Caterpillar Compensating To Plant Defenses

Protease Inhibitors

Broadway et al. (1989) demonstrated that chronic ingestion of proteinase inhibitors by caterpillars’ larva causes significant elevation of the level of tryptic activity. Similarly, Gog et al. (2010) revealed that Helicoverpa zea subsists nicotine and trypsin protease inhibitors by modulating expression of cytochrome P450 enzymes. Pest insects such as Helicoverpa sp. produce trypsin in their midgut which can counteract and overcome different defensive proteinase inhibitors in plants which are toxic to insects. Feeding on serine protease inhibitors present in soybean leaves results in hyperproduction of chymotrypsin and elastase-like enzymes in Helicoverpa zea.

Sriniva et. al. in 2006 did research focusing on deciphering the changes in protease specificities and activities arising from altered amino acids at the active site and found a diverse array of complexities in their structure and function. Helicoverpa zea use multiple isomers to offset plant protease inhibitors. For example, serine proteases exert diverse isoforms in the gut of Helicoverpa armigera and alone is known to contain about twenty different types of active serine protease isoforms. The proteolytic enzyme of the pest target protease inhibitors by stimulating the production of the insensitive isoform of protease-like HzTrypsin-S, an isoform of trypsin. Even the 1, 000-fold excess of inhibitors cannot resist this alternate form. Upadhyay et al. (2012) has also found the similar result in their study that seven different peptides selected from trypsin digested total protein has similarity. Other examples suggested the adaptation of Helicoverpa zea to derogate plant defense. Bayes et al. (2005) discovered a B-type carboxypeptidase from Helicoverpa zea (CPBHz) which showed unresponsive to potato carboxypeptidase inhibitor, a defensive plant protease inhibitor. In addition, gene expression is not always the same throughout the life cycle of Helicoverpa sp. Dong et al. (2007) exposed some genes that expressed differentially during larval molting and metamorphosis of Helicoverpa armigera and show versatile functions and gene expression extensively altered due to adaptation to different feeding habitat.

Unlike many insects such as aphids, honey bees, and termites, caterpillars lack gut microbiome except leaf-derived microorganisms suggesting its typical simple structured gut not suitable for microbial colonization. Caterpillars also do not have gastric caeca and most nutrients are absorbed in the midgut. The midgut tissues also have another importance as Jacqualine Bonnie (2006) showed the predilection to defensive regurgitate nature of caterpillar which is highly linked with midgut structure. Under herbivore attack, plants become responsive by changing transcriptional genes which encode enzymes of primary metabolism and accumulates secondary metabolites such as toxic alkaloid nicotine.

Modifying in central metabolism includes but not necessarily limited to cell wall remodeling, induction of protease inhibitors, reduction in photosynthesis and enhanced water loss from around sites (Aldea et. al. , 2005). There are many plant defense proteins that play role in the insect gut which includes threonine deaminase, arginase, leucine, aminopeptidase, serine protease inhibitor, trypsin protease inhibitor, lectins, Mir1 protease, β-glycosidase. Among them lectins are carbohydrate binding protein which disrupt peritrophic matric of midgut and thereby dissolute midgut protection.

Detoxification

The polyphagous Helicoverpa zea encounters a broad range of plant allelochemicals in its many host plants such as coumarin, indole-3-carbinol, xanthotoxin which induce its detoxification genes and diets containing coumarin or xanthotoxin significantly reduced the mortality rates of caterpillars exposed to the insecticides, diazinon and carbaryl. Metabolic resistance is the key way for herbivore insects to escape the toxins such as nicotine. The enzymes responsible for the metabolism, also known as detoxification, are the cytochrome P450 monooxygenase (P450s), glutathione transferase, and carboxylesterase. It is hypothesized that these detoxification enzymes in saliva starts detoxification on the feeding site or the leaf surface during chewing which needs to be confirmed.

Another experiment showed that more immunity related gene are stimulated in the fat body of Helicovrepa when become exposed with fungi or bacteria on plant tissues. Roxanne et. al. in 1989 proved that chronic ingestion of proteinase inhibitors by caterpillars’ larva causes significant elevation of the level of tryptic activity. Similarly, Musser et. al. in 2010 revealed that Helicoverpa zea subsists nicotine and trypsin protease inhibitors by modulating expression of cytochrome P450 enzymes. Pest insects such as Helicoverpa sp. produce trypsin in their midgut which can counteract and overcome different defensive proteinase inhibitors in plants which are toxic to insects. Feeding on serine protease inhibitors present in soybean leaves results in hyperproduction of chymotrypsin and elastase-like enzymes in Helicoverpa zea. Thus, we can see that some protease enzymes like trypsin and chymotrypsin have multiple functions like digestion as well as detoxification. Helicoverpa zea can offset the challenge of protease inhibitor which targets the proteolytic enzyme of the pest by alternating the insensitive isoform of protease-like HzTrypsin-S, an isoform of trypsin. This alternate form is not inhibited by the 1, 000-fold excess of inhibitors. Santosh et. al. in 2012 has also found the similar result in their experiment and figured out with denovo sequencing data that seven different peptides selected from trypsin digested total protein has similarity. Many other examples are suggesting the adaptation of Helicoverpa zea to derogate plant defense. Alex et. al. in 2005 discovered a B-type carboxypeptidase from Helicoverpa zea (CPBHz) which showed unresponsive to potato carboxypeptidase inhibitor, a defensive plant protease inhibitor.

There is a correlation between the large number of detoxification genes and insecticide sensitivity. Claudianos et al. (2006) enlightened on demonstrated that fewer protein coding genes in honeybees had nearly in comparison with Drosophila and Anopheles of about tenfold s fewer in numbers resulting in less CYP4 P450s genes than Drosophila or Anopheles and this resulted and more in higher sensitivity to insecticides for bees. Again, expression of genes is not always the same throughout the life cycle of Helicoverpa sp. Du-Juan et. al. in 2007 exposed some genes that expressed differentially during larval molting and metamorphosis of Helicoverpa armigera.