Transport and Accumulation of IAA-¹⁴C in Tumour-formingNicotianaHybrids

The polar transport of indol-3yl-acetic acid (IAA-2-14C) in stem explants and decapitated shoots of tumour-prone Nicotiana hybrids (2n, 3n, and 4n) was compared with that in the normal, non-tumorous parent species N. glauca and N. langsdorffii. The total uptake of the auxin from donor blocks was greatest in the hybrids and N. glauca. The velocity of the basipetal movement of IAA-14C was the same in all species tested, i.e. 8 mm/h. The transport capacity for the hormone, however, was decreased in the three tumour-prone hybrids. Gas chromatography showed that between 70 and 90 per cent of the transported auxin was present in the form of IAA, between 10 and 30 per cent in the form of indol-3yl-aldehyde (IAld). The basipetal transport exceeded the acropetal transport in young (third) intemodes of all plants studied, whereas in older stem segments (tenth intenodes) the reverse was found. The polarity of auxin transport was less well expressed in the tumorous hybrids. Blocking the active transport by pre-treatment of stem cuttings with 2,4-dinitrophenol (2,4-DNP) caused a drastic reduction in the polar IAA-14C movement; in all plants tested the auxin transport was reduced to the same low level. The accumulation of auxin at the base of cuttings was higher in N. glauca and the 2n hybrid than in N. langsdorffii, i.e. about seven times higher after 1-h and three times higher after 12-h transport experiments. The release of 14C from the cutting into an agar receiver block, however, was markedly reduced in the 2n hybrid, whereas in N. glauca the labelled substances moved more freely into the receiver blocks. Differences in the capacity for the accumulation and the release of IAA-14C in hybrid and N. glauca stem tissues were studied using decapitated greenhouse plants wounded by incision above the fourth internode. Accumulation of the auxin occurred only above the wound-cut in hybrid plants. This observation is consistent with the view that tumour formation on hybrid stems occurs at sites of wounding. Our data suggest an elevated auxin level to be present during tumour initiation at these sites. These results on polar transport and accumulation of IAA-14C in tumorous Nicotiana plants together with our previous data on various endogenous auxins suggest that the induction of neoplastic growth in tobacco plants is correlated with increased auxin levels and an accumulation of the hormone at sites of wounding.