Iron uptake and metabolism are tightly regulated in both plants and animals. In Arabidopsis (Arabidopsis thaliana), BRUTUS (BTS), which contains three hemerythrin (HHE) domains and a Really Interesting New Gene (RING) domain, interacts with basic helix-loop-helix transcription factors that are capable of forming heterodimers with POPEYE (PYE), a positive regulator of the iron deficiency response. BTS has been shown to have E3 ligase capacity and to play a role in root growth, rhizosphere acidification, and iron reductase activity in response to iron deprivation. To further characterize the function of this protein, we examined the expression pattern of recombinant ProBTS::b-GLUCURONIDASE and found that it is expressed in developing embryos and other reproductive tissues, corresponding with its apparent role in reproductive growth and development. Our findings also indicate that the interactions between BTS and PYE-like (PYEL) basic helix-loop-helix transcription factors occur within the nucleus and are dependent on the presence of the RING domain. We provide evidence that BTS facilitates 26S proteasome-mediated degradation of PYEL proteins in the absence of iron. We also determined that, upon binding iron at the HHE domains, BTS is destabilized and that this destabilization relies on specific residues within the HHE domains. This study reveals an important and unique mechanism for plant iron homeostasis whereby an E3 ubiquitin ligase may posttranslationally control components of the transcriptional regulatory network involved in the iron deficiency response.

Glycerol-3-phosphate (G3P) is an important metabolite that contributes to the growth and disease-related physiologies of prokaryotes, plants, animals and humans alike. Here we show that G3P serves as the inducer of an important form of broad-spectrum immunity in plants, termed systemic acquired resistance (SAR). SAR is induced upon primary infection and protects distal tissues from secondary infections. Genetic mutants defective in G3P biosynthesis cannot induce SAR but can be rescued when G3P is supplied exogenously. Radioactive tracer experiments show that a G3P derivative is translocated to distal tissues, and this requires the lipid transfer protein, DIR1. Conversely, G3P is required for the translocation of DIR1 to distal tissues, which occurs through the symplast. These observations, along with the fact that dir1 plants accumulate reduced levels of G3P in their petiole exudates, suggest that the cooperative interaction of DIR1 and G3P orchestrates the induction of SAR in plants.

No abstract

Soybean (Glycine max) RPG1-B (for resistance to Pseudomonas syringae pv glycinea) mediates species-specific resistance to P. syringae expressing the avirulence protein AvrB, similar to the nonorthologous RPM1 in Arabidopsis (Arabidopsis thaliana). RPM1-derived signaling is presumably induced upon AvrB-derived modification of the RPM1-interacting protein, RIN4 (for RPM1-interacting 4). We show that, similar to RPM1, RPG1-B does not directly interact with AvrB but associates with RIN4-like proteins from soybean. Unlike Arabidopsis, soybean contains at least four RIN4-like proteins (GmRIN4a to GmRIN4d). GmRIN4b, but not GmRIN4a, complements the Arabidopsis rin4 mutation. Both GmRIN4a and GmRIN4b bind AvrB, but only GmRIN4b binds RPG1-B. Silencing either GmRIN4a or GmRIN4b abrogates RPG1-B-derived resistance to P. syringae expressing AvrB. Binding studies show that GmRIN4b interacts with GmRIN4a as well as with two other AvrB/RPG1-B-interacting isoforms, GmRIN4c and GmRIN4d. The lack of functional redundancy among GmRIN4a and GmRIN4b and their abilities to interact with each other suggest that the two proteins might function as a heteromeric complex in mediating RPG1-B-derived resistance. Silencing GmRIN4a or GmRIN4b in rpg1-b plants enhances basal resistance to virulent strains of P. syringae and the oomycete Phytophthora sojae. Interestingly, GmRIN4a- or GmRIN4b-silenced rpg1-b plants respond differently to AvrB-expressing bacteria. Although both GmRIN4a and GmRIN4b function to monitor AvrB in the presence of RPG1-B, GmRIN4a, but not GmRIN4b, negatively regulates AvrB virulence activity in the absence of RPG1-B.

Water stress-induced spikelet sterility limits rice production under upland conditions. The causes of spikelet sterility under drought stress are poorly understood. In this study the role of antioxidant defence management in drought-induced spikelet sterility was investigated in two rice (Oryza sativa) genotypes differing in drought resistance. Drought-resistant N22 genotype showed less water stress-induced spikelet sterility when compared to the susceptible N118 genotype under upland conditions. The N22 panicles maintained higher RWC and turgor potential and lower H 2 O 2 levels across the developmental stages under water stress than that of N118 panicles. Drought-induced enhancement in superoxide dismutase (SOD, EC 1.15.1.1) activity coupled with higher ascorbate (AsA), glutathione (GSH) content and enhanced ascorbate peroxidase (APX, EC 1.11.1.11) and glutathione reductase (GR, EC 1.6.4.2) activities resulted in lower H 2 O 2 levels in N22 panicles. In contrast, insufficient enhancement in SOD, APX and GR activities resulted in relatively higher H 2 O 2 levels under water stress in N118 panicles. The N22 panicles exhibited a higher number of SOD and APX isozymes in comparison with N118 panicles that might provide better reactive oxygen species scavenging. Hence it is concluded that well-equipped antioxidant defence plays an important role in minimizing water stress-induced spikelet sterility in upland rice.

Wheat (Triticum aestivum L.) seedlings of a drought‐resistant cv. C306 were subjected to severe water deficit directly or through stress cycles of increasing intensity with intermittent recovery periods (drought acclimation). The antioxidant defense in terms of redox metabolites and enzymes in leaf cells, chloroplasts, and mitochondria was examined in relation to ROS‐induced membrane damage. Drought‐acclimated seedlings modulated growth by maintaining favorable turgor potential and RWC and were able to limit H2O2 accumulation and membrane damage as compared with non‐acclimated plants during severe water stress conditions. This was due to systematic upregulation of H2O2‐metabolizing enzymes especially ascorbate peroxidase (APX, EC 1.11.1.11) and by maintaining ascorbate–glutathione redox pool in acclimated plants. By contrast, failure in the induction of APX and ascorbate–glutathione cycle enzymes makes the chloroplast susceptible to oxidative stress in non‐acclimated plants. Non‐acclimated plants protected the leaf mitochondria from oxidative stress by upregulating superoxide dismutase (SOD, EC 1.15.1.1), APX, and glutathione reductase (GR, EC 1.6.4.2) activities. Rewatering led to rapid enhancement in all the antioxidant defense components in non‐acclimated plants, which suggested that the excess levels of H2O2 during severe water stress conditions might have inhibited or downregulated the antioxidant enzymes. Hence, drought acclimation conferred enhanced oxidative stress tolerance by well‐co‐ordinated induction of antioxidant defense both at the chloroplast and at the mitochondrial level.

BackgroundDeveloping methods for protecting organisms in metal-polluted environments is contingent upon our understanding of cellular detoxification mechanisms. In this regard, half-molecule ATP-binding cassette (ABC) transporters of the HMT-1 subfamily are required for cadmium (Cd) detoxification. HMTs have conserved structural architecture that distinguishes them from other ABC transporters and allows the identification of homologs in genomes of different species including humans. We recently discovered that HMT-1 from the simple, unicellular organism, Schizosaccharomyces pombe, SpHMT1, acts independently of phytochelatin synthase (PCS) and detoxifies Cd, but not other heavy metals. Whether HMTs from multicellular organisms confer tolerance only to Cd or also to other heavy metals is not known.Methodology/Principal FindingsUsing molecular genetics approaches and functional in vivo assays we showed that HMT-1 from a multicellular organism, Caenorhabditis elegans, functions distinctly from its S. pombe counterpart in that in addition to Cd it confers tolerance to arsenic (As) and copper (Cu) while acting independently of pcs-1. Further investigation of hmt-1 and pcs-1 revealed that these genes are expressed in different cell types, supporting the notion that hmt-1 and pcs-1 operate in distinct detoxification pathways. Interestingly, pcs-1 and hmt-1 are co-expressed in highly endocytic C. elegans cells with unknown function, the coelomocytes. By analyzing heavy metal and oxidative stress sensitivities of the coelomocyte-deficient C. elegans strain we discovered that coelomocytes are essential mainly for detoxification of heavy metals, but not of oxidative stress, a by-product of heavy metal toxicity.Conclusions/SignificanceWe established that HMT-1 from the multicellular organism confers tolerance to multiple heavy metals and is expressed in liver-like cells, the coelomocytes, as well as head neurons and intestinal cells, which are cell types that are affected by heavy metal poisoning in humans. We also showed that coelomocytes are involved in detoxification of heavy metals. Therefore, the HMT-1-dependent detoxification pathway and coelomocytes of C. elegans emerge as novel models for studies of heavy metal-promoted diseases.

Wheat (Triticum aestivum L.) seedlings of a drought-resistant cv. C306 were subjected to severe water deficit directly or through stress cycles of increasing intensity with intermittent recovery periods. The antioxidant defense in terms of redox metabolites and enzymes in root cells and mitochondria was examined in relation to membrane damage. Acclimated seedlings exhibited higher relative water content and were able to limit the accumulation of H(2)O(2) and membrane damage during subsequent severe water stress conditions. This was due to systematic up-regulation of superoxide dismutase, ascorbate peroxidase (APX), catalase, peroxidases, and ascorbate-glutathione cycle components at both the whole cell level as well as in mitochondria. In contrast, direct exposure of severe water stress to non-acclimated seedlings caused greater water loss, excessive accumulation of H(2)O(2) followed by elevated lipid peroxidation due to the poor antioxidant enzyme response particularly of APX, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and ascorbate-glutathione redox balance. Mitochondrial antioxidant defense was found to be better than the cellular defense in non-acclimated roots. Termination of stress followed by rewatering leads to a rapid enhancement in all the antioxidant defense components in non-acclimated roots, which suggested that the excess levels of H(2)O(2) during severe water stress conditions might have inhibited or down-regulated the antioxidant enzymes. Hence, drought acclimation conferred enhanced tolerance toward oxidative stress in the root tissue of wheat seedlings due to both reactive oxygen species restriction and well-coordinated induction of antioxidant defense.