HO-Responsive Hormonal Status Involves Oxidative Burst Signaling and Proline Metabolism in Rapeseed Leaves

Drought alters the level of endogenous reactive oxygen species (ROS) and hormonal status, which are both involved in the regulation of stress responses. To investigate the interplay between ROS and hormones in proline metabolism, rapeseed ( L.) plants were exposed to drought or exogenous HO (Exo-HO) treatment for 10 days. During the first 5 days, the enhanced HO concentrations in drought treatment were associated with the activation of superoxide dismutase (SOD) and NADPH oxidase, with enhanced ABA and SA levels, while that in Exo-HO treatment was mainly associated with SA-responsive POX. During the latter 5 days, ABA-dependent ROS accumulation was predominant with an upregulated oxidative signal-inducible gene () and leading to the activation of ABA synthesis and the signaling genes ( and ). During the first 5 days, the enhanced levels of P5C and proline were concomitant with SA-dependent -mediated signaling in both drought and Exo-HO treatments. In the latter 5 days of drought treatment, a distinct enhancement in and expression led to higher proline accumulation compared to Exo-HO treatment. These results indicate that SA-mediated P5C synthesis is highly activated under lower endogenous HO levels, and ABA-mediated -dependent proline accumulation mainly occurs with an increasing ROS level, leading to activation as a hypersensitive response to ROS and proline overproduction under severe stress.