrate Reductases Autoregulates Nitric Oxide Biosynthesis in Arabidopsis

Nitric oxide (NO) is a regulator of growth, development, and stress responses in living organisms. Plant nitrate reductases (NR) catalyze the reduction of nitrate to nitrite or, alternatively, to NO. In plants, NO action and its targets remain incompletely understood, and the way NO regulates its own homeostasis remains to be elucidated. A significant transcriptome overlapping between NO-deficient mutant and NO-treated wild type plants suggests that NO could negatively regulate its biosynthesis. A significant increase in NO content was detected in transgenic plants overexpressing NR1 and NR2 proteins. In turn, NR protein and activity as well as NO content, decreased in wild-type plants exposed to a pulse of NO gas. Tag-aided immunopurification procedures followed by tandem mass spectrometry allowed identifying NO-triggered post-translational modifications (PTMs) and ubiquitylation sites in NRs. Nitration of tyrosine residues and S-nitrosation of cysteine residues affected key amino acids involved in binding the essential FAD and molybdenum cofactors. NO-related PTMs were accompanied by ubiquitylation of lysine residues flanking the nitration and S-nitrosation sites. NO-induced PTMs of NRs potentially inhibit their activities and promote their proteasome-mediated degradation. This auto-regulatory feedback loop may control nitrate assimilation to ammonium and nitrite-derived production of NO under complex environmental conditions.

Comparison with Arabidopsis reveals optimal nitrogen allocation strategy and mechanism in Chorispora bungeana, a cryophyte with strong freezing tolerance

The stress responses of plant compete for resources with growth and development. Resource allocations among these processes may have been optimized in plants adapted to natural habitats. Here, nitrogen (N) allocations were compared in leaves of Arabidopsis and Chorispora bungeana, a cryophyte with strong freezing tolerance. The results showed that the two species differed not only in N partitions among N forms and allocations among leaves, but also in their responses to cold stress. Interestingly, leaf protein contents were enhanced in C. bungeana while reduced in Arabidopsis, though the N allocations to leaves were reduced in both plants upon cold stress. Profoundly, when grown at warm temperature, contents of total free amino acids (FAAs) in leaves of C. bungeana were 6-11 times higher than those in Arabidopsis. In contrast, cold treatment induced FAAs accumulation in leaves of Arabidopsis without having significant effect in any leaf of C. bungeana. Considerable discrepancy was also found between the two species in the expressions of nitrate transporter genes and the activities of nitrate assimilation enzymes. Correlation and network analysis showed that most NPFs were clustered in a single network module and had loose relations with protein synthesis in Arabidopsis, while they were distributed in different modules in a decentralized network in C. bungeana. Therefore, our results reveal that C. bungeana may have optimized its N allocation strategy by producing and storing amino acids as efficient N reserve and adopting a decentralized network for N utilization, which may equip the plant with powerful capabilities for environmental adaptions.

An Insight into the Factors Influencing Specificity of the SUMO System in Plants

Due to their sessile nature, plants are constantly subjected to various environmental stresses such as drought, salinity, and pathogen infections. Post-translational modifications (PTMs), like SUMOylation, play a vital role in the regulation of plant responses to their environment. The process of SUMOylation typically involves an enzymatic cascade containing the activation, (E1), conjugation (E2), and ligation (E3) of SUMO to a target protein. Additionally, it also requires a class of SUMO proteases that generate mature SUMO from its precursor and cleave it off the target protein, a process termed deSUMOylation. It is now clear that SUMOylation in plants is key to a plethora of adaptive responses. How this is achieved with an extremely limited set of machinery components is still unclear. One possibility is that novel SUMO components are yet to be discovered. However, current knowledge indicates that only a small set of enzymes seem to be responsible for the modification of a large number of SUMO substrates. It is yet unknown where the specificity lies within the SUMO system. Although this seems to be a crucial question in the field of SUMOylation studies, not much is known about the factors that provide specificity. In this review, we highlight the role of the localisation of SUMO components as an important factor that can play a vital role in contributing to the specificity within the process. This will introduce a new facet to our understanding of the mechanisms underlying such a dynamic process.

Knockdown of OsSAE1a affects the growth and development and phosphate homeostasis in rice

SUMOylation is a post-translational modification process that comprises a tandem enzymatic cascade, i.e., maturation, activation, conjugation, and ligation of a small ubiquitin-like modifier, which triggers the modulated activities and transport of the cellular proteins to other areas of the cell. In Oryza sativa (rice), OsSIZ1/2 encoding E3 SUMO ligase exerts regulatory influences on Pi homeostasis and developmental responses. However, the role of OsSAE1a, SUMO E1 activating enzyme, in regulating phosphate (Pi) utilization and/or growth and development is not known in rice and was thus investigated in this study. The qRT-PCR assay revealed a constitutive and variable spatiotemporal expression pattern of OsSAE1a in the vegetative and reproductive tissues and was comparable in the root and shoot grown under different Pi regimes. RNAi-mediated suppression of OsSAE1a exerted variable effects on the concentrations of Pi and total P in different tissues, uptake and distribution of ³²Pi, and relative expression levels of several genes that play pivotal roles in the maintenance of Pi homeostasis. The effects of the mutation in OsSAE1a were also evident in the vegetative and reproductive traits of rice during growth in a hydroponic system and pot soil, respectively. Overall, these results suggest a broad-spectrum role of OsSAE1a in the maintenance of Pi homeostasis and regulating growth and development.

SIZ1-Mediated SUMO Modification of SEUSS Regulates Photomorphogenesis in Arabidopsis

Small ubiquitin-like modifier (SUMO) post-translational modification (SUMOylation) plays essential roles in regulating various biological processes; however, its function and regulation in the plant light signaling pathway are largely unknown. SEUSS (SEU) is a transcriptional co-regulator that integrates light and temperature signaling pathways, thereby regulating plant growth and development in Arabidopsis thaliana. Here, we show that SEU is a substrate of SUMO1, and that substitution of four conserved lysine residues disrupts the SUMOylation of SEU, impairs its function in photo- and thermomorphogenesis, and enhances its interaction with PHYTOCHROME-INTERACTING FACTOR 4 transcription factors. Furthermore, the SUMO E3 ligase SIZ1 interacts with SEU and regulates its SUMOylation. Moreover, SEU directly interacts with phytochrome B photoreceptors, and the SUMOylation and stability of SEU are activated by light. Our study reveals a novel post-translational modification mechanism of SEU in which light regulates plant growth and development through SUMOylation-mediated protein stability.

Nitrogen Depletion Blocks Growth Stimulation Driven by the Expression of Nitric Oxide Synthase in Tobacco

Nitric oxide (NO) is a messenger molecule widespread studied in plant physiology. Latter evidence supports the lack of a NO-producing system involving a NO synthase (NOS) activity in higher plants. However, a NOS gene from the unicellular marine alga Ostreococcus tauri (OtNOS) was characterized in recent years. OtNOS is a genuine NOS, with similar spectroscopic fingerprints to mammalian NOSs and high NO producing capacity. We are interested in investigating whether OtNOS activity alters nitrogen metabolism and nitrogen availability, thus improving growth promotion conditions in tobacco. Tobacco plants were transformed with OtNOS under the constitutive CaMV 35S promoter. Transgenic tobacco plants expressing OtNOS accumulated higher NO levels compared to siblings transformed with the empty vector, and displayed accelerated growth in different media containing sufficient nitrogen availability. Under conditions of nitrogen scarcity, the growth promoting effect of the OtNOS expression is diluted in terms of total leaf area, protein content and seed production. It is proposed that OtNOS might possess a plant growth promoting effect through facilitating N remobilization and nitrate assimilation with potential to improve crop plants performance.

SUMO E3 Ligase SIZ1 stabilizes MYB75 to regulate anthocyanin accumulation under high light conditions in Arabidopsis

Sumoylation is one of post-translational modification (PTM) in which SUMO (small ubiquitin-like modifier) are covalently conjugated to protein substrates through a range of biochemical steps. This paper presents evidence that SUMO E3 ligase SIZ1 positively regulates anthocyanin accumulation. Loss-of-function siz1 mutant seedlings exhibit anthocyanin accumulation-reduced phenotype under high light conditions. Moreover, SIZ1 interacts and sumoylates MYB75/PAP1, a key transcription factor in anthocyanin accumulation. Loss-of-function siz1 or K246R substitution in MYB75 blocked SIZ1-mediated sumoylation in vitro and in vivo. Anthocyanin accumulation in mutant myb75-c can not be rescued by expressing MYB75^K246R, but expression of wild-type MYB75^WT complements the mutant phenotype. It suggested that sumoylation is important for MYB75 function. We further prove that sumoylation is essential for MYB75 protein stability. And SIZ1 is involved in the light-induced accumulation of anthocyanins. Our findings reveal an important role for sumoylation of MYB in regulation of anthocyanin accumulation in plants.

Nitrate and hormonal signaling crosstalk for plant growth and development

Nitrate is an essential macronutrient for plants, a primary nitrogen source in natural and human-made ecosystems. Nitrate can also act as a signaling molecule that directs genome-wide gene expression changes with an impact on plant metabolism, physiology, growth and development. Nitrate and phytohormone signaling pathways crosstalk to modulate growth and developmental programs in a multifactorial manner. Nitrate-signaling controls plant growth and development using molecular mechanisms that involve phytohormone-signaling pathways. In contrast, many phytohormones modulate or impact nitrate signaling in interconnected pathways. In this review, we explore recent progress in our understanding of well-documented connections between nitrate and phytohormones such as auxin, cytokinin and abscisic acid. We also discuss recent studies connecting nitrate to other phytohormones such as ethylene, salicylic acid, gibberellins and brassinosteroids. While many molecular details remain to be elucidated, a number of core signaling components at the intersection between nitrate and the major hormonal pathways have been described. We focus on established interactions of nitrate and different hormonal pathways to bring about cellular, growth and developmental processes in Arabidopsis thaliana.

E3 SUMO ligase AtSIZ1 regulates the cruciferin content of Arabidopsis seeds

Arabidopsis thaliana E3 SUMO ligase SIZ1 (AtSIZ1) controls vegetative growth and development, including responses to nutrient deficiency and environmental stresses. Here, we analyzed the effect of AtSIZ1 and its E3 SUMO ligase activity on the amount of seed proteins. Proteomic analysis showed that the level of three major nutrient reservoir proteins, CRUCIFERIN1 (CRU1), CRU2, and CRU3, was reduced in the siz1-2 mutant compared with the wild type. However, quantitative real-time PCR (qRT-PCR) analysis showed that transcript levels of CRU1, CRU2, and CRU3 genes were significantly higher in the siz1-2 mutant than in the wild type. Yeast two-hybrid analysis revealed direct interaction of AtSIZ1 with CRU1, CRU2, and CRU3. The sumoylation assay revealed that CRU2, and CRU3 proteins were modified with a small ubiquitin-related modifier (SUMO) by the E3 SUMO ligase activity of AtSIZ1. Additionally, high-performance liquid chromatography (HPLC) analysis showed that the amino acid content was slightly higher in siz1-2 mutant seeds than in wild type seeds. Taken together, our data indicate that AtSIZ1 plays an important role in the accumulation and stability of seed storage proteins through its E3 ligase activity.

Sumoylation of BRI1-EMS-SUPPRESSOR 1 (BES1) by the SUMO E3 Ligase SIZ1 Negatively Regulates Brassinosteroids Signaling in Arabidopsis thaliana

Brassinosteroids (BRs), a group of plant steroid hormones, participate in the regulation of plant growth and developmental processes. BR functions through the BES1/BZR1 family of transcription factors, however, the regulation of the BES1 activity by post-translational modifications remains largely unknown. Here, we present evidence that the SUMO E3 ligase SIZ1 negatively regulates BR signaling pathway. T-DNA insertion mutant siz1-2 shows BL (Brassinolide, the most active BR) hypersensitivity and BRZ (Brassinazole, a BR biosynthesis inhibitor) insensitivity during hypocotyl elongation. In addition, expression of BES1-dependent BR-response genes is hyper-regulated in siz1-2 seedlings. The siz1-2bes1-D double mutant exhibits longer hypocotyl than bes1-D. Moreover, SIZ1 physically interacts with BES1 in vivo and in vitro and mediates the sumoylation of BES1. A K302R substitution in BES1 blocks its sumoylation mediated by SIZ1 in plants, indicating that K302 is the principal site for SUMO conjugation. Consistently, we find that sumoylation inhibits BES1 protein stability and activity. Taken together, our data show that the sumoylation of BES1 via SIZ1 negatively regulates the BR signaling pathway.

The nitric oxide suppressed Arabidopsis mutants- Atnoa1 and Atnia1nia2noa1-2 produce nitric oxide in MS growth medium and on uranium exposure

The mutants Atnoa1 and Atnia1nia2noa1-2 having a defective chloroplast developmental process, showed enhanced chlorophyll levels when they were grown on Murashige and Skoog (MS) medium and on exposure with uranium (U) on Hoagland medium. Thus we hypothesized that these mutants probably produced NO in MS medium and on exposure with U. Wild-type Col-0, Atnoa1, Atnia1nia2noa1-2 plants were cultured on modified Hoagland and 1/10 MS media and NO generation in the roots of these mutants was monitored using NO selective fluorescent dyes, DAF-2DA and Fl2E. Both Atnoa1 and Atnia1nia2noa1-2 triple mutants produced NO as observed by increases in DAF-2T and Fl2E fluorescence when these mutants were grown on MS medium but not on Hoagland medium. In presence of NO scavenger, methylene blue (MB, 200 μM), DAF-2T and Fl2E fluorescence was completely abolished. On the other hand treatment of the plants with 25 μM U triggered NO generation. U-treated Atnoa1 and Atnia1nia2noa1-2 plants upregulated genes (POR B, POR D, CHL D) involved in the chlorophyll biosynthesis. From these results it was concluded that Atnoa1 and Atnia1nia2noa1-2 are conditional NO producers and it appears that NO generation in plants substantially depends on growth medium and NIA1, NIA2 or NOA1 does not appear to be really involved in NO generation in MS medium or after U exposure.

Exploring the regulatory levels of SUMOylation to increase crop productivity

SUMOylation is an essential post-translational modification that affects several cellular processes, from gene replication to stress response. Studies using the SUMO (de)conjugation machinery have provided evidence regarding its potential to improve crop performance and productivity under normal and adverse conditions. However, the pleiotropic effect of SUMOylation can be a disadvantage in both situations, especially when considering unpredictable environmental conditions caused by climate changes. Here, we discuss the pleiotropic effects caused by disrupting the SUMOylation machinery, and new strategies that may help to overcome pleiotropy. We propose exploring the several regulatory levels of SUMOylation recently revealed, including transcriptional, post-transcriptional regulation by alternative splicing, and post-translational modifications. These new findings may provide valuable tools to increase crop productivity.

Proteolysis and nitrogen: emerging insights

Nitrogen (N) is a core component of fertilizers used in modern agriculture to increase yields and thus to help feed a growing global population. However, this comes at a cost to the environment, through run-off of excess N as a result of poor N-use efficiency (NUE) by crops. An obvious remedy to this problem would therefore be the improvement of NUE, which requires advancing our understanding on N homeostasis, sensing, and uptake. Proteolytic pathways are linked to N homeostasis as they recycle proteins that contain N and carbon; however, emerging data suggest that their functions extend beyond this simple recycling. Here, we highlight roles of proteolytic pathways in non-symbiotic and symbiotic N uptake and in systemic N sensing. We also offer a novel view in which we suggest that proteolytic pathways have roles in N homeostasis that differ from their accepted function in recycling.