AM fungi might affect the root morphology of maize by increasing indole-3-butyric acid biosynthesis

Inoculation of maize (Zea mays L,) with the arbuscular mycorrhizal (AM) fun gus Glomus intraradices resulted in a distinct root phenotype ca 10 days af ter inoculation. Although the fresh weight of inoculated and control roots was about the same, the AM-inoculated roots showed a significant increase i n the percentage of lateral fine roots. This increase coincided with an inc rease in free indole-3-butyric acid (IBA) as well as an increase in IBA syn thesis. At later time points (31 days after inoculation), the free IBA cont ent was not increased in infected roots; however, the fraction of bound IBA increased compared to controls. The phenotype of mycorrhizal maize roots c ould be mimicked by IBA applied exogenously to non-mycorrhizal roots. Addit ion of trifluoro-IBA (TFIBA), an inhibitor of IBA-induced root growth and l ateral root induction, simultaneously with IBA resulted in a phenotype rese mbling that of untreated controls. In roots treated with TFIBA the inoculat ion with AM fungi did not increase the formation of fine roots. The TFIBA t reatment also reduced endogenous free IBA and the AM infection rate in myco rrhizal roots. The results are discussed with respect to a possible role of IBA in the establishment of AM sym biosis.