Miiller, J. 1993. Indole-3-butyric acid in plants: occurrence, synthesis, metabolism and transport. -Physiol. Plant. 88: 382-389.Indole-3-butyric acid (IBA) was recently identified by GC/MS analysis as an endogenous constituent of various plants. Plant tissues contained 9 ng g~ fresh weight of free IBA and 37 ngg 'fresh weight of total IBA, compared to 26 ngg ' and 52 ngg ' fresh weight of free and total indole-3-acetic acid (lAA), respectively. IBA level was found to increase during plant development, but never reached the level of lAA. It is generally assumed that the greater ability of IBA as compared with lAA to promote rooting is due to its relatively higher stability. Indeed, the concentrations of lAA and IBA in autodaved medium were reduced by 40% and 20%, respectively, compared with filter sterilized controls. In liquid medium. IAA was more sensitive than IBA to non-biological degradation. However, in all plant tissues tested, both auxins were found to be metabolized rapidly and conjugated at the same rate with amino acids or sugar. Studies of auxin transport showed that IAA was transported faster than IBA. The velocities of some of the auxins tested were 7.5 mm h' for IAA, 6.7 mm h ' for naphthaleneacetic acid (NAA) and only 3.2 mm h ' for IBA. Like IAA, IBA was transported predominantly in a basipetal direction (polar transport). After application of 'H-IBA to cuttings of various plants, most of the label remained in the bases of the cuttings. Easy-to-root cultivars were found to absorb more of the auxin and transport more of it to the leaves. It has been postulated that easy-to-root, as opposed to the difficult-to-rool cultivars, have the ability to hydrolyze auxin conjugates at the appropriate time to release free auxin which may promote root initiation. This theory is supported by reports on increased levels of free auxin in the bases of cuttings prior to rooting. The auxin conjugate probably acts 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 lo oxidation, and thus deactivation. The efficiency of the two auxins in root induction therefore seems to depend on the stability of their conjugates. The higher rooting promotion of IBA was also ascribed to the fact that its level remained elevated longer than that of IAA, even though IBA was metabolized in the tissue. IAA was converted to IBA by seedlings of corn and Arabidopsis. The K^ value for IBA formation was low (approximately 20 \iM), indicating high affinity for the 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 suggested that IBA is formed by the acetylation of IAA with acetyl-CoA in the carboxyi position via a biosynthetic pathway analogous to the primary steps of fatty acid biosynthesis, where acetyl moieties are transferred to an acceptor molecule. Incubation of the soluble enzyme fraction from ...

Data suggest that HTR can perform high-volume and -intensity exercise training, reaching exercise performance comparable to or even exceeding values of sedentary or moderately trained healthy subjects.

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