Organelles and phytohormones: a network of interactions in plant stress responses

Phytohormones are major signaling components that contribute to nearly all aspects of plant life. They constitute an interconnected communication network to fine-tune growth and development in response to the ever-changing environment. To this end, they have to coordinate with other signaling components, such as reactive oxygen species and calcium signals. On the one hand, the two endosymbiotic organelles, plastids and mitochondria, control various aspects of phytohormone signaling and harbor important steps of hormone precursor biosynthesis. On the other hand, phytohormones have feedback actions on organellar functions. In addition, organelles and phytohormones often act in parallel in a coordinated matter to regulate cellular functions. Therefore, linking organelle functions with increasing knowledge of phytohormone biosynthesis, perception, and signaling will reveal new aspects of plant stress tolerance. In this review, we highlight recent work on organelle-phytohormone interactions focusing on the major stress-related hormones abscisic acid, jasmonates, salicylic acid, and ethylene.

Identification of Novel Inhibitors of a Plant Group A Protein Phosphatase Type 2C Using a Combined In Silico and Biochemical Approach

Type 2C protein phosphatases (PP2Cs) of group A play a significant role in the regulation of various processes in plants including growth, development, ion transport, and stress acclimation. In this study, we selected potential PP2C group A inhibitors using a structure-based virtual screening method followed by biochemical and in vitro validation. Over twenty million chemical compounds from the ZINC database were used for docking studies. The precision of the calculations was increased by an induced-fit docking protocol and the molecular mechanics/generalized Born surface area (MM/GBSA) method, which yielded approximate values for the binding energy of the protein-ligand complex. After clustering and ranking their activity, the top-ranking compounds were tested against PP2C group A members in vitro and their in vivo activity was also explored. Phosphatase activity assays identified two compounds with significant inhibitory activity against ABI1 protein ranging from around 57 to 91% at a concentration of 100 μM. Importantly, this in vitro activity correlated well with in vivo inhibition of seed germination, as expected for PP2C inhibitors. The results should promote the design of novel inhibitors with improved potency against ABI1-like and other PP2Cs that might be used in agriculture for the protection of crops against stress.

A Role for Barley Calcium-Dependent Protein Kinase CPK2a in the Response to Drought

Increasing the drought tolerance of crops is one of the most challenging goals in plant breeding. To improve crop productivity during periods of water deficit, it is essential to understand the complex regulatory pathways that adapt plant metabolism to environmental conditions. Among various plant hormones and second messengers, calcium ions are known to be involved in drought stress perception and signaling. Plants have developed specific calcium-dependent protein kinases that convert calcium signals into phosphorylation events. In this study we attempted to elucidate the role of a calcium-dependent protein kinase in the drought stress response of barley (Hordeum vulgare L.), one of the most economically important crops worldwide. The ongoing barley genome project has provided useful information about genes potentially involved in the drought stress response, but information on the role of calcium-dependent kinases is still limited. We found that the gene encoding the calcium-dependent protein kinase HvCPK2a was significantly upregulated in response to drought. To better understand the role of HvCPK2a in drought stress signaling, we generated transgenic Arabidopsis plants that overexpressed the corresponding coding sequence. Overexpressing lines displayed drought sensitivity, reduced nitrogen balance index (NBI), an increase in total chlorophyll content and decreased relative water content. In addition, in vitro kinase assay experiments combined with mass spectrometry allowed HvCPK2a autophosphorylation sites to be identified. Our results suggest that HvCPK2a is a dual-specificity calcium-dependent protein kinase that functions as a negative regulator of the drought stress response in barley.

Arabidopsis ABA-Activated Kinase MAPKKK18 is Regulated by Protein Phosphatase 2C ABI1 and the Ubiquitin-Proteasome Pathway

Phosphorylation and dephosphorylation events play an important role in the transmission of the ABA signal. Although SnRK2 [sucrose non-fermenting1-related kinase2] protein kinases and group A protein phosphatase type 2C (PP2C)-type phosphatases constitute the core ABA pathway, mitogen-activated protein kinase (MAPK) pathways are also involved in plant response to ABA. However, little is known about the interplay between MAPKs and PP2Cs or SnRK2 in the regulation of ABA pathways. In this study, an effort was made to elucidate the role of MAP kinase kinase kinase18 (MKKK18) in relation to ABA signaling and response. The MKKK18 knockout lines showed more vigorous root growth, decreased abaxial stomatal index and increased stomatal aperture under normal growth conditions, compared with the control wild-type Columbia line. In addition to transcriptional regulation of the MKKK18 promoter by ABA, we demonstrated using in vitro and in vivo kinase assays that the kinase activity of MKKK18 was regulated by ABA. Analysis of the cellular localization of MKKK18 showed that the active kinase was targeted specifically to the nucleus. Notably, we identified abscisic acid insensitive 1 (ABI1) PP2C as a MKKK18-interacting protein, and demonstrated that ABI1 inhibited its activity. Using a cell-free degradation assay, we also established that MKKK18 was unstable and was degraded by the proteasome pathway. The rate of MKKK18 degradation was delayed in the ABI1 knockout line. Overall, we provide evidence that ABI1 regulates the activity and promotes proteasomal degradation of MKKK18.

Involvement of genes encoding ABI1 protein phosphatases in the response of Brassica napus L. to drought stress

In this report we characterized the Arabidopsis ABI1 gene orthologue and Brassica napus gene paralogues encoding protein phosphatase 2C (PP2C, group A), which is known to be a negative regulator of the ABA signaling pathway. Six homologous B. napus sequences were identified and characterized as putative PP2C group A members. To gain insight into the conservation of ABI1 function in Brassicaceae, and understand better its regulatory effects in the drought stress response, we generated transgenic B. napus plants overexpressing A. thaliana ABI1. Transgenic plants subjected to drought showed a decrease in relative water content, photosynthetic pigments content and expression level of RAB18- and RD19A-drought-responsive marker genes relative to WT plants. We present the characterization of the drought response of B. napus with the participation of ABI1-like paralogues. The expression pattern of two evolutionarily distant paralogues, BnaA01.ABI1.a and BnaC07.ABI1.b in B. napus and their promoter activity in A. thaliana showed differences in the induction of the paralogues under dehydration stress. Comparative sequence analysis of both BnaABI1 promoters showed variation in positions of cis-acting elements that are especially important for ABA- and stress-inducible expression. Together, these data reveal that subfunctionalization following gene duplication may be important in the maintenance and functional divergence of the BnaABI1 paralogues. Our results provide a framework for a better understanding of (1) the role of ABI1 as a hub protein regulator of the drought response, and (2) the differential involvement of the duplicated BnaABI1 genes in the response of B. napus to dehydration-related stresses.

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 regulated 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 (abi1td) than in wild-type plants. Importantly, an increase in stress-induced ethylene production in the abi1td mutant was compensated by a higher ascorbate redox state and elevated antioxidant activities. Overall, the results of this study provide evidence 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.

Total protein, selected protein fractions and chemical elements in the colostrum and milk of mares (Short Communication)

Abstract. The study material consisted of the colostrum and milk of five noble half-blood mares housed under the same environmental conditions. Colostrum samples were collected 24 h after foaling, whereas milk samples were taken five times, every 30 days, with the first sampling on lactation day 30 (1st month). In the samples were determined the concentrations of total protein, pre-albumins, albumins, α- and β-globulins, immunoglobulins and calcium (Ca), magnesium (Mg) and zinc (Zn). The presence of pre-albumins was observed only in mare colostrum. Total protein concentration was at the same level in the colostrum and in the milk in the 1st lactation month, whereas protein fraction concentrations underwent dynamic changes during the 1st month of lactation. When compared to the values determined in the colostrum, the level of α-globulins increased 8 times during lactation, whereas that of immunoglobulins decreased almost 4 times. The highest concentrations of Ca, Mg and Zn were found in mare colostrum, with a decrease in the concentration of all chemical elements in the 1st month of lactation, confirmed statistically in case of Mg and Zn.

Met-enkephalin in the liver as a marker of hepatocellular damage in chronic viral hepatitis type B and C

PURPOSE

The aim of the study was to assess the liver Met-enkephalin concentration in chronic viral hepatitis type B and C as well as in liver cirrhosis in order to estimate the role of opioid system in pathogenesis of liver disease.

MATERIAL AND METHODS

The concentration of Met-enkephalin was examined in liver tissue of 103 consecutive patients with chronic hepatitis type B and C. Control group consisted of uninfected patients. Met-enkephalin concentration was analyzed in relation to the degree of hepatic necroinflammatory activity and the extent of fibrosis estimated by histopathological examination of liver bioptates and compared to such parameters as age, sex and concomitant diseases.

RESULTS

Significant differences in Met-enkephalin concentration were found between cases with advanced fibrosis (stage 3 and 4 acc. to Batts and Ludwig classification) and cases with fibrosis classified as stage 2 (p < 0.05). Met-enkephalin concentration was higher in HCV infected patients in comparison to HBV infected patients (p < 0.05) and uninfected controls (0.05 < p < 0.1). There wasn't found any correlation between Met-enkephalin level and necroinflammatory activity in the liver, age, sex and concomitant diseases.

CONCLUSIONS

Met-enkephalin concentration measurement in the liver tissue seams to be a useful method for differentiation of stage 2 from stages 3 and 4 of severe liver fibrosis. There is increased concentration of Met-enkephalin in liver tissue in HCV infected patients in comparison to HBV infected or uninfected individuals. The degree of necroinflammatory activity in the liver as well as sex and age of patients with chronic hepatitis do not correlate with changes in opioid system.

Einfluss eines 11-monatigen Trainings auf die Ergebnisse des "Ängstlichkeitstests" von jungen Halbblut-Hengsten

Abstract. Title of the paper: Modifying influence of 11-months training on results of ‘timidity’ test of young half bred stallions 78 three-year-old stallions of warm blooded Polish breeds and foreign breeds, kept at the training Centres, were investigated during the period 1998–2001. Stallions had to qualify for a 100-days training, after they passed it, they had to perform an 8-month training program in a central station. To estimate the degree of nervous balance of horses "timidity" a (optical) test was used. Stallions were putt to the tests three times: after qualifying to Training Centre, after completion 100-days training, after completion 8-month training. Stereotypes of stallions maintenance were graded from points 1 to 10. Stallions were divided onto three groups: hyper excitable (1.0–4.0 p), medium excitable (4.1–6.0 p), calm (6.1–10.0 p). Applied "timidity" test showed considerable usefulness to fast and evident opinion of equilibration of nervous system. Rising score in "timidity" test after 100-days as well as 8-months shows the training process had a modifying influence on stallions’ nervous excitability in this period. Largest nervous equilibration was observed at the offspring's of Holstein breed sires’, most excitable after the whole training period were offspring's of noble half bred sires. The highest score of nervous balance after 8-month training was observed at offspring of English thoroughbred sires.