To monitor iron delivery to bacterial symbionts living in root nodules, the Balk lab has developed an iron-responsive reporter.
The bacterial lux (luciferase) operon was placed under the control of the mbfA promoter (membrane-bound ferritin). This promoter is active in bacteria during nodule development and regulated by iron.
We used the iron-responsive reporter to demonstrate that two related Vacuolar iron Transporter-Like (VTL) proteins of the host plant Medicago truncatula are required for iron delivery to the bacterial symbionts in nodules. Knockout mutants of VTL4 and VTL8 are affected in nodule development and nitrogenase activity (acetylene reduction) and have decreased iron reporter signals.
VTL8 is closely related to SEN1 in Lotus japonicus, and is the main route of iron delivery during the final differentation stage of the bacteriods. In contrast, VTL4 plays a much less essential role early in the infection process (Walton JW et al. 2020, New Phytologist).