Moving to the Field: Plant Innate Immunity in Crop Protection

In natural ecosystems, disease is not the rule, but a rare outcome in the spectrum of plant–microbe interaction, since plants have developed, during their evolutionary history, various defence strategies to face pathogens. Therefore, in this evolutionary arms race, plants have (co)evolved a complex set of defence mechanisms to counteract pathogen challenging and, in most cases, prevent infection. As animals, plants are able to recognize and distinguish between self, non-self, and altered self, by their innate immune system, thus activating a battery of defence reactions. When a pathogen becomes able to overcome these defences, disease ceases to be the exception [1]. Though a comprehensive discussion on the plant immune system is beyond the scope of this editorial, the molecular mechanisms involved in the plant immunity have been recently reviewed [2–6]. Kørner et al. [7] emphasized the cross-talk between endoplasmic reticulum (ER) stress signaling pathways and immune responses in plants. In particular, IRE1 (inositol requiring enzyme 1) is a conserved ER stress sensor protein identified as a transcriptional regulator of ER genes and involved in immunity and programmed cell death (PCD). In their review article, Wang et al. [8] focused on the role of NADPH oxidases, the major source of apoplastic reactive oxygen species (ROS) under both normal and stress conditions, in mediating PCD and plant immune response. However, with the advent of the agro-ecosystem, this equilibrium was altered because of human activities such as intensive farming, monoculture, and varietal selection. In this context, diseases that damage crops have to be managed by different control strategies integrated into pest management programs. According to Regulation (EC) N◦ 1107/2009, a plant protection product generally contains more than one component, and the active constituent against pathogens/pests/weeds is referred to as active substance [9]. Plant protection products are usually used for (i) protecting plants or plant products against damaging organisms; (ii) influencing the plant growth (plant growth regulators); and (iii) preventing growth or eradicating undesired plants (weeds). Nowadays, chemical control represents the most used and effective strategy in crop protection, with a variety of agrochemicals available to control plant diseases, pests, and weeds, such as fungicides, insecticides, and herbicides. In this scenario, the use of elicitors and plant activators represents a novel and promising strategy in crop protection, as an alternative to conventional agrochemicals that exert direct toxic effects on noxious organisms. Indeed, elicitors and plant activators trigger the plant’s own defence mechanisms by stimulating the plant innate immune system, differently from conventional pesticides. Alexandersson et al. [10] provided a current summary of plant resistance inducers that have been successfully used in Solanaceae species to protect against pathogens.