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.

The fungus Gibberella fujikuroi is used for the commercial production of gibberellins (GAs), which it produces in very large quantities. Four of the seven GA biosynthetic genes in this species encode cytochrome P450 monooxygenases, which function in association with NADPH-cytochrome P450 reductases (CPRs) that mediate the transfer of electrons from NADPH to the P450 monooxygenases. Only one cpr gene (cpr-Gf) was found in G. fujikuroi and cloned by a PCR approach. The encoded protein contains the conserved CPR functional domains, including the FAD, FMN , and NADPH binding motifs. cpr-Gf disruption mutants were viable but showed a reduced growth rate. Furthermore, disruption resulted in total loss of GA 3 , GA 4 , and GA 7 production, but low levels of non-hydroxylated C 20 -GAs (GA 15 and GA 24 ) were still detected. In addition, the knock-out mutants were much more sensitive to benzoate than the wild type due to loss of activity of another P450 monooxygenase, the detoxifying enzyme, benzoate p-hydroxylase. The UV-induced mutant of G. fujikuroi, SG138, which was shown to be blocked at most of the GA biosynthetic steps catalyzed by P450 monooxygenases, displayed the same phenotype. Sequence analysis of the mutant cpr allele in SG138 revealed a nonsense mutation at amino acid position 627. The mutant was complemented with the cpr-Gf and the Aspergillus niger cprA genes, both genes fully restoring the ability to produce GAs. Northern blot analysis revealed co-regulated expression of the cpr-Gf gene and the GA biosynthetic genes P450-1, P450-2, P450-4 under GA production conditions (nitrogen starvation). In addition, expression of cpr-Gf is induced by benzoate. These results indicate that CPR-Gf is the main but not the only electron donor for several P450 monooxygenases from primary and secondary metabolism.

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Nine biological species, or mating populations (MPs), denoted by letters A to I, and at least 29 anamorphic Fusarium species have been identified within the Gibberella fujikuroi species complex. Members of this species complex are the only species of the genus Fusarium that contain the gibberellin (GA) biosynthetic gene cluster or at least parts of it. However, the ability of fusaria to produce GAs is so far restricted to Fusarium fujikuroi, although at least six other MPs contain all the genes of the GA biosynthetic gene cluster. Members of Fusarium proliferatum, the closest related species, have lost the ability to produce GAs as a result of the accumulation of several mutations in the coding and 5 noncoding regions of genes P450-4 and P450-1, both encoding cytochrome P450 monooxygenases, resulting in metabolic blocks at the early stages of GA biosynthesis. In this study, we have determined additional enzymatic blocks at the first specific steps in the GA biosynthesis pathway of F. proliferatum: the synthesis of geranylgeranyl diphosphate and the synthesis of ent-kaurene. Complementation of these enzymatic blocks by transferring the corresponding genes from GA-producing F. fujikuroi to F. proliferatum resulted in the restoration of GA production. We discuss the reasons for Fusarium species outside the G. fujikuroi species complex having no GA biosynthetic genes, whereas species distantly related to Fusarium, e.g., Sphaceloma spp. and Phaeosphaeria spp., produce GAs.