Exogenous Proline Maintains Cell Wall Structure and Membrane Integrity in Rice Seedlings Under Cr(VI) Stress Associated with Regulation of Proline-Rich Proteins

Cr(VI) pollution severely damages plant cell wall structure and membrane integrity. Proline-rich proteins (PRPs), key cell wall structural components, rely on proline as their biosynthetic precursor. Pro accumulation has been shown to positively correlate with PRP abundance, suggesting a direct biochemical link between exogenous Pro [Pro(exo)] application and PRP-mediated cell wall function. However, how Pro(exo) regulates PRP expression to maintain cell wall structural integrity and membrane stability in rice seedlings under Cr(VI) stress remains unclear. In this study, the subcellular distribution of Cr, electrolyte leakage (EL), cell wall thickness, and the expression of PRP genes in rice seedlings were evaluated. The results revealed that Cr(VI) stress induced significant increases in EL and changes in cell wall thickness in rice seedling cells (p < 0.05). The application of Pro(exo) significantly mitigated EL and increased cell wall thickness (p < 0.05). Furthermore, Pro(exo) markedly increased the sequestration of Cr within the seedling root cell walls while reducing its distribution in the cytoplasm and organelles of seedling shoot cells (p < 0.05). Phylogenetic analysis of 48 OsPRP genes, on the basis of their homology with seven functionally characterized PRPs from other species, revealed 12 candidate genes in rice potentially involved in the regulation of cell membrane stability, cell wall assembly, thickening, and integrity. Subsequent qRT-PCR analysis and gene expression variation factor calculation revealed that Pro(exo) significantly promoted the expression of OsPRP1.1, OsPRP3, OsRePRP2.1, OsHyPRP18, and OsHyPRP27 in roots and that of OsHyPRP27 in the shoots of rice seedlings under Cr(VI) stress. These findings indicate that Pro(exo) enhances Cr compartmentalization within the root cell wall and improves membrane stability, thereby contributing to Cr(VI) tolerance in plants through the modulation of these key PRP genes. Our findings provide novel insights into the mechanism by which Pro(exo) regulates PRP expression to affect plant cellular structural stability and heavy metal tolerance.