Plant innate immunity against invasive biotrophic pathogens depends on the intracellular defense regulator ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1). We show here that Arabidopsis thaliana EDS1 interacts in vivo with another protein, SENESCENCE-ASSOCIATED GENE101 (SAG101), discovered through a proteomic approach to identify new EDS1 pathway components. Together with PHYTOALEXIN-DEFICIENT4 (PAD4), a known EDS1 interactor, SAG101 contributes intrinsic and indispensable signaling activity to EDS1-dependent resistance. The combined activities of SAG101 and PAD4 are necessary for programmed cell death triggered by the Toll-Interleukin-1 Receptor type of nucleotide binding/leucine-rich repeat immune receptor in response to avirulent pathogen isolates and in restricting the growth of normally virulent pathogens. We further demonstrate by a combination of cell fractionation, coimmunoprecipitation, and fluorescence resonance energy transfer experiments the existence of an EDS1-SAG101 complex inside the nucleus that is molecularly and spatially distinct from EDS1-PAD4 associations in the nucleus and cytoplasm. By contrast, EDS1 homomeric interactions were detected in the cytoplasm but not inside the nucleus. These data, combined with evidence for coregulation between individual EDS1 complexes, suggest that dynamic interactions of EDS1 and its signaling partners in multiple cell compartments are important for plant defense signal relay.

Summary• Enhanced Disease Susceptibility1 (EDS1) is an important regulator of plant basal and receptor-triggered immunity. Arabidopsis EDS1 interacts with two related proteins, Phytoalexin Deficient4 (PAD4) and Senescence Associated Gene101 (SAG101), whose combined activities are essential for defense signaling. The different sizes and intracellular distributions of EDS1-PAD4 and EDS1-SAG101 complexes in Arabidopsis leaf tissues suggest that they perform nonredundant functions.• The nature and biological relevance of EDS1 interactions with PAD4 and SAG101 were explored using yeast three-hybrid assays, in vitro analysis of recombinant proteins purified from Escherichia coli, and characterization of Arabidopsis transgenic plants expressing an eds1 mutant (eds1 L262P ) protein which no longer binds PAD4 but retains interaction with SAG101.• EDS1 forms molecularly distinct complexes with PAD4 or SAG101 without additional plant factors. Loss of interaction with EDS1 reduces PAD4 post-transcriptional accumulation, consistent with the EDS1 physical association stabilizing PAD4. The dissociated forms of EDS1 and PAD4 are fully competent in signaling receptortriggered localized cell death at infection foci. By contrast, an EDS1-PAD4 complex is necessary for basal resistance involving transcriptional up-regulation of PAD4 itself and mobilization of salicylic acid defenses.• Different EDS1 and PAD4 molecular configurations have distinct and separable functions in the plant innate immune response.

SummaryIn addition to the role of the cell wall as a physical barrier against pathogens, some of its constituents, such as pectin-derived oligogalacturonides (OGA), are essential components for elicitation of defence responses. To investigate how modifications of pectin alter defence responses, we expressed the fruit-specific Fragaria · ananassa pectin methyl esterase FaPE1 in the wild strawberry Fragaria vesca. Pectin from transgenic ripe fruits differed from the wild-type with regard to the degree and pattern of methyl esterification, as well as the average size of pectin polymers. Purified oligogalacturonides from the transgenic fruits showed a reduced degree of esterification compared to oligogalacturonides from wild-type fruits. This reduced esterification is necessary to elicit defence responses in strawberry. The transgenic F. vesca lines had constitutively activated pathogen defence responses, resulting in higher resistance to the necrotropic fungus Botrytis cinerea. Further studies in F. vesca and Nicotiana benthamiana leaves showed that the elicitation capacity of the oligogalacturonides is more specific than previously envisaged.

Summary The enlargement of receptacle cells during strawberry (Fragaria × ananassa) fruit development is a critical factor determining fruit size, with the increase in cell expansion being one of the most important physiological processes regulated by the phytohormone gibberellin (GA). Here, we studied the role of GA during strawberry fruit development by analyzing the endogenous content of bioactive GAs and the expression of key components of GA signalling and metabolism. Bioactive GA1, GA3 and GA4 were monitored during fruit development, with the content of GA4 being extremely high in the receptacle, peaking at the white stage of development. Genes with high homology to genes encoding GA pathway components, including receptors (FaGID1(GIBBERELLIN‐INSENSITIVE DWARF1)b and FaGID1c), DELLA (FaRGA(REPRESSOR OF GA) and FaGAI(GA‐INSENSITIVE)), and enzymes involved in GA biosynthesis (FaGA3ox) and catabolism (FaGA2ox), were identified, and their expression in different tissues and developmental stages of strawberry fruit was studied in detail. The expression of all of these genes showed a stage‐specific pattern during fruit development and was highest in the receptacle. FaGID1c bound GA in vitro, interacted with FaRGA in vitro and in vivo, and increased GA responses when ectopically expressed in Arabidopsis. This study thus reveals key elements of GA responses in strawberry and points to a critical role for GA in the development of the receptacle.