Partial submergence-induced adventitious root emergence in cucumber requires CsRBOHB-mediated ROS production

Emergence of adventitious roots (ARs) in partially submerged plants is a crucial morphological adaptation to flooding stress, although the underlying molecular mechanisms are not well understood. In the present research, we investigated the role of CsRBOHB-mediated reactive oxygen species (ROS) production in cucumber (Cucumis sativus L.). ROS specifically accumulated in the vascular tissues of partially submerged hypocotyls. Respiratory burst oxidase homologs (RBOHs) are enzymes that generate ROS. We found that the cucumber RBOH gene, CsRBOHB, was specifically induced in vascular cells by partial submergence. Knockout of CsRBOHB inhibited AR emergence, while overexpression of CsRBOHB enhanced it. Downregulation of an RBOHB homolog, SlRBOHB, in partially submerged tomato also inhibited AR emergence. Additionally, cucumber calcium-dependent protein kinase 2 (CsCDPK2) directly activated CsRBOHB in cucumber hypocotyls during AR emergence and silencing it inhibited AR emergence under partially submerged conditions. These findings indicate that AR emergence in partially submerged cucumber is regulated by a CsCDPK-CsRBOHB pathway.