Mutual interaction of auxin and cytokinins in regulating correlative dominance

Correlative dominance requires correlative signals from a dominant to a dominated organ. Auxins, particularly IAA, and cytokinins are obviously important components of this correlative system. Using a vegetative pea shoot and a generative apple and tomato fruit system it can be demonstrated that dominant organs always export more IAA and have a higher H-3-IAA transport capacity and velocity compared to dominated organs. In both systems the dominant organ can be replaced by the application of auxin, e.g. NAA, which maintains the differences in IAA export. This is an indication that similar regulatory mechanisms control dominance in both of these diverse systems. The possibility of replacing a dominant organ by auxin also makes it unlikely that growth of that organ or allocation of nutrients regulates the correlative inhibition of the dominated organ. It is suggested that differences in IAA export from, and transport capacities of, dominant and dominated shoots, may be explained by a mechanism of auxin transport autoinhibition (ATA), whereby the earlier and stronger export of IAA from the dominant shoot inhibits auxin export from the dominated shoot at the point where the two auxin streams converge. This hypothesis was tested with explants of pea, apple and tomato. It was shown that the basal application of cold IAA significantly reduced endogenous as well as exogenous IAA transport through these explants. Since the reduced IAA transport of dominated organs was not followed by an accumulation of IAA in the auxin producing subtending organ, it was concluded that IAA biosynthesis was possibly reduced and/or IAA conjugation stimulated. This could have been one of the determinants of their growth inhibition. ATA might also explain how the unidirectional IAA signal may affect the growth rate of organs even lateral or acropetal to its transport pathway and thus polar IAA-transport becomes a ``multidirectional'' signal. From the experiments demonstrated it seems that ATA is a sufficient mechanism to impose growth inhibition in the dominated organ, without the need of other regulators. However, to release dominated organs from dominance cessation of ATA may not be sufficient and cytokinins are obviously a powerful antagonist to auxins. Their repeated exogenous application turns dominated lateral buds into strongly growing organs which ultimately may even dominate the previously dominant apex. These lateral shoots finally gain a strong IAA export capacity and inhibit, by ATA, IAA export from the hitherto dominant apex. In other experiments it was shown that interruption of polar IAA transport leads to a strong increase in root derived cytokinins. This can largely be prevented, in a concentration dependent manner, by the application of auxin, indicating that basipolar auxin may control cytokinin production in the roots and its possible delivery to lateral buds. In turn, the increased delivery of cytokinins to the lateral buds promotes a strong increase in IAA production and export. Thus there is a strong mutual interaction between auxin production in the shoots and cytokinin production in the roots, which may be important in regulating the balance between root and shoot growth.