Abstract
Lignin and polyphenols are important cellular components biosynthesized through phenylpropanoid pathway. Phenylpropanoid pathway in plants is regulated by some important transcription factors including R2R3 MYB transcription factors. In this study, we report the cloning and functional characterization of a banana R2R3-MYB transcription factor (MusaMYB31) by overexpression in transgenic banana plants and evaluated its potential role in regulating biosynthesis of lignin and polyphenols. Sequence analysis of MusaMYB31 indicated its clustering with members of subgroup 4 (Sg4) of R2R3MYB family which are well known for their role as repressors of lignin biosynthesis. Expression analysis indicated higher expression of MusaMYB31 in corm and root tissue, known for presence of highly lignified tissue than other organs of banana. Overexpression of MusaMYB31 in banana cultivar Rasthali was carried out and four transgenic lines were confirmed by GUS histochemical staining, PCR analysis and Southern blot. Histological and biochemical analysis suggested reduction of cell wall lignin in vascular elements of banana. Transgenic lines showed alteration in transcript levels of general phenylpropanoid pathway genes including lignin biosynthesis pathway genes. Reduction of total polyphenols content in transgenic lines was in line with the observation related to repression of general phenylpropanoid pathway genes. This study suggested the potential role of MusaMYB31 as repressor of lignin and polyphenols biosynthesis in banana.
Highlights
Lignin is an important macromolecule with roles in providing mechanical strength to the plant, supporting water transport through xylem tissue and biotic stress tolerance [1]
Apart from C1 and C2 motifs, recently an additional motif known as C3-motif/ZnF-like has been identified in the c-terminal of certain repressors belonging to R2R3 MYB transcription factors [14]
Transcription factors regulate the time and stage specific production of several of these metabolites. Important among these are members belonging to subgroup4 of R2R3-MYB transcription factors, which have been associated with repression of phenylpropanoid pathway including lignin and polyphenols biosynthesis [12, 10, 15]
Summary
Lignin is an important macromolecule with roles in providing mechanical strength to the plant, supporting water transport through xylem tissue and biotic stress tolerance [1]. Lignin biosynthesis during secondary cell wall development is regulated by transcription factors belonging to NAC and R2R3 MYB gene families [6]. ZmMYB42 down-regulate several genes of lignin and flavonoid biosynthesis pathway and reducing lignin content besides qualitatively altering lignin composition [2]. Another maize gene, ZmMYB31 repress many genes involved in the synthesis of monolignols resulting in reduction of lignin content in transgenic plants increasing their cell wall degradability [13]. Switchgrass (Panicumvirgatum) MYB transcription factor, PvMYB4 can down-regulate the expression of monolignol pathway genes by binding to AC-elements in their promoter and its overexpression cut down the lignin biosynthesis in transgenic switchgrass [15]
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