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Abstract
Process of senescence includes multiple steps involving break-down of chlorophyll
to degrade photosynthetic machinery. In this study, we showed that a stress-associated
NAC transcription factor MpSNAC67 regulates senescence by promoting chlorophyll-catabolic
genes. MpSNAC67 encodes a transcriptional activator and its promoter activity is restricted
to vascular tissue of banana. Expression of MpSNAC67 showed positive responses to
multiple abiotic stress conditions suggesting that MpSNAC67 is a stress associated
NAC transcription factor. Transgenic banana lines overexpressing MpSNAC67 showed highly
senesced phenotype including yellowing and de-greening of leaves similar to etiolated
leaves. Transgenic leaves possessed low chlorophyll content and failed to retain normal
chloroplast morphology including loss of granum thylakoid, non-uniform chloroplast
membrane and increased number as well as size of plastoglobulins. In a gel shift assay
MpSNAC67 could retard the mobility of chlorophyll catabolic genes such as PAO-like
(Pheophorbide-a-oxygenase), HCAR-like (hydroxymethyl chlorophyll-a-reductase), NYC/NOL-like
(Chlorophyll-b-reductase) as well as ORS1-like (a SenNAC). Expression of these genes
were highly elevated in transgenic lines which indicate that MpSNAC67 is a positive
regulator of senescence in banana and exercise its effect by regulating the expression
of chlorophyll catabolic genes and ORS1.