Resistance of grapevine and potato against phytoplasmas

The bacterium Candidatus Phytoplasma solani is responsible for a widespread and problematic grapevine disease named Bois Noir. The very same phytopathogenic phytoplasma also infects plants from the nightshade family, including tomato and especially potato causing there stolbur disease. Evidence for resistance against Ca. Phytoplasma solani has not been reported neither in grapevine nor in potato, but differences in susceptibility between cultivars have been observed on the basis of disease incidence, symptom severity, as well as plants ability to recover. As there is no highly effective treatment against phytoplasma infection in grapevine or potato available at the moment and current strategies are relying on pesticide use against transmitting vectors which are active for several weeks, obtaining tolerant cultivars is crucial for future sustainable production and reduction of pesticide use. Loss of function of a plant susceptibility gene involved in compatible interaction that is required by the pathogen for the infection can limit the ability of the pathogen to cause disease. Previous transcriptome analysis in phytoplasma infected grapevine suggests the candidate S gene DMR6 as a promising target for mutagenesis with a good perspective that its deactivation will increase the resistance to phytoplasma and possibly other pathogens. The gene is responsible for deactivation of the plant hormone salicylic acid, which is a crucial component of plant defence against biotrophic pathogens. In the project potato and grapevine lines defective in DMR6 function and other S genes with potentially related functions will be evaluated for their resistanceolerance to Ca. Phytoplasma solani after graft and insect infection. Disease development will be monitored phenotypically and by evaluating phytoplasma titer to obtain knowledge on the role of DMR6 in plant defense against phytoplasma disease. RNA sequencing will be performed in these lines and controls with and without Ca. Phytoplasma solani to analyse the overall effect of DMR6 function on (defense) gene expression. Expression will be verified by quantitative RT- PCR in time series experiments. The experiments will lead to knowledge on the role of the DMR6 S gene family for phytoplasma infection and disease development. Moreover, expression analysis will reveal the degree of pleiotropic effects on gene expression and the effects phytoplasma infection has on this expression pattern. Altogether, this can open up possibilities for better understanding of plant defense and lead to breeding programs and techniques to obtain improved potato and grapevine cultivars.

The bacterium 'Candidatus Phytoplasma solani' (the stolbur phytoplasma) is associated with the widespread grapevine disease named "Bois Noir". The very same phytopathogenic phytoplasma also infects plants from the nightshade family, including tomato and especially potato as well as celery plants causing there "stolbur" disease. There is no evidence for true resistance against stolbur, but differences in susceptibility between cultivars have been observed on the basis of disease incidence, symptom severity and the plant's ability to recover. As there is no highly effective treatment against phytoplasma infection in grapevine or potato available at the moment. Current strategies are relying on pesticide use against transmitting vectors and we have shown that the efficacy of pesticide use has dependent on disease pressure limited value. Hence, obtaining tolerant cultivars is crucial for future sustainable production and reduction of pesticide use. Loss of function of a plant susceptibility gene involved in compatible interaction that is required by the pathogen for the infection can limit the ability of the pathogen to cause disease. Deactivation of the candidate S gene DMR6 potentially increases the resistance to phytoplasma and possibly other biotrophic pathogens. In the frame of the project in cooperation with partners mutagenized DRM6 potatoes and grapevines in both variants have been infected with Phytoplasma by infected insect vectors in a designed choice set up. Results show increased tolerance in two grapevine lines. In potato and grapevine graft infection was less successful and did not result in sufficient numbers of infected plants. RNA isolation and RNA sequencing was performed to analyse the overall effect of DMR6 function on defense gene expression. The strength and reproducibility of the expression pattern will be further evaluated and estimate the trade-off of enhanced tolerance or susceptibility to specific pathogens and growth with the aim to lead to breeding programs and techniques to obtain improved potato and grapevine cultivars. Moreover, this study focussed on comprehensive investigation of the physiological roles of potential phytoplasma effectors and pathogenicity factors in grapevine. Bioinformatic analysis identified several candidate proteins, some showing strain-specific differences explaining differences in disease severity caused by different phytoplasma strains. Key proteins linked to pathogenicity, were found to be exposed to plant cells and implicated in altering host metabolism, particularly carbohydrate metabolism and the ascorbate-glutathione cycle. Additionally, a surface protein was linked to the formation of autophagosomes in plant cells. Overall, the study confirms that effectors of the stolbur phytoplasma manipulate plant metabolism to support infection and replication.