Pathways of flower infection and pollen-mediated dispersion of Pseudomonas syringae pv. actinidiae, the causal agent of kiwifruit bacterial canker

Abstract

Flowers can provide a protected and nutrient-rich environment to the epiphytic microflora, thus representing a sensible entry point for pathogens such as Pseudomonas syringae pv. actinidiae (Psa). This bacterium can colonize both male and female Actinidia flowers, causing flower browning and fall, and systemic invasion of the host plant, eventually leading to its death. However, the process of flower colonization and penetration into the host tissues has not yet been fully elucidated. In addition, the presence of Psa in the pollen from infected flowers, and the role of pollination in the spread of Psa requires confirmation.The present study employed a Psa strain constitutively expressing the fluorescent GFPuv protein, to visualize in vivo flower colonization. Microscopy observations were performed by means of confocal laser scanning and wide-field fluorescent microscopy, and were coupled with the study of Psa population dynamics by quantitative PCR (q-PCR). The pathogen was shown to colonize stigmata, move along the stylar furrow, and penetrate the receptacles via the style or nectarhodes. Once the receptacle was invaded, the pathogen migrated along the flower pedicel and became systemic. Psa was also able to colonize the anthers epiphytically and endophytically. Infected male flowers produced contaminated pollen, which could transmit Psa to healthy plants. Finally, pollinators (Apis mellifera and Bombus terrestris) were studied in natural conditions, showing that, although they can be contaminated with Psa, the pathogen’s transmission via pollinators is contrasted by its short survival in the hive.

Understanding flower-bud rot development caused by <i>Pseudomonas syringae </i>pv. <i>actinidiae </i>in green-fleshed kiwifruit

“…It appears, therefore, that Psa colonisation begins in the outer flower parts (sepals) and spreads into the inner parts of the flower (petals, anthers and ovaries) as the disease progresses. This inwards movement of Psa was also suggested by Donati et al (2018), who also detected Psa on asymptomatic buds when studying the pathway of infection in kiwifruit flowers.…”

Pathogens Associated to Kiwifruit Vine Decline in Italy

“…Since KVDS symptom assessment accounts for leaf margin desiccation, wilting and phylloptosis which are not specific, to clarify the influence of the irrigation regime on the disease incidence and severity, fungal pathogens were reisolated from roots of inoculated vines and their population enumerated. In fact, the presence of an endophytic population of the pathogen inside a target organ is a precise method to estimate disease incidence [27]. By taking into consideration the occurrence of fungal reisolation, a different situation emerged where the higher incidence of fungi corresponds to the higher irrigation volumes (Figure 6a,b).…”

Inheritance of seed and rhizosphere microbial communities through plant–soil feedback and soil memory

“…Microorganisms, mostly fungi and bacteria, enter the seeds through the outer seed coat or pollen granules, or naturally through infiltration of nectarthodes in flowers (Underwood et al, 2007;Frank et al, 2017). However, the mechanisms underlying microbial migration into seeds have only been studied with a limited number of pathogenic bacteria (Donati et al, 2018). Although microorganisms within seeds are confined to seed coats, they are transferred to the soil through cotyledon defoliation and root development after seed germination, which serves as a basis for the establishment of the soil microbial community in association with the indigenous soil microbiota (Nelson et al, 2018;Fig.…”