Fact sheet 6: Plant defense response

Anupoma Niloya Troyee

Starting with general aspects of plant stress, this chapter focuses on plant’s defense response to biotic and abiotic stresses, resistance traits, crosstalk in response(s) to stress, phases of plant stress and recently studied correlation of stress to epigenetics.

In the natural environment, plants as sessile organisms are constantly exposed to a wide range of stresses. In biological context, stress can be denoted as any unfavorable condition that exerts a disadvantageous influence on the metabolism, growth or development of an organism [1]. Factors that induce stress in plants can have multiple physical, chemical or biological origins, such as extreme temperatures, drought, limited availability of light, non-optimal mineral composition or soil contamination, pathogen attack, lack of symbiotic partners, interactions with other plants, parasites or herbivores are among them. Stress(es) for plants have been classified also in varied ways, namely, according to the type of factors that cause the stress (biotic or abiotic), effect of the stress (positive or negative), or persistence of the stress (short or long term). For combating stress plants have developed a plethora of protective mechanisms or defensive responses including developmental and morphological adaptations, specific signaling and defense pathways, as well as innate and acquired immunity. Plant stress responses have also different phases and usually involve complex physiological, biochemical and molecular level reactions [2, 3]. Since stress is a major driver of evolution in plants and may have a huge impact on plant breeding and cultivation, it has become important to understand the plant stress responses.

As one of the most significant defense responses, a broad  group of structural, chemical, and indirect resistance traits are observed through evolutionary race between plants and herbivores (i.e., ‘coevolution')[4, 5]. For instance, morphological features like waxes, trichomes, spinescence, raphids, pubescence, sceleropylly are well known physical resistance traits that have evolved in many different plants as a stress response to biotic stress like herbivore attack [8]. These resistance traits are costly implying frequently a reduction in growth and reproduction, a trade-off with critical consequences for plants that have evolved sophisticated mechanisms to balance it. [6, 7]. For adapting unfavorable environmental conditions, plants develop crosstalk among signaling networks during specific stress responses that activate ion channels, kinase cascades, production of reactive oxygen species (ROS), accumulation of hormones such as salicylic acid (SA), ethylene (ET), jasmonic acid (JA) and abscisic acid (ABA) or signal transduction by protein [9, 10]. Epigenetic factors have emerged also as key regulators of the defense response in plants and several research reports have demonstrated, for instance, the important role of small non-coding RNAs in the post transcriptional gene regulatory networks in response to biotic and abiotic stress[11, 13]. Also, chromatin regulators also have been reported to be involved in the regulation of stress-responsive gene networks under different abiotic stress conditions, e.g. histone modification on the drought-inducible genes are changed in response to drought stress [16]. Increasing evidences suggest that heritable variation in DNA methylation and histone modification can cause significant variation in plant defense responses [14, 15]. Additionally, activity of small RNAs and its role in environmental stress plants to modify respective gene is also taken to discussion as a constituent of plant stress responses [11].With the progression of time, the integration of epigenetics with genetics studies has revealed new areas of interactions and epigenetic mechanisms are suggested as one of the as possible mechanisms for ecological stress memory  [12]. So, in a stress-directed way epigenetic marks may control gene evolution that possibly permit initiation of new adaptive alleles on genetic and epigenetic levels. Although current data leave no doubt that throughout plant’s life cycle, it is able to perceive, to process, and to translate different stressful stimuli into adaptive defense responses, we are still far from understanding the underlying epigenetic and genetic role in the physiological and molecular mechanisms involved in them. As a final point, more focus should be given on filling the gaps of epigenetics study related to diverse plant stresses and its response.

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