Expression analysis of MusaNAC68 transcription factor and its functional analysis by overexpression in transgenic banana plants

NAC (NAM, ATAF and CUC) proteins are plant-specific transcription factors regulating development and responses to stress. One of NAC proteins from banana is NAC68, and expression analysis indicated its positive association to stress conditions. The 5'-proximal region of MusaNAC68 was isolated and sequence analysis indicated presence of stress related cis-elements and cis-elements involved in auxin-signaling. Expression of MusaNAC68 was maximum in roots and positively correlated with application of alpha-naphthaleneacetic acid. Nuclear localization of MusaNAC68 was determined by fusion of green-fluorescent protein with MusaNAC68 and transiently overexpressing in banana embryogenic cells. Transgenic lines were marginally taller and displayed more abundant roots than control along with altered expression of auxin-responsive genes like auxin-responsive factors and IAA/Aux (indoleacetic acid-induced protein) genes. Transgenic plants overexpressing MusaNAC68 displayed higher activity of polyphenol oxidase and rapid browning of plant extract. Transgenic lines showed better tolerance to stress induced by NaCl and mannitol and produced more shoot biomass. Leaf disc assay showed that transgenic lines retain more chlorophyll and lower malondialdehyde than control under salinity and drought. Transgenic line constitutively overexpressing MusaNAC68 showed elevated expression of many stress-responsive genes indicating its involvement in salinity and drought tolerance in banana.