A Role for Zinc in Plant Defense Against Pathogens and Herbivores

Abstract

Pests and diseases pose a threat to food security, which is nowadays aggravated by climate change and globalization. In this context, agricultural policies demand innovative approaches to more effectively manage resources and overcome the ecological issues raised by intensive farming. Optimization of plant mineral nutrition is a sustainable approach to ameliorate crop health and yield. Zinc is a micronutrient essential for all living organisms with a key role in growth, development, and defense. Competition for Zn affects the outcome of the host-attacker interaction in both plant and animal systems. In this review, we provide a clear framework of the different strategies involving low and high Zn concentrations launched by plants to fight their enemies. After briefly introducing the most relevant macro- and micronutrients for plant defense, the functions of Zn in plant protection are summarized with special emphasis on superoxide dismutases (SODs) and zinc finger proteins. Following, we cover recent meaningful studies identifying Zn-related passive and active mechanisms for plant protection. Finally, Zn-based strategies evolved by pathogens and pests to counteract plant defenses are discussed.

Keywords: Zn-triggered organic defenses; plant enemies; zinc deprivation; zinc hyperaccumulation; zinc toxicity.

Figure 1

Low- and high-Zn conditions created by plants to confront pests, pathogens, and herbivores. Low-Zn conditions when resulted from a poor Zn diet can trigger the synthesis of organic defenses against a wide variety of plant enemies. Active Zn-sequestering/efflux from/to the extracellular media reduces/increases Zn availability, causing deficiency/toxicity to pathogens. The presence of high Zn concentrations in the above-ground parts of Zn hyperaccumulating species causes Zn toxicity to plant attackers.

Figure 2

Zinc concentration that caused a 50% inhibition of Alternaria brassicicola growth in vitro (Alternaria brassicicola EC₅₀) and theoretical correspondence between Zn leaf concentration and Zn concentration in solution in Noccaea caerulescens plants grown at 12 and 102 µM Zn (A). Plant and pathogen response curves to metal concentration. A higher optimal metal concentration for the plant fitness than for the pathogen can lead to protection against diseases by elemental defense (B).

Figure 3

Zn and camalexin, a phytoalexin essential for A. thaliana resistance against Alternaria brassicicola, showed a joint effect in the Arabidopsis response to Alternaria. High leaf Zn concentration could not substitute the role of camalexin in the Arabidopsis camalexin-deficient mutant, pad3, infected with A. brassicicola. Nonetheless, in the wild type, Zn supplementation greatly enhanced the JA-ET-dependent defense signaling pathway and the expression of PAD3, an enzyme that catalyzes the last step in camalexin synthesis.