Dissecting the role of iron in the interaction between the host plant tomato and ‘Candidatus Phytoplasma solani’

Phytoplasmas are prokaryotic plant pathogens that colonize the sieve elements of the host plant, causing alteration in phloem function and impairment of assimilate translocation. Despite the huge impact on agriculture and the lack of effective curative strategies, mechanisms underlying plant host-phytoplasma interaction are still largely unexplored. In particular, no knowledge is available on the role of iron (Fe) in this interaction. Iron is an essential element for most living organisms, and competition for it can lead, as already observed in different pathosystems, to the development of an Fe-withholding response by plants that changes Fe distribution and trafficking. In the current study, we investigated on the role of Fe in the interaction between tomato and ‘Candidatus Phytoplasma solani’ by analyzing healthy plants (H/+Fe), Fe-starved plants (H/-Fe), phytoplasma-infected plants (I/+Fe) and phytoplasma-infected/Fe-starved plants (I/-Fe). Firstly, an experimental system was set up so that phytoplasma infection and occurrence of Fe deficiency symptoms were concomitant. Then, high-throughput RNA-sequencing focused on midrib-enriched tissue was conducted to profile leaf transcriptome changes in both stresses. We found that most of differentially regulated genes in common to I/+Fe and H/-Fe plants encode proteins involved in photosynthetic light reactions, in porphyrin and chlorophyll metabolism, and in carotenoid biosynthesis. These similarities supported the hypothesis that phytoplasma might induce alteration in cellular Fe homeostasis. Even if no significant difference in total Fe concentration emerged when comparing H and I plants under both nutritional conditions, the phytoplasma presence caused local modifications of Fe distribution visible by Perls’-DAB staining, with a shift from the leaf lamina to the site of infection (the phloem). Similar to healthy (H/+Fe), Fe dots were localized to the phloem in the infected leaves (I/+Fe), but lacked in xylem parenchyma cells similar to H/-Fe leaves. Moreover, in both stresses the mesophyll palisade cells of the leaf lamina had fewer Fe dots than in H/+Fe condition. We examined the activity of genes involved in the Fe uptake and Fe homeostasis in roots. Under Fe-sufficient conditions, the phytoplasma apparently did not alter the acquisition mechanism. Under Fe-deficient conditions, the phytoplasma reduced the expression of all the examined genes, except for FRO1. These findings suggest that, under Fe-deficient conditions, the presence of phytoplasmas may compromise the communication of the Fe status between leaves and roots, possibly by the interference with the synthesis or transport of a promotive signal.