Abstract
BackgroundCinnamyl 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.ResultsHere 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.ConclusionsCmCADs but CmCAD4 are positively regulated by ABA, H2O2 and JA under drought stress and participate in lignin synthesis.
Highlights
Cinnamyl alcohol dehydrogenase (CAD) is an important enzyme functions at the last step in lignin monomer synthesis pathway
Drought-induced abscisic acid (ABA), Hydrogen peroxide (H2O2) and Methyl jasmonic acid (MeJA) could be inhibited by the corresponding inhibitor or scavenger ABA and H2O2 are frequently reported as important signal molecules in regulating downstream gene expression and metabolism under drought stress [6, 28]
To investigate the changes of ABA, H2O2 and jasmonic acid (JA) in melon stems under drought stress, contents of ABA, H2O2 and JA were determined and found the signal molecules were all induced by Polyethylene glycol (PEG) treatment (Fig. 1a-c)
Summary
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. Previous studies reported that JA can induce CAD gene expression [12] and promote lignin deposition together with reactive oxygen species (ROS) [13, 14]. While the regulation of ABA to lignification still remains controversial, Mohr and Cahill (2007) [17] reported that ABA inhibited lignin synthesis and SA accumulation in Arabidopsis leaves, studies found that ABA increased expressions of lignin biosynthesis genes (phenylalanine ammonia (PAL), cinnamate 4hydroxylase (C4H), 4-coumarate-CoA ligase (4CL5), CAD, CCR, ferulate 5-hydroxylase (F5H1)) and activities of lignifying enzymes (PAL, peroxidase (POD)), promoted the accumulation of lignin [18,19,20]. Positive correlations were observed between ABA, JA, SA and lignification in mature and senescence tissues [21, 22]
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