Abstract
Simple SummaryPhytoplasmas are a group of phloem-restricted phytopathogens that attack a huge number of wild and cultivated plants, causing heavy economic losses. They are transmitted by phloem-feeding insects of the order Hemiptera; the transmission process requires the vector to orally acquire the phytoplasma by feeding on an infected plant, becoming infective once it reaches the salivary glands after quite a long latency period. Since infection is retained for all of the insect’s life, acquisition at the nymphal stage is considered to be most effective because of the long time needed before pathogen inoculation. This work provides evidence for the reduced latency period needed by adults of the phytoplasma vector Euscelidius variegatus from flavescence dorée phytoplasma acquisition to transmission. Indeed, we demonstrate that adults can become infective as soon as 9 days from the beginning of phytoplasma acquisition. Our results support a reconsideration of the role of adults in phytoplasma epidemiology, by indicating their extended potential ability to complete the full transmission process.Phytoplasma transmission takes place by insect vectors through an Acquisition Access Period (AAP), Latency Period (LP) and Inoculation Access Period (IAP). Generally, phytoplasmas are believed to be transmitted more efficiently by nymphs because they need a long LP to reach the salivary glands before becoming infective. The transmission can start from adults as well, but in this case a long LP may exceed the insect’s lifespan. However, previous evidence has indicated that adults can undergo a shorter LP, even though little knowledge is available regarding the phytoplasma temporal dynamics during this period. Here, we investigate the minimum time required by the phytoplasma to colonize the vector midgut and salivary glands, and finally to be inoculated into a plant. We used the leafhopper Euscelidius variegatus to investigate the life cycle of flavescence dorée phytoplasma (FDP). Phytoplasma-free E. variegatus adults were left on broad beans (BBs) infected with FDP for an AAP of 7 days. Subsequently, they were individually transferred onto a healthy BB for seven different IAPs, each one lasting 24 h from day 8 to 14. Molecular analyses and fluorescence in situ hybridization were performed for FDP detection. FDP was found in the leafhopper midgut from IAP 1 with an infection rate reaching 50%, whereas in the salivary glands it was found from IAP 2 with an infection rate reaching 30%. FDP was also detected in BBs from IAP 4, with infection rates reaching 10%. Our results represent an important step to further deepen the knowledge of phytoplasma transmission and its epidemiology.
Highlights
Phytoplasmas are phloem-limited, wall-less bacteria of the class Mollicutes, associated with diseases affecting over one thousand cultivated and wild plant species [1]
The aim of this research was to assess the temporal dynamics of the phytoplasma transmission process, starting the Acquisition Access Period (AAP) with adult specimens, identifying the minimum period required for phytoplasma to infect the insect midgut and salivary glands, and the shortest Latency Period (LP) with successful transmission that can be recorded
Healthy E. variegatus adults were used for the AAP on 10 FD-infected broad beans (BBs), in which the phytoplasma concentration ranged from 1.16 × to 2.96 × flavescence dorée phytoplasma (FDP) GU/sample, with an average load of
Summary
Phytoplasmas are phloem-limited, wall-less bacteria of the class Mollicutes, associated with diseases affecting over one thousand cultivated and wild plant species [1]. Phytoplasma diseases in general are known and described throughout almost the entire world, the different groups and subgroups are not evenly distributed geographically [2] They are associated with many plants, both herbaceous and woody [3]; the symptoms attributable to these pathogenic agents are usually flower malformation (virescence, phyllody), yellowing, stunting, growth aberration, and decline [4]. Their relevance in agriculture has increased over the last few decades, and many diseases are identified as related to phytoplasmas. More than 75% of the known leafhopper vector species are included in the Cicadellidae subfamily Deltocephalinae; a number of vector species are present within other subfamilies [8,9]
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