Localized expression of a proline-rich protein gene in juvenile and mature ivy petioles in relation to rooting competence

Cell division and root initiation of excised juvenile, mature and half-expanded mature (My) leaf petioles of Hedera helix L. cultured in vitro were studied to determine whether these processes were correlated with localized expression of a proline-rich protein (PRP) gene. Petioles of all three types showed cell divisions at day 5 of culture in auxin-treated petioles but not in non-auxin-treated petioles. No cell division occurred in non-auxin-treated petioles even after day 9 of culture. Juvenile and one population of My auxin-treated petioles formed root primordia after 9 days of culture. Mature petioles and another population of My petioles formed only callus in response to auxin. The spatial and temporal expression pattern of a gene encoding a PRP was analyzed by in situ hybridization. The PRP mRNA was not detectable in petioles of any developmental phase immediately after excision. In both juvenile and mature petioles the PRP mRNA preferentially accumulated in the phloem parenchyma, the inner cortex adjacent to the phloem, and in cells surrounding ducts. Cell division was not required for PRP gene expression since both auxin-treated and non-treated juvenile and mature petioles had expression. Steady state levels of PRP mRNA were much lower in juvenile relative to mature petioles cultured in vitro. Auxin treatment reduced the steady state levels of PRP mRNA in My petioles but not in mature or juvenile petioles. These data are consistent with an inverse relationship between competence to form adventitious roots and PRP mRNA levels in the specific cell types from which root primordia form. Alternatively, the PRP mRNA level may serve as a molecular marker for developmental plasticity for root initiation.