Plants have been successful in colonizing most environments and when doing so they have to face tough competition not just from other plants but also from animals! So how do plants protect themselves from herbivores? Interactions between plants and insect herbivores are important determinants of plant productivity. In response to the attack, plants have evolved a range of defenses to reduce the threat of injury and loss of productivity.
Did you know that crop losses from damage caused by arthropod pests can exceed 18% annually? Identifying the defensive traits expressed by plants to deter herbivores or limit herbivore damage, and understanding the underlying defense mechanisms, is crucial for crop scientists to exploit plant defensive traits in crop breeding. So it is important to understand how plants fight out the attack on them from herbivores. By studying and replicating the secret ways that plants have adapted to fight various pests and herbivores can actually help in improving yields in plants and cultivation.
According to Rachel S. Meyer and Ashley E. DuVal of McGill University, with an estimated at 2500 species globally domesticated agricultural crops as of now, we can use an artificial selection of desirable traits that enhance yield and quality of the harvested product. This is extremely critical while breeding for set agronomic targets in high input environments with an aim to successfully increased global crop productivity.
Another significant use of understanding the natural fighting mechanism of plants would be to reduce the dependence of insecticides and harmful pesticides in food crops which in turn reduces the health hazards in our food. We also end up reducing water pollution.
The defense mechanism of plants against herbivores can be broadly categorized into three different styles.
- Physical Barriers
- Chemical attack on Pest Settling and Feeding
- Reduced Plant Palatability
Let us examine them in detail to get a better understanding.
1. Physical Barriers: Some plants have adapted themselves to tolerate these damages and reproduce again, whereas other plants have grown traits which defend them from predators. Plants can grow a variety of defenses, such as thorns, and thick or pointy leaves. In this resistance mechanism, the plant structural traits, like trichomes, spinescence, waxy cuticles, sclerophylly, etc., can act as a physical barrier to arthropod pest attachment, feeding and oviposition. The plant cuticle and trichome density are two traits of particular focus in crop protection. Epicuticular waxes form a slippery film or crystals that prevent pests from attaching to the plant surface. Trichomes can prevent pest attachment and limit pest movement on crops. The needle-like thorns present on the plant discourage the predators from consuming it.
2. Chemical attack on Pest Settling and Feeding:
Did you know when a plant is attacked, it can signal its friends and alert them to be careful and build up their chemical defense system? This is because a herbivore feeding and oviposition can induce plant defense, including emission of herbivore-induced plant volatiles (HIPVs). Production of HIPVs signals herbivore presence that can attract natural enemies of the pest and even signal herbivore threat and induce defense responses in neighboring plants. Chemical defenses include compounds with chemical properties that directly deter herbivores from feeding on a plant. A classic example would be when wheat plants modified to produce insect alarm pheromone both repelled aphids and attracted their natural enemies in controlled conditions.
3. Reduced Plant Palatability: This method of defense is more direct and practical to stop not just the current attacker but also deter any future herbivore. Many plants deposit unsavory granular minerals in tissues that deter insect attack and feeding. According to Fergus P. Massey, Department of Biology and Environmental Science, University of Sussex, a well-known example is silica accumulation in grasses (up to 2-5% silica by mass), which is abrasive, damaging herbivore feeding structures, and reducing digestibility. By depositing silica in its tissues, it is thought to increase the abrasiveness of plant tissues, causing increased tooth or mouthpart wear and thereby acting as a feeding deterrent.
Fergus P. Massey observed that when attacked the silica content of grass leaves was up to seven times higher for those plants in the high silica treatment for all species. Can you imagine this – up to 7 times more! This would mean plants actually training the animal not to come their way!
So plants have a tremendous capacity to adapt to the situations and have various secrets up their sleeve that we might not even know yet. As scientists keep studying the interesting defense mechanisms, we will bring you more interesting science facts from the world of plants.
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