Herbicidal and antioxidant responses of transgenic rice overexpressing Myxococcus xanthus protoporphyrinogen oxidase

Microbial HemG-type protoporphyrinogen IX oxidase enzymes for biotechnology applications in plant herbicide tolerance traits

BACKGROUND

Protoporphyrinogen IX oxidase (PPO)-inhibiting herbicides act by inhibiting a key enzyme in the heme and chlorophyll biosynthetic pathways in plants. This enzyme, the PPO enzyme, is conserved across plant species. However, some microbes are known to utilize a unique family of PPO enzymes, the HemG family. This enzyme family carries out the same enzymatic step as the plant PPO enzymes, but does not share sequence homology with the plant PPO enzymes.

RESULTS

Bioinformatic analysis was used to identify putative HemG PPO enzyme variants from microbial sources. A subset of these variants was cloned and characterized. HemG PPO variants were characterized for functionality and tolerance to PPO-inhibiting herbicides. HemG PPO variants that exhibited insensitivity to PPO-inhibiting herbicides were identified for further characterization. Expression of selected variants in maize, soybean, cotton and canola resulted in plants that displayed tolerance to applications of PPO-inhibiting herbicides.

CONCLUSION

Selected microbial-sourced HemG PPO enzyme variants present an opportunity for building new herbicide tolerance biotechnology traits. These traits provide tolerance to PPO-inhibiting herbicides and, therefore, could provide additional tools for farmers to employ in their weed management systems. © 2019 Society of Chemical Industry.

The role of reactive oxygen species in signalling from chloroplasts to the nucleus

The coordination of chloroplast function with the rest of cellular activity requires a continual stream of communication from this organelle to the nucleus. Chloroplasts are major sites of the production of reactive oxygen species (ROS) as either by-products of the reduction of molecular oxygen (O(2)) or its excitation in the presence of highly energised pigments. Such ROS, while potentially damaging to the cell, are also important initiators or transducers of signals from these organelles to the nucleus in response to environmental cues. ROS can initiate such retrograde signalling pathways that trigger either programmed cell death or adjustment to changed conditions. Such different outcomes have implications for the way in which signal transduction by ROS is accomplished and is the subject of this review. In response to mild-stress situations, and as a consequence of their reactivity or because of their containment by cellular antioxidant systems, it is proposed that ROS engage with or initiate signalling at or very near their site of production. In contrast, under more extreme conditions, ROS are proposed to diffuse away from their site of production and consequently elicit a different set of signalling events.

Jasmonic acid, protein phosphatase inhibitor, metals and UV-irradiation increased momilactone A and B concentrations in the moss Hypnum plumaeforme

Momilactone A and B have been found only in rice and the moss, Hypnum plumaeforme, although both plants are taxonomically quite distinct. The endogenous concentrations of momilactone A and B in H. plumaeforme were 58.7 and 24.3 microg/g dry weight, respectively. UV-irradiation increased the concentrations of momilactone A and B. The concentrations of momilactone A and B, respectively, became 14- and 15-fold greater than those of non-UV-irradiated control. CuCl2 and FeCl2 (1 mmol/L) treatments also increased momilactone A and B concentrations by 2.7- to 6.1-fold and 2.9- to 6.3-fold, respectively. In addition, the protein phosphatase inhibitor, cantharidin, and jasmonic acid increased momilactone A and B concentrations in H. plumaeforme. Cantharidin acts as an elicitor and jasmonic acid is an important signaling molecule regulating inducible defense genes against the pathogen infection. Momilactone A and B, respectively, were increased 12- and 11-fold by 200 micromol/L cantharidin, and 14- and 15-fold by 100 micromol/L jasmonic acid compared with non-treated controls. As momilactone A and B are phytoalexins, these compounds may play an important role in defense responses against biotic and abiotic stress conditions in H. plumaeforme.

Transgenic plants for phytoremediation of herbicides

Herbicides are economically important, but the non-point pollution that they cause may disrupt the surrounding environment. Phytoremediation of herbicides has been well studied using conventional plants. Transgenic plants produced for metabolizing herbicides and long-persisting pollutants can be used for phytoremediation of foreign chemicals in contaminated soil and water. The genes involved in the metabolism of chemical compounds can be isolated from various organisms, including bacteria, fungi, plants, and animals, and these genes are then introduced into candidate plants. Transgenic plants expressing mammalian P450s and the other enzymes showed tolerance and phytoremediation activity toward target herbicides. Transgenic plants can also enhance the absorption and detoxification of pollutants, thereby aiding the phytoremediation of contaminated environments.

UV-induced momilactone B accumulation in rice rhizosphere

UV-irradiation increased the concentration of momilactone B in shoots and roots of rice seedlings, and increasing the irradiation increased the concentration. The concentration in 90-min UV-irradiated shoots and roots, respectively, was 31.8- and 3.6-fold higher than that in non-irradiated shoots and roots. After UV-irradiation the concentration of momilactone B in rice shoots decreased. There was, however, an accumulation of momilactone B in the medium in which UV-irradiated seedlings were grown. Five days after UV-irradiation, momilactone B in the medium was at a level 2.5 times greater than on day 0, which was 47% of momilactone B in the seedlings, suggesting that rice may actively secrete momilactone B into medium. Therefore, UV-irradiation increased not only production of momilactone B in rice seedlings but also secretion of momilactone B into rice rhizosphere. As momilactone B acts as an antimicrobial and allelopathic agent, secretion of momilactone B into the rhizosphere may provide a competitive advantage for root establishment through local suppression of soil microorganism and inhibition of the growth of competing plant species.

A mitochondrial protein homologous to the mammalian peripheral-type benzodiazepine receptor is essential for stress adaptation in plants

The cloning of abiotic stress-inducible genes from the moss Physcomitrella patens led to the identification of the gene PpTSPO1, encoding a protein homologous to the mammalian mitochondrial peripheral-type benzodiazepine receptor and the bacterial tryptophane-rich sensory protein. This class of proteins is involved in the transport of intermediates of the tetrapyrrole biosynthesis pathway. Like the mammalian homologue, the PpTSPO1 protein is localized to mitochondria. The generation of PpTSPO1-targeted moss knock-out lines revealed an essential function of the gene in abiotic stress adaptation. Under stress conditions, the PpTSPO1 null mutants show elevated H(2)O(2) levels, enhanced lipid peroxidation and cell death, indicating an important role of PpTSPO1 in redox homeostasis. We hypothesize that PpTSPO1 acts to direct porphyrin precursors to the mitochondria for heme formation, and is involved in the removal of photoreactive tetrapyrrole intermediates.

Crystal structure of protoporphyrinogen oxidase from Myxococcus xanthus and its complex with the inhibitor acifluorfen

Protoporphyrinogen IX oxidase, a monotopic membrane protein, which catalyzes the oxidation of protoporphyrinogen IX to protoporphyrin IX in the heme/chlorophyll biosynthetic pathway, is distributed widely throughout nature. Here we present the structure of protoporphyrinogen IX oxidase from Myxococcus xanthus, an enzyme with similar catalytic properties to human protoporphyrinogen IX oxidase that also binds the common plant herbicide, acifluorfen. In the native structure, the planar porphyrinogen substrate is mimicked by a Tween 20 molecule, tracing three sides of the macrocycle. In contrast, acifluorfen does not mimic the planarity of the substrate but is accommodated by the shape of the binding pocket and held in place by electrostatic and aromatic interactions. A hydrophobic patch surrounded by positively charged residues suggests the position of the membrane anchor, differing from the one proposed for the tobacco mitochondrial protoporphyrinogen oxidase. Interestingly, there is a discrepancy between the dimerization state of the protein in solution and in the crystal. Conserved structural features are discussed in relation to a number of South African variegate porphyria-causing mutations in the human enzyme.