The Arabidopsis Iron-Sulfur (Fe-S) Cluster Gene MFDX1 Plays a Role in Host and Nonhost Disease Resistance by Accumulation of Defense-Related Metabolites.

Until recently, genes from the iron-sulfur (Fe-S) cluster pathway were not known to have a role in plant disease resistance. The Nitrogen Fixation S (NIFS)-like 1 (NFS1) and Mitochondrial Ferredoxin-1 (MFDX1) genes are part of a set of 27 Fe-S cluster genes induced after infection with host and nonhost pathogens in Arabidopsis. A role for AtNFS1 in plant immunity was recently demonstrated. In this work, we showed that MFDX1 is also involved in plant defense. More specifically, Arabidopsis mfdx1 mutants were compromised for nonhost resistance against Pseudomonas syringae pv. tabaci, and showed increased susceptibility to the host pathogen P. syringae pv. tomato DC3000. Arabidopsis AtMFDX1 overexpression lines were less susceptible to P. syringae pv. tomato DC3000. Metabolic profiling revealed a reduction of several defense-related primary and secondary metabolites, such as asparagine and glucosinolates in the Arabidopsis mfdx1-1 mutant when compared to Col-0. A reduction of 5-oxoproline and ornithine metabolites that are involved in proline synthesis in mitochondria and affect abiotic stresses was also observed in the mfdx1-1 mutant. In contrast, an accumulation of defense-related metabolites such as glucosinolates was observed in the Arabidopsis NFS1 overexpressor when compared to wild-type Col-0. Additionally, mfdx1-1 plants displayed shorter primary root length and reduced number of lateral roots compared to the Col-0. Taken together, these results provide additional evidence for a new role of Fe-S cluster pathway in plant defense responses.

The N-end rule pathway regulates pathogen responses in plants.

To efficiently counteract pathogens, plants rely on a complex set of immune responses that are tightly regulated to allow the timely activation, appropriate duration and adequate amplitude of defense programs. The coordination of the plant immune response is known to require the activity of the ubiquitin/proteasome system, which controls the stability of proteins in eukaryotes. Here, we demonstrate that the N-end rule pathway, a subset of the ubiquitin/proteasome system, regulates the defense against a wide range of bacterial and fungal pathogens in the model plant Arabidopsis thaliana. We show that this pathway positively regulates the biosynthesis of plant-defense metabolites such as glucosinolates, as well as the biosynthesis and response to the phytohormone jasmonic acid, which plays a key role in plant immunity. Our results also suggest that the arginylation branch of the N-end rule pathway regulates the timing and amplitude of the defense program against the model pathogen Pseudomonas syringae AvrRpm1.

Secondary metabolites in plants: transport and self-tolerance mechanisms.

Plants produce a host of secondary metabolites with a wide range of biological activities, including potential toxicity to eukaryotic cells. Plants generally manage these compounds by transport to the apoplast or specific organelles such as the vacuole, or other self-tolerance mechanisms. For efficient production of such bioactive compounds in plants or microbes, transport and self-tolerance mechanisms should function cooperatively with the corresponding biosynthetic enzymes. Intensive studies have identified and characterized the proteins responsible for transport and self-tolerance. In particular, many transporters have been isolated and their physiological functions have been proposed. This review describes recent progress in studies of transport and self-tolerance and provides an updated inventory of transporters according to their substrates. Application of such knowledge to synthetic biology might enable efficient production of valuable secondary metabolites in the future.

Impact of the PGPB Enterobacter radicincitans DSM 16656 on growth, glucosinolate profile, and immune responses of Arabidopsis thaliana.

Plant growth-promoting bacteria (PGPB) affect plant cellular processes in various ways. The endophytic bacterial strain Enterobacter radicincitans DSM 16656 has been shown to improve plant growth and yield in various agricultural and vegetable crops. Besides its ability to fix atmospheric nitrogen, produce phytohormones, and solubilize phosphate compounds, the strain is highly competitive against native endophytic organisms and colonizes the endorhizosphere in high numbers. Here, we show that E. radicincitans inoculation of the noncrop plant Arabidopsis thaliana promotes plant growth. Furthermore, high performance liquid chromatography (HPLC) analysis revealed that bacterial inoculation slightly decreased amounts of aliphatic glucosinolates in plant leaves in a fast-growing stage but increased these compounds in an older phase where growth is mostly completed. This effect seems to correlate with developmental stage and depends on the nitrogen requirement. Additionally, nitrogen deficiency studies with seedlings grown on medium containing different nitrogen concentrations suggest that plant nitrogen demand can influence the intensity of plant growth enhancement by E. radicincitans. This endophyte seems not to activate stress-inducible mitogen-activated protein kinases (MAPKs). Analyzing transcription of the defense-related genes PR1, PR2, PR5, and PDF1.2 by quantitative real time polymerase chain reaction (qPCR) revealed that E. radicincitans DSM 16656 is able to induce priming via salicylic acid (SA) or jasmonate (JA)/ethylene (ET) signaling pathways to protect plants against potential pathogen attack.

Sulfur-containing secondary metabolites from Arabidopsis thaliana and other Brassicaceae with function in plant immunity.

Biosynthesis of antimicrobial secondary metabolites in response to microbial infection is one of the features of the plant immune system. Particular classes of plant secondary metabolites involved in plant defence are often produced only by species belonging to certain phylogenetic clades. Brassicaceae plants have evolved the ability to synthesise a wide range of sulfur-containing secondary metabolites, including glucosinolates and indole-type phytoalexins. A subset of these compounds is produced by the model plant Arabidopsis thaliana. Genetic tools available for this species enabled verification of immune functions of glucosinolates and camalexin (A. thaliana phytoalexin), as well as characterisation of their respective biosynthetic pathways. Current knowledge of the biosynthesis of Brassicaceae sulfur-containing metabolites suggests that the key event in the evolution of these compounds is the acquisition of biochemical mechanisms originating from detoxification pathways into secondary metabolite biosynthesis. Moreover, it is likely that glucosinolates and Brassicaceae phytoalexins, traditionally considered as separate groups of compounds, have a common evolutionary origin and are interconnected on the biosynthetic level. This suggests that the diversity of Brassicaceae sulfur-containing phytochemicals reflect phylogenetic clade-specific branches of an ancient biosynthetic pathway.

Chemical warfare or modulators of defence responses - the function of secondary metabolites in plant immunity.

In plants, a host's responses to an attempted infection include activation of various secondary metabolite pathways, some of which are specific for particular plant phylogenetic clades. Phytochemicals that represent respective end products in plant immunity have been stereotypically linked to antimicrobial properties. However, in many cases, owing to the lack of unequivocal evidence for direct antibiotic action in planta, alternative functions of secondary metabolites should be considered. Correspondingly, recent findings have identified novel, and rather unexpected, functions of phytochemicals in plant immunity that mediate regulatory pathways for conserved defence responses. It also seems likely that these conserved responses can be regulated by clade-specific phytochemicals.

Sub-cellular immunolocalization of the glucosinolate sinigrin in seedlings of Brassica juncea.

Polyclonal rat antibodies were raised to a bovine serum albumin-sinigrin conjugate and used to immunolocalize sinigrin (2-propenylglucosinolate) in imbibed seeds and developing seedlings of Brassica juncea. (L.) Czern. Sinigrin was localized to protein bodies in aleurone-like cells but shown to be absent from myrosin cells. Double labelling techniques were used to co-localize both myrosinase (beta-thioglucoside glucohydrolase, EC 3.2.3.1) and sinigrin. Myrosin grains were labelled only with the anti-myrosinase antibody, but aleurone cells were labelled with both anti-myrosinase and anti-sinigrin antibodies. High-performance liquid chromatographic analysis of conventionally fixed and dehydrated seed tissues (4 h post imbibition in water), indicated a high proportion of sinigrin was retained in fixed tissues. Over a time course of 100 h, protein bodies within aleurone-like cells degraded, fused to form the cell vacuole and lost all myrosinase labelling but retained residual sinigrin labelling. The degradation of protein bodies corresponded to a decrease in retention of sinigrin in the fixed tissues. The results describe for the first time the co-localization of a plant enzyme and its substrate, a secondary metabolite.

Acute exacerbation of hepatitis due to reactivation of hepatitis B virus with mutations in the core region after chemotherapy for malignant lymphoma.

A 43-year-old Japanese man who was positive for hepatitis B surface (HBs) antigen and HB e antibody, underwent chemotherapy for non-Hodgkin's lymphoma. After the chemotherapy he suffered from acute exacerbation of hepatitis because of reactivation of HBV. Recovery was achieved with interferon-alpha, glucagon-insulin therapy, and plasma exchange. Mutations were detected in codons 97, 100, 129, and 131 of the core region of HBV. The peptide encoded from the core region including such mutations possibly had greater antigenicity to induce cytotoxic T cell activity in the host. Core region mutations may be a crucial cause of the acute exacerbation of hepatitis B seen after chemotherapy.