Venus flytrap carnivorous lifestyle builds on herbivore defense strategies

Carnivory is not restricted to the realm of animal life; the plant kingdom also has flesh eaters (Darwin 1875; Adamec 1997; Krol et al. 2012). This remarkable trait has evolved independently at least six times in plants (Albert et al. 1992; Ellison and Gotelli 2009). Techniques to catch prey are as diverse as pitcher, sticky, and snap traps. Remarkably, some of these have evolved convergently in different clades. However, the carnivory lifestyle appears costly for the plant (Givnish et al. 1984), implying that the metabolic price deters a more frequent use of plant carnivory than its invention. Assuming that plant carnivory is based on readily accessible traits, Charles Darwin suggested routes as to how plant carnivory was gradually acquired (Darwin 1875). Nonetheless, despite increasing insights into the phylogeny of the carnivorous plants, current models mainly focus on the morphological and anatomical traits of the typical organs. Knowledge about the molecular and physiological roots of carnivory is essentially lacking, as are unbiased, data-driven approaches aimed at unraveling the molecular mechanisms involved in plant carnivory. Here, we focus on one of the most prominent carnivorous plants, Dionaea muscipula, which belongs to the Caryophyllales order. At least four of the carnivorous families—Nepenthaceae, Drosophyllaceae, Dioncophyllaceae, and Droseraceaea—can be unequivocally placed within the Caryophyllales (Heubl et al. 2006). Their presence within a single clade indicates a common ancestor and argues that sticky traps are the most ancient trap form within this clade; the snap traps found in Dionaea and Aldrovanda are derived from a sticky trap. Current phylogenetic reconstructions indicate that this type of trap evolved only once within the common ancestor of these two species (Cameron et al. 2002). Today, the Venus flytrap, D. muscipula, is only found natively in the Green Swamp of North and South Carolina. The leaf at the end of a photosynthetically active petiole of Dionaea develops into a green bilobed snap trap, with the inner trap surface equipped with peculiar mechanosensitive hairs. These 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, Bohm et al. (2016b) 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). Interestingly, mechano-electric stimulation can be substituted by direct administration of the touch hormone jasmonic acid (JA), suggesting that the number of APs translates into a chemical signal that roughly informs the plant about the size and nutrient content of a struggling prey. Although Darwin recognized early on that Dionaea’s animal prey consumption is based on electrical excitability (Burdon-Sanderson 1872) and fast flytrap biomechanics, the molecular mechanism of these animal-like features still remains poorly understood. Despite recent efforts in the molecular analysis of various carnivorous plant species, no lifestyle-specific genes have as yet been identified (Ibarra-Laclette et al. 2011, 2013; Leushkin et al. 2013; Fleischmann et al. 2014; Barta et al. 2015; Cao et al. 2015; Carretero-Paulet et al. 2015a,b; Stephens et al. 2015; Tran et al. 2015). To gain mechanistic insights into the molecular processes underlying the carnivory syndrome, we combined ultrastructural, physiological, and proteomic analyses with thorough transcriptome sequencing to analyze the prey-dependent changes in gross gene expression patterns and nutrient transport in relation to the endocrine biology of the glands.