Figure 4.

GA Increases the Elasticity of Hypocotyls.

(A) and (C) Hypocotyls were frozen and thawed then subjected to cycles of application and removal of force (Figures 2B and 2E).

(A) The average magnitude of strain incurred by 2 (2d) and 3 DAS (3d), control (Con), and GA-treated seedlings, and uniconazole (Uni)-treated seedlings at a force of 5 mN is shown. Bright-field images were collected every 645 ms, and strain was computed from regions that were tracked in the images using the ACMEtracker software (see Methods). Bars show means ± se (n ≥ 4, at least five oscillations were made). The strain for 2d GA differs significantly from the 2d Con (P = 0.041), and Uni differs from 3d GA (P = 0.025).

(B) AFM-based elastic stiffness obtained from living, plasmolysed, and 2 and 3 DAS seedlings, control, or treated with GA. Bars show means ± se. For each hypocotyl, three areas of 50 × 100 μm were indented from at least two independent samples. GA and Con differ significantly at 2 d (P < 2e-16, n > 400) and 3 d (P < 2e-16, n > 300). Different letters indicate means differ significantly (P < 0.05).

(C) Average strain was computed from rapid oscillation at a range of forces for 3 DAS seedlings. As above, batches of 5 to 10 oscillations were made with 30 s at zero force in between forces (as shown in Figures 2A and 2D). Strain was computed from images as described for (A). The force strain curves were compared with a linear and Hill-type model (Table 2).