Priming of plant resistance by natural compounds. Hexanoic acid as a model

Abstract

Some alternative control strategies of currently emerging plant diseases are based on the use of resistance inducers. This review highlights the recent advances made in the characterization of natural compounds that induce resistance by a priming mechanism. These include vitamins, chitosans, oligogalacturonides, volatile organic compounds, azelaic and pipecolic acid, among others. Overall, other than providing novel disease control strategies that meet environmental regulations, natural priming agents are valuable tools to help unravel the complex mechanisms underlying the induced resistance (IR) phenomenon. The data presented in this review reflect the novel contributions made from studying these natural plant inducers, with special emphasis placed on hexanoic acid (Hx), proposed herein as a model tool for this research field. Hx is a potent natural priming agent of proven efficiency in a wide range of host plants and pathogens. It can early activate broad-spectrum defenses by inducing callose deposition and the salicylic acid (SA) and jasmonic acid (JA) pathways. Later it can prime pathogen-specific responses according to the pathogen's lifestyle. Interestingly, Hx primes redox-related genes to produce an anti-oxidant protective effect, which might be critical for limiting the infection of necrotrophs. Our Hx-IR findings also strongly suggest that it is an attractive tool for the molecular characterization of the plant alarmed state, with the added advantage of it being a natural compound.

Keywords: Botrytis cinerea; hexanoic acid; natural inducers; oxidative stress; priming; vitamins.

FIGURE 1

Treatment with natural compounds increases plant protection against future stresses.

FIGURE 2

Root treatment with hexanoic acid protects tomato plants against Botrytis cinerea infection. Four-week-old tomato plants (cv. Ailsa Craig) were treated with 0.6 mM hexanoic acid, 0.5 mM SA, and 0.5 mM BABA under hydroponic conditions. Control plants were treated with water. Lesion diameter was measured at 72 h after inoculation. Data show the lesion diameter (mm) ± SE (n = 20). Different letters represent statistically significant differences (p < 0.05; least significant difference test). Figure modified from Vicedo et al. (2009).

FIGURE 3

Treatment with hexanoic acid reduces the disease symptoms in tomato fruits infected with Botrytis cinerea. Tomato fruits (cv. Ailsa Craig) harvested at different ripening stages were wounded and inoculated with 5 μL of a Botrytis cinerea conidia suspension in each wound. 24 h later, when the first symptoms of infection are already visible, twenty fruits were sprayed with Hx 20 mM. Control fruits were sprayed with sterile water. Three days after treatment fruits were scored for symptoms by measuring the lesion diameter. Statistically significant differences are indicated with (*p-value < 0.05). Representative images of the infected fruits are also provided. Figure from Leyva Pérez (2008).

FIGURE 4

Model for the Hx priming effect on plant defense mechanisms against Botrytis cinerea. Black arrows indicate responses in untreated plants upon Botrytis infection. Orange arrows indicate induced responses in Hx-treated plants upon Botrytis infection. Hx-treatment increases Botrytis-induced responses enhancing callose, OPDA, JA and JA-Ile accumulation; potentiating transcript accumulation of genes like WRKYs, protease inhibitors and PRs, and inducing anti-oxidant, ROS-scavenging and detoxifying mechanisms. Hx, by counteracting the massive ROS accumulation induced by the fungus, alleviates the oxidative imbalance associated with Botrytis infection. Abbreviations: JA-Ile, jasmonoyl-isoleucine; GSH/GSSG, reduced/oxidized glutathione ratio; AsA/DAsA, reduced/oxidized ascorbate ratio; GR, glutathione reductase; GSTs, glutathione-S-transferases.