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.

Priming Bean Seedlings to Boost Natural Plant Defenses Against Common Bacterial Wilt: Leaf Architecture, Leaf area, Foliage Water Content, and Plant Biomass Results (Part 3)

“…Vicinal, structured water also increases heat tolerance which was explained in more detail in Part 2 of this series [2]. Structured water has lower vapor pressure properties, as explained in Part 2, which means it changes from a liquid to a gas phase more slowly than free water.…”

“…This second explanation correlates the increased free salicylic acid (SA) concentrations for both EB formulations with a possible increase in intracellular bound water for both treatments. As mentioned in Part 1 of this series, both EB formulations increased free SA levels when measured at 5 DAT for the chemical treatments [1]. Please review Part 1 for a full explanation of how the EB formulation generated an oxidative burst that resulted in the free SA peaks in the foliage tissue at about 5 DAT.…”

“…Of these three priming factors, it seems intuitive that plants would have the ability to respond less robustly to a single primer application versus repeated primer applications. In Part 1 of this series, the single primer treatments generated a double-peaked curve in free salicylic acid (SA) concentration in leaf tissue [1]. These peak SA concentrations subsided back to baseline levels within 24 hours after the CFF injection date and five days after the primer treatments for both the CFF inoculated and non-inoculated plants.…”

“…However, there is some evidence that the intracellular water levels were increased, which in turn suggests that the levels of bound water also increased. In Part 2 of this article series, one explanation was offered for the assumed increase in intracellular water contents [2]. This explanation suggested that the EB formulations temporarily reduced the chlorophyll density, thereby allowing red light to be absorbed by water molecules which results in higher levels of bound water inside the cells.…”

“…This article (Part 3 is the last of three articles (Part 1 -3) involving a greenhouse study that evaluated the effects of chemical inducers on kidney bean seedlings inoculated with a vascular wilt disease. The first article [1] reported on the salicylic acid results, while the second article (Part 2) [2] reported on the gas exchange and fluorescent results of this study. This two-year study was divided into three parts to keep each article readable and focused on specific findings.…”