Genome-wide Identification and Characterization of Respiratory Burst Oxidase Homolog Genes in Six Rosaceae Species and an Analysis of Their Effects on Adventitious Rooting in Apple

Adventitious root formation is essential for plant propagation, development, and response to various stresses. Reactive oxygen species (ROS) are essential for adventitious root formation. However, information on Respiratory Burst Oxidase Homolog (RBOH), a key enzyme that catalyzes the production ROS, remains limited in woody plants. Here, a total of 44 RBOH genes were identified from six Rosaceae species (Malus domestica, Prunus avium, Prunus dulcis 'Texas', Rubus occidentalis, Fragaria vesca and Rosa chinensis), including ten from M. domestica. Their phylogenetic relationships, conserved motifs and gene structures were analyzed. Exogenous treatment with the RBOH protein inhibitor diphenyleneiodonium (DPI) completely inhibited adventitious root formation, whereas exogenous H2O2 treatment enhanced adventitious root formation. In addition, we found that ROS accumulated during adventitious root primordium inducing process. The expression levels of MdRBOH-H, MdRBOH-J, MdRBOH-A, MdRBOH-E1 and MdRBOH-K increased more than two-fold at days 3 or 9 after auxin treatment. In addition, cis-acting element analysis revealed that the MdRBOH-E1 promoter contained an auxin-responsive element and the MdRBOH-K promoter contained a meristem expression element. Based on the combined results from exogenous DPI and H2O2 treatment, spatiotemporal expression profiling, and cis-element analysis, MdRBOH-E1 and MdRBOH-K appear to be candidates for the control of adventitious rooting in apple.