Drought-induced ABA, H₂O₂ and JA positively regulate CmCAD genes and lignin synthesis in melon stems

Cinnamyl alcohol dehydrogenase (CAD) is an important enzyme functions at the last step in lignin monomer synthesis pathway. Our previous work found that drought induced the expressions of CmCAD genes and promoted lignin biosynthesis in melon stems.

Here we studied the effects of abscisic acid (ABA), hydrogen peroxide (H₂O₂) and jasmonic acid (JA) to CmCADs under drought stress. Results discovered that drought-induced ABA, H₂O₂ and MeJA were prevented efficiently from increasing in melon stems pretreated with fluridone (Flu, ABA inhibitor), imidazole (Imi, H₂O₂ scavenger) and ibuprofen (Ibu, JA inhibitor). ABA and H₂O₂ are involved in the positive regulations to CmCAD1, 2, 3, and 5, and JA is involved in the positive regulations to CmCAD2, 3, and 5. According to the expression profiles of lignin biosynthesis genes, ABA, H₂O₂ and MeJA all showed positive regulations to CmPAL2-like, CmPOD1-like, CmPOD2-like and CmLAC4-like. In addition, positive regulations were also observed with ABA to CmPAL1-like, CmC4H and CmCOMT, with H₂O₂ to CmPAL1-like, CmC4H, CmCCR and CmLAC17-like, and with JA to CmCCR, CmCOMT, CmLAC11-like and CmLAC17-like. As expected, the signal molecules positively regulated CAD activity and lignin biosynthesis under drought stress. Promoter::GUS assays not only further confirmed the regulations of the signal molecules to CmCAD1~3, but also revealed the important role of CmCAD3 in lignin synthesis due to the strongest staining of CmCAD3 promoter::GUS.

CmCADs but CmCAD4 are positively regulated by ABA, H₂O₂ and JA under drought stress and participate in lignin synthesis.