April 29, 2015

Tiny Monster

The aquatic carnivorous bladderwort Utricularia gibba

Plants can move. Plant signal transduction research is frequently contextualized as follows: plants have to be particularly sensitive to changes in their environment, as they are sessile and are unable to move to a different location when conditions are unfavorable. Though technically correct, this statement is a bit simplistic. Plants can move towards or away from things—they just do it by directional growth rather than locomotion. In addition, a few very specialized plants actually do move in the way that is familiar to us. The closing of the Venus flytrap is probably the most famous example.

Utricularia gibba. Another carnivorous plant that exhibits movement is the aquatic bladderwort U. gibba. Multiple tiny traps that grow along the length of free-floating (rootless) stems are used to catch prey (mostly zooplankton). U. gibba is becoming an exciting model system for the study trap function, genome size, and the development of complex plant structures. U. gibba is so fast and easy to grow that it is a favorite of fans of carnivorous or bog plants and can be purchased easily (we ordered from California Carnivores).

Credit: Enrique Ibarra-Laclette and Claudia Anahí Pérez-Torres

A compact genome. Despite having undergone 3 whole genome duplications and encoding over  28,000 genes, the U. gibba genome is only 82 Mb. For comparison, the Arabidopsis thaliana genome is almost twice the size but has the same number of genes. This compation appears to be the result of very little noncoding DNA or transposons. Its small, sequenced genome makes U. gibba an interesting case study for those interested in genome structure and evolution. It also makes it a potentially useful molecular genetic model system!

Trap function. The U. gibba trap is a tiny oval bladder with an entrance at one end. This entrance is covered by a “trapdoor” and surrounded by “trigger trichomes” (see image to right). In its resting state, the bladder is thought to be under negative pressure. When a prey interacts with the trigger trichomes, the trapdoor opens and the animal is aspirated into the trap. This firing is one of the fastest movements recorded in plants, occurring in 10-15 miliseconds. After firing, water is pumped back out of the trap and it returns to its concave shape within about 30 minutes.

Mechanism of trap firing. One question that remains unresolved in the literature is whether the trigger is 1) strictly mechanical or 2) potentiated by electrochemical signaling--by an increase in action potential, for example. A well-regarded book from the 1940s presents evidence for the former, but at least one later publication (Aust. J. biol. Sci., 1973, 26, 1115-26)  questions those conclusions. I am hoping to test these ideas this summer at the MBL Physiology Course and during my sabbatical in Spring 2016, so stay tuned!

Sources:
Adamec, 2011
Ibarra-Laclette et al., 2013
Coen Lab Website