Professional Education

  • Bachelor of Arts, Gustavus Adolphus College (2012)
  • Doctor of Philosophy, Iowa State University (2017)

Lab Affiliations

All Publications

  • D2O Labeling to measure active biosynthesis of natural products in medicinal plants AICHE JOURNAL Nett, R. S., Guan, X., Smith, K., Faust, A., Sattely, E. S., Fischer, C. R. 2018; 64 (12): 4319–30

    View details for DOI 10.1002/aic.16413

    View details for Web of Science ID 000449983800019

  • A Third Class: Functional Gibberellin Biosynthetic Operon in Beta-Proteobacteria. Frontiers in microbiology Nagel, R., Bieber, J. E., Schmidt-Dannert, M. G., Nett, R. S., Peters, R. J. 2018; 9: 2916


    The ability of plant-associated microbes to produce gibberellin A (GA) phytohormones was first described for the fungal rice pathogen Gibberella fujikuroi in the 1930s. Recently the capacity to produce GAs was shown for several bacteria, including symbiotic alpha-proteobacteria (α-rhizobia) and gamma-proteobacteria phytopathogens. All necessary enzymes for GA production are encoded by a conserved operon, which appears to have undergone horizontal transfer between and within these two phylogenetic classes of bacteria. Here the operon was shown to be present and functional in a third class, the beta-proteobacteria, where it is found in several symbionts (β-rhizobia). Conservation of function was examined by biochemical characterization of the enzymes encoded by the operon from Paraburkholderia mimosarum LMG 23256T. Despite the in-frame gene fusion between the short-chain alcohol dehydrogenase/reductase and ferredoxin, the encoded enzymes exhibited the expected activity. Intriguingly, together these can only produce GA9, the immediate precursor to the bioactive GA4, as the cytochrome P450 (CYP115) that catalyzes the final hydroxylation reaction is missing, similar to most α-rhizobia. However, phylogenetic analysis indicates that the operon from β-rhizobia is more closely related to examples from gamma-proteobacteria, which almost invariably have CYP115 and, hence, can produce bioactive GA4. This indicates not only that β-rhizobia acquired the operon by horizontal gene transfer from gamma-proteobacteria, rather than α-rhizobia, but also that they independently lost CYP115 in parallel to the α-rhizobia, further hinting at the possibility of detrimental effects for the production of bioactive GA4 by these symbionts.

    View details for PubMedID 30546353

    View details for PubMedCentralID PMC6278637

  • phytohormone with wide distribution in the bacterial rice leaf streak pathogen Xanthomonas oryzae pv. oryzicola. New phytologist Nagel, R., Turrini, P. C., Nett, R. S., Leach, J. E., Verdier, V., Van Sluys, M., Peters, R. J. 2017; 214 (3): 1260-1266


    Phytopathogens have developed elaborate mechanisms to attenuate the defense response of their host plants, including convergent evolution of complex pathways for production of the GA phytohormones, which were actually first isolated from the rice fungal pathogen Gibberella fujikuroi. The rice bacterial pathogen Xanthomonas oryzae pv. oryzicola (Xoc) has been demonstrated to contain a biosynthetic operon with cyclases capable of producing the universal GA precursor ent-kaurene. Genetic (knock-out) studies indicate that the derived diterpenoid serves as a virulence factor for this rice leaf streak pathogen, serving to reduce the jasmonic acid-mediated defense response. Here the functions of the remaining genes in the Xoc operon are elucidated and the distribution of the operon in X. oryzae is investigated in over 100 isolates. The Xoc operon leads to production of the bioactive GA4 , an additional step beyond production of the penultimate precursor GA9 mediated by the homologous operons recently characterized from rhizobia. Moreover, this GA biosynthetic operon was found to be widespread in Xoc (> 90%), but absent in the other major X. oryzae pathovar. These results indicate selective pressure for production of GA4 in the distinct lifestyle of Xoc, and the importance of GA to both fungal and bacterial pathogens of rice.

    View details for DOI 10.1111/nph.14441

    View details for PubMedID 28134995

  • Characterization of CYP115 As a Gibberellin 3-Oxidase Indicates That Certain Rhizobia Can Produce Bioactive Gibberellin A(4) ACS CHEMICAL BIOLOGY Nett, R. S., Contreras, T., Peters, R. J. 2017; 12 (4): 912-917
  • Elucidation of gibberellin biosynthesis in bacteria reveals convergent evolution NATURE CHEMICAL BIOLOGY Nett, R. S., Montanares, M., Marcassa, A., Lul, X., Nagel, R., Charles, T. C., Hedden, P., Rojas, M. C., Peters, R. J. 2017; 13 (1): 69-74


    Gibberellins (GAs) are crucial phytohormones involved in many aspects of plant growth and development, including plant-microbe interactions, which has led to GA production by plant-associated fungi and bacteria as well. While the GA biosynthetic pathways in plants and fungi have been elucidated and found to have arisen independently through convergent evolution, little has been uncovered about GA biosynthesis in bacteria. Some nitrogen-fixing, symbiotic, legume-associated rhizobia, including Bradyrhizobium japonicum-the symbiont of soybean-and Sinorhizobium fredii-a broad-host-nodulating species-contain a putative GA biosynthetic operon, or gene cluster. Through functional characterization of five unknown genes, we demonstrate that this operon encodes the enzymes necessary to produce GA9, thereby elucidating bacterial GA biosynthesis. The distinct nature of these enzymes indicates that bacteria have independently evolved a third biosynthetic pathway for GA production. Furthermore, our results also reveal a central biochemical logic that is followed in all three convergently evolved GA biosynthetic pathways.

    View details for DOI 10.1038/NCHEMBIO.2232

    View details for Web of Science ID 000393267200015

    View details for PubMedID 27842068

  • Labeling Studies Clarify the Committed Step in Bacterial Gibberellin Biosynthesis ORGANIC LETTERS Nett, R. S., Dickschat, J. S., Peters, R. J. 2016; 18 (23): 5974-5977


    Bacteria have evolved gibberellin phytohormone biosynthesis independently of plants and fungi. Through (13)C-labeling and NMR analysis, the mechanistically unusual "B" ring contraction catalyzed by a cytochrome P450 (CYP114), which is the committed step in gibberellin biosynthesis, was shown to occur via oxidative extrusion of carbon-7 from ent-kaurenoic acid in bacteria. This is identical to the convergently evolved chemical transformation in plants and fungi, suggesting a common semipinacol rearrangement mechanism potentially guided by carbon-4α carboxylate proximity.

    View details for DOI 10.1021/acs.orglett.6b02569

    View details for Web of Science ID 000389396100002

    View details for PubMedID 27934361