2019/05/01 by Laurent Augusto, Nicolas Fanin, Mark R. Bakker · 1 citation
Agricultural and Biological Sciences · #Mycorrhizal Fungi and Plant Interactions #Plant and Biological Electrophysiology Studies #Biocrusts and Microbial Ecology #Nutrient #Biology #Ecosystem #Weathering #Ecology #Botany #Organic matter
paper · doi:10.1111/1365-2435.13325
openalex publication_date 2019/05/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/29
Over geological times, plants have developed collaborative strategies with micro-organisms to acquire scarce nutrients in the environment (Martin, Uroz, & Barker, 2017). Among them, symbiotic associations with mycorrhizal fungi promote plant growth through increasing soil exploration and/or through accelerating the decomposition of organic matter (Lindahl & Tunlid, 2015), while symbiotic associations with bacteria enable increasing nitrogen availability through dinitrogen fixation from the atmosphere (Hartwig, 1998). About two decades ago, Jongmans et al. (1997) also reported on observations where mycorrhizal fungi in association with trees were able to weather minerals directly. Later on, the “rock-eating” mycorrhizal fungi model was confirmed across a wide range of ecosystems and plant functional groups, highlighting that fungi are particularly efficient in weathering soil minerals, especially in soils impoverished in nutrients (Blum et al., 2002; Hoffland et al., 2004; van Schöll et al., 2008). Multiple lines of evidence in nutrient-poor ecosystems have also led to the conclusion that plants can obtain scarce nutrients themselves, for example through the development of specialised root systems such as dense packing of root hairs that form cluster roots, or dauciform roots (Lambers, Brundrett, Raven, & Hopper, 2010; Lambers, Raven, Shaver, & Smith, 2008), or through the formation of a myco-heterotrophic lifestyle (McKendrick, Leake, & Read, 2000), or through a carnivorous strategy by getting nutrients from the digestion of small animals (Ellison & Gotelli, 2001). Yet, we know only little about whether and how root systems are able to accelerate the release of essential nutrients from bedrock into ecosystems, and even less about the regulatory mechanisms set-up by these “rock-eating” plants. Recently, Perakis and Pett-Ridge (2019) demonstrated that the production of nitric acid in the rooting zone of N-fixing trees increased mineral weathering, likely because of the acidity induced by the N fixation process. However, many plants are not associated with symbiotic bacteria and/or mycorrhizal fungi and the mechanisms used to release minerals from rock by those species are still unclear. In the present issue of Functional Ecology, Sales-Teodoro and her colleagues report new observations on a specialised type of plant roots from the Velloziaceae monocotyledonous family, which is able to weather a highly resistant quartzite rock to directly mobilise phosphorus through physical and chemical interactions involving exudation of carboxylates. This new type of root has not been previously described, suggesting that we are still far from understanding the ins and outs of all plant strategies implemented to acquire nutrients from rocks in the environment. This new functional type of root was found in the Campos Rupestres area in one Brazilian OCBIL region (i.e., “old climatically buffered and infertile landscapes”) characterised by an extreme level of nutrient scarcity. In this mesothermic zone, the authors have observed that some species in the Velloziaceae family were able to grow directly on quartzite rocks. This behaviour is no less than surprising because quartzite is a rock derived from quartz sandstone and, as such, is a nutrient-poor substrate hard to penetrate. The main question of the Sales-Teodoro's group was to determine how these plants were able to take up nutrients, in particular phosphorus that is typically lacking in this OCBILs (Oliveira et al., 2015). To do so, Sales-Teodoro and her colleagues set up a fairly simple, but elegant, study design. They collected plant individuals from two non-mycorrhizal species, each plant being established on a quartzite rock. In the laboratory, rocks and plants were analysed and observed using a battery of tests involving different methodologies from chemical analyses to microscopy (X-ray fluorescence, micro-XRF, liquid chromatography, mass spectrometer). The conclusions of these investigations are clear: some species of the Velloziaceae family are able “to produce root tunnels into the rock,” and to forage phosphorus by excreting carboxylates enabling mineral weathering. This mechanism is possible due to physical and chemical interactions via a specialised non-mycorrhizal root that was undescribed until now. Sales-Teodoro's group called their rock-eating root the “vellozioid” root. The weathering process involved, which is the release of carboxylates, was already known for other types of specialised roots (Lambers et al., 2008), but the morphological and physiological root adaptations described here are intriguing: the fibrous roots that enable the root growth into the rocks are rather classical root traits, while the specialised root zones (with a suberised exodermis, extremely long root hairs, vesicles and invaginations) remind more symbiotic structures. Even though this type of root specialisation may be of limited relevance for biogeochemical cycles at the global scale, it is quantitatively important for OCBIL regions. Also, this finding is a good example of the astonishing diversity of the functional adaptations of plants to nutrient limitation. Other yet unknown examples of plant adaptations probably exist elsewhere. Sales-Teodoro's study hence directly highlights how necessary it is to continue exploring the functional diversity in the plant kingdom. Notably, there are large uncertainties about what root traits are prominent in driving the acquisition of nutrients, and about the existence of the acquisition–conservation trade-off (the “Root Economics Spectrum”; Erktan, McCormack, & Roumet, 2018). Indirectly, this study also highlights the need for diversity in research approaches. Modern scientists are expected to test complex hypotheses using costly equipment and methods. But well-conducted observations, in combination with relevant methods of analysis, are still highly useful and can lead to new discoveries, as was done in this study. Much effort will be necessary to understand the evolutionary factors influencing the functional adaptation of vellozioid roots to rocks and the coupling to other biogeochemical cycles, such how nutrients are obtained and resorbed-recycled during the continuous iterative process of the growth of ephemeral roots into the rocks. This also begs the question whether the functional adaptation of vellozioid roots favours biodiversity in these weathered ecosystems because of niche differences, which are often considered as the main drivers of long-term species coexistence by preventing competitive exclusion. Although these questions remain unresolved, the study of Teodoro et al. (2019) is outstanding and we believe that new studies should inspire and build their experimental design upon this work, from the observations in the field to chemical analyses in the laboratory.