Plant stomata represent the main battlefield for host plants and the pathogens that enter plant tissues via stomata. Septoria spp., a group of ascomycete fungi, use host plant stomata for invasion and cause serious damage to agricultural plants. There is no evidence, however, showing the involvement of stomata in defense systems against Septoria infection. In this study, we isolated Septoria gentianae 20-35 (Sg20-35) from Gentiana triflora showing gentian leaf blight disease symptoms in the field. Establishment of an infection system using gentian plants cultured in vitro enabled us to observe the Sg20-35 infection process and estimate its virulence in several gentian cultivars or lines. Sg20-35 also entered gentian tissues via stomata and showed increased virulence in G. triflora compared with G. scabra and their interspecific hybrid. Notably, the susceptibility of gentian cultivars to Sg20-35 was associated with their stomatal density on the adaxial but not abaxial leaf surface. Treatment of EPIDERMAL PATTERNING FACTOR-LIKE 9 (EPFL9/STOMAGEN) peptides, a small secreted peptide controlling stomatal density in Arabidopsis thaliana, increased stomatal density on the adaxial side of gentian leaves as well. Consequently, treated plants showed enhanced susceptibility to Sg20-35. These results indicate that stomatal density on the adaxial leaf surface is one of the major factors determining the susceptibility of gentian cultivars to S. gentianae and suggest that stomatal density control may represent an effective strategy to confer Septoria resistance.
Tombusviruses have been identified in several crops, which include gentian virus A (GeVA), in Japanese gentians. In this study, we isolated another tombusvirus, Sikte waterborne virus strain C1 (SWBV-C1) from Japanese gentian. Although SWBV-C1 and GeVA are not closely related among tombusviruses, SWBV-C1, like GeVA, showed host-specific low-temperature-preferred multiplication in gentians and Arabidopsis. The use of in vitro transcripts from full-length cDNA clones of SWBV-C1 genomic RNA as inocula confirmed these properties, which indicates that the identified genomic RNA sequences encode viral factors underlying characteristic SWBV-C1 features.
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