Brassinosteroid stimulation of hypocotyl elongation and wall relaxation in pakchoi (Brassica chinensis cv Lei-Choi)

Hypocotyl elongation of pakchoi (Brassica chinensis cv Lei-Choi) was stimulated by applying 300 ng of brassinosteroid (2α,3α,22β,23β-tetrahydroxy-24β-methyl-B-homo-7-oxa- 5α-cho-lestan-6-one, BR) in 1 μL of 50% ethanol to the apex of hypocotyls. BR had its greatest effect on elongation of the apical 3-mm region below the cotyledonary node (75% stimulation) between 6 and 18 h after treatment. Stress/strain (Instron) analysis of this 3-mm region revealed that plastic and elastic components of extension were not significantly different between BR-treated and control seedlings. In pressure-block experiments, the initial rate of relaxation was 2-fold faster in BR-treated plants as compared with controls, whereas after 125 min the total amount of relaxation and the relaxation rate were the same for the two treatments. Osmotic pressure of cell sap expressed from this 3-mm region showed a large decrease (28%) in BR-treated seedlings compared to the controls. We conclude that BR stimulates growth in pakchoi by accelerating the biochemical processes that cause wall relaxation, without inducing a large change in wall mechanical properties.