From Roots to Leaves: The Capacity of Micromonospora to Colonize Different Legume Tissues

An important number of Micromonospora strains have been reported from nitrogen-fixing root nodules of legume and actinorhizal plants. However, the question of whether this bacterium can also be found in other parts of these plants remains unanswered. More than 150 strains were recovered from different Lupinus angustifolius and Pisum sativum tissues, including leaves, stems, roots, and nodules. In all, 97% of the isolates were identified by 16S ribosomal RNA gene sequence in the target genus and were associated with 27 different Micromonospora spp. Plant-polymer-degrading enzymes are suspected to play a role in the colonization of plants. To this end, bacterial enzymatic activity assays for amylases, cellulases, chitinases, pectinases, and xylanases were determined. All strains produced xylanases and pectinases, while 98.6, 98, and 94.6% of them produced amylases, cellulases, and chitinases, respectively. The most productive strains included seven isolates from P. sativum and one from L. angustifolius. Strain Micromonospora lupini ML01-gfp was used to determine its capacity to reach and colonize different plant organs using P. sativum as the plant model. Stem and leaf samples were monitored by optical and fluorescence microscopy to locate the tagged strain. These results strongly suggest that Micromonospora is able not only to infect nitrogen-fixing nodules but also to reach other parts of the host plant, especially the leaves.