Plant Cuttings

This image of leaf-cutter ants Atta cephalotes (Bug World, Bristol Zoo, England) [The grey piece is a wire binding the rope.] by Adrian Pingstone is in the public domain.

The natural world is replete examples of co-operations between members of the different kingdoms of living things (Regina Bailey). For example, algae (kingdom Protista) and polyps (kingdom Animalia) in the coral partnership, and fungi (kingdom Fungi) with angiosperms (kingdom Plantae (Tara Jo Holmberg) in mutually-beneficial mycorrhiza (Angela O’Callaghan). All are fascinating relationships and great demonstrations of how unlike organisms can get along together and survive in the world.

One of the most complex inter-kingdom interactions is that defined by the activity of ants that ‘farm’ fungi*. These insects harvest plant material that is used to nourish fungi, which use the vegetation as their food source. In turn the ants consume the fungi and thereby sustain their own growth and development**. The common English name for these industrious invertebrates is leaf-cutter ants (Jaymi Heimbuch), because that’s probably the most well-known – and visible – thing that they do.

While most of us are probably happy just to accept that leaf-cutter ants cut up leaves, it’s always nice to discover that some people are more curious and want to know more about the phenomenon. The suitably inquisitive team of Daniela Römer et al. (J Exp Biol (2023) 226(12): jeb244246; https://doi.org/10.1242/jeb.244246)*** have investigated this behaviour, and solved the puzzle of how the ants excise leaf portions with such astonishing precision.

Leaf-cutter ants (Atta sexdens) (Alex Byrne) from colonies maintained at the University of Würzburg (Germany) [but originally from Brazil] were used in the study. But, although the ants were supplied with fresh leaves of blackberry/bramble (Rubus fruticosus) and privet (Ligustrum vulgare), the team used ‘pseudoleaves’ for the study. These pseudoleaves were fashioned from plastic sheeting known as Parafilm****. Although leaf-cutter ants readily use this as a substitute for proper leaves (Flavia Roces, Animal Behaviour 40(6): 1181-1183, 1990; https://doi.org/10.1016/S0003-3472(05)80185-X), Römer et al. took the precaution of exposing the pseudoleaves to an environment enriched in rose oil vapour, or slightly smearing them with crushed bramble leaves, to increase acceptance of the proxy leaves by the ants. Additionally, leaves of different thickness were constructed by arranging multiple layers of Parafilm on top of each other. The normal pseudoleaf-cutting behaviour of the ants was recorded as movies, as was any differences in this ability after using various ‘interventions’ such as lifting their legs away from the ‘leaf’, and removing sensory hairs from the ants’ necks.

After experiments were undertaken, data collected, and results analysed, Römer et al. (2023) propose that “ants do not use a single fixed reach to determine the size of the fragments cut, but display a plastic mode of cutting that relies on sensory information provided by at least two different feedback loops. Feedback from the limbs, predominantly the hind legs, appears to be the main mechanism providing directional information of the location of the leaf edge, while the subordinate guidance of the cutting trajectory by feedback from the head’s movements exerts control over the cutting trajectory to a lesser extent”. Evidently, these industrious invertebrates ‘use their heads’ when cutting leaf fragments – as well as their legs.

* This ant ‘fungal-farming’ behaviour (more here, here, here, here, Jennifer Tsang, Ada McVean, Emily Crotteau, and Bryan Handwerk) is not only sophisticated (e.g. with use of antibiotics (Hongjie Li et al., PNAS 115(42): 10720-10725, 2018; https://doi.org/10.1073/pnas.1809332115; Ayush Pathak et al., Trends in Ecology & Evolution 34: 974-976, 2019; https://doi.org/10.1016/j.tree.2019.08.007), but has been practised for a very long time, maybe upto 60 million years (Joe Caspermeyer; Jennifer Tsang). In this respect, it’s not surprising that there are those who consider that ants were the first farmers or agriculturalists (Phoebe Tjandra). And, if not the first, certainly did so long before humans began to develop their own system of agriculture approx. 12,000 years ago (Bryan Handwerk; Jackson Landers).

** Although the relationship between leaf-cutting and fungus-farming ants and fungi – plus the plants that supply the leaves – may look quite complicated and involves three different kingdoms of living things, it’s actually even more complicated/remarkable than that. A fourth kingdom – the Bacteria (Tara Jo Holmberg) – is also involved via the ant’s use of bacteria-derived antibiotics (Hongjie Li et al., PNAS 115(42): 10720-10725, 2018; https://doi.org/10.1073/pnas.1809332115; Ayush Pathak et al., Trends in Ecology & Evolution 34: 974-976, 2019; https://doi.org/10.1016/j.tree.2019.08.007) that help to keep the myrmyco-agricultural system disease-free. And, there’s an additional prokaryotic connection/dimension with discovery of the involvement of nitrogen-fixing bacteria (Stephen Wagner; Vasyl Cherlinka) whose biochemical activity “facilitates the cultivation of specialized fungal crops by leaf-cutter ants” (Adrián Pinto-Tomás et al., Science 326(5956): 1120-1123, 2009; doi: 10.1126/science.1173036). You ‘couldn’t make it up’(!)

*** For more scicomm insights into this work, see Kathryn Knight, and here.

**** Nothing to do with this leaf-cutter ant item, but it’s related to Parafilm, and deals with plant biology, and is therefore a good example of a ‘plant connection’ that Mr P Cuttings is quite fond of, is this story highlighted by Jeff Mulhollem. Entitled “Type of plastic film on high tunnels can filter sunlight, influence plant growth”, Mulhollem’s article understandably caught my attention. It’s a scicomm item that provides background and context to the research by Fritzner Pierre et al. (Horticulturae 2024, 10(1), 33; doi: 10.3390/horticulturae10010033). Pierre et al. investigated whether the quality of the plastic used in constructing polytunnel (Sean Barker, Elizabeth Waddington) growth environments had an effect on aspects of the growth, development, and behaviour of green and purple basil (Ocimum basilicum L.) cultivars. Four types of polyethylene films with varying light diffusion and ultraviolet (UV) radiation transparency levels were investigated: TuffLite IV (TIV), KoolLite Plus (KLP), UV-transparent (UVT), and UV-opaque (UVO). As you might have expected [scientific papers usually report that something’s happened and been discovered, rather than nothing was found], the type of plastic had significant effects on the growth of both basil cultivars: “Green basil generally exhibited higher fresh and dry biomass compared to purple basil. Leaf area, stem, leaf, and total plant biomass were influenced by the plastic cover, with UVO and UVT films resulting in higher biomass production. The plastic covering films showed varying effects on the mineral content, total chlorophyll, carotenoids, total phenolic compounds, and antioxidant activity, with UVO and UVT films often resulting in improved nutritional quality compared to traditional films. Furthermore, covering films influenced the downy mildew severity on both cultivars and the UVT film consistently limited the severity of the foliar disease in both genotypes” (Pierre et al., 2024). With a wide range of different outcomes dependent upon choice of film, the researchers usefully conclude that “Overall, this study highlights the importance of selecting appropriate plastic covering films with varying levels of UV transparency in high tunnel production systems”.