Proteomic analysis of the effects of exogenous calcium on hypoxic-responsive proteins in cucumber roots

BACKGROUND: Hypoxia acts as a plant stress factor, particularly in cucumbers plants under hydroponic culture. Calcium is involved in stress signal transmission and in the growth of plants. To determine the effect of exogenous calcium on hypoxic-responsive proteins in cucumber (Cucumis sativus L. cv. Jinchun No.2) roots, proteomic analysis was performed using two-dimensional electrophoresis (2-DE) and mass spectrometry. RESULTS: Cucumber roots were used to analyze the influence of hypoxia on plants. The expressions of 38 protein spots corresponding to enzymes were shown to change in response to hypoxia. Of these, 30 spots were identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF/TOF MS analysis). The proteins were categorized according to functional groups, including glycolysis, the tricarboxylic acid (TCA) cycle, fermentative metabolism, nitrogen metabolism, energy metabolism, protein synthesis and defense against stress. Exogenous calcium appeared to alleviate hypoxic stress via these metabolic and physiological systems. Western blotting was used to analyze the accumulation of alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC); calcium further increased the expression of ADH and PDC under hypoxia. In addition, semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) was used to assess the transcript levels of differentially expressed proteins. CONCLUSIONS: Exogenous calcium enhanced the expression of enzymes involved in glycolysis, the TCA cycle, fermentative metabolism, nitrogen metabolism, and reactive oxygen species (ROS) defense in plants under hypoxia. Calcium appears to induce hypoxic tolerance of cucumber seedlings. These phenomena have prompted us to further investigate the mechanisms by which cucumbers respond to exogenous calcium under hypoxia.