GSMMs consider all metabolic possibilities given the available substrates. By changing the substrates and their uptake rates in the media you can dictate what’s available in the model. However, for most metabolites the uptake rates are unknown. This can be solved setting these fluxes as unconstrained in the GSMM. This also enforces decision to not include all root components but only the most prevalent. You don’t want that rare compounds have a huge impact on the model predictions. This does imply that the model predictions could be slightly negative: with including the other available compounds the system may work even better.
To get the most realistic approximation for BananaGuard in the soil the M9 media composition was used with carbon as limiting nutrient. The carbon source is based on the exudates of banana roots and consists of 34% sucrose and 66% divided over multiple amino acids