The consumption of lettuce is associated with an increased risk of ingesting nitrate, a naturally occurring and potentially harmful compound that can have adverse effects on human health. Hydroponic cultivation systems serve as effective tools for regulating nutrient solutions and nitrogen availability, which are essential for controlling nitrate levels. However, the techniques for reducing nutrient levels need to be appropriately calibrated based on lettuce growth responses and their interactions with the environment and growing conditions. Previous studies have demonstrated that plant probiotic actinomycetes can alleviate nutritional stress in various crops. However, there is a noticeable gap in research concerning the effects of actinomycetes on hydroponically grown lettuce, particularly under nutrient-limiting conditions. This study aimed to evaluate the effectiveness of the actinomycete Streptomyces thermocarboxydus S3 in enhancing lettuce growth in a nutrient-restricted hydroponic system. The results indicated that the detrimental effects of nutrient stress on lettuce were mitigated by the inoculation of lettuce with S. thermocarboxydus S3. This mitigation was evident in various growth parameters, including leaf count, shoot length, and the fresh and dry weights of both shoots and roots. In the presence of nutritional stress, S. thermocarboxydus S3 likely mitigated the negative effects on lettuce by reducing hydrogen peroxide levels, presumably through the synthesis of H₂O₂-scavenging enzymes. Furthermore, S. thermocarboxydus S3 successfully survived and colonized lettuce roots. Therefore, the inoculation of lettuce with S. thermocarboxydus S3 offers significant advantages for promoting lettuce growth in nutrient-limited hydroponic systems.

Spent mushroom substrate (SMS), a lignocellulosic by-product of mushroom cultivation, represents a promising renewable carrier for microbial bioinoculants as well as agricultural waste valorization. This study evaluated SMS as a carrier for the plant growth-promoting actinobacterium Streptomyces gardneri MC1-2 and assessed its effects on tomato growth and soil bacterial communities. Three formulations were prepared: SMS alone (F1), SMS supplemented with 1% (w/w) carboxymethyl cellulose (CMC) (F2), and talc supplemented with 1% CMC and 1.5% CaCO₃ (F3). S. gardneri MC1-2 exhibited in vitro traits commonly associated with plant-beneficial activity, including indolic compound production, siderophore production, and inorganic phosphate solubilization. During 30 days of storage at 4, 30, and 40 °C, both SMS-based formulations maintained viable populations at a 3 Log CFU reduction, whereas the talc-based formulation lost viability within seven days. Among the formulations, F2 showed the best overall performance and was selected for the pot experiment. Direct inoculation with S. gardneri MC1-2 promoted tomato growth comparably to chemical fertilizer, while F2 strongly supported growth in sterile soil, where S. gardneri MC1-2 established and dominated the bacterial community. In non-sterile soil, reduced bacterial establishment suggested that competition with native microbiota limited inoculant performance. These findings demonstrate that SMS can serve as an effective carrier for Streptomyces-based bioinoculants with potential applications for sustainable crop production.