PLANT PHYSIOLOGY , Vol 106, Issue 2 469-476, Copyright © 1994 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Effects of the Indole-3-Acetic Acid (IAA) Transport Inhibitors N-1-Naphthylphthalamic Acid and Morphactin on Endogenous IAA Dynamics in Relation to Compression Wood Formation in 1-Year-Old Pinus sylvestris (L.) Shoots
B. Sundberg, H. Tuominen and CHA. Little
Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, S-901 83 Umea, Sweden (B.S., H.T.)
Both N-1-naphthylphthalamic acid (NPA) and
methyl-2-chloro-9-hydroxyfluorene-9-carboxylic acid (CF) inhibit the polar
transport of indole-3-acetic acid (IAA) and, therefore, are attractive
tools for investigating IAA's role in the regulation of plant growth.
Ringing an intact conifer shoot with lanolin containing NPA or CF induces
the formation of compression wood above the ring. This induction has been
attributed to a postulated accumulation of IAA above the application site
of the IAA transport inhibitor, but the validity of this postulation has
never been confirmed. Using gas chromatography-selected ion monitoring-mass
spectroscopy with [13C6]IAA as an internal standard, we measured the levels
of endogenous free and conjugated IAA in 1-year-old Pinus sylvestris (L.)
shoots ringed with NPA or CF. The level of free IAA was dramatically
decreased below the ring, indicating that the polar transport of endogenous
IAA was inhibited by the treatment. However, the free IAA level above the
ring, where compression wood was formed, was also slightly lower than in
control shoots. The lack of IAA accumulation above the site of the IAA
transport inhibitor could not be explained by an increase in IAA
conjugation. Furthermore, the turnover of [2-14C]IAA, measured using
high-performance liquid chromatography with on-line radioactivity
monitoring, was the same in NPA-treated and control shoots. The decrease in
IAA level above a NPA or CF ring is attributed to these substances being
transported acropetally and interfering with polar IAA transport along the
shoot. It is concluded that compression wood formation above a NPA or CF
ring is not associated with an overall increase in cambial region IAA level
or increased IAA turnover. Instead, we suggest that acropetally transported
NPA and CF induce compression wood formation by interacting with the NPA
receptor in differentiating tracheids, thereby locally increasing IAA in
these cells.
