Brassinolide as a modulator of the activities of cell wall loosening proteins

Abstract

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 pH4.5 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 (f) 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.