## 1 Introduction  
It is widely known that there is a close relationship between a high input of fertilizers and pesticides in agro-ecosystems and numerous adverse effects, such as soil degradation, water pollution, biodiversity loss, climate change, and lastly a reduced quality of the food produced. In light of these threats, more environmentally friendly methods of soil enrichment and crop protection are being sought compared with those commonly used in agricultural practices today [1, 2, 3, 4]. The inclusion of microorganism-based products in crop management systems due to the specific role of these rhizosphere organisms in plant performance (plant health and vigor) appears to be one option for reducing the amount of agrochemicals used [1, 5, 6, 7, 8, 9, 10, 11]. Soil biota, such as plant growth-promoting rhizobacteria (PGFR) and arbuscular mycorrhizal fungi (AMF), individually and/or in co-inoculation (as a bacterial-fungal inoculum to combine the benefits of each) have been tested as a means of increasing the plant nutritional status [8; 12; 13; 14; 15], water status [16; 17; 18] or the host's resistance to biotic stress factors [8; 11; 19], including soil replant disease [20]. Nitrogen fixation, phosphorus solubilization, siderophore production, a reduction in soil nutrient loss, and the effect on soil structure have been indicated to be key factors regulating nutrient uptake by plants, thus decreasing the need for chemical fertilizers [6; 11; 21]. In turn, plant nutritional status directly affects plant health and the quality of plant-derived food. Since both the above-mentioned biotic and abiotic stresses may disturb the equilibrium between the production and scavenging of reactive oxygen species (ROS), activation of the antioxidant defense system(s) of host plants is considered to be one of the interrelationships between plants and microorganisms [17; 22; 23; 24]. A plant antioxidant defense network influenced by PGPR and/or fungal inoculum can be both enzymatic and/or non-enzymatic in nature, with a direct effect on the plant chemical composition [12; 25; 26; 27]. Furthermore, activation of the secondary metabolism by microorganisms may have an additional effect on the internal quality of the final products, including a greater quantity of health-promoting phytochemicals such as flavonoids, anthocyanins, vitamins, phytoestrogens, and carotenoids [2; 7; 13; 28]. The potential role of AMF in maintaining fruit quality in future climate change scenarios has also been discussed [29]. However, studies on the phytochemical content of mycorrhizal food crops still cover only a limited number of species, such as _Solanum Lyopersicum_ (tomato), _Capsicum annuum_ (pepper), _Allium cepa_ (onion), _Cynama carduculus_ (artichoke), _Lacuca sativa_ (lettuce), _Fragaria_ spp. (strawberry) and _Zea mais_ (maize) [2; 30]. Only a small number of cultivars have been analysed in these studies, despite cultivated species being characterized by a great number of cultivars with a high variation in internal quality traits that, in turn, are subject to the influence of several external factors [31; 32]. Very little data in the literature can be found on fruit nutraceuticals due to the biofertilizer used [28] including apple [7; 14], whereas the diversity and content of phytochemicals in plant products are affected by different variables [32], such as plant genotype, weather, and agronomic factors (management, soil mineral nutrients, drought, salinity, harvest season, and agrochemicals). Hence, research on microorganisms' mechanism of action and their impact on the quality of horticultural products is still deficient and at times contradictory. Apples, the object of this study, are undisputedly a leading horticultural product worldwide, but conversion from conventional to a more sustainable or organic production currently recommended is difficult, especially due to pest and disease control [33].