Insectivorous Plants - Part 2

Sundews (D. Rhodes)

The sundew plants pictured above were used by me as a novel "biological control" to suppress shore flies in the Purdue University Horticulture Greenhouse during my vegetable crops class. Shore flies are a nuisance pest feeding on algae growing on greenhouse soils (Shore flies in the greenhouse).

In my previous post (Insectivorous Plants - Part 1), I focused on the Venus flytrap (Dionaea muscipula). Charles Darwin labeled this plant ‘one of the most wonderful plants in the world’ (Gibson and Waller (2009)). Darwin was also fascinated by the sundew (Drosera) ... "‘I care more for Drosera than the origin of species ... it is a wonderful plant, or rather a most sagacious animal. I will stick up for Drosera to the day of my death.’" (Letter from Charles Darwin to Asa Gray (Jones, 1923)) (quote from: Gibson and Waller (2009)).

The Venus flytrap and sundews are members of the same angiosperm family (Droseraceae), and it is thought that snap-traps of the Venus flytrap evolved from a common ancestor with sundews (Drosera spp.). Sundews are generally known for possessing active flypaper traps with sticky tentacles (Cameron et al. (2002), Poppinga et al. (2013)). The tentacles (stalked glands) on the leaves' surface produce a sticky polysaccharide mucilage (Nakamura et al. (2013)) that is inspiring development of novel bioadhesives (Huang et al. (2015)). A possible intermediate species of the same family with a snap-trap is the waterwheel plant (Aldrovanda vesiculosa). The latter is an aquatic plant and so the traps are submerged (Gibson and Waller (2009), Poppinga et al. (2013)). Aldrovanda shares many reproductive characters with sundews (Drosera) (Cameron et al. (2002)).

The traps of many species of Drosera not only capture prey by using their glue tentacles, but also wrap around their prey over several hours and days (Poppinga et al. (2013)). This "wrapping" phenomenon can be seen in the images above.

Certain Drosera species have fast tentacles. "Drosera glanduligera, a sundew from southern Australia, features a sophisticated catapult mechanism: prey animals walking near the edge of the sundew trigger a touch-sensitive snap-tentacle, which swiftly catapults them onto adjacent sticky glue-tentacles; the insects are then slowly drawn within the concave trap leaf by sticky tentacles." (Poppinga et al. (2012)). This response occurs within a fraction of a second, similar to the speeds reported for Venus flytrap snap-traps (Poppinga et al. (2012)).

As in the Venus flytrap, the main function of the sticky traps of sundews is to obtain nitrogen and other nutrients, enhancing the rate of photosynthesis per unit leaf mass or area and hence increasing photosynthetic efficiency (Behie et al. (2013), Pavlovic et al. (2014), Pavlovic and Saganova (2015)). Phosphate is critical for photosynthesis in plants and phosphate may be one of the main nutrients acquired by Drosera from its prey that facilities increased photosynthetic efficiency (Pavlovic et al. (2014)). Reliance on prey-derived nitrogen by sundews seems to decrease with increasing nitrogen supply from other sources such as atmospheric N deposition (Millett et al. (2012)).

Sundews excrete various enzymes that serve to digest chitin (endochitinases; Jopcik et al. (2017)) and protein (proteases; Butts et al. (2016)), releasing amino acids that serve as nitrogen sources for plant growth. The sundew Drosera capensis appears to prefer protein over chitin as a nitrogen source. "Plants fed on chitin derived 49% of the leaf nitrogen from chitin, while those fed on the protein bovine serum albumin (BSA) derived 70% of its leaf nitrogen from this" (Pavlovic et al. (2016)). Tentacles of in vitro-grown round-leaf sundew (Drosera rotundifolia) show induction of chitinase activity upon mimicking the presence of prey with chitin (Matusíková et al. (2005)). Nutrient uptake by the plant may occur via specific membrane-bound transport proteins and channels, and also may be facilitated by a process known as endocytosis, enabling absorption and intracellular digestion of whole proteins (Adlassnig et al. (2012)).

As in the Venus flytrap (see (Insectivorous Plants - Part 1), electrical signals and jasmonates play a critical role in both sensing and coordinating the digestion of insect prey (Krausko et al. (2017), Mithöfer et al. (2014)). Jasmonic acid (JA) and jasmonic acid-isoleucine conjugate (JA-Ile) production is elicited by wounding, insect prey capture and oral secretions from insects (Mithöfer et al. (2014)). Prey-induced electrical signals (action potentials (APs)) are most likely involved in initiating jasmonate accumulation (Krausko et al. (2017)). Jasmonate accumulation then induces expression of digestive enzymes in the digestive fluids of sundews (Nakamura et al. (2013), Krausko et al. (2017)).

How do sundews avoid capturing their insect pollinators? Like the Venus flytrap many Drosera species have evolved long scapes (racemes/flower stalks) that separate the flowers from the trap rosettes and make the flowers more visible to pollinators (Anderson (2010)). Behavioural experiments have indicated that both white color and spatial separation between flowers and traps may aid in reducing pollinator entrapment while capturing prey (el-Sayed et al. (2016)). In a species of sundew that has flowers that are adjacent to their traps (Drosera auriculata) el-Sayed et al. (2016) "identified chemical signals emanating from flowers that comprised an eight-component blend, while the plant’s traps emitted a unique four-component blend. The floral odour attracted both pollinator and prey insects, while trap odour only attracted prey." Thus, Drosera species may utilize visual, spatial, and chemical signals to spare flower visitors while trapping prey insects. The flowers of Drosera auriculata emit a blend of terpenoids (including alpha-pinene, B-pinene and limonene) and benzenoids/phenylpropanoids (benzaldehyde, benzyl alcohol, phenylacetaldehyde, phenylacetaldehyde and phenylethanol). The traps, however, emit linalool, geranyl acetone, B-farnesene and plumbagin (el-Sayed et al. (2016)).

I will return to the special role of plumbagin in the Droseraceae in a future post in this series.

A final comment on sundews. It was recently found that the larvae of the predatory flower fly Toxomerus basalis (Diptera: Syrphidae: Syrphinae) scavenge on the sticky leaves of several carnivorous sundew species in Brazil (Fleischmann et al. (2016)). These flower flies literally steal the sundew's lunch, and this type of thievery has been coined "kleptoparasitism"!

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