Unveiling Root Nodulation in Tribulus terrestris and Roystonea regia via Metagenomics Analysis

Root nodule symbiosis is commonly observed in plant orders like Fabales, Fagales, Cucurbitales, and Rosales within the rosid I clade of angiosperms. However, the formation of root nodule-like structures in Zygophyllaceae and Roystonea regia has been reported but not well understood. Our study aimed to clarify this by conducting morphological and metagenomic analyses on root samples from Tribulus terrestris (Zygophyllaceae) and Roystonea regia (Arecaceae) collected from West Asia and the Caribbean, respectively. We found nodule-like structures in both plant species, but these lacked the typical features of nodules seen in legumes, such as symbiosomes and fixation threads. Metagenomic analysis revealed the presence of potential nitrogen-fixing bacteria. However, these bacteria belong to the Gammaproteobacteria and Bacilli, which are not typically associated with nodulation. The observed structures differed notably from traditional nodules, suggesting a unique development influenced by the endophytes residing in the intercellular spaces.