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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.

Coexpression of a plant NB-LRR-type resistance (R) gene and corresponding viral avirulent (Avr) gene introduced in Nicotiana benthamiana using Agrobacterium tumefaciens confers hypersensitive response (HR). Such Agrobacterium-mediated transient gene expression methods have contributed to the identification of new plant R genes and facilitated the analysis of their functions. Here we describe a model method, by which several tobamovirus R genes from Solanaceous plants have been successfully identified and characterized molecularly.

The Arabidopsis membrane protein ACCELERATED CELL DEATH 6 (ACD6) and the defense signal salicylic acid (SA) are part of a positive feedback loop that regulates the levels of at least 2 pathogen-associated molecular patterns (PAMP) receptors, including FLAGELLIN SENSING 2 (FLS2) and CHITIN ELICITOR RECEPTOR (LYSM domain receptor-like kinase 1, CERK1). ACD6- and SA-mediated regulation of these receptors results in potentiation of responses to FLS2 and CERK1 ligands (e.g. flg22 and chitin, respectively). ACD6, FLS2 and CERK1 are also important for callose induction in response to an SA agonist even in the absence of PAMPs. Here, we report that another receptor, EF-Tu RECEPTOR (EFR) is also part of the ACD6/SA signaling network, similar to FLS2 and CERK1.