Alexander H. Purcell

Research

      My research concerns the ecology and evolution of three-way interactions among bacteria, insects, and plants. We continued pioneering progress in the study of mutualistic symbioses in a model system centered around the pea aphid, where both bacteria and insect are obligate partners. My lab's emphasis has been the "secondary symbionts" of the pea aphid. We showed that these parasitic bacteria are common but not present in all pea aphids nor essential to their survival, as are the primary symbionts. In fact the secondary symbionts can be harmful (pathogens) or beneficial (mutualists) to the aphid host, depending on the aphid's host plant and temperatures. These auxiliary bacteria are transovarially transmitted at high rates, so in essence they make up part of the aphid's genome. The pea aphid can have 6 heritable genomic units in addition to its own chromosomal DNA (4 bacteria, a phage that attacks one bacterium, and mitochondria), a far more complex situation than generally assumed.

      Pytoplasmas and spiroplasmas are bacteria that are very diverse and commonly occur in insects, though they are usually overlooked because of their extremely small size and lack of a cell wall. My research on insect-associated microbes in this group has mainly involved those causing disease to crop plants, but most members of this class (Mollicutes) do not cause disease. Their roles in insects and plants are little understood. My work has helped to develop control methods for diseases caused by these organisms in tree cherry and peach, mostly through improved understanding of vector ecology.

      During the past 10 years, I returned to research on the insect-transmitted bacterium, Xylella fastidiosa, which causes Pierce's disease of grape and other less well known (in California) plant diseases. This bacterium occurs in natural areas far from agriculture. Pierce's disease continues in epidemic form in coastal California and has now entered an extremely dangerous phase because of the introduction to California of a new vector species that threatens to eliminate numerous vineyards in some parts of the state. The new vector, the glassy-winged sharpshooter, now threatens the survival of some wine-producing districts of southern California. Its recent detection in the Central Valley threatens to cause extremely high losses to disease in grapes and almonds in northern California, with some local extinction of these crops. My research on X. fastidiosa continues as the main focus of our lab.

      One of the major difficulties in working with Pierce's disease in the field is that multiple years are needed to collect meaningful data on the relationship of insects to disease spread. Lab experiments take months to complete because of the months-long incubation period for disease symptoms to appear. With collaborators, we developed and implemented a method of drastically reducing populations of the principal Pierce's disease vector in coastal California by managing riparian vegetation that serves as the primary habitat of this insect. These reductions have been greater than those achieved by insecticide treatments of riparian areas. At the same time we have the opportunity to improve many degraded riparian habitats with revegetation. Because our research required the removal and replacement of trees, we are only now at the publication stage. Similarly, over three years of field and lab studies of the population behavior of X. fastidiosa in vines in the field is now being readied for publication.

      I am collaborating with colleagues in Brazil to capitalize on the expected completion of the genome sequence of X. fastidiosa, which will be published this fall. Over the past few years, I have visited Brazil annually to consult with colleagues there on the citrus variegated chlorosis disease caused by X. fastidiosa. We recently published a paper on why vector transmission from citrus to citrus of this strain of the bacterium is so low compared to other Xylella diseases. With the genome project, we hope to understand the biofilm characteristics of this bacterium, especially how it attaches in the high velocity fluid environment of the vectors' mouthparts, a phenomenon that is critical to understanding vector transmission. Many of the genes in X. fastidiosa that have already been identified in the genome project are homologous with attachment and movement genes in related bacteria. We are making exciting progress towards understanding how freezing in eliminates X. fastidiosa from grapevines. We have developed tools to study how the bacterium moves systemically in plants, a key characteristic of its ability to cause plant disease. We are also exploring biological control of X. fastidiosa with bacterial viruses and antagonistic bacteria.

Major research accomplishments

Xylella fastidiosa

• Part of team to first culture Xylella fastidiosa and prove it caused Pierce's disease, contradicting long-held views that it was a virus or another bacterium.

• Showed that the bacterium Xylella fastidiosa is transmitted from the foreguts of its insect vectors.

• With collaborators, demonstrated that spring infections of grape with Xylella fastidiosa were the most critical for establishing chronic Pierce's disease (lethal). Later infections do not persist until the following year. This also explains variation among varieties in resistance to Pierce's disease.

• Explained the lack of vine to vine spread of Pierce's disease.

• Developed vegetation management and restoration methods to reduce Pierce's disease near riparian habitats.

• Discovered that freezing was therapeutic for Pierce's disease and suggested why the disease is restricted geographically.

• Demonstrated the variety of fates that Xylella fastidiosa can have in numerous plant species, suggested their role in the epidemiology of this bacterium.

• Proved that a new strain of Xylella fastidiosa causes oleander leaf scorch disease.

Phytoplasmas and Spiroplasmas

• Provided evidence that peach yellow leaf roll disease had a psyllid vector rather than a leafhopper vector, as had long been assumed.

• Established ecological and epidemiological data on X-disease of peach and cherry in California.

• With collaborators, determined that removing diseased trees was the most effective control method for X-disease of cherry.

• Showed that phytoplasmas can multiply in non-vectors and have dramatic beneficial effects on non-vectors.

Transovarially-transmitted parasitic bacteria

• Cultured a secondary symbiotic" bacterium from a leafhopper, showed how it was transmitted horizontally via plants without multiplying in plants and that it was pathogenic to its host.

• With Deqiao Chen, described two secondary symbionts of pea aphid and rose aphid, their occurrence in California pea aphids, and that effects on host aphids varied from harmful to beneficial depending on host plant and temperature.