A dual biosensor for early detection of lanthanum-induced hidden stress in soybean

The widespread use of rare earth elements (REEs) has led to their accumulation in crops, threatening plant health and agricultural productivity. Early detection of hidden stress and subvisible plant damage is therefore crucial for sustainable agriculture. In this study, we systematically investigated the impact of lanthanum [La(III)], a representative REE, on plant health using soybean, a major economic crop, as a model system. A dual-component living-cell biosensor based on soybean leaves was developed to monitor in real-time the changes in electron transfer impedance (R_et) associated with two La(III)-binding proteins: extracellular vitronectin-like protein (VN), and plasma membrane-anchored arabinogalactan proteins (AGPs). When soybean leaves were exposed to 40-100 μM La(III), significant subvisible cellular damage occurred, despite the absence of visible symptoms, indicating a state of hidden stress. This was evidenced by a 115.9 %-259.68 % increase in malondialdehyde levels and a 2.39 %-26.27 % decrease in chlorophyll content. Concurrently, the dual biosensor detected a 74.34 %-151.98 % increase in R_et for VN and a 47.32 %-104.68 % increase for AGPs, demonstrating its ability to sensitively capture early physiological alterations. At La(III) concentrations exceeding 100 μM, visible leaf damage emerged, covering 31 % of the leaf surface, accompanied by a 306.57 % increase in malondialdehyde content and a 34.75 % reduction in chlorophyll levels. Under these conditions, R_et increased by 155.43 % for VN and 108.77 % for AGPs. These results indicate that the biosensor enables early, sensitive detection of REE-induced hidden stress in crops before visible symptoms occur, offering a promising tool for proactive agricultural monitoring and sustainable food security.