Stimulation of border cell production in response to increased carbon dioxide levels

Abstract

Field soil atmospheres have higher CO(2) and lower O(2) concentrations compared with ambient atmosphere, but little is known about the impact of such conditions on root exudation patterns. We used altered levels of CO(2) and O(2) relative to ambient conditions to examine the influence of the atmosphere on the production of root border cells by pea (Pisum sativum) root tips. During germination, atmospheres with high CO(2) and low O(2) inhibited root development and border cell separation in pea seedlings. Later in development, the same atmospheric composition stimulated border cell separation without significantly influencing root growth. Increased CO(2), not low O(2), was responsible for the observed stimulation of border cell number. High CO(2) apparently can override endogenous signals that regulate the number of border cells released from pea roots into the rhizosphere. The same conditions that stimulated border cell production in pea had no such effect in alfalfa (Medicago sativa).

Figure 1

Effects of high CO₂/low O₂ atmospheres on border cell numbers during germination. Pea seeds were germinated under the indicated test atmospheres. In each treatment, 12 seedlings were collected and border cells were harvested and counted individually for each specific root length. The experiment was performed twice. Values are the means of 24 replicates and bars represent se. White bars, 0.03%CO₂:21%O₂; light gray bars, 3% CO₂:18%O₂; dark gray bars, 6%CO₂:15%O₂.

Figure 2

Effects of high CO₂/low O₂ atmospheres on root growth of pea (A) and alfalfa (B) seedlings with established roots. Pea and alfalfa seeds were germinated and grown under ambient atmospheres in an incubator. Seedlings with established roots (25 mm for pea and 20 mm for alfalfa) were transferred to the indicated test atmospheres. Root lengths of 15 seedlings were measured every day for 3 d. The experiment was performed twice. Values are the means of 30 replicates and bars represent se.

Figure 3

Effects of high CO₂/low O₂ atmospheres on border cell separation from pea. Pea seeds were germinated and grown under ambient atmospheric conditions in a controlled-temperature incubator, and seedlings with established roots (25 mm) with border cells were transferred into ambient (0.03% CO₂:21% O₂) and high CO₂/low O₂ (6% CO₂:15% O₂) atmospheric conditions. Seedlings from both treatments were collected 3 d later, root tips were immersed in 1 mL of distilled water for 1 min, and the separation of border cells was observed under a dissecting microscope.

Figure 4

Different effects of high CO₂/low O₂ atmospheres on border cell separation from pea (A) and alfalfa (B). Pea and alfalfa seeds were germinated and grown under ambient atmospheric conditions in a controlled-temperature incubator and seedlings with established roots (25 mm for pea and 20 mm for alfalfa) and a whole set of border cells were transferred into indicated test atmospheres. Seedlings were collected and border cell numbers were counted from each treatment before (0) and 1, 2, and 3 d after the treatment. There were three replicates in each treatment and each sample was the average of five seedlings. The whole test was performed twice. Values are the means of six replicates and bars represent se.

Figure 5

Effects of increased CO₂ (A) or decreased O₂ (B) on border cell numbers of pea. Pea seeds were germinated and grown under ambient atmosphere in an incubator. Pea seedlings with established roots (25 mm) were exposed to the indicated increased CO₂ or decreased O₂ treatments. A certain number of seedlings were collected and border cell numbers were counted from each treatment before (0) and 1, 2, and 3 d after treatment. There were three replicates in each treatment and each sample was the average of five seedlings. The whole test was performed twice. Values are the means of six replicates and bars represent se. White bars, 0.03%CO₂:21%O₂; light gray bars, 3% CO₂:21%O₂; dark gray bars, 6%CO₂:21%O₂.