Symbiotic bacteria associated with legume tree species in a Mexican tropical dry forest

The ubiquitous dominance of legumes across successional stages in tropical dry forests is partly attributed to their ability to fix symbiotically atmospheric nitrogen. However, the diversity and composition of microorganisms with the potential to fix N 2 or produce phytohormones in association with different host tree species is often overlooked in forest ecosystems. In this study, we analyze the composition and diversity of bacteria associated with three common legume tree species in a tropical dry forest in northwestern Mexico. The study legumes are the pioneer Acacia cochliacantha Humb. & Bonpl. ex Willd., the generalist Lysiloma divaricatum (Jacq.) J.F.Macbr. and the late-successional L. watsonii Rose. We collected nodules from roots of seedlings grown in a greenhouse to determine their microbial composition using the 16S rRNA gene. We also describe strain characteristics and nodule diversity and relate nodule weight to seedling shoot and root length. Four species of bacteria were associated with different host plant species: Achromobacter mucicolens with A. cochliacantha , Beijerinckia fluminensis with L. divaricatum and Bacillus subtilis and Variovorax paradoxus with L. watsonii . A high proportion of variance in shoot length was explained by nodule weight in A. cochliacantha ( R 2 = 0.84), followed by L. divaricatum ( R 2 = 0.76) and L. watsonii ( R 2 = 0.69). The symbiotic association with growth-promoting bacteria likely enhances the ability of our studied plant species to colonize diverse environments and maintain a widespread distribution. Identifying key roles of symbiotic bacteria in legume tree species should be a priority to understand and restore belowground processes linked to aboveground patterns in tropical dry forests.