RESUMO
Many fungal pathogens are carried and transmitted by seeds. These pathogens affect germination and seed quality. Their transmission from the germinating seed to seedling causes many diseases in crops. Seed defense mechanisms during germination are poorly documented. RNA-seq experiments were used to describe the molecular mechanisms involved in seed interaction with a necrotrophic fungus. Here the Arabidopsis thaliana/Alternaria brassicicola pathosystem was used to perform dual-transcriptomic approach. Arabidopsis thaliana seeds and necrotrophic fungus transcripts were identified at critical germination and seedling establishment stages. Total RNA was extracted from healthy and infected germinating seeds and seedlings at 3, 6 and 10 days after sowing. Transcript libraries were made and sequenced, then fungal and plant short reads were mapped and quantified respectively against Arabidopsis thaliana and Alternaria brassicicola reference transcriptomes. This dual-transcriptomic approach revealed that 3409, 7506 and 8589 Arabidopsis thaliana genes showed a differential expression at respectevely 3, 6 and 10 days after sowing between healthy and infected seeds, including 1192 genes differentially expressed at the three studied stages. Moreover, in this experiement, we also identified the dynamic of the transcript changes occurring at the same stages in the necrotrophic fungus concomitantly during germination and seedling establishment.
RESUMO
The transmission of seed-borne pathogens by the germinating seed is responsible for major crop diseases. The immune responses of the seed facing biotic invaders are poorly documented so far. The Arabidopsis thaliana/Alternaria brassicicola patho-system was used to describe at the transcription level the responses of germinating seeds and young seedling stages to infection by the necrotrophic fungus. RNA-seq analyses of healthy versus inoculated seeds at 3 days after sowing (DAS), stage of radicle emergence, and at 6 and 10 DAS, two stages of seedling establishment, identified thousands of differentially expressed genes by Alternaria infection. Response to hypoxia, ethylene and indole pathways were found to be induced by Alternaria in the germinating seeds. However, surprisingly, the defense responses, namely the salicylic acid (SA) pathway, the response to reactive oxygen species (ROS), the endoplasmic reticulum-associated protein degradation (ERAD) and programmed cell death, were found to be strongly induced only during the latter post-germination stages. We propose that this non-canonical immune response in early germinating seeds compared to early seedling establishment was potentially due to the seed-to-seedling transition phase. Phenotypic analyses of about 14 mutants altered in the main defense pathways illustrated these specific defense responses. The unexpected germination deficiency and insensitivity to Alternaria in the glucosinolate deficient mutants allow hypothesis of a trade-off between seed germination, necrosis induction and Alternaria transmission to the seedling. The imbalance of the SA and jasmonic acid (JA) pathways to the detriment of the JA also illustrated a non-canonical immune response at the first stages of the seedling.
RESUMO
Seed dormancy and timing of its release is an important developmental transition determining the survival of individuals, populations, and species in variable environments. Medicago truncatula was used as a model to study physical seed dormancy at the ecological and genetics level. The effect of alternating temperatures, as one of the causes releasing physical seed dormancy, was tested in 178 M. truncatula accessions over three years. Several coefficients of dormancy release were related to environmental variables. Dormancy varied greatly (4-100%) across accessions as well as year of experiment. We observed overall higher physical dormancy release under more alternating temperatures (35/15 °C) in comparison with less alternating ones (25/15 °C). Accessions from more arid climates released dormancy under higher experimental temperature alternations more than accessions originating from less arid environments. The plasticity of physical dormancy can probably distribute the germination through the year and act as a bet-hedging strategy in arid environments. On the other hand, a slight increase in physical dormancy was observed in accessions from environments with higher among-season temperature variation. Genome-wide association analysis identified 136 candidate genes related to secondary metabolite synthesis, hormone regulation, and modification of the cell wall. The activity of these genes might mediate seed coat permeability and, ultimately, imbibition and germination.