Obtaining essential nutrients is a task that all plants have to perform. The majority of plants manage perfectly well via obtaining nutrients through rooting systems from the soil or ground material. A small proportion of plant species obtain essential nutrients through other means. One of these is parasitism of other plant species, to either a partial or total degree. An even smaller number of plant species have resorted to another extreme method to obtain essential nutrients – carnivory. Carnivory has evolved independently several times in plants in different forms. Carnivorous plants tend to be found in low-nutrient environments, which has presumably been a key evolutionary driving force in the emergence of this nutrient-obtaining strategy. Advances in, and wider availability of, DNA sequencing technology in recent years mean that the genomes, or draft versions of them, of several carnivorous plant species are now available. Access to these genomes will allow us to understand in a manner previously that was previously impossible how carnivory in plants arises at a genetic level and how closely related it may be to other processes performed by a wider variety of plant species. An international consortium of authors have recently taken advantage of this in a paper published Open Access in Current Biology.
The authors analysed the genomes of three species of carnivorous plants belonging to Droseraceae, one of the largest families of carnivorous plants. Two of the species, Dionea muscipula (Venus Fly Trap) and Aldrovanda vesiculosa, have touch-sensitive snap traps. The other, Drosera spatulate, has tentacle-based traps. The authors identify several groups of genes that are specifically expanded in the carnivorous species. These include groups of enzymes likely involved in prey digestion or production of metabolites involved in prey attraction, as well as genes related to vesicle transport (an important part of the release of digestive components and subsequent nutrient uptake). In addition to the expansion of certain groups of genes, the authors identify that carnivory is associated with loss of gene content related to other functions. One such loss is genes associated with root function, perhaps unsurprising when carnivorous plants have seemingly shunned root-based nutrient uptake in favour of carnivory.
Following on from this, the authors profiled the expression of genes in different tissues in one of the carnivorous species, Di. muscipula, including in different parts of the carnivorous structures. Interestingly, expression of some genes usually associated with root function is strong in the traps of Di. muscipula. The authors state: ‘This situation strongly suggests that genes used for prey-derived nutrient absorption in Di. muscipula were recruited from the root, the organ engaged with soil nutrient exploration and absorption in non-carnivorous plants’. In other words, while Di. muscipula has resorted to a rather unusual strategy to obtain nutrients, it may have used features common to other land plants to develop this. In support of this, the authors also find genes expressed in the rim of Di. muscipula traps that are similar to those used in non-carnivorous plants to attract pollinating insects, implying that multiple features of carnivory may have been built up using already-available genes.
It is already known that some genes involved in plant carnivory are not constitutively expressed, but are only activated once prey has been caught. This includes, for example, genes encoding nutrient transporters. The authors investigated how such genes may be specifically activated to support carnivory and found that these genes seem to be targets of certain types of transcription factors, which control gene activity. These transcription factors seem to be specifically expressed in the activated traps of Di. muscipula and are also encoded by the genomes of the two other carnivorous plant species. These transcription factors also exist in non-carnivorous plant species and are involved in responses to stress or attack. This suggests that carnivorous plants may have re-purposed both the actual machinery required for carnivory, as well as mechanisms to regulate and optimise its use, from other more general processes.
As the authors conclude: ‘Thus, the path to carnivory could have been open to most plants. To the relief of the animal kingdom, only a select few have evolved along this route and became green hunters.’ Perhaps we should all be grateful for this!
Cover image from Scott Darbey/Wikimedia Commons