Arabidopsis stress associated protein 9 mediates biotic and abiotic stress responsive ABA signaling via the proteasome pathway

Arabidopsis thaliana Stress Associated Protein 9 (AtSAP9) is a member of the A20/AN1 zinc finger protein family known to play important roles in plant stress responses and in the mammalian immune response. Although SAPs of several plant species were shown to be involved in abiotic stress responses, the underlying molecular mechanisms are largely unknown, and little is known about the involvement of SAPs in plant disease responses. Expression of SAP9 in Arabidopsis is up‐regulated in response to dehydration, cold, salinity and abscisic acid (ABA), as well as pathogen infection. Constitutive expression of AtSAP9 in Arabidopsis leads to increased sensitivity to ABA and osmotic stress during germination and post‐germinative development. Plants that overexpress AtSAP9 also showed increased susceptibility to infection by non‐host pathogen Pseudomonas syringae pv. phaseolicola, indicating a potential role of AtSAP9 in disease resistance. AtSAP9 was found to interact with RADIATION SENSITIVE23d (Rad23d), a shuttle factor for the transport of ubiquitinated substrates to the proteasome, and it is co‐localized with Rad23d in the nucleus. Thus, AtSAP9 may promote the protein degradation process by mediating the interaction of ubiquitinated targets with Rad23d. Taken together, these results indicate that AtSAP9 regulates abiotic and biotic stress responses, possibly via the ubiquitination/proteasome pathway.     

The microfilament cytoskeleton plays a vital role in salt and osmotic stress tolerance in Arabidopsis

Although recent studies have suggested that the microfilament (MF) cytoskeleton of plant cells participates in the response to salt stress, it remains unclear as to whether the MF cytoskeleton actually plays an active role in a plant's ability to withstand salt stress. In the present study, we report for the first time the role of MFs in salt tolerance of Arabidopsis thaliana . Our experiments revealed that Arabidopsis seedlings treated with 150 m m NaCl maintained MF assembly and bundle formation, whereas treatment with 250 m m NaCl initially induced MF assembly but subsequently caused MF disassembly. A corresponding change in the fluorescence intensity of MFs was also observed; that is, a sustained rise in fluorescence intensity in seedlings exposed to 150 m m NaCl and an initial rise and subsequent fall in seedlings exposed to 250 m m NaCl. These results suggest that MF assembly and bundles are induced early after salt stress treatment, while MF polymerization disappears after high salt stress. Facilitation of MF assembly with phalloidin rescued wild-type seedlings from death, whereas blocking MFs assembly with latrunculin A and cytochalasin D resulted in few survivors under salt stress. Pre-treatment of seedlings with phalloidin also clearly increased plant ability to withstand salt stress. MF assembly increased survival of Arabidopsis salt-sensitive sos2 mutants under salt stress and rescued defective sos2 mutants. Polymerization of MFs and its role in promoting survival was also found in plants exposed to osmotic stress. These findings suggest that the MF cytoskeleton participates and plays a vital role in responses to salt and osmotic stress in Arabidopsis .     

Arabidopsis MAPK signaling pathways and their cross talks in abiotic stress response

Journal of Plant Biochemistry and Biotechnology - Plants, being sessile in nature have developed refined mechanisms to rapidly sense changing environment and protect themselves from environmental...     

Mitogen-activated protein kinase signaling in plants under abiotic stress

Mitogen-activated protein kinase cascade is evolutionarily conserved signal transduction module involved in transducing extracellular signals to the nucleus for appropriate cellular adjustment. This cascade consists essentially of three components, a MAPK kinase kinase (MAPKKK), a MAPK kinase (MAPKK) and a MAPK connected to each other by the event of phosphorylation. These kinases play various roles in intra- and extra-cellular signaling in plants by transferring the information from sensors to responses. Signaling through MAP kinase cascade can lead to cellular responses including cell division, differentiation as well as responses to various stresses. MAPK signaling has also been associated with hormonal responses. In plants, MAP kinases are represented by multigene families and are involved in efficient transmission of specific stimuli and also involved in the regulation of the antioxidant defense system in response to stress signaling. In the current review we summarize and investigate the participation of MAPKs as possible mediators of various abiotic stresses in plants.Key words: abiotic stress, cross talk, mitogen-activated protein kinases, heat map, MAPK signaling, signal transduction, stress signaling     

The role of ABA and MAPK signaling pathways in plant abiotic stress responses

As sessile organisms, plants have developed specific mechanisms that allow them to rapidly perceive and respond to stresses in the environment. Among the evolutionarily conserved pathways, the ABA (abscisic acid) signaling pathway has been identified as a central regulator of abiotic stress response in plants, triggering major changes in gene expression and adaptive physiological responses. ABA induces protein kinases of the SnRK family to mediate a number of its responses. Recently, MAPK (mitogen activated protein kinase) cascades have also been shown to be implicated in ABA signaling. Therefore, besides discussing the role of ABA in abiotic stress signaling, we will also summarize the evidence for a role of MAPKs in the context of abiotic stress and ABA signaling.     

Heat shock protein 101 plays a crucial role in thermotolerance in Arabidopsis

Plants are sessile organisms, and their ability to adapt to stress is crucial for survival in natural environments. Many observations suggest a relationship between stress tolerance and heat shock proteins (HSPs) in plants, but the roles of individual HSPs are poorly characterized. We report that transgenic Arabidopsis plants expressing less than usual amounts of HSP101, a result of either antisense inhibition or cosuppression, grew at normal rates but had a severely diminished capacity to acquire heat tolerance after mild conditioning pretreatments. The naturally high tolerance of germinating seeds, which express HSP101 as a result of developmental regulation, was also profoundly decreased. Conversely, plants constitutively expressing HSP101 tolerated sudden shifts to extreme temperatures better than did vector controls. We conclude that HSP101 plays a pivotal role in heat tolerance in Arabidopsis. Given the high evolutionary conservation of this protein and the fact that altering HSP101 expression had no detrimental effects on normal growth or development, one should be able to manipulate the stress tolerance of other plants by altering the expression of this protein.     

Ascorbate plays a key role in alleviating low temperature-induced oxidative stress in Arabidopsis

Low temperature has a negative impact on plant cells and results in the generation of reactive oxygen species (ROS). In order to study the role of ascorbate under chilling stress, the response of an ascorbate-deficient Arabidopsis thaliana mutant vtc2-1 to low temperature (2°C) was investigated. After chilling stress, vtc2-1 mutants exhibited oxidative damage. An increase in the H₂O₂ generation and the production of thiobarbituric acid reactive substances (TBARS), and a decrease in chlorophyll content, the maximal photochemical efficiency of PSII (F_v/F_m) and oxidizable P₇₀₀ were also noted. The ratio of ascorbate/dehydroascorbate and reduced glutathione/oxidzed glutathione in the vtc2-1 mutants were reduced, compared with the wild type (WT) plants. The activities of antioxidant enzymes, such as catalase (CAT) and ascorbate peroxidase (APX), and soluble antioxidants were lower in the vtc2-1 mutants than those in WT plants. These results suggested that the ascorbate-deficient mutant vtc2-1 was more sensitive to chilling treatment than WT plants. The low temperature-induced oxidative stress was the major cause of the decrease of PSII and PSI function in the vtc2-1 mutants. Ascorbate plays a critical role of defense without which the rest of the ROS defense network is unable to react effectively.     

Plantacyanin plays a role in reproduction in Arabidopsis

Plantacyanins belong to the phytocyanin family of blue copper proteins. In the Arabidopsis (Arabidopsis thaliana) genome, only one gene encodes plantacyanin. The T-DNA-tagged mutant is a knockdown mutant that shows no visible phenotype. We used both promoter-beta-glucuronidase transgenic plants and immunolocalization to show that Arabidopsis plantacyanin is expressed most highly in the inflorescence and, specifically, in the transmitting tract of the pistil. Protein levels show a steep gradient in expression from the stigma into the style and ovary. Overexpression plants were generated using cauliflower mosaic virus 35S, and protein levels in the pistil were examined as well as the pollination process. Seed set in these plants is highly reduced mainly due to a lack of anther dehiscence, which is caused by degeneration of the endothecium. Callose deposits occur on the pollen walls in plants that overexpress plantacyanin, and a small percentage of these pollen grains germinate in the closed anthers. When wild-type pollen was used on the overexpression stigma, seed set was still decreased compared to the control pollinations. We detected an increase in plantacyanin levels in the overexpression pistil, including the transmitting tract. Guidance of the wild-type pollen tube on the overexpression stigma is disrupted as evidenced by the growth behavior of pollen tubes after they penetrate the papillar cell. Normally, pollen tubes travel down the papilla cell and into the style. Wild-type pollen tubes on the overexpression stigma made numerous turns around the papilla cell before growing toward the style. In some rare cases, pollen tubes circled up the papilla cell away from the style and were arrested there. We propose that when plantacyanin levels in the stigma are increased, pollen tube guidance into the style is disrupted.