INDOLE-3-BUTYRIC ACID IN PLANTS - OCCURRENCE, SYNTHESIS, METABOLISM AND TRANSPORT

Indole-3-butyric acid (IBA) was recently identified by GC/MS analysis as an endo-genous constituent of various plants. Plant tissues contain ed 9 ng g-1 fresh weight of free IBA and 37 ng g-1 fresh weight of tot al IBA, compared to 26 ng g-1 and 52 ng g-1 fresh weight of free and t otal indole-3-acetic acid (IAA), respectively. IBA level was found to increase during plant development, but never reached the level of IAA. It is generally assumed that the greater ability of IBA as compared w ith IAA to promote rooting is due to its relatively higher stability. Indeed, the concentrations of IAA and IBA in autoclaved medium were re duced by 40% and 20%, respectively, compared with filter sterilized co ntrols. In liquid medium, IAA was more sensitive than IBA to non-biolo gical degradation. However, in all plant tissues tested, both auxins w ere found to be metabolized rapidly and conjugated at the same rate wi th amino acids or sugar. Studies of auxin transport showed that IAA wa s transported faster than IBA. The velocities of some of the auxins te sted were 7.5 mm h-1 for IAA, 6.7 mm h-1 for naphthaleneacetic acid (N AA) and only 3.2 mm h-1 for IBA. Like IAA, IBA was transported predomi nantly in a basipetal direction (polar transport). After application o f H-3-IBA to cuttings of various plants, most of the label remained in the bases of the cuttings. Easy-to-root cultivars were found to absor b more of the auxin and transport more of it to the leaves. It has bee n postulated that easy-to-root, as opposed to the difficult-to-root cu ltivars, have the ability to hydrolyze auxin conjugates at the appropr iate time to release free auxin which may promote root initiation. Thi s theory is supported by reports on increased levels of free auxin in the bases of cuttings prior to rooting. The auxin conjugate probably a cts as a 'slow-release' hormone in the tissues. Easy-to-root cultivars were also able to convert IBA to IAA which accumulated in the cutting bases prior to rooting. IAA conjugates, but not IBA conjugates, were subject to oxidation, and thus deactivation. The efficiency of the two auxins in root induction therefore seems to depend on the stability o f their conjugates. The higher rooting promotion of IBA was also ascri bed to the fact that its level remained elevated longer than that of I AA, even though IBA was metabolized in the tissue. IAA was converted t o IBA by seedlings of corn and Arabidopsis. The K(m) value for IBA for mation was low (approximately 20 muM), indicating high affinity for th e substrate. That means that small amounts of IAA (only a fraction of the total IAA in the plant tissues) can be converted to IBA. It was su ggested that IBA is formed by the acetylation of IAA with acetyl-CoA i n the carboxyl position via a biosynthetic pathway analogous to the pr imary steps of fatty acid biosynthesis, where acetyl moieties are tran sferred to an acceptor molecule. Incubation of the soluble enzyme frac tion from Arabidopsis with H-3-IBA, IBA and UDP-glucose resulted in a product that was identified tentatively as IBA glucose (IBGlc). IBGlc was detected only during the first 30 min of incubation, showing that it might be converted rapidly to another conjugate.