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Accumulated evidence suggests that peptides play diverse roles in various aspects of plant growth and development. Therefore, many research groups have developed and utilized new methods for the identification of peptides in different organisms. Nevertheless, the number and diversity of identified peptides remain lower than expected. In addition, the number of identified peptides is underestimated because of their low endogenous concentrations and, consequently, technical difficulties with their isolation. Additionally, while some types of peptides undergo functional maturation via proteolytic processing or post-translational modifications, which are important for their biological activity, most of the enzymes involved in the maturation of peptides have not yet been identified. Together, these factors indicate that the investigation of peptides is in its beginning stages in plants, as in other organisms. This review summarizes the biosynthesis mechanisms, identification tools, post-translational modifications, and biological functions of plant peptides, and the importance of post-translational modifications in peptide activity.

Triterpenoid saponins are major secondary metabolites in soybean; they are classified into group A and DDMP saponins. Group A saponins cause bitterness and an astringent aftertaste, whereas DDMP saponins are less bitter and more beneficial to human health. Therefore, a need exists for genetic improvement to develop bitter-free soybeans without losing other beneficial saponins. In this study, an ethyl methanesulfonate (EMS)–induced mutant, PE1327, was isolated and characterized as having a reduced level of group A saponins by chromatography. The PE1327 phenotype, which exhibits no accumulation of group A saponins, resembles that of a previously reported sg-5 wild soybean mutant whose biosynthesis is controlled by the Sg-5 (Glyma.15G243300) gene. The Sg-5 gene sequence in PE1327 revealed a single-nucleotide polymorphism (G1036A) that causes an amino acid change from Glu346 to Lys346 in the sg-5 protein. Multiple alignment analysis of cytochrome P450 enzymes from numerous organisms revealed that Glu346 is highly conserved in eukaryotes, including plants and animals, but not in prokaryotes. The predicted 3D model of the Sg-5 protein showed that the Glu346 residue is located in the J helix and is likely involved in the linkage between the J and K helices and the stabilization of the J-K loop, suggesting that the amino acid substitution from Glu346 to Lys346 in the PE1327 mutant results in hypofunction of Sg-5. Co-segregation analysis revealed that the Sg-5 locus is tightly linked to reduced group A saponin biosynthesis in PE1327 and that the novel sg-5 variant is recessive to Sg-5.

Background Seed size has been extensively studied in crop plants, as it determines crop yield. However, the mechanism of seed development remains elusive. In this study, we explored the mechanism of seed development in rice (Oryza sativa L.), and identified proteins affecting seed size. Results Proteomic analysis showed that glyceraldehyde 3-phosphate dehydrogenase, chitinase 14 (CHT14), and phosphoglycerate kinase (PGK) accumulated to high levels in the seeds of the natural japonica rice mutant Oochikara, which carries a loss-of-function mutation in the grain width 2 (GW2) gene; GW2 encodes a RING-type E3 ubiquitin ligase. In vitro pull-down and ubiquitination assays showed that CHT14 and PGK directly interacted with GW2 but were not ubiquitinated by GW2. Immunoblot analysis revealed that protein disulfide isomerase-like 1-1 accumulated to high levels in young developing seeds of the gw2 mutant compared with the wild type. Histochemical β-glucuronidase staining showed strong expression of GW2 in leaf and root tissues but weak expression in leaf sheaths and internodes. In addition, transformation of the green fluorescent protein (GFP) gene under the control of the GW2 promoter in rice revealed GFP expression in the aleurone layer of seeds. Conclusions Collectively, these results suggest that GW2 regulates seed size through direct interactions with proteins involved in carbohydrate metabolism by modulating their activity or stability and controlling disulfide bond formation in various proteins during seed development. Additionally, GW2 participates in vegetative as well as reproductive growth, and protects the seed from pathogen attack.

Soybean [Glycine max (L.) Merr.] is a major agricultural crop and generally known as a salt-sensitive crop. In a previous study, GmSALT3 was identified as a salt tolerance gene in soybean, and its nine haplotypes (H1 to H9) were reported in Chinese soybean accessions. In the present study, we aimed to identify new haplotypes of GmSALT3 in soybean accessions and to develop molecular markers for selection of salt-tolerant and -sensitive accessions. To do so, we examined genomic variations in the GmSALT3 coding region of 216 accessions of G. max and G. soja from Korea, China, and Japan. As a result, 40 different haplotypes, including three known haplotypes (H1, H2, and H5), were identified. Performing salt tolerance tests for the haplotypes, we were able to classify them into salt-tolerant (8) and salt-sensitive (32) categories. We also found that more variations in the haplotype composition of G. soja accessions exist than in that of G. max accessions. Quantitative expression analysis showed that almost all of the salt-tolerant haplotypes had much higher levels of GmSALT3 expression than the salt-sensitive haplotypes did. Finally, we developed molecular markers and applied them to screen salt tolerance of soybean accessions. The molecular markers performed well with an accuracy of 98.8% in identifying phenotypes of soybean accessions.

Flower color is one of the key traits, which has been widely considered for genetic studies on soybean. A variety of flower colors, such as dark purple, purple, purple blue, purple throat, magenta, pink, near white, and white, has been identified in cultivated soybean (Glycine max). Out of the 19,649 soybean accessions deposited in the United States Department of Agriculture-Germplasm Resources Information Network database, 67 % have purple flowers, 32 % have white flowers, and merely 1 % have flowers with different colors. In contrast, almost all accessions of wild soybean (Glycine soja) have only purple flowers. Flavonoids, mainly anthocyanins, are the most common pigments contributing to flower coloration in soybean. In the recent decades, the flavonoid biosynthesis pathway for anthocyanins has been well established, and some of the genes controlling flower color in soybean have been identified and characterized. Flower pigmentation of soybean is mainly controlled by six independent loci (W1, W2, W3, W4, Wm, and Wp) along with the combination of various other factors such as anthocyanin structure, vacuolar pH, and co-pigments. In this review, we summarize the current status of genetic and molecular regulation of flower pigmentation in cultivated and wild varieties of soybean.

Zinc finger proteins compose one of the largest transcription factor families in plants, but only a handful have been functionally specified in plant growth. In this study, a semidominant mutant, Arabidopsis thaliana dwarf1 (Atdwa1), was discovered from activation tagging screening using the SKI015 TDNA vector. The Atdwa1 mutant displayed severe dwarfism and loss of apical dominance, as well as other pleiotropic defects, such as earlier flowering, fewer leaves, and shorter sliliques than those of the wild-type plant. Thermal asymmetric interlaced-polymerase chain reaction analysis revealed that activated T-DNA was inserted into the At5g54330 gene, disrupting its coding sequence. We also demonstrated that At5g54330 expression was nullified by the insertion, whereas expression of its downstream genes, At5g54340, At5g54350, and At5g54360, were activated. A database search revealed that all three downstream genes encode proteins related to C2H2-type zinc finger proteins. Overexpression of each gene resulted in dwarfism similar to that of Atdwa1. These findings indicate that Atdwa1 mutant was caused by synergistic manifestation of concomitant activation of At5g54340 to At5g54360, and that the zinc finger proteins may play a role in regulation of plant growth and development.

Rice leaf blight, which is caused by the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo), results in huge losses in grain yield. Here, we show that Xoo-induced rice leaf blight is effectively controlled by niclosamide, an oral antihelminthic drug and molluscicide, which also functions as an anti-tumor agent. Niclosamide directly inhibited the growth of the three Xoo strains PXO99, 10208 and K3a. Niclosamide moved long distances from the site of local application to distant rice tissues. Niclosamide also increased the levels of salicylate and induced the expression of defense-related genes such as OsPR1 and OsWRKY45, which suppressed Xoo-induced leaf wilting. Niclosamide had no detrimental effects on vegetative/reproductive growth and yield. These combined results indicate that niclosamide can be used to block bacterial leaf blight in rice with no negative side effects.

Expression patterns of AtBSMT1 encoding an S-adenosyl-L-methionine: salicylic acid carboxyl methyltransferase from Arabidopsis thaliana were analyzed in AtBSMT1 promoter::β-glucuronidase (GUS) transgenic Arabidopsis. AtBSMT1 was expressed in flowers and siliques and induced under specific biotic/abiotic stress conditions. The results indicated that the induction of AtBSMT1 is regulated by plant disease response and jasmonic acid signaling.

We have previously shown that overexpression of BrERF4 (BrassicarapaETHYLENE-RESPONSIVEFACTOR4) increases salt and drought tolerance in Arabidopsis plants, and also retarded organ growth. In the present study, we investigated in detail the leaf growth retardation phenotype at the cellular level. Results showed that BrERF4-overexpressing Arabidopsis plants developed small leaves by reducing their cell size but not the cell number. Detailed kinematic analysis revealed that changes in cell size appeared from the very early stages of leaf development, directly affecting the size of leaf organs. RT-PCR analysis showed that expression of expansin genes was reduced in the overexpressors, whereas expression of the cell cycle gene, CYCB1;1, was not altered at all. In addition, overexpression of AtMYB44, another stress-related transcription factor gene, reduced leaf growth, which also resulted from reduction in cell size but not in cell number. These results suggest that overexpression of those transcription factors negatively affects cell expansion during leaf growth without altering cell number. We discuss about the advantages that the BrERF4- or AtMYB44-induced cell expansion retardation confers on plants under natural environmental adversity.

We here show that OsPrMC3 affects seed yields by regulating tillering. OsPrMC3 is highly expressed in leaves and mature seeds, although its expression is detected in all tissues, and its mutant osprmc3 has more tillers and less grain, indicating its crucial role in determining grain yield as a tillering inhibitor.

The development of soybean varieties that lack the β-conglycinin α′ subunit is an attractive goal because the β-conglycinin α′ subunit exerts a negative influence on nutrition and tofu gelation, and is also a major allergen. We sought to develop a co-dominant DNA marker for the β-conglycinin α′ subunit (Cgy-1) gene for use in marker-assisted selection (MAS). We identified a deleted sequence responsible for the null allele of Cgy-1 in a soybean variety lacking the β-conglycinin α′ subunit known as ‘Keburi’. The deletion spanned 12,998 bp and included Cgy-1 and its incomplete tandem duplicate on chromosome 10. Based on the Cgy-1 sequence from both the Williams 82 soybean reference sequence and the Keburi variety, a set of three allele-specific primers was designed for multiplex PCR assay. These primers enabled allelic discrimination by the sizes of PCR products. This amplified two distinct DNA fragments of 913 and 649 bp in Williams 82 and Keburi, respectively. The practicality of the developed co-dominant marker for Cgy-1 was also confirmed by amplification in five other soybean varieties including three wild types and two mutants. The heterozygosity of the F1 plants at the Cgy-1 locus was ascertained using our novel co-dominant marker. This PCR-based co-dominant marker is capable of detecting the presence or absence of β-conglycinin α′ subunit for soybean marker assisted breeding system.

The composition and distribution of seed storage proteins are important factors for eating quality such as flavor and texture in rice. The levels of rice protein disulfide isomerase and rice chaperone binding protein proteins were examined in rice cultivars which have different eating qualities. Results showed that their levels have no direct correlation with eating quality.

The Arabidopsis resistance (R) RPS2 gene confers resistance to Pseudomonas syringae (P. syringae) strains that express the avirulence (avr) avrRpt2 gene [Kunkel et al., 1993]. When Arabidopsis plants containing RPS2 are challenged with P. syringae bacteria carrying avrRpt2, the plants activate disease resistance responses, including a specialized type of programmed cell death known as the hypersensitive response (HR) [Kunkel et al., 1993; Hammond-Kosack and Jones, 1997]. The interaction between RPS2 and avrRpt2 products leads to an excellent model for pathogen-induced resistance responses and signal transduction events. As a typical resistance protein of the nucleotide-binding site-leucine-rich repeat (NBS-LRR) class, the RPS2 protein, consisting of 909 amino acids, contains several possible functional peptide motifs, including an Nterminal hydrophobic region, a putative leucine zipper (LZ), a NBS and a LRR (Fig. 1A) [Hammond-Kosack and Jones, 1997]. Additional motifs found in NBS-LRR resistance genes, such as GLPL and KMH motifs, are present in the RPS2 sequence [Traut, 1994]. The LZ and LRR domains have been implicated in regards to proteinprotein interactions (Alber, 1992; Kobe and Deisenhofer, 1994). The conserved central NBS domain is hypothesized to bind either to ATP or GTP involving kinase activities, containing 3 motifs: the P loop (or kninase-1a), kinase-2 and less conserved kinase-3a [Traut, 1994]. These structural features regarding the RPS2 protein suggest that it is involved in signal transduction processes for disease resistance responses. For studies of the molecular function of RPS2 and signaling by RPS2-avrRpt2 interaction, about 20 mutants including rps2-101C have so far been isolated [Kunkel et al., 1993; Mindrinos et al., 1994; Axtell et al., 2001]. In order to study on the spatial structure and function of RPS2, we tried to isolate numerous additional mutants, defective in RPS2-avrRpt2 dependent cell death, by using Axtell et al. [2001]’s method with minor modifications. Transgenic Col-0 Arabidopsis seeds, possessing a dexamethasone-inducible avrRpt2 transgene (pTA7001avrRpt2) [McNellis et al., 1998], were obtained from Dr. Brian J. Staskawicz (University of California, Berkeley, CA), and then mutagenized with 0.1% ethyl methane sulfonate (Sigma, St. Louis, MO). M2 plants based on the self-fertilization of M1 seeds were grown on soil. When dexamethasone (30 μM) was sprayed onto 2-week-old M2 plants, almost were yellowish in color, had aborted growth rates and eventually died a few days later (Fig. 1B). To identify the presence of the avrRpt2 transgene in the surviving plants, genomic DNAs were isolated from leaves, then the polymerase chain reaction (PCR) was performed by using two specific primers (data not shown). The upstream and downstream sequences were 5'-ATGAAAATTGCTCCAGTTGCCAT-3' and 5'-TTAG CGGTAGAGCATTGCGTGT-3', respectively. Twenty-six mutants out of the surviving plants were screened by PCR. Some mutations were expected to eliminate expression of the avrRpt2 transgene due to mutations in the dexamethasone-inducible expression system (pTA7001 expression cassette). To test this possibility, we performed a Northern blot analysis (Fig. 1C). Total RNA was isolated from leaves taken 1 day after application of dexamethasone. The accumulation of avrRpt2 transcripts was detected in 7 out of 26 mutants as well as rps2-101C as a positive control line. To examine whether 7 mutants showed the ability to mount an HR in response to avrRpt2, we performed a secondary screening procedure by inoculating mutant leaves with the P. syringae pv. maculicola strain DG6 (carrying avrRpt2) at OD600=0.1 [Guttman and Greenberg, 2001]. The typical resistance response to this interaction *Corresponding author Phone: +82-53-950-7753; Fax: +82-53-958-6880 E-mail: jtsong68@knu.ac.kr

It was known that long after far-red light 1 (LAF1), a positive phytochrome A (phyA) signal transducer, is ubiquitinated by constitutive photomorphogenic 1 (COP1) in vitro and stabilizes long hypocotyl in far-red 1 (HFR1), another phyA signal transducer, through direct interaction. Our results imply that LAF1 stabilizes phyA by inhibiting or blocking COP1 activity, or by directly interacting with phyA, resulting in the stimulation of phyA signaling.

Tillering in rice (Oryza sativa L.) is an important agronomic trait that enhances grain production. A tiller is a specialized grain-bearing branch that is formed on a non-elongated basal internode that grows independently of the mother stem. Transgenic rice over-expressing the transcription factorOsTB1, a homologue of maizeTB1 (Teosinte Branched 1), exhibits markedly reduced lateral branching without the propagation of axillary buds being affected. However, the tillering mechanism remains unknown. Therefore, to further understand that mechanism, we applied proteomics methodology to isolate the proteins involved. Using two-dimensional gel electrophoresis and mass spectrometry, our analysis of the basal nodes from two rice cultivars that differ in their numbers of tillers showed that a rice serine proteinase inhibitor, OsSerpin, accumulates in great amounts in high-tillering ‘Hwachung’ rice. Northern blot analysis revealed that much moreOsSerpin transcript is found in ‘Hwachung’ than in relatively low-tillering ‘Hanmaeum’, likely because of high levels of transcription. Therefore, our data suggest that OsSerpin content determines the extent of lateral branching.

We cloned a salicylic acid/benzoic acid carboxyl methyltransferase gene, OsBSMT1, from Oryza sativa. A recombinant OsBSMT1 protein obtained by expressing the gene in Escherichia coli exhibited carboxyl methyltransferase activity in reactions with salicylic acid (SA), benzoic acid (BA), and de-S-methyl benzo(1,2,3)thiadiazole-7-carbothioic acid (dSM-BTH), producing methyl salicylate (MeSA), methyl benzoate (MeBA), and methyl dSM-BTH (MeBTH), respectively. Compared to wild-type plants, transgenic Arabidopsis overexpressing OsBSMT1 accumulated considerably higher levels of MeSA and MeBA, some of which were vaporized into the environment. Upon infection with the bacterial pathogen Pseudomonas syringae or the fungal pathogen Golovinomyces orontii, transgenic plants failed to accumulate SA and its glucoside (SAG), becoming more susceptible to disease than wild-type plants. OsBSMT1-overexpressing Arabidopsis showed little induction of PR-1 when treated with SA or G. orontii. Notably, incubation with the transgenic plant was sufficient to trigger PR-1 induction in neighboring wild-type plants. Together, our results indicate that in the absence of SA, MeSA alone cannot induce a defense response, yet it serves as an airborne signal for plant-to-plant communication. We also found that jasmonic acid (JA) induced AtBSMT1, which may contribute to an antagonistic effect on SA signaling pathways by depleting the SA pool in plants.

The identification of molecular markers and marker-aided selection are essential to the efficient breeding of drought-tolerant plants. However, because that characteristic is controlled by many quantitative trait loci, such markers that can screen and trace desirable barley genotypes in a segregating population or germplasm have not yet been determined. Relative water content has been used to estimate drought tolerance in plants because it is highly correlated with the drought index of yield. To develop reliable gene-specific markers for identifying tolerant versus susceptible genotypes, we performed suppression subtractive hybridization to identify candidate genes. We used two domestic barley cultivars, one having the highest RWC (drought-tolerant ‘Chalbori’) and the other having the lowest (drought-susceptible ‘Daebaekbori’). In response to dehydration at the early seedling stage, rapid upregulation ofDehydrin3 (Dhn3) andDhn4 occurred in the drought-tolerant genotypes, but not in the susceptible ones. Similar results were obtained with mature plants growing under frequent drought stress in the greenhouse. In addition,Dhn3 andDhn4 conferred higher drought tolerance when they were over-expressed in transgenicArabidopsis. Thus, in addition to using assessments of RWC, we propose thatDhn3 andDhn4 expressions can serve as drought-induced gene-specific markers to determine drought-tolerant barley genotypes at the seedling stage.

Photosynthetic organisms exhibit a green color due to the accumulation of chlorophyll pigments in chloroplasts. Mg-protoporphyrin IX chelatase (Mg-chelatase) comprises three subunits (ChlH, ChlD and ChlI) and catalyzes the insertion of Mg(2+) into protoporphyrin IX, the last common intermediate precursor in both chlorophyll and heme biosyntheses, to produce Mg-protoporphyrin IX (MgProto). Chlorophyll deficiency in higher plants results in chlorina (yellowish-green) phenotype. To date, 10 chlorina (chl) mutants have been isolated in rice, but the corresponding genes have not yet been identified. Rice Chl1 and Chl9 genes were mapped to chromosome 3 and isolated by map-based cloning. A missense mutation occurred in a highly conserved amino acid of ChlD in the chl1 mutant and ChlI in the chl9 mutant. Ultrastructural analyses have revealed that the grana are poorly stacked, resulting in the underdevelopment of chloroplasts. In the seedlings fed with aminolevulinate-dipyridyl in darkness, MgProto levels in the chl1 and chl9 mutants decreased up to 25% and 31% of that in wild-type, respectively, indicating that the Mg-chelatase activity is significantly reduced, causing the eventual decrease in chlorophyll synthesis. Furthermore, Northern blot analysis indicated that the nuclear genes encoding the three subunits of Mg-chelatase and LhcpII in chl1 mutant are expressed about 2-fold higher than those in WT, but are not altered in the chl9 mutant. This result indicates that the ChlD subunit participates in negative feedback regulation of plastid-to-nucleus in the expression of nuclear genes encoding chloroplast proteins, but not the ChlI subunit.

Plant nitrite reductase (NiR) catalyzes the reduction of nitrite to ammonium. We examined if NiR activity is regulated by sumoylation, and found NiR was not sumoylated by E3 small ubiquitin-related modifier ligase AtSIZ1. However, its transcript level decreased in the siz1-2 mutant, indicating that AtSIZ1 does not directly control NiR activity.

Far-red light regulates many aspects of seedling development, such as inhibition of hypocotyl elongation and the promotion of greening, acting in part through phytochrome A (phyA). The RING motif protein COP1 is also important because cop1 mutants exhibit constitutive photomorphogenesis in darkness. COP1 is present in the nucleus in darkness but is gradually relocated to the cytoplasm upon illumination. Here we show that COP1 functions as an E3 ligase ubiquitinating both itself and the myb transcription activator LAF1, which is required for complete phyA responses. In transgenic plants, inducible COP1 overexpression leads to a decrease in LAF1 concentrations, but is blocked by the proteasome inhibitor MG132. The coiled-coil domain of SPA1, a negative regulator of phyA signalling, has no effect on COP1 auto-ubiquitination but facilitates LAF1 ubiquitination at low COP1 concentrations. These results indicate that, in darkness, COP1 functions as a repressor of photomorphogenesis by promoting the ubiquitin-mediated proteolysis of a subset of positive regulators, including LAF1. After the activation of phyA, SPA1 stimulates the E3 activity of residual nuclear COP1 to ubiquitinate LAF1, thereby desensitizing phyA signals.

We have characterized a gene specifically expressed in the floral nectaries of Brassica campestris L. ssp. pekinensis. Differential screening led to the isolation of a floral nectary-specific cDNA clone. Northern hybridization indicated that its mRNA transcript is 1450 nucleotides long and specific to the flower base. In situ hybridization and immunolocalization showed that its mRNA and protein are localized specifically to both the lateral and median nectaries of flowers. The cDNA codes for a 43.8 kDa polypeptide 392 amino acids long. The protein was named nectarin1 (NTR1) after floral nectary protein. NTR1 was located in the cytoplasm of nectariferous cells in the nectaries and was also observed in nuclei at a much lower level. The level of the transcript increases with flower development, especially during nectary development, but decreases abruptly with the opening of the flower. Genomic Southern blot analysis indicated that at least three copies of homologous genes were present in the genome of B. campestris, but that only a single copy was present in both Arabidopsis thaliana and Lycopersicon esculentum. The deduced amino acid sequence of NTR1 shows similarity to S-adenosyl-L-methionine:salicylic acid carboxyl methyltransferase of Clarkia breweri which is expressed mostly in petals. The function of the gene is speculated to be involved in the methylation of a plant secondary metabolite in the floral nectaries.

A cDNA clone, RGA1, was isolated by using a GPA1 cDNA clone of Arabidopsis thaliana G protein alpha subunit as a probe from a rice (Oryza sativa L. IR-36) seedling cDNA library from roots and leaves. Sequence analysis of genomic clone reveals that the RGA1 gene has 14 exons and 13 introns, and encodes a polypeptide of 380 amino acid residues with a calculated molecular weight of 44.5 kDa. The encoded protein exhibits a considerable degree of amino acid sequence similarity to all the other known G protein alpha subunits. A putative TATA sequence (ATATGA), a potential CAAT box sequence (AGCAATAC), and a cis-acting element, CCACGTGG (ABRE), known to be involved in ABA induction are found in the promoter region. The RGA1 protein contains all the consensus regions of G protein alpha subunits except the cysteine residue near the C-terminus for ADP-ribosylation by pertussis toxin. The RGA1 polypeptide expressed in Escherichia coli was, however, ADP-ribosylated by 10 microM [adenylate-32P] NAD and activated cholera toxin. Southern analysis indicates that there are no other genes similar to the RGA1 gene in the rice genome. Northern analysis reveals that the RGA1 mRNA is 1.85 kb long and expressed in vegetative tissues, including leaves and roots, and that its expression is regulated by light.

Small ubiquitin-related modifier (SUMO) is a small polypeptide that modulates protein activity and regulates hormone signalling, abiotic and biotic responses in plants. Here we show that AtSIZ regulates nitrogen assimilation in Arabidopsis through its E3 SUMO ligase function. Dwarf plants of siz1-2 flower early, show abnormal seed development and have high salicylic acid content and enhanced resistance to bacterial pathogens. These mutant phenotypes are reverted to wild-type phenotypes by exogenous ammonium but not by nitrate, phosphate or potassium. Decreased nitrate reductase activity in siz1-2 plants resulted in low nitrogen concentrations, low nitric oxide production and high nitrate content in comparison with wild-type plants. The nitrate reductases, NIA1 and NIA2, are sumoylated by AtSIZ1, which dramatically increases their activity. Both sumoylated and non-sumoylated NIA1 and NIA2 can form dimers. Our results indicate that AtSIZ1 positively controls nitrogen assimilation by promoting sumoylation of NRs in Arabidopsis., Posttranslational modification of proteins by small ubiquitin-related modifier is a response to stress signalling in plants. Here, the Arabdiposis protein SIZ1 is shown to cause SUMOylation of nitrate reductases 1 and 2 and to increase their activity, suggesting that SIZ1 controls nitrate uptake via SUMOylation.