INFLUENCE OF VESICULAR-ARBUSCULAR MYCORRHIZA ON PHYTOHORMONE BALANCES IN MAIZE (ZEA-MAYS L)

The phytohormones abscisic acid (ABA), cytokinin (zeatin riboside) and indole-acetic-acid (IAA) were quantitatively determined in maize infected by the Glomus vesicular-arbuscular mycorrhizal (VAM) isolate T6 and in non-colonized controls. Measurements by indirect ELISA showed considerably higher levels of free ABA in VAM-infected than in control plants at all growth stages assayed (40, 60, 80, 110-day-old plants). In contrast, the amount of zeatin riboside was the same in infected and non-infected plants both for roots and shoots with the exception of the late plant growth phase (110-day-old plants), which showed an enhanced zeatin riboside level in VAM-colonized maize. Auxin (IAA) content, determined by a classical bioassay (C2H4-formation by pea epicotyl sections), was essentially the same in infected and non-infected roots. The concentrations of gibberellins could not accurately be determined by the release of alpha-amylase from barley endosperm, since ABA from roots and shoots had overriding antagonistic effects in this assay. However, the test independently showed that the ABA-levels were higher in VAM-colonized roots of maize than in non-infected controls. As the plants had been grown under phosphate limitation, the increased levels of ABA in colonized roots could have resulted from a better P(i)-supply acquired by the fungi or more likely from colonization of the roots by the fungi.