CONICET | Buscador de Institutos y Recursos Humanos

Expression of the chloroplast electron shuttle ferredoxin is induced by light throughmechanisms that partially depend on sequences lying in the coding region of the gene,complicating its manipulation by promoter engineering. Ferredoxin expression is alsodown-regulated under virtually all stress situations, and it is unclear if light-dependentinduction and stress-dependent repression proceed through the same or similarmechanisms. Previous reports have shown that expression of a cyanobacterialflavodoxin in tobacco plastids results in plants with enhanced tolerance to adverseenvironmental conditions such as drought, chilling and xenobiotics (Tognetti et al.2006). The protective effect of flavodoxin was linked to functional replacement offerredoxin, suggesting the possibility that tolerant phenotypes might be obtained bysimply increasing ferredoxin contents. To bypass endogenous regulatory constraints, wetransformed tobacco plants with a ferredoxin gene from Anabaena sp. PCC7120, which2has only 53% identity with plant orthologs. The cyanobacterial protein was able tointeract in vitro with ferredoxin-dependent plant enzymes and to mediate NADP+photoreduction by tobacco thylakoids. Expression of Anabaena ferredoxin wasconstitutive and light-independent. However, homozygous lines accumulating 3-foldhigher ferredoxin levels than the wild-type failed to show enhanced tolerance tooxidative stress and chilling temperatures. Under these adverse conditions, Anabaenaferredoxin was down-regulated even faster than the endogenous counterparts. Theresults indicate that: i) light- and stress-dependent regulations of ferredoxin expressionproceed through different pathways, and ii) overexpression of ferredoxin is not analternative to flavodoxin expression for the development of increased stress tolerance inplants.