Transgenic banana plants overexpressing MusabZIP53 display severe growth retardation with enhanced sucrose and polyphenol oxidase activity

bZIP transcription factors are involved in diverse cellular processes including stress response pathways in plants. In the present study, we identified a bZIP gene, MusabZIP53, from banana EST database and subsequently characterized it by overexpression in transgenic banana plants. Expression profiling in native banana plants proved that MusabZIP53 was strongly up-regulated by cold and drought stress and by ABA treatment in both leaf and root tissues. Transgenic banana plants constitutively overexpressing MusabZIP53 displayed growth retardation from early stages of transformation/regeneration protocol and mature greenhouse hardened transgenic plants displayed a distinct dwarf phenotype. Genes belonging to several families known to be involved in abiotic stress perception and mitigation were found to be differentially regulated in these transgenic plants. These included genes coding for dehydration response element binding proteins, late embryogenesis abundant proteins, anti-oxidant enzymes, aquaporins, polyphenol oxidases, Aux/IAA proteins and proteins involved in amino acid metabolism. Presence of a conserved untranslated ORF in the 5' UTR region of MusabZIP53 gene and detection of enhanced levels of sucrose in the leaves of transgenic MusabZIP53 overexpressing banana plants indicated an important role for this bZIP gene in sucrose homeostasis in banana plants. Further, strong up-regulation of four polyphenol oxidase coding transcripts in MusabZIP53 overexpressing plants coupled with induction of these transcripts in banana leaves by cold and ABA treatments pointed towards possible involvement of MusabZIP53 in the control of total polyphenol oxidase activity in banana plants.