Secondary metabolites are the compounds that a plant produces that don’t have an immediate role in the growth or reproduction of a plant. These would be things like pigments for petals, or anti-herbivore defences. Secondary metabolites are the tools that give plants the ability to do more than sit passively in their environment. But some plants may have more need for tools than others. Christopher Hatcher and colleagues have been examining the function of secondary metabolites in carnivorous plants.
The review goes through the secondary metabolites found in carnivorous plants, rather like an insect would. So it naturally starts with drawing food towards the plant.
How do you attract prey?
Hatcher and colleagues start by examining volatile organic compounds (VOCs). These are compounds that become gases at ambient temperature and are often used by plants to give them their perfume. The team found a lot of research on VOCs and plants use combinations of many of them to produce their own unique attractors. “Kreuzwieser et al. (2014) identified more than 60 different VOCs released by Dionaea muscipula (Venus flytrap),” write Hatcher and colleagues. “Their comparison of VOC concentrations before and after prey capture suggested that 20 compounds were directly related to prey attraction, through mimicry of fruit and flowering scents.”
There is far more to VOC production than stink and hope. Like a hunter selecting the right lure for his prey, N. rafflesiana produces different scents to attract different prey to its traps. Higher traps are more geared flying insects, while lower traps attract ants.
The complexity of VOCs means they could either have more than one function or else have been repurposed from other tasks to attraction. Hatcher and colleagues refer to research that shows some VOCs might stun insects, other scents can act as insecticides or help with digestion.
For pigmentation, the authors point out that red isn’t likely to be aimed at invertebrates, but they also have ultraviolet patterning. “Other functions for trap pigments have been suggested, such as photoprotection or light stress, as seen in Drosera rotundifolia and nutrient stress as observed in Dionaea muscipula and Drosera spathulata.
How do you capture prey?
Carnivorous plants produce plenty of tools to trap prey. Waxes are common in Nepenthes, while other plants like Drosera produce glues to stick to the prey. Roridula also has glue, but this produced in an entirely different way. The chemicals produced work to trap the prey, but they don’t work in isolation.
One of the more surprising features that Hatcher and colleagues note “Plumbagin is a secondary metabolite found in all genera of the Nepenthales and rarely outside this group of plants, though its specific function in carnivory in these species is yet to be fully determined.” The authors note that it is found on the rim of Nepenthes and may work with other features to have an anaesthetic effect on prey. Adding a drugging effect could help make the other features of a plant more deadly.”
Once prey is caught, other metabolites signal that digestion must start. Jasmonates are a common chemical used in defensive systems, and also common for signalling in carnivorous plants. It’s jasmonates that trigger a response on Drosera telling the plant where to bend its leaf to surround its prey.
How do you digest prey?
Hatcher and colleagues start the coverage of digestion metabolites by discussing how jasmonates trigger the production of digestive enzymes. While the enzymes help draw nutrients in, there is also a danger that digestion could also provide access to pathogens.
The authors highlight the importance of naphthoquinones and phenolics. “In non-carnivorous plant species these secondary metabolites act as antibacterial protection against pathogenic attack on plant tissue. In carnivorous plants they appear to have been co-opted to preserve and aid in the digestion of prey by preventing decay from bacteria as well as functioning to protect the plant tissue during the process of prey breakdown.”
Hatcher and colleagues say that plumbagin, as found on the rims of the pitcher plants, is found in every genus of the Nepenthales, regardless of whether the plant has a pitcher trap or not. They argue that in digestion plumbagin protects the plant from pathogens. “If plumbagin and its isomers are critical in protecting the plant during digestion or enhancing prey digestion, it would explain why these compounds have persisted regardless of trap morphology.”
Secondary metabolites as a tool for taxonomy
While Hatcher and colleagues have identified many different metabolites, there are also some shared chemicals between differing species. Development of different secondary metabolites could lead to speciation as plants develop new tools to attract and deal with prey. But also examining the metabolites species share can help reveal evolutionary relationships between plants.
“We hypothesize that metabolite diversity provided a mechanism for the evolution of carnivory in plants,” the authors conclude, “and that this continued diversity facilitates rapid evolution into new environments. Due to the multiple times carnivory has independently evolved and the general restriction of carnivorous plants to high-stress environments, these plants are an ideal system to investigate whether metabolic diversity may have been or is a way for novel traits to evolve and be maintained.”