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Plants interacting with mutualistic fungi (MF) or antagonistic fungi (AF) can form associations with bacteria. We assessed whether the performance gain conferred by mutualistic bacteria to fungal‐associated plants is affected by the interaction between symbiont traits, type of bacterial‐protective traits against AF and abiotic/biotic stresses. Results showed that (A) performance gain conferred by bacteria to MF‐associated plants was greater when symbionts promoted distinct rather than similar plant functions, (B) bacterial‐based alleviation of the AF's negative effect on plants was independent of the type of protective trait, (C) bacteria promoted a greater performance of symbiotic plants in presence of biotic, but not abiotic, stress compared to stress‐free situations. The plant performance gain was not affected by any fungal‐bacterial trait combination but optimised when bacteria conferred resistance traits in biotic stress situations. The effects of bacteria on fungal‐associated plants were controlled by the interaction between the symbionts' functional traits and the relationship between bacterial traits and abiotic/biotic stresses. [ABSTRACT FROM AUTHOR]

Epichloë festucae uses a siderophore‐mediated system to acquire iron, which is important to maintain endophyte–grass symbioses. Here we investigate the roles of the alternative iron acquisition system, reductive iron assimilation (RIA), via disruption of the fetC gene, which encodes a multicopper ferroxidase, either alone (i.e., ΔfetC) or in combination with disruption of the gene sidA, which encodes a siderophore biosynthesis enzyme (i.e., ΔfetC/ΔsidA). The phenotypic characteristics of these mutants were compared to ΔsidA and wild‐type (WT) strains during growth under axenic culture conditions (in culture) and in symbiosis with the host grass, perennial ryegrass (in planta). Under iron deficiency, the colony growth rate of ΔfetC was slightly slower than that of WT, while the growth of ΔsidA and ΔfetC/ΔsidA mutants was severely suppressed. Siderophore analyses indicated that ΔfetC mutants hyperaccumulate ferriepichloënin A (FEA) at low iron concentrations and ferricrocin and FEA at higher iron concentrations. When compared to WT, all mutant strains displayed hyperbranching hyphal structures and a reduced ratio of Epichloë DNA to total DNA in planta. Furthermore, host colonization and vertical transmission through infection of the host seed were significantly reduced in the ΔfetC/ΔsidA mutants, confirming that high‐affinity iron uptake is a critical process for Epichloë transmission. Thus, RIA and siderophore iron uptake are complementary systems required for the maintenance of iron metabolism, fungal growth, and symbiosis between E. festucae and perennial ryegrass. [ABSTRACT FROM AUTHOR]

Few studies have evaluated the effects of bacteria that form associations with fungi on plant-microbial symbioses. We investigated the effects of a novel symbiotic bacterium (E226) of Epichloë , a well-researched mutualistic fungal endophyte of grasses, on perennial ryegrass (Lolium perenne) also associated with Epichloë. We hypothesised that E226 would promote plant growth but would not interfere with the in-planta growth of Epichloë and its production of antiherbivore alkaloids. E226, identified as Micrococcus luteus, formed an ectosymbiotic relationship with Epichloë when cultured on artificial media (i.e., bacterial cells were located on the hyphal surface) and possessed plant-growth promoting traits identified through whole genome analysis. Seeds of L. perenne , already colonised by an Epichloë strain, were inoculated with E226. The bacterium established within the seedlings, and the abundance of M. luteus populations initially increased with plant age. In agreement with our hypotheses, plant inoculation with E226 enhanced the production of leaves and did not affect the growth of Epichloë or the concentration of Epichloë -derived alkaloids. The putative abilities of E226 to solubilise phosphate and produce vitamins and metabolic cofactors may explain the observed plant growth promotion. • Micrococcus luteus strain E226 was an ectosymbiont of Epichloë endophytes. • E226 promoted the growth of plants. • E226 did not compromise the Epichloë growth and alkaloid production. • E226 promoted the plant growth likely via enhancing the host nutrition. [ABSTRACT FROM AUTHOR]

Pseudomonas syringae pv. actinidiae ICMP 18884 biovar 3 (Psa3) produces necrotic lesions during infection of its kiwifruit host. Bacterial growth in planta and lesion formation are dependent upon a functional type III secretion system (T3S), which translocates multiple effector proteins into host cells. Associated with the T3S locus is the conserved effector locus (CEL), which has been characterized and shown to be essential for the full virulence in other P. syringae pathovars. Two effectors at the CEL, hopM1 and avrE1, as well as an avrE1‐related non‐CEL effector, hopR1, have been shown to be redundant in the model pathogen P. syringae pv. tomato DC3000 (Pto), a close relative of Psa. However, it is not known whether CEL‐related effectors are required for Psa pathogenicity. The Psa3 allele of hopM1, and its associated chaperone, shcM, have diverged significantly from their orthologs in Pto. Furthermore, the CEL effector hopAA1‐1, as well as a related non‐CEL effector, hopAA1‐2, have both been pseudogenized. We have shown that HopM1 does not contribute to Psa3 virulence due to a truncation in shcM, a truncation conserved in the Psa lineage, probably due to the need to evade HopM1‐triggered immunity in kiwifruit. We characterized the virulence contribution of CEL and related effectors in Psa3 and found that only avrE1 and hopR1, additively, are required for in planta growth and lesion production. This is unlike the redundancy described for these effectors in Pto and indicates that these two Psa3 genes are key determinants essential for kiwifruit bacterial canker disease. [ABSTRACT FROM AUTHOR]