Priming Bean Seedlings to Boost Natural Plant Defenses Against Common Bacterial Wilt: Salicylic Acid Responses to Chemical Primers (Part 1)

Abstract

This greenhouse study evaluated the effects of two chemical inducers for priming kidney bean seedlings against a bacterial wilt disease. This study's central premise was that chlorine dioxide's oxidant properties would mimic the signaling properties of radical oxygen species, thereby initiating a cascade of molecular plant defenses, including the synthesis of salicylic acid (SA). This signaling agent then initiates a cascade of pre-defense activities to provide a more rapid and robust natural defense against pathogen attacks. This factorial study included two levels for a foliar chlorine dioxide treatment and two for a bacterial wilt inoculation treatment. The two plant response variables were free and conjugated salicylic acid levels sampled in leaf tissue over two collection dates. Half of the 96 plants were inoculated with a bacterial culture that causes common bean wilt disease. Leaf tissue was harvested 17 to 32 h and 960 h after the wilt inoculation to determine the temporal dynamics of SA due to chemical treatments. Also, PCR tests were used to verify wilt presence in the inoculated plants. Inoculation of the wilt disease did not affect free SA when leaf tissue was sampled from 17 to 32 h. after wilt inoculation. However, chlorine dioxide applied at 400 mg/l and sampled at 20 h after inoculation resulted in a 15-fold increase in free SA over the control. Also, chlorine dioxide applied at 400 mg/l with leaf tissue sampled at 26 h after inoculation resulted in a 33-fold increase in conjugate SA levels compared to the control plants. Leaf tissue sampled at 960 h after the inoculation showed no free SA differences among the chemical treatments. However, the inoculated plant had a 15.9-fold increase in free SA compared to the non-inoculated plants. The priming effect on kidney bean seedlings using a single chlorine dioxide foliage application temporarily increased free and conjugate SA. The free and conjugate SA levels for the non-inoculated plants returned to baseline levels when sampled at 960 h. These results indicate that primed plants elevate SA up to several weeks with a slow decline back to baseline levels. Stem injection of the bacterial wilt bypassed the immunity mechanisms present in leaves, which significantly increased the wilt injury levels. Stem injection negated much of the foliar defenses, which overshadowed the priming effects of the chemical treatments on plant immunity and foliar defenses. The second leaf sampling on newly formed leaves reveals elevated SA levels in the inoculated plants but not in the non-inoculated plants.