Abstract
BackgroundSeed transmission constitutes a major component of the parasitic cycle for several fungal pathogens. However, very little is known concerning fungal or plant genetic factors that impact seed transmission and mechanisms underlying this key biological trait have yet to be clarified. Such lack of available data could be probably explained by the absence of suitable model pathosystem to study plant-fungus interactions during the plant reproductive phase.ResultsHere we report on setting up a new pathosystem that could facilitate the study of fungal seed transmission. Reproductive organs of Arabidopsis thaliana were inoculated with Alternaria brassicicola conidia. Parameters (floral vs fruit route, seed collection date, plant and silique developmental stages) that could influence the seed transmission efficiency were tested to define optimal seed infection conditions. Microscopic observations revealed that the fungus penetrates siliques through cellular junctions, replum and stomata, and into seed coats either directly or through cracks. The ability of the osmosensitive fungal mutant nik1Δ3 to transmit to A. thaliana seeds was analyzed. A significant decrease in seed transmission rate was observed compared to the wild-type parental strain, confirming that a functional osmoregulation pathway is required for efficient seed transmission of the fungus. Similarly, to test the role of flavonoids in seed coat protection against pathogens, a transparent testa Arabidopsis mutant (tt4-1) not producing any flavonoid was used as host plant. Unexpectedly, tt4-1 seeds were infected to a significantly lower extent than wild-type seeds, possibly due to over-accumulation of other antimicrobial metabolites.ConclusionsThe Arabidopsis thaliana-Alternaria brassicicola pathosystem, that have been widely used to study plant-pathogen interactions during the vegetative phase, also proved to constitute a suitable model pathosystem for detailed analysis of plant-pathogen interactions during the reproductive phase. We demonstrated that it provides an excellent system for investigating the impact of different fungal or plant mutations on the seed transmission process and therefore paves the way towards future high-throughput screening of both Arabidopsis and fungal mutant.
Highlights
Seed transmission constitutes a major component of the parasitic cycle for several fungal pathogens
While an inverse relation between aggressiveness and vertical transmission has been shown for some pathogens [3], the ability to transmit by seed may have significant advantages for pathogens, such as long-term survival, maximum opportunity for progeny infection and long distance dissemination [4,5]
This could be probably explained by the difficulty to carry out such experiments under field conditions, due to the reduced number of reproduction cycles per year and the lack of available model pathosystem to study plant-fungus interactions during the plant reproductive phase
Summary
Seed transmission constitutes a major component of the parasitic cycle for several fungal pathogens. Very little is known concerning fungal or plant genetic factors that impact seed transmission and mechanisms underlying this key biological trait have yet to be clarified Such lack of available data could be probably explained by the absence of suitable model pathosystem to study plant-fungus interactions during the plant reproductive phase. Iacomi-Vasilescu et al [7] recently reported that Alternaria brassicicola strains deficient in a group III osmosensor histidine kinase were highly jeopardized in their ability to infect radish seeds This was interpreted as a consequence of their failure to overcome severe osmotic stress conditions consecutive to the gradual decrease in the water potential in maturing seeds. This could be probably explained by the difficulty to carry out such experiments under field conditions, due to the reduced number of reproduction cycles per year and the lack of available model pathosystem to study plant-fungus interactions during the plant reproductive phase
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