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.     

High temperature-induced abscisic acid biosynthesis and its role in the inhibition of gibberellin action in Arabidopsis seeds

Suppression of seed germination at supraoptimal high temperature (thermoinhibiton) during summer is crucial for Arabidopsis (Arabidopsis thaliana) to establish vegetative and reproductive growth in appropriate seasons. Abscisic acid (ABA) and gibberellins (GAs) are well known to be involved in germination control, but it remains unknown how these hormone actions (metabolism and responsiveness) are altered at high temperature. Here, we show that ABA levels in imbibed seeds are elevated at high temperature and that this increase is correlated with up-regulation of the zeaxanthin epoxidase gene ABA1/ZEP and three 9-cis-epoxycarotenoid dioxygenase genes, NCED2, NCED5, and NCED9. Reverse-genetic studies show that NCED9 plays a major and NCED5 and NCED2 play relatively minor roles in high temperature-induced ABA synthesis and germination inhibition. We also show that bioactive GAs stay at low levels at high temperature, presumably through suppression of GA 20-oxidase genes, GA20ox1, GA20ox2, and GA20ox3, and GA 3-oxidase genes, GA3ox1 and GA3ox2. Thermoinhibition-tolerant germination of loss-of-function mutants of GA negative regulators, SPINDLY (SPY) and RGL2, suggests that repression of GA signaling is required for thermoinibition. Interestingly, ABA-deficient aba2-2 mutant seeds show significant expression of GA synthesis genes and repression of SPY expression even at high temperature. In addition, the thermoinhibition-resistant germination phenotype of aba2-1 seeds is suppressed by a GA biosynthesis inhibitor, paclobutrazol. We conclude that high temperature stimulates ABA synthesis and represses GA synthesis and signaling through the action of ABA in Arabidopsis seeds.     

拟南芥prr5突变体对ABA的响应

以拟南芥为材料,统计PRRs （pseudo-response regulators）突变体 prr5及其野生型经ABA处理后的萌发率、根长和NaCl处理后的萌发率,并采用实时定量PCR方法,对不同浓度ABA处理的拟南芥幼苗中的PRR5基因表达进行分析.结果表明：prr5突变体对ABA弱敏感,其种子萌发率比野生型显著或极显著增高,主根比野生型长,且PRR5基因表达受ABA抑制.同时,NaCl处理后,prr5的萌发率比野生型极显著增高.因此,推测prr5可能为ABA信号通路相关基因.     

Arabidopsis RNA-binding protein UBA2a relocalizes into nuclear speckles in response to abscisic acid

The Arabidopsis thaliana RNA binding protein UBA2a is the closest homologue of the Vicia faba AKIP1 (56% identity). Like AKIP1, UBA2a is a constitutively-expressed nuclear protein and in response to ABA it is also reorganized within the nucleus in "speckles" suggesting a possible role of this protein in the regulation of mRNA metabolism during ABA signaling. AKIP1 interacts with, and is phosphorylated by, the upstream ABA-activated protein kinase AAPK. We have investigated if a pathway similar to that described in Vicia faba also exists in Arabidopsis. Our results showed that despite the resemblance between the corresponding Vicia and Arabidopsis proteins, it appears that the function of UBA2a is independent of OST1 phosphorylation.     

Conversion of perianth into reproductive organs by ectopic expression of the tobacco floral homeotic gene NAG1.

Mutations in the AGAMOUS (AG) gene of Arabidopsis thaliana result in the conversion of reproductive organs, stamens and carpels, into perianth organs, sepals and petals. We have isolated and characterized the putative AG gene from Nicotiana tabacum, NAG1, whose deduced protein product shares 73% identical amino acid residues with the Arabidopsis AG gene product. RNA tissue in situ hybridizations show that NAG1 RNA accumulates early in tobacco flower development in the region of the floral meristem that will later give rise to stamens and carpels. Ectopic expression of NAG1 in transgenic tobacco plants results in a conversion of sepals and petals into carpels and stamens, respectively, indicating that NAG1 is sufficient to convert perianth into reproductive floral organs.     

Abscisic acid and osmotic stress or slow drying independently induce desiccation tolerance in mutant seeds of Arabidopsis thaliana

Action of endogenous abscisic acid (ABA) is absent in the ABA-deficient and -insensitive double mutant ( aba-1abi3–1 ) seeds of Arabidopsis thaliana . Thus, responses to osmotic stress and dehydration can be studied without interference of endogenous ABA. Seeds of this double mutunt are viable hut desiceation-intolerant. However, desiccation tolerance can he induced by either (1) slow dehydration of immature seeds; (2) treatment of immature seeds with osmotica or; (31 due to the leakiness of the ABA-insensitivty mutation, by application of exogenous ABA. Consequently it is concluded that either ABA or osmotic- or dehydration-stress and related gene expression meets the minimal requirements for acquisition of desiccation tolerance in seeds of Arabidopsis thalianna .     

SDIR1 is a RING finger E3 ligase that positively regulates stress-responsive abscisic acid signaling in Arabidopsis

Ubiquitination plays important roles in plant hormone signal transduction. We show that the RING finger E3 ligase, Arabidopsis thaliana SALT- AND DROUGHT-INDUCED RING FINGER1 (SDIR1), is involved in abscisic acid (ABA)-related stress signal transduction. SDIR1 is expressed in all tissues of Arabidopsis and is upregulated by drought and salt stress, but not by ABA. Plants expressing the ProSDIR1-beta-glucuronidase (GUS) reporter construct confirmed strong induction of GUS expression in stomatal guard cells and leaf mesophyll cells under drought stress. The green fluorescent protein-SDIR1 fusion protein is colocalized with intracellular membranes. We demonstrate that SDIR1 is an E3 ubiquitin ligase and that the RING finger conservation region is required for its activity. Overexpression of SDIR1 leads to ABA hypersensitivity and ABA-associated phenotypes, such as salt hypersensitivity in germination, enhanced ABA-induced stomatal closing, and enhanced drought tolerance. The expression levels of a number of key ABA and stress marker genes are altered both in SDIR1 overexpression and sdir1-1 mutant plants. Cross-complementation experiments showed that the ABA-INSENSITIVE5 (ABI5), ABRE BINDING FACTOR3 (ABF3), and ABF4 genes can rescue the ABA-insensitive phenotype of the sdir1-1 mutant, whereas SDIR1 could not rescue the abi5-1 mutant. This suggests that SDIR1 acts upstream of those basic leucine zipper family genes. Our results indicate that SDIR1 is a positive regulator of ABA signaling.     

Induction of RAB18 gene expression and activation of K+ outward rectifying channels depend on an extracellular perception of ABA in Arabidopsis thaliana suspension cells

Important progress has been made regarding the characterization of the ABA signalling components using genetic and molecular approaches (Leung and Giraudat, 1998). However, we do not yet know the mechanism of ABA perception. Conflicting results concerning the site of ABA perception have been published. The prevailing view is that since ABA controls many responses, different sites of perception for ABA might exist. In order to establish the cellular localisation of the ABA receptors in Arabidopsis thaliana suspension cells, we developed two physiological tests based upon the capacity of impermeant ABA-BSA conjugate to mimic permeant free ABA effects. We show that purified ABA-BSA conjugate is able to trigger RAB18 gene expression and that this response is strictly due to the natural (+)-ABA enantiomer. The rate of RAB18 gene expression was independent of the level of ABA uptake by the cells. Using the voltage-clamp technique we show that ABA-BSA, similarly to ABA, evokes a membrane depolarization and activates time- and voltage-dependent outward rectifying currents (ORC). We demonstrate that these ORC are due to a K+ efflux as assessed by tail currents and specific inhibition by both tetraethylammonium (TEA) and Ba2+. These observations provide evidence in favour of an extracellular site for ABA perception.