Boron-mediated lignin metabolism in response to aluminum toxicity in citrus (Poncirus trifoliata (L.) Raf.) root.

Aluminum (Al) toxicity has conspicuous detrimental effects on citrus production whereas boron (B) has been shown to alleviate its toxicity. Lignin plays a critical role in the cell wall extensibility and root elongation under stressed conditions. Hence, the interaction between B and Al on cell wall structure and lignin-related metabolic pathway was investigated in root of trifoliate orange (Poncirus trifoliata (L.) Raf.) seedlings. The results showed B supply considerably decreased the Al content in root, particularly in cell wall, and reduced Al-induced damage on growth-related parameters and thickness of cell wall. Boron application decreased the hydrogen peroxide (HO), malondialdehyde (MDA), and lignin contents in the Al-treated root, which prevents the inhibitory effects of Al on the root length. Moreover, metabonomics results showed that B addition resulted in the reduction of metabolites involved in the lignin biosynthesis pathways (phenylpropanoid metabolic) i.e., shikimic acid, tyrosine, caffeic acid, chlorogenic acid, coniferyl alcohol, sinapinic acid, sinapaldehyde, and sinapyl alcohol, as well as distinctively restrain the activities of lignin biosynthesis-related enzymes (4-coumarate-CoA ligase (4CL), cinnamyl-alcohol dehydrogenase (CAD)) under Al toxicity. Collectively, our findings suggest that the positive effects of B on the resistance of Al toxicity may be it reduces Al accumulation in the cell wall, lignin biosynthesis, and cell wall thickness, thereby increasing the extensibility and elasticity of cell wall and thus promoting root elongation.