Rosemary Loria was born and raised in Detroit, Michigan. She received her B.S. degree in 1974 from Lyman Briggs College, Michigan State University, and her M.S. (1977) and Ph.D. (1980) degrees also from Michigan State University. In 1980, Loria became an assistant professor in the Department of Plant Pathology at Cornell University. She was promoted to the rank of associate professor in 1986 and professor in 1996. At Cornell, Loria was initially stationed at the Long Island Horticultural Research and Extension Center in Riverhead, New York, where she worked on a variety of diseases of potato. In 1988, her research program moved to the main campus in Ithaca and began focusing on potato scab caused by Streptomyces scabies and other Streptomyces species. In 2011, she moved her program to the University of Florida, where she became professor and chair of the Department of Plant Pathology.
During the past 30 years, experimentally amenable gram-negative bacteria have become preeminent models in molecular plant pathology, while the equally important but more recalcitrant gram-positive plant pathogens have received less attention. Loria’s research on gram-positive Streptomyces species has played a major role in correcting this imbalance and is providing new tools and a picture of virulence that is as comprehensive and mechanistically detailed as that for any gram-negative phytopathogen. Her early work on S. scabies and related species focused on taxonomy and population dynamics, but in 1995, she published a paper in Phytopathology describing the production of the known S. scabies phytotoxin thaxtomin A in culture and a tuber slice bioassay for detecting thaxtomins and identifying pathogenic Streptomyces strains. With these tools in hand, Loria began exploring the biosynthesis and mode of action of thaxtomin, the existence of novel virulence factors, and the genetics (and eventually genomics) of virulence in Streptomyces.
Because thaxtomin is a dipeptide (composed of phenylalanine and a nitrated tryptophan), Loria and her team postulated that its synthesis involved a nonribosomal peptide synthetase (NRPS). Following this hunch, they successfully used degenerate oligonucleotide primers corresponding to a conserved NRPS domain to clone the txtA and txtB genes. By mutating txtA and then complementing the mutation, they obtained the first genetic evidence that thaxtomin production plays a critical role in pathogenesis. They then sequenced DNA in the regions flanking txtA and txtB, which included a gene encoding a protein with high similarity to mammalian nitric oxide synthases (NOS). Loria’s team, including collaborators Donna M. Gibson (USDA-ARS) and Brian R. Crane (Cornell), reported in 2004 in Nature that the Streptomyces NOS is essential for thaxtomin production; they subsequently reported in Nature Chemical Biology that TxtE, a unique cytochrome P450, uses NO and O2 to catalyze the nitration of tryptophan; the resulting 4-nitrotryptophan represents a novel NRPS substrate for thaxtomin production.
Loria also noted that the ability of thaxtomin to suppress seedling growth, inhibit cytokinesis, and cause plant cell hypertrophy suggested disruption of cell wall biosynthesis. This hypothesis was confirmed through collaboration with Chris R. Somerville, at Stanford University, which revealed thaxtomin to inhibit 14C-glucose uptake into the cellulosic fraction of Arabidopsis thaliana cell walls. Interestingly, cellotriose is released from rapidly growing plant tissues, and the Loria group discovered that cellotriose induces thaxtomin production in pathogenic Streptomyces species, which is consistent with the susceptibility of these tissues.
Loria and her team also determined that the genetic basis for thaxtomin production is associated with a pathogenicity island that is present in the few Streptomyces species that are plant pathogenic. The horizontal transfer of this pathogenicity island likely underlies the emergence of new pathogenic species, such as S. turgidiscabies, and suggests limits on the durability of cultural practices to manage potato scab. The Loria group was able to observe, under laboratory conditions, the mobilization of the S. turgidiscabies pathogenicity island to the nonpathogen S. diastatochromogenes and, with it, the ability to produce thaxtomin and colonize excised potato tuber tissue. The 2005 Molecular Microbiology paper describing this remarkable finding was also the first report of a pathogenicity island in a gram-positive plant pathogen.
The Loria group has identified additional virulence determinants, which appear to be horizontally acquired and are represented in the S. turgidiscabies pathogenicity island that harbors the thaxtomin biosynthesis genes. These include (i) Nec1, a secreted protein that is produced by most scab-causing Streptomyces species (but not by other gram-positive plant pathogens) and triggers necrosis in a wide range of plant species; (ii) cytokinin biosynthesis genes homologous to the Rhodococcus fascians plant fasciation (fas) operon; (iii) a secreted saponinase that is predicted to cleave defense-associated plant saponin glycosides that can complex with sterols in eukaryotes, a homolog of which is found in another gram-positive plant pathogen, Clavibacter michiganensis subsp. michiganensis; and (iv) genes directing the biosynthesis of coronafacic acid that are homologous to those contributing to phytotoxin coronatine production in Pseudomonas syringae. Thus, Loria’s work with Streptomyces is providing a broader view of virulence mechanisms in gram-positive and other bacterial plant pathogens.
Loria has also held major administrative positions at Cornell and the University of Florida. At Cornell, she served as chair of the College of Agriculture and Life Sciences Faculty Senate from 1997 to 1998. She served for 8 months in 1999 as associate director of the Office for Research and of the Cornell University Agricultural Experiment Station and then, from 1999 to 2005, as chair of the Department of Plant Pathology at Ithaca. In addition, from 2002 to 2005, she was director of the Northeast Plant Diagnostic Network. Since 2011, she has been serving as chair of the Department of Plant Pathology at the University of Florida. She has maintained a remarkably productive research program during her administrative service and, therefore, has achieved remarkable success both in research and administration.
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