Banana plants (Musa acuminata L.) are exposed to various biotic and abiotic stresses that affect their production worldwide. Banana plants respond to these stresses, but their responses to combined stresses are unique and differ from those to various individual stresses. This study reported the effects of the mycorrhizal colonization of banana roots and/or infection with root rot on the transcriptional expression of the responsive factor JERF3 and stress-responsive genes (POD, PR1, CHI, and GLU) under different salinity levels. Different transcriptional levels were recorded in response to the individual, dual, or triple treatments. All the applied biotic and abiotic stresses triggered the transcriptional expression of the tested genes when individually applied, but they showed different influences varying from synergistic to antagonistic when applied in combinations. The salinity stress had the strongest effect when applied in combination with the biotic stress and/or mycorrhizal colonization, especially at high concentrations. Moreover, the salinity level differentially affects the banana responses under combined stresses and/or mycorrhizal colonization in addition, the mycorrhizal colonization of banana plantlets improved their growth, photosynthesis, and nutrient uptake, as well as greatly alleviated the detrimental effects of salt and infection stresses. In general, the obtained results indicated that the responses of banana plantlets under the combined stresses are more complicated and differed from those under the individual stresses depending on the crosstalks between the signaling pathways.

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Rhizoctonia root rot is one of the most destructive diseases affecting pea crops, resulting in up to 75% loss. In this study, the biocontrol activity of seaweed (Ascophyllum nodosum) extract at 1, 2, and 3% and/or mycorrhization of pea roots was investigated against Rhizoctonia root rot under greenhouse conditions. In addition, their effects on the transcriptional, physiological, ultrastructural, and growth status of pea plants were also studied. The results showed that the mycorrhizal colonization of pea roots and the application of the seaweed extract at 3% synergistically overexpressed the responsive factor (JERF3) recording 18.2-fold, and the defense-related genes peroxidase (23.2-fold) and chitinase II (31.8-fold). In addition, this treatment improved the activity of the antioxidant enzymes POD and PPO, increased the phenolic content in pea roots, and triggered multiple hypersensitivity reactions at the ultrastructural level of the cell, leading to a 73.1% reduction in disease severity. Moreover, a synergistic growth-promoting effect on pea plants was also observed. The photosynthetic pigments in pea leaves were enhanced in response to this dual treatment, which significantly improved their yield (24 g/plant). The inducing effect of mycorrhizal colonization on plant resistance and growth has been extensively studied. However, developing improved and synergistically acting biological agents for plant disease control and growth promotion as alternatives to the chemical fungicides is crucial for safety and food security. Based on these results, it can be concluded that the mycorrhizal colonization of pea roots and soaking their seeds in the A. nodosum extract at 3% have a promising and improved biocontrol activity against R. solani, and a growth-promoting effect on pea plants. However, field applications should be evaluated prior to any use recommendations.

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most devastating diseases of wheat worldwide. This study dealt with investigating the biocontrol activity of mycorrhizal colonization and/or application of Streptomyces viridosporus HH1 against stripe rust of wheat. In vitro, data showed that S. viridosporus HH1 had a strong anti-spore germination effect on uredospores of P. striiformis (90% reduction). A 84.2% reduction in the disease severity was achieved in the infected wheat plants, which were colonized with mycorrhizal fungi and were sprayed with S. viridosporus HH1. Molecular investigation revealed that mycorrhizal colonization and application of S. viridosporus HH1 upregulated the defensive genes ethylene response factor protein 3 (JERF3) by 20.7-fold, chitinase II (13.6-fold), and β-1,3-glucanase (9.8-fold). Furthermore, mycorrhizal colonization and application of S. viridosporus HH1 led to a significant increase in the activity of the antioxidant enzymes peroxidase (52.3%), polyphenol oxidase (106.9%), and catalase (87.9%). The phenolic content also increased due to mycorrhizal colonization and application of S. viridosporus HH1, while the lipid peroxidation decreased in the infected wheat leaves. A mitigation in the adverse effects of infection on the photosynthetic pigments in wheat leaves was noticed. Observations from transmission electron microscopy showed that different hypersensitivity reactions were induced on the ultrastructural level in the infected wheat cells due to this treatment. In addition, a significant growth-promoting effect was also reported. It can be concluded that mycorrhizal colonization and spraying with S. viridosporus HH1 have a promising biocontrol activity against stripe rust in wheat. Field evaluation is necessary in the future studies before the use recommendation.

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