Interactions between the stolbur phytoplasma infection and the phloem functions in tomato

In higher plants, the allocation of photoassimilates is controlled by the phloem. Hormones, ions and macromolecules are also transported by phloem making it a superhighway for long distance signaling. The driving force for phloem mass flow results from the loading of carbohydrates in source organs. Key factors involved in the metabolism, loading and transport of sugars have been identified, and play a fundamental role in plant development. Such phloem functions can be hijacked by plant pathogens for their own benefit during infection and systemic colonization. Tomato is a model plant to investigate phloem activity and the relationships between source and sink organs. Transgenic lines altered in the expression of sucrose transporter genes (SUT1 and SUT2) are available (Hackel et al., 2006). Such plants showed abnormal development and modulation of genes involved in sucrose metabolism and transport. Phytoplasmas, which are phloem-restricted pathogens, are known to impair phloem transport causing severe consequences to the entire plant (Christensen, 2005). We have investigated the effect of ‘Candidatus phytoplasma solani’ infection on wild type and transgenic antisense SUT1 and SUT2 tomato lines. We have set up complementary approaches, including observations of plant anatomy and phloem organization, analyses of phloem sap metabolite profiles and studies on gene expression. The analyses revealed phloem hyperproliferation in infected plants and modulation of genes coding for proteins involved in sugars metabolism and callose synthesis. Moreover, one of the antisense lines showed a delay in symptoms appearance. The implications for plant phytoplasma interactions will thus be presented