Diversity and colonization strategies of arbuscular mycorrhizal fungi in Mediterranean semiarid environments

Resumen

The maintenance of soil resources is a key factor for optimizing the stability and productivity of natural ecosystems and also to prevent erosion and minimize the effects of environmental stresses. The role of soil microorganisms is crucial for the integrity of terrestrial ecosystems and the development of climate change mitigation strategies. Among them, arbuscular mycorrhizal fungi (AMF) are one of the most influential groups in the maintenance of the stability of terrestrial ecosystems. They establish mycorrhizal associations (named arbuscular mycorrhizas) with most of the terrestrial plant species. AMF colonize the root system and improve plant growth by increasing nutrient and water uptake. Moreover, they usually protect plants against biotic and abiotic stresses. In turn, the fungi receive carbon compounds from photosynthesis. Mycorrhizal symbioses also improve soil structure and play a strong influence on plant productivity and diversity and therefore on ecosystem development. There are three main types of propagules from which AMF can colonize new plants: spores, fragments of mycorrhizal roots and fungal hyphae growing in the soil (extraradical mycelium, ERM). Mediterranean environments of Southeast Spain are characterized by particular climate conditions such as scarce and irregular rainfall, a long, dry and hot summer and human-mediated degradation activities, which may synergistically act as driving-forces able to promote desertification processes. In these environments, AMF can play a key role by improving plant establishment and tolerance against nutrient deficiency, drought and other environmental stresses characteristically involved in soil degradation. AMF distribution patterns are still unclear and differences detected in AMF diversity among sites have been attributed to differences in the associated host plant species, soil physicochemical characteristics or intrinsic properties of AMF species, as dispersal ability. In order to better understand the behavior of AMF and the patterns affecting their distribution in natural ecosystems, the general objective of this Doctoral Thesis was raised. Such objective is to increase the knowledge about the colonization strategies and diversity of AMF associated to plant communities and threatened plant species in Mediterranean environments. The sites selected for this study are located in two protected areas belonging to the Baetic Range (Southern Spain): Sierra de Baza and Sierra Nevada Natural Parks, both of them located in the Granada province. Roots and rhizosphere soil of the five most common plant species from the basophilic scrub in Sierra de Baza were sampled: Genista cinerea (Vill.) DC., Lavandula latifolia Medik., Rosmarinus officinalis L., Thymus mastichina L. and Thymus zygis L. Moreover, roots of Narcissus nevadensis, an endangered and endemic plant species from Sierra de Baza and Sierra Nevada, were also sampled. Narcissus nevadensis grows in wet meadow habitats showing different edaphic and physico-chemical characteristics. In Sierra de Baza Natural Park, the AMF community composition of the different propagule fractions associated with the five plant species from the target shrubland was identified using 454pyrosequencing of the small subunit (SSU) rRNA gene. By other hand, seasonal changes in the AMF community composition present in the three propagule types were monitored by using a terminal restriction fragment length polymorphism (TRFLP) approach of the SSU rRNA gene. Moreover, the composition of the AMF community in the roots of rosemary (Rosmarinus officinalis L.), inoculated with the three different propagule types, was also analyzed by using TRFLP of the SSU rRNA gene. Finally, the AMF community associated with individual N. nevadensis plants was determined using 454 pyrosequencing of the SSU rRNA gene. A total of 96 virtual taxa (VT) were recorded among the five shrubland plant species, including 31 novel and non-previously described VT. AMF communities differed significantly among the propagule fractions. A significant effect of host plant species on AMF community composition was detected only among root samples. The clear pattern of AMF distribution between the different propagule forms showed a taxonomic basis at the level of family, suggesting different life strategies: those allocating their biomass mainly into the roots (belonging to the Glomeraceae family), the spores (Pacisporaceae and Paraglomeraceae) or the ERM (Diversisporaceae and Gigasporaceae) fractions. Our results show that differences of AMF taxa in the biomass allocation patterns among propagules are maintained throughout the year. The AMF community composition in roots was significantly different between spring and autumn; however, no significant differences were detected in the AMF communities detected in soil propagules (spores and ERM). By other hand, the AMF communities established in the rosemary plants was significantly different according to the type of propagule used as inoculums, suggesting different colonization strategies for the different AMF families involved and reinforcing the results previously obtained. Whereas members of the Glomeraceae and Claroideoglomeraceae families predominated when colonized roots were used as inoculum, members of the Pacisporaceae and Diversisporaceae were mainly detected when spores or ERM, respectively, were the inoculum source. Regarding N. nevadensis, a total number of 81 VT were detected, including nine novel VT not previously detected in the database neither in the former study developed in Sierra de Baza. The AMF community composition significantly differed between the two studied locations and, to a lesser extent, by soil characteristics. The Glomeraceae family was the most frequently detected in association with N. nevadensis at every location but it was less frequent in Sierra de Baza. The highest VT richness, as well as the highest number of novel and rare VT were detected in Sierra Nevada alkaline soils. This fact, together with the high homogeneity detected in the AMF communities in this site, suggests that it is quite well preserved and that represents an important reservoir of AMF diversity that must be specially protected. The joint analysis of the AMF community colonizing the roots of plant species from the basophilic scrub and N. nevadensis growing in alkaline soils from Sierra de Baza, indicated that the composition of AMF communities were significantly different according to plant species, plant growth forms and habitats. Despite the high differences among the habitat and plant growth of the target plant species, a ubiquitous group of AMF VT was detected in the roots of all studied species, belonging mainly to the Glomeraceae and Claroideoglomeraceae families. These fungi appear to be generalist fungi. However, other VT, probably specialist fungi, were exclusively detected in some of the studied habitats. In summary, this Doctoral Thesis offers clear detailed evidence from natural systems that AMF taxa are differentially allocated among propagule types and these differences are maintained through the year. It is also shown that AMF taxa differ in their ability to colonize new plants from the different AMF propagule types as well as the existence of some generalist AMF fungi, with a ubiquitous distribution, and other specialist AMF. These results altogether reveal important differences in the life-history strategies among AMF. The results of the present Doctoral Thesis provide relevant information to interpret the studies of AMF diversity and to take into account in future restoration programmes aimed to increase the resilience of the plant species against environmental stresses in the Mediterranean area.