Differential Responses of Phenolics and Lignin Metabolism to Excess Iron and Zinc in Monocot and Dicot Roots Treated by Silicon

Cell wall structure/composition differences between dicots and Poales may affect their responses to metals and Silicon. Impacts of excess Fe, Zn and their interactions with Si on phenolics/lignin metabolism of monocot and dicot roots were investigated. Monocot (rice and wheat) and dicot (canola and cotton) plants were exposed to excess Fe (150 mg L⁻¹) or excess Zn (150 µg L⁻¹) with or without Si (1.5 mM) and assessed for growth and phenol/lignin metabolism in root apical (AP) and basal parts (BP). Excess Fe compromised plant biomass, but Si improved it. Excess Zn provoked similar responses except in canola. In the monocot root AP, excess Fe increased phenylalanine ammonia lyase (PAL), cell wall peroxidase (POD) and polyphenol oxidase (PPO) activities. Phenolics accumulated mostly in monocots roots under excess Fe whereas lignin accumulated in nearly all roots and similar but weaker effects observed under excess Zn. Under excess Fe, Si stimulated cell wall POD activity and declined phenolics in monocot roots. The corresponding responses were much weaker in dicot roots. FTIR data differentiated altered cell wall functional groups in monocots and dicots after treatments, however, monocots data were more diverse than dicots. Si is more efficient to substitute for lignin metabolism in monocots than in dicots under excess Fe. Response of phenolic/lignin metabolism to heavy metal stress and Si depends on both plant and heavy metal species. Si application brings about more changes in the root cell wall of monocots than dicots through modulation of the root cell wall structure and function.