Silicon Modulates the Chloroplast Proteome to Enhance Drought Tolerance in Soybean

Soybeans are highly susceptible to drought stress, which significantly impairs their growth and yield. Silicon (Si) supplementation has emerged as a promising strategy to mitigate drought-induced damage in plants. We investigated changes in the physiological and chloroplast proteomes in soybeans under drought stress, both with and without Si supplementation. Soybean plants were grown under controlled conditions and subjected to drought stress. The treatments included Si application (sodium silicate), sodium chloride control, and water control. Chloroplast proteins were extracted from control and Si-treated plants and analyzed using two-dimensional gel electrophoresis and mass spectrometry. Plants treated with Si showed improved drought tolerance, exhibiting reduced leaf rolling and wilting, while the control plants experienced significant wilting under drought conditions. Photosynthetic performance, measured by quantum efficiency of photosystem II and chlorophyll content, was better maintained in Si-supplemented plants under drought. However, stomatal conductance and transpiration were similarly reduced across all drought treatments. We detected 15 Si-responsive protein spots corresponding to 13 unique chloroplast proteins that were differentially expressed in response to Si supplementation. These identified proteins include those involved in photosynthesis, such as Rubisco activase isoforms, oxygen-evolving enhancer proteins, and PsbP domain-containing protein, as well as stress response proteins like dehydrin and 20 kDa chaperonin. Si treatment upregulated Rubisco activase isoforms, oxygen-evolving enhancer proteins, PsbP domain-containing protein, and 20 kDa chaperonin, which are typically reduced under drought. Si treatment maintained a higher glutamine synthetase level under drought stress. Gene ontology and KEGG pathway analyses revealed that Si-modulated proteins are associated with photosynthesis, energy metabolism, and nitrogen metabolism under drought stress. Our findings demonstrate that Si supplementation alleviates drought stress in soybean by preserving chloroplast function and enhancing the expression of photosynthetic proteins and enzymes, as well as key stress-responsive proteins. This research provides insights into the molecular mechanisms of Si-induced drought tolerance in soybeans and highlights potential targets for developing drought-resilient soybean cultivars.