A comparison of oligogalacturonide- and auxin-induced extracellular alkalinization and growth responses in roots of intact cucumber seedlings

Abstract

Oligogalacturonic acid (OGA) affects plant growth and development in an antagonistic manner to that of the auxin indole-3-acetic acid (IAA), the mechanism by which remains to be determined. This study describes the relationship between IAA and OGA activity in intact cucumber (Cucumis sativus) seedlings. Both OGA and IAA induced rapid and transient extracellular alkalinization; however, the characteristics of the OGA and IAA responses differed in their kinetics, magnitude, calcium dependence, and region of the root in which they induced their maximal response. IAA (1 microM) induced a saturating alkalinization response of approximately 0.2 pH unit and a rapid reduction (approximately 80%) in root growth that only partially recovered over 20 h. OGAs, specifically those with a degree of polymerization of 10 to 13, induced a maximal alkalinization response of 0.48 pH unit, but OGA treatment did not alter root growth. Saturating concentrations of OGA did not block IAA-induced alkalinization or the initial IAA-induced inhibition of root growth but allowed IAA-treated roots to recover their initial growth rate within 270 min. IAA-induced alkalinization occurs primarily in the growing apical region of the root, whereas OGA induced its maximal response in the basal region of the root. This study demonstrates that OGA and IAA act by distinct mechanisms and that OGA does not simply act by inhibition of IAA action. These results also suggest that IAA-induced extracellular alkalinization is not sufficient to account for the mechanism by which IAA inhibits root growth.

Figure 1

Time courses of the alkalinization response of the incubation medium of intact cucumber seedlings induced by OGA pool or IAA. Data represents the mean + se of at least three separate experiments (some error bars are smaller than symbols). The following treatments were added at time zero to 50 2-d-old seedlings in 10 mL of incubation medium: 50 μg mL⁻¹ OGA pool (⋄), 30 μg mL⁻¹ OGA pool (▵), 15 μg mL⁻¹ OGA pool (□), 7.5 μg mL⁻¹ OGA pool (○), 1 μm IAA (♦), 0.2 μm IAA (▴), 0.05 μm IAA (▪), 0.02 μm IAA (●), and no addition (X).

Figure 2

Effect of the DP of OGA on their ability to induce alkalinization in the incubation medium of intact cucumber seedlings. Each OGA fraction was tested at four concentrations in at least three separate experiments. The activity of each OGA treatment was calculated by the ratio of the pH increase induced by that treatment divided by the average alkalinization response induced by saturating concentrations of OGA pool in the same experiment (pH/pH_max). The data presented represent the mean ± se of pH/pH_max at a concentration of 2.84 μm as determined by an analysis of covariance model. The letters represent groups that are significantly different based on Bonferroni adjusted multiple comparisons (see text for P values).

Figure 3

Growth rates of the roots of intact cucumber seedlings treated with IAA and/or OGA. The roots of ten 2-d-old cucumber seedlings were bathed in 18 mL of incubation medium in hydroponic growth chambers. Each point represents the mean ± se of the growth rate measured at 90′ intervals in at least four separate experiments. The growth rate at time zero represents the average growth rate measured over the preceding 3 h. A, Growth rate of roots treated with 1 μm IAA (▪; n = 6), 0.2 μm IAA (▴; n = 4), and untreated roots (●; n = 6). B, Growth rate of roots treated with 50 μg mL⁻¹ OGA pool at −60 min followed by addition at 0 min of 1 μm IAA (▪; n = 6), 0.2 μm IAA (▴; n = 4), or no additional treatment (●; n = 4).

Figure 4

Representative time courses of OGA- and IAA-induced alkalinization in hydroponic growth chambers under the same conditions described for Figure 3. OGA pool (50 μg mL⁻¹) was added at −60 min and 1 μm IAA was added at 0 min. The treatments are: OGA pool followed by IAA (●), OGA pool alone (⋄), IAA alone (▪), and no addition (X).