A RADIAL CONCENTRATION GRADIENT OF INDOLE-3-ACETIC-ACID IS RELATED TOSECONDARY XYLEM DEVELOPMENT IN HYBRID ASPEN

The radial distribution pattern of indole-3-acetic acid (IAA) was dete rmined across the developing tissues of the cambial region in the stem of hybrid aspen (Populus tremula L. x Populus tremuloides Michx). IAA content was measured in consecutive tangential cryosections using a m icroscale mass spectrometry technique. Analysis was performed with wil d-type and transgenic trees with an ectopic expression of Agrobacteriu m tumefaciens IAA-biosynthetic genes. In all tested trees IAA was dist ributed as a steep concentration gradient across the developing tissue s of the cambial region. The peak level of IAA was within the cambial zone, where cell division takes place. Low levels were reached in the region where secondary wall formation was initiated. The transgenic tr ees displayed a lower peak level and a wider radial gradient of IAA co mpared with the wild type. This alteration was related to a lower rate of cambial cell division and a longer duration of xylem cell expansio n in the transgenic trees, resulting in a decreased xylem production a nd a larger fiber lumen area. The results indicate that IAA has a role in regulating not only the rate of physiological processes such as ce ll division, but also the duration of developmental processes such as xylem fiber expansion, suggesting that IAA functions as a morphogen, c onveying positional information during xylem development.