Chapter 13 - Uptake and Transport of Hormones

• Published in: Plant Growth and Development pp. 285 - 302
• Main Author: Lalit M. Srivastava
• Format: Book Chapter
• Language: English
• Published: Elsevier Inc 2002
• ISBN: 012660570X; 9780126605709
• DOI: 10.1016/B978-012660570-9/50155-6

Uptake of undissociated hormones by passive diffusion occurs freely across cell membrane, subject only to concentration gradient and the molecular characteristics of the diffusing species. Auxins and abscisic acid (ABA) are also taken up via their respective carrier proteins. The motive force for such uptake, at least for auxins, is provided by the proton gradient between apoplast and symplast established by the activity of proton ATPases. Hormones and some of their metabolites are translocated long distance in the plant in vascular tissues at high velocities and in the direction in which xylem or phloem streams are translocated. Among the natural hormones, only IAA and indolebutyric acid (IBA) also show a highly directed polar transport, which is strictly basipetal both in shoots and roots and occurs in parenchyma cells at rates three to four times higher than those for diffusion. The polar transport of IAA has enormous implications for the normal growth and development of plants and responses to gravi- and phototropic stimuli. A genetic approach has led to the cloning of genes for both a putative auxin influx carrier and an auxin efflux carrier. Several genes encoding the efflux carrier have been isolated already and more are likely to follow; it is also likely that the influx carrier is encoded by multiple genes. Cloning of these genes is an important breakthrough, which is likely to lead to a clear understanding of the polar transport of auxins, as well as the natural factors that regulate it.