Bruce C. Kirkpatrick
Bruce C. Kirkpatrick was born in San Jose, CA. He received a B.S. degree in biological sciences from the University of California, Irvine, in 1980 and M.S. and Ph.D. degrees in plant pathology from the University of California, Berkeley, in 1983 and 1986, respectively. In 1986, he was appointed assistant professor in the Department of Plant Pathology, University of California, Davis, and was promoted to associate professor in 1992. The primary focus of his research has been molecular detection and characterization of mycoplasma-like organisms (MLOs) and development of control strategies for MLO-diseases of fruit and nut trees. Western X-disease MLO (WXMLO) caused heavy losses in cherry and peach orchards. Dr. Kirkpatrick’s approach to the problem has been systematic, first developing techniques to detect and characterize the pathogens involved, studying their epidemiology, and developing management strategies to minimize losses.
Plant-pathogenic MLOs have been nearly impossible to manipulate in the laboratory. Electron microscopy (EM) has been used to detect MLOs in diseased plants but cannot distinguish MLOs pleiomorphically or morphologically. MLO-specific serological reagents have been produced, but they have limited utility for comparing pathologically diverse MLOs. In 1987, Dr. Kirkpatrick was the first to develop methods for isolating and cloning MLO DNA by raising WX-MLO– infected leafhoppers and isolating partially purified fractions containing the MLO. DNA was isolated from the MLO-enriched fraction, and an MLO-specific, low-density DNA band was recovered. This DNA was cloned, and fragments of the WX-MLO chromosome were identified by differential hybridization with healthy and WX-MLO plant and leafhopper DNA probes. Cloned fragments of WX-MLO DNA were used as hybridization probes to detect the pathogen in field collections of herbaceous and woody plants, as well as infected insect vectors. Hybridization analyses with cloned portions of the WX-MLO genome provided the first insights into the genetic relationships between MLOs that infect plants. The ability to isolate and clone MLO DNA paved the way for genetic characterization of other plant MLOs and enabled cloning and sequencing of evolutionarily conserved genes, which allowed definitive classification.
In 1989, Dr. Kirkpatrick and his students began cloning and sequencing the 16S ribosomal RNA (rRNA) genes of several MLOs to determine the phylogenetic relationships among phytopathogenic MLOs and other members of the class Mollicutes. In 1989, researchers published the first full-length sequence of an MLO 16S rRNA. Analyses of the sequence, as well as sequences of the WX- and western aster yellows MLOs determined in Kirkpatrick’s laboratory, clearly established that MLOs are members of the class Mollicutes, but they are phylogenetically different from previously characterized mollicutes. Cloning and sequencing of full-length 16S rRNA genes is a laborious task that limits the total number of MLOs that can be rapidly characterized. Dr. Kirkpatrick investigated the possibility of using other evolutionary markers to more rapidly classify MLOs. He found that phylogenetic relationships based on the sequences of the spacer region (SR) separating the 16S and the 23S rRNA genes agreed with relationships based on full-length 16S rRNA sequences. Because the SR was only one-fifth the size of the 16S rRNA, it was much easier to characterize MLOs. As a result, the phylogenetic relationships of more than 70 MLOs recently have been determined in his laboratory.
In 1993, collaborative research by Kirkpatrick and H. Neimark at SUNY, Brooklyn, resulted in the first isolation and characterization of full-length MLO chromosomes. This technique, involving linearization of chromosomes and separation by pulse-field gel electrophoresis (PFGE) contributed important information on MLO genomes and provided a source of MLO DNA completely free of contaminating host DNA. The ability to characterize MLO chromosomes by PFGE also permitted the construction of the first genomic map of an MLO.
In the field, Dr. Kirkpatrick used molecular probes in epidemiological studies to identify important plant reservoirs and insect vectors of the WX-MLO in California. Collaborative studies done over several years with colleagues at UC Berkeley and Davis determined the population dynamics of the primary WX-MLO insect vectors. Management guidelines were established, and the current incidence of WXMLO in sweet cherry orchards in California is low. Dr. Kirkpatrick’s recent investigations on peach yellow leafroll (PYLR) showed that two genetically distinct MLOs induce symptoms in peach trees. One of the MLOs is genetically similar to the WX-MLO, whereas the other is very similar to the MLO that causes pear decline disease. Most PYLR-diseased trees in California are infected with the pear decline-type MLO and management strategies to prevent its insect vector from infecting peach are currently being evaluated.
Dr. Kirkpatrick welcomes collaboration and has hosted scientists from around the world. He is more interested in the advancement of the MLO field than in personal advancement and routinely provides researchers with unpublished procedures and data. He uses his enthusiasm and influence to promote innovative research on MLOs and is a recognized leader. Dr. Kirkpatrick’s accomplishments have stimulated research on phytopathogenic MLOs throughout the world. Dr. Kirkpatrick is the chair of the MLO working team of the International Organization for Mycoplasmology (IOM) and is a member of the Subcommittee on Taxonomy of Mollicutes. The IOM working team has proposed that the more definitive term, Phytoplasma, be adopted in place of MLO.
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