The Overlooked Challenge in Coastal Saline-Alkaline Soils: Potential Risk of Copper Toxicity Constraining Rice Production

• Coastal saline-alkaline conditions promote copper toxicity in rice. • In such soils, copper bioavailability increases while iron, manganese, and zinc decrease. • Copper uptake genes are paradoxically upregulated in rice despite high copper levels. • Copper toxicity is a critical but overlooked constraint for rice in coastal saline-alkali soils. Coastal saline-alkaline soils are a vital resource for expanding global food production, yet their use for growing crops like rice faces significant challenges. While salinity and alkalinity are well-known stresses, the full scope of constraints on rice growth needs to be better understood for effective land reclamation. This study examines four rice varieties grown in normal and reclaimed coastal saline-alkaline soils, analyzing their growth, nutrient content, and physiological responses. We found that saline-alkaline stress severely hindered rice development, reducing tillering and seed-setting rates. Notably, while the levels of essential micronutrients like iron (Fe), manganese (Mn), and zinc (Zn) decreased in plant tissues, copper (Cu) accumulated to strikingly high levels, increasing 3.10-4.05 times in roots and 1.95-2.43 times in shoots compared to plants in normal soil. In response to this Cu accumulation, plants activated detoxification mechanisms, increasing the production of metal-chelating compounds and the expression of their related genes. Soil analysis confirmed that soluble Cu was 1.27-1.42 times more available in the saline-alkaline soil. The expression of general metal transporters (e.g., OsIRT1, OsNRAMP5, OsYSL6 ) was up-regulated; however, a paradoxical significant elevation in the expression of OsCOPT1 , a Cu uptake gene typically suppressed under high Cu conditions, was observed. Concurrently, genes involved in Cu detoxification ( OsHMA5 and OsYSL16 ) showed divergent expression patterns. Collectively, our findings demonstrate that Cu toxicity, associated with the observed ion imbalance, is a critical factor exacerbating saline-alkaline stress in rice. This study provides critical insights for developing strategies to reduce metal toxicity and improve rice yields in these challenging environments.