The apoplastic copper amine oxidase AtAO1 mediates jasmonic acid-induced protoxylem differentiation in Arabidopsis roots

Polyamines are involved in key developmental processes and stress responses. Copper amine oxidases (CuAOs) oxidize the polyamine putrescine (Put) producing an aldheyde, ammonia and hydrogen peroxide (H2O2). The Arabidopsis At4g14940 gene (AtAO1) encodes an apoplastic CuAO expressed at the early stages of vascular tissue differentiation in roots. Here, its role in root development and xylem differentiation was explored by pharmacological and forward/reverse genetic approaches. The analysis of AtAO1 expression pattern in roots by a promoter::GFP-GUS fusion revealed a strong gene expression in the protoxylem at the transition, elongation and maturation zones. Methyl jasmonate (MeJA) induced AtAO1 gene expression in vascular tissues, especially at the transition and elongation zones. Early protoxylem differentiation was observed upon MeJA treatment along with Put level decrease and H2O2 accumulation in wild-type (WT) roots, whereas Atao1 loss-of-function mutants were unresponsive to the hormone. The H2O2 scavenger N,N1-dimethylthiourea (DMTU) reversed the MeJA-induced early protoxylem differentiation in WT seedlings. Likewise, Put, which had no effect on Atao1 mutants, induced early protoxylem differentiation in WT, this event being counteracted by DMTU-treatment. Consistently, AtAO1 over-expressing plants showed lower Put levels and early protoxylem differentiation concurrent with H2O2 accumulation in the root zone where the first protoxylem cells with fully developed secondary wall thickenings are found. These results show that the H2O2 produced via the AtAO1-driven Put oxidation plays a role in MeJA signaling leading to early protoxylem differentiation in root