Endophytic fungi associated with wild lima bean (Phaseolus lunatus L.) and sympatric plant species in Southern Mexico

Background/Question/Methods

Fungal endophytes live symptomless in plant tissues. These cryptic organisms have been found in all plant species investigated to date and show a bewildering diversity. It is has been hypothesized that they are key players in natural and agricultural ecosystems. Despite their ubiquity and hyperdiversity, the ecological function of endophytes remains widely elusive except in a few temperate grass species. In these grasses, endophytes produce toxic alkaloids which heavily impact herbivores. However, there is almost no information on the function of endophytes in other plants than grasses. Using lima bean (Fabaceae: Phaseolus lunatus) and sympatric species (Fabaceae: Phaseolus microcarpus and Convolvulaceae: Ipomoea indica) at natural sites in southern Mexico we addressed the question of how specific fungal endophytes are to host plant species and to sites. Fungal endophytes were cultured on agar plates and consecutively identified using ribosomal internal transcribed spacer (ITS) sequences. Lima bean is a particularly suitable system for this research approach as it represents a model plant for chemical ecology research as well as an important crop plant throughout the New World. 

Results/Conclusions

We obtained a total of 205 high quality ITS sequences that were grouped into 36 operational taxonomic units (OTUs; 97% sequence similarity). A total of twelve fungal orders was detected with the Glomerellales (51.2%) being the most dominant in the entire dataset, followed by Diaporthales (11.7%), Pleosporales (9.3%), Botryosphaeriales (4.4%), Xylariales (3.9%), Capnodiales (3.4%), Trichosphaeriales (3.4%), Sordariales (1.0%), Corticiales (1.0%), Hypocreales (1.0%), Agaricales (0.5%), Magnaporthales (0.5%). Host plant species had a greater influence on fungal community composition than site. For example, the Glomerellales were particularly dominant in Phaseolus lunatus and less so in the other two plant species. Fourteen OTUs had at least three representative sequences in our entire dataset. Eight of these OTUs were not detected in I. indica, while one was specific to I. indica and absent in the fabaceous plants. OTUs, which were detected in P. lunatus were present in P. microcarpus with the exception of two incidents demonstrating that the same endophytes often colonize related hosts. This diversity analysis is a first important step for functional studies on the ecological effects of endophytic fungi. Many of the taxa identified have been shown to produce secondary compounds, which might have effects on plant herbivores or pathogens.