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

Phytoplasmas are plant pathogenic bacteria belonging to the Mollicutes group. They are responsible for important diseases in many crops, such as Stolbur in tomato, grapevine and lavender. Phytoplasmas are phloem-restricted pathogens, transmitted by grafting, insects or dodder. Phytoplasmas are known to impair phloem transport causing severe consequences to the entire plant (Christensen et al., 2005). Because it is not possible to cultivate phytoplasmas in vitro and because the phloem tissue is difficult to observe and analyze, many traits of the interactions with host plants are still unclear and need to be investigated. The phloem is a complex tissue controlling the allocation of photoassimilates from source to sink organs. Hormones, ions and macromolecules are also transported making this tissue a superhighway for long distance signaling. The phloem transport network consists of sieve tubes, which are the primary site of multiplication of phytoplasmas. The phloem sap content is mainly composed of sugars, such as sucrose, in addition to amino acids, organic acids and a range of secondary metabolites involved in signaling and defense. The driving force for phloem mass flow of sieve tubes 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 have been proposed to be hijacked by phytoplasmas for their nutrition and multiplication during infection and systemic colonization. Our aim is to investigate the trophic relationships between the phytoplasma and its host. Tomato is a model plant to study 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). Moreover it has been shown that tomato plants infected by phytoplasmas showed abnormal development and modulation of genes involved in sucrose metabolism and transport (De Marco et al., 2016). We investigated the effect of ‘Candidatus phytoplasma solani’ infection, the stolbur phytoplasma, on wild type and transgenic SUT1 and SUT2 tomato antisense lines. We combined the observations of plant anatomy and phloem organization, analyses of phloem sap metabolite profiles and studies on gene expression. The results and their implications for plant phytoplasma interactions will be presented