Plant Signaling and Phytoplasma Response

Candidatus Phytoplasma is a genus of small wall-less, phloem-dwelling bacteria associated with more than 100 plant diseases world-wide. Phytoplasmas are transmitted by phloem sucking hemipterous insects belonging to Auchenorrhyncha and Psyllidae. The most problematic phytoplasma diseases in Europe are affecting fruit trees of the rose family and grapevines. These grapevine yellows diseases are associated with distinct phytoplasma species causing flavescence dorée (FD) and bois noir (BN; caused by stolbur phytoplasma). Due to the phloem restriction and bacterial nature, control of phytoplasma diseases is challenging. Also the management of the reservoir-hosts such as bindweed and the partly unknown vectors is a difficult task. However, grapevines might recover from phytoplasma symptoms for unclear reasons. Previous contacts to pathogens, to beneficial microorganism and chemicals acting as elicitors and secondary messengers can induce resistance in plants. In this state, plants respond to subsequent infections of a wide array of pathogens by enhanced defense responses leading to reduction of symptoms and milder infections. Very little is known, if induced resistance and treatment with beneficial microorganisms are also effective against phytoplasmas. Also, it is unclear, which plant defense pathways are involved in resistance to phytoplasmas and in the phenomenon of recovery. In the current project proposal we plan to investigate if inducing resistance in plants will lead to conditions limiting phytoplasma growth and proliferation. To test the hypothesis, plants will be treated with resistance inducing chemicals or beneficial bacteria and graft-inoculated with phytoplasma infected scions. Samples taken in time series will allow the determination of phytoplasma DNA content with quantitative PCR in various distances of the graft junction. The involvement of different pathways contributing to plant defense responses will be also established using tomato mutants impaired in defense and hormone signaling. These investigations should help to enlighten the role of active plant defense in phytoplasma infections. Closely related but distinct stolbur phytoplasma strains occur in different herbaceous host and with distinct geographic distribution. Genome sequencing and comparison of these strains should give valuable information of phytoplasma genes and sequence elements involved in host specifications and specific virulence. Phytoplasma gene expression studies of these genes and genes with signaling sequences encoding for excreted proteins will allow evaluation of potential effectors necessary for the establishment of phytoplasma in host and vector interactions.

The stolbur phytoplasma is the cause of black wood "Bois noir" of grapevine and is transmitted through phloem-sucking insects of the order Hemiptera from herbaceous reservoir plants to grapevine. The disease causes economic damage in European winegrowing regions. Phytoplasmas cannot be cultured in a conventional manner in the laboratory and therefore the characterization of the pathogen is difficult. Within the project, different strains were characterized with cultivation independent methods and 14 different genotypes were described for Austria. Due to the genetic characterization and a high incidence of the stolbur transmitting Hyalesthes obsoletus, it could be shown that in the current bois noir epidemic, phytoplasmas are transmitted from nettle to grapevine and a single genotype is responsible for more than two-thirds of the diseased grapevines. The elucidation of the propagation path has important implications for possible disease control strategies. The most common genotype and 5 other strains were transferred by means of insects on experimental plants (tomatoes and Madagascar periwinkle) and the strains were completely sequenced. The different strains show distinct symptoms in the experimental plants. Together with the genome data this provides interesting evidence for the analysis of factors which are responsible for the symptoms caused by the disease. In this context, also expression analyzes were performed, which point to important factors in the interaction phytoplasma - plant and insect vector. Using the experimental plants, laboratory experiments were carried out which show the effect of plant hormones and plant-associated bacteria on the disease development. In summary, the results eventually lead to a better understanding of the mechanisms responsible for the symptom manifestations in phytoplasma - infected plants.