How-to-guide.md

-## Define a root to build the taxonomy tree
+## Define a root taxon from which to load the taxonomy tree
 
-By default, MKBDR load the NCBI taxonomy tree using "vertebrae" as a root taxon ( Taxon ID = 7742).
+By default, MKBDR load the NCBI taxonomy tree using "vertebrae" as a root taxon ( Taxon ID = 7742) covering only vertabrates taxonomy.
 It is possible to define a different root taxon to load the taxonomy tree using the option `--root_ncbi_taxon_id`.
 
 For instance if you are working on plants, you can use the node "Viriplantae" (Taxon ID = 33090) as a root to load your ncbi taxonomy tree.