Plant Defense Mechanisms

Over the course of 350 million years, plants and insects have coevolved to counter each other’s defensive strategies. A variety of physical and biochemical mechanisms are used almost intuitively to help plants deter herbivorous pests. Chemical signals and compounds emitted by plants can be exploited by herbivores, predators and parasitoids to locate resources. In this review, we will look at the ways plants interact with their herbivore counterparts. We will also delve into how science is using herbivore signaling and compounds as a natural form of pesticide.

INTRODUCTION

Adaptations evolved by plants, improve their survival and reproductive success by reducing herbivore impact. The earliest land plants can be found dated back in the fossil record at 450 million years (Erlich and Raven, 1964). The appearance of angiosperms, or flowering plants, in the Cretaceous Period lead to an explosive expansion of insect diversity. In response, plants developed surveillance systems to recognize the herbivorous pests. Induced reactions can be stimulated by touch, encouraging the production of secondary plant metabolites that influence changes in growth, behavior and survival.

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PHYSICAL DEFENSE: SPECIALIZED ORGANS AND TISSUES

All plant tissues come equipped with structural barriers that limit the attachment and invasion of pathogens. The primary cell wall provides structural support and regulates turgor pressure (Keegstra, 2010). Structural fibers including cellulose, microfibrils, and cross-linking glycans contained in the wall enhance support function. This function shifts a bit in the presence of a pathogen, where the wall will thicken to provide additional strength. During a microbial attack, plants deposit callose between the cell membrane and cell wall. Papillae composed of callose deposits impede cellular penetration at the site of infection. Plants cells also have specialized receptors that signal an alarm in the presence of a pathogen. Elicitor compounds found in the saliva of herbivorous insects bind with receptors imbedded in the cell wall, initializing an oxidative catalyst reaction. Digestive enzymes are released, damaging the invading organisms.

Highly specialized cells including idioblasts, sclerenchyma cells, pigmented cells, and stinging cells, further refine plant defense approach. Toxic chemicals or sharp crystals contained in idioblast cells tear apart the mouthparts of feeding herbivores. Sclerenchyma cells, found in fruit of pears and apples, have highly lignified cellular walls that form bundles of fortified tissue (Zhang et al, 2017). Bitter-tasting compounds found in pigmented cells make plant parts undesirable to eat, Stinging cells imbedded in the waxy cuticle of some weedy species, break off upon contact depositing irritating toxins. Some cells of this variety contain prostaglandins, used to further amplify the pain response. This diverse armory of defensive structurse allow plants to persist while under constant attack.

BIOCHEMICAL DEFENSE

Plant chemical defense compounds can be divided into two main categories, primary and secondary metabolites. Structural compounds such as proteins, sugars, nucleic acids, and amino acids are categorized as primary metabolites. Secondary metabolites are not directly involved in major metabolic processes. When triggered, these compounds may be applied to either quantitative or qualitative defense (Yang et al, 2018). Qualitative metabolites are small toxic molecules, found throughout the tissues of young leaves and buds. These toxins interfere with herbivore metabolism and are reaction specific. They can be rapidly created with little energetic investment and are not dosage dependent. Non-adaptive generalist herbivores are impacted the greatest by this group of compounds. In contrast, quantitative defense compounds are present in high concentrations and are equally as effective against specialist and generalist herbivores (Kushalappa, Yogendra, & Karre, 2016). Energetic investment is much larger in this group due to the extended formation and transport time required.

Defense compounds can be further divided into large chemical groups. Nitrogen containing alkaloids are bitter in taste and include nicotine, caffeine, cocaine and morphine. High levels of caffeine produced by coffee seedlings inhibit the germination of seeds in close proximity. This inhibition of growth to optimize space and resources is known as allelopathy (Berton, Yang and Weston, 2003). Phenolics are another large compound produced primarily through shikimic acid and malonic acid pathways (Bhattacharya, Stood, & Citovsky 2010). These compounds are generally used for stress protection and play an important role in plant development and structural support.

ARTIFICIAL APPLICATIONS OF PLANT DEFENSE

The interactions between plants and herbivores help to determine overall productivity in vegetation. Worldwide, the number of domesticated crop plants is estimated at 2500 species. Consumption by foliage, sap and root feeding insects have reduced net plant productivity by 20 percent globally (Agrawal, 2011). Crop loss attributed to pests, weeds and disease is estimated even higher at an astonishing 40 percent. A decrease in energy directed towards growth often comes consequently, usually as an overall decrease in plant mass and fitness. Selecting genes that increase resilience and provide resistance to herbivore attack will increase overall yield and production. Understanding the mechanisms behind physical defense, plant persistence and herbivore induced volatiles allow scientists to alter plants in a way that benefits the agriculture community.

One of the most difficult herbivorous insects to control on cultivated plants are aphids, from the superfamily Aphidoidea (Nalam, Loius and Shaw. 2019) Members of this group are specialists, feeding on the phloem of plants. The number of species in existence is estimated at 4000, with 250 labeled as highly destructive. As an r-selective species, aphids have a high reproductive output, with a short generation time. The decline of host plant fitness triggers the production of winged morphs, allowing them to disperse over large areas.

Exploitation of insect defense mechanisms and compounds may be the key to chemical free plant defense. Previous studies have performed successful trials in the expression of insect defense traits in crop plants (Bruce et al, 2015). The manipulation of pheromones and compounds is a sustainable pest management strategy the would reduce the use of broad-spectrum pesticides and the need for seasonal pesticide application.

PHYSICAL DEFENSE MECHANISMS

Physical plant defense structures impede herbivory by preventing consumption or attachment. Bark, waxy cuticles, thorns and spines provide the first line of defense against herbivore attack. Thorns and spines, in particular, are located on the leaves or stems of the host plant. Upon ingestion, these structures damage the mouth parts and internal structures of the herbivore. If these structures are compromised by mechanical damage, plants may employ the use of secondary metabolites and induce volatiles. In addition to defensive structures, plants may employ biotic defensive strategies. The tree selected for this study provides food and/or shelter for potential predators. In exchange, predators are given protection (Calixto, Lange, and Del-Claro, 2015). Extrafloral nectaries produce a sweet liquid containing sugars, lipids, amino acids, alkaloids, phenols and volatile organic compounds. This nectar attracts several arthropods predators, including ants. Past research suggests that the presence of ants on foliage increases defense against herbivores (Lange and Del-Claro, 2014).

In my primary article, the authors explore the effect of the three most evident defenses found in the leaves of Qualea multiflora (Vochysiaceae) during foliar sprouting (Calixto, Lange and Del-Claro, 2015). The presence of trichomes, foliar toughness and the activity of extrafloral nectaries were examined to quantify the impact on foliar sprouting. Foliar herbivory at each leaf stage was also recorded. Combined, this data would aid the authors in determining whether biotic and abiotic plant defense against herbivores operate both independently and in conjunction with one another. To support their hypothesis, Calixto and colleagues formulated four questions, 1) do these defenses vary during leaf development, 2) what the period of peak activity of each type of defense is, 3) do peak effectiveness of defenses overlap, and 4) does leaf area loss caused by herbivores vary depending on leaf age.

Qualea multiflora (Vochysiaceae) is one of the most abundant trees inhabiting the Brazilian savannah (Appolinario and Schiavini, 2018). Data was collected on Q. multiflora from October 2010 to January 2011, in the Ecological Reserve of the Clube Caca e Pesca Itororo De Uberlandia, MG Brazil. eExtrafloral nectaries in the leaf petiole, non-glandular trichomes in the leaf blade, and the attraction of ants to it’s extrafloral nectaries, made this species a strong candidate for the authors’ study.

To gather data on defense and herbivory, the authors tagged 19 Q. multiflora individuals carrying the same selected traits. All tagged plants showed signs of stem resprouting and had a height of 3 m. The first 3 leaf producing branches were tagged on each plant and then collected in December. Leaves were put in categorizes from 1 to 5, with 1 having the newest growth and 5 having the oldest growth. Manual counters were then used to count the number of trichimes in the medial region of the abaxial leaf surface near to midrib. Leaves were photographed and measurements were taken representing lost foliar area. The dimensions of each leaf were also taken. Extrafloral necataries were accounted for by observing the presence or absence of bland necrosis in the branches.

Productivity of EFNs was quantified by scoring thres distinct branches from nine plants and evaluateing them daily.

A second paper on the topic H. perezi infection in blue crab populations investigates how size and molt stage influence the susceptibility and timing of disease transmission (Lycett, Chung and Pitula, 2018). Seasonal shifts in H. perezi prevalence have been linked to other external factors such as pH, temperature, salinity and dissolved oxygen (Lycett and Pitula, 2017). Openings in the carapace during molting may leave young crabs vulnerable to infection. Lycett and colleagues (2018) sought out to determine if the rate and prevalence of infection observed in young crabs during seasonal peaks could be distinguished from other size classes.

In conclusion, Lycett (2018) determines that all size classes can harbor a range of infection, with larger crabs exhibiting higher rates of parasitic proliferation and disease progression. Juvenile crabs showed an increased prevalence of disease in the fall months, whereas medium crabs were more vulnerable to infection in the summer months.