Author : Moh Hari Rusli

Summary : Cadmium (Cd) contamination in agricultural soils poses a serious threat to crop productivity and food safety because rice ( Oryza saliva L.), one of the world's major staple crops, readily accumulates Cd in edible grains. Recent studies have demonstrated that L-3,4-dihydroxyphenylalanine (L-DOPA), a naturally occurring non-proteinogenic amino acid, activates iron (Fe)-deficiency responses and enhances Cd tolerance in Arabidopsis thaliana. However, whether a similar mechanism operates in rice remains largely unknown. This study investigated the physiological and molecular effects of exogenous L-DOPA on Fe homeostasis and Cd accumulation in rice through physiological characterization, elemental analysis, reverse transcription quantitative PCR (RT-qPCR), and transcriptomic analysis. Rice seedlings (Oryza saliva ssp. japonica cv. Kitaake) were grown hydroponically in half-strength Kimura B nutrient solution and treated with L-DOPA (100, 250, or 500 µM), CdCl2 (10 µM), or their combinations. Plant growth, chlorophyll content, biomass accumulation, developmental progression, and elemental concentrations were evaluated. Gene expression analyses were performed using RT-qPCR and RNA sequencing to elucidate the molecular responses associated with L-DOPA and Cd treatments. L-DOPA exhibited concentration-dependent effects on rice growth. Under non-stress conditions, high L-DOPA concentrations inhibited shoot and root growth while simultaneously increasing relative chlorophyll content. Under Cd stress, however, low L-DOPA concentrations partially alleviated Cd-induced reductions in root growth, biomass accumulation, and developmental delay, whereas higher concentrations exerted phytotoxic effects. Long-term experiments further demonstrated that moderate L-DOPA concentrations accelerated early reproductive development under normal conditions, while low L-DOPA partially mitigated Cd-induced developmental delays under hydroponic culture. Transcriptomic and RT-qPCR analyses further revealed that L-DOPA modulated the expression of genes associated with Fe homeostasis and metal transport, suggesting extensive transcriptional reprogramming in response to combined L-DOPA and Cd treatments. Together with elemental analyses, these findings indicate that L-DOPA influences physiological adaptation to Cd stress through modulation of Fe-related regulatory pathways. Overall, this study demonstrates that L­DOPA functions as a concentration-dependent regulator of rice growth and Cd responses. At appropriate concentrations, L-DOPA partially alleviates Cd toxicity while modulating Fe homeostasis and associated transcriptional networks. These findings provide new insights into the molecular interactions between Fe nutrition and Cd stress and establish a foundation for developing environmentally compatible strategies to reduce Cd accumulation and improve rice production in contaminated agricultural systems.