Plant Defense Response

Anupoma Niloya Troyee

Overview

‌As sessile organisms of divergent life-history, plants are constantly exposed to a wide range of environmental fluctuations of abiotic conditions (e.g., temperature, drought, precipitation, nutrients) and biotic interactions (e.g., herbivory, pest, mycorrhiza) that can affect their growth, reproduction, and survival. When fluctuations of abiotic or biotic factors become extreme, plants experience stress, and their responses vary across life history, traits, and environmental context. Both for ecology and plant breeding/cultivation, it has become important to understand the means of plant stress responses because changes in plant productivity affect both biodiversity and agriculture. Natural populations can show differences in performance when they are exposed to changes in environmental conditions, partly because of their genetic variation but also because of their epigenetic variation. A general overview of the different types of plant stress factors, their physiological effects, and individual plant defense response mechanisms to overcome their impacts have been previously reported mainly for model and crop species (Mosa et al. 2017; Liza M. Holeski et al. 2012). In earlier chapters, we have discussed phenotypic plasticity and life-history traits in a more general way. In this chapter, we focus on the plant defense response to stresses from the viewpoint of epigenetics in order to understand the full depiction of the plant defense response.

‌

Understanding the role of epigenetic regulation in plant stress responses in the ecological context of natural populations has been a topic that received wide attention in ecological research (Thiebaut et al. 2019; Richards, Alonso, et al. 2017). For untangling the contribution of genotypes and epigenotypes to the stress response in natural populations, our current knowledge needs to be improved by conferring the epigenetic contribution to different types of stress that involves plant phenotypic variation in key traits of populations (Balao et al. 2018). For example, the diversity of life history (reported in Chapter 2) plays a role in the epigenetic regulation of plant stress responses for species with different longevities (annual, perennial, long-lived, etc.) and type of reproduction (sexual, asexual) (Alonso, Medrano et al. 2019). However, to date, the adaptive and evolutionary importance of epigenetic variation in terms of the plant stress response has only rarely been addressed. The incorporation of epigenetics for understanding the plant defense response is only starting (Balao et al. 2018). Different key traits, analysis of varied plant tissue types (root, leaf, bud, etc.), stress relevant features, and 'epiphenotypes' along with DNA sequence information should be analyzed to have more empirical information, which is needed to comprehend plant defense response in natural populations.

In natural plant populations, the most relevant features of environmental stress are the stress intensity, the occurrence frequency, and its predictability, as they will determine the most successful defense strategy to overcome its negative impact on plant performance. These features are properties of the natural environment of a population and are usually not taken into consideration in studies with model species. The defense strategies that plants can develop in nature after multidimensional biotic and abiotic stress exposure are highly related to the environments in which they live. In general, the intensity and frequency of stress factors tend to be inversely correlated, and the defense strategy is most likely selected by the dominant stress features the plants experience (derived from Grativol et al. 2012 and Walter et al. 2013). Further, the predictability of stress or the reliability of the environmental cue is important that plants can prepare a suitable plastic response, either within- or across generations (Reed et al. 2010). And, at which stage of lifespan a cue is received may also dictate the response strategy for different stresses. Studies suggested that when the environmental cue is persistent, plants also show stress-induced or environmentally induced defense mediated by epigenetic variation that could be transmitted over generations (Mauch-Mani et al. 2017). Both within and across generations, the stability of phenotypic responses depends on the degree and the predictability of environmental variation and on the (epi)genetic architecture (Herman et al. 2014). Therefore, the defense strategies of plants are selected according to the particular stress features in their natural environment, and they are specific for individuals or populations within a species and can also differ from species to species.

‌Plants use stress features as cues to fine-tune their defense responses so that they can minimize detrimental effects on plant fitness (Brown and Rant 2013). We can describe three general ways a plant can respond in accordance to the frequency and intensity of the stress: tolerance, avoidance, and induced defense (Walter et al. 2013; Grativol et al. 2012) (Fig 1).

‌

i) Induced defense: Plants show enhanced activation of induced defense when the intensity of stress is low but is encountered more frequently. Under such circumstances, plant performance will be higher if they have a plastic and reversible defense mechanism and, through prior frequent stress cues, an improved responsiveness or priming/acclimation (Fig 1). This means plants acquire this kind of defense by receiving frequent stimuli from pathogens, arthropods, chemicals, and abiotic cues that can trigger the establishment of priming (Mauch-Mani et al. 2017).

‌ii) Tolerance: Plants can show a neutral response of tolerance or a constitutive strategy where plants perform uniformly (same fitness and reproduction) regardless of the intensity and the frequency of stress occurrence. e.g., exposure to heavy metals or a high concentration of salts select plants with higher tolerance against that specific factor (Yaish 2013).

‌iii) Avoidance: Plants show avoidance or an escape strategy only during less frequent stress events of strong intensity because this strategy is usually associated with high costs. For example, fires, flooding, or complete defoliation events often favor avoidance by re-sprouting from below-ground shoots (Boyko & Kovalchuk 2008, "Epigenetic Control of Plant Stress Response"; Gong and Zhang, 2014).

Otherwise, plants also can have negative responses or damage after stress and eventually can suffer greater damage or even complete collapse when stress is recurrent.

Fig 1. Schematic representation of the relationship between plant performance and stress frequency and intensity according to the three plant defense strategies explained within the text. The two axes are continuous and relative. The X-axis denotes the usually inverse relationship between frequency and intensity for any given stress factor, and the Y-axis denotes plant performance, with zero being the average performance for the standard conditions at a certain environment. The dashed-dotted, straight, and dashed lines indicate the induced, the tolerance, and the avoidance plant defense strategy, respectively.

This ability of the immobile plants to survive under fluctuating conditions is sometimes aided by epigenetic mechanisms that can store information at a potentially low cost (Boyko & Kovalchuk 2008, "Epigenetic Control of Plant Stress Response"; Kranner et al. 2010; Grativol et al. 2012). Epigenetic regulation involves histone variants, histone post-translational modifications, small RNA, and DNA methylation that together alter the chromatin structure and determine changes in individual phenotypes without changing the DNA sequence. In plants, methylation of the 5th carbon of the DNA nuclein base cytosine (DNA methylation hereafter) is found within three sequence contexts along the genome: CG, CHG (H = A, T, C), and CHH. DNA methylation regulates the activation and movement of transposable elements and the expression of genes (see Chapter 6 for details). Furthermore, different histones (H2A, H2B, H3, and H4) can be covalently modified at different positions (mostly lysine and arginine residues) by different chemical marks (see Chapter 7 for details). Finally, small regulatory RNAs (sRNAs; approximately 21–24 nt in size, see Chapter 8 for details) also have emerged as important regulators of gene expression. The main aim of this chapter is to review how these epigenetic factors can contribute to the plants' stress responses and discuss how to fill the gaps in our current understanding, as well as how to untangle the genetic and epigenetic contributions.