Abstract
Cinnamyl alcohol dehydrogenase (CAD) is a key enzyme in lignin biosynthesis. However, little was known about CADs in melon. Five CAD-like genes were identified in the genome of melons, namely CmCAD1 to CmCAD5. The signal peptides analysis and CAD proteins prediction showed no typical signal peptides were found in all CmCADs and CmCAD proteins may locate in the cytoplasm. Multiple alignments implied that some motifs may be responsible for the high specificity of these CAD proteins, and may be one of the key residues in the catalytic mechanism. The phylogenetic tree revealed seven groups of CAD and melon CAD genes fell into four main groups. CmCAD1 and CmCAD2 belonged to the bona fide CAD group, in which these CAD genes, as representative from angiosperms, were involved in lignin synthesis. Other CmCADs were distributed in group II, V and VII, respectively. Semi-quantitative PCR and real time qPCR revealed differential expression of CmCADs, and CmCAD5 was expressed in different vegetative tissues except mature leaves, with the highest expression in flower, while CmCAD2 and CmCAD5 were strongly expressed in flesh during development. Promoter analysis revealed several motifs of CAD genes involved in the gene expression modulated by various hormones. Treatment of abscisic acid (ABA) elevated the expression of CmCADs in flesh, whereas the transcript levels of CmCAD1 and CmCAD5 were induced by auxin (IAA); Ethylene induced the expression of CmCADs, while 1-MCP repressed the effect, apart from CmCAD4. Taken together, these data suggested that CmCAD4 may be a pseudogene and that all other CmCADs may be involved in the lignin biosynthesis induced by both abiotic and biotic stresses and in tissue-specific developmental lignification through a CAD genes family network, and CmCAD2 may be the main CAD enzymes for lignification of melon flesh and CmCAD5 may also function in flower development.
Highlights
The lignification of tissues is thought to play a critical role in specialized conducting and supporting tissues of plants, facilitating water transport, providing mechanical strength, and defense against biotic and abiotic stresses [1,2,3,4,5,6]
The presence of functional domains was checked via NCBI’s Conserved Domain Database (CDD), and only those sequences having the same functional domains of Cinnamyl alcohol dehydrogenase (CAD) with other proved/putative species were selected as our target genes
By ExPASy tools, we found that the longest protein of CADs
Summary
The lignification of tissues is thought to play a critical role in specialized conducting and supporting tissues of plants, facilitating water transport, providing mechanical strength, and defense against biotic and abiotic stresses [1,2,3,4,5,6]. Lignification is a complex process, which involves many intermediates and enzymes [7]. Reports on the identification of CAD enzymes have showed the CAD in gymnosperm is encoded by a single gene [9,10]. CAD is encoded by a multigene family in angiosperm species. Complete sets of CAD genes and CAD-like genes have been identified in the genomes of model species and non-model plants [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]
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