Analysis of CmCADs and three lignifying enzymes in oriental melon (‘CaiHong7’) seedlings in response to three abiotic stresses

Abstract

Abstract Cinnamyl alcohol dehydrogenase (CAD; EC 1.1.1.195), a key enzyme in lignin monomer synthesis pathway, is responsible for the conversion of cinnamyl aldehydes to cinnamyl alcohols. Our previous work identified five CAD genes in the melon genome and named CmCAD1 to CmCAD5, respectively. The further promoter analysis of CmCADs revealed elements involved in stress response and signaling pathway. However, what abiotic stresses the genes respond to remain unclear. Thus three abiotic stresses were conducted on oriental melon seedlings (‘CaiHong7’) as drought, salt and wound. In our study, lignin deposition was significantly promoted in stems and roots, and stem sections showed deeper staining with phloroglucinol-HCl under the three abiotic stresses than the control, indicating a higher lignin accumulation promoted by abiotic stress. Activities of CAD and peroxidase (POD; EC 1.11.1.7), as well as phenylalanine ammonia-lyase (PAL; EC 4.3.1.5), were generally induced to significant levels by the abiotic stresses, implying that lignin synthesis pathway may be activated. CmCAD2 was significantly up-regulated in stems and roots under salt treatment but was most strongly induced in whole seedlings under drought stress. Also, CmCAD2 was significantly induced by wound treatment as well as the other CmCADs. However, the other four CmCAD genes exhibited tissue-specific inductions under drought and salt stresses. Expression analysis suggested that CmCAD2 might be a major gene in lignin deposition responding to the abiotic stresses.