Moon-Soo Chung

A putative novel transcription factor, AtSKIP, is involved in abscisic acid signalling and confers salt and osmotic tolerance in Arabidopsis

We identified and functionally characterized the AtSKIP gene (At1g77180), an Arabidopsis homologue of SNW/SKIP, under abiotic stresses. Although the SNW/SKIP protein has been implicated as a critical transcription cofactor, its biological functions have yet to be reported in any plant. Recently, we have isolated Salt-tolerance genes (SATs) via the overexpression screening of yeast with a maize cDNA library. One of the selected genes (SAT2) appeared to confer elevated tolerance to salt. Maize SAT2 cDNA encodes a homologue of the human SNW/SKIP transcriptional coregulator. Treatment with salt, mannitol and abscisic acid induced AtSKIP expression. Ectopic expression of the AtSKIP gene modulated the induction of salt tolerance, dehydration resistance and insensitivity towards abscisic acid under stress conditions. By contrast, atskip antisense lines displayed reduced tolerance to abiotic stresses during germination. Moreover, a decrease in AtSKIP expression resulted in an abnormal phenotype. We further determined that the AtSKIP protein activated the transcription of a reporter gene in yeast. Green fluorescent protein-tagged AtSKIP was localized in the nuclei of both onion cells and transgenic Arabidopsis cells. Taken together, these results suggest that AtSKIP functions as both a positive regulator and putative potential transcription factor in the abiotic stress signalling pathway.

The R-R-Type MYB-Like Transcription Factor, AtMYBL, is Involved in Promoting Leaf Senescence and Modulates an Abiotic Stress Response in Arabidopsis

Functional analysis of a putative novel transcription factor Arabidopsis MYB-like protein designated AtMYBL, which contains two predicted DNA-binding domains, was performed. The physiological role of the R-R-type MYB-like transcription factor has not been reported in any plant. Analyses of an AtMYBL promoter-β-glucuronidase (GUS) construct revealed substantial gene expression in old leaves and induction of GUS activity by ABA and salt stress. AtMYBL-overexpressing plants displayed a markedly enhanced leaf senescence phenotype. Moreover, the ectopic expression of the AtMYBL gene was very significantly influential in senescence parameters including Chl content, membrane ion leakage and the expression of senescence-related genes. Although the seed germination rate was improved under ABA and saline stress conditions in the AtMYBL-overexpressing plants, decreased salt tolerance was evident compared with the wild type and atmybl RNA interference lines during later seedling growth when exposed to long-term salt stress, indicating that AtMYBL protein is able to developmentally regulate stress sensitivity. Furthermore, AtMYBL protein activated the transcription of a reporter gene in yeast. Green fluorescent protein-tagged AtMYBL was localized in the nuclei of transgenic Arabidopsis cells. Taken together, these results suggest that AtMYBL functions in the leaf senescence process, with the abiotic stress response implicated as a putative potential transcription factor.

Physiological characterization of the Arabidopsis thaliana Oxidation-related Zinc Finger 1, a plasma membrane protein involved in oxidative stress

The CCCH-type zinc finger proteins are a superfamily containing tandem zinc-binding motifs involved in many aspects of plant growth and development. However, the precise role of these proteins involved in plant stress tolerance is poorly understood. This study was to examine the regulatory and functional role of the CCCH-type zinc finger protein, AtOZF1 (At2g19810), under oxidative stress. Interestingly, the AtOZF1 protein was localized in the plasma membrane. The AtOZF1 transcripts were highly induced by treatment with hydrogen peroxide, abscisic acid and salinity. The AtOZF1-overexpressing plants were relatively resistant to oxidative stress than wild-type and T-DNA insertion mutant atozf1. Malondialdehyde, a decomposition product of lipid peroxidation, accumulated in atozf1 mutants more than in wild-type and AtOZF1-overexpressing plants. Furthermore, atozf1 mutants displayed lower activities of catalase and guaiacol peroxidase, higher chlorosis, and down-regulated expression of antioxidant genes under oxidative stress. Taken together, these observations demonstrate that AtOZF1 is required for the tolerance of Arabidopsis to oxidative stress.

Isolation and functional characterization of the Arabidopsis salt-tolerance 32 (AtSAT32) gene associated with salt tolerance and ABA signaling

Recently, we have isolated salt-tolerance genes (SATs) on the basis of the overexpression screening of yeast with a maize cDNA library from kernels. One of the selected genes [salt-tolerance 32 (SAT32)] appears to be a key determinant for salt stress tolerance in yeast cells. Maize SAT32 cDNA encodes for a 49-kDa protein, which is 41% identity with the Arabidopsis salt-tolerance 32 (AtSAT32) unknown gene. Arabidopsis Transfer-DNA (T-DNA) knockout AtSAT32 (atsat32) altered root elongation, including reduced silique length and reduced seed number. In an effort to further assess salinity tolerance in Arabidopsis, we have functionally characterized the AtSAT32 gene and determined that salinity and the plant hormone ABA induced the expression of AtSAT32. The atsat32 mutant was more sensitive to salinity than the wild-type plant. On the contrary, Arabidopsis overexpressing AtSAT32 (35S::AtSAT32) showed enhanced salt tolerance and increased activity of vacuolar H(+)-pyrophosphatase (V-PPase, EC 3.6.1.1) under high-salt conditions. Consistent with these observations, 35S::AtSAT32 plants exhibited increased expression of salt-responsive and ABA-responsive genes, including the Rd29A, Erd15, Rd29B, Rd22 and RAB18 genes. Therefore, our results indicate that AtSAT32 is involved in both salinity tolerance and ABA signaling as a positive regulator in Arabidopsis.

The atrzf1 mutation of the novel RING-type E3 ubiquitin ligase increases proline contents and enhances drought tolerance in Arabidopsis

The covalent attachment of ubiquitin to proteins plays a fundamental role in the regulation of cellular function through biological events involving abiotic or biotic stress responses, immune responses, and apoptosis. Here, we characterize the biological function of the Arabidopsis thaliana RING Zinc Finger 1 (AtRZF1) in dehydration response. AtRZF1 was significantly reduced by drought stress. The atrzf1 mutant was less sensitive to osmotic stress than the wild-type during early seedling development, whereas transgenic plants overexpressing AtRZF1 were hypersensitive, indicating that AtRZF1 negatively regulates drought-mediated control of early seedling development. Moreover, the ectopic expression of the AtRZF1 gene was very significantly influential in drought sensitive parameters including proline content, water loss, membrane ion leakage and the expression of dehydration stress-related genes. AtRZF1 is a functional E3 ubiquitin ligase, and its conserved C3H2C3-type RING domain is likely important for the biological function of AtRZF1 in drought response. Together, these results suggest that the E3 ligase AtRZF1 is an important regulator of water deficit stress during early seedling development.

Direct suppression of a rice bacterial blight (Xanthomonas oryzae pv. oryzae) by monoterpene (S)-limonene.

Rice bacterial blight, caused by Xanthomonas oryzae pv. oryzae (Xoo), is a severe disease of rice plants. Upon pathogen infection, rice biosynthesizes phytoalexins, including diterpenoids such as momilactones, phytocassanes, and oryzalexins. However, information on headspace volatiles in response to Xoo infection is limited. We have examined headspace volatile terpenes, induced by the infection of Xoo, and investigated their biological roles in the rice plant. Monoterpenes α-thujene, α-pinene, sabinene, myrcene, α-terpene, and (S)-limonene and sesquiterpenes cyclosativene, α-copaene, and β-elemene were detected from 1-week-old Xoo-infected rice seedlings, by solid-phase microextraction–gas chromatography–mass spectrometry. All monoterpenes were constitutively released from rice seedlings before Xoo infection. However, (S)-limonene emission was further elicited after exposure of the seedlings to Xoo in coincidence with upregulation of limonene synthase gene (OsTPS20) transcripts. Only the stereospecific (S)-limonene [and not (R)-limonene or other monoterpenes] severely inhibited Xoo growth, as confirmed by disc diffusion and liquid culture assays. Rice seedlings showed suppressed pathogenic symptoms suggestive of resistance to Xoo infection after foliar treatment with (S)-limonene. Collectively, our findings suggest that (S)-limonene is a volatile phytoanticipin, which plays a significant role in suppressing Xoo growth in rice seedlings.

Plasma neutrophil gelatinase‐associated lipocalin levels are positively associated with diabetic retinopathy in patients with Type 2 diabetes

To assess the relationship between plasma neutrophil gelatinase‐associated lipocalin (NGAL) levels and diabetic retinopathy in patients with Type 2 diabetes.

Functional switching of ascorbate peroxidase 2 of rice (OsAPX2) between peroxidase and molecular chaperone

Ascorbate peroxidase (APX) is a class I haem-containing peroxidase, which catalyses the conversion of H2O2 to H2O and O2 using ascorbate as the specific electron donor. APX plays a central role in the elimination of intracellular reactive oxygen species (ROS) and protects plants from the oxidative damage that can occur as a result of biotic and abiotic stresses. At present, the only known function of APX is as a peroxidase. However, in this study, we demonstrate that Oryza sativa APX2 also operates as a molecular chaperone in rice. The different functions of OsAPX2 correlate strongly with its structural conformation. The high-molecular-weight (HMW) complexes had chaperone activity, whereas the low-molecular-weight (LMW) forms displayed predominantly APX activity. The APX activity was effectively inhibited by sodium azide, which is an inhibitor of haem-containing enzymes, but this did not affect the protein’s activity as a chaperone. Additionally, the OsAPX2 conformational changes could be regulated by salt and heat stresses and these stimulated OsAPX2 dissociation and association, respectively. Our results provide new insight into the roles of APXs.

Loss of Ribosomal Protein L24A (RPL24A) suppresses proline accumulation of Arabidopsis thaliana ring zinc finger 1 (atrzf1) mutant in response to osmotic stress

Proline (Pro) metabolism in plants is involved in various cellular processes mediated during abiotic stress. However, the Pro-regulatory mechanisms are unclear. We used a suppressor mutation technique to isolate novel genes involved in the regulation of Pro metabolism in Arabidopsis. Using atrzf1 as a parental plant for T-DNA tagging mutagenesis, we identified a suppressor mutant, termed proline content alterative 21 (pca21), that displayed reduced Pro contents compared with the atrzf1 under osmotic stress conditions. Genomic Thermal Asymmetric Interlaced (TAIL)-PCR revealed pca21 harbored an inserted T-DNA in the region of At2g36620 that encodes Ribosomal Protein L24A. In general, the pca21 mutant partially suppressed the insensitivity of atrzf1 to osmotic stress and abscisic acid during seed germination and early seedling stage. Additionally, the pca21 mutant had increased MDA content and lower expression of several Pro biosynthesis-related genes than the atrzf1 mutant during drought condition. These results suggest that pca21 acts as partial suppressor of atrzf1 in the osmotic stress response through the Pro-mediated pathway.

Physiological Characterization of an AtPGR from Arabidopsis Involved in Pathogen Resistance

The AtPGR gene is induced by pathogen infection, jasmonic acid and salicylic acid treatment and may therefore play a role in plant defense responses. Arabidopsis thaliana Plasma membrane Glucose-responsive Regulator (AtPGR) was previously isolated from Arabidopsis, which confers glucose insensitivity on plants. To study its biological functions directly, we have characterized both loss-of-function RNAi mutant and gain-of-function transgenic overexpression plants for AtPGR in Arabidopsis. The AtPGR-overexpressing plants displayed enhanced resistance to a virulent strain of the bacterial pathogen Pseudomonas syringae as measured by a significant decrease in both bacterial growth and symptom development as compared to those in wild-type and RNAi plants. The enhanced resistance in the gain-of-function transgenic plants was associated with increased induction of SA-regulated PDF1.2 and JA-regulated PR1 by the bacterial pathogen. Thus, pathogen-induced AtPGR plays a positive role in defense responses to P. syringae.