Mechanisms for recognizing pathogens: plants vs. animals
I found an interesting article on Science Daily this evening. As we learned in Chapter 14 (Innate Immunity,) the point of contact between our immune system and a pathogen are the patterns found on the surface of that pathogen that are different from our cells. Antigens are similar in concept, just much more specific, so that antigens differ between species of microorganisms (and even between individuals of the same species, as is the case with the antigens that dictate organ rejections in human beings), and patterns (such as peptidoglycan or Lipid A) may be things found on a group of pathogens, but not on our cells.
The Science Daily article references a recent review article from one of the premier scientific journals, Science. The article examines the genomes of a variety of plant and animals for the presence of pattern recognizing proteins, that serve a similar function to the Toll-like receptors originally found in Drosophila, the common fruit fly. Recall from our discussion in class, that absence of Toll-like receptors in a mutant fruit fly made that fly more susceptible to fungal infections, as seen in this picture. Sequencing of the human genome identified clear homologs of Toll-like receptors that serve a similar function. Now Drosophila only has a single Toll-like protein, which will recognize the patterns from fungi and bacteria, while humans have multiple TLR proteins, each recognizing a different pattern.
Here’s the interesting part: in examining plant genomes, researchers have found that the model plant Arabidopsis thaliana have dozens of these pattern recognition proteins, while humans only have between 10 and 15 distinct proteins. Oryza sativa (rice) has potentially hundreds of these proteins. Some of the high numbers of these proteins in plants is due to the structure of plant genomes; the process of domestication by human beings over the past 5000 years has resulted in the duplication of their genomes multiple times. The corn we farm today bears little resemblance to the maize raised by ancient Americans, and that in turn bears little resemblance to the native plant.
This raises some interesting questions! First, what sort of things will plants have to worry about as pathogens? Second, how do they deal with pathogens? And third, why would they ever need all of these pattern recognition proteins, when the humble fruit fly can get by with just one? Bonus opportunity here over Thanksgiving Break: answer some of the above questions for bonus points! Sorry, one bonus per customer for each of the questions.