6-Hydroxy-5-nitrobenzo[d]oxazol-2(3H)-one—A degradable derivative of natural 6-Hydroxybenzoxazolin-2(3H)-one produced by Pantoea ananatis

Figures & data

Figure 1. The color of 6-hydroxy-5-nitrobenzo[d]oxazol-2(3H)-one depends on pH: pH 3 greenish; pH 7 yellow; pH 8 orange; pH 9–10 wine-red. Pantoea ananatis cultures, producing the compound from BOA-6-OH, change the color from yellow to orange due to the pH value which increases over time. The anionic, orange form of the compound is degraded, as shown by the HPLC chromatograms (left: analysis of the yellow medium immediately after adding synthetic nitro-BOA-6-OH (black points) before the medium turned to orange. After 3 h (right), only traces of the compound are left.

Figure 2. Enzymatic nitration by Pantoea ananatis (P.a.) exuded protein, horseradish peroxidase (HRP) and Abutilon (Ab) root surface proteins in presence of nitrite; P.a. and HRP also in the presence of nitrate. No enz. = no enzyme was added to the assay. Ab/B-6-OH: root surface proteins from seedlings pre-incubated with BOA-6-OH. Means ± SD are shown; asterisk(s) indicate significant differences (t-test, * = p < 0.05; *** = p < 0.0001).

Figure 3. 6-Hydroxy-5-nitrobenzo[d]oxazol-2(3H)-one (NO2-BOA-6-OH) inhibits the growth of Lepidium sativum (I50 values: lowest at pH 6.0 with 20 µM for root growth and 10 µM for the shoot growth). As expected, A. theophrasti is rather insensitive. The I50 values for nitrated BOA-6-OH increased with pH from averaged 300 µM at pH 4.0 and 5.0 to 1.7 mM at pH 6.0 and 2 mM at pH 7.0 for root growth. The averaged I50 value of 0.7 mM for Abutilon shoot growth was approximately the same for all tested pH values. Means ± SD are shown; different letters indicate significant differences (t-test, a = p<0.05; b = <0.005; c = p<0.0001).