Integrin-like Protein Is Involved in the Osmotic Stress-induced Abscisic Acid Biosynthesis in Arabidopsis thaliana

We studied the perception of plant cells to osmotic stress that leads to the accumulation of abscisic acid （ABA） in stressed Arabidopsis thaliana L. cells. A significant difference was found between protoplasts and cells in terms of their responses to osmotic stress and ABA biosynthesis, implying that cell wall and/or cell wall-plasma membrane interaction are essential in identifying osmotic stress. Western blotting and immunofluorescence localization experiments, using polyclonal antibody against human integrin β1, revealed the existence of a protein similar to the integrin protein of animals in the suspension-cultured cells located in the plasma membrane fraction. Treatment with a synthetic pentapeptide, Gly-Arg-Gly-Asp-Ser （GRGDS）, which contains an RGD domain and interacts specifically with integrin protein and thus blocks the cell wall-plasma membrane interaction, significantly inhibited osmotic stress-induced ABA biosynthesis in cells, but not in protoplasts. These results demonstrate that cell wall and/or cell wall-plasma membrane interaction mediated by integrin-Iike proteins played important roles in osmotic stress-induced ABA biosynthesis in Arabidopsis thaliana.     

在ipt-GUS转基因拟南芥中双组分信号传导基因应答体内细胞分裂素的增加

ipt—GUS转录融合基因在拟南芥植物中表达,其体内细胞分裂素的含量可达到野生型的20-30倍。从拟南芥种子萌发后的6、12、20和30d四个时间分析了植物体内细胞分裂素含量的提高对其双组分信号传导系统中基因的影响。研究发现：细胞分裂素受体基因CRE1比CKI1基因更容易被增加的植物细胞分裂素诱导表达。拟南芥植物细胞分裂素反应调节基因ARR4和ARR5在植物发育的不同时期应答植物体内增加的植物细胞分裂素,ARR4的应答反应比ARR5早,种子萌发后的第6天幼苗真叶形成初期,ARR4基因被明显涛导;而ARR5的应答反应在幼苗真叶形成后的几个时间段均能检测到,并且在种子萌发后的第20天,花枝形成开始时特别明显。在双组分信号传导途径中,从受体到反应调节基因传导磷酸基团的传导基因AHP4在幼苗发育的后期种子萌发后的第20和30天,应答植物体内增加的植物细胞分裂素,并且在花枝形成初期比较明显。     

Heat Shock Factors HsfB 1 and HsfB2b Are Involved in the Regulation of Pdf1.2 Expression and Pathogen Resistance in Arabidopsis

In order to assess the functional roles of heat stress-induced class B-heat shock factors in Arabidopsis, we investigated T-DNA knockout mutants of AtHsfB1 and AtHsfB2b. Micorarray analysis of double knockout hsfB1/hsfB2b plants revealed as strong an up-regulation of the basal mRNA-levels of the defensin genes Pdfl.2a/b in mutant plants. The Pdfexpression was further enhanced by jasmonic acid treatment or infection with the necrotrophic fungus Alternaria brassicicola. The single mutant hsfB2b and the double mutant hsfB 1/B2b were significantly improved in disease resistance after A. brassicicola infection. There was no indication for a direct interaction of Hsf with the promoter of Pdf1.2, which is devoid of perfect HSE consensus Hsf-binding sequences. However, changes in the formation of late HsfA2-dependent HSE binding were detected in hsfB1/B2b plants. This suggests that HsfB1/B2b may interact with class A-Hsf in regulating the shut-off of the heat shock response. The identification of Pdfgenes as targets of Hsf-dependent negative regulation is the first evidence for an interconnection of Hsf in the regulation of biotic and abiotic responses.     

Disposal of chloroplasts with abnormal function into the vacuole in Arabidopsis thaliana cotyledon cells

Summary. Autophagy is a process in which cell membrane rearrangement allows for the sequestration and degradation of part of the cytoplasm. Many protein components of the autophagic mechanism and their corresponding genes have been identified in yeast cells by molecular genetics, and this has enabled researchers to identify homologues of these genes in mammalian and plant systems. Autophagy is involved in the starvation response in which part of the cytoplasm is degraded in order to produce essential substrates to allow the cell to survive during extreme substrate-limiting conditions. However, autophagy may also be important as a quality control mechanism in normal cells. By screening Arabidopsis thaliana T-DNA insert mutants, we isolated an A. thaliana mutant that lacks the AtTIC40 gene and found that the cotyledon cells of this mutant contained undeveloped plastids. Moreover, many toluidine-stained particulate structures were found in the vacuoles of these mutant cells. The images from electron microscopy suggested that some of these particulate structures were partially degraded chloroplasts. Furthermore, oil bodies were found in the cotyledon cells of mutant and wild-type plants, which suggests that the mutant seedlings were not starved under the experimental conditions. These results may indicate that under nutrient-sufficient conditions, plant cells remove abnormal plastids by autophagy and that this mechanism is involved in the quality control of organelles.Present address: BioResource Center, Tsukuba Institute, Institute of Physical and Chemical Research (RIKEN), Tsukuba, Japan.Present address: Genomics Sciences Center, Yokohama Institute, Institute of Physical and Chemical Research (RIKEN), Yokohama, Japan.Correspondence and reprints: School of Food and Nutritional Sciences, University of Shizuoka, 52-1 Yada, Shizuoka 422-8526, Japan.     

Nucleotide Variation in the NCED3 Region of Arabidopsis thaliana and its Association Study with Abscisic Acid Content under Drought Stress

Drought tolerance is a comprehensive quantitative trait that is being understood further at the molecular genetic level. Abscisic acid （ABA） is the main drought-induced hormone that regulates the expression of many genes related to drought responses. 9-cis-epoxycarotenoid dioxygenase （NCED3） is thought to be a key enzyme in ABA biosynthesis. In this paper, we measured the ABA content increase under drought stress, and sequenced and compared the sequence of AtNCED3 among 22 Arabidopsis thaliana accessions. The results showed that the fold of ABA content increase under drought stress was highly variable among these accessions. High density single nucleotide polymorphism （SNP） and insertion/deletion （indel） were found in the AtNCED3 region, on average one SNP per 87.4 bp and one indel per 502 bp. Nucleotide diversity was significantly lower in the coding region than that in non-coding regions. The results of an association study with ANOVA analysis suggested that the 274th site （P←→S） and the 327th site （P←→R） amino acid variations might be the cause of ABA content increase of 163av accession under drought stress.     

Arabidopsis Protein Phosphatase 2C ABI1 Interacts with Type I ACC Synthases and Is Involved in the Regulation of Ozone-Induced Ethylene Biosynthesis

Ethylene plays a crucial role in various biological processes and therefore its biosynthesis is strictly regu- lated by multiple mechanisms. Posttranslational regulation, which is pivotal in controlling ethylene biosynthesis, impacts 1-aminocyclopropane 1-carboxylate synthase （ACS） protein stability via the complex interplay of specific factors. Here, we show that the Arabidopsis thaliana protein phosphatase type 2C, ABI1, a negative regulator of abscisic acid signaling, is involved in the regulation of ethylene biosynthesis under oxidative stress conditions. We found that ABI1 interacts with ACS6 and dephosphorylates its C-terminal fragment, a target of the stress-responsive mitogen-activated protein kinase, MPK6. In addition, ABI1 controls MPK6 activity directly and by this means also affects the ACS6 phosphorylation level. Consistently with this, ozone-induced ethylene production was significantly higher in an ABI1 knockout strain （abiltd） than in wild-type plants. Importantly, an increase in stress-induced ethylene production in the abiltd mutant was compen- sated by a higher ascorbate redox state and elevated antioxidant activities. Overall, the results of this study provide evi- dence that ABI1 restricts ethylene synthesis by affecting the activity of ACS6. The ABI1 contribution to stress phenotype underpins its role in the interplay between the abscisic acid （ABA） and ethylene signaling pathways.     

The Effects of Regulatory Proteins RcsA and RcsB on the Expression of the Vibrio fischeri lux Operon in Escherichia coli

A study was made of the effect of RcsA and RcsB on the Vibrio fischeri lux expression in Escherichia coli. RcsA suppressed the LuxR activity and thereby inhibited expression of the lux genes coding for luciferase and reductase. In osmotic shock, RcsA–RcsB activated lux expression and, consequently, the bioluminescence of E. coli cells in the early log phase.