The role of brassinolide (BR) in acid-induced cell wall loosening was studied using
lkb dwarf pea (
Pisum sativum L.), a BR mutant. Cell wall loosening at pH 4.5 was analyzed by creep measurements of internode segments from
lkb and
LKB (wild type) seedlings stored in 50% glycerol at −15°C (G-segments). Although cell wall extensibility (φ) was not significantly different between
LKB and
lkb, the yield threshold (y) was distinctly lower in
LKB than in
lkb, suggesting that some load-bearing bonds in the cell wall of
LKB are more easily broken than in
lkb. In G-segments from BR-treated, growth stimulated
lkb seedlings y was lowered, and the pattern of the creep rate change was similar to that of
LKB. BR applied
in vitro to
lkb G-segments also lowered y. In heat-treated
LKB G-segments no acid-induced cell wall loosening was observed. However, it was restored by addition of a crude cell wall protein fraction. In heat-treated
lkb G-segments, acid-induced cell wall loosening could be observed only in the presence of both the protein fraction and BR. It is suggested that BR is necessary for acid-induced cell wall loosening and that BR acts as a modulator of the activity of wall loosening proteins which are limiting the yield threshold.
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