Enhanced Seed Protein Yield in Hydroponically Grown Rice via Silica Hydrogel Application

The production of plant-made pharmaceuticals (PMPs) using transgenic rice in controlled environments requires high protein yields to offset operational costs. Unlike other crops, rice has a high demand for silicon (Si). While rice accumulates Si, practical and stable methods for Si supplementation in hydroponic systems are limited by pH instability and nutrient imbalances. In this study, we developed a simple, passive strategy to supply Si by submerging silica-hydrogel fertilizer (SiHF) in the nutrient reservoir. Compared with traditional silicon sources, this approach does not exacerbate pH instability and minimizes the risk of nutrient imbalances. We evaluated its effects on plant growth, yield, phenology, and protein productivity in hydroponically grown rice. SiHF markedly increased shoot biomass and grain yield, with the optimal SiHF concentration identified at 500 g/10 L. Although root dry weight decreased, the water uptake efficiency per unit root mass significantly increased, promoting greater biomass allocation to shoots. The heading stage was advanced by up to 4 days (e.g., from 54 to 50 days) across different concentration groups. Regarding grain quality, protein concentration decreased, likely due to the dilution effect of the substantial increase in grain yield. However, the total protein yield per plant increased 1.98-fold, without remarkably altering the protein composition profile. Ultimately, this SiHF-based method of Si supplementation optimizes biomass allocation and total protein productivity in rice while accelerating the reproductive transition, without requiring complex nutrient management. This approach offers a practical strategy for establishing a foundational baseline for future PMP production efficiency in rice-based plant factory systems.