Efficient adventitious root formation in Petunia hybrida cuttings: when signals meet resources

Each year, in Europe several hundreds of million cuttings are rooted for production of ornamental young plants. This involves a complex chain including cutting production at low latitude sites and storage and transport of cuttings to rooting stations. Using Petunia hybrida as model, we analysed indole-3-acetic acid (IAA) levels, different metabolites, enzyme activities and gene expression to investigate the contribution of auxin-related pathways, of carbohydrate and nitrogen metabolism and the interaction of both to adventitious root (AR) formation. Blocking of polar auxin transport (PAT) eliminated the 24 h-peak of IAA, reduced the activities of cell wall and vacuolar invertases in the rooting zone and severely inhibited AR formation. Under standard rooting conditions genes controlling PAT and signalling via the Aux/IAA-ARF interaction showed phase-specific regulation indicating particular functions during the induction and differentiation of ARs. Enhanced AR formation after cold dark storage of cuttings was associated with rooting zone-dominated activation of cell wall invertase and induction of one particular member of the cell wall invertase-encoding gene family. Further, amino acids accumulated in cutting tissues under darkness. Moreover, higher levels of IAA were found in the stem base under dark compared to cultivation under light, which was further associated with upregulation of genes controlling one auxin efflux transporter and components of the auxin signalling cascade. These results strongly suggest that a coordinated remobilization of nitrogen within the cutting, establishment of the new sink in the rooting zone and activation of auxin-related pathways contribute to the induction and formation of root meristemoids under the dark phase and to accelerated AR differentiation and growth when the cuttings are subsequently planted.