Insectivorous Plants - Part 1

A great way to engage young scientists in plant biology is to introduce them to insectivorous (carnivorous) plants. These are fascinating organisms that capture and devour their insect prey, mostly for their nitrogen content. Insectivorous plants evolved to thrive in nitrogen poor environments where N availability severely limits growth of other (non-insectivorous) plant species. These environments include boggy soils and swamps. By capturing and digesting insects, insectivorous plants acquire essential nitrogen-rich amino acids derived from break down of insect protein. This strategy of nitrogen acquisition requires several biochemical innovations which I will discuss in depth below and in subsequent posts in this series. This strategy of insect consumption, however, poses a bit of a dilemma for some insectivorous plants if they are also insect pollinated! How do they avoid killing their own pollinators? I will try to answer this question for each of the species to be discussed.

History

Charles Darwin was one of the pioneers in the study of these plants and I would refer the reader to his book (Insectivorous Plants (1875)) for a historical perspective on these organisms.

In this series I would like to focus on 3 common insectivorous plants, the Venus flytrap, sundew, and the pitcher plant, and discuss some of the more recent findings on their highly specialized adaptations (both morphological and biochemical). Today's post will focus on the Venus flytrap.

Venus Flytrap

Venus flytrap (D. Rhodes)

The Venus flytrap Dionaea muscipula, a member of the Droseraceae family of angiosperms, has evolved modified leaves which possess mechanosensors. The tips of Dionaea leaves have developed into bi-lobed snap trap-type capture organs, equipped with a number of mechano-sensitive hairs and a densely packed array of glands (Böhm et al. (2016)).

Insects are attracted to the capture organs in part by a fruity volatile blend (emitted mostly by starving flytraps) that is comprised of a number of volatile organic compounds (VOCs). Over 60 VOCs, including terpenes, benzenoids, and aliphatics, were emitted by Dionaea, predominantly in the light and under nitrogen starvation conditions (Kreuzwieser et al. (2014)). Clearly this is a strategy that is similar to the use of floral scent by other angiosperms to attract pollinators (see my series on Floral Scent - Part 1, Part 2, Part 3, Part 4, and Part 5). One insect lured by the fruity aroma of the Venus flytrap is the fuit fly Drosophila melanogaster (Kreuzwieser et al. (2014)).

The traps of the Venus flytrap are tinged red and this may also provide visual cues for insect attraction. Moreover the peripheral zone (with carbohydrate secreting glands) is UV-absorbing while the trapping zone is UV reflecting (discussed in Jurgens et al. (2009), with reference to photographs presented by Joel et al. (1985)). This UV contrast may serve an important role in attracting the attention of insect prey. Because the traps are arranged in a rosette above the ground, the traps may also capture crawling insects. The flowers of the Venus flytrap are elevated several inches above the rosette of traps and are white in color, and tend to attract a different range of insects (Youngsteadt et al. 2018). It remains unclear however, whether the floral scent of the Venus flytrap flowers differs substantially from the scent emitted from the trap.

The Youngsteadt et al. 2018 paper was recently featured in Science News (Zielinski (2018)), and Youngsteadt is quoted as saying ... "different scents or colors produced by flowers and traps might lure in different species to each structure. That’s another area for future study. While attraction to scent and color are well documented for traps, little is now known about those factors for the flowers."

The trap mechanism is succinctly described by Bemm et al. (2016) as follows:

The trap "hairs allow Dionaea to recognize prey by transducing a mechanical stimulation into an electrical signal known as action potential (AP). The first mechano-electric stimulation of a trigger hair sets the trap to an 'attention mode.' In other words, a one touch-induced AP is memorized but does not close the trap. With a second AP, the Dionaea trap closes within a fraction of a second (Forterre et al. (2005), Escalante-Perez et al. (2014)), locking the prey between the two trap lobes. Prey, when trying to escape, repeatedly touch the mechano-sensors, thereby eliciting repetitive firing of APs. In a very recent study, Böhm et al. (2016) showed that the Venus flytrap can count the number of APs generated, thus 'memorizing' how often an insect has touched it and preventing false alarms. While two APs trigger fast trap closure, more than five APs result in the capture organ becoming hermetically sealed. Numerous glands that cover the inner surface of the stomach start expressing genes that encode enzymes involved in decomposing the prey into its nutrient building blocks (Schulze et al. (2012))." (Bemm et al. (2016)).

The digestive fluid is acidic (pH 4.0) and contains various enzymes: chitinases, lipases, phosphatases and peptidases (Risør et al. (2015)). Chitinases are specialized enzymes that degrade chitin in the cuticle (chitin-rich outer layer) of insects and spiders. The degradation of prey proteins is performed primarily by cysteine endopeptidases (Risør et al. (2015)).

Schulze et al. (2012) show that insect-capture of Dionaea traps and subsequent digestion of prey is modulated by the phytohormones abscisic acid (ABA) and jasmonates, primarily jasmonic acid (JA). JA is a wound/touch triggered hormone in plants, and appears to be engaged in initiating a massive change in gene expression facilitating prey digestion and nutrient uptake by the trap (Bemm et al. (2016), Böhm et al. (2016)). "The touch hormone jasmonic acid (JA) signaling pathway is activated after the second stimulus, while more than three APs are required to trigger an expression of genes encoding prey-degrading hydrolases; this expression is proportional to the number of mechanical stimulations." (Böhm et al. (2016)).

Sodium ions released from the dead/dying insect are taken up by sodium-selective channels, and the sodium ions so acquired may contribute to the plant's ability to maintain its cellular osmotic potential and turgor (Böhm et al. (2016)).

Please feel free to comment or ask questions below. I will try to respond as soon as possible