Nitrogen is an essential soil nutrient for plants, and lack of nitrogen commonly limits plant growth. Soil nitrogen is typically available to plants in two inorganic forms: nitrate and ammonium. To better understand how nitrate and ammonium differentially affect plant metabolism and development, we performed transcriptional profiling of the shoots of ammoniumsupplied and nitrate-supplied Arabidopsis (Arabidopsis thaliana) plants. Seven genes encoding class III glutaredoxins were found to be strongly and specifically induced by nitrate. RNA silencing of four of these glutaredoxin genes (AtGRXS3/4/5/8) resulted in plants with increased primary root length (approximately 25% longer than the wild type) and decreased sensitivity to nitratemediated inhibition of primary root growth. Increased primary root growth is also a well-characterized phenotype of many cytokinin-deficient plant lines. We determined that nitrate induction of glutaredoxin gene expression was dependent upon cytokinin signaling and that cytokinins could activate glutaredoxin gene expression independent of plant nitrate status. In addition, crosses between "long-root" cytokinin-deficient plants and "long-root" glutaredoxin-silenced plants generated hybrids that displayed no further increase in primary root length (i.e. epistasis). Collectively, these findings suggest that AtGRXS3/4/5/8 operate downstream of cytokinins in a signal transduction pathway that negatively regulates plant primary root growth in response to nitrate. This pathway could allow Arabidopsis to actively discriminate between different nitrogen sources in the soil, with the preferred nitrogen source, nitrate, acting to suppress primary root growth (vertical dimension) in concert with its well-characterized stimulatory effect on lateral root growth (horizontal dimension).

Glutaredoxins (GRXs) are small oxidoreductase enzymes that can reduce disulfide bonds in target proteins. The class III GRX gene family is unique to land plants, and Arabidopsis thaliana has 21 class III GRXs, which remain largely uncharacterized. About 80% of A. thaliana class III GRXs are transcriptionally regulated by nitrate, and several recent studies have suggested roles for these GRXs in nitrogen signaling. Our objective was to functionally characterize two nitrate‐induced GRX genes, AtGRXS5 and AtGRXS8, defining their roles in signaling and development in the A. thaliana root. We demonstrated that AtGRXS5 and AtGRXS8 are primarily expressed in root and shoot vasculature (phloem), and that the corresponding GRX proteins display nucleo‐cytosolic subcellular localization. Ectopic expression of AtGRXS8 in transgenic plants caused major alterations in root system architecture: Normal primary root development, but a near absence of lateral roots. RNA sequencing demonstrated that the roots of AtGRXS8‐overexpressing plants show strongly reduced transcript abundance for many primary nitrate response genes, including the major high‐affinity nitrate transporters. Correspondingly, high‐affinity nitrate uptake and the transport of nitrate from roots to shoots are compromised in AtGRXS8‐overexpressing plants. Finally, we demonstrated that the AtGRXS8 protein can physically interact with the TGA1 and TGA4 transcription factors, which are central regulators of early transcriptional responses to nitrate in A. thaliana roots. Overall, these results suggest that AtGRXS8 acts to quench both transcriptional and developmental aspects of primary nitrate response, potentially by interfering with the activity of the TGA1 and TGA4 transcription factors.

A petrochemical plant producing terephthalic acid faced a saturation of its wastewater treatment facilities due to an increase in production. In fact, the plant has been growing in the last years, and the effluents have been treated by reproducing the original activated sludge design. However, due to lack of space, as well as energy consumption and sludge production reaching a certain level, the plant considered other options for coping with the new effluent flow and organic load. Based on the authors' previous experience with this wastewater, the consultant designed a process consisting in modifying an existing pond, in order to add an anaerobic step previous to the aerobic tanks already in operation. The anaerobic pond is a three stage process, all included in the same adapted basin, with a distribution system in the bottom of each stage that creates an upflow pattern. Terephthalic acid wastewater is a mixture of several organic acids, with different anaerobic degradation kinetics, acetic and benzoic acids being more rapidly removed; the staged design takes this into account. The first two stages have a plastic floating cover (5,813 m 3 and 8,719 m 3 volume, respectively), while the third one is a conventional UASB type reactor (6,276 m 3 volume) with a gas-liquid-solid separation device on top. The design wastewater flow is 230 m 3 /h, with 10,300 mg/l COD, a pH of 4.5 and a temperature of 40 0 C. There is an effluent recycling pump (510 m 3 /h) in order to control upflow velocities and eventual acidification problems in the first two stages. The reactor, seeded with anaerobically adapted waste sludge from the aerobic plant, is now under start up, with the expected performance.

No abstract

Glutaredoxins are oxidoreductase enzymes that can regulate the activities of target proteins through the reversible breakage of disulfide bonds. We recently identified a group of seven glutaredoxin genes that are upregulated by nitrate and control primary root length in the model plant Arabidopsis thaliana. The objective of this study was to characterize the gene expression patterns and subcellular localization of three nitrate‐regulated glutaredoxins: AtGRXS5, AtGRXS6, and AtGRXS8. Reporter lines consisting of the glutaredoxin promoter sequences fused to the β‐glucuronidase (GUS) gene were used to study glutaredoxin gene expression. Colorimetric GUS assays and histological sectioning showed that AtGRXS5, AtGRXS6, and AtGRXS8 are expressed exclusively in the phloem of A. thaliana roots and leaves. GUS activity assays demonstrated that AtGRXS6 and AtGRXS8 are strongly and specifically upregulated by nitrate. In contrast, AtGRXS5 gene expression was activated by both ammonium and nitrate. Nitrate concentrations as low as 50 μM activated glutaredoxin gene expression, with no further increase in expression at higher concentrations of nitrate. To characterize the subcellular localization of AtGRXS5, AtGRXS6, and AtGRXS8, translational fusions between the glutaredoxin gene and yellow fluorescent protein (YFP) were expressed in transgenic Arabidopsis and tobacco (Nicotiana benthamiana) plants. Preliminary results demonstrated that all three glutaredoxins are localized to cell nuclei as well as small, yet unidentified structures in the cytoplasm. Overall, these studies will further explain how plants control the growth of their root system to maximize the uptake of nitrogen from the soil, a behavioral response that is highly significant in agriculture.Support or Funding InformationNSF IOS grant 1651584NIH RISE GM‐64783This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.