miR397 regulates cadmium stress response by coordinating lignin polymerization in the root exodermis in Kandelia obovata

Secondary lignification of the root exodermis of Kandelia obovata is crucial for its response to adversity such as high salinity and anaerobic environment, and this lignification is also effective in blocking cadmium transport to the roots. However, how the differences in lignification of root exodermis at different developmental stages respond to Cd stress and its regulatory mechanisms have not been revealed. In this study, after analyzing the root structure and cell wall thickness using a Phenom scanning electron microscope as well as measuring cadmium content in the root cell wall, we found that the exodermis of young and mature roots of K. obovata responded to Cd stress through the polymerization of different lignin monomers, forming two different mechanisms: chelation and blocking. Through small RNA sequencing, RLM-5′-RACE and dual luciferase transient expression system, we found that miR397 targets and regulates KoLAC4/17/7 expression. The expression of KoLAC4/17 promoted the accumulation of guaiacyl lignin during lignification and enhanced the binding of cadmium to the cell wall. Meanwhile, KoLAC7 expression promotes the accumulation of syringyl lignin during lignification, which enhances the obstruction of cadmium and improves the tolerance to cadmium. These findings enhance our understanding of the molecular mechanisms underlying the differential lignification of the root exodermis of K. obovata in response to cadmium stress, and provide scientific guidance for the conservation of mangrove forests under heavy metal pollution. Environmental Implication Coastal wetlands are important enrichment sites for heavy metals. It affects plant nutrient uptake and normal physiological metabolism, but lignification of the root exodermis of Kandelia obovata improves cadmium tolerance. Understanding the relationship between the process of lignification of the exodermis and cadmium tolerance is beneficial to the conservation of mangrove plants. In this study, we found that the differences in lignin polymerization in the exodermis of young and mature roots resulted in both adsorption and blocking mechanisms of cadmium. Different members of the laccase gene family regulated the differences in lignin polymerization,which regulation by miR397 in K. obovata.