Effect of Benzothiadiazole on Transmission of X-Disease Phytoplasma by the Vector Colladonus montanus to Arabidopsis thaliana, a New Experimental Host Plant

Phytoplasmas are cell-wall-less members of the bacterial class Mollicutes that are obligate parasites of plants and insect vectors. Phytoplasmas cause hundreds of plant diseases, several of which have worldwide agricultural significance (17). The plant host range of some phytoplasma strains may be very wide. This is the case for X-disease phytoplasma (Xp). Although X-disease is an economic disease of stone fruit trees such as cherry and peach (10,18), the pathogen infects a wide range of herbaceous plants (3,6,12,14) in both eastern and western North America. Using the polyphagous leafhopper (Hemiptera: Cicadellidae) vector Colladonus montanus (Van Duzee) (20), Frazier and Jensen (6) and Jensen (12) experimentally transmitted the western strain of Xdisease causal agent (then incorrectly assumed to be a virus) from infected celery to 12 different plant species. Chiykowski and Sinha (3) reported 17 host species of an eastern strain of X-disease vectored by Paraphlepsius irroratus (Say). We have recorded disease symptoms from numerous other plant species inoculated with infectious leafhopper vectors (A. H. Purcell, unpublished). The small and fast-growing crucifer Arabidopsis thaliana has become widely used in plant genetic studies, and its genome has been completely sequenced (2). Moreover, the small-sized and fastmaturing A. thaliana has numerous ecotypes that are available for researchers. A. thaliana may provide a useful tool to increase knowledge of molecular details of plant–phytoplasma interactions (5). To date, the beet leafhopper-transmitted virescence (BLTV) phytoplasma (5,8) and Spiroplasma citri (4,5) are recognized as mollicute pathogens of A. thaliana. Because phytoplasmas have never been cultured, molecular genetic methods have become indispensable tools for studies of phytoplasma biology and pathology. Systemic acquired resistance (SAR) is an inducible resistance mechanism in plants that provides resistance against a broad spectrum of plant pathogens: viruses, fungi, and bacteria. This mechanism of resistance is activated in several plant species by natural attacks of pathogens. Alternatively, chemicals (elicitors) have been developed to apply directly to plant tissues (13,15) to activate SAR. To our knowledge, no data are available on the effect of SAR on phytoplasma infections. This report describes our use of the leafhopper vector C. montanus and Xpinfected celery as a source of inoculum for the leafhopper, to test A. thaliana as an experimental host of Xp. Additionally using the elicitor benzo-(1,2,3)-thiadiazole-7carbothioic acid S-methyl ester (BTH), an inducer of SAR in A. thaliana (15), we assessed the effect of SAR in the Xp infection of A. thaliana inoculated by infective C. montanus.