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​2020 APS Lee M. Hutchins Award​

This award is presented ​to the author or authors of published research on basic or applied aspects of diseases of perennial fruit plants (tree fruits, tree nuts, small fruits, and grapes, including tropical fruits, but excluding vegetables).

Caroline Roper
University of California, Riverside

Caroline Roper earned her BS degree from the University of South Carolina and her PhD in plant pathology from the University of California, Davis. She is currently a faculty member at University of California, Riverside in the Department of Microbiology and Plant Pathology. She is internationally recognized as one of the leaders in the research field of vector-transmitted bacterial diseases of fruit plants, including Pierce's disease of grapevine, and citrus Huanglongbing.

Roper's nomination for the Lee M. Hutchins award is based on a series of seven papers published in APS journals, including Molecular Plant-Microbe Interactions (MPMI) and Phytobiomes, authored by Roper within the last 10 years. Roper's research program is centered on understanding the biology of insect-vectored bacterial plant pathogens that live in the vascular tissue of plants specifically in the context of two specialty crops, citrus and grape, that are vital to the world agricultural economy. In grapes, she studies how the bacterial causal agent of Pierce's Disease (PD), Xylella fastidiosa, interacts with its host plant and how the host plant responds to this bacterial invasion in its xylem tissue. X. fastidiosa makes a wonderful model system for studying the biology of xylem-dwelling pathogens because it is limited to the xylem tissue of the plant. Because of this, one can truly dissect specific bacterial mechanisms of invasion and plant responses in the xylem. The xylem is a fascinating arena of plant-microbe interactions because the xylem is primarily non-living at maturity, but yet produces robust responses to pathogen ingress. One of her earliest manuscripts published in MPMI identified the first pathogenicity factor for this important pathogen. It was a foundational study on how X. fastidiosa relies on plant cell wall degrading activity to breach xylem pit membranes. Since then, Roper has proceeded to further elucidate the role of plant cell wall degrading enzymes in the biology of bacterial pathogens, which was published in MPMI as well. Most recently, her latest article in MPMI was highlighted in The American Phytopathological Society's (APS) International Society of Molecular Plant Microbe's Interactions newsletter. In addition, Roper's research on X. fastidiosa has elegantly shown that the pathogen uses a portion of its cell surface (lipopolysaccharide O-antigen) to shield itself from the grape immune system to ensure successful xylem invasion. In fact, this camouflaging technique effectively masks the bacterium from initially being detected as a biotic stress to the plant and, as such, the plant does not deploy effective immune responses to combat the pathogen. This work was published in MPMI and was chosen as a senior editor's pick to highlight that issue of the journal.

In addition to studying the binary interactions that pathogens engage in with their hosts, Roper incorporates studies of the host-pathogen interactions in the broader context of the entire microbiome (total microbial community) associated with that plant. Her research focus in this area is to identify microbes that behave as either allies or adversaries to the host during the pathogen invasion process in the context of PD in grapevines. Interestingly, in vineyards with high incidence of PD, there are examples of individual vines exhibiting either no symptoms or very mild PD symptoms yet are surrounded by heavily infected vines. These asymptomatic vines as referred to as “disease-escaped." All vines in a vineyard are genetically identical (clonal) so the differences in tolerance to PD are unlikely to be attributed to the genetics of the plant. She and colleagues identified microorganisms that negatively correlate with X. fastidiosa abundance in the grapevine endosphere, particularly a group of Pseudomonads. She has published this work in the inaugural issue of the new APS journal, Phytobiomes. Representatives of these Pseudomonads are now being tested as biocontrol agents against Pierce's disease in the shared ecological niche of the xylem. She has also found that several associated fungal endophytes produce natural products that are inhibitory to X. fastidiosa.

Roper is also applying these same comprehensive microbiome studies to understand citrus Huanglongbing (HLB), a serious disease that is threatening to end citrus production in the United States, particularly in Florida. HLB has not yet established itself in California, so time is of the essence to understand what impact HLB may have on California citriculture to prevent the huge crop losses that are occurring in Florida. Like in the grapevine-PD system, there are trees that develop symptoms slower than their clonal counterparts (termed survivor trees) despite being under tremendous disease pressure in Florida. Roper is characterizing the microbiomes of these trees and has published this work in the Phytobiomes journal. Roper also serves as an associate editor for Phytobiomes. Her contribution to controlling HLB disease attracted a $5M research grant from the United States Department of Agriculture National Institute of Food and Agriculture. Her goal is to determine if microorganisms are responsible for the HLB-tolerance exhibited by these survivor trees, and if they can be harnessed for disease control.

In summary, the goal of Roper's research program is to understand the biology of the intricate interactions of pathogens with their hosts and other microbes that make up the plant host's microbiome. She is solidly dedicated to promoting the field of plant pathology on an international research platform and in the classroom from K-12 to higher education. In her own laboratory, she is training the next generation of scientists in agriculture and is continually amazed at their innovative and new ideas that are pushing the research in her lab forward as well as in the field of plant pathology. She is grateful for the hard work and dedication of all the students, postdoctoral scholars, and senior scientists that she has had the opportunity to work with thus far in her career.​

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