A vast microbial community inhabits in the rhizosphere, among which, specialized bacteria known as Plant Growth-Promoting Rhizobacteria (PGPR) confer benefits to host plants including growth promotion and disease suppression. PGPR taxa vary in the ways whereby they curtail the negative effects of invading plant pathogens. However, a cumulative or synergistic effect does not always ensue when a bacterial consortium is used. In this review, we reassess the disease-suppressive mechanisms of PGPR and present explanations and illustrations for functional diversity and/or stability among PGPR taxa regarding these mechanisms. We also provide evidence of benefits when PGPR mixtures, rather than individuals, are used for protecting crops from various diseases, and underscore the critical determinant factors for successful use of PGPR mixtures. Then, we evaluate the challenges of and limitations to achieving the desired outcomes from strain/species-rich bacterial assemblages, particularly in relation to their role for plant disease management. In addition, towards locating additive or synergistic outcomes, we highlight why and how the benefits conferred need to be categorized and quantified when different strains/species of PGPR are used in combinations. Finally, we highlight the critical approaches needed for developing PGPR mixtures with improved efficacy and stability as biocontrols for utilization in agricultural fields.

Peanut (Arachis hypogaea Linn. cv: Luhua 11) and tomato (Lycopersicon esculentum Mill. cv: Zhongshu 4) were inoculated with arbuscular mycorrhizal fungi (AMF) Funneliformis mosseae BEG167 (Fm), Rhizophagus intraradices BEG141 (Ri), and Glomus versiforme Berch (Gv), and/or Spodoptera exigua (S. exigua) under greenhouse conditions. Results indicated that feeding by S. exigua had little influence on colonization of peanut plants by AMF, but improved colonization of tomato by Fm and Gv. Feeding by S. exigua had little influence on leaf net photosynthetic rate, transpiration rate, and stomatal conductance of nonmycorrhizal peanut plants but significantly improved net photosynthetic rate and transpiration rate of mycorrhizal plants of both hosts. AMF with or without S. exigua inoculation improved host plant photosynthetic characteristics, growth, and hormone status. Fm showed maximum beneficial effects, followed by Gv. The concentrations and ratios of phytohormones abscisic acid (ABA), indole-3-acetic acid (IAA), gibberellin (GA), zeatin riboside (ZR), and jasmonic acid (JA) in the leaves of the host plants were changed due to the interaction between AMF and S. exigua. Generally, AMF with or without S. exigua inoculation increased the concentrations of GA, ZR, and JA and the ratios of IAA/ABA, GA/ABA, ZR/ABA, and IAA + GA + ZR/ABA, while feeding by S. exigua on nonmycorrhizal plants showed the opposite effect. The concentration of JA in the leaves of peanut and tomato inoculated with Fm or Fm + S. exigua was 1.9 and 1.9 times and 2.5 and 2.7 times, respectively, greater than that of the controls inoculated with neither. There was a negative correlation between the JA concentration and the survival percentage of S. exigua larva. We conclude that indirect interactions between AMF and insect herbivores changed the photosynthetic and hormone characteristics, and ratios of phytohormones, thereby revealing mechanisms of belowground-aboveground interactions.

Arbuscular mycorrhizal (AM) fungi in coalmine spoil, island forest and saline soils were enriched in pot culture with maize (Zea mays L.), tobacco (Nicotiana tabacum L.), white clover (Trifolium repens Linn.) and silverweed cinquefoil (Potentilla anserina L.). Based on spores, there were more species of AM fungi in the coalmine spoil (15 species, 3 genera), than in the forest soil (11 species, 4 genera) and the saline soil (5 species, 2 genera). In the trap cultures, the total of 28 species in Acaulospora, Gigaspora, Glomus, and Sclerocystis detected in the original soils were all recovered with at least one of the four trap plants. The highest spore and species numbers were recovered in trap cultures of T. repens inoculated with coalmine spoil. Glomus constrictum and Glomus multicaule were the dominant species associated with N. tabacum grown in saline soil and forest soil. The dominant species of AM fungi on the four hosts was Acaulospora mellea, which had over 90% of the spore incidence in pot trap culture in coalmine spoil. It is suggested that there be selectivity between host plants and AM fungi. The number of species of AM fungi detected was influenced by host plants under certain conditions and white clover was generally the optimal host plant to detect diversity of AM fungi.

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