The 35S promoter is a really strong constitutive promoter. Just expressing the OxO gene from it could be enough of a metabolic cost as to be noticeably growth i

The 35S promoter is a really strong constitutive promoter. Just expressing the OxO gene from it could be enough of a metabolic cost as to be noticeably growth inhibitory, much less the effect of the unintentional gene knockout. It looks like they have a good plan for future engineering: Induced expression only in cells in the infected part of the tree.