Nitric oxide mediates nitrate induced alleviation of waterlogging stress in cucumber.

Effects of waterlogging (WL) stress and its mitigation by nitrate (NO3-) were investigated in a commercial cucumber hybrid line. WL resulted in a shift toward anaerobic metabolism in roots and development of adventitious roots (ARs). In WL stressed roots exodermal, extracellular H2O2 accumulation and protein nitration were detected. Shoot growth was retarded with signs of oxidative stress. Nitrate content decreased in both root and shoot. In the root WL upregulated several genes implicated in hypoxic response, including transcripts of CsRAP2.3, CsRBOHs and the CsHem3 phytoglobin genes. Nitrate supplementation restored shoot growth and mitigated oxidative stress. Fermentative metabolism was reduced in the roots. Nitrate levels increased substantially, expression of NRT1 genes was induced, supporting improved NO3- uptake and delivery. According to chemical analysis and fluorescent microscopy NO accumulated in the vascular cylinder of roots. NO depletion by cPTIO confirmed a role for NO in stress mitigation. cPTIO treatment decreased AR formation and foliar nitrate levels. It decreased protein nitration and strongly downregulated nitrate transporters' gene expression in roots. Overall, our work revealed nitrate triggered, NO mediated alleviation of WL stress in cucumber. Resolution of WL induced energy crisis in the roots by nitrate treatment was attributed to parallel effects of anaerobic respiration and AR formation. Shoot growth repression was probably alleviated by improved root functions, including elevated nitrate transport. Expression of NRT1 nitrate transporter genes was induced by nitrate, and this induction was found NO dependent in WL stressed cucumber.