Moreover, a nodule-specific expression pattern has been reported for a putative plant copper
transporter in M. truncatula (Fedorova et al., 2002). In line with these findings, we show here that L. japonicus plants harboring mature functional N2-fixing nodules accumulate miR397 in both nodule and leaf tissues, a process reverted when higher Cu2+levels are supplied
in the growth medium. Based on these collective data, it seems likely that nodules become major Cu2+sinks in the plant at the onset of bacterial N2fixation. Subsequent regulation of nutrient allocation and prioritization would then rely on the host molecular machinery,
including miR397, which could thus play an important role in ensuring N2fixation and nodule functionality.This would explain the codependence of L. japonicus miR397 induction on functional symbiosis and Cu2+ availability. Indeed, plants harboring 28-dpi fully N2-fixing nodules show strongly enhanced miR397 levels when compared with the earlier symbiotic stages, where N2fixation-associated Cu2+demands are predicted to be less pronounced. Accordingly, elevated Cu2+levels in the growth medium are sufficient for a marked reduction in the local and systemic accumulation of miR397 at the later nodule stages. Nevertheless, we cannot ex-
clude the possibility that it is the onset of nodule senescence, setting in at 4 to 6 wpi under our conditions, that activates the Cu2+-deprivation miR397 systemic network through an unknown molecular mechanism.