De-etiolation causes a phytochrome-mediated increase of polyamine oxidase expression in outer tissues of the maize mesocotyl: a role in the photomodulation of growth and cell wall di erentiation

The photomodulation of polyamine oxidase (PAO) expression during de-etiolation of the maize (Zea mays L.) mesocotyl was used as the experimental model to investigate a possible correlation with the photomod- ulation of growth and with wall di€erentiation and sti€ening. The accumulation of PAO transcript and enzyme activity were enhanced by light treatment in cortical and epidermal (outer) tissues of the mesocotyl. Histochemical analysis revealed that this phenomenon is mostly due to the increased level of PAO activity in epidermal and sub-epidermal tissues. The photomodu- lation of PAO activity upon de-etiolation in outer tissues is mediated by phytochrome. A close correlation was found between the time course of red-light-elicited increase of PAO activity and that of growth inhibition in the outer tissues of the apical, growing zone of the mesocotyl. Light exposure of etiolated, sub-apical me- socotyl segments resulted in a higher production of hydrogen peroxide (H2O2) in the incubation medium compared with segments incubated in the dark. The latter phenomenon was inhibited by the speci®c PAO inhibitor guazatine. A short pre-treatment of mesocotyl and coleoptile segments with 1 mM spermidine inhibited IAA-induced elongation growth, this phenomenon be- ing reversed by catalase. Pre-treatment with catalase alone resulted in a higher extent of IAA-induced elongation. Moreover, pre-incubation with 1,3-diamino- propane, a product of spermidine oxidation catalysed by PAO, had no e€ect on IAA-induced elongation growth of either coleoptile or mesocotyl segments, while H2O2 pre-treatment was e€ective. These results indicate that PAO activity is important in producing H2O2 in vivo for peroxidase-catalysed wall-sti€ening reactions and may be involved in the modulation of growth and cell wall di€erentiation in the maize mesocotyl.