Seed treatments with beneficial microorganisms are growing due to the reduction in the use of synthetic agrochemicals and the low application cost of microbial inoculants in comparison with other field application methods. Seed treatments include film coating, pelleting and priming. Film coating involves the application of a thin layer of beneficial coating material directly on the seed surface, with limited variation of seed shape, size, and weight. Film coating is usually adopted for applying on seed surface many products containing pesticides, fertilizers, hormones, and/or biostimulants. Some authors prefer to use the terms ‘seed treatment’ when biologicals are applied, and ‘seed coating’ for seed application of agrochemicals such as pesticides. Seed pelleting consists in covering the seeds with external material like talc and clay powders that are retained onto the seeds by binding agents, increasing seed weight, size, and shape. The external material can also include agrochemicals or biologicals to stimulate or protect germinating seeds. Seed priming is a process of controlled seed hydration with water (hydro-priming), with saline solutions (osmo-priming), phytohomones, (hormonal priming) or biological entities (bio-priming), or with mixtures containing solid material and water (matric-priming). Seed priming allows a controlled seed imbibition and triggers pre-germinative metabolic processes (de novo synthesis of nucleic acids and proteins, ATP production, sterols and phospholipids accumulation, or activation of DNA repair and antioxidant mechanisms). Seed imbibition needs to be stopped before overcoming the first reversible stage of germination thus avoiding radical protrusion through the seed coat. Several beneficial microorganisms belong to arbuscular mycorrhizal (AM) fungi, Trichoderma spp., rhizobia, and plant growth promoting bacteria are currently applied as seed treatments in cereals, vegetables, oil and seed pulse crops, and fiber and forage crops (Cardarelli et al., 2022). Arbuscular mycorrhizal (AM) fungi alone or in consortium with other beneficial microorganisms like Trichoderma spp. are applied on seed surface of large seeds (e.g., wheat, corn, pumpkin) by film coating or on seed surface of small seeds by seed pelleting (e.g., lettuce, basil) for enhancing nutrient uptake, crop resistance to environmental stress, and product quality

Fig. 1. Wheat seeds treated (Left) and untreated (Right) with AM fungi and Trichoderma atroviride

Colla et al. (2015) reported that durum wheat seeds (Triticum durum Desf.) treated by film coating with a consortium of endophytic microorganisms consisting of AM fungi [Rhizoglomus intraradices BEG72 (former Glomus intraradices), Funneliformis mosseae BEG 234 (former G. mosseae)], and Trichoderma atroviride MUCL 45632 significantly increased leaf number (+28.6%), and shoot (+23.1%) and root (+64.2%) dry biomass of 17-old seedlings in comparison with untreated control. In an open field trial, the use of film-coated wheat seeds with these beneficial fungi led to a significant increase of grain yield by 32 and 8%, during the first and second growing cycle, respectively, compared to the untreated seeds. The above findings can be explained by the total rainfall which was higher than the 30-years mean value in the second growing cycle (899.8 vs 591.9 mm), whereas the first growing cycle was much drier (313.1 mm) than the historical mean (591.9 mm). It is well-known that AM fungi enhance plants’ stress tolerance against various environmental stresses including drought. During the first growing cycle, wheat plants could have experienced drought stress which was mitigated by seed inoculation with beneficial fungi. Grain quality of wheat in terms of protein content, K, P, Fe and Zn concentrations were also enhanced by seed inoculation with AM fungi and Trichoderma atroviride. Moreover, seed inoculation with beneficial fungi reduced the variation of grain yield between the two growing cycles in comparison to untreated control. The increase in grain yield and yield stability in the film-coating seed treatment was associated with an increased of macro and micronutrient in leaves, higher leaf chlorophyll content and photosynthetic activity.

Fig. 2.Wheat plants from seeds untreated and treated

In another field trial (Rouphael et al., 2017), globe artichoke [Cynara cardunculus var. scolymus (L.) Fiori] seeds of two cultivars were film-coated with a consortium of AM fungi (Rhizophagus intraradices BEG72, and Funneliformis mosseae BEG 234), and Trichoderma atroviride MUCL 45362 and field tested in comparison with untreated seeds. Treated and untreated seeds of both cultivars were sown in polystyrene plug trays filled with a peat/perlite mixture and thus seedlings were transplanted in open field after 21 days from sowing.

Fig. 3.Artichoke trial with two rows of plants from coated seeds showing early production

Seed treatment with beneficial fungi enhanced total yield of globe artichoke heads in comparison with untreated control. The above findings were attributed to the increase of 1) nutrient uptake and translocation through greater effective root area as well as to the colonization level of AM fungi; 2) solubilization of trace elements by Trichoderma atroviride; 3) production in the root zone of volatiles, small peptides, and metabolites with hormone activities (i.e., indole-3-acetic acid) or analogues. Moreover, the results demonstrated that nutritional value of artichoke heads in terms of antioxidant activity, total phenolics, caffeoylquinic acids and flavonoids was enhanced by film-coating the seeds with a consortium of beneficial fungi containing R. intraradices, F. mosseae and T. atroviride.

In conclusion, seed treatment with microbial-biostimulants like AM fungi is the most economical and efficient way of applying beneficial microorganisms in cereals, pulses, oilseeds, and vegetable crops. Agronomic trials worldwide demonstrated that seed treatments with microbial-biostimulants can enhance crop yield, quality traits of product and resource use efficiency especially under adverse environmental conditions. However, to maximize the benefits of seed treatments with beneficial microorganisms it is necessary to identify the most appropriate microbial strains and application rate for each crop and growing condition. Moreover, a deep understanding of interactions among microbial inoculants, resident microflora, and environmental factors is pivotal for optimizing the agronomic performances of seed treatment.