Introduction
Urban areas represent ca. 0.5% of the land area in the world, in which around 55% of the human population currently lives (Seto et al. , 2012; Liu et al., 2020). However it is projected that by 2030, the urban coverage will increase by 50%, mainly due to the fast urbanization occurring in developing countries (Seto et al. , 2012). This rapid expansion of the urban ecosystem around the world can affect the ecology and evolution of urban dwellers by imposing new and extreme environmental conditions, such as impervious surfaces, high temperature, high pollution (soil, air, water, light, and sound), and high nutrient availability (Kaye et al. , 2006; Grimm et al. , 2008; Menberg et al. , 2013). Changes in abiotic urban attributes can cause alterations in the biotic component of the urban environment, for instance, by affecting the abundance and diversity of species within urban communities (Grimm et al., 2008), or by reducing the intensity or affecting the nature of ecological interactions (e.g. shift from mutualism to antagonisms or vice versa) (Miles et al. 2019; Irwin et al., 2020). In this way, disentangling the roles of urban abiotic factors on the ecology of biotic interactions is key to understanding the causal relationship that drives adaptation to urban environments. For instance, soil nutrient enrichment in urban environments can alter the soil microbiota that plays a central role in plant physiology, above-ground interactions, and plant fitness (Mejia-Alva et al. , 2018; Irwin et al. , 2020).
Urban abiotic conditions can also alter the microbiota communities (Weerasundara et al. , 2017), its relationship with plants (Linet al. , 2021), and potentially condition the function of the urban ecosystem (Harris, 1991). Urban soils are often highly enriched due to atmospheric deposition, combustion, importation of food, and fertilizer application (Harris, 1991; Kaye et al. , 2006). These factors condition the interaction among nutrients, for instance, high calcium (Ca) concentrations can increase soil pH, which in turn can enhance potassium (K), phosphorous (P), and nitrogen (N) availability (Osman, 2013). Specifically, direct and indirect effects between soil components that ultimately alter P and N availability can cause important alterations in the mutualistic plant-arbuscular mycorrhizal fungi (AMF) interaction by altering the cost-benefit balance that drives this interaction (Salvioli di Fossalunga & Novero, 2019; Irwin et al., 2020).
The mutualistic arbuscular mycorrhiza interaction is 460 myr old, and 80% of the plant species can form an association with arbuscular mycorrhizal fungi (AMF) (Smith & Smith, 2011). AMF are obligated symbionts, which provide increased availability of N and P to plants in exchange for carbon (Smith & Smith, 2011), as well as conferring the plant with resistance to various types of biotic and abiotic stresses, and increasing plant survival and reproduction (Ramos-Zapata et al., 2010; Mejia-Alva et al. , 2018; Wu & Zou, 2017). Nevertheless, this association, which can be measured as the AMF-colonization rate, can be disrupted by increasing P and N soil concentrations (Salvioli di Fossalunga & Novero, 2019), which reduces the benefits that plants obtain from the association with AMF (Smith & Smith, 2011). Such reduction in plant-AMF association can be expected in urban environments if P and N are enriched (Kaye et al., 2006). Interestingly, while previous studies have found that plants in urban soils interact less with AMF (Tyburska et al. , 2013), and that urban soils show lower spore richens, diversity, and density, than rural soils (Cousinset al., 2003), there are few studies that explore the urban soil properties that affect the plant-AMF association (Egerton-Warburton & Allen, 2000; Wiseman & Wells, 2005; Buil et al. , 2021).
Ruellia nudiflora (Acanthaceae) is a perennial herb that interacts with AMF and can grow in urban and rural environments (Tripp, 2007; Ramos-Zapata et al., 2010). Previous research has detected genetic variation in R. nudiflora for this association as well as AMF positive effects on fitness (Ramos-Zapata et al. , 2010; Mejia-Alva et al., 2018). For instance, AMF increased by 18% plant size and 15% fruit production of R. nudiflora (Mejia-Alvaet al., 2018), and indirectly increased fitness by reducing seed predation from the moth Tripudia paraplesia (Noctuidae) by 50% (Mejía-Alva et al. , 2018). All this evidence suggests that AMF can play a key role in the incursion of R. nudiflora into urban environments and become a useful model to understand the ecological and environmental effects of the urban environment on plant-AMF interactions.
The main goal of this research is to investigate the effects of urbanization on the mutualistic relationship between R. nudifloraplants and AMF, and to test the prediction that the mutualistic interaction plant-AMF should be reduced in enriched urban soil (Irwinet al., 2020). For this, we focus on answering three questions. 1) Do urban and rural soils differ in their abiotic properties and inR. nudiflora plant traits (e.g., biomass, root shape)? 2) DoR. nudiflora ’s AMF-colonization rates, spore density, and diversity vary between rural and urban environments? And, lastly, 3) How are AMF-colonization rates on R. nudiflora associated with soil properties, and to what extent are these associations affected by urbanization?