Carnivorous plants inhabit nutrient-poor environments, where prominent targets of prey capture are organic nitrogen and phosphorus. Some carnivorous plants also acquire carbon from their victims. A new report focusing on Utricularia, the bladderwort, demonstrates that carbon assimilated from photosynthesis is paradoxically secreted into the trapping environment, where it may help to support a mutualistic bacterial community. This bacterial community may also secrete allelochemicals that attract microcrustaceans which bear a strong overt resemblance to bladderwort traps. Furthermore, Utricularia and its sister genus Genlisea share anomalous molecular evolutionary features, such as highly increased rates of nucleotide substitution and dynamic evolution of genome size, from approximately 60–1500 megabases depending on the species or even population. A mechanistic hypothesis, based on the mutagenic action of reactive oxygen species (ROS) is proposed to underlie these phenomena, involving error-prone repair at the level of DNA bases and double-strand breaks. It is argued that these plants are prime candidates for further research on the complexities of plant physiology associated with carnivory, metagenomic surveys of trap microbial communities, novel plant nitrogen/nutrient utilization pathways, the ecology of prey attraction, whole-plant and trap comparative development, and, finally, evolution of the minimal angiosperm genome.
‘What a wonderful and long-continued series of variations must have led up to the perfect “trap” in Utricularia, while at any stage of the process the same end might have been gained by a little development of roots and leaves, as in 9999 plants out of 10 000!’
Alfred Russel Wallace, from a letter to Charles Darwin dated 21 July 1875 (Marchant, 1916, p. 234)
The carnivorous bladderwort, (Utricularia, Lentibulariaceae), a favourite of Darwin's (Darwin, 1875), has proved to be an excellent system for studying biological processes related to plants trapping animals for nutritive purposes. Carnivorous plants are typically inhabitants of nutrient-poor environments, and it is well known that organic nitrogen and phosphorus are prominent targets of prey digestion (Ellison and Gotelli, 2009). It has also been shown that some carnivorous plants acquire carbon from their prey, in addition to that provided by photosynthesis (Juniper et al., 1989; Ellison and Gotelli, 2009). In this issue, Dagmara Sirová et al. report on the paradoxical secretion of photosynthetically absorbed carbon into the Utricularia bladder trap environment (Sirová et al., 2009a). Given this capacity, it is easy to imagine that carbon acquired from carnivory may also make its way into the negatively-pressured, liquid-filled bladders. Sirová's team used 13C labelling experiments to document that assimilated carbon is first allocated to apical growth, as expected for a plant in active development, but that as a function of shoot age, more and more soluble carbon is released into the bladder environment than into the vegetative structures that bear them. Still, the highest absolute rates of carbon release occur in the youngest traps, which is understandable, given that they will have the longest triggering/resetting lifetimes. Sirová et al. compare the phenomenon observed to carbon exudation by roots into the rhizosphere, which permits regulatory influence on local microbial communities. The intriguing issue with Utricularia is that the plant is rootless! Previous histochemical studies have detected the excretion of sugars from bladders, but the investigators were unable to uncover any reason for this phenomenon (Cheema et al., 1992).
Sirová and collaborators (Sirová et al., 2009b) have previously demonstrated that a large microbial community is associated with living bladderwort traps, and their hypothesis was that these bacteria play a mutualistic role in a complex food web that exists in and around the bladders. They compared bladderworts with corals, which ‘cultivate’ food from a microecosystem they create themselves by local water filtration (Ferrier-Pagès et al., 1998). In addition to bacteria, Utricularia traps collect a diverse flora and fauna of microplankton, as well as detritus. Dissolution of this ‘periphyton’ into vital nutrients in the enclosed bladder environment would certainly be aided by bacterial enzymes. In the work presented in this issue, the suggestion is made that carbon secretion and enhanced periphyton utilization in the sealed bladder environment, (i.e. after prey stimulation causes water engulfing and trap closure) permits bladderworts to live relatively competition-free in habitats otherwise occupied by rooted macrophytes. Regardless of the strength of such an effect, Sirová et al.'s carbon allocation experiments break new ground in pointing attention to the complexity of Utricularia's amazing bladders (Sirová et al., 2009a).
The nutrition of Utricularia still holds many mysteries. Do these trap-dwelling bacteria secrete allelochemicals in addition to serving the roles discussed by Sirová et al.? Cohn (1875) first proposed a chemical prey attractant for Utricularia, and bladder trichome secretions have since been shown to attract epibiotic rotifers (Wallace, 1978). Darwin (1875), noted yet another insight: aquatic Utricularia bladders bear a striking resemblance to microcrustaceans. The bladder shape, surface reticulations, stalk, and especially the antennae and bristles resemble microcrustacean anatomy. Interestingly, the bladders most closely resemble the littoral zone cladocerans (bosminids and chydorids) that are frequently found or overrepresented in bladders (Guiral and Rougier, 2007; Alkhalaf et al., 2009); see the silhouettes of clade-specific trap forms in Fig. 1. Moreover, experiments reveal that the cladoceran-like structures of bladders significantly improve the capture rates of cladocerans (Meyers and Strickler, 1979; Harms, 1999; Jobson and Morris, 2001). Some fish apparently selectively graze bladders (Walker, 2004). Utricularia has even been reported to capture and kill small fish, but there is a dearth of evidence for predation on fish in nature (Moseley, 1884; Gudger, 1947). The study of the ecomorphology of Utricularia is still immature, but the simple ‘mousetrap’ analogy for Utricularia is clearly too simple.
Other new lines of evidence put Utricularia biology in the spotlight. In 2006, Lubomír Adamec, a coauthor of the featured Sirová et al. (2009a) article, found that Utricularia bladders have immensely greater respiration, while exhibiting far lower photosynthetic rates than vegetative tissues (Adamec, 2006). The respiratory chain of mitochondria, normally coupled to electron transport, is one of the principal means that cells gain their energy for performing various activities. Electron transport drives a chemiosmotic pump that causes sequestration of protons in the mitochondrial intermembrane space, whereafter these positive charges enter the mitochondrial lumen to catalyse the phosphorylation of adenosine diphosphate into the cellular ‘currency’ of energy, adenosine triphosphate, (ATP). The ‘gatekeeper’ of the rate of oxidative phosphorylation is the enzyme cytochrome c oxidase (COX), positioned one step before ATP synthase. Two years before Adamec's report, Jobson and colleagues reported on the possibility that, due to specific amino acid positions under positive Darwinian selection, COX structure and function may be altered in Utricularia and some species of its sister genus, Genlisea (the corkscrew plant), to elevate energy production during times of need (Jobson et al., 2004). Specific hypotheses were that the uniquely changed residues [two contiguous cysteines (C) found only in one other organism in the entire National Center for Biological Information database] could alter the dissociation kinetics between COX and cytochrome c, and this hypothesis was later extended by Liisa Laakkonen and colleagues with a model whereby a redox switch at a vicinal C-C disulphide could actually cause a conformational change that would interfere with proton pumping, thus decoupling that process with electron transport (Laakkonen et al., 2006). In this way, the intermembrane space was envisioned as a capacitor holding massive positive charge until ATP was needed, i.e. to pump water out of traps after their firing.
But sequestering protons has its cellular consequences. Oxidative phosphorylation is an imperfect process; electrons sometimes leak into the mitochondrial lumen to reduce oxygen only partially, forming reactive oxygen species (ROS) instead of the normal and harmless water molecule. Moreover, the leakiness of the electron transport chain will increase when a greater charge potential exists between the intermembrane space and the lumen, as with the capacitor hypothesis above. ROS can be extremely harmful to cells, producing damage to DNA at both the nucleotide and whole-helix levels. Repair is inherently an error-prone process, and an expensive one for cells. Damage at the nucleotide level must either be repaired, or the mutations fixed. ROS can also lead to double-strand breaks, which, in plants, can lead either to deletions or to insertions of DNA via homologous or non-homologous end-joining (Puchta, 2005; Pacher et al., 2007). Intriguingly, and fitting quite well with the ROS hypothesis above, data suggest that both processes may be occurring rampantly in bladderworts.
Richard Jobson and Victor Albert first reported in 2002 that nucleotide substitution rates in Utricularia and Genlisea were radically sped up relative to their sister group, the carnivorous butterworts (Pinguicula, a sticky-leaf trapper), across the genomes of all three cellular compartments: the mitochondrial, plastid, and nucleus (Jobson and Albert, 2002; see also Müller et al., 2005). This systemic effect was, at the time, attributed to enhanced speciation rates among the bladderworts, with their incredible diversity of body plans (Taylor, 1989), but the ROS mutation hypothesis presented here is a simpler causative explanation.
Now to wax ecstatic, an amazing twist on Utricularia and Genlisea was recently uncovered by Johann Greilhuber and colleagues: some species possess the smallest haploid angiosperm genomes known, at c. 60 megabases (Mb), less than one-half that of Arabidopsis, with many bacterial-size chromosomes that vary widely in number between species (Greilhuber et al., 2006). There are still two more punch lines: (i) Utricularia and Genlisea species range in genome size from approximately 60 Mb to 1500 Mb, and (ii), as studied by Todd Michael, one of the species with the very smallest genomes, (Genlisea aurea) ranges from 60 Mb to 90 Mb to 120 Mb among three different geographically separated populations! Some preliminary work also suggests some populations of bladderworts have smaller than 60 Mb genomes. The dynamism of bladderwort and corkscrew-plant genome size suggests a minimal genome hypothesis whereby the downstream mechanisms that may be leading to DNA double-strand breaks, as indicated by the COX C-C amino acid motif, drive the turnover of non-essential genome space. This hypothesis could explain the wide span of genome sizes in the G. aurea populations and provide a platform to understand the evolution and specialization of genome structure in relation to nutrient acquisition. Despite the rather enormous genome size differences among such closely related genera, species, and populations, all Lentibulariaceae manage to make inflorescences and flowers, i.e. these genomic disparities must not interfere with the majority of normal plant processes. Clearly then, minimal bladderwort and corkscrew-plant genomes will provide insight into the minimal set of angiosperm genes.
Bladderworts grow in such a diversity of habitats, ranging from wet terrestrial to affixed on rocks in streams, attached at lake bottoms, floating aquatic, epiphytic in cloud forests, and even within the tanks of tree-dwelling bromeliads, and they bear their bladders species-specifically, but so heterogeneously, on stolon-like rhizoids, at leaf tips, on leaf edges, from inflorescence floats or leaf lamina surfaces, or even in entire leaf positions during embryonic shoot development (Taylor, 1989). As such, Sirová et al.’s article (Sirová et al., 2009a) reminds us that Utricularia and Genlisea are prime candidates for further research on the complexities of plant physiology associated with carnivory, metagenomic surveys of periphyton communities (see a first experiment in Fig. 2), novel plant nitrogen/nutrient utilization pathways, the ecology of prey attraction, whole-plant and bladder comparative ontogeny, and, finally, evolution of the minimal plant genome.