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Degradation dynamics of photosynthetic pigments in flavescence dorée-infected grapevine (Vitis vinifera) leaves is linked to phytoplasma titre

Phytoplasmas, bacteria belonging to the genus ‘Candidatus Phytoplasma’, are known to be pathogenic for a wide variety of plant species. One of highly impacted plant cultures by phytoplasmas is grapevine (Vitis vinifera L.). Grapevine yellows are serious diseases affecting viticultural production worldwide. One of them is caused by the flavescence dorée phytoplasma (ribosomal group 16SrV), which is spread by the vector, leafhopper Scaphoideus titanus. Phytoplasmas are known to secrete effector molecules, which affect the physiology of their hosts, in order to increase their replication and spread. It is known that the flavescence dorée phytoplasma heavily affects photosynthetic pigments of grapevine, causing severe chlorosis in infected leaves. In this study, we sampled the leaves of symptomatic and asymptomatic plants to evaluate the dynamics of photosynthetic pigment content during the development of infection. Using real-time PCR, we confirmed the infection with the flavescence dorée phytoplasma in symptomatic plants and quantitatively determined the phytoplasma titre. Additionally, by amplification, sequencing and phylogenetic analysis of map gene amplicons, we detected two flavescence dorée genotypes in sampled grapevine leaves, M38 and M54, both belonging to the mapFD2 cluster. Spectrophotometrically, we analysed the effect of the flavescence dorée phytoplasma infection on photosynthetic pigment content in grapevine and its dynamics during the development of infection and whether it is impacted by phytoplasma genotype. Results revealed that M38 impacted photosynthetic pigments more severely than M54 genotype. Interestingly, M38 was present in lower count in grapevine leaves than the M54. However, for both genotypes, it was revealed that the highest reduction in photosynthetic pigments was at the second time point, which was also when the phytoplasma titre was the highest. This potentially links the phytoplasma titre with severity of chlorosis symptoms due to the degradation of photosynthetic pigments. These results contribute to resolving the mechanisms of flavescence dorée phytoplasma pathogenicity on the level of photosynthetic pigments and provide details for the search of potential flavescence dorée phytoplasma effectors which target photosynthetic pigments.

chlorosis, FDp, photosynthesis, phytoplasmas, plant-pathogen interactions

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