Drought alters the level of endogenous reactive oxygen species (ROS) and hormonal status, which are both involved in the regulation of stress responses. To investigate the interplay between ROS and hormones in proline metabolism, rapeseed (Brassica napus L.) plants were exposed to drought or exogenous H₂O₂ (Exo-H₂O₂) treatment for 10 days. During the first 5 days, the enhanced H₂O₂ concentrations in drought treatment were associated with the activation of superoxide dismutase (SOD) and NADPH oxidase, with enhanced ABA and SA levels, while that in Exo-H₂O₂ treatment was mainly associated with SA-responsive POX. During the latter 5 days, ABA-dependent ROS accumulation was predominant with an upregulated oxidative signal-inducible gene (OXI1) and MAPK6, leading to the activation of ABA synthesis and the signaling genes (NCED3 and MYC2). During the first 5 days, the enhanced levels of P5C and proline were concomitant with SA-dependent NDR1-mediated signaling in both drought and Exo-H₂O₂ treatments. In the latter 5 days of drought treatment, a distinct enhancement in P5CR and ProDH expression led to higher proline accumulation compared to Exo-H₂O₂ treatment. These results indicate that SA-mediated P5C synthesis is highly activated under lower endogenous H₂O₂ levels, and ABA-mediated OXI1-dependent proline accumulation mainly occurs with an increasing ROS level, leading to ProDH activation as a hypersensitive response to ROS and proline overproduction under severe stress.

Controlled Environment Agriculture (CEA) offers a protected system for agricultural production; however, it remains vulnerable to diseases, particularly root diseases such as Pythium root rot and Fusarium wilt. Sustainable and eco-friendly agricultural practices using plant-beneficial microbes can help mitigate these harmful diseases. These microbes produce natural antibiotics and promote induced systemic resistance (ISR), which enhances nutrient uptake, stress tolerance, and disease resistance. While plant-beneficial microbes have been applied in conventional cropping systems, they have yet to be fully integrated into CEA-based systems. Oxygen availability in the root zone is critical for the functionalities of beneficial microorganisms. Insufficient levels of dissolved oxygen (DO) can hinder microbial activity, lead to the accumulation of harmful compounds, and cause stress to the plants. Contemporary aeration technologies, such as novel oxygenated nanobubble (ONB) technology, provide better oxygen distribution and promote optimal microbial proliferation, enhancing plant resilience. Hydroponic and soilless substrate-based systems of CEA production have significant potential to integrate beneficial microbes, increase crop yields, prevent diseases, and improve resource use efficiency. This review aims to summarize the significance of DO and the potential impact of novel ONB technology in CEA for managing root zone diseases while increasing crop productivity and sustainability.

In plants, prolonged drought induces oxidative stress, leading to a loss of reducing potential in redox components. Abscisic acid (ABA) is a representative hormonal signal regulating stress responses. This study aimed to investigate the physiological significance of dimethylthiourea (DMTU, an H₂O₂ scavenger) in the hormonal regulation of the antioxidant system and redox control in rapeseed (Brassica napus L.) leaves under drought stress. Drought treatment for 10 days provoked oxidative stress, as evidenced by the increase in O₂^•− and H₂O₂ concentrations, and lipid peroxidation levels, and a decrease in leaf water potential. Drought-induced oxidative responses were significantly alleviated by DMTU treatment. The accumulation of O₂^•− and H₂O₂ in drought-treated plants coincided with the enhanced expression of the NADPH oxidase and Cu/Zn-SOD genes, leading to an up-regulation in oxidative signal-inducible 1 (OXI1) and mitogen-activated protein kinase 6 (MAPK6), with a concomitant increase in ABA levels and the up-regulation of ABA-related genes. DMTU treatment under drought largely suppressed the drought-responsive up-regulation of these genes by depressing ABA responses through an antagonistic interaction with salicylic acid (SA). DMTU treatment also alleviated the drought-induced loss of reducing potential in GSH- and NADPH-based redox by the enhanced expression of glutathione reductase 1 (GR1) and up-regulation of oxidoreductase genes (TRXh5 and GRXC9). These results indicate that DMTU effectively alleviates drought-induced oxidative responses by suppressing ABA-mediated oxidative burst signaling in an antagonistic regulation of SA.

Controlled Environment Agriculture (CEA) offers a protected system for agricultural production; however, it remains vulnerable to diseases, particularly root diseases such as Pythium root rot and Fusarium wilt. Sustainable and eco-friendly agricultural practices, including the use of plant-beneficial microbes, can help mitigate these harmful diseases. These microbes produce natural antibiotics and promote Induced Systemic Resistance (ISR), which enhances nutrient uptake, stress tolerance, and disease resistance. While plant-beneficial microbes have been applied in conventional cropping systems, they have yet to be fully integrated into CEA-based systems. Oxygen availability in the root zone is critical for the functionalities of beneficial microorganisms. Insufficient levels of dissolved oxygen (DO) can hinder microbial activity, lead to the accumulation of harmful compounds, and cause stress to the plants. Contemporary aeration technologies, such as novel oxygenated nanobubble technology, provide better oxygen distribution and promote optimal microbial proliferation, enhancing plant resilience. Hydroponic and soilless substrate-based systems of CEA productions have significant potential to integrate beneficial microbes, increase crop yields, reduce diseases, and improve resource use efficiency. This review aims to summarize the significance of dissolved oxygen and the potential impact of novel nanobubble technology in CEA for managing root zone diseases while increasing crop productivity and sustainability.