Proline drives cross-linking between extensins and pectin in the cell wall to combat hexavalent chromium stress in rice plants

The cell wall (CW) serves as a protective barrier against toxic metals, with extensins (EXTs), proline (Pro), and hydroxyproline-rich glycoproteins playing key roles. These components regulate CW synthesis and assembly and can cross-link with pectin (Pec) to reinforce the CW structure. In this study, we investigated how exogenous proline [Pro(exo)] promotes cross-linking between EXTs and Pec in CW to mitigate hexavalent chromium [Cr(VI)] stress in rice plants. Our physio-biochemical analyses revealed that Pro(exo) significantly modulated the relative growth rate and root phenotype of the rice seedlings under Cr(VI) stress. It also influenced the synthesis of CW components, altered the structure and composition of CW macromolecules, and enhanced Cr sequestration by CWs. Real-time quantitative PCR analysis demonstrated that Pro(exo) regulates the expression of OsSULTR1;2, thereby affecting Cr translocation between the roots and shoots of rice seedlings. This regulation alleviated the inhibitory effects of Cr(VI) stress on growth. In addition, Pro(exo) stimulated the expression of several EXT, proline hydroxylases, and hydroxyproline O-arabinosyltransferase genes, promoting the formation of EXT–Pec cross-links. Furthermore, Pro(exo) activated genes involved in CW signaling pathways, enhancing the CW response to Cr(VI) stress signals, and effectively triggering adaptive strategies in rice plants. Overall, our findings provide a preliminary understanding of the molecular mechanisms by which Pro(exo) mediates EXT–Pec cross-linking, thereby influencing the CW structure, function, and signal transduction processes.