Manipulation of Plant’s Defense by Herbivores Poses Challenge to Entomologists

Plants have inherent defensive mechanism which protects them from their predators and phytopathogens. Their inability to move is largely complemented by the defensive adaptations which help the plants to survive and reproduce successfully. One of the most important defensive mechanisms is production of secondary metabolites. These metabolites are the chemical substances which are not involved in the normal photosynthetic process of the plants. They are alternatively known as allelochemicals and they can influence the behavior, growth, or survival of herbivores. Allelochemicals act as toxins to herbivores and in some cases also reduce the digestibility of plant cell wall.

Some external structural features on the stem and the leaves (prickles, spines, thorns etc) discourage herbivory to a very large extent. These mechanical defensive mechanisms deter, injure, or kill the grazer. Some defensive substances like wax, resins, lignins, etc. are released on the plants’ epidermis and prevent the herbivores from feeding on them. Apart from these, sensitivity to touch shown by Mimosa pudica is another example of defense mechanism in plants.

Herbivores form a primary link in the food chain, as they are the source of energy for organisms ranking higher in the food chain. Plants are the only source of food for herbivores and this led to the co-evolution of adaptations, which help them to acquire their food, irrespective of the defense mechanisms in plants. Some of these adaptations are:

• synthesis of enzymes which can reduce the effect of secondary metabolites
• existence of internal symbiotic bacteria, fungi, or protozoa which help to break down the plant cellulose so that it can be digested by the herbivore
• mechanical adaptations like incisors in moths which helps them to eat soft leaves.

Amongst these adaptations, host manipulation is the most important mechanism by which herbivores feed on the plants in a better way. According to the recent research conducted by Renato Almeida Sarmento and his team, invasive spider mite Tetranychus evansi, manipulates its host (tomato), by interfering with signaling pathways involved in its defense mechanism [1].

Primary hosts for T. evansi are solanaceous crops like tomato, potato, tobacco etc. These mites puncture the leaves while feeding, leading to yellowing or whitening of leaves. Subsequently the leaves fall down and in case of severe attack the plant dies. The study conducted by Sarmento, suggests that T. evansi, manipulates the host defense mechanism by suppressing the induction of the salicylic acid and jasmonic acid signalling routes which are involved in plant defenses in tomato plant. Salicylic acid and jasmonic acid play a vital role in plants immunity. Spider mites reduce the levels of inducible defense compounds and volatiles (example, proteinase inhibitors) which play a crucial role in plants defense.

Crops like tomato and potato find a widespread use across the world. Control of pathogens affecting these crops is a vital area of research. The result of the research by Sarmento, provides a better understanding of the plant-herbivore interactions. It opens the gateway for further research in two directions:

• Biological control of Tetranychus evansi at an affordable cost to the farmer.
• Developing crop varieties which are able to combat the manipulations by herbivores.

Reference:

[1] Sarmento R, Lemos F et al. A herbivore that manipulates plant defense. Ecol Lett. 2011 March 14(3): 229-236.