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The sesquiterpenoid capsidiol is the major phytoalexin produced by Nicotiana and Capsicum species. Capsidiol is produced in plant tissues attacked by pathogens and plays a major role in postinvasion defense by inhibiting pathogen growth. Using virus-induced gene silencing-based screening, we identified two Nicotiana benthamiana (wild tobacco) genes encoding functionally redundant full-size ABCG (PDR-type) transporters, Nb -ABCG1/PDR1 and Nb -ABCG2/PDR2 , which are essential for resistance to the potato late blight pathogen Phytophthora infestans. Silencing of Nb- ABCG1/2 compromised secretion of capsidiol, revealing Nb-ABCG1/2 as probable exporters of capsidiol. Accumulation of plasma membrane-localized Nb-ABCG1 and Nb-ABCG2 was observed at the site of pathogen penetration. Silencing of EAS (encoding 5- epi -aristolochene synthase), a gene for capsidiol biosynthesis, reduced resistance to P. infestans , but penetration by P. infestans was not affected. By contrast, Nb- ABCG1/2 -silenced plants showed reduced penetration defense, indicating that Nb-ABCG1/2 are involved in preinvasion defense against P. infestans. Plastidic GGPPS1 (geranylgeranyl diphosphate synthase) was also found to be required for preinvasion defense, thereby suggesting that plastid-produced diterpene(s) are the antimicrobial compounds active in preinvasion defense. These findings suggest that N. benthamiana ABCG1/2 are involved in the export of both antimicrobial diterpene(s) for preinvasion defense and capsidiol for postinvasion defense against P. infestans. [ABSTRACT FROM AUTHOR]

Key requirements for microbes to initiate and establish mutualistic symbiotic interactions with plants are evasion of potential host defense responses and strict control of microbial growth. Reactive oxygen species (ROS) produced by a specific NADPH oxidase isoform, NoxA, regulate hyphal growth in the mutualistic interaction between the fungal endophyte Epichloë festucae and its grass host Lolium perenne. Unlike mammalian systems, little is known about the fungal NADPH oxidase complex and its response to differentiation signals. We identify an E. festucae p67phox-like regulator, NoxR, dispensable in culture but essential in planta for the symbiotic interaction. Plants infected with a noxR deletion mutant show severe stunting and premature senescence, whereas hyphae in the meristematic tissues show increased branching leading to increased fungal colonization of pseudostem and leaf blade tissue. Inhibition of ROS production or overexpression of noxR recapitulates the hyperbranching phenotype in culture. NoxR interacts in vitro with the small GTP binding protein RacA and requires a functional RacA binding site to complement the noxR mutant and restore the wild-type plant interaction phenotype. These results show that NoxR is a key regulator of NoxA in symbiosis, where it acts together with RacA to spatially regulate ROS production and control hyphal branching and patterning. [ABSTRACT FROM AUTHOR]

Although much is known about the signals and mechanisms that lead to pathogenic interactions between plants and fungi, comparatively little is known about fungus-plant mutualistic symbioses. We describe a novel role for reactive oxygen species (ROS) in regulating the mutualistic interaction between a clavicipitaceous fungal endophyte, Epichloë festucae, and its grass host, Lolium perenne. In wild-type associations, E. festucae grows systemically in intercellular spaces of leaves as infrequently branched hyphae parallel to the leaf axis. A screen to identify symbiotic genes isolated a fungal mutant that altered the interaction from mutualistic to antagonistic. This mutant has a single-copy plasmid insertion in the coding region of a NADPH oxidase gene, noxA. Plants infected with the noxA mutant lose apical dominance, become severely stunted, show precocious senescence, and eventually die. The fungal biomass in these associations is increased dramatically, with hyphae showing increased vacuolation. Deletion of a second NADPH oxidase gene, noxB, had no effect on the E. festucae-perennial ryegrass symbiosis. ROS accumulation was detected cytochemically in the endophyte extracellular matrix and at the interface between the extracellular matrix and host cell walls of meristematic tissue in wild-type but not in noxA mutant associations. These results demonstrate that fungal ROS production is critical in maintaining a mutualistic fungus-plant interaction. [ABSTRACT FROM AUTHOR]